Hydrothermal Carbonization of Biomass for Hydrochar Production: Mechanisms, Process Parameters, and Sustainable Valorization

Abstract

Hydrothermal carbonization (HTC) represents a promising thermochemical method for converting wet biomass under moderate aqueous conditions into carbon-rich materials, characterized by specific attributes. Notwithstanding the increasing interest surrounding HTC, the current literature remains fragmented regarding the precise mechanisms by which process parameters influence hydrochar formation, its properties, and sustainable utilization. Consequently, the primary objective of this review is to systematically elucidate the fundamental mechanisms that govern HTC, to identify key parameters impacting hydrochar yield and quality, and to assess the sustainability and prospective contributions of HTC within the context of circular economy principles. This paper elaborates on the reaction pathways of hydrolysis, dehydration, decarboxylation, and aromatization that dictate the structural alterations and carbon densification of hydrochars. It emphasizes the roles of temperature, residence time, solid/liquid ratio, catalysts, and feedstock composition in jointly determining hydrochar yield, elemental composition, aromaticity, porosity, and energy density. Additionally, recent advancements, including microwave-assisted HTC, catalytic modifications, and post-activation techniques, are reviewed to enhance hydrochar functionality for applications in energy, adsorption, catalysis, and soil enhancement. Challenges remain regarding the scale-up of the process, reactor design, standardization of hydrochar properties, and the sustainable management or valorization of process water. This review integrates mechanistic insights with recent technological progress to position HTC as a versatile and sustainable method for producing high-value hydrochars, thereby underscoring its potential role in future biorefineries and circular economy initiatives.

1. Introduction

The escalating global demand for energy, alongside the depletion of fossil fuel reserves and the escalation of greenhouse gas emissions, underscores the critical necessity for sustainable energy alternatives. Within this framework, biomass emerges as a plentiful renewable source of carbon, serving as a versatile feedstock that can be transformed into high-value fuels and chemicals through advanced conversion technologies, thus providing a more efficient solution compared to direct combustion [1,2]. Biomass can be transformed into a wide array of innovative products through various conversion processes, such as pyrolysis, gasification, hydrothermal treatments, and biochemical conversions. These processes present a viable and sustainable alternative to conventional fossil resources for energy generation, thereby enhancing energy production while fostering environmental sustainability through reduced dependence on fossil fuels. Among these diverse processes, thermochemical transformation methods, including hydrothermal carbonization (HTC), are particularly effective for converting wet biomass into a carbon-rich solid fuel or a valuable material known as hydrochar. Such advancements not only improve the viability of biomass as an energy resource but also position it as a vital component in the transition toward a greener and more sustainable energy future. HTC operates at relatively moderate temperatures (180–260 °C) under self-generated pressures (ranging from 2 to 4 MPa) in an aqueous medium, facilitating the carbonization of biomass without the necessity for energy-intensive drying of the feedstock [3,4,5]. This feature renders HTC a cost-effective and environmentally friendly technology for biomass utilization and waste treatment [6,7,8,9]. For instance, lignocellulosic feedstocks processed under HTC conditions at temperatures of 453–533 K and approximately 4 MPa yield a dark, friable solid hydrochar suitable for use as a solid fuel or carbon material [10].

During the hydrothermal carbonization (HTC) process, biomass experiences a multifaceted series of reactions that encompass hydrolysis, dehydration, decarboxylation, aromatization, and condensation [11]. These transformations decompose biopolymers such as cellulose, hemicellulose, and lignin into smaller organic fragments and water-soluble intermediates, which subsequently reorganize and polymerize to form the solid hydrochar structure. The aqueous phase generated during the process captures numerous oxygen-rich decomposition products, whereas the solid hydrochar becomes progressively enriched in carbon. The key operational parameters, particularly reaction temperature and residence time, substantially influence the progression of the HTC process and the resulting characteristics of the hydrochar product [12,13]. Elevated HTC temperatures generally facilitate the dehydration and decarboxylation reactions, resulting in hydrochars that exhibit reduced hydrogen and oxygen content alongside a more aromatic structure. Conversely, lower temperatures tend to promote the retention of oxygen functional groups on the surface of the char [14]. Therefore, hydrochars produced at lower temperatures typically possess a higher concentration of hydrophilic functional groups, rendering them suitable for applications such as adsorption and soil conditioning. In contrast, hydrochars obtained at elevated temperatures show enhanced calorific values and fuel properties attributable to increased carbonization [15]. Similarly, reaction time exerts an important influence: a prolonged residence time generally diminishes the overall yield of hydrochar (due to the continued conversion of solids into liquids/gases) while enhancing the extent of carbonization and aromatic character of the hydrochar. Carbonization in an aqueous medium also promotes the generation of oxygenated surface functional groups on hydrochar, which may be advantageous for specific applications such as pollutant adsorption [8,16]. Hydrothermal carbonization (HTC) presents numerous pragmatic benefits compared to traditional dry thermal conversion methods, such as direct combustion or pyrolysis, for the processing of biomass. Foremost among these advantages is the technique’s capability to handle high-moisture biomass without necessitating prior drying, which significantly lessens the energy demand and overall costs associated with the process [8]. Moreover, the hydrothermal conditions inherent to HTC mitigate the generation of air pollutants; this typically results in minimal-to-negligible emissions of toxic gases, as the majority of volatile compounds are retained in the liquid phase rather than released into gaseous forms [17]. This outcome necessitates less stringent air emissions control, thereby enhancing the environmental sustainability of this technology.

HTC operations are relatively straightforward, and they are often conducted within sealed pressure vessels utilizing water solely as the reaction medium. This process can achieve a high solid product yield characterized by a low ash content, surpassing what is typically observed in combustion methods [18]. Collectively, these attributes render HTC a flexible platform for the sustainable treatment of waste biomass. The method’s capacity to produce cleaner solid fuels while accommodating wet, heterogeneous feedstocks positions it favorably in comparison to many conventional biomass conversion techniques with respect to both feedstock adaptability and overall process efficacy. The hydrochar generated is energy-dense and facilitates easy handling, transportation, and storage, akin to coal. Furthermore, the omission of a drying stage and the exothermic nature of certain HTC reactions contribute to enhanced energy efficiency relative to alternative methods, thus further decreasing operational costs [8].

The hydrochar produced via HTC exhibits unique physicochemical characteristics that frequently surpass those of the original biomass. Typically, hydrochar is hydrophobic and friable, which aids in its separation from process water and facilitates further dewatering or drying if required. It possesses a higher carbon content, elevated heating value, and reduced O/C and H/C ratios when compared to the initial biomass due to the concentration of carbon in the solid phase, along with the expulsion of some oxygen and hydrogen into the liquid and gas phases. Consequently, on a mass basis, hydrochar is more energy-dense (measured in MJ/kg) than the original biomass, with enhanced combustion properties, including improved ignition and uniform burning behavior [19,20]. The energy densification achieved through HTC can be substantial; research has indicated that the higher heating value of biomass is significantly increased to yield a solid fuel comparable in energy content to lignite coal. In summary, the hydrochar obtained through HTC demonstrates enhanced energy density and improved combustion characteristics compared to raw biomass. In addition, hydrochar commonly exhibits hydrophobic behavior and higher fixed carbon content, while typically possessing higher mineral ash content than raw biomass as a result of inorganic matter concentration during HTC. This makes it suitable for storage without concerns related to biodegradation while allowing for cleaner combustion [20]. The improved fuel characteristics, coupled with the favorable mechanical properties of hydrochar’s friable nature, allow for easy grinding into a powder, rendering it an appealing renewable solid fuel or potential coal substitute. Beyond its application as fuel, the presence of surface functional groups, such as carboxyl, carbonyl, and hydroxyl groups that are either inherited or developed during HTC, can be beneficial for various environmental applications. For example, hydrochar has been evaluated as a sorbent for pollutants in water and soil, utilizing its porous structure and surface chemistry to bind heavy metals and organic contaminants. Due to these properties, hydrochar has applications in several sectors, including carbon sequestration (as a stable carbon reservoir when integrated into soils), soil enhancement (serving as a conditioner to improve soil moisture retention and nutrient levels), and wastewater treatment (functioning as an adsorbent to extract impurities) [8,19,20]. Initially, the primary aim of HTC was energy generation, utilizing hydrochar as a biocoal. However, recent explorations have expanded its potential into various high-value applications, such as environmental pollutant remediation, catalyst or electrocatalyst support, electrode materials for energy storage, and even precursors for nanomaterials utilized in sensors or biocatalysis [21]. This widening scope accentuates the versatile nature of hydrochar and its prospective uses beyond its role as a fuel source.

In this review, the primary objective is to offer a detailed and methodical evaluation of hydrothermal carbonization (HTC) as a viable method for the conversion of biomass. The review elucidates the essential reaction mechanisms underlying HTC, assesses the influence of critical operating parameters on the yield and properties of hydrochar, and explores aspects of sustainability as well as challenges and opportunities for valorization. The paper commences with a discussion of the foundational principles and mechanisms of HTC and subsequently analyzes the impact of various process conditions on the characteristics of hydrochar. This is followed by an overview of the physicochemical properties of hydrochars derived from diverse feedstocks. Ultimately, the review highlights recent developments, potential applications, prevailing challenges, and future directions for research in this field.

2. Fundamentals and Reaction Mechanisms of Hydrothermal Carbonization (HTC)

In the process of hydrothermal carbonization (HTC), the generation of solid hydrochar is accompanied by the production of a byproduct known as process water (PW) or HTC liquid effluent. During conventional batch HTC operations, a considerable volume of water is introduced alongside the biomass, and subsequently, a significant portion of this water remains in liquid form post-reaction, containing dissolved organic materials. This process water is a considerable fraction of the overall output and is characterized by a diverse array of dissolved substances. In addition to water, HTC also results in the formation of a minor gaseous phase predominantly comprising carbon dioxide (CO₂) from decarboxylation reactions, along with lesser amounts of methane (CH₄), hydrogen (H₂), and other trace gases. The distribution of yields between hydrochar, process water, and gas is influenced by the specific HTC conditions as well as the characteristics of the biomass feedstock employed [22]. Generally, milder HTC conditions tend to facilitate higher solid yields, whereas more extreme conditions, such as elevated temperatures or extended reaction times, tend to elevate the proportion of organic compounds transitioning into the liquid and gaseous states [22]. These distinct and important reaction stages, along with the comprehensive overall HTC transformation pathway, are systematically illustrated in Figure 1. This informative figure provides a detailed and comprehensive visual summary of the sequential chemical conversions that take place from raw biomass all the way to hydrochar. It effectively highlights the key mechanistic steps involved, which include hydrolysis, dehydration, decarboxylation, and aromatization. Additionally, it showcases the corresponding structural and compositional evolution that occurs during the entire HTC process. This visual representation serves as a valuable resource for understanding the underlying processes that contribute to the transformation of biomass into hydrochar.

Before discharge, remediation of the liquid byproduct may be essential, utilizing strategies such as aerobic biological treatment, anaerobic digestion, adsorption, or advanced oxidation methods to eliminate or neutralize pollutants [24]. In addition, methods to valorize this liquid are under exploration to enhance the overall sustainability of the HTC process. For example, the organic-rich water can be reintegrated into subsequent HTC cycles (partial recirculation), contributing both to water reuse and enabling further reactions of dissolved organics, thereby mitigating contaminant loads with repeated cycles. Another strategy involves utilizing the process water as a feedstock for generating value-added products: investigations have focused on fermenting the HTC liquid to produce biogas or ethanol, recovering volatile fatty acids for use as chemical precursors, or leveraging the nutrient-rich liquid as a medium for algal cultivation or other bioprocesses [25,26]. Initially, these liquid-phase outputs were regarded strictly as waste in need of disposal, thus incurring additional treatment costs and increasing the overall environmental impact of HTC. However, recent studies offer a more optimistic perspective in which process water is seen as a secondary resource stream that is ripe for energy or material recovery. By implementing such strategies, recycling process water and recovering beneficial components, the efficiency and ecological sustainability of the HTC process may be notably enhanced, moving towards a zero-waste framework [25].

In alignment with the initiatives regarding process water valorization, it is essential to recognize that analogous endeavors have been thoroughly examined within hydrothermal liquefaction (HTL) systems, which are characterized by the presence of bioactive organic compounds in their aqueous phase. Recent research findings indicate that HTL process water, which is infused with antimicrobial elements, can serve as an effective plant protection agent. This efficacy is observed both independently and in conjunction with biocontrol agents such as Trichoderma virens, demonstrating its capacity to inhibit soil-borne pathogens like Verticillium dahliae, while concurrently fostering plant growth [27]. Although hydrothermal liquefaction (HTL) and hydrothermal carbonization (HTC) represent differing methodologies, these findings underscore the potential of hydrothermal process waters to be regarded as valuable resources rather than mere waste products. Given the chemical and nutrient complexities inherent in HTC process water, it remains plausible that analogous valorization strategies could be pursued in relation to HTC, contingent upon thorough investigations that affirm its composition, safety, and agronomic viability.

While this review primarily concentrates on the generation, characteristics, and valorization of hydrochar, it is essential to briefly address the fate and prospective applications of the liquid byproduct produced during hydrothermal carbonization, commonly referred to as process water. This aqueous phase is generally marked by elevated chemical oxygen demand and consists of a complex mixture of dissolved organic compounds, including low-molecular-weight organic acids, phenolics, furans, and other oxygenated intermediates [19,22,23]. In addition to traditional biological treatment or disposal methods, various alternative valorization pathways have been proposed to enhance the overall sustainability of the hydrothermal carbonization process. These pathways include the partial recirculation of process water into subsequent hydrothermal carbonization cycles, anaerobic digestion for biogas production, and thermochemical upgrading methods such as hydrothermal reforming or gasification. Furthermore, thermocatalytic strategies utilizing heterogeneous catalysts under hydrothermal conditions have been investigated to transform dissolved organics into value-added fuels or chemicals. Although a comprehensive assessment of these strategies is outside the remit of this review, their inclusion provides a broader understanding of the hydrothermal carbonization process and underscores potential avenues for integrated resource recovery.

In studying thermochemical conversion of biomass, it is crucial to distinguish hydrothermal carbonization (HTC) from other methods like pyrolysis and torrefaction, which produce char with different properties [28,29]. Pyrolysis and torrefaction are dry thermal processes that occur in low- or no-moisture conditions, and they are often compared to HTC for their role in producing solid fuels. Pyrolysis heats biomass to 400–600 °C in the absence of oxygen (in an inert atmosphere), resulting in three main products: solid biochar, condensable vapors forming bio-oil, and non-condensable syngas. Fast pyrolysis optimizes bio-oil, while slow pyrolysis maximizes biochar. Torrefaction, or mild pyrolysis, operates at 200–320 °C, effectively removing water and volatiles to create a dry, energy-dense product, known as torrefied biomass [18]. Both pyrolysis and torrefaction require dry feedstocks with moisture under 20%, unlike HTC, which uses water in the process.

Gasification operates at temperatures above 700 °C, using limited oxygen or steam to convert biomass into a combustible gas mixture of H₂, CO, CH₄, and CO₂, with some ash or char [30,31]. Both gasification and combustion convert biomass carbon into gases (CO₂ or CO), while HTC and pyrolysis retain more carbon in solid forms, like hydrochar or biochar. The chemistry of HTC diverges from dry pyrolysis due to its aqueous environment and lower temperatures, promoting reactions such as hydrolysis and solvolysis. Hydrochar, the solid byproduct of HTC, differs significantly from biochar, typically having higher H/C and O/C ratios, indicating its formation at lower temperatures with less dehydration and decarboxylation. Hydrochar produced at 200–250 °C retains more aliphatic structures and oxygen-containing groups, while biochar generated at 500–600 °C is mainly aromatic with fewer functional groups [32,33]. The elevated oxygen content and diversity of functional groups in hydrochar contribute to its more hydrophilic and chemically reactive nature compared to biochar, which is generally hydrophobic and inert, owing to its graphitic, polyaromatic structure [34].

Fresh hydrochars generally display mildly acidic properties (pH < 7) due to retained acidic functional groups, like carboxylic and phenolic groups, while biochars from high-temperature pyrolysis tend to be alkaline because of reduced acidic groups and increased basic mineral ash components (e.g., calcium, potassium, and magnesium carbonates). Morphologically, hydrochars are often spherical and uniform in size, formed from dissolved precursors via nucleation in water, whereas biochars retain the heterogeneous porous structure of the biomass or appear as irregular, flaky carbon aggregates. Typically, hydrochars have lower porosity and surface area compared to high-temperature biochars or activated carbons. While increasing pyrolysis temperature can enhance biochar surface area initially, excessively high temperatures may cause pore wall collapse, reducing surface area. Hydrochars made at lower temperatures in water tend to have more condensed structures with blocked micropores, leading to modest surface areas. High surface area applications, like adsorption, may require hydrochars to undergo additional activation. Despite this, hydrochar’s unique chemical properties may give it advantages over biochar in applications like catalysis and polar contaminant adsorption, while biochar’s stability suits it for soil amendment and carbon sequestration. In conclusion, although both hydrochar and biochar are carbon-rich solids derived from biomass, their dissimilar production pathways give rise to varied properties and potential applications [35,36].

As depicted in Figure 2, hydrothermal carbonization (HTC) transpires through a multifaceted array of interrelated reaction mechanisms influenced by the fundamental composition of lignocellulosic biomass and the specific processing parameters employed. During the HTC process, the components of biomass, namely cellulose, hemicellulose, and lignin, undergo a series of transformations that include hydrolysis, dehydration, decarboxylation, and polymerization. These reactions culminate in the production of hydrochar, which exhibits an increasing degree of aromaticity and structural organization. Beyond the reaction mechanisms outlined in this section, the comprehensive conceptual framework of the hydrothermal carbonization (HTC) process is encapsulated in Figure 2. This figure merges the structural composition of biomass, the transformation pathway during the hydrothermal carbonization process, and the concurrent evolution of physicochemical characteristics, including ion exchange sites, pore structure, chemisorption capacity, and surface functional groups. Furthermore, it underscores the potential for these inherent properties to be further customized through post-HTC surface functionalization or activation treatments, consequently facilitating the engineering of hydrochars for targeted environmental and energy applications.

Through the manipulation of process parameters, hydrothermal carbonization (HTC) facilitates the customization of hydrochar characteristics to fulfill particular requirements. Extensive investigations have demonstrated that modifications in temperature, duration, biomass-to-water ratio, and even the initial pH of the reaction can significantly affect both the yield and quality of the hydrochar produced [37]. For instance, increasing the HTC temperature from 180 °C to 250 °C generally results in a reduction in hydrochar yield, as a greater portion of biomass is converted into liquid and gaseous products. However, this temperature elevation correlates with an enhancement in the carbon content and heating value of the hydrochar, as more oxygen is released in the form of CO₂ or CO [36]. Similarly, an increase in residence time from approximately 1 h to 8 h can lead to further carbonization of hydrochar, resulting in a higher fixed carbon fraction and greater aromaticity, albeit at the cost of yield. The composition of the biomass itself also plays a critical role in determining hydrochar properties; feedstocks abundant in lignin typically yield more solid char and exhibit higher carbon content compared to those rich in cellulose or starch, due to the more favorable recombination of lignin fragments into char. Given this potential for adjustment, researchers can tailor hydrochar for various applications by selecting suitable HTC conditions [38,39].

Furthermore, post-processing techniques can significantly modify the properties of hydrochar. There exists a substantial body of research focused on the activation and functionalization of hydrochar to enhance its surface area and functional characteristics. Both chemical activation, utilizing agents such as KOH, H₃PO₄, or ZnCl₂, and physical activation, achieved through high-temperature steam or CO₂ treatment, can establish a network of micropores within the hydrochar, substantially increasing its surface area and efficiency for adsorption applications. Techniques for surface functionalization, including the oxidation of hydrochar with acids or the incorporation of heteroatoms such as nitrogen or sulfur, have been demonstrated to improve its capacity to adsorb specific contaminants. For example, the introduction of nitrogen-containing functional groups can enhance the hydrochar’s adsorption of heavy metals through chelation mechanisms [40].

Additionally, innovations in the HTC process itself are being explored. Co-hydrothermal carbonization (co HTC) involves the simultaneous processing of biomass with additives or various waste streams, leading to the generation of co-hydrochars with synergistically enhanced properties. An exemplary case is the combination of high-protein waste with lignocellulosic biomass, which can result in the incorporation of nitrogen functionalities directly during the HTC process. Another advancing method is microwave-assisted HTC, which utilizes microwave energy to heat the biomass water blend rapidly and uniformly. This technique has the potential to reduce reaction times and offer greater precision in heating. Reports indicate that microwave-assisted HTC can yield hydrochar with equivalent or superior characteristics in a significantly shorter timeframe compared to conventional heating methods, though the scalability of this technology is still under investigation [41]. These advancements seek to enhance process efficiency, lower energy consumption, and further refine the material attributes of hydrochar for advanced utilization.

In light of the aforementioned context, this review will meticulously analyze the influence of hydrothermal carbonization (HTC) process parameters on the structural and functional characteristics of hydrochar derived from lignocellulosic biomass. Additionally, it will address the operational challenges and prospects for enhancing HTC technology. The objective is to furnish a thorough comprehension of how hydrothermal carbonization can be optimized and implemented for energy generation, resource recovery, and environmental applications, while also identifying outstanding research gaps warranting future investigation. By evaluating recent advancements, including comparative analyses between hydrochar and conventional biochar, innovative approaches such as co HTC and microwave-assisted HTC, and methodologies for managing process water, a more comprehensive understanding of the potential contributions of HTC to sustainable bioenergy and waste remediation strategies can be achieved. This discourse aspires to provide valuable insights for researchers and engineers regarding the current landscape of HTC and to steer future innovations within this domain.

Key takeaways: Hydrothermal carbonization (HTC) of biomass involves reactions in the aqueous phase, converting biomass polymers into carbon-rich solids through hydrolysis, deoxygenation, and condensation. Under subcritical water, polysaccharides like cellulose and hemicellulose turn into soluble sugars and oligomers, which then undergo dehydration and decarboxylation, losing water (H₂O) and carbon dioxide (CO₂). Residual fragments undergo polymerization and aromatization, forming a carbon network. Elevated water temperature alters its characteristics, acting as a weak acid catalyst and non-polar solvent, enhancing hydrolysis and hydrophobic intermediate transport. This leads to carbon densification, with hydrochar becoming rich in aromatic structures while releasing oxygen and hydrogen into the liquid and gas phases. Consequently, HTC-derived hydrochars have lower O/C and H/C ratios than feedstocks and exhibit coal-like properties. The intensity of HTC, determined by reaction temperature and duration, significantly influences reaction equilibrium, affecting yield and product quality. Higher temperatures (from 180 °C to above 250 °C) and longer reaction times accelerate dehydration and decarboxylation, improving carbon content and heating value but reducing solid yield due to biomass solubilization. Milder conditions produce chars with higher oxygenated functional groups, which are suitable for applications like adsorption and soil amendment, whereas severe HTC yields dense chars with high energy content comparable to lignite coal. Increased temperature or reaction time boosts carbonization (aromaticity and energy density) while reducing mass yield through prolonged dehydration and decarboxylation. Operational variables further affect these pathways; the solid-to-liquid ratio influences organic intermediate concentration, impacting reaction rates and polymerization. An optimal solid-to-liquid ratio (1:5 to 1:10) balances effective hydrolysis with sufficient organic compounds for char formation. Conversely, overly dilute systems (1:50) lead to porous hydrochars at the cost of yield, while low water content may hinder carbonization by limiting heat transfer. These conflicting findings in the literature arise from the balance of water concentration and polymerization dynamics. Feedstock composition also introduces variability; lignin-rich materials yield more aromatic hydrochar, while nutrient-rich substrates produce hydrochars with higher ash and nitrogen content. Catalysts can alter reaction equilibrium, with in situ acids or metal salts accelerating dehydration and polymerization, enhancing carbon content and heating value (HHV), while strong bases could inhibit certain pathways. Overall, the properties of final hydrochar result from a complex interplay of mechanisms and conditions. Key reactions of hydrolysis, dehydration, decarboxylation, and aromatization are influenced by temperature, duration, water content, catalysts, and feedstock. By adjusting these variables, practitioners can control HTC outcomes. Higher temperatures and longer durations increase carbon densification at the expense of yield, whereas lower temperatures and more water preserve functional groups, enhancing suitability for adsorption. Inconsistencies in the literature regarding solid-to-liquid ratios illustrate this complexity; optimal conditions require balancing reaction kinetics and mass transfer. This mechanistic view clarifies why extreme HTC conditions boost aromatic carbon content and calorific value while reducing yield. Understanding these trade-offs aids in optimizing HTC, enabling practitioners to select temperature, duration, water loading, and catalysts strategically to achieve desired hydrochar quality.

3. Influence of Process Parameters on Hydrochar Yield and Properties

The chemical and physical characteristics of hydrochars can vary considerably and show a wide range of diversity depending on the specific operating conditions that are applied during the hydrothermal carbonization (HTC) process [8]. Therefore, a comprehensive and thorough understanding of the influence of each critical parameter is absolutely crucial for effectively optimizing process efficiency and achieving the desired hydrochar quality and the desired functionality. Among the many influencing factors, reaction temperature and residence time undoubtedly play the most dominant roles, as they directly and significantly affect the extent of dehydration, decarboxylation, and aromatization reactions that occur during the HTC process. However, it is essential to note that the type and composition of the feedstock, the solid-to-liquid ratio utilized, the reaction pressure applied, and the presence of catalysts or various chemical additives also exert significant and notable effects on the carbon yield, elemental composition, and the overall physicochemical properties of the resulting hydrochar product.

As illustrated in

Table 1. Influence of key process parameters on the physicochemical properties of hydrochars produced from various biomass feedstocks.

Biomass (Feedstock)	Temperature (°C)	Time (min.)	Solid/Liquid Ratio	Fixed Carbon (wt.%)	Ash (%)	Carbon Content (wt.%)	HHV (MJ kg−1)	Surface Area (m2/g)	Reference
Industrial digestate (IDT)	200–240	10–60	1:4–1:10	1.80–4.60	52.80–64.20	20.50–27.50	9.70–13.10	5–22	[42]
Sugarcane residues	200–240	80–240	1:10–1:8	18.00–35.00 (%)	12.00	-	21.43–24.94	9	[43]
Potato peel (organic and mineral acid)	180	300	1:6	-	0.09–1.70	58.60–72.40	22.00–29.80	-	[44]
Antibiotic fermentation residues	120–270	45	1:10	-	6.80	-	24.50	-	[45]
Potato peel waste	150–220	60–300	1:10	-	-	42.95–44.65	-	-	[46]
Medical waste (MW)	240–280	45	1:10	5.90	-	78.90	38.99	-	[47]
Chenopodium botrys	150–180	360–600	1:10	-	-	-	-	13–59	[48]
Sorghum	180–240	30–240	1:10	-	3.50–4.00	49.10–60.50	20.20–25.20	-	[49]
Sago (Metroxylon spp)	200–300	60	1:50	11.30–26.42	1.00–2.40	38.54–53.55	19.48–23.88	4–180	[50]
Brewer’s spent grain (BSG)	220–280	60	1:5	23.14–27.07	1.72–2.28	61.32–66.91	34.70–35.36	-	[51]
Traditional Chinese medicine residue	200–250	240–720	1:5	37.32–43.90	3.74–4.38	60.78–70.81	20.36–25.12	-	[52]
Waste coir substrate	180–300	60	1:5	36.12–47.94	4.59–8.19	51.95–70.32	19.55–24.93	-	[53]
Stevia rebaudiana	185–275	30–90	1:10	20.79–34.27	1.68–6.72	62.11–75.59	26.95–36.61	-	[54]
Colombian plantain peels	150–230	120–240	1:1–1:6	-	-	-	-	-	[55]
Olive stone	200–240	120–600	1:10–1:20	27.3–39.10	0.56–1.02	50.13–57.53	20.04–23.00	-	[56]
Corn stalk	180–300	20–80	1:4	-	-	71.87–80.52	22.03–22.45	-	[57]
Woodchip (Fe(NO3)3·9H2O)	240	60	1:4	42.95–44.31	-	-	22.10–30.05	-	[58]
Swine manure	200–280	0–60	1:20–1:4	8.60–15.70	36.10–48.90	27.40–37.40	10.90–16.00	-	[59]
Banana stalk	160–200	60–180	1:10	5.00–44.30	6.70–19.00	33.00–48.50	15.60–18.90	-	[60]
Tobacco stalk	180–260	60–720	1:10	15.19–48.75	3.05–7.49	46.22–65.24	18.78–27.18	1–11	[61]
Food waste (Acetic acid)	180–260	120–360	2:10–1:10	11.78–38.81	2.97–6.59	43.89–72.88	18.05–32.56	-	[62]

‘-’ indicates data not reported in the cited source.

, hydrochars produced at elevated temperatures and extended residence times typically demonstrate an increase in fixed carbon content, improved higher heating values (HHV), and a decrease in volatile matter, which collectively suggest superior fuel characteristics. In contrast, lower hydrothermal carbonization (HTC) temperatures or shorter reaction durations tend to preserve oxygen-containing surface functionalities, leading to materials with enhanced hydrophilicity and greater adsorption capacity. Additionally, variations in the solid-to-liquid ratio significantly impact the concentration of organic intermediates in the aqueous phase, thereby influencing the carbonization rate and the development of the pore structure in the hydrochar. The incorporation of catalysts or acids can similarly facilitate dehydration and condensation reactions, resulting in increased carbon densification and the formation of aromatic structures. A visual summary of these relationships is presented in Figure 3, which illustrates how the combined effects of feedstock characteristics and HTC operating conditions dictate hydrochar yield and physicochemical properties.

In a recent study on the hydrothermal carbonization of industrial digestate, Oliveira et al. systematically examined the effects of temperature, residence time, and solid-to-liquid ratio on hydrochar yield and properties [42]. The results demonstrated that HTC proceeds through dehydration, decarboxylation, and polymerization reactions, leading to the gradual transformation of organic matter into carbon-rich solids. Increasing temperature and reaction time reduced hydrochar yield due to enhanced decomposition and solubilization of intermediates, whereas higher feedstock concentrations favored solid formation through intensified condensation reactions. The severity index (S₀), integrating temperature and time, proved to be an effective indicator of reaction intensity, correlating inversely with yield but positively with fixed carbon content and thermal stability. These findings confirm that careful tuning of process parameters enables control over the physicochemical characteristics and potential applications of the produced hydrochars.

Kurniawan et al. (2026) optimized the hydrothermal carbonization (HTC) of sugarcane residues to evaluate how temperature, reaction time, and biomass-to-water ratio affect hydrochar yield and fuel quality [43]. The HTC process involves dehydration, decarboxylation, and aromatization, converting lignocellulosic structures into carbon-rich solids. Elevating temperature (200–240 °C) and residence time (80–240 min) reduced hydrochar yield from 79% to 57% due to decomposition, but improved the higher heating value (21–25 MJ kg⁻¹) and fixed carbon content. The biomass-to-water ratio had a minimal effect, favoring higher yields at 0.15 B/W (biomass-to-water ratio). Optimal conditions (223–229 °C, 80 min, and 0.15 B/W) achieved around 65% yield and HHVs of 23.5 MJ kg⁻¹, showcasing increased aromaticity and low sulfur content, indicating coal-like quality. The study highlights temperature as the main factor influencing HTC severity and energy density, with shorter times and moderate dilution enhancing recovery and carbonization efficiency.

Naeem et al. (2026) studied the influence of organic and inorganic acids on the hydrothermal carbonization (HTC) of potato peel at 180 °C for 5 h, aiming to understand how acidity affects hydrochar yield and structure [44]. The HTC mechanism involved starch hydrolysis, sugar dehydration into furans, and polymerization into carbonaceous solids. Weak organic acids like acetic and lactic acid slightly increased hydrochar yield (37–40%) by promoting polymerization, while strong acids like HCl and H₂SO₄ reduced yield (16–20%) due to enhanced hydrolysis and decomposition. Elemental analysis showed that acetic and formic acids enhanced deoxygenation, producing carbon-rich hydrochars (C > 70%, HHV ≈ 29 MJ kg⁻¹), while lactic acid retained more oxygen, and H₂SO₄ resulted in oxygen-rich solids (O ≈ 36.6%) with lower heating value (~22 MJ kg⁻¹). FTIR and DRIFTS analyses indicated that HCl and H₂SO₄ facilitated aromatization, forming stable phenolic –OH groups while reducing carbonyl functionalities. These results highlight how acid type and strength significantly alter the HTC reaction network, affecting hydrochar yield, composition, and energy density.

Lai et al. studied co-hydrothermal carbonization (co HTC) of antibiotic fermentation residues (AFRs) with a Camellia oleifera shell (COS) to improve coalification, demineralization, and denitrogenation of hydrochars [45]. The HTC process involved hydrolysis, dehydration, decarboxylation, and aromatization, transforming AFRs’ nitrogen-rich biopolymers and COS’s lignocellulosic parts into dense, carbon-rich solids. Temperature influenced the process: hydrolysis was dominant below 180 °C, while carbonization prevailed above 210 °C, enhancing fuel quality and promoting aromatic condensation of lignin-derived intermediates. The optimal condition was at 220 °C for 45 min, yielding co-hydrochars with improved properties, HHV ≈ 24.5 MJ kg⁻¹, ash 6.81 wt %, nitrogen 4.49 wt %, and notable synergistic effects (coalification +7–23%, demineralization −41%, and denitrogenation +21%). This synergy resulted from Mannich and Maillard cross-reactions between amino acids and COS-derived compounds, aiding organic matter retention and nitrogen stabilization. COS also generated organic acids, lowering pH, enhancing demineralization efficiency, and improving combustion performance, while decreasing NO_x emissions by 67%. This study shows how feedstock composition and temperature affect coalification, mineral migration, and nitrogen transformation.

Nguyen et al. (2025) proposed an integrated biorefinery model utilizing potato peel waste hydrolysate (PPH) to cultivate Spirulina sp. mixotrophically, followed by hydrothermal carbonization (HTC) of lipid-extracted biomass [46]. The research focused on optimizing cultivation and HTC parameters to enhance biomass, lipid yield, and hydrochar production. The culture with 10% PPH showed optimal biomass productivity (59.84 mg L⁻¹ d⁻¹) and lipid content (≈20%) due to monosugar availability boosting metabolic activity. Biodiesel from this culture had predominant C₁₆-C₁₈ fatty acids with increased monounsaturated components like C18:1 (14.37%). The lipid-extracted biomass underwent HTC between 150 and 220 °C for 60–300 min, where temperature and time were crucial for carbonization. The best hydrochar yield (46.2%) occurred at 190 °C for 120 min. Higher temperatures led to secondary decomposition, while the hydrochar produced had improved carbon content (44.65%) and energy density. Moderate HTC conditions favored reactions enhancing biochar’s physicochemical properties.

Yan et al. (2025) assessed hydrothermal carbonization (HTC) and vapor thermal carbonization (VTC) for converting mixed medical waste into quality solid fuels [47]. Both methods relied on hydrolysis, dehydration, decarboxylation, and polymerization, transforming components like plastics into carbon-rich solids. HTC outperformed VTC, yielding hydrochars with higher fixed carbon (up to 78.9%) and calorific value (38.99 MJ kg⁻¹). While HTC produced stronger energy release and stability, VTC improved homogeneity and pore formation. Temperature significantly influenced fuel properties: 240–280 °C reduced volatile matter but enhanced deoxygenation. Elemental migration analysis showed efficient removal of Na⁺, K⁺, and Cl⁻ ions, with HTC achieving high alkali removal rates and VTC excelling in dechlorination. The study revealed that the reaction medium and temperature determine carbonization intensity and contaminant immobilization in waste-derived hydrochars.

Ekinci et al. (2025) synthesized activated carbon from Chenopodium botrys biomass via two methods: direct chemical activation (Cb-AC) and hydrothermal pretreatment followed by activation (Cb-HC-AC) [48]. The hydrothermal carbonization (HTC) pretreatment at 150–180 °C for 6–10 h facilitated hydrolysis, dehydration, and decarboxylation, enhancing the hydrochar with oxygenated functional groups and promoting mesopores. NaOH activation then utilized these sites, resulting in a micro/mesoporous structure with high surface functionality. The optimal conditions, NaOH activator, 600 °C, 60 min, and 1:1 ratio, yielded an iodine number of ≈1100 mg/g, indicating extensive micropore formation. Adsorption tests showed Cb-HC-AC having a methylene blue (MB) capacity of 140.83 mg/g, surpassing Cb-AC’s 77.15 mg/g, with kinetics dominated by chemisorption. Thermodynamically, Cb-HC-AC’s adsorption was endothermic (ΔH° = +31.17 kJ/mol), while Cb-AC’s was exothermic (ΔH° = −25.89 kJ/mol). Electrochemical tests indicated that hydrothermal treatment enhanced conductivity, with Cb-HC-AC achieving 441 F/g at 0.2 A/g, 93% retention after 5000 cycles, and 16.94 Wh/kg energy density, outperforming Cb-AC. These results underscore the potential of C. Botrys-derived carbon in adsorption and electrochemical applications.

Mehrez et al. systematically studied the hydrothermal carbonization (HTC) of heavy metal (HM)-contaminated sorghum to enhance fuel quality and evaluate the environmental impact of the resulting liquid effluents [49]. The HTC process, conducted at 180–240 °C for 0.5–4 h, followed a reaction sequence involving hydrolysis, dehydration, and decarboxylation, transforming lignocellulosic structures into carbon-rich hydrochars. Higher temperatures improved deoxygenation and carbonization, increasing the higher heating value (HHV) from 20.2 to 25.2 MJ kg⁻¹ and carbon content from 49.1% to 60.5%, while solid yield dropped from 80% to 62% due to increased volatile release. Combustion tests found that the sample produced at 200 °C for 0.5 h (HC200-0.5) had optimal fuel properties, with a high ignition index (Di = 4.24 × 10⁻⁴% min⁻¹ °C⁻²) and combustibility index (CCI = 21.07 × 10⁻⁷%² min⁻² °C⁻³), and a burnout temperature (Tb) of 460 °C, indicating effective combustion and thermal stability. FTIR analysis showed that higher temperatures favored the formation of aromatic structures and oxygen-containing groups, which is consistent with dehydration and condensation. Heavy metal removal was most effective at 180–200 °C for 0.5 h, with 60% of Mn and 50% of Zn removed, while Cd, Pb, and Ni had limited removal (<30%), and higher temperatures led to the re-stabilization of metals in the carbon matrix. Despite improved hydrochar quality, the liquid phase was highly contaminated (pH ≈ 3.9; TOC ≈ 15 g L⁻¹; strong phytotoxicity, GI = 0%), necessitating treatment before disposal. The study highlighted the importance of temperature and residence time in controlling HTC efficiency and emphasized the need for integrated management of toxic effluents for sustainable biomass valorization.

Rambli et al. developed a microwave-assisted hydrothermal carbonization (MWHTC) process to convert sago waste into hydrochar, which is examined for glycerol etherification [50]. The HTC reactions, hydrolysis, dehydration, decarboxylation, and polymerization occurred under subcritical water at 200–250 °C for 1 h, producing carbon-rich solids with improved fixed carbon content (22.1–26.4%) and lower O/C (0.999–1.432) and H/C (0.007–0.117) ratios, indicating enhanced carbonization and hydrophobicity. Microwave irradiation facilitated rapid heating, enhancing carbon densification while maintaining structural integrity. The hydrochar yield was 29.8% at 200 °C, surpassing traditional heating. BET surface area increased from 57.9 m² g⁻¹ (conventional) to 179.8 m² g⁻¹ (microwave + acid activation), with pH changing from 4.5 (raw biomass) to 6.6–9.6, indicating loss of acidic groups. FTIR and XRD analyses revealed the removal of hydroxyl, carbonyl, and carboxyl groups with temperature and partial graphitization of the carbon matrix. The sulfonated catalyst (M-Hab) achieved 99% glycerol conversion and 59.9% selectivity toward glycerol tert butyl ethers (GTBE) at 110 °C, performing as well as or better than commercial resins like Amberlyst-15. It maintained stable activity for three cycles before gradual deactivation due to pore fouling. The study shows that microwave power and treatment temperature effectively influence the production of active, sustainable hydrochar catalysts for biofuel synthesis.

Liu et al. studied the hydrothermal carbonization (HTC) of protein-rich brewer’s spent grain (BSG) to understand hydrochar formation and improve fuel characteristics [51]. Experiments at temperatures from 220 to 280 °C for 60 min showed that temperature significantly influenced dehydration, decarboxylation, and aromatization processes. As temperatures increased, volatile matter decreased, while fixed carbon increased from 23.14 to 27.07 wt%. The hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios dropped from 1.44 and 0.32 at 220 °C to 1.25 and 0.25 at 280 °C, reflecting a higher degree of carbonization. The catalytic role of H₃O⁺ ions from water self-ionization promoted bond cleavage and polymerization, which raised the sp²/sp³ carbon ratio from 1.13 to 1.49, indicating better aromatic ordering. Fourier transform infrared (FTIR) spectroscopy showed a decrease in -OH and -COOH groups with rising temperatures, highlighting dehydration’s dominance. Scanning electron microscopy (SEM) images revealed microspheres forming between 250 and 280 °C due to condensation reactions. The best fuel properties were found in sample HC-280, which had a higher heating value (HHV) of 35.4 MJ/kg, a low ash content of 1.72 wt%, and improved hydrophobicity, making it suitable for pyrolysis. Hydrochar formation occurred through two pathways: a liquid-phase mechanism from hydrolysis of proteins and carbohydrates, and a solid-state mechanism from lignin and cellulose restructuring. This study confirmed that temperature-controlled HTC is a viable method for converting BSG into carbon-rich hydrochars for energy recovery and advanced materials production.

Yao et al. examined the hydrothermal carbonization (HTC) of Glycyrrhiza uralensis residues to assess fuel characteristics and combustion behavior [52]. Conducted at 200–250 °C for 4–12 h, the HTC process included chloroform extraction of biocrude. Key reactions, such as hydrolysis, dehydration, and decarboxylation, enhanced carbon content (from 42.6% to 66.9%) and reduced O content (to 18.4%), improving fuel quality. Hydrochar yield decreased as temperature increased (from 38.05% at 200 °C to 25.39% at 250 °C), while biocrude yield rose to 15.8%, indicating intensified decomposition. The higher heating value (HHV) of hydrochar reached 29.37 MJ kg⁻¹, with biocrude achieving 30.91–35.04 MJ kg⁻¹, underscoring its high calorific value. Fixed carbon (FC) increased and volatile matter (VM) decreased with temperature, improving combustion stability. FTIR results showed a loss of hydroxyl and carboxyl groups, along with greater aromaticity, while GC-MS demonstrated that biocrude consisted mainly of n-alkanes (C₂₁-C₃₄) and related alkenes. Thermogravimetric analysis highlighted a three-stage combustion pattern, with higher ignition temperatures for hydrochars, confirming thermal stability. The optimal sample, H-225-12, showed a HHV of 27.9 MJ kg⁻¹, CCI of 3.61 × 10⁻⁷, and Ea ≈ 249 kJ mol⁻¹, reflecting efficient carbonization. Additionally, synergistic effects between biocrude and hydrochar during combustion resulted in HHV and CCI values exceeding predictions, with maximum synergy at 225 °C for 12 h. The study concluded that temperature governs hydrochar yield, fuel reactivity, and combustion synergy, while longer residence times promote carbon formation and coupling, leading to enhanced fuel properties.

Woo et al. evaluated the hydrothermal carbonization (HTC) of waste coconut coir from controlled environment agriculture (CEA) to improve fuel characteristics and reduce contaminants [53]. HTC experiments at 180–300 °C for 60 min under subcritical water triggered hydrolysis, dehydration, and decarboxylation, producing carbon-rich hydrochar. Carbon content increased from 49.77% to 70.32% as the temperature rose, while hydrogen and oxygen decreased to 4.79% and 22.28%, respectively. Fixed carbon increased by ~15% (up to 47.94%), and volatile matter decreased to 43.87%, confirming carbonization progress. The higher heating value (HHV) improved from 17.99 to 24.93 MJ kg⁻¹, and energy density rose from 1.09 to 1.39, though hydrochar yield fell from 74% to 49.6% due to devolatilization. Combustion analyses showed ignition temperature increased from 276.8 °C to 354.8 °C, indicating enhanced thermal stability. The volatile ignitability (VI) rose from 11.01 to 19.90 MJ kg⁻¹, with the combustibility index stabilizing around 23.2 MJ kg⁻¹. X-ray fluorescence confirmed reduced problematic inorganics such as K₂O (from 8.33% to 1.75%) and Cl (from 0.83% to 0.32%). The hydrochars had atomic H/C and O/C ratios similar to lignite, with improved combustion stability and calorific performance. The study highlighted HTC temperature’s role in carbonization, fuel quality, and ash behavior, enabling the transformation of CEA-derived coir waste into high-grade renewable solid fuel, aiding South Korea’s carbon neutrality goals.

Alper investigated acrylic acid’s (AA) catalytic role in hydrothermal carbonization (HTC) of Stevia rebaudiana biomass to optimize fuel properties and liquid composition [54]. HTC reactions were performed at 215 °C for 60 min, modifying AA concentrations (0.25, 0.50, and 1.00 mol L⁻¹). The HTC processes included hydrolysis, dehydration, decarboxylation, and polymerization, converting lignocellulosic structures into carbon-rich solids. The highest hydrochar yield (48.5%) was at 0.25 mol L⁻¹ AA, while increased acid concentrations enhanced deoxygenation and aromatization but reduced solid yield due to degradation. Analyses showed fixed carbon increased from 20.79% to 34.27%, with the O/C ratio dropping from 0.67 (raw biomass) to 0.21, confirming deoxygenation and improved fuel quality. Hydrochars’ higher heating value (HHV) ranged from 26.95 to 36.61 MJ kg⁻¹, peaking at 1.00 mol L⁻¹ AA (32.20 MJ kg⁻¹). Spectroscopic and structural analyses indicated higher aromatic C=C vibrations and fewer hydroxyl groups, with XRD showing partial graphitization and SEM revealing porous morphologies. Thermogravimetric analysis indicated that HC15 had the highest thermal stability and residual carbon. GC–MS analysis of the aqueous phase showed increased phenolic compounds, with phenol content rising from 19.47% (no catalyst) to 40.92% (1.00 mol L⁻¹ AA), highlighting acrylic acid’s role in lignin depolymerization. The liquid phase’s TOC (14,280–28,728 mg L⁻¹) and COD (chemical oxygen demand) (43,227–113,920 mg L⁻¹) showed high organic loading and the need for post-treatment, suggesting recovery potential for phenolics or biogas. The study concluded that acrylic acid significantly enhances carbonization efficiency, energy density, and chemical diversity, promoting sustainable valorization of Stevia rebaudiana waste into biofuels and phenolic chemicals.

Vélez Gómez et al. investigated the hydrothermal carbonization (HTC) of Dominico Harton plantain peels, a high-moisture residue in Colombia, focusing on how temperature, moisture content, and residence time influence hydrochar yield, composition, and energy potential [55]. Experiments were performed at 150–230 °C, with moisture levels of 50–85% and reaction times of 2–4 h under autogenous pressure. Results showed that as the temperature rose, hydrochar yield decreased from 83% to 26%, while fixed carbon content increased from 19.3% to 47.1%, indicating stronger carbonization. Elemental analysis revealed that carbon content grew from 40.8% (raw) to 68.0%, with reduced oxygen and hydrogen levels, pointing to dehydration and decarboxylation as key reactions. The higher heating value (HHV) increased from 16.4 MJ kg⁻¹ (feedstock) to 27.7 MJ kg⁻¹ at 230 °C and 85% moisture, achieving a 1.69 energy densification ratio, showcasing effective energy upgrading. Van Krevelen analysis confirmed the shift in composition from carbohydrate-like to lignite-like, driven by dehydration and decarboxylation. Thermogravimetric analysis indicated improved thermal stability and decreased mass loss with higher temperatures, correlating with increased aromatic condensation. The process water exhibited acidic pH (3.8–5.0), high COD (24,000–93,000 mg O₂ L⁻¹), and notable 5-HMF levels (up to 1418 μg mL⁻¹ at 190 °C), along with phenolic content and antioxidant activity, suggesting potential for biochemicals. The study concluded that temperature and biomass moisture are crucial in affecting HTC severity and energy densification, with moderate-to-high temperatures under high moisture producing carbon-rich hydrochars and valuable process water.

Gallego Mena et al. optimized the hydrothermal carbonization (HTC) of olive stones for carbon-rich hydrochar and activated carbon production, aiming at CO₂ capture [56]. The HTC process spanned 200–240 °C for 2–8 h with various water-to-biomass ratios under autogenous pressure, assessing the impact of process severity on yield, fuel quality, and structural changes. The classical HTC pathway involved hydrolysis, dehydration, decarboxylation, and aromatization, resulting in stabilized aromatic carbon structures through the breakdown of hemicellulose and cellulose and lignin condensation. Increasing temperature and time led to lower hydrochar yields (62.8%) but heightened fixed carbon content (48.73%) and higher heating value (≈23 MJ kg⁻¹), attributed to intensified dehydration and deoxygenation. FTIR analysis indicated a decrease in hydroxyl and carbonyl groups while forming aromatic C=C bonds, and TGA showed improved thermal stability with reduced mass loss at greater carbonization. The optimized hydrochar underwent ZnCl₂ activation (1:6 ratio, 700 °C, N₂), yielding a porous material with a BET surface area of 1281 m² g⁻¹ and total pore volume of 0.67 cm³ g⁻¹, primarily mesoporous. SEM and TEM images illustrated a transition from compact microspherical structures to a porous network. XPS and XRD demonstrated partial graphitization and oxygenated functional groups that enhanced CO₂ adsorption. The final activated carbon achieved CO₂ adsorption of 2.78 mmol g⁻¹ at 273 K, comparable to KOH-activated carbons but through a more eco-friendly method. This study revealed that temperature, residence time, and water-to-biomass ratio significantly influence carbonization and physicochemical attributes of olive stone hydrochars, while ZnCl₂ activation promotes micro-mesoporous structures effective for gas adsorption, underscoring the potential of optimized HTC combined with chemical activation for sustainably converting agricultural waste into high-performance CO₂ adsorbents.

Zhang et al. investigated the hydrothermal carbonization (HTC) of corn stalks to assess the effects of temperature, reaction time, and solid–liquid ratio on the composition, chemistry, microstructure, and fuel quality of resultant hydrochars [57]. HTC experiments spanned temperatures of 180–300 °C, durations of 20–80 min, and solid–liquid ratios of 1–4% in a 5 MPa reactor. Higher carbonization temperatures and longer durations enhanced dehydration and dehydroxylation, converting organic intermediates into amorphous carbon and increasing carbon content from ≈48 wt % to 74 wt %, while reducing oxygen to ≈26 wt %. The higher heating value (HHV) rose from 16.25 MJ kg⁻¹ (raw biomass) to 23.6 MJ kg⁻¹, nearing anthracite levels. FTIR analyses showed the loss of hydroxyl and carbonyl groups, with C=C and C–O–C bonds forming, indicating dehydration and aromatization. SEM-EDS results revealed that temperature and time significantly impacted microstructure and composition, and higher parameters yielded finer, porous particles (<2 µm) with increased carbon content. Excessive solid–liquid ratios (≥4%) hampered KCl dissolution and carbonization efficiency. Optimal conditions (260 °C, 40 min, and S/L (solid-to-liquid ratio) = 2%) resulted in C ≈ 76.5 wt %, HHV = 22.3 MJ kg⁻¹, and particle size < 50 µm, showing anthracite-like stability. The study emphasizes that temperature, time, and moderate solid–liquid ratios are crucial for optimizing carbonization and enhancing fuel quality, promoting a sustainable method to convert agricultural residues into high-quality solid fuels.

Wang et al. studied catalysts in biomass hydrothermal carbonization (HTC) to improve hydrochar quality for blast furnace fuels [58]. They carbonized waste wood chips at 210–270 °C under 5 MPa for 60 min, using deioniPWzed and circulating water, plus two catalysts: Fe(NO₃)₃·9H₂O (acidic) and CaO (basic). Findings showed higher temperatures increased hydrolysis and dehydration, reducing yield from 75% to 53%, but raised the higher heating value (HHV) from 26.2 to 30.05 MJ kg⁻¹ and fixed carbon from 40.7% to 44.3%, especially with 4 wt% Fe(NO₃)₃·9H₂O. In contrast, CaO decreased fixed carbon to 32% and HHV to 22 MJ kg⁻¹. The use of circulating water demonstrated autocatalytic behavior, improving polymerization and slightly enhancing HHV and yield due to organic acids. SEM and FTIR analyses revealed that Fe(NO₃)₃·9H₂O and circulating water promoted porosity, while CaO produced smoother surfaces with less aromaticity. Raman spectroscopy indicated that Fe-catalyzed samples had higher structural ordering. Thermogravimetric analysis showed Fe(NO₃)₃·9H₂O improved combustion performance, while CaO lowered combustion temperatures. The study concluded that temperature, Fe-based catalysts, and circulating water improved carbonization and combustion reactivity of hydrochar, while CaO inhibited formation by increasing alkalinity. The optimized approach resulted in high-quality hydrochars (HHV ≈ 30 MJ kg⁻¹), viable as low-carbon substitutes for bituminous coal, supporting carbon-neutral metallurgy.

Xiong et al. studied the hydrothermal carbonization (HTC) of swine manure to assess how temperature, residence time, and solid-to-liquid ratio affect hydrochar yield, fuel quality, and process water content [59]. Experiments were performed at 200–280 °C, for 0–60 min, and at 0.05–0.25 g mL⁻¹ solid–liquid ratios. Findings indicated that temperature significantly influenced HTC performance. As the temperature rose, hydrochar yield decreased from 58.7% to 50.2%, while fixed carbon content increased from 9.2% to 15.7%, and the higher heating value (HHV) improved from 12.4 to 16.0 MJ kg⁻¹. The optimal conditions were established at 260 °C, 0.1 g mL⁻¹, and 30 min, yielding 53.2% hydrochar with an HHV of 15.9 MJ kg⁻¹. Elemental analysis showed that higher temperatures enhanced dehydration and decarboxylation, leading to increased carbon content and reduced H/C and O/C ratios, demonstrating greater aromaticity. Process water had high N (≈900–1100 mg L⁻¹) and K (≈1400–1500 mg L⁻¹) levels, and COD decreased from 13.4 to 7.7 g L⁻¹. Heavy metals like Zn, Cu, Pb, Cr, and Cd were present in the hydrochar, while their soluble fractions in water were reduced significantly. Ultimately, HTC is effective for converting swine manure into carbon-rich hydrochars, though post-treatment of the process water is necessary due to its organic load and metals.

Islam et al. investigated the hydrothermal carbonization (HTC) of banana stalks to create carbon-rich solid biofuel, analyzing the effects of temperature and reaction time on yield and fuel quality [60]. Experiments were conducted at temperatures from 160 to 200 °C for durations of 1–3 h, with the resulting hydrochars characterized by elemental, proximate, FTIR, and thermogravimetric analyses. Hydrochar yield decreased from 75.3% to 57.8% as temperature and time increased, mainly due to cellulose and hemicellulose decomposition. Fixed carbon content rose from 16.9% to 44.3%, while ash and volatile matter decreased, showcasing improved carbonization through dehydration and decarboxylation reactions. The higher heating value (HHV) increased from 15.6 MJ kg⁻¹ (raw biomass) to 18.1–18.9 MJ kg⁻¹, demonstrating significant fuel-quality upgrades. Elemental analysis showed carbon content increased (42.7–48.5%) and oxygen decreased (≈45–49%), shifting the material toward lignite-like characteristics. FTIR spectra indicated hydroxyl and carbonyl peak reductions, alongside new aromatic C=C vibrations. TGA revealed a three-stage thermal degradation pattern with key mass loss between 220 and 360 °C, indicating the role of hemicellulose and cellulose in volatile release. The study concluded that banana stalk hydrochar, especially at 180 °C for 2 h, exhibited optimal properties (HHV ≈ 18.6 MJ kg⁻¹) and was suitable as a sustainable solid fuel, whether used alone or co-fired with coal, due to its energy efficiency, low emissions, and thermal stability.

Cai et al. investigated the hydrothermal carbonization (HTC) of tobacco stalks, focusing on how temperature and residence time influence hydrochar yield, composition, and combustion performance [61]. Conducted at 180–260 °C for 1–12 h in a batch reactor, the results showed that as temperature and time increased, carbon content rose from 46.2% to 65.2%, while hydrogen and oxygen decreased, indicating dehydration and decarboxylation. Hydrochar yield decreased from 80% to 41%, reflecting increased decomposition at higher severity. Fixed carbon content increased from 15.2% to 48.8%, and the higher heating value (HHV) improved from 18.7 MJ kg⁻¹ to 27.2 MJ kg⁻¹, indicating effective energy densification. FTIR spectra indicated a loss of hydroxyl and carbonyl groups and the formation of aromatic structures, confirming reactions during HTC. SEM images showed damage to the lignocellulosic texture of raw stalks, replaced by porous surfaces with microspherical carbon at 260 °C after 8–12 h of treatment. BET analysis revealed low overall surface areas (1–11 m² g⁻¹). Thermogravimetric analysis (TGA) identified three combustion stages: moisture loss, devolatilization, and fixed carbon oxidation, with improved thermal stability for HTC samples. Activation energy increased from 46.1 kJ mol⁻¹ (raw) to a range of 43.7–85.8 kJ mol⁻¹ for hydrochars, showing enhanced stability and slower combustion reactivity. The study affirmed that temperature and residence time are key parameters influencing HTC severity, carbonization degree, and combustion behavior, with optimized hydrochars exhibiting coal-like fuel quality, showcasing HTC’s potential for transforming tobacco stalks into sustainable, high-energy-density solid fuels.

Fallah et al. investigated the process parameters during hydrothermal carbonization (HTC) of food waste to optimize hydrochar yield and fuel quality [62]. Experiments were performed at temperatures from 180 to 260 °C for 2–6 h, using biomass-to-liquid (B/L) ratios of 10–20 wt% and landfill leachate chemical oxygen demand (COD) levels of 10,000–20,000 mg L⁻¹. The HTC process transformed raw food waste into carbon-rich hydrochars through various reactions, including hydrolysis and dehydration. Results indicated that increased temperature and time reduced hydrochar yield (from 68.0% to 45.0%) but raised fixed carbon (from 11.8% to 38.8%) and higher heating value (HHV) (from 18.0 to 32.6 MJ kg⁻¹). Elemental analysis showed carbon content increased from 43.9% to 72.9%, with oxygen dropping from 40.6% to 12.6%, which highlighted improved carbonization and deoxygenation. Compositional changes led to reduced H/C and O/C ratios, indicating a shift toward lignite-like materials. Thermogravimetric analysis (TGA) showed raw feedstock had distinct decomposition stages, while hydrochars had lower mass loss and higher thermal stability due to increased aromaticity. The use of landfill leachate as a medium enhanced hydrolysis and polymerization, improving carbonization efficiency. Adding acetic acid (10 wt %) as a catalyst raised fixed carbon to 37.2% and HHV to 32.1 MJ kg⁻¹, although the yield slightly decreased. Under optimal conditions (225 °C, 4.5 h, 20 wt % B/L, and 10,000 mg L⁻¹ COD), a hydrochar yield of 55.3% and an HHV of 30.15 MJ kg⁻¹ was achieved, showcasing significant carbonization and energy densification. Fallah et al. concluded that temperature and time were key parameters in the HTC process, while leachate composition and acetic acid catalysis improved hydrochar quality, yielding a renewable solid fuel comparable to low rank coal.

In the aggregated studies on hydrothermal carbonization (HTC), it is evident that temperature and residence time play pivotal roles in the processes of carbon densification and fuel enhancement. An escalation in processing severity, defined by a temperature range of approximately 160–300 °C and a residence time of 0.5 to 12 h, corresponds to a systematic increase in fixed carbon content (from approximately 5% to around 45–49% by weight) and overall carbon content (rising from approximately 27–40% to about 70–80% by weight). Concurrently, the higher heating value (HHV) experiences an increase from approximately 10–18 MJ kg⁻¹ to about 30–39 MJ kg⁻¹, often achieving or exceeding the values associated with low-rank coal, despite accompanying yield losses [42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62]. Moderate solid-to-liquid ratios, specifically in the range of 1:5 to 1:10, typically facilitate a favorable balance between carbon recovery and deoxygenation. Conversely, more dilute systems, such as those with a ratio of 1:50, promote the development of porosity and enhance surface area (up to approximately 180 m² g⁻¹) but may result in diminished yield. For lignocellulosic feedstocks, ash content is significantly reduced (to 1–4% by weight), whereas nutrient and metal-rich residues such as digestate, manures, and medical waste tend to maintain higher ash levels and yet still show considerable improvement in carbon enrichment and HHVs. In summary, the optimal operational parameters for the production of carbon-rich and energy-dense hydrochars with customizable surface characteristics are identified as a temperature range of 200–260 °C, a residence time between 1 and 6 h, and a solid-to-liquid ratio of 1:5 to 1:10.

Key takeaways: The cumulative evidence examined in this section illustrates that the efficacy of hydrothermal carbonization (HTC) is primarily influenced by reaction temperature and residence time. These two factors jointly dictate the magnitude of dehydration, decarboxylation, and aromatization reactions. An increase in process severity systematically promotes carbon densification, which results in a higher fixed carbon content and lower hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, as well as notable enhancements in higher heating value (HHV). This often results in hydrochar quality that is comparable to lignite-like fuels. However, such improvements are invariably associated with a reduction in solid yield, indicating an inherent trade-off between energy enhancement and mass recovery. This yield-quality compromise represents a recurring and mechanistically explicable trend within the literature, corroborating the notion that optimization of HTC cannot be successfully approached through a singular parameter framework.

The reviewed literature further indicates that the solid-to-liquid (S/L) ratio exerts a non-linear and at times contradictory effect on HTC results. Target S/L ratios in the moderate range of approximately 1:5 to 1:10 typically facilitate effective hydrolysis and condensation, yielding a balanced quality of fuel along with satisfactory carbon recovery. Conversely, systems that are excessively dilute may enhance porosity and surface area development, albeit often at the expense of yield. In contrast, highly concentrated slurries may inhibit hydrolysis and heat transfer, thereby limiting carbonization efficiency. The inconsistencies reported in various studies do not represent fundamental contradictions but rather reflect interactions between kinetic and mass transfer phenomena, as well as feedstock-dependent chemical behavior.

The composition of the feedstock also introduces systematic variability into the HTC process. Biomass sources that are rich in lignin and structurally aromatic components tend to produce hydrochars that are more carbon-dense and thermally stable. In contrast, wastes that are high in protein and nutrients often generate hydrochars with elevated ash and nitrogen contents, which may affect both combustion characteristics and the suitability for subsequent applications. Additionally, the introduction of catalysts and chemical additives can significantly alter the reaction network; however, their effects are not always advantageous. While acidic conditions generally foster deoxygenation and carbonization, strongly alkaline environments can destabilize the formation of aromatic networks and hinder carbon densification, underscoring the necessity for a mechanism-oriented approach rather than an empirical one in catalyst selection. Moreover, despite the established benefits of temperature in enhancing hydrochar quality, numerous inconsistencies remain in the literature, particularly concerning the effects of water loading, catalytic environments, and substrates rich in minerals. These discrepancies suggest that our current understanding remains predominantly empirical, necessitating more systematic investigations that link mechanistic pathways to macroscopic results. In summary, HTC behavior is dictated by a complex interplay among thermal severity, aqueous-phase chemistry, feedstock composition, and process configuration. Thus, future optimization strategies must explicitly consider these interdependencies and trade-offs, rather than relying solely on the adjustment of isolated parameters.

In addition to providing summaries of individual studies, this review establishes a cohesive conceptual framework to elucidate the often inconsistent patterns observed in the hydrothermal carbonization (HTC) literature. The analysis suggests that the perceived contradictions, particularly related to the solid-to-liquid ratio, reaction severity, and the balance between hydrochar yield and quality, stem not from experimental variability but from transitions between different reaction regimes. Under conditions characterized by lower concentrations, HTC processes are primarily influenced by aqueous-phase hydrolysis and solubilization, which promote the development of porosity and retention of functional groups while compromising solid yield. Conversely, in more concentrated systems, liquid-phase polymerization and solid-state condensation take precedence, leading to increased carbon densification, enhanced aromaticity, and improved fuel quality, albeit at the cost of surface functionality. These observations elucidate the absence of a universally “optimal” HTC condition and clarify the limitations of single-parameter optimization approaches. Instead, the properties of hydrochar are the result of a complex interplay involving thermal severity, water availability, feedstock chemistry, and the prevailing reaction pathways. By explicitly correlating mechanistic processes with macroscopic results, this synthesis enhances the understanding of HTC beyond mere empirical trend analysis and offers a rationale based on mechanistic principles for the targeted modification of hydrochar for either energy-focused or functionality-driven purposes.

4. Physicochemical Properties of Hydrochar

As depicted in Figure 4, hydrothermal carbonization (HTC) initiates a sequence of interrelated physicochemical changes that collectively influence the structural, chemical, and elemental features of the resultant hydrochar. The nature of the feedstock, which can include lignocellulosic residues, protein-rich substrates, materials derived from waste, and mixtures processed together, dictates the activation of specific reaction pathways. These pathways encompass dehydration, decarboxylation, deamination, aromatization, and polymerization. Such processes lead to the gradual densification of carbon content, the reduction of oxygenated functional groups, the evolution of pore structures, and the emergence of microspherical or graphene-like architectures, particularly in the context of microwave-assisted HTC. Simultaneously, significant elemental trends are observed, characterized by an increase in carbon content and a decrease in the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, along with the retention or incorporation of heteroatoms such as nitrogen or phosphorus. These trends contribute to the stabilization and functional enhancement of hydrochar. The transformation pathways illustrated in Figure 4 thereby offer a mechanistic framework for understanding the variations in fuel properties, aromaticity, mineral retention, and surface chemistry that are evident among the various hydrochars summarized in

Table 2. Comparative physicochemical evolution of hydrochars produced from diverse biomass and waste feedstocks under varying HTC conditions: structural transformations, elemental composition shifts, and key carbonization outcomes.

Feedstock and Conditions	Structural and Chemical Features	Elemental Composition	Key Findings	R
Cotton stalks 180–240 °C, 4 h, 1:10 (w/v)	Increasing temperature reduced yield but enhanced carbonization and fuel quality.	C content increased while H/C and O/C ratios declined, indicating stronger carbonization and aromaticity.	Higher T improved fuel quality but reduced yield.	[63]
Dairy manure (DM) Microwave-assisted HTC at 180–260 °C, 0.5–14 h,	Higher T increased C and HHV, reduced O, and enhanced carbonization and energy density.	Rising T and time enhanced dehydration–deoxygenation, increasing carbon content and energy efficiency.	Microwave-assisted HTC enhanced carbonization and porosity, yielding mesoporous microspheres and graphene-like sheets.	[64]
Forest waste (FW) 200–280 °C, 1 h, 1:5 (w/v)	Porosity and surface area increased then declined with T due to pore collapse. Rising T caused dehydration decarboxylation, loss of –OH/C=O groups, and partial lignin decomposition.	Fixed carbon rose 14.9 → 45.1%, carbon 47.9 → 70.8%, O and H decreased; H/C ↓ and O/C ↓, showing enhanced carbonization and coalification.	Elevated T produced energy-dense, stable hydrochar with improved carbon order despite lower yield	[65]
Low-rank coal 250–340 °C, 1 h, 1:2 (w/w)	BET surface area declined and pores enlarged due to collapse. Aliphatic chains shortened, aromaticity increased, and oxygenated groups diminished.	Carbonization increased (C, FC↑; O, O/C, H/C↓), improving coal rank and energy density	Higher T promoted carbonization, aromatization, and impurity removal, yielding stable, low-volatile HTC coal.	[66]
Cocoa bean shell residues 180 °C; 24 h; 1:5 (w/w)	HTC E showed highest surface area and mesoporosity. Dehydration, decarboxylation reduced –OH/C=O groups and formed aromatic C=C; activation enhanced porosity and induced N-doping.	C content increased and O, H/C, O/C ratios decreased, indicating strong carbonization	HTC activation enhanced aromatization and pore formation; ethanol increased C, and water enlarged pores	[67]
Food waste digestate solids ( 220–260 °C; 30–60 min; 1:1 (w/w)	Color darkened, and pore collapse occurred with higher T. Dehydration, decarboxylation weakened –OH/C=O bands, indicating cellulose degradation and increased aromaticity.	Higher T enriched carbon and lowered O/C–H/C ratios, evidencing coalification and aromatic carbon growth.	Higher T reduced yield but improved HHV and stability; N moved to liquid, P retained as apatite.	[68]
Microalgae + corn stalk 200–280 °C, 120 min,	FeCl3/NH4Cl increased surface area; melamine reduced porosity. Dehydration, aromatization dominated; Fe3+ promoted deamination, and NH4Cl/melamine induced amidation without triazine residues.	C↑ (61.8–74.4%), O↓; NH4Cl yielded highest N (8.9%), FeCl3 the lowest.	FeCl3 promoted N migration to liquid; NH4Cl and melamine increased hydrochar N and yield via two stage transformation.	[69]
Jatropha curcas fruit residues 220 °C, 2 h, 10 wt %	Particle density increased (up to 1.64 g cm−3); pore evolution was governed by lignin and size. Dehydration, decarboxylation weakened –OH/C–O bands, forming aromatic, hydrophobic hydrochars.	C ↑ (50.7→61.9%), O ↓ (37.8→29.1%); H/C ↓ (0.13 → 0.10 approx.); O/C ↓ (0.56→0.33). FHK: lower C gain (max 12%); SSH: +20%.	SSH showed higher yield, HHV, and ED than FHK; smaller particles enhanced ash reduction and mineral leaching	[70]
Chlorella vulgaris biomass 180–250 °C; 0.5–4 h; 1:100;	Carbon densification doubled with higher T; fixed C↑, volatiles↓, ash↑. Dehydration, decarboxylation reduced –OH/C–O and enhanced C=C, C=O, yielding aromatic, hydrophobic hydrochars (optimum at 210 °C)	C↑ (27.4→~60%), O↓ (59.7→25–40%), H/C and O/C↓; condensation and aromatization enhanced carbonization.	Yield ↓, HHV ↑ (~2×), energy yield ≤ 76.6%. Higher ignition T improved safety; nutrient-rich aqueous phase supported 92% microalgae regrowth.	[71]
Pig manure 200 °C and 240 °C, 3 h, 1:10	Mg citrate enhanced pore formation and fiber opening, reduced ash (↓ 4–20%), and increased C (+2–19%). Dehydration, decarboxylation intensified aromatic C=C and C–O bands; Mg–O and Si–O–Mg structures formed.	C↑ (+19%), ash and O/C–H/C↓, indicating enhanced aromatization and structural stability.	Mg citrate improved N/P retention and struvite recovery (>90% N, >85% P), enhancing hydrolysis, stabilization, and economic yield.	[72]
Dominico Harton plantain peels 150–230 °C, 2–4 h,	Increasing T formed denser, porous hydrochars; FC↑, VM↓, ash↓, ED = 1.69. Dehydration–decarboxylation dominated, generating furfural/5-HMF (190 °C) and phenolics (170–190 °C); aromatization intensified above 210 °C.	C↑, O and O/C–H/C↓, confirming enhanced aromaticity and structural stability	Yield↓ with T; high C and ED indicate solid fuel potential; 210–230 °C gives optimal stability and efficiency.	[55]
Rice straw (RS), Wheat straw (WS), and Corn straw (CS) 180 °C and 260 °C; 1 h, 1:10 (w/v);	Temperature increase transformed fibrous structures into porous, microspherical carbon via polymerization and gas release. Enhanced dehydration and decarboxylation weakened –OH/C=O bands and strengthened aromatic C=C, indicating oxygen removal and aromatization.	C↑ and H/C–O/C↓ confirmed dehydration, deoxygenation; Van Krevelen indicated higher coalification and hydrophobicity.	HTC at 280 °C produced carbon-rich, aromatic hydrochar via dehydration, aromatization, suitable for energy conversion.	[73]
Styrofoam (SF) + Sawdust (SD) 180 °C, 200 °C, 220 °C; 60 min; 1:1	Thermal analysis showed three degradation stages (≤ 233 °C, 200–386 °C, 445–598 °C) reflecting gradual carbon densification. FTIR confirmed dehydration–decarboxylation via weakened –OH/C=O and aromatic C–H bands, indicating increased aromaticity	Progressive C enrichment and O loss (↓H/C, O/C) enhanced carbonization, coalification, and hydrophobic character.	Co HTC improved HHV (28.8→29.8 MJ/kg), raised fixed carbon, and lowered ash. Water recirculation enhanced yield via polymerization and gas release effects	[74]

.

The comparative examination of hydrochars derived from various biomass and waste feedstocks under different hydrothermal carbonization (HTC) conditions, as delineated in Table 2, indicates uniform physicochemical transformation patterns dictated by dehydration, decarboxylation, and aromatization processes. Notably, the analysis encompasses a wide range of feedstocks, including lignocellulosic residues (such as cotton stalks, forest waste, and straws from rice, wheat, and corn), nutrient-dense substrates (including dairy manure, pig manure, and Chlorella biomass), as well as mixed feedstock systems (like combinations of microalgae with corn stalks and Styrofoam with sawdust). An increase in both temperature and residence time consistently promotes a transition towards more condensed, carbon-rich, aromatic structures. A discernible trend is observed, wherein higher HTC temperatures facilitate the cleavage of hydroxyl, carbonyl, and ether bonds, as indicated by the attenuation of -OH, C=O, and C–O absorption bands, alongside the corresponding amplification of aromatic C=C signatures. This escalating deoxygenation, which can be monitored via Fourier transform infrared spectroscopy (FTIR) and CHO elemental analysis, is reflected in diminishing H/C and O/C ratios, signifying intensified carbonization and coalification processes. Feedstocks characterized by elevated lignin content, such as forest residues and Jatropha fruit shells, are likely to yield more stable aromatic char matrices; conversely, substrates rich in proteins or nitrogen (such as microalgae, pig manure, and dairy manure digestate) tend to engage in additional reactions involving deamination, Maillard-type condensation, and the incorporation of nitrogen functionalities into the carbon matrix. The application of microwave-assisted HTC is noted to enhance deoxygenation kinetics, resulting in the formation of mesoporous carbon spheres and graphene-like nanosheets within shorter reaction periods. Similarly, co HTC systems, exemplified by blends of microalgae with corn stalks and Styrofoam with sawdust, exhibit synergistic effects resulting from complementary decomposition pathways: intermediates derived from polysaccharides encourage polymerization and pore development, while fragments originating from proteins or plastics assist in the incorporation of heteroatoms or promote carbon densification. These co-processing methodologies frequently lead to improvements in yield and higher heating value (HHV), as well as structural stability, while also influencing the retention of heteroatoms (e.g., NH₄Cl-induced nitrogen doping in algal systems or struvite-mediated phosphorus retention in hydrochars from pig manure). Elemental analyses consistently reveal that increasing temperature is associated with rising carbon content (often increasing from approximately 30–50% to over 60–75%) alongside substantial reductions in both oxygen and hydrogen fractions, reinforcing the predominance of dehydration and decarboxylation pathways. Concurrent advancements in fixed carbon content, decreases in volatile matter, and heightened aromatic character collectively produce hydrochars with enhanced fuel properties, thermal stability, and hydrophobic characteristics. For specific feedstocks, such as plantain peels, forest waste, and Chlorella biomass, this structural transformation is characterized by the emergence of microspherical morphologies and pore development, which may subsequently collapse under extreme thermal conditions. In conclusion, the findings emphasize that while the nature of the precursor biomass, ranging from lignin-rich and cellulose-rich to protein-rich and polymer-contaminated, affects the distribution of HTC pathways, the fundamental mechanisms driving hydrochar development remain constant: progressive dehydration, decarboxylation, and polymerization culminating in the formation of condensed aromatic networks that exhibit improved fuel quality and structural integrity. Collectively, these results underscore the capacity of HTC to be tailored for the production of specific carbon materials by judiciously adjusting parameters such as temperature, residence time, feedstock composition, catalyst use, and co-processing techniques.

Hydrothermal carbonization (HTC) effectively transforms various wet biomasses into carbon-rich hydrochars with enhanced fuel properties. Increasing HTC temperature (180–280 °C) elevates the carbonization degree, resulting in higher carbon content and improved calorific value. For instance, hydrochar from forest residues saw carbon content increase from ~48% at 200 °C to ~71% at 280 °C, tripling fixed carbon from ~15% to 45% [65]. Cotton stalk HTC exhibited similar trends, with rising carbon and declining H/C and O/C ratios, indicating dehydration and aromatization [63]. These changes can greatly enhance heating values; microalgal hydrochar’s heating value nearly doubled when the temperature increased from 180 °C to 250 °C [71]. For plantain peels, hydrochar achieved an HHV of ~27.7 MJ/kg at 230 °C, nearing low-rank coal energy levels [55]. However, maximizing fuel quality results in lower solid yields, as yield dropped from over 80% to ~26% under severe conditions. Finding optimal HTC conditions involves a balance between energy content and char recovery, with moderate conditions retaining ~70–76% of energy in the char [55,71].

Physicochemical transformations: The superior fuel characteristics exhibited by hydrochar are fundamentally attributed to specific physicochemical transformations that occur during hydrothermal carbonization (HTC). The primary reactions involved in this process include dehydration and decarboxylation, which facilitate the removal of oxygen in the form of H₂O and CO₂, subsequently producing a solid that is enriched in carbon and aromatic compounds. Fourier transform infrared (FTIR) spectroscopy analyses commonly reveal that, with an increase in HTC temperature or duration, the peaks corresponding to O–H and C=O functional groups diminish or vanish, while new signals indicative of aromatic C=C and C–H bonds emerge [68,74]. This observation suggests the development of polyaromatic structures resulting from the degradation of cellulose and hemicellulose, along with partial depolymerization of lignin [65,68]. Concurrently, there is a notable decrease in the atomic hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios of hydrochars as a function of intensification, which signifies a transition towards coalification and increased hydrophobicity. A Van Krevelen diagram analysis conducted on agricultural straw demonstrated that hydrochars generated at a temperature of 260 °C are positioned closer to the lignite/coal region when contrasted with hydrochars from 180 °C [73]. Across various lignocellulosic feedstocks, such as agricultural straws and wood waste, an escalation in HTC temperature consistently corresponds to a reduction in the O/C and H/C ratios, often by 40–60%, and an increase in the fixed carbon fraction, indicating the condensation of biomass into a more thermally stable state [65,68,73]. Additionally, even low-rank coal subjected to HTC treatment at temperatures ranging from 250 to 340 °C has been shown to undergo further aromatization, characterized by the shortening of aliphatic chains, loss of oxygenated groups, and enhancements in both the coal’s rank and energy density, as reflected by decreases in H/C and O/C ratios [66]. These chemical transformations culminate in hydrochars with markedly elevated ignition temperatures and diminished volatile matter, thereby improving their safety and combustion properties. For example, hydrochar derived from Chlorella at a temperature of 210 °C exhibited a substantial reduction in volatile content and a doubling of fixed carbon, resulting in a fuel that is less reactive but demonstrates enhanced energy density and is safer to handle than its raw algal counterpart [71].

Structural and surface characteristics: Hydrothermal carbonization (HTC) significantly modifies the physical structure of carbonized solids. Under moderate HTC conditions, porosity is enhanced through the decomposition of biopolymers and the release of gases, which create voids within the material. For instance, hydrochar derived from forest biomass initially exhibited an increase in surface area with rising temperature; however, temperatures exceeding 280 °C resulted in pore collapse and a subsequent reduction in surface area [65]. This observation indicates the existence of an optimal HTC range that maximizes porosity prior to the onset of structural contraction. The inherent characteristics of the feedstock also play a crucial role in pore formation. In the case of Jatropha fruit residues, larger lignin-rich particles displayed a minimal carbon increase (~12%) and maintained higher ash content, while smaller shell fractions achieved approximately 20% greater carbon content, yielding chars with enhanced energy density and reduced ash due to more efficient mineral leaching [70]. The particle density of Jatropha hydrochar reached up to 1.64 g cm⁻³ at 220 °C, reflecting the development of a dense, carbonized framework [70]. Furthermore, the implementation of alternative heating techniques or pretreatment processes can further alter the hydrochar structure. For example, microwave-assisted HTC applied to dairy manure produced mesoporous carbon microspheres and graphene-like sheets, suggesting that rapid and uniform heating can facilitate the formation of distinctive carbon architectures [66]. Similarly, the incorporation of a post HTC activation phase or the utilization of organic co-solvents can enhance surface characteristics. A notable study demonstrated that cocoa shell hydrochar synthesized in an ethanol–water medium (HTC E), followed by activation, exhibited the greatest surface area and substantial mesoporosity, as well as nitrogen doping stemming from protein content [67]. The presence of ethanol fostered improved carbon retention, thereby increasing the carbon content of the char, whereas the use of water alone led to greater pore development, highlighting the capacity of process additives to fine-tune pore architecture [67]. In summary, the combined processes of thermal decomposition and restructuring driven by autogenous pressure through HTC produce hydrochars with adjustable porosity and morphology; however, extreme conditions are more likely to promote bulk densification rather than increases in surface area.

Co-hydrothermal carbonization (co-HTC) and the role of additives: co-HTC entails the simultaneous conversion of multiple biomass types in one operation, leveraging chemical interactions between them. This process is not merely additive; it often shows synergistic effects, leading to hydrochars with superior stability, carbon retention, and functional properties compared to those from single feedstocks. The synergy is due to the reactivity of cellulose, hemicellulose, and lignin during HTC. Hydrolysis of cellulose and hemicellulose yields soluble sugars and organic acids, which lower pH and catalyze reactions to enhance char formation. Lignin-derived fragments support structural growth and condensation. Additionally, in nitrogen- and mineral-rich environments, reactions and catalytic effects aid heteroatom incorporation and stabilize porous structures. Consequently, co-HTC produces hydrochars with improved aromaticity and enriched nitrogen and mineral properties, demonstrating the significant role of biomass interactions. Contemporary research has investigated the utilization of co HTC, which entails processing mixed feedstocks alongside chemical additives to enhance the quality of hydrochar and optimize resource recovery. The combination of high-protein or high-ash substrates with carbon-dense biomass has been found to produce a synergistically superior char. For instance, the co-HTC of microalgae in conjunction with corn stalks, representing a protein-rich and lignocellulosic feedstock, demonstrated that the introduction of various nitrogen- or metal-based additives significantly affected the resultant product’s characteristics [69]. Specifically, the application of ferric iron (FeCl₃) acted as a catalyst for the deamination of proteins, facilitating the migration of nitrogen into the aqueous phase. Conversely, ammonium chloride (NH₄Cl) and melamine (an organic nitrogen-rich compound) promoted the integration of nitrogen within the hydrochar matrix, leading to a nitrogen content in the hydrochar of approximately 8.9% [69]. These additives also influenced the pore architecture: FeCl₃ contributed to an increase in surface area by enhancing char aeration, while melamine appeared to obstruct pore access, resulting in a decrease in surface area, although it raised the nitrogen concentration within the char [69].

In another study, the incorporation of magnesium citrate into pig manure during the HTC process served dual functions as both a catalyst and a nutrient stabilizer. The magnesium additive facilitated the degradation of fibrous components and the formation of mineral complexes, yielding a hydrochar characterized by a reduction in ash content (by 4–20%) and an enhancement in carbon content (by 2–19%) when compared to HTC processes that did not utilize additives [72]. Notably, greater than 90% of the nitrogen and more than 85% of the phosphorus present in the manure were retained within the solid product in the form of struvite-like Mg–NH₄–P compounds [72]. This retention of nutrients positions the hydrochar as a viable candidate for use in fertilizers or soil amendments, exhibiting slow-release nutrient properties while simultaneously diminishing nutrient losses to the liquid phase.

Moreover, co-HTC involving biomass and recalcitrant waste has shown considerable potential for improvement. A specific investigation that combined waste Styrofoam, a highly volatile synthetic polymer, with sawdust at temperatures ranging from 180 to 220 °C produced a co-hydrochar with enhanced fuel properties relative to individual feedstock char outputs. Analysis of thermogravimetric profiles revealed a more complete carbonization process when the materials were co-processed, resulting in a co-HTC product exhibiting increased fixed carbon content and a slight rise in higher heating value (HHV), from 28.8 to 29.8 MJ/kg [74]. Importantly, the ash content of the blended char was lower than that of pure sawdust char, likely due to dilution effects and interactions that transferred inorganics into the liquid phase, a phenomenon corroborated by findings from other co HTC studies. Such advancements underscore the efficacy of co-feeding complementary materials, leading to the production of a solid fuel characterized by diminished ash content and superior combustion attributes [74]. Additionally, the same investigation indicated that the practice of recycling process water during successive HTC cycles resulted in an augmented hydrochar yield through secondary char formation from dissolved organics, thereby significantly enhancing carbon recovery [74].

Valorization of hydrothermal carbonization (HTC) products: A significant benefit of the hydrothermal carbonization process lies in its potential for the valorization of both the resulting solid hydrochar and the liquid byproducts. The fuel characteristics of solid hydrochar can be enhanced, as previously described; additionally, it has applications within environmental contexts. Due to its elevated fixed carbon content and porous architecture, hydrochar can be employed for carbon sequestration or as an adsorbent. Furthermore, when supplemented with nutrients, such as magnesium or inherent ash, it can serve functions as a fertilizer or a soil amendment [72]. In various feedstocks, inorganic nutrients often concentrate within the hydrochar matrix; for instance, phosphorus derived from food waste digestate can precipitate as stable hydroxyapatite minerals within the char at elevated HTC temperatures [68]. This process of phosphorus immobilization mitigates leaching risks and indicates that such char may facilitate a gradual release of phosphorus in soil environments.

The liquid byproduct, commonly referred to as “HTC process water,” typically contains a high concentration of dissolved organic substances, including organic acids, furan derivatives, and phenolic compounds, along with ammonia and other mineral nutrients originating from the feedstock. This aqueous effluent is characterized by a low pH level (approximately 3.8 to 5.0) and exhibits an exceptionally high chemical oxygen demand, often registering tens of thousands of mg L⁻¹ due to the soluble carbon content [55]. Instead of disposing of this stream as waste, researchers have pinpointed potential avenues for the recovery of valuable compounds or the reuse of the water. For instance, HTC processing of plantain peels yielded process water with significant concentrations of platform chemicals like 5-hydroxymethylfurfural (up to around 1.4 g L⁻¹) and various phenolic compounds, in addition to measurable antioxidant activity [55]. These bios-derived chemicals present the opportunity for extraction for applications in pharmaceuticals or fine chemicals, or as precursors for resin manufacturing, thereby establishing an additional valorization pathway.

Another strategy entails recycling the nutrients present in the aqueous phase. In the context of microalgal HTC, the process water demonstrated the capability to support the regrowth of fresh microalgae, achieving approximately 92% of the biomass yield comparable to that produced with a fresh growth medium [71]. This exemplifies a closed-loop system in which the nitrogen and phosphorus released during the HTC process, elements that would typically necessitate wastewater treatment, are recycled to foster new biomass growth, thus enhancing the sustainability of the approach. Lastly, as previously mentioned, reusing the process water in subsequent HTC batches not only ensures responsible management of wastewater but also has the potential to augment hydrochar yields by transforming dissolved organics into additional solid carbon [74]. Collectively, these valorization strategies associated with HTC outputs, enhancing hydrochar for use as a clean solid fuel or functional material and extracting energy or nutrients from the liquid phase, underline the contribution of HTC to sustainable biomass refineries. The advancements reported in recent studies [55,63,64,65,66,67,68,69,70,71,72,73,74] elucidate that by adjusting process parameters (including temperature, duration, feedstock combinations, and catalysts), HTC can be optimized to yield hydrochars with fuel characteristics akin to coal, alongside high-value co-products, thereby fostering an alignment of energy production with waste reduction and resource recovery within a circular bioeconomy.

The comparative assessment of various hydrochars outlined in this section provides compelling evidence that, despite marked differences in feedstock composition, hydrothermal carbonization (HTC) consistently instigates similar physicochemical transformation pathways across all biomass categories. The data presented in Table 2 corroborates that increased severity of HTC, primarily influenced by temperature and residence time, systematically advances dehydration, decarboxylation, and aromatization reactions. This progression leads to enhanced carbon densification, a decrease in the presence of oxygenated functional groups, and a notable reduction in the hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios. This pervasive transition towards more condensed and aromatic carbon structures is fundamental to the improved fuel quality, hydrophobicity, and thermal stability observed in nearly all hydrochars. While the characteristics of the feedstock influence the scale of these phenomena, they do not alter their overall trajectory. Residual lignin tends to yield more structurally stable aromatic chars, while feedstocks rich in nitrogen or containing proteins are subject to additional deamination and Maillard-like reactions, facilitating the incorporation of heteroatoms into the carbon framework. Additionally, microwave-assisted HTC not only accelerates deoxygenation but also supports the formation of mesoporous carbon spheres, whereas co HTC systems manifest synergistic effects that bolster carbon retention, enhance porosity, and integrate heteroatoms. Elemental analyses consistently indicate carbon content levels between 60 and 75%, accompanied by significant reductions in both hydrogen and oxygen fractions, reinforcing the predominance of carbonization pathways independent of the initial biomass type. Moreover, structural analyses encompassing micropore and mesopore evolution, microsphere formation, and partial graphitization illustrate the capacity of HTC to yield carbon materials tailored for specific applications. Although extreme conditions may lead to pore collapse, moderate HTC parameters frequently encourage the development of functional porosity and surface characteristics favorable for applications such as adsorption, catalysis, or electrochemical utilization. Collectively, the findings presented in this section affirm that HTC serves as a robust and flexible thermochemical process capable of transforming a diverse array of biomass and waste feedstocks into carbon-rich solids with superior physicochemical attributes. The consistent mechanisms of transformation observed, alongside the adaptability afforded by variations in temperature, reaction duration, and feedstock selection, position HTC as a promising and versatile technology for generating high-quality solid fuels and value-added carbon materials.

Key takeaways: The comparative examination of various hydrochars presented in this section illustrates that, despite considerable diversity in biomass sources, compositions, and processing methodologies, hydrothermal carbonization (HTC) instigates a notably uniform sequence of physicochemical transformations. These transformations are primarily influenced by dehydration, decarboxylation, aromatization, and polymerization reactions. An increase in the severity of HTC systematically facilitates carbon densification, reduces hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, curtails oxygenated surface functionalities, and encourages coalification, ultimately resulting in hydrochars with characteristics akin to lignite in terms of stability and energy density. Nevertheless, this section also indicates that these advantages do not manifest in a universally linear or boundless manner. Excessive thermal intensity may precipitate pore collapse, impair surface area, and compromise solid yield, thereby challenging the assertion that “higher temperature is always better,” which is neither mechanistically sound nor technologically optimal. Instead, the data favors the concept of a “functional optimization” window, wherein adequate severity fosters aromatic ordering and hydrophobicity while circumventing structural damage and material loss. The composition of the feedstock introduces further complexity: materials rich in lignin tend to favor the development of stable aromatic structures, whereas substrates rich in nitrogen and minerals are susceptible to additional deamination, Maillard-type condensation, nutrient immobilization, and ash–mineral interactions that can beneficially, or occasionally detrimentally, modify the final chemistry and performance of hydrochar. This necessity emphasizes the importance of interpreting HTC results through both mechanistic chemistry and feedstock-specific reactivity, rather than relying solely on empirical observation. Notably, the section also elucidates that process intensification methods, such as microwave-assisted HTC, co-HTC strategies, and additive-assisted processing, function as active chemical design tools rather than mere incremental modifications. The application of microwave heating enhances deoxygenation kinetics and fosters mesoporous and partially graphitized structures, while co-HTC consistently exhibits synergistic characteristics, improving carbon retention, stabilizing structural integrity, promoting heteroatom incorporation, and, in some cases, enhancing ash management. Additives such as magnesium-based compounds reveal a dual functional role by improving carbon quality while simultaneously stabilizing nutrients into recoverable mineral forms, thereby directly associating HTC chemistry with the functionalities of a circular bioeconomy. These findings underscore the perspective that HTC should not be perceived merely as a carbonization process but as a tunable reaction platform where structure, chemistry, and application potential can be systematically engineered. Nonetheless, despite the strong consistency in qualitative transformation patterns, the quantitative variability among studies reflects a continued reliance on case-specific experimentation within the field. There is an evident necessity for more standardized evaluation frameworks, mechanistic interpretations spanning diverse feedstocks, and application-driven optimizations. Overall, this section substantiates the assertion that HTC exhibits both robustness and versatility: it consistently produces condensed, energy-dense, and functionally adaptable carbon materials, while also providing significant potential for targeted customization via parameter control, co-processing, and additive engineering.

5. Functional Materials

Owing to several benefits, including the inexpensive raw materials, extensive microporosity, large surface areas, and their excellent chemical and thermal stability, carbon materials derived from HTC are applied in various fields, such as energy storage (e.g., batteries, supercapacitors), electrocatalysis, heterogeneous catalysis, gas storage, and water treatment [75,76]. A graphic representation of the HTC process from biomass and the products’ possible uses is given in Figure 5.

5.1. Heterogeneous Catalysis

HTC-derived carbon materials have found various applications in catalysis. They can act as catalysts on their own, primarily serving as solid acid catalysts by imparting strong Brønsted acidity due to the presence of sulfonated groups on their surfaces. Another common application of these materials is as a support for catalysts. Their adjustable surface polarity and area enhance the attachment of metal nanoparticles, which can be utilized in different chemical reactions. Hydrothermal carbon is particularly noteworthy in this context because its properties help reduce the leaching of active species compared to traditional activated carbons. Given that hydrothermal carbon can be removed through combustion in air at relatively mild temperatures, it can serve as an agent that directs structure formation [75].

The incorporation of sulfonated groups into carbon materials synthesized via HTC results in the presence of sulfonic acid groups (Figure 6), which allows for the creation of a solid acid catalyst suitable for catalytic reactions. These materials can be easily filtered and recovered. Typically, these materials are produced by the treatment of porous carbon with concentrated sulfuric acid at elevated temperatures [75]. Gan et al. investigated a carbonaceous catalyst derived from lignin that is highly acidic, created using Masson pine alkaline lignin through hydrothermal carbonization in the presence of acrylic acid, followed by sulfonation with concentrated sulfuric acid. This SO₃H-functionalized lignin-based solid acid demonstrated remarkable efficiency for catalyzing the hydrolysis of cellulose into reducing sugars within a [BMIM]Cl-H₂O solvent system. The catalyst LHAC-SO₃H-40, prepared by incorporating 40 wt% acrylic acid, exhibited competitive catalytic performance for cellulose hydrolysis, achieving a total reducing sugar yield of 75.4% in a [BMIM]Cl-H₂O solvent system at a mass ratio of 100:1, surpassing traditional solid acid catalysts [77].

Numerous sulfonated hydrothermal carbon catalysts have been utilized in esterification processes, particularly in the production of biodiesel. One example is the catalyst developed by Ribeiro et al. Ribeiro et al. applied esterification using açai seeds (Euterpe oleracea), as a source of sulfonated carbon spheres, as a green catalyst. With a conversion rate ranging from 91% to 88% over the course of five cycles, the catalyst made by hydrothermal carbonization in water performed the best, whereas the catalyst made with the help of a catalyst achieved conversion rates of 89–82% over the same five cycles [78]. Yadav et al. studied using the low-temperature hydrothermal carbonization process to prepare a solid acid catalyst from areca nut husk for use in the production of biodiesel at low temperatures. The catalyst’s reusability was tested for up to four consecutive cycles, and on the fourth catalytic cycle, a high 85% conversion of oleic acid (OA) to methyl oleate was obtained [79]. Araujo et al. investigated the hydrothermal carbonization of acai seed using the in situ functionalization technique and applied it to the esterification process between methanol and oleic acid. Catalysts with a high density of sulfonic groups and good catalytic activity in the oleic acid esterification reaction were successfully produced using the single-step synthesis approach. In the esterification reaction of oleic acid and methanol, one of the catalysts that was simultaneously carbonized and sulfonated at 130 °C for six hours performed better, with a 95% conversion at a reaction temperature of 100 °C, reaction time of one hour, catalyst loading of three weight percent, and a 12:1 methanol-to-oleic acid molar ratio. After five reaction cycles, the best one’s recyclability was satisfactory, lowering conversion by just 18%. Consequently, the authors emphasized that the in situ functionalization method was better than the postgrafting functionalization method for creating catalysts based on the acai seed [80]. Hybridizing functional inorganic materials during the hydrothermal carbonization (HTC) of biomass is beneficial for producing functional catalyst systems. When the HTC process occurs in the presence of noble metal salts, these can be reduced on-site by the aldehyde groups from the carbohydrates and form intermediates, which lead to carbon materials that contain metallic nanoparticles. Given that these nanoparticles are hydrophobic, they tend to reside in the hydrophobic core of the carbon material, specifically at the center of the carbon sphere. Through this method, Pd^o nanoparticles were synthesized within a carbon matrix, which provides an environment characterized by polar, oxygenated, hydrophilic groups along the channels leading to the catalytic particles [81]. This composite system demonstrated effective catalytic activity for the selective hydrogenation of phenol into cyclohexanone [82]. The conventional method for synthesizing cyclohexanone still requires a two-step process that includes the reduction of phenol to cyclohexanol, followed by its dehydrogenation to yield cyclohexanone. The industry favors a direct approach, but it can only be conducted in the vapor phase under relatively extreme conditions [83]. In comparison to the commercially available hydrophobic charcoal-supported catalyst, the more hydrophilic HTC catalyst exhibited good selectivity in water for partial hydrogenation, predominantly producing cyclohexanone.

The catalytic system can be enhanced not only by adding metal salts and precursors during the hydrothermal carbonization process but also through the inclusion of organic monomers that are soluble in water, which can either modify the support or create metal-free catalytic activity. Therefore, by conducting hydrothermal carbonization of glucose alongside various organic monomers such as acrylic acid [84] or vinyl imidazole [85], carbon materials were produced that exhibited enhanced organic functionality. In this scenario, the carbon precursor (carbohydrate) serves as the foundational structure, providing thermal or mechanical stability to the system, while the organic monomer, introduced in small quantities, contributes the desired functionality. This approach represents a blend of a biomass-derived, sustainable material with elements of petrochemistry, with the predominant fraction derived from sustainable sources.

For the synthesis of catalysts, this organic modification is ideally paired with the silica templating method discussed earlier, resulting in high surface areas along with advantageous pore connectivity. This approach results in the formation of a mesoporous carbon framework that has imidazole groups present on its surface [86]. Imidazoles themselves are not particularly strong bases or nucleophiles, which restricts their catalytic applications [81]. Conversely, imidazolium ions, particularly those derived from imidazolium-based ionic liquids, have numerous catalytic uses either independently [87] or as stable carbenes [88]. To evaluate the accessibility of the imidazole-grafted functions for catalysis, the material underwent alkylation by being refluxed overnight in toluene with one mass equivalent of butyl bromide. The success of the alkylation was confirmed by energy dispersive X-ray spectroscopy (EDX), which indicated the presence of bromine in the solids (up to 1.5 atom%). The catalytic performance of this quaternary carbon material was assessed in two reactions recently shown to be accelerated by imidazolium halides: (i) the aromatization of unsaturated six-membered rings (particularly Diels–Alder condensation products) [89], and (ii) Knoevenagel and aldol condensations [86].

The Diels–Alder reaction between naphthoquinone and cyclohexadiene, conducted with a metal-free carbon catalyst, nearly reaches full conversion of the reactant, while anthraquinone (the re-aromatization byproduct) constitutes about 25 mol% of the total products. Likewise, both the Knoevenagel reaction of benzaldehyde with malononitrile and the aldol reaction of benzaldehyde with acetophenone were successfully carried out in high yields under relatively mild conditions and with excellent selectivity [86].

Karod et al. focused on the use of bentonite, an inexpensive and plentiful clay mineral, as an in situ heterogeneous catalyst for two thermochemical conversion methods: pyrolysis and HTC. Avocado seeds were mixed with 20 wt% bentonite clay and subjected to pyrolysis at 600 °C and hydrothermal carbonization at 250 °C, which are common conditions noted in the literature. During pyrolysis, the presence of bentonite clay facilitated Diels–Alder reactions that converted furans into aromatic compounds, thereby reducing the oxygen content in the bio-oil and yielding a fuel that is more compatible with existing infrastructure. The HTC-derived char, with higher FC content, was proposed to exhibit greater stability than char from pyrolysis for environmental applications [90].

Liang and Yang created a carbon-based acid catalyst using hydrothermal carbonization from glucose at milder conditions, resulting in a higher yield. When compared to Nafion and Amberlyst-15, this catalyst demonstrated a high acidity of up to 2.0 mmol/g. With its characteristics of strong acidity and low cost, as well as high thermal and chemical stability, the catalyst displayed excellent performance in acetalization, achieving an average yield of over 93%, suitability for large-scale applications, and impressive reusability [65].

Yan et al. investigated the HTC of wood-derived sugars to produce carbon-encapsulated iron nanoparticles and their use as catalysts for the Fischer–Tropsch synthesis (FTS) process. Consequently, nanoparticles demonstrated a great degree of selectivity in the production of C₅ hydrocarbons, recording an 89.5% CO conversion rate and a 65% hydrocarbon selectivity when reacting with biomass-derived syngas at 290 °C [91].

Diaz de Tuesta conducted extensive research on the valorization of sugarcane, malt, and chia seed bagasse by converting them into pyrochars, hydrochars, and activated carbons through pyrolysis, HTC, and sequential HTC and pyrolysis for the wet peroxide oxidation of micro-pollutants. Hydrochars demonstrated weak catalytic activity in the investigated oxidation process and limited porosity development when compared to other materials. As demonstrated by the hydrochar made with iron, the use of iron as a chemical agent in the activation of the carbon materials increased their catalytic activity, since Fe catalysts were produced [92].

It has been widely acknowledged that the supporting materials of heterogeneous catalysts have a significant impact on their performance. Because of their great surface area and usefulness, investigating biomass-derived HTC carbon composites and colloids as metal particle supports for various catalytic applications was a simple trial. The production of Pt-/Pd-loaded carbon microspheres (CMS) as catalysts for direct methanol fuel cells (DMFCs), a promising, low-cost liquid energy source for the future, was described by researchers [93]. Considering the cyclic voltammogram results, the freshly prepared Pt@CMS and Pd@CMS showed better catalytic activity for methanol oxidation in alkaline media than commercial Pt (Pd)/carbon black. Also, some groups have studied different carbon composites from HTC coated with Pt nanoparticles, showing higher performance for methanol oxidation [94,95,96,97,98]. Nevertheless, as all of these catalysts had “simple” structures, they were unable to address the issue of methanol diffusion, which both poisons the catalyst and results in a mixed potential.

A thrilling step toward generating a methanol-tolerant cathodic catalyst composed of mesoporous carbon with widely dispersed core–shell Pt@C nanoparticles in the nanochannels (Pt@C/MC) was reported by Wen et al. For methanol-tolerant oxygen electroreduction, the product produced by the HTC process was found to exhibit high catalytic activity and outstanding stability. The thin mesoporous carbon layer on the Pt nanoparticles was reported to allow oxygen to reach the activity sites while preventing methanol from performing in the same way, and is thought to be the reason for the great stability by the authors [99].

Yu and colleagues utilized the HTC technique to create “hybrid fleece” structures, which were composed of homogeneous carbonaceous nanofibers embedded with noble metal nanoparticles. Significantly, the resulting hybrid nanostructures function as effective catalysts for the 100% conversion of CO to CO₂ at low temperatures. This was ascribed to their highly specific metal surface area, binary carbon–metal interactions, and unique nano- and microstructure. It needs to be highlighted that the catalysis performance in the second round of experiments was superior to the first [100].

In heterogeneous catalysis, the performance of a catalyst is often dictated by its specific surface area (SSA), which provides the platform for active site dispersion. Raw hydrochars typically possess very low surface areas compared to biochars [18,101], primarily due to the “hydrothermal” nature of their synthesis, which favors the formation of solid carbon spheres rather than a porous network. While pyrolysis (biochar) involves the release of volatiles that create “escape channels” or pores, HTC involves the dehydration and polymerization of monomers in a liquid phase. However, the recent literature in heterogeneous catalysis has shifted toward viewing hydrochar not as a finished catalyst, but as a highly functionalized precursor that can be engineered into high-surface-area supports [18,101].

To make hydrochars viable for heterogeneous catalysis, the literature emphasizes chemical and physical activation methods:

• Chemical activation: The most common method to achieve high SSA is treating hydrochar with activating agents like KOH, ZnCl₂, or FeCl₃ [102,103]. This creates a hierarchy of micro- and mesopores.
• In situ templating (hard and soft templates): Researchers often use templates during the HTC process to prevent the formation of dense microspheres [104]. In hard templates, after the carbon is formed, the silica is etched away with HF or NaOH, leaving behind a carbon skeleton with a controlled pore size [105]. In soft templates, it is typical to use surfactants (like Pluronic F127) to guide the polymerization of carbon precursors into ordered mesoporous structures [106].
• Graphene oxide (GO) assistance: Studies have shown that adding small amounts of graphene oxide during HTC acts as a 2D template [107,108,109]. This may prevent the “clumping” of carbon, and it results in flake-like or platelet-like structures that significantly boost the initial SSA before any further chemical activation.

Despite the lower initial SSA, the catalysis literature often prefers hydrochar for specific reactions because it has some advantages, like high oxygen content, hydrophilicity, and use in green synthesis. The “messy” surface of hydrochar is actually an advantage. Carboxylic and phenolic groups act as anchoring sites for metal precursors, preventing the agglomeration of active catalyst particles [110]. In terms of hydrophilicity, hydrochars perform better than the naturally hydrophobic biochars because they allow better contact between the water-bound reactants and the catalyst surface in aqueous-phase catalysis [111]. As a last advantage, HTC is conducted at lower temperatures (180–250 °C) and does not require drying the feedstock, making the overall catalyst production more energy-efficient, which makes it suitable for green synthesis [112]. Unactivated hydrochar’s low surface area inherently limits its capacity as a high-activity heterogeneous catalyst support in reactions where accessible surface sites dominate. However, chemical/thermal activation and functionalization can dramatically change this picture, enabling hydrochars to approach or even rival traditional supports, especially in catalytic systems where surface chemistry and functional group interactions play a significant role.

According to the studies above, the application of hydrothermal carbonization-derived carbon catalysts from biomass has found a wide range of types of catalytic processes. Sulfonated hydrothermal carbons prepared from agricultural wastes (e.g., açai seeds, areca nut husk, and glucose) show high activity, reusability, and stability in esterification reactions for biodiesel production, often achieving conversions above 85–95%. In situ functionalization during HTC is highlighted as superior to post-synthesis methods for introducing sulfonic groups. The incorporation of metal salts during HTC enables the formation of carbon-supported metal nanoparticles (e.g., Pd, Fe, and Pt), which demonstrate effective performance in reactions such as selective hydrogenation, Fischer–Tropsch synthesis, methanol oxidation, and oxygen reduction. Organic monomers and templating strategies further enhance surface functionality and porosity, enabling metal-free catalysis and improved selectivity in reactions like Diels–Alder, Knoevenagel, and aldol condensations. Overall, biomass-derived HTC carbons are presented as sustainable, versatile, and high-performance catalyst supports or active catalysts with significant potential for industrial and energy-related applications.

5.2. Adsorption

The discharge of effluents from a variety of industries, including the production of paint, textiles, paper, leather, and cosmetics, contributes significantly to water contamination as a result of industrialization. With improved removal efficiency, quick reaction times, and ease of use, adsorption has become a very effective and economical method for treating wastewater. The improved adsorption potential of hydrochar, which has a greater capacity and quicker action than biomass, has been the subject of numerous investigations. Hydrochar was created from biomass using the HTC procedure to evaluate its efficacy in terms of contaminants from synthetic wastewater [113]

Saini et al. investigated the enhanced adsorption potential of hydrochar produced via HTC of sunflower stalks (biomass) for removing methylene blue (MB) dye from synthetic wastewater. Their results showed that hydrochar, due to its greater surface area and pore volume, exhibited a superior adsorption capacity (49.37 mg/g) compared to the raw biomass (24.24 mg/g) under optimal conditions [113]. Khoshbouy et al. studied obtaining sludge-based activated hydrochars (SAC) via HTC followed by physical or chemical activation, and their application for the removal of methylene blue (MB), a basic dye, from aqueous solution. Adsorption capacity results were recorded as the maximum monolayer MB adsorption capacity for the prepared hydrochar, physically activated SAC, and chemically activated SAC as 63.3, 122.4, and 588.2 mg/g, respectively, demonstrating superior performance compared to commercial activated carbons [114]. Liu et al. prepared a novel rich carbon material, ‘pyro hydrochar,’ derived from corn straw and corncob via sequential hydrothermal carbonization and pyrolysis, and compared the adsorption performance of biochar, hydrochar, and pyro hydrochar for contaminants such as atrazine, Cd²⁺, and Cr(VI). They concluded that pyro hydrochar, due to its large specific surface area and rich oxygenated functional groups, is a promising and economic adsorbent candidate [115]. Koottatep et al. focused on the use of hydrochar derived from the HTC of fecal sludge as an adsorbent for copper removal in wastewater treatment. They compared the adsorption capacity of HTC hydrochar and KOH-modified hydrochar for copper removal, finding that the chemically modified hydrochar had a maximum adsorption capacity of 18.6 mg Cu/g hydrochar [116]. Hao et al. used activated carbons prepared from HTC waste biomass with respect to the adsorption of carbon dioxide. They investigated physically activated carbons (PAC) and chemically activated carbons (CAC) derived from HTC biomass, detailing their preparation and pore characteristics. As a result, they highlighted that PAC from HTC grass cuttings and horse manure exhibited the largest ultramicropore volumes, which are crucial for CO₂ capture from flue gas [117]. In another study, researchers focused on the preparation of hydrochar from “Salacca zalacca” peels HTC for use as an adsorbent. Their results showed that the adsorption capacity for Congo red dye increased significantly from 33.003 mg/g for the raw peels to 133.333 mg/g for the HTC-derived hydrochar [118]. Chandrasekar et al. investigated the synthesis of hydrochar using catalytic HTC of sawdust (biomass precursor) for the adsorption of emerging contaminants in multicomponent systems. In their adsorption efficiency results, the hydrochar achieved maximum removal percentages of 92.4% for ibuprofen, 85.4% for sulfamethoxazole, and 82% for bisphenol A. They concluded that the synthesized hydrochar possesses a porous structure, a high specific surface area of 114.84 m²·g⁻¹, and various oxygen-containing functionalities [119]. Genli et al. explored the preparation of mesoporous activated carbon (AC) through potassium hydroxide activation of hydrochar, which was derived from the HTC of chickpea stem (CS), a type of biomass, and its application in removing MB dye from aqueous solutions in a batch system. They recorded that after five consecutive adsorption–desorption cycles, HTC CSAC maintained the reuse efficiency of 77.86% [120]. In another study, researchers compared hydrochar, produced via HTC of rice husk biomass, and pyrochar, produced via slow pyrolysis, as adsorbent materials for the removal of methylene blue, iodine, and copper ions from aqueous solution. They concluded that hydrochars have a much higher adsorption capacity for methylene blue, iodine, and copper ions than pyrochars, despite having a relatively lower surface area, because of ion exchange and complexation [121]. Stefanelli et al. studied the use of activated hydrochars prepared from sewage sludge through HTC for the removal of pollutants in gaseous and aqueous environments, specifically investigating ciprofloxacin (CIP) adsorption capacity. They stated that chemical activation of HTC-treated sewage sludge provides a promising method to convert waste into valuable low-cost adsorbents with high surface areas [122].

Using Fenton and biological oxidation processes, Zhang et al. offered a viable method for the valorization of ferrous and activated sludge generated during the treatment of textile wastewater. These sludges were used in a hydrothermal method to create the nano-level magnetic biochar catalyst. The extra biological sludge and ferric sludge produced during dyeing wastewater treatment were turned into a magnetic biochar composite (MBC) under ideal HTC conditions in order to address sludge disposal and management issues. Compared to BC made by a single biological sludge process under identical HTC circumstances, the created MBC with ferric sludge mixing contained paramagnetic Fe₃O₄, had a smaller diameter of around 200 nm, a smaller pore size, a bigger specific surface area, and a higher degree of carbonization. Furthermore, it was discovered that the biochar and Fe₃O₄ in the MBC were strongly chemically bonded together, giving the MBC the ability to recycle magnetically. It was identified as a good catalyst due to its stable, high MB degradation performance in a Fenton reaction following recycling [123].

The studies that we have reviewed above demonstrated that the effectiveness of hydrochar and activated hydrochar produced via hydrothermal carbonization (HTC) of various biomass and waste materials as adsorbents and catalysts for environmental remediation. HTC-derived hydrochars show significantly higher adsorption capacities than raw biomass and, in many cases, outperform commercial activated carbons for removing dyes, heavy metals, pharmaceuticals, gases (CO₂), and emerging contaminants from water and air. Physical and chemical activation, as well as post-treatments such as pyrolysis or metal incorporation, substantially enhance surface area, pore structure, and functional groups, leading to improved adsorption and reusability. Additionally, waste-derived hydrochars, including those from sewage sludge and industrial sludges, can be converted into high-value magnetic biochar catalysts with strong recyclability and catalytic performance in advanced oxidation processes. Overall, HTC is highlighted as a sustainable and versatile approach for transforming biomass and waste into efficient, low-cost adsorbents and catalysts for pollution control.

5.3. Soil Amendment and Agricultural Applications

One new mitigation method that has drawn a lot of attention is the conversion of biomass into biochar and its application to soil. The kind and pretreatment of the biomass or feedstock utilized, as well as the temperature, pressure, and other parameters during conversion, vary among the several known processes for producing biochar. Therefore, the physical and chemical properties of chars made using various techniques vary greatly, as does their capacity to sequester carbon [124,125]. Recently, HTC has been proposed as an easy, affordable, and efficient method of raising biomass’s C content [126]. This carbonization technique is especially useful for large-scale wet biomass that is unsuitable for other carbonization techniques, like sewage sludge, industrial bio-wastes, and green household wastes [127,128,129,130].

Malghani et al. investigated the use of hydrochar, which is created by hydrothermally carbonizing biomass, as a soil additive to raise soil C. Following the results, the addition of hydrochar C in levels initially sufficient to increase the total C content of the soil by 20–30% showed significant effects on both plant development and soil characteristics. Even a year after the hydrochar amendment was applied, the measurements of total organic C, total nitrogen, dissolved organic carbon (DOC), water content, and microbial biomass increased. At the conclusion of their research, they emphasized the need for a deeper comprehension of the relationship between the charring process, the structure of the char that was produced, and its reactivity in various soils [125]. Xia et al. examined the effectiveness, risk assessment, and possible mechanisms of using modified hydrochar to immobilize heavy metals in contaminated soils. Hydrochar was made from pinewood sawdust. The modified hydrochar and pristine hydrochar were tested to immobilize the heavy metals (HMs) of Pb and Cd in polluted soils. The modified hydrochar was made using a simple one-pot lime-assisted hydrothermal synthesis method. Due to the modified hydrochar’s increased surface functionality and non-crystalline qualities, higher pH level, and improved electronegativity, the results demonstrated that the modified hydrochar greatly outperformed pristine hydrochar in immobilizing Pb and Cd [131]. The hydrothermal carbonization of biomass from landscape management was the main emphasis of Röhrdanz et al. They emphasized how the circumstances of the HTC process affected the capacity for cation exchange and water retention. The water-holding capacity (WHC) and cation exchange capacity (CEC) of hydrochars are five-to-ten times higher than those of quartz sand. The CEC of hydrochars made under harsher HTC conditions is similar to that of Cambisol. Highest WHC and CEC following HTC were recorded at 15 min and 180 °C. It is necessary to find a balance between high WHC and CEC and great biological stability. After completing their research, they declared that more research was necessary [132]. Using a pot experiment, Lang et al. examined the remediation impact and possible processes of various hydrochars from cow manure (H-CM), corn stalk (H-CS), and Myriophyllum aquaticum (H-MA) at two levels (0.5% and 1.0%) in soil contaminated with oxytetracycline (OTC). The kind of feedstock had a greater influence on the soil microbial community’s reaction to the hydrochar amendment than the amount of hydrochar. The addition of hydrochar improved the microbial metabolism of the soil, including the digestion of carbohydrates and amino acids. According to the redundancy analysis, the abundances of OTC-degrading bacteria (proteobacteria, arthrobacter, and sphingomonas) in all hydrochar-amended soils were positively correlated with the TCA cycle. By changing the characteristics of the soil, boosting OTC-degrading bacteria, and encouraging microbial metabolism, the hydrochar amendment hastened the clearance of OTC from the soil and decreased plant uptake in the soil Chinese cabbage system [133]. De Castro e Silva investigated the use of hydrochars to enhance soil characteristics and the growth of perennial ryegrass (Lolium perenne L.). Using 60 kg P₂O₅ ha⁻¹ of pig manure and maize digestates, generated hydrochars, and triple super phosphate (TSP), a traditional P fertilizer, soil incubation and plant-growing tests were carried out for 12 weeks. The alkaline circumstances had an impact on the potential availability of micronutrients; hydrochars produced a slower, more gradual release of P, whereas plant-available P was 1.5 times greater than TSP. As a result of their increased aromaticity and porosity, hydrochars from co-digestate with a 4 h residence period achieved comparable dry weight (DW) and macronutrient absorption as mono-digestate and TSP. Pig manure mono-digestate was a cheap and efficient phosphorus fertilizer. In comparison to mono-digestate and TSP, hydrochars enhanced soil characteristics but did not increase crop yields [134]. Wang et al. worked on hydrochars produced by treating municipal sewage sludge using hydrothermal carbonization. Investigations were conducted into the effects of reaction temperature (180–300 °C) and reaction time (2–15 h) on the hydrochars’ structural properties, as well as alterations and release risk of common pharmaceuticals and personal care products (PPCPs) in the hydrochars. The decarboxylation reaction was the main process during the conversion of sludge to hydrochars, and reaction temperature had a greater impact on hydrochar characteristics than reaction duration. Compared to biochars, the sludge hydrochars exhibited greater yields, carbon recovery rates, polarity, and reduced aromaticity. With the exception of caffeine and acetaminophen, the hydrothermal treatment successfully decreased the load of PPCPs in sludge hydrochars. Because of the alteration of N-containing precursors, the hydrochars made at intermediate and high temperatures (240 and 300 °C) showed higher caffeine contents than the initial sludge [135]. In a study, researchers investigated the potential of sewage sludge (SS) with rice husk (RH) and wheat straw (WS) co HTC to form hydrochar and aqueous phase (AP) as substitutes for fuel and chemical fertilizer, respectively. The output of co HTC-based hydrochar and greater heating value rose by 10.9–21.6% and 4.2–182.7%, respectively, when compared to single SS hydrochar, reaching a maximum of 72.6% and 14.7 MJ/kg. Co HTC has been shown to enhance hydrochar’s safe handling, storage, and transportation, as well as its combustion efficiency [136]. Si et al. optimized the liquid–solid ratio (LSR) of HTC and comprehensively examined the stability, potential for soil application, and financial advantages of maize stover-based hydrochar under various LSRs. The outcomes revealed that while the element content, thermal stability, carbon fixation capacity, specific surface area, pore volume, and functional group type were not significantly impacted, the total amount of dissolved organic carbon in hydrochars increased by 55.0% when LSR decreased from 10:1 to 2:1. As LSR dropped to 1:1, incomplete carbonization caused the hydrochar’s specific surface area and pore volume to drop by 61.8% and 70.9%, respectively. Enhanced soil application potential was indicated by the gray relation, which demonstrated the highest relation degree of 0.80 and 0.70 for hydrochar generated at LSR of 10:1 and 2:1, respectively [137]. In order to immobilize multi-contaminated soils, Xia et al. created a unique amino-functionalized hydrochar substance (NH2–HCs). The findings demonstrated that the application of NH₂ HCs considerably (p < 0.05) enhanced the organic content, pH value, and cation exchange capacity of the soil. Cu, Pb, and Cd bioavailable concentrations were reduced by 96.2%, 52.2%, and 15.5% in the contaminated soil upon the injection of NH2-HCs, while the leaching toxicity of Cu, Pb, and Cd was significantly reduced by 98.1%, 31.3%, and 30.4%, respectively. The majority of reduced exchangeable Cu, Pb, and Cd were converted into their less accessible oxidizable and residual components. According to a potential ecological risk assessment, the element Cd was responsible for the majority of the overall risks in soils treated with NH₂-HCs. The mechanism investigation revealed that the immobilization of heavy metals was mostly dependent on surface complexation, chemical chelation, and cation–pi interaction of NH₂–HCs. The use of NH₂-HCs greatly enhanced plant growth and decreased metal accumulation, as further shown by pot studies [138]. Alessandrino investigated nutrient and heavy metal release in a siliciclastic riverine sandy soil (Typic Psamments) using a variety of amendments, including two types of hydrochar, two types of lignite, and two types of cactus powder. According to the outcomes, hydrochar, lignite, and cactus powder showed promise for increasing soil fertility, boosting microbial dissolved organic oxygen decomposition, and improving nutrient content. There was a significant metal release from the two varieties of hydrochar and cactus powder [139]. In order to examine practical HTC reaction conditions for handling food waste (FW), Xu et al. examined an integrated pilot-scale HTC system. In both greenhouse and field trials, the application of appropriately diluted aqueous phase (AP) and the composted solid hydrochar (HC) greatly enhanced plant growth and nutrient availability, which were on par with commercial chemical fertilizer and soil amendment. Their results showed that the HTC of FW combined with the products’ agricultural use produced net-negative carbon emissions of −0.28 t CO₂e t⁻¹, which was significantly less than the other FW treatment options [140]. Iqbal et al. aimed to assess how soil fertility and maize productivity were affected by tillage techniques, nitrogen management plans, and acidified hydrochar. Four replications and a split-plot arrangement were employed in the randomized complete block design experiment. Either deep or shallow tillage was applied to the main plots. Nitrogen (120 kg ha⁻¹) from farmyard manure (FYM) and urea was applied to three subplots: control, 33% FYM + 67% urea (MU), and 80% FYM + 20% urea (MF). Sub-sub plots were treated with acidified hydrochar treatments H₀ (no hydrochar) and H₁ (with hydrochar, 2 t ha⁻¹). Deep tillage and coordinated nitrogen management thereby improve soil characteristics and maize productivity. Their results emphasized how crucial it is to choose the right nitrogen and tillage techniques in addition to adding hydrochar for sustainable maize production [141]. Chang and Huang employed hydrothermal carbonization to create silicon-doped liquid fertilizer from waste biomass that was high in silicon. Five distinct treatments were used in the experiment: silicon carbide fertilizer, rice husks, peanut shells, sugarcane extract, water, and unadulterated soil. According to the statistical comparison, adding peanut shell significantly affected the germination of cabbage seeds and accelerated the growth rate of the plant when the silicon carbide concentration was raised. Crop development was impeded when the liquid fertilizer’s silicon carbide level exceeded 12% [142]. Using a 63-day soil column experiment, Wang et al. implemented a cattle manure hydrochar (CHC) at 220 °C to investigate its effects and mechanisms on CH₄ and N₂O emissions, tomato growth, and fruit quality in a coastal soil in comparison with corresponding hydrochars derived from plant straws, namely sesbania straw hydrochars (SHC) and reed straw hydrochars (RHC). The findings demonstrated that CHC was more effective than RHC and SHC at lowering the potential for global warming. Three hydrochars at 3% (w/w) greatly enhanced the dry biomass of tomato shoot and fruit for plant growth by 12.4–49.5% and 48.6–165%, respectively. In addition, CHC had the greatest promoting effect on tomato shoot and fruit dry biomass, followed by SHC ≈ RHC. When compared to CK, the application of SHC, CHC, and RHC considerably increased the tomato’s sweetness, with CHC (54.4%) > RHC (35.6%) > SHC (22.1%) [143]. Hou et al. used a soil column experiment with two hydrochar types (sawdust-derived hydrochar (SDH) and microbial-aged hydrochar (A SDH)) at two application rates (5, 15; (w/w)) to investigate the impact of hydrochar addition on grain yield from low-fertility soils and the corresponding CH₄ and N₂O emissions. The findings demonstrated that adding hydrochar clearly raised rice production. The hydrochar’s substrate supply was the primary cause of the N₂O emissions, with the presence of denitrifiers (functional genes) having less of an impact. The cumulative N₂O emissions were considerably reduced by 26.32% to 36.84% when hydrochar amendment was applied at a low rate (5; SDH05, A-SDH05). Furthermore, it was revealed that the hydrochar amendment did not raise the CH₄ emissions because of the substrate constraint; the cumulative emissions, which ranged from 11.1 to 12.8 g m⁻², were comparable to those from the control. Due to the high yield and low N₂O emissions, the greenhouse gas intensity from the hydrochar-treated soils (SDH05, A-SDH05, and A-SDH15) was much lower than that of the control in terms of grain yield and global warming potential [144]. Chang and Huang examined hydrothermal carbonization as a precursor to agricultural biomass that contains silicon (sugarcane exocarp). To add value, cellulose was added to a bio-nutrient solution at high pressure (15 atm) and high temperature (200 ± 10 °C). Either the field or the greenhouse was used for this experiment. According to this study, the bio-nutrient solution with amino acid oligosaccharide as the primary fertilizer may be applied directly to leaves and effectively fertilize roots. Humic acid liquid fertilizer, a liquid organic soil enhancer, is a bio-nutrient solution with 17.85% humic acid (humic acid > 10%), which can encourage crop growth. Fresh crop weights increased by at least 113%, while field crop stem lengths grew by up to 40% [145]. In order to recover nutrients from biogas slurry (BS), Deng et al. studied the hydrothermal carbonization of a variety of agricultural and forestry wastes, including yellow bamboo, green bamboo, peanut shell, wood meal, peanut straw, wheat straw, rice husk, and maize straw. Green bamboo was determined to be the best biomass material for nitrogen recovery, recovering 46.64% of N after HTC at 250 °C for three hours. Peanut straw was shown to be the best at recovering phosphorus; after HTC, P in the liquid residue was undetectable. Nevertheless, HTC was unable to extract potassium from BS using any kind of biomass that was examined. Since cellulose and hemicellulose provided the majority of the C-N active binding site for the synthesis of aliphatic amines, nitrogen recovery efficacy was strongly connected with the overall cellulosic content of biomass. Because metal phosphate precipitates were formed, higher concentrations of metal salts in biomass feedstock facilitated phosphorus recovery. A wide range of pore diameters was produced by the hydrochars derived from various biomass samples, which was essential for the slow-release properties of nutrients and the immobilization of microorganisms that promoted plant growth. A pot experiment further showed that maize straw hydrochar significantly outperformed chemical fertilizer in terms of holding water and nutrients, which was essential for improving soil fertility over the long run [146]. In another study, pig manure (PM) and invasive plants (Solidago canadensis) were used as feedstocks, with magnesium citrate as a catalyst, to investigate the physicochemical properties and nutrient transformation of HTC products. Results demonstrated that magnesium citrate promoted the waste biomass’s nutrition cycle. The addition of magnesium citrate improved the hydrochars’ pH and carbon content while lowering their ash level, making them more suitable for use in agricultural soils. Additionally, it increased the hydrochars’ degree of charring, which made it easier for proteins, cellulose, and hemicellulose to hydrolyze. This procedure promoted the further stability of organic nitrogen while increasing the amount of accessible nitrogen (NH₄⁺-N, NO₃⁻-N) and the proportion of stabilized phosphorus in hydrochars [72]. Yan et al. investigated the utilization of hydrochar made from biomass waste from duck farms for a maize cultivation experiment and found that hydrochar efficiently increased the soil’s nutrient supply while also stimulating maize development. The application of hydrochar resulted in a 78–253% increase in the organic carbon content of the soil, which was primarily composed of condensed aromatic structure compounds, carbohydrates, and CHON-type lignin. In the meantime, hydrochar significantly altered the oxygenated functional groups and aromatic structures of the soil, resulting in the formation of additional soil macroaggregates. Furthermore, it was shown that hydrochar improved the quantity of soil microbes [147].

Consequently, studies showed that hydrochar amendments enhance soil organic carbon, nutrient availability, water-holding capacity, cation exchange capacity, and microbial activity, leading to improved plant growth, crop yield, and quality. Modified and functionalized hydrochars (e.g., lime-assisted, amino-functionalized, metal-doped, or acidified) demonstrate strong capabilities for immobilizing heavy metals, reducing contaminant bioavailability, and remediating soils polluted with antibiotics and pharmaceuticals. HTC processing conditions, feedstock type, and application rates strongly influence hydrochar properties, stability, and environmental performance. Hydrochars derived from sewage sludge, animal manure, food waste, and agricultural residues also enable nutrient recovery, slow-release fertilization, and reductions in greenhouse gas emissions (CH₄ and N₂O), while supporting carbon sequestration and net-negative emissions. Overall, hydrochar is highlighted as a versatile, low-cost, and sustainable soil amendment that improves soil fertility, mitigates environmental risks, and supports climate-smart agricultural practices in the related studies.

5.4. Activated Carbon and Carbon-Derived Materials

Hydrochar has been shown to have a variety of sorption capabilities and to be an inexpensive adsorbent for pollutants in aqueous solutions [92,126]. The manufacturing conditions and feedstocks were the primary determinants of the variations in sorption capacity [10]. Puccini et al. prepared activated carbons by KOH chemical activation of hydrochars obtained from hydrothermal carbonization of green waste and municipal solid waste. The effects of precursors and different KOH/char mass ratios on the physical characteristics of the activated carbon were investigated. Methylene blue and iodine numbers were determined to evaluate the adsorption properties of the obtained activated carbons. As a result, the application of KOH-activated hydrochar as activated carbon appears to be promising [148]. Plavniece et al. studied HTC and pyrolysis as methods for carbonizing biomass (birch wood chips and lignocellulosic pyrolysis tar) precursors for activated carbon synthesis. They explored how the carbonization method (HTC vs. pyrolysis) affects the structure and porosity of the resulting activated carbon materials. Results of the study showed that the carbonization method has little effect on the activated carbons’ specific surface area values; however, it allows for the regulation of pore size distribution [149]. Falco et al. examined how the precursor and HTC temperature affected the final activated carbons’ (ACs’) porous characteristics, noting that very microporous ACs may be produced regardless of the precursor. The porosity development and the micropore size distribution (MPSD) of the ACs were shown to be significantly influenced by the HTC temperature. By adjusting the HTC temperature, the ACs’ MPSD was tuned. These materials demonstrated the efficacy of this synthesis strategy in transforming a low-value lignocellulosic biomass into a functional carbon material with high performance in gas storage applications. Promising preliminary results in gas storage (i.e., CO₂ capture and high-pressure CH₄ storage) were obtained [150]. Zhao et al. investigated the efficiency of recycling phosphate from digestate through HTC and subsequent production of a high-surface-area activated carbon. They reported that the activated carbon generated from acid-leached hydrochar exhibited higher microporosity and superior adsorption capacity when they used methylene blue [151]. Khoshbouy et al. studied the preparation of sludge-based activated hydrochars (SAC) using HTC followed by physical or chemical activation methods (or KOH) to enhance the surface area and characteristics of the resulting material. According to their results, the activated hydrochars, specifically KOH-activated C SAC, demonstrated a superior maximum monolayer MB adsorption capacity (588.2 mg/g) compared to commercial activated carbons, highlighting their application as carbon-derived materials [114]. Hao et al. examine activated carbon prepared from HTC waste biomass with respect to the adsorption of carbon dioxide. They compared physically activated carbon (PAC) and chemically activated carbon (CAC) derived from HTC biomass, detailing their preparation methods and resulting pore structures. The results showed that PACs exhibited a large CO₂ adsorption capacity, rapid adsorption, excellent cyclability, and that the ultramicropore volume is crucial for CO₂ capture from flue gas [117]. Vallejo et al. optimize the production of activated carbons from HTC biomass using potassium hydroxide (KOH) and phosphoric acid (H₃PO₄) as activating agents, using a factorial experimental design. This is according to the outcomes of using KOH, which yielded superior iodine numbers but lower mass yields, with H₃PO₄, which resulted in higher mass retention with moderate iodine numbers [152]. El Ouadrhiri et al. explain the conversion of solid waste (SW) from the essential oil extraction industry into a carbonaceous material using citric acid-catalyzed hydrothermal carbonization (c-HTC) coupled with chemical activation. They optimized the HTC process using an I-optimal design and response surface methodology (RSM) to maximize carbon retention rate and hydrochar mass yield. As a result, the characteristics of the resulting material, activated hydrochar (AHCop), included a specific surface area of 989.81 m²·g⁻¹ and its potential for removing organic pollutants like methylene blue [153]. Rodríguez Correa et al. examine the influence of the carbonization process, specifically pyrolysis and HTC, and the KOH mixing method on the textural properties and adsorption capacity of AC derived from bamboo. The study compared activated carbons from impregnated hydrochar and pyrochars, finding that the former had the highest hydrogen adsorption capacity. They concluded that HTC operates at lower temperatures and can be expected to have better economic indices when considered as the carbonization step for activated carbon production, given similar yields and slight differences in textural properties compared to pyrolysis [154]. Xie et al. developed a novel, coupled hydrothermal carbonization pyrolysis self-activation process for fabricating biomass-derived activated carbon without external activators or pre-pulverization. They utilized the endogenous water within the hydrothermally treated precursor and the CO₂ generated during pyrolysis as synergistic intrinsic activators to achieve pore formation. They underlined that the resulting bamboo-activated carbon exhibits a specific surface area of 1556 m²·g⁻¹ and a substantial equilibrium benzene adsorption capacity of 141.6 mL·g⁻¹ [155].

These studies highlighted that subsequent physical or chemical activation, particularly with KOH, H₃PO₄, or acid leaching, significantly enhances surface area, porosity, and adsorption capacity. While the choice of precursor has limited influence on specific surface area, HTC conditions (especially temperature) and activation strategy strongly control pore size distribution and microporosity, which are critical for applications such as dye removal, gas storage, and CO₂ capture. HTC-derived activated carbons often outperform commercial activated carbon in methylene blue, iodine, hydrogen, and benzene adsorption, while offering advantages such as lower processing temperatures, higher carbon retention, and improved economic feasibility compared to pyrolysis. Overall, HTC is highlighted as an efficient and versatile carbonization route for converting low-value biomass and waste into advanced activated carbons for environmental and energy-related applications.

6. Functionalization of HTC Products

Hydrothermal carbonization was utilized as a pretreatment method to convert high-oxygen-content lignocellulosic biomass, an otherwise non-valuable renewable material, to a more condensed functional material for higher-value applications. Surface functional groups on activated carbons play a key role in the adsorption of toxic inorganic and organic contaminants from aqueous media. HTC, run under mild conditions, maintains the functional groups present on the surface of the raw material in an effective manner [156,157].

HTC has been investigated by numerous researchers for the simultaneous carbonization and functionalization of precursors in the presence of certain chemical agents. Sevilla et al. prepared a highly functionalized carbonaceous material (hydrochar) via hydrothermal carbonization of two types of biomasses (eucalyptus sawdust and barley straw) at 250 °C. They reported that the hydrochar products have a high degree of aromatization and contain a large amount of oxygen-containing groups, including carbonyl, carboxylic, hydroxyl, quinone, and ester groups, as confirmed by spectroscopic techniques. They highlighted the presence of oxygen functionalities on the surface of the hydrochar particles that lead to high water affinity and hydrophilic properties [158]. By co-processing terminal amino hyperbranched polymer and walnut shell biomass in hydrothermal carbonization conditions, Ghadikolaei et al. developed a series of functionalized hydrochars with a high density of nitrogen-containing functional groups. Their research demonstrated the benefit of the HTC method for covalently attaching the hyperbranched polymer’s functional groups to the hydrochar backbone and regulating the size and porosity of the resulting hydrochars by modifying the process’s primary control variables. A thorough investigation was conducted into the variables that affect the Cr(VI) adsorption behavior, including pH, contact time, dose, and competing ions [159]. With the goal of producing hydrochar with a high adsorption capacity using the HTC process, Kohzadi et al. investigated wheat straw, a carbon-rich precursor and a typical agricultural waste in Sanandaj. Artificial neural networks (ANNs) were used to examine the impacts of several interfering parameters, such as pH, MB concentration, and adsorbent dosage, on adsorption modeling. The hydrochars were tested as adsorbents to remove MB from aqueous solution. Hydrochar treated with KOH (0.1 M) had the highest MB removal effectiveness. A change towards humification was indicated by the observed hydrogen-to-carbon ratio (H/C) and oxygen-to-carbon ratio (O/C), suggesting the impact of KOH addition during the hydrochar generation process [160]. Yan et al. explored the impact of post-treatment methods, including HCl and NaOH washing, HNO₃ oxidization, and low-temperature thermal heating, on the redox properties of hydrochar derived from wheat straw and Spartina alterniflora biomass. They investigated how these modification methods affected the reducing capacity (RC) of hydrochar, noting that HNO₃ and thermal oxidization significantly increase RC, while NaOH washing has a negative effect on detail. In conclusion, they identified the origins of the reducing capacity of hydrochar as oxygen-containing functionality, humic-like matter, and persistent free radicals (PFRs), demonstrating how modification tunes these sources [80]. Jiang et al. examined the preparation of modified hydrochars (HC) derived from corn straw through functionalization with H₃PO₄ and polyethyleneimine (PEI). They explained the main adsorption mechanisms for Pb(II) onto the modified hydrochars as ion exchange and hydrogen bonding for H₃PO₄-HC, and coordination bonding and hydrogen bonding for PEI-HC in detail [161]. Chhabra et al. focused on the preparation of hydrochar from low-cost biomass waste via hydrothermal carbonization. They investigated the process of functionalizing hydrochar with three different Brønsted acidic moieties: sulfonic, nitrate, and phosphate, to create heterogeneous catalysts. They utilized orange peel as the biomass source for hydrochar synthesis before subsequent acid functionalization [162]. Kozyatnyk and Yakupova investigated the effects of various chemical (hydrochloric acid, sodium hydroxide, and ethylenediaminetetraacetic acid solutions) and physical (microwave irradiation and hydrothermal processing) treatments on the surface properties of hydrochar (a product of hydrothermal carbonization, or HTC). The group analyzed surface changes in hydrochar using X-ray photoelectron spectroscopy, revealing more variable changes in oxygen-containing functional groups, notably in carbonyl functionalities, after treatments. According to their results, microwave and hydrothermal treatments were effective in introducing oxygen-containing functional groups to the hydrochar surface, such as increasing C–O and C=O groups, which can enhance reactivity and hydrophilicity [163]. Unur studied HTC as a pretreatment method to convert high-oxygen-content lignocellulosic biomass (hazelnut shells) into a more condensed functional material. The researcher explained that HTC allows for self-functionalization of biomass during carbonization without the need for additional chemical agents, preserving high oxygen content and functionality. The researcher presented a sustainable approach where hydrochar from hazelnut shells is successively calcined under an inert atmosphere to yield hydrophobic nano-architectured carbon, or in the presence of a porogen to yield highly porous hydrophilic carbon [157].

Kim et al. synthesized nitrogen (N) heteroatom-doped hydrochar derived from corncob via NH₄Cl-aided hydrothermal carbonization (HTC). They used the N-doped hydrochar for the simultaneous adsorption and removal of divalent copper (Cu(II)) and hexavalent chromium (Cr(VI)) in aqueous solutions. They focused on the synergistic effect between coexisting Cu(II) and Cr(VI), which significantly promotes the adsorption affinity of the N-doped hydrochar through mechanisms like electrostatic shielding, cation bridging, and redox reactions [164]. Prieto et al. studied the potential for further enhancement of biomass-based hydrochars through the incorporation of extrinsic nanoparticles like graphene, carbon nanotubes, nanodiamonds, and metal oxides. They searched the relatively unexplored use of transition metal dichalcogenides (TMDCs) with hydrochars, noting preliminary findings demonstrating highly competitive capacitances. They reported that thiourea-doped activated carbon from cellulose resulted in a high specific surface area and capacitance, while chitosan demonstrated high capacitance and good cyclability due to O and N atom-doping [165]. Sun et al. prepared phosphorus-doped porous carbons (MBPs) using cypress sawdust as a biomass precursor through phosphoric acid-assisted hydrothermal carbonization (HTC), followed by KOH activation. They proposed a two-step synergistic strategy for synthesizing phosphorus-doped porous biochars using cedar sawdust as a biomass precursor, integrating H₃PO₄-assisted hydrothermal carbonization and subsequent KOH activation to achieve in situ phosphorus doping and pore structure optimization. They highlighted the successful introduction of surface functional groups, such as P–O, P=O, and C–P, which enhanced the surface polarity of the carbon matrix and contributed to improved chemical adsorption capacity [166]. Rustamaji et al. studied modifying hydrochar derived from oil palm from empty fruit bunches using thiourea solution to produce N, S co-doped activated carbon. They focused on the process of hydrochar modification by impregnation with thiourea, which acts as a dual source for nitrogen and sulfur functionalities, followed by activation at 800 °C. According to results, increasing the thiourea-doping ratio increased the nitrogen content (up to 17.19%) but decreased the sulfur content of the resulting activated carbon [167].

Rizwan et al. studied a novel approach by directly incorporating zeolite catalysts (HZSM-5 and USY) into the HTC of sewage sludge (SS), followed by pyrolysis of the derived hydrochar (HC). The selective production of CO and H₂ was achieved through temperature-dependent pyrolysis between 500 and 900 °C. HZSM-5 facilitated an increase in CO production to 54.18%, whereas USY boosted the CO yield up to 35.6%. The optimal product distribution was achieved by strategically incorporating zeolite catalysts, allowing for precise control of N and O functionality and promoting selective syngas and chemical precursor yield. This innovative catalyst-mediated HTC Py cascade offers unique control over pyrolytic products by introducing an efficient pathway for transforming problematic SS into green energy carriers, thus bridging the gap between environmental sustainability and feasible industrial utilization [168]. A de Mora et al. explored the use of hydrochar derived from the HTC of anaerobic digestion sludge as a precursor for developing metal-free and metal-doped carbon-based catalysts. They investigated the catalytic performance of the synthesized materials for the catalytic wet air oxidation (CWAO) of an HTC aqueous liquor, aiming to produce a valuable stream for anaerobic digestion. They gave results including KOH and chloride salts (FeCl₃, ZnCl₂, and CuCl₂), to enhance the surface area and incorporate active metal species into the hydrochar for catalytic processes. The KOH as an activating agent increased the surface area of hydrochar up to ca. 1000 m²/g of the BET surface area. The employment of CuCl₂ and FeCl₃ as activating agents allows Cu- and Fe-rich doped materials of remarkable surface areas with 49.1 and 42.5 wt% of each metal, respectively [169]. Kostyniuk y Likozar used various zeolite catalysts (H-ZSM-5, H-Beta, H–Y, H-USY, and H-Mordenite) during wet torrefaction (WT-a form of hydrothermal carbonization) of wood cellulose pulp residue (WCPR), aiming for the simultaneous one-pot generation of value-added products (bio-ethanol and levulinic acid). According to their results, the addition of zeolite catalysts enhances carbonization efficiency, with the WT + Mordenite sample achieving the highest carbon content of 71.5% and a higher heating value of 27.3 MJ/kg [160]. Mota et al. prepared a novel, sustainable solid acid catalyst through the HTC of carrageenan, a biomass-derived sulfated polysaccharide, and used it in the conversion of glycerol to solketal, achieving 95–98% conversion within 6 h. Their results showed that the catalytic activity of the biomass-derived HTC material is similar to that of the commercial sulfonated resin catalyst Amberlyst-15 and superior to many other commercial solid acids for the target reaction [170].

According to the studies given, HTC inherently yields oxygen-rich, hydrophilic, and aromatized carbon materials, while post-treatments and in situ modifications—such as acid/base washing, oxidation, heteroatom doping (N, P, S), polymer grafting, nanoparticle incorporation, and catalyst-assisted HTC—enable precise control of functional groups, redox properties, and adsorption mechanisms. Functionalized hydrochars demonstrate high performance in the adsorption of dyes, heavy metals, and oxyanions, heterogeneous catalysis, energy storage, and syngas production, often rivaling or surpassing commercial materials. The studies highlight that adsorption and catalytic behavior are governed by surface functionalities, redox-active sites, and pore structure, which can be systematically tuned through HTC conditions and additives. Overall, HTC is presented as a versatile and sustainable platform for converting low-value biomass and waste into highly functional carbon materials for environmental remediation, catalysis, and energy applications.

7. Sustainability Assessment

This part of the study examines the sustainability assessment of HTC, its integration within the circular economy framework, and its implications for environmental sustainability, technoeconomic feasibility, operational challenges, and future research. As depicted in Figure 7, hydrothermal carbonization enhances various sustainability dimensions, such as resource efficiency, the advancement of clean energy, responsible consumption, and the development of sustainable urban systems. These interrelated advantages resonate with the overarching principles of the circular economy and underscore HTC’s potential to mitigate waste, improve material recovery, and foster low-carbon development pathways. Consequently, this section investigates HTC not solely as a thermochemical conversion process but also as a crucial mechanism for promoting sustainability, taking into account its environmental efficacy, technoeconomic practicality, implementation obstacles, and the requisite avenues for future investigative efforts [9].

7.1. Sustainability Development

HTC has recently gained attention as a potential substitute for biomass processing to produce products with added value. It is a highly effective method used to convert biomass, particularly wet biomass waste, into valuable products such as solid biofuel, liquid, and gaseous products in a way that supports the circular economy approach [10]. HTC eliminates the need to dry waste biomass by using water as both a reactant and a medium. This significantly reduces energy consumption. Energy (biochar), soil remediation, carbon removal, chemicals, and advanced materials are just a few of the many uses for HTC technology. Additionally, by converting waste into valuable products, HTC offers sustainability potential in terms of waste management and low-carbon products [8,171,172,173].

The United Nations adopted the 2030 Sustainable Development Goals (SDGs) in 2015 as a global roadmap that encourages development that is balanced in terms of social, economic, and environmental aspects. This framework provides a comprehensive structure consisting of 17 main goals and 169 subgoals that address a wide range of issues, including poverty reduction, climate change mitigation, access to clean energy, and the development of sustainable consumption and production processes [174].

HTC is closely linked to the United Nations’ global Sustainable Development Goals. This robust and eco-friendly technology reduces reliance on fossil fuels by enabling the production of high-energy biofuels, directly contributing to SDG 7 (Affordable and Sustainable Energy). Recycling waste as input into the system directly aligns with SDG 12 (Responsible Consumption and Production) and hastens the transition to a circular economy. It helps to achieve SDG 13 (Climate Action) by reducing methane emissions from decomposing waste and enabling long-term carbon storage [175,176,177,178]. Recovering critical nutrients such as nitrogen and phosphorus from wastewater, it helps to achieve SDGs 2 (Zero Hunger) and 6 (Clean Water and Sanitation) [172,173]. Furthermore, HTC’s circular approach and ability to produce multiple products ensure resource efficiency, economic value creation, and a lower environmental footprint. These also contribute to SDGs 8 (Decent Work and Economic Growth), 9 (Industry, Innovation and Infrastructure), and 11 (Sustainable Cities and Communities) [179]. All of these elements clearly demonstrate the importance of evaluating and disseminating HTC technology in the context of sustainable development.

7.2. Circular Economy

The circular economy is a production and consumption model in which materials and products are reused, shared, renewed, and recycled for as long as possible. This extends the usefulness of materials and products. In practice, this entails minimizing waste. This approach is opposed to the linear economic model of ‘take make dispose’, which is based on large quantities of low-cost, readily available materials and energy. Reusing and recycling products would help to reduce natural resource consumption, landscape and habitat disruption, and biodiversity loss. Another advantage of the circular economy is a reduction in overall annual greenhouse gas emissions [179,180,181].

In the presence of water, HTC transforms biomass waste, including sewage sludge, municipal solid wastes, food waste, algae, agricultural waste, and livestock manure, into energy carriers, soil remediation, chemical materials, adsorbents, photocatalysts, biosensors, and nanomaterials, which are among the various products obtained in this process [182,183,184,185,186,187,188,189,190,191]. These products directly support the circular economy.

Resource efficiency is ensured by processing the wastes to create new and valuable products rather than discarding them. For this purpose, conventional dry processes like gasification or pyrolysis are also employed; however, HTC is a more advantageous method of turning bio-wastes with high moisture content into products [192,193]. HTC eliminates the need for pre-drying and uses less energy by processing biomass in water and at lower temperatures [179].

HTC provides viable options for energy production, carbon sequestration, and waste management on an industrial scale. Several countries, mostly in Europe, have established industrial HTC facilities that incorporate circular economy concepts into waste management [173,194].

7.3. Environmental Sustainability

HTC reduces environmental burdens by turning waste biomass into energy and value-added products. This approach provides numerous environmental benefits, such as waste management, energy production, carbon sequestration, and preservation of the environment [195,196]. HTC promotes environmental sustainability by reducing the consumption of natural resources and the need for regular storage [179]. Reducing greenhouse gas emissions is another way HTC contributes to environmental sustainability. By quickly stabilizing the biomass, it reduces the methane emissions that come from the breakdown of organic waste in landfills. Furthermore, the resulting hydrochar provides carbon sequestration when applied as a soil conditioner by retaining carbon in a more stable form [197]. HTC allows high-moisture waste to be processed without pre-drying, saving water and energy. This technology has low energy consumption and operating costs because it does not require high temperature and pressure conditions [24].

The environmental benefits of this technology are reported in the literature in qualitative terms; however, understanding the full scope of the environmental sustainability of HTC requires a comprehensive and systematic approach. In this context, life cycle assessment (LCA), which addresses all direct and indirect impacts within the system boundaries, should be used to reveal the environmental performance of HTC. When analyzing environmental sustainability using the ISO 14040/14044 international standards [198,199], LCA provides a framework that employs the ‘cradle to grave’ approach. This method investigates the environmental impacts of all stages of a product, process, or service, including raw material extraction and supply, transportation, production, use, and waste management [200].

LCA is used by professionals, manufacturers, and decision makers to compare emerging technologies to alternatives in terms of energy efficiency, material consumption, carbon footprint, and other environmental impacts [201]. For HTC technology to be applied at an industrial scale, environmental impacts must be examined throughout the life cycle. As a result, there has been an increase in the number of LCA studies modeling the environmental effects of the newly developed HTC process.

Most of these studies have compared life cycle environmental impacts of HTC with other biological and thermochemical waste conversion systems, namely anaerobic digestion (AD), composting, pyrolysis, gasification, and incineration [202]. HTC typically has less of an impact on the environment than more conventional techniques. Unlike conventional thermochemical methods, the HTC process does not require raw material drying, which has the potential to save significant amounts of energy. The study conducted by Mannarino et al. [203] revealed that HTC demonstrated better environmental performance than composting in 13 out of the 16 impact categories examined. According to a different study [204], the global warming potential of HTC fell between −0.014 and −0.032 kg CO₂ eq. per kilo of wet organic waste. While HTC achieves the best results in terms of global warming potential, it performs the worst of all technologies tested in the abiotic resource depletion potential. In most cases, energy recovery incineration facilities perform better environmentally than HTC; HTC, on the other hand, performs better than landfills and similarly to AD and composting.

The integrated use of HTC with other waste management technologies has been the subject of life cycle-based environmental sustainability analyses in some studies, and the findings have demonstrated that combining HTC with other techniques has a significant impact on environmental effects. The integration of the HTC + AD (−141.46 kg CO₂ eq./ton food waste)-integrated system for food waste has demonstrated a significantly lower environmental impact than AD alone (−38.87 kg CO₂ eq./ton food waste) [205]. Another study compared the environmental effects of using HTC and AD in tandem with incineration [206]. When compared to incineration, the integration of HTC + AD produced negative values in the global warming potential, acidification, eutrophication, human, and ecotoxicity categories, indicating a significant environmental benefit. On the other hand, incineration produces a high environmental burden, especially when it comes to human toxicity and marine ecotoxicity, but it has positive effects in each category. In terms of photochemical oxidation, fossil fuel consumption, and other impact categories, HTC + AD performs more sustainably overall.

The literature also includes LCA studies that examine how various biomass types, such as food waste, sewage sludge, olive pomace, cotton stalks, date palm branches, microalgae, and orange peel perform environmentally in the HTC process. The environmental performance of three distinct biomasses in the HTC process was compared by Ugolini et al. [207]. With a positive global warming impact of 17.9 kg CO₂ eq./ton, paper biological sludge offers a more limited environmental benefit than other biomasses. On the other hand, when exposed to HTC, olive pomace and orange peel show a strong potential for reducing greenhouse gas emissions, with very high negative carbon footprint values of −1290 and −1301 kg CO₂ eq./ton, respectively.

Studies comparing the environmental effects of various applications of hydrocarbons, liquid fractions, and gas products from the HTC process, such as energy production, soil remediation, water treatment, and nutrient recovery, are also included in the literature. LCA studies show that HTC can reduce greenhouse gas emissions and environmental burdens, particularly by producing hydrocarbons that can be used to replace fossil fuels [208]. Rahman et al. [209] determined that HTC has a lower environmental impact than biostabilization and storage, as well as increased environmental benefits, particularly because using hydrochar instead of coal for energy production reduces fossil fuel consumption. The HTC, when combined with electricity generation, had a lower environmental impact by reducing global warming, freshwater ecotoxicity, and photochemical ozone formation. According to a study assessing the environmental impacts of various hydrochar applications [210], hydrochar use can reduce global warming by 28% and freshwater ecotoxicity by 47% in wastewater treatment, as well as the eutrophication effect in soil fertilization by 54% and the global warming in soil improvement by 34%. It also demonstrates that hydrochar produced at 160 °C has the lowest environmental impact when used in combination.

According to the LCA studies related to HTC, the environmental performance of HTC varies considerably depending on the type of biomass and how the products are used, as well as when compared to conventional waste management techniques. It is stressed that integrated applications and process optimization can improve HTC’s environmental sustainability, especially when it comes to resource utilization, energy and nutrient recovery, and the effects of climate change.

7.4. Economic Sustainability

The economic analysis of HTC technology is a growing area of interest in the waste management and resource recovery sector, with more studies being conducted in recent years. The primary data sets used in economic analyses for HTC technology are the capital expenditure (CAPEX) and operating expenditure (OPEX) costs [208]. HTC facilities require a significant investment in equipment such as reactors, heat exchangers, dryers, and centrifuges operating under high temperature and pressure. For example, the total investment cost for a facility with a capacity of 5317 tons per year has been estimated to be around EUR 1.77 million. When the investment costs are examined, the reactor and heaters are found to be the most expensive parts of the system [211]. The literature also provides capital cost estimates for plants with higher capacities. A co-hydrothermal carbonization (Co HTC) plant with a 110 MWe capacity, which processes coal and biomass concurrently in subcritical water, is estimated to have a total capital cost of USD 12.7 million, with the biggest capital expenditures going toward pumping, heat exchangers, and filtration [212]. Scrinzi et al. [213] computed CAPEX in research using production conditions. The reported investment cost for a pilot plant that uses HTC to produce biofuel from pine sawdust is USD 6391 (for an operating condition of 190 °C). The investment cost is USD 15,500 for a high-temperature scenario (such as 250 °C).

The main OPEX items in the HTC process consist of energy, maintenance, and labor. HTC, unlike other thermochemical methods, saves energy and money because wet biomass does not need to be pre-dried first. In an industrial-scale HTC plant, the production cost for waste materials like grape pomace under optimal conditions (220 °C, 1 h) was calculated to be EUR 157/ton, while the break-even selling price was EUR 200/ton. These attributes make hydrocarbon wood pellets competitive [214].

Economic costs are just as important as environmental effects in processes that assess HTC integration. HTC + AD exhibits a 37.0% increase in CAPEX when compared to AD when evaluating the financial burden of integrating HTC into the AD system. Since investments in other sections are almost equal, this increase results from the addition of all equipment required for the HTC reaction and hydrocarbon drying sections [208].

8. Challenges, Limitations, and Future Perspectives

HTC is a promising method for converting wet biomass into valuable, carbon-rich products that can be used in energy production, agriculture, and environmental improvement. According to recent research, HTC is rapidly progressing from laboratory to pilot and industrial scales, with a particular emphasis on waste management and energy recovery applications [109]. The commercialization potential of HTC technology is demonstrated by the fact that many commercial and pilot-scale facilities, especially those operating throughout Europe, have integrated into circular economy models. HTC technology has been scaled to pilot and commercial scale by companies like TerraNova Energy and SunCoal Industries in Germany, and AVA-CO₂ in Switzerland. These businesses produce hydrocarbons with a high energy density by processing high-moisture biomass, such as sewage sludge, agricultural residues, and organic waste, without the need for drying [214].

Despite its potential, some challenges and limitations must be addressed before this technology can be successfully applied in the industry. This section of the study discusses HTC’s challenges and limitations while also assessing the technology’s future potential.

8.1. Challenges in Scaling up HTC Processes

There are a number of difficulties in transferring HTC technology from the laboratory- and pilot-scale to the industrial-scale. The largest challenge is designing safe and effective reactors for industrial scale, even though these studies are typically conducted in batch mode. It is especially important to control parameters like temperature, pressure, time, and biomass water ratio [215]. It is critical to provide homogeneous mass and heat transfer in high-pressure and high-temperature reactors in order to carbonize large volumes of biomass piles consistently and efficiently. Continuous operation causes engineering problems with the consistent feeding and discharge of wet and viscous raw materials into the high-pressure system. These technical challenges, combined with the risk of corrosion and the need for complex heat recovery systems, drive up the plant’s investment and operating costs and slow down its progress [216,217].

Process water in HTC technology contains a high concentration of organic and inorganic contaminants. At laboratory and pilot scales, this water is relatively simple to handle; however, for the industrial scale, it is a major issue. Treatment, recovery, and disposal of this water on an industrial scale is a challenge that must be addressed [215,218].

Furthermore, heat losses and energy efficiency are important considerations in industrial-scale HTC plants. Efficient heating and reactor insulation increase process efficiency and have a direct impact on operating costs. Therefore, energy management is considered a critical factor in HTC’s industrial-scale viability [219,220].

In addition, regulations, quality standards, and acceptance criteria for using hydrocarbons as fuel or in soil applications vary by country. This is another factor limiting the broad acceptance of HTC technology [216]. HTC’s industrial applications offer a substantial opportunity; however, they still need to be developed in terms of process integration, water/byproduct management, energy management, and policy-regulation aspects. HTC’s contribution to sustainable waste management and the carbon cycle is expected to grow as a result of advancements in these areas [12,221].

8.2. Controlling Product Quality Variability in HTC

The quality of the products obtained from the HTC process and the system’s efficiency depend on parameters such as process conditions, raw materials, biomass/water ratio, catalysts, and auxiliary chemicals. If the optimum conditions are not maintained, product quality and yield can be negatively affected [222,223,224].

The type and composition of waste or biomass used in HTC processes are one of the most important factors influencing product properties. For example, combining lignocellulosic biomass with sludge or organic waste increases the calorific value and combustion properties of hydrochar [225,226]. Temperature, pressure, and reaction time are important process parameters that affect product quality. While long reaction times can increase the degree of carbonization and energy content while decreasing solid product yield, high temperatures can result in lower solid product yields. Additionally, the product’s morphology, energy density, and stability are shaped by process variables like pH, biomass/water ratio, particle size, and the use of catalysts or additives [222,223,227].

8.3. Emerging Directions for Future HTC Research

HTC is a growing technology that transforms waste or biomass into high-value products. HTC demonstrates a relatively new, renewable, and innovative process. HTC processing of biomass is similar to conventional thermochemical processes; however, HTC operates at elevated pressure and can process feeds with moisture content ranging from 75 to 90% [192]. The high moisture content of the feedstock can be directly utilized by HTC, which eliminates energy-intensive drying steps and lowers operating costs. Because of its high calorific value, the resulting hydrochar can be used in place of fossil fuels as an adsorbent, solid fuel, or soil conditioner.

The global increase in HTC-related publications and patents, as well as the increase in industrial facilities and corporate activities, demonstrates that this technology will play an important role in the coming years [9,228]. This technology, which is compatible with the circular economy and provides both environmental and economic benefits, will become increasingly important in sustainable energy, material production, and waste management in the future [228]. Furthermore, current trends suggest that this technology will be used in multi-product biorefinery systems, as well as a transformation tool to support climate policies [9].

To fully realize this potential, HTC must improve the sustainable management of process water, develop large-scale, continuously operable reactor designs, and increase the economic competitiveness of its products. Progress in these areas is viewed as critical for HTC to become a widely adopted, mature, and industrially accepted technology in the future [9]. Technology evaluations must be integrated with policy and market analyses in order for HTC-related work to more firmly establish the industrial and economic viability of this technology. In this way, HTC can become a dependable component of sustainable bioeconomy and carbon management strategies rather than just a lab technology.

9. Conclusions

This review provides a thorough examination of the hydrothermal carbonization (HTC) process, illustrating its ability to convert various biomass feedstocks into valuable carbonaceous materials. The study delineates the principal reaction mechanisms involved—namely hydrolysis, dehydration, decarboxylation, and aromatization—and quantifies the influence of specific HTC parameters, including temperature, residence time, solid-to-liquid ratio, the presence of catalysts, and the chemical composition of the feedstock, on hydrochar yield and its resultant properties. A comparative evaluation of lignocellulosic residues, nutrient-dense waste materials, microalgae, and co-HTC systems has uncovered both general carbonization patterns and feedstock-specific phenomena (such as nitrogen integration, mineral transport, and synergistic effects during carbonization). Furthermore, the review includes an overview of recent advancements in hydrothermal carbonization technologies, such as microwave-assisted methods, the use of catalytic additives, and post-activation treatments, all of which enhance the characteristics of hydrochar (including its aromatic content, porosity, and surface chemistry) for diverse applications in fuel production, adsorption, catalysis, and soil enhancement. The collective insights suggest that HTC exhibits considerable adaptability: by operating under relatively mild, aqueous conditions, it facilitates the production of coal-like, carbon-dense hydrochars from moist organic waste without necessitating energy-intensive drying processes. Consequently, HTC not only yields customizable carbon products but also contributes to sustainability efforts through improved water management, circular economy principles, and carbon sequestration by reclaiming value from waste materials.

Nevertheless, significant research gaps persist in this field. Among the foremost challenges are the following:

• Fundamental Mechanistic Gaps: The comprehensive reaction chemistry underlying HTC, including kinetics, the formation of intermediates, and polymerization pathways, remains inadequately characterized, particularly for innovative approaches like co-HTC or microwave-assisted HTC. Without a deeper mechanistic understanding, the development of predictive models and effective process control measures is hampered.
• Reactor Engineering and Scalability: Most existing HTC research has been conducted using small-scale batch reactors. The design of continuous pilot-scale systems that feature effective heat integration is critical for practical applications. Current batch-to-batch variability, along with heat losses, must be addressed through novel reactor designs.
• Process Water Treatment: HTC generates a process effluent that is typically acidic and laden with organics and nutrients. Robust methodologies for treating or valorizing this aqueous byproduct are currently lacking. Further investigation is required into nutrient recycling, energy or chemical recovery from the effluent, and the potential integration of HTC with wastewater treatment solutions, such as coupled anaerobic digestion, to manage this byproduct efficiently.
• Standardization of Hydrochar Quality: There is no consensus on the characterization standards or quality metrics for hydrochar. Inconsistent protocols for proximate and ultimate analyses, surface chemistry assessments, and calorific value determinations hinder the comparability of cross-study results. The establishment of standardized production methodologies and analytical protocols is imperative for the consistent evaluation of hydrochar properties.
• Techno-Economic and Life-Cycle Evaluation: System-wide analyses of HTC methodologies are still limited. Comprehensive techno-economic evaluations and life-cycle assessments are essential to elucidate cost factors, energy balances, and environmental repercussions on a larger scale. In the absence of such studies, benchmarks for HTC against alternative waste management and energy strategies are difficult to ascertain.

To address these challenges and leverage the potential of HTC, future research endeavors should focus on the following avenues:

• Continuous Reactor Development: The design and implementation of continuous HTC systems, such as tubular or screw reactors, should be pursued to optimize heat recovery and material handling. Pilot-scale experiments are needed to bridge the gap between laboratory findings and industrial applications, validating performance during sustained operation.
• Integration into Biorefinery Processes: HTC may be effectively combined with other complementary treatments such as anaerobic digestion, pyrolysis, and gasification within integrated biorefinery frameworks. Such synergies could improve overall energy and nutrient recovery, for instance, by utilizing HTC effluent as feed for anaerobic digestion or repurposing residual heat for process integration. The recycling of process water as feedstock or fermentation medium could minimize waste generation.
• Hydrochar Activation and Functionalization: Advancing post-HTC processing methods is essential to enhance the functionality of hydrochar. Techniques such as physical or chemical activation (utilizing steam, CO₂, KOH, etc.), heteroatom doping, and templating can yield highly porous activated carbons tailored for specific applications, such as electrodes for energy storage or catalysts for pollutant degradation. Research should aim to optimize these methodologies to devise hydrochars designed for particular uses.
• Standardized Protocols and Metrics: The development of consensus protocols for HTC processing and hydrochar evaluation is necessary. This includes establishing standardized feedstock preparation methods, reaction protocols, and analytical techniques (such as elemental analysis, BET surface area assessment, functional-group quantification, and measurement of heating values). Additionally, benchmark comparisons of hydrochar against traditional materials like biochar and activated carbon should be conducted to enhance clarity and utility in this field.

In summary, hydrogen thermochemical cycles (HTC) represent a promising and adaptable thermochemical approach. Nevertheless, the successful transition from experimental investigation to reliable, large-scale, and economically viable technology will hinge on the resolution of existing scientific, technical, and environmental obstacles.