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Article

Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver

by
Bryan Chiguano-Tapia
1,*,
Elena Diaz
1,2,
M. Angeles de la Rubia
1,2 and
Angel F. Mohedano
1,2,*
1
Department of Chemical Engineering, Universidad Autonoma de Madrid, 28049 Madrid, Spain
2
Institute for Advanced Research in Chemistry, Universidad Autonoma de Madrid, 28049 Madrid, Spain
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(11), 5022; https://doi.org/10.3390/su17115022
Submission received: 24 March 2025 / Revised: 26 May 2025 / Accepted: 28 May 2025 / Published: 30 May 2025
(This article belongs to the Section Waste and Recycling)
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Abstract

The management through co-hydrothermal carbonization (co-HTC) of swine manure (SM) and soybean hulls (SH), a by-product of animal feeding, is established as a strategy for their material and/or energy recovery. The effect of hydrothermal carbonization (HTC) temperature (210–240 °C) and mass ratio (1:0, 1:1, 1:3, 0:1) on hydrochar characteristics revealed that an improved hydrochar (C (51–59%), HHV (21–24 MJ/kg), N (~2%), S (~0.3%), and ash (<9%)) is produced with respect to hydrochar obtained from individually treated wastes. Regarding biofuel characteristics, hydrochar obtained from the SM/SH mass ratio (1:3) at 240 °C complied with the requirements of the ISO/TS 17225-8:2023 (N < 2.5%; S < 0.3%; HHV > 17 MJ/kg; ash < 12%) and showed high energy content (23.2 MJ/kg) and a greater thermal stability than the hydrochar obtained from individual wastes. Hydrochar retained relatively high amounts of nutrients such as phosphorus (6.5–9.7 g/kg), potassium (2.0–3.5 g/kg), and calcium (9–20 g/kg), which supports their use as soil improvers. Moreover, all hydrochar fulfill the standards (Spanish Royal Decrees 1051/2022, 824/2024 and EU Regulation 2019/1009) for sustainable nutrition in agriculture soils in terms of heavy metals concentration. The co-HTC of swine manure and soybean hulls demonstrated a promising transformation of waste materials into biofuel and/or soil improvers.

Graphical Abstract

1. Introduction

Given the limitations of traditional livestock and agricultural waste management strategies, there is a need to consider new and innovative technologies that allow a better use of waste. Among others, thermochemical treatment technologies stand out for their accessibility and the production of high-value-added products [1]. Hydrothermal carbonization (HTC) emerges as a promising and efficient technology, since it works under mild conditions and requires less energy consumption. HTC operates in a temperature range of 180–250 °C and 10–40 bar. This process usually takes place in a high-pressure reactor system, under subcritical water conditions, which provide the necessary environment for biomass conversion. The biomass is treated in the presence of water, which acts as both a solvent and a medium for heat transfer. The reactor design can vary, with batch and continuous flow reactors being the most common configurations. In a batch reactor, biomass and water are heated to the desired temperature and pressure, allowing for a controlled carbonization reaction over a set time. The continuous flow reactor, on the other hand, allows for continuous feeding and processing of biomass, making it suitable for large-scale operations [2,3]. Under these conditions, HTC converts biomass into a carbon-rich solid product known as hydrochar (HC). In addition to hydrochar, HTC generates a liquid phase (process water, PW) containing dissolved organic compounds and nutrients, as well as a small fraction of gases, primarily CO2 [4]. The versatility of HTC lies in its ability to process different kinds of biomass while enhancing the properties of the resulting hydrochar, making it a potential alternative for energy production and/or soil improver applications [5,6]. Furthermore, process water may contain valuable nutrients and carbon compounds, creating opportunities for nutrient recovery [7] or further treatment through anaerobic digestion [8] and dark fermentation [9] to produce methane and hydrogen, respectively. The proper management of all HTC products is essential to maximize resource recovery and minimize environmental impact.
HTC employs the inherent moisture of the waste to drive chemical reactions that occur during the treatment, making it suitable for managing high-moisture wastes avoiding pre-drying steps. However, when the moisture content of the waste is extremely high, HTC treatment can become impractical due to the dilution of organic compounds, the increase in the aqueous phase, and the reduction in conversion efficiency [10]. On the other hand, waste with low moisture content may require additional water to reach optimal conditions, potentially leading to operational challenges in the reactor [11].
To address these challenges, co-hydrothermal carbonization (co-HTC) emerges as a promising strategy to optimize the HTC process, improving the characteristics of the products. Several studies have shown that co-HTC takes advantage of the complementary properties of different types of biomasses. For instance, Malool et al. explored a co-HTC strategy using sugarcane bagasse and digested sewage sludge to enhance the Pb (II) adsorption capacity of hydrochar (up to 85% adsorption in the first 15 min) by adjusting the feedstock ratio and process parameters to optimize conversion efficiency [12], highlighting the capacity of co-HTC to tailor the surface chemistry for adsorption applications. Similarly, co-HTC of olive stone and grass pruning demonstrated water saving capability, where high moisture grass (80%) provided some of the water needed for the process, resulting in more efficient treatment with enhanced HHV (25–30 MJ/kg) values without compromising hydrochar performance (yields > 60%) [13]. In a related study, the recirculation of PW during the co-HTC of orange peel and fennel plant waste improved hydrochar recovery, increasing the solid mass yield by 4–11%. This outcome reflects a synergistic effect between feedstocks and process reuse strategies, highlighting the potential of integrating PW management to enhance carbon recovery and overall process efficiency [14]. Another example of co-HTC is the treatment of sludge and food waste, which highlights the influence of feedstock composition on sulfur retention and transformation pathways within the hydrochar matrix. Even though the resulting hydrochar exhibits a relatively low higher heating value (HHV < 17 MJ/kg), which limits its direct application as a biofuel, it can be further modified or enriched with energy-dense additives or functionalized for alternative uses, such as in soil amendment or pollutant adsorption systems [15]. These studies illustrate the versatility of co-HTC to improve the process and overcome challenges associated with raw materials while improving the characteristics of HTC products.
Swine manure (SM), which usually presents a moisture content higher than 80% [16], has traditionally been used as a natural fertilizer due to its high organic matter and nutrient content [17,18]. However, the intensification of pig farming has led to the excessive accumulation of SM, creating significant management and environmental challenges. Its high nitrogen and phosphorus content contributes to soil and water pollution, particularly through nitrate leaching, which can cause eutrophication in aquatic ecosystems [19,20]. Additionally, SM storage is a source of greenhouse gas (GHG) emissions due to the release of methane and ammonia, which contribute to climate change and air pollution [21]. Spain concentrates approximately 25.4% of the pig farming production in the European Union [22], which translates into large amounts of swine waste (approximately 61 Mt/year of SM) [23]. Given the environmental challenges associated with its management, exploring alternative valorization strategies becomes essential. Ipiales et al. reported the co-HTC treatment of SM with lignocellulosic material [24]. In this research, SM with high levels of N, S, ashes, and low HHV was co-processed with garden and park waste (GPW) to obtain an enhanced hydrochar with lower N, S, and ashes and higher HHV values making this product more suitable for biofuel applications. Similar research was reported by Lang et al. (2018) and Xiong et al., (2024) in whose studies the quality of the hydrochar is improved effectively, reducing the bioavailability of heavy metals expanding its potential into agricultural applications [25,26]. Another promising lignocellulosic material that can be explored in co-HTC processes is soybean hulls (SH), a by-product generated in the pig farming sector [27,28]. The soybean production in Spain is around 4200 tons per year, and approximately 8% of the production corresponds to SH [29]. Soybean hulls, rich in cellulose, hemicellulose, and lignin [30,31] and with a moisture content below 10% [32], present an opportunity to improve the valorization of SM. This can create a potential synergy between waste management and agricultural by-product utilization. Since soybean feed is commonly used in swine diets, this material is readily available in pig-breeding regions such as Spain, making it an ideal candidate for co-HTC processes to improve hydrochar quality while addressing environmental and waste management challenges. Moreover, as both wastes are generated within the same agricultural system (pig farm), their co-processing avoids the need for additional transportation or handling of external biomass sources, enhancing the feasibility and sustainability of the proposed treatment.
The aim of this study is to analyze the synergistic effect of co-HTC of SM and SH at different mass ratios (i.e., SM/SH 1:0, 1:1, 1:3, and 0:1) under temperatures of 210 °C and 240 °C. The research focuses on the analysis of hydrochar properties to determine the optimal conditions for maximizing hydrochar quality for biofuel and soil improver applications. On the other hand, the properties of PW were analyzed to suggest future valorization pathways by minimizing environmental impact. By integrating two co-located agricultural wastes into a single valorization treatment strategy, this study addresses key gaps in HTC research and proposes a sustainable solution for swine-producing regions. The findings of this work contribute to proper waste management from livestock farming, supporting a circular economy in agricultural and agro-industrial sectors.

2. Materials and Methods

2.1. HTC and Co-HTC Experiments

Feedstocks, dehydrated SM, and SH were provided by a pig farm in Avila (Spain). The solid fraction of SM was separated using a mechanical solid/liquid separator and sun-dehydrated on the farm. SM and SH were dried at 105 °C (ASTM E1756) [33] until constant weight, showing moisture values of 69% and 11%, respectively. Feedstocks were stored as received until HTC and co-HTC experiments. HTC and co-HTC experiments were carried out in a 2 L electrically heated Parr reactor (Parr 4530, PARR Instruments, Moline, IL, USA). The experiments were performed at 210 °C and 240 °C under autogenous pressure during 1 h [34] at a constant heating rate of 5 °C/min. HTC reactions were loaded with 800 g of mixture with 10% solid weight on a dry basis. To determine the synergistic effect of co-HTC, various SM/SH mass ratios were prepared as follows: 1:0, 1:1, 1:3, and 0:1. After each reaction, the hydrochar and process water were separated by centrifugation at 3700 rpm for 30 min. The wet hydrochar dried at 105 °C until its constant weight was homogenized with mortar and stored for further analysis. The process water was filtered through a 0.45 µm nylon filter and stored at 4 °C for further analysis.
Hydrochar yield (YHC) was calculated using Equation (1), where WHC is the hydrochar mass and WFeedstock is the feedstock mass, both on a dry basis.
YHC = WHC/WFeedstock · 100
Similarly, the yield of process water (YPW) was determined through Equation (2), as the ratio of total solids (TS) in the PW (WTSPW) to the mass waste feedstock on a dry basis. The gas fraction was estimated by the difference between YHC and YPW.
YPW = WTSPW/WFeedstock · 100
The resulting hydrochars were labeled as HC-x:y-Temperature, where “HC” stands for hydrochar, “x:y” represents the SM to SH mass ratio, and “Temperature” indicates the reaction temperature. For instance, HC-1:1-210 refers to the hydrochar obtained from a mixture of mass ratio SM:SH 1:1 at 210 °C. Likewise, the process water was labeled as PW-x:y-Temperature, following the same notation (e.g., PW-1:1-210).

2.2. Characterization of Feedstock, HTC and Co-HTC Products

2.2.1. Feedstock and Hydrochar Characterization

Ultimate analysis of the samples was performed with a CHNS analyzer (LECO CHNS-932; Geleen, The Netherlands). The mineral and heavy metal composition of the hydrochar was determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) with a Thermo iCAP 6500 Duo ICP-OES Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). A thermogravimetric analyzer (TG 209, F3, Netzsch, Selb, Germany) was used to conduct the proximate analysis, determining moisture, ash content, and volatile matter (VM) content in accordance with ASTM standards D3173-11, D3174-11, and D3175-11 [35,36,37]. The fixed carbon (FC) was determined by the difference between VM and ashes on a dry basis. The oxygen was calculated by difference between C, H, N, S, and ashes. The higher heating value (HHV) in MJ/kg of hydrochars were determined with the formula of Channiwala and Parikh (Equation (3)) [38]. Elemental composition (C, H, N, S, O) is presented in percentage on a dry basis.
HHV = 0.349 · C + 1.178 · H − 0.015 · N + 0.101 · S − 0.103 · O − 0.0211 · ash
The calculation of energy yield (EY) was performed as follows [39]
EY = YHC · HHVHC/HHVFeedstock
where HHVHC represents the resulting hydrochar HHV, while HHVFeedstock denotes the HHV of the feedstock on a dry basis.
The synergistic effect of co-HTC treatment in hydrochars was evaluated from calculated value (CV) and synergistic coefficient (SC) values using Equations (5) and (6).
CV = (SMvalue · mass ratio) + (SHvalue · mass ratio)
where CV (calculated values) represents the predicted parameter value for the co-HTC process. SMvalue and SHvalue correspond to the analyzed parameters of SM and SH, respectively (e.g., hydrochar yield in wt.%, fixed carbon in wt.%, carbon content in wt.%, etc.).
SC = (PV − CV)/CV
where SC quantifies the synergistic coefficient of co-HTC, PV represents the measured parameter value (experimental) obtained from the co-HTC process, and CV corresponds to the calculated theoretical value from Equation (5). A positive SC indicates an enhancement in the parameter beyond the expected value, whereas a negative SC suggests an antagonistic effect.
The biochemical composition of the feedstock was determined by lipids, proteins, and fibers content (cellulose, hemicellulose, and lignin). Lipids were quantified using the Soxhlet extraction method with a 2:1 dichloromethane–methanol mixture for 3 h [40], followed by solvent removal via rotary evaporation. Protein content was assessed following method 4500 E [41] using a TKN-365 Dist Line instrument (Büchi Labortechnik AG, Flawil, Switzerland). The lignocellulosic composition of SM and SH was analyzed using the Van Soest method to determine acid detergent fiber (ADF) and neutral detergent fiber (NDF) [42].
The textural characteristics of hydrochars were assessed through CO2 adsorption isotherms at 0 °C using a TriStar II 3020 adsorption system (Micromeritics Instrument Corp., Norcross, GA, USA). Prior to analysis, approximately 0.1 g of each sample was degassed under vacuum at 150 °C for a minimum of 10 h. The micropore volume, surface area, and pore size distribution were determined using the Dubinin–Astakhov model.

2.2.2. Process Water Characterization

The pH value and conductivity were determined using a Hach SensionN+ PH3 pH meter and a Sension+ EC7, respectively. Total solids (TS) and volatile solids (VS) were analyzed following the APHA Standard Methods 2540 B and 2540 E, respectively [41]. Total organic Carbon (TOC) was measured with a Shimadzu TOC-VCPN analyzer (Shimadzu TOC analyzers, Tokyo, Japan), which applies high-temperature catalytic combustion followed by NDIR detection, in accordance with the Standard Methods 5310 B [41]. TKN nitrogen, ammonia nitrogen (NH4-N), was measured using method 4500 E [41] with a TKN-365 Dist Line instrument (Büchi Labortechnik AG, Flawil, Switzerland). Total nitrogen (TN) was measured by CHNS analyzer (LECO CHNS-932; Geleen, The Netherlands).

2.2.3. Combustion Analysis

Combustion parameters of hydrochars were derived from thermogravimetric (TG) and derivative thermogravimetric (DTG) curves. These parameters include ignition temperature in °C (Ti) and burnout temperature in °C (Tb). Furthermore, the comprehensive combustion index in %/min2K3 (CCI) was determined as follows [43]
CCI = (DTGmax ∙ DTGmean)/(Ti2 ∙ Tb)
where DTGmax and DTGmean represent the maximum weight loss and mean weight loss, respectively. The analysis on the calculation methods, combustion parameters, and kinetic models were based on previous research [44,45,46,47,48]. In addition, slagging and fouling indexes of the ashes were assessed based on the methodology described by [49,50].

2.2.4. Statistical Analysis

The characteristics of hydrochars were statistically analyzed using the Statistical Package for the Social Sciences (SPSS) software (IBM, version 28.0). Data normality was assessed through the Kolmogorov–Smirnov test, while the Kruskal–Wallis test was applied to non-normally distributed data. Multiple comparisons were conducted to identify significant differences between the produced hydrochars.

3. Results and Discussion

3.1. Characterization of Feedstock and HTC/Co-HTC Products

The main characteristics of SM and SH were summarized in Table 1. Their elemental composition revealed a comparable carbon and fixed carbon content. Both feedstocks SM and SH showed high volatile material (>80%). While SM exhibited comparable HHV value to SH, their ash content differed significantly, with values of 12.9% and 3.4%, respectively. In terms of structural components, both feedstocks presented similar amounts of hemicellulose but different concentrations of cellulose, with SH reaching 44.5%, a typical characteristic of lignocellulosic waste, in agreement with values reported by Yoo et al. [51]. Additionally, the elevated lignin concentration observed in SM may be linked to the specific diet provided to the pigs on the farm, which likely influences the composition of their manure [52]. These observations reflect the origin and nature of the feedstocks (animal-based and plant-based) and the influence of their composition and behavior during HTC.
Figure 1 depicts the hydrochar and process water yields on a dry basis during the treatment process. A decreasing trend in hydrochar yield was observed with decreasing SM mass at both temperatures. At 210 °C, the hydrochar yield for raw SM (HC-1:0-210) reached 70.6%, whereas the raw SH (HC-0:1-240) showed a lower yield of 46.2%. A similar trend was observed at 240 °C, where the hydrochar yield decreased from 67.8% (HC-1:0-240) to 46.2% (HC-0:1-240). The co-HTC treatment showed slight variations at both 210 °C and 240 °C, with hydrochar recovery of approximately 65% and 55%, respectively. From Equation (5) the expected yields for the 1:1 and 1:3 mixtures were determined at both temperatures. As shown in Figure 1, the HC-1:3-240 sample exhibited a slightly higher hydrochar yield than the predicted one, which, when quantified using the Equation (6) for the synergy coefficient (SC), this increment represented an 11% improvement over the expected value. Similarly, the 1:1 and 1:3 mixtures at 210 °C, as well as the 1:1 mixture at 240 °C, showed smaller enhancements compared to their predicted values.
The total solids in the process water increased with SH content, from 11.6% to 16.9% at 210 °C, and from 11.9% to 20.6% at 240 °C. The lower values correspond to the TS of treated SM, whereas the higher values were associated with treated SH. The increase in TS in the process water with a higher proportion of SH in the co-HTC treatment can be explained based on the initial composition of the feedstock. SH has a significantly higher VM content (91.0% compared to 80.2% in SM) and a lower ash content (3.4% compared to 12.9% in SM), indicating that it is more likely to decompose thermally and release soluble compounds into the process water [53]. Furthermore, SH has a higher cellulose content than SM and similar hemicellulose content. These components are more prone to hydrolyze at elevated temperatures, generating soluble products such as sugar oligomers and monomers [53,54]. Additionally, the lower lignin content in SH indicates a less recalcitrant biomass structure, which contributes to diluting the overall lignin concentration in the treated mixture. This reduction in lignin content facilitates the dissolution of organic material into the process water, potentially enhancing the efficiency of co-HTC [54].
The proximate and ultimate analyses of HTC and co-HTC hydrochars were shown in Figure 2a,b, highlighting the impact of treatment temperature and feedstock composition on carbonization process. The HTC of the pure feedstock showed an important increase in carbon content with rising treatment temperature. Carbon content increased from 52.0% and 48.8% to 57.9% and 57.0% for HC-1:0 and HC-0:1 at 210 °C and 240 °C, respectively. This represents an enhancement of approximately 30% in carbon content relative to the feedstock, particularly in samples treated at higher temperatures. However, statistical analysis (Table S1) revealed that the mixtures treated at 240 °C were largely comparable, whereas HC-1:3-240 showed a significant difference from the other samples. Samples treated at 210 °C exhibit lower nitrogen content in the 1:1, 1:3, and 0:1 mass ratio samples, while the 1:0 sample maintains nitrogen levels close to 1.5%. At 240 °C, most samples show a nitrogen content of 2%, except for sample 1:3, which reaches 1.9%. Additionally, significant variations in nitrogen composition are observed among the samples. A notable increase in fixed carbon was observed with rising treatment temperatures, particularly in co-HTC samples. At 240 °C, FC values ranged between 22% and 25%, significantly higher than the samples treated at 210 °C, which only reached between 7% and 9%. Similar results were reported by Ipiales et al. [24] and Lang et al. [25] for the co-HTC of SM with lignocellulosic matter, demonstrating that increasing the proportion of lignocellulosic material improves carbon retention. The volatile matter did not show a substantial improvement at 210 °C for SM/SH mixture. On the other hand, at 240 °C the VM improved significantly, reaching lower values around 70%. This behavior is consistent with typical HTC reaction pathways, where dehydration, decarboxylation, and polymerization reactions are intensified at high temperatures, leading to carbon enrichment and a reduction in volatile components in the solid phase [55]. Figure 2a illustrates the absence of a synergistic effect for FC and VM in the co-HTC treatment at 210 °C, as the experimental VM values were significantly higher than expected, while FC values were lower. In contrast, a modest synergistic effect was observed at 240 °C for both parameters. Regarding ash content, the obtained values closely aligned with the expected ones, with the HC-1:3-240 sample exhibiting a positive synergistic effect by achieving a reduction in ash content. In Figure 2b, the elemental analysis data for co-HTC closely aligned with the expected values, with a slight synergistic contribution in specific cases. The nitrogen content of the hydrochars showed slight variations depending on treatment temperature and feedstock composition. In general, HTC treatments reduced the nitrogen content compared to the initial SM, which can be attributed to the decomposition of N-containing compounds into volatile species such as ammonia and amines. The increase in nitrogen content after the co-HTC treatment at 240 °C can be attributed to the formation of heterocyclic nitrogen compounds that promote the formation of secondary hydrochar [56,57].
The textural properties of hydrochars were analyzed through CO2 adsorption isotherms (Figure 3) and evaluated with the Dubinin–Astakhov method. The results showed that the hydrochars present microporous structures ranging from 1.6 nm to 1.9 nm and micropore volumes between 0.07 and 0.15 cm3/g (Table S2). The lower temperature treatment showed that the mixture of SM and SH in 1:1 (268 m2/g) and 1:3 (235 m2/g) ratios achieved the highest DA surface areas compared to pure treated hydrochars, and a positive synergistic effect of 60% and 30%, respectively, was observed. On the other hand, samples obtained at 240 °C exhibit DA surface areas between 147 m2/g and 204 m2/g, with HC-1:0-240 showing the highest surface area, while HC-0:1-240 had the lowest. The hydrochars from the 1:1 and 1:3 mixtures at 240 °C displayed similar textural characteristics but with lower surface area values than their counterparts at 210 °C. These findings suggest that the co-HTC treatment improves the textural properties of hydrochar, particularly at 210 °C, due to the balance between the lignin content of SM and SH. The higher proportion of SH promotes surface area development, which contributes to greater structural rigidity and porosity retention during hydrothermal treatment, while the presence of SM may influence the overall stability and product structure [58]. Additionally, the decrease in surface area at 240 °C may be attributed to increased thermal degradation and partial collapse of micropores, reducing overall textural properties [59]. Secondary hydrochar tends to exhibit lower thermal stability compared to primary hydrochar [57], which is consistent with the behavior observed in this study. Therefore, the formation of secondary hydrochar could also contribute to the reduction in surface area and to the changes in combustion behavior detected at higher treatment temperatures.
The process water of HTC and co-HCT (Table 2) was characterized by an acidic pH of approximately 4.3 and conductivity ranging from 5.2 to 7.9 mS/cm, with PW-0:1-210 and PW-0:1-240 showing the highest values. Total solids increased as the SH ratio in the mixture increased from 15 g/L to 26 g/L. Higher treatment temperatures (240 °C) also contributed to TS increase, likely due to the solubilization of organic compounds [53]. Volatile solids (VS) were highest in the 0:1 ratio at both temperatures, while the lowest values were observed in SM samples. Total organic carbon (TOC) showed a slight increasing trend (12 g/L to 17 g/L) with SH content, although no direct influence of temperature was observed. Similar occurred with the total Kjeldahl nitrogen (TKN) and ammonia nitrogen (NH3-N).

3.2. Analysis of Hydrochars for Biofuel Applications

Despite swine manure and soybean hulls exhibiting promising characteristics for biofuel applications, certain properties—like FC, VM, ash content, nitrogen and sulfur content, and ash composition (Table 1), as established by ISO 17225-8:2023 [60]—limit their direct use as biofuels, requiring further processing to enhance their combustion efficiency and energy yield [12,26]. In this context, the Van Krevelen diagram (Figure 4) was used to evaluate the coalification degree of hydrochars through their H/C and O/C ratios, which provide information on their transformation during HTC and co-HTC treatments. The HTC and co-HTC treatments primarily promote decarboxylation and dehydration processes, which play a key role in the transformation of feedstock into hydrochars [26]. It has been observed that the H/C ratio substantially improves with the HTC and co-HTC treatment with respect to the starting wastes, suggesting a higher degree of aromatization. As the temperature increases, the degradation of volatile compounds and the breakdown of components of cellulose and hemicellulose promote the aromatization of the material [61]. A higher degree of aromatization was observed for the SM hydrochar at 240 °C, placing it in a range close to sub-bituminous type coals. The carbonization of hydrochars produced by co-HTC achieved peat-like characteristics at 210 °C and lignite-like characteristics at 240 °C for the HC-1:1-240 sample, while HC-1:3-240 showed mixed peat and lignite characteristics. The hydrochars from co-HTCs of mass ratios 1:1 and 1:3 at 210 °C behaved more like peat materials. Furthermore, the amount of lignocellulosic material in the feedstock shows a clear trend toward promoting dehydration reactions in the studied mixtures, further influencing the texture of the hydrochars and fuel properties [62]. These findings suggest the potential of co-HTC to optimize both energy recovery and structural integrity of biofuels, underscoring its viability as a treatment strategy.
The ISO/TS 17225-8:2023 standard, which defines the requirements for graded thermally treated and densified biomass fuels in commercial and industrial applications, indicates that the raw feedstocks do not fully comply with the required criteria for key parameters such as VM content (<75%), ash content (<12%), nitrogen (<2.5%), sulfur (<0.3%), and a minimum HHV of 17 MJ/kg. In this context, SM exceeds the ash content limit, which can lead to slagging and fouling issues during combustion [63]. However, SM meets the nitrogen requirement and presents a HHV above 17 MJ/kg, making it a viable energy source despite these drawbacks. On the other hand, SH has a nitrogen content above 2.5%, which may contribute to NOx emissions [64] and its high VM content may affect combustion stability. Given these limitations, using SM and SH individually as biofuels presents challenges. Then, the co-HTC process of these wastes offers a promising approach to enhancing their fuel properties. By leveraging the high carbon content of SM and the structural benefits of SH, co-HTC can optimize the balance between VM, ash content, and HHV. The HTC and co-HTC treatments effectively reduced ash, sulfur, and nitrogen contents, bringing them below the allowed limits (Figure 2). However, the VM of the hydrochars obtained in co-HTC at 210 °C exceeded the allowed limit. In contrast, hydrochars from co-HTC at 240 °C presented values of 69% and 71% for 1:1 and 1:3 mixtures, respectively. Notably, the hydrochars obtained from co-HTC at 240 °C successfully meet the requirements set by ISO/TS 17225-8:2023, making them a suitable alternative for use as a biofuel. Figure 5 illustrates the enhancements in the HHV of hydrochars compared to the raw feedstocks. Hydrochars produced at 210 °C exhibited an HHV of approximately 22 MJ/kg, while treatment at 240 °C resulted in a significant increase, reaching ~25 MJ/kg. In both cases, the HHV values were consistent with the trend expected from HTC processes, confirming that co-HTC maintained the energy performance of the hydrochars without compromising their calorific value. These results fall within the typical HHV range reported for hydrochars produced from similar biomasses, which is generally between 22 and 24 MJ/kg according to Lang et al. [25]. The energy recovery efficiency, calculated using Equation (4), showed a decreasing trend as the SH proportion increased in the mixture at both temperatures. In contrast, the co-HTC treatment demonstrated improved energy recovery efficiency, yielding a positive synergistic effect ranging from 4% to 10%. This trend suggests that the higher lignocellulosic content of SH leads to increase degradation and solubilization of organic compounds during hydrothermal treatment, thereby reducing the retention of energy-dense components in the solid phase [65]. In contrast, SM-rich samples exhibit greater energy recovery, likely due to their higher initial carbon content and lower susceptibility to solubilization [66]. These findings highlight the influence of feedstock composition on the efficiency of hydrochar production and suggest that optimizing the SM/SH ratio in co-HTC can enhance biofuel potential.
Table 3 summarizes combustion and ash analyses of hydrochars. The ignition and burnout temperatures of the hydrochars showed clear trends influenced by both treatment temperature and feedstock composition. In general, hydrochars with high SM content exhibited the highest Ti values, indicating greater thermal stability, while those from pure SH had the lowest Ti, suggesting easier ignition behavior. The co-HTC process for both temperatures resulted in intermediate Ti values, with slight variations depending on the mixture ratio. The Ti of the HC-1:1-240 slightly decreased to 261 °C, suggesting increased reactivity, while the HC-1:3-240 showed a higher Ti (283 °C), closer to that of HC-1:1-210 sample. Regarding Tb, higher SM proportions tend to increase in Tb, with HC-1:0-210 and HC-1:0-240 samples reaching the highest values. Meanwhile, samples with higher SH content exhibit lower Tb, 473 °C and 500 °C for HC-0:1-210 and HC-0:1-240, respectively. The co-HTC produces hydrochars with intermediate Tb values between SM and SH hydrochars.
This trend in ignition and burnout temperatures directly influences the combustion behavior of hydrochars, making it essential to evaluate additional parameters such as the combustion coefficient index (CCI). Generally, higher CCI values indicate a less stable combustion process, as the material is more likely to ignite suddenly or burn too aggressively, leading to unstable combustion [43]. CCI parameter revealed that HTC of SM for both 210 °C and 240 °C temperatures may show more stable combustion than SH. Mixtures with 1:1 mass ratio (HC-1:1-210 and HC-1:1-240) showed higher CCI values than the SM- and SH-treated samples, indicating that these hydrochars are more prone to quick combustion. The higher VM content in these samples (around 85%) likely contributes to this behavior, as higher VM content is typically associated with faster combustion rates due to the ease with which volatile compounds are released [67]. On the other hand, samples with higher SH content such as HC-1:3-210 and HC-1:3-240 present lower CCI values (1.8 and 1.0, respectively). These data lead to a positive synergistic contribution of SH, which leads to enhanced combustion stability, reflected in the lower CCI compared to the 1:1 mass ratio hydrochar samples. Thus, the higher combustion stability observed in these samples highlights their potential for biofuel applications, where maintaining a constant heat output over extended periods is essential for efficient energy production.
In addition to the influence of Ti and Tb on combustion behavior and the analysis of CCI values, the tendency of hydrochars to form slag and fouling deposits during combustion (Table 3) is another critical factor to consider for their practical application as biofuels. The slagging and fouling analysis of ashes is helpful to predict if the ashes of a material will molten and adhere to a surface. The alkali index (AI) showed a possible slagging and fouling effect as SH content increased for AI values between 0.17 and 0.34 [50]. The acid base ratio (Ra/b) above 0.5 shows considerable presence of alkaline oxides which increase the risk of slagging. In this regard, increasing SH content also increases the presence of alkaline oxides. The slagging index (SI) showed that co-HTC hydrochars present a high slagging propensity (2.0 < SI < 2.6) when burning, but HTC of SM exhibits extreme slagging risk (SI > 2.6) due to the high ash content, demonstrating the efficiency co-HTC treatment. The fouling index (FI) showed that all hydrochars presented a high risk of fouling (0.6 < FI < 40). These results suggest that co-HTC hydrochars may be more controllable in terms of slag formation, although the high risk of fouling in all samples indicates the need for additional considerations in combustion systems. The improved combustion stability observed in these materials highlights their potential for use in biofuel applications [63].

3.3. Evaluation of Hydrochars for Soil Improvement Applications

Hydrochars are suitable candidates to be employed as soil improvers [68] due to the essential nutrients like sodium, potassium, phosphorus, nitrogen, calcium, and magnesium. These nutrients show low solubility under hydrothermal conditions, which means that most of the elements present in the feedstock remain in the hydrochar [23]. Table 4 and Table 5 summarize the mineral composition and heavy metals present in the studied hydrochars. It is observed that the temperature does not represent a key parameter in the concentration of mineral species; rather, the feedstock composition plays a more significant role. Aluminum content remains relatively constant across all samples, while calcium, iron, sodium, and phosphorus decrease as the proportion of SH increases. These findings suggest that co-HTC can be strategically used to optimize the nutrient composition of hydrochars for soil amendment purposes. By adjusting the SM/SH ratio, it is possible to enhance the retention of key macronutrients while mitigating excessive losses of essential elements like phosphorus. Moreover, co-HTC can improve the stability and carbon retention of hydrochars, contributing to soil health and long-term carbon sequestration [26].
In Spain, Royal Decrees 1051/2022 and 824/2024 establish the guidelines for sustainable soil nutrition and set maximum allowable limits for heavy metals in organic soil improvers (Cd < 10 mg/kg, Cu < 1 g/kg, Ni < 0.3 g/kg, Pb < 0.75 g/kg, Zn < 2.5 g/kg, Cr < 1 g/kg). Additionally, the EU Regulation 2019/1009 defines the criteria for placing fertilizing products on the market, including the specifications for materials intended as soil improvers. In this sense, all hydrochars comply with both EU and Spanish regulations, as can be seen in Table 5. Based on these results, further essays can be developed to study the suitability of co-HTC for soil improver applications [69,70].

4. Conclusions

This study evaluates the hydrochars obtained from HTC and co-HTC treatments of two wastes generated in pig farms: swine manure and soybean hulls, a biowaste of pig diet. The results demonstrated that both temperature (210 °C and 240 °C) and mass ratio significantly enhanced the properties of hydrochars. The treatment of SM and SH in different ratios, particularly at a 1:3 mass ratio at both temperatures, yielded hydrochars with improved properties, including a higher FC content, lower VM and ash content, as well as lower nitrogen and sulfur concentrations compared to hydrochars derived from feedstocks. Additionally, an increase in HHV aligns these improvements with the requirements of ISO/TS 17225-8:2023. Combustion analysis confirmed the viability of co-HTC hydrochars, highlighting their great stability. However, slagging and fouling assessments suggest that additional treatments may be necessary to mitigate operational challenges.
The analysis of the mineral and heavy metal content supports the potential use of co-HTC hydrochars as soil amendments, complying with current regulatory standards in Spain and the EU. In future studies, it would be valuable to explore the long-term effects of co-HTC hydrochars on soil health, fertility, and crop growth. Specific investigations into the bioavailability of heavy metals and the impact on soil microbial communities would provide further insights into their effectiveness and safety as soil amendments.
To enhance the value of co-HTC hydrochars, future research should focus on the valorization of the co-HTC process water. Possible applications include fertigation, precipitation of mineral salts rich in phosphorus, and the production of methane through anaerobic digestion. Such valorization would contribute to a circular economic framework, mitigating the environmental impact of waste and supporting sustainable agricultural practices, especially in areas where both types of waste are generated in the same location.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17115022/s1, Table S1: Proximate and ultimate analyses of hydrochars.; Table S2: Textural properties of hydrochars analyzed by CO2 adsorption isotherms using the Dubinin–Astakhov method. Figure S1: Schematic setup of HTC/co-HTC process.

Author Contributions

Conceptualization, B.C.-T., E.D. and A.F.M.; Methodology, B.C.-T. and E.D.; Investigation, B.C.-T., E.D. and A.F.M.; Writing—original draft preparation, B.C.-T. and E.D.; Writing—review and editing, B.C.-T., E.D., M.A.d.l.R. and A.F.M.; Supervision, E.D. and A.F.M.; Funding acquisition, E.D., M.A.d.l.R. and A.F.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Spanish MCIN/AEI/10.13039/501100011033, European Union “NextGenerationEU/PRTR” (TED2021-130287B-I00 and PID2022-138632OB-I00) and Comunidad de Madrid (TEC-2024/BIO-177).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not Applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Acknowledgments

B. Chiguano-Tapia acknowledges research grand from Spanish MCIN and PREP2022-000792. Authors acknowledge A. Vázquez-Bueno for her valuable help.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
HTCHydrothermal carbonization
Co-HTCCo-hydrothermal carbonization
SMSwine manure
SHSoybean hull
HCHydrochar
PWProcess water
TSTotal solids
FCFixed carbon
VMVolatile matter
TOCTotal organic carbon
TNTotal nitrogen
TKNTotal Kjeldal nitrogen
CVCalculated values

References

  1. Mei, C.; Cheng, M.; Xie, M.; Yang, R.; Liu, T.; Huang, Z.; Zhou, T.; Zhao, Y.; Liu, Z.; Li, B. Recent Advances in Thermochemical Conversion Technology for Anaerobic Digestate from Food Waste. Bioresour. Technol. 2024, 413, 131527. [Google Scholar] [CrossRef] [PubMed]
  2. Heidari, M.; Norouzi, O.; MacDermid-Watts, K.; Acharya, B.; Zhang, Y.; Dutta, A. Product Evaluation of Hydrothermal Carbonization of Biomass: Semi-Continuous vs. Batch Feeding. Biomass-Convers. Biorefinery 2022, 12, 15–25. [Google Scholar] [CrossRef]
  3. Nizamuddin, S.; Baloch, H.A.; Griffin, G.J.; Mubarak, N.M.; Bhutto, A.W.; Abro, R.; Mazari, S.A.; Ali, B.S. An Overview of Effect of Process Parameters on Hydrothermal Carbonization of Biomass. Renew. Sustain. Energy Rev. 2017, 73, 1289–1299. [Google Scholar] [CrossRef]
  4. Qureshi, S.S.; Premchand; Javed, M.; Saeed, S.; Abro, R.; Mazari, S.A.; Mubarak, N.M.; Siddiqui, M.T.H.; Baloch, H.A.; Nizamuddin, S. Hydrothermal Carbonization of Oil Palm Trunk via Taguchi Method. Korean J. Chem. Eng. 2021, 38, 797–806. [Google Scholar] [CrossRef]
  5. Ipiales, R.P.; Pimentel-Betancurt, D.; Diaz, E.; de la Rubia, A.; Rodríguez, J.J.; Mohedano, A.F. Energy Recovery from Garden and Park Waste by Hydrothermal Carbonization with Process Water Recycling. ACS Sustain. Chem. Eng. 2024, 12, 5229–5240. [Google Scholar] [CrossRef]
  6. Gebretsadkan, A.A.; Belete, Y.Z.; Krounbi, L.; Gelfand, I.; Bernstein, R.; Gross, A. Soil Application of Activated Hydrochar Derived from Sewage Sludge Enhances Plant Growth and Reduces Nitrogen Loss. Sci. Total Environ. 2024, 949, 174965. [Google Scholar] [CrossRef]
  7. Qaramaleki, S.V.; Mohedano, Á.F.; Coronella, C.J. Phosphorus Recovery from Aqueous Product of Hydrothermal Carbonization of Cow Manure. Waste Manag. 2023, 168, 301–310. [Google Scholar] [CrossRef]
  8. Sarrion, A.; Ipiales, R.P.; de la Rubia, M.A.; Mohedano, A.F.; Diaz, E. Chicken Meat and Bone Meal Valorization by Hydrothermal Treatment and Anaerobic Digestion: Biofuel Production and Nutrient Recovery. Renew. Energy 2023, 204, 652–660. [Google Scholar] [CrossRef]
  9. Diez, M.P.; Barahona, E.; de la Rubia, M.A.; Mohedano, A.F.; Diaz, E. Valorization of Process Water from Hydrothermal Carbonization of Food Waste by Dark Fermentation. Int. J. Hydrogen Energy 2024, 89, 1383–1393. [Google Scholar] [CrossRef]
  10. Zhang, Q.; Cao, Y.; He, M.; Lei, H.; Song, H.; Alessi, D.S.; Tsang, D.C.W. Improved Energy Recovery from Yard Waste by Water-Starved Hydrothermal Treatment: Effects of Process Water and Pressure. Bioresour. Technol. 2024, 394, 130211. [Google Scholar] [CrossRef]
  11. Kambo, H.S.; Minaret, J.; Dutta, A. Process Water from the Hydrothermal Carbonization of Biomass: A Waste or a Valuable Product? Waste Biomass Valorization 2018, 9, 1181–1189. [Google Scholar] [CrossRef]
  12. Ebrahim Malool, M.; Keshavarz Moraveji, M.; Shayegan, J. Co-Hydrothermal Carbonization of Digested Sewage Sludge and Sugarcane Bagasse: Integrated Approach for Waste Management, Optimized Production, Characterization and Pb(II) Adsorption. Alex. Eng. J. 2023, 74, 79–105. [Google Scholar] [CrossRef]
  13. García-Morato, R.; Román, S.; Ledesma, B.; Coronella, C. Co-Hydrothermal Carbonization of Grass and Olive Stone as a Means to Lower Water Input to HTC. Resources 2023, 12, 85. [Google Scholar] [CrossRef]
  14. Picone, A.; Volpe, M.; Codignole Lùz, F.; Malik, W.; Volpe, R.; Messineo, A. Co-Hydrothermal Carbonization with Process Water Recirculation as a Valuable Strategy to Enhance Hydrochar Recovery with High Energy Efficiency. Waste Manag. 2024, 175, 101–109. [Google Scholar] [CrossRef]
  15. Wang, Z.; Huang, J.; Wang, J.; Hu, Z.; Xu, M.; Qiao, Y. Co-Hydrothermal Carbonization of Sludge and Food Waste for Hydrochar Valorization: Effect of Mutual Interaction on Sulfur Transformation. Sci. Total Environ. 2023, 905, 167318. [Google Scholar] [CrossRef]
  16. Pampuro, N.; Dinuccio, E.; Balsari, P.; Cavallo, E. Gaseous Emissions and Nutrient Dynamics during Composting of Swine Solid Fraction for Pellet Production. Appl. Math. Sci. 2014, 8, 6459–6468. [Google Scholar] [CrossRef]
  17. Köninger, J.; Lugato, E.; Panagos, P.; Kochupillai, M.; Orgiazzi, A.; Briones, M.J.I. Manure Management and Soil Biodiversity: Towards More Sustainable Food Systems in the EU. Agric. Syst. 2021, 194, 103251. [Google Scholar] [CrossRef]
  18. Zhang, Y.; Jiang, Y.; Wang, S.; Wang, Z.; Liu, Y.; Hu, Z.; Zhan, X. Environmental Sustainability Assessment of Pig Manure Mono- and Co-Digestion and Dynamic Land Application of the Digestate. Renew. Sustain. Energy Rev. 2021, 137, 110476. [Google Scholar] [CrossRef]
  19. da Rosa, G.M.; Gabriel, M.; da Silva, J.C.; Mendonça, A.M.; Junior, J.A.C.; Wastowski, A.D. Leaching of the Different Forms of Nitrogen by the Application of Poultry Litter, Swine Waste, and Mineral Nitrogen on Corn Cultures (Zea mays L.). Environ. Qual. Manag. 2018, 28, 131–138. [Google Scholar] [CrossRef]
  20. Raj, C.; Chakraborty, D.; Watts, D.B.; Horvath, T.; Ketterings, Q.M.; Blersch, D.; Tomasek, A.A.; Cordoba, B.C.; Prasad, R. Impact of Broiler Litter and Swine Liquid Manure on Nutrient Loss in Runoff from Three Consecutive One Acre-Inch Rainfall Events. Heliyon 2024, 10, e40062. [Google Scholar] [CrossRef]
  21. Zhuang, M.; Shan, N.; Wang, Y.; Caro, D.; Fleming, R.M.; Wang, L. Different Characteristics of Greenhouse Gases and Ammonia Emissions from Conventional Stored Dairy Cattle and Swine Manure in China. Sci. Total Environ. 2020, 722, 137693. [Google Scholar] [CrossRef] [PubMed]
  22. EUROSTAT. Agri-Environmental Indicator—Livestock Patterns; EUROSTAT: Luxembourg, 2023.
  23. Ipiales, R.P.; Díaz, E.; Mohedano, A.F.; de la Rubia, M.A. Material and Energy Recovery from Animal Manure by Hydrothermal Carbonization. In Reference Module in Materials Science and Materials Engineering; Elsevier: Amsterdam, The Netherlands, 2024. [Google Scholar]
  24. Ipiales, R.P.; Mohedano, A.F.; Diaz-Portuondo, E.; Diaz, E.; de la Rubia, M.A. Co-Hydrothermal Carbonization of Swine Manure and Lignocellulosic Waste: A New Strategy for the Integral Valorization of Biomass Wastes. Waste Manag. 2023, 169, 267–275. [Google Scholar] [CrossRef]
  25. Lang, Q.; Guo, Y.; Zheng, Q.; Liu, Z.; Gai, C. Co-Hydrothermal Carbonization of Lignocellulosic Biomass and Swine Manure: Hydrochar Properties and Heavy Metal Transformation Behavior. Bioresour. Technol. 2018, 266, 242–248. [Google Scholar] [CrossRef] [PubMed]
  26. Xiong, W.; Luo, Y.; Shangguan, W.; Deng, Y.; Li, R.; Song, D.; Zhang, M.; Li, Z.; Xiao, R. Co-Hydrothermal Carbonization of Lignocellulosic Biomass and Swine Manure: Optimal Parameters for Enhanced Nutrient Reclamation, Carbon Sequestration, and Heavy Metals Passivation. Waste Manag. 2024, 190, 174–185. [Google Scholar] [CrossRef]
  27. Świątkiewicz, M.; Witaszek, K.; Sosin, E.; Pilarski, K.; Szymczyk, B.; Durczak, K. The Nutritional Value and Safety of Genetically Unmodified Soybeans and Soybean Feed Products in the Nutrition of Farm Animals. Agronomy 2021, 11, 1105. [Google Scholar] [CrossRef]
  28. Gaffield, K.N.; Goodband, R.D.; DeRouchey, J.M.; Tokach, M.D.; Woodworth, J.C.; Denny, G.; Gebhardt, J.T. A Review of Soybean Processing Byproducts and Their Use in Swine and Poultry Diets. Transl. Anim. Sci. 2024, 8, txae063. [Google Scholar] [CrossRef]
  29. MAPA Estadística Digital. Available online: https://servicio.mapama.gob.es/es/estadistica/temas/estadistica-digital/powerbi-cultivos.aspx (accessed on 25 April 2025).
  30. Cassales, A.; de Souza-Cruz, P.B.; Rech, R.; Záchia Ayub, M.A. Optimization of Soybean Hull Acid Hydrolysis and Its Characterization as a Potential Substrate for Bioprocessing. Biomass Bioenergy 2011, 35, 4675–4683. [Google Scholar] [CrossRef]
  31. Martinez-Burgos, W.J.; Porto de Souza Vandenberghe, L.; Karp, S.G.; Murawski de Mello, A.F.; Thomaz Soccol, V.; Soccol, C.R. Microbial Lipid Production from Soybean Hulls Using Lipomyces Starkeyi LPB53 in a Circular Economy. Bioresour. Technol. 2023, 372, 128650. [Google Scholar] [CrossRef]
  32. Riaz, M.N. Soy Beans: Processing. In Encyclopedia of Food and Health; Elsevier Inc.: Amsterdam, The Netherlands, 2015; pp. 48–53. ISBN 9780123849533. [Google Scholar]
  33. ASTM E1756; Standard Test Method for Determination of Total Solids in Biomass. ASTM International: West Conshohocken, PA, USA, 2020.
  34. Sarrion, A.; Diaz, E.; de la Rubia, M.A.; Mohedano, A.F. Fate of Nutrients during Hydrothermal Treatment of Food Waste. Bioresour. Technol. 2021, 342, 125954. [Google Scholar] [CrossRef]
  35. ASTM D3173-11; Standard Test Method for Moisture in the Analysis Sample of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2011.
  36. ASTM D3174-11; Standard Test Method for Ash in the Analysis Sample of Coal and Coke from Coal. ASTM International: West Conshohocken, PA, USA, 2011.
  37. ASTM D3175-11; Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke. ASTM International: West Conshohocken, PA, USA, 2011.
  38. Channiwala, S.A.; Parikh, P.P. A Unified Correlation for Estimating HHV of Solid, Liquid and Gaseous Fuels. Fuel 2002, 81, 1051–1063. [Google Scholar] [CrossRef]
  39. Park, K.Y.; Lee, K.; Kim, D. Characterized Hydrochar of Algal Biomass for Producing Solid Fuel through Hydrothermal Carbonization. Bioresour. Technol. 2018, 258, 119–124. [Google Scholar] [CrossRef] [PubMed]
  40. Cequier-Sánchez, E.; Rodríguez, C.; Ravelo, Á.G.; Zárate, R. Dichloromethane as a Solvent for Lipid Extraction and Assessment of Lipid Classes and Fatty Acids from Samples of Different Natures. J. Agric. Food Chem. 2008, 56, 4297–4303. [Google Scholar] [CrossRef]
  41. APHA. Standard Methods for the Examination of Water and Wastewater, 21st ed.; American Public Health Association: Washington, DC, USA, 2005. [Google Scholar]
  42. Hindrichsen, I.K.; Kreuzer, M.; Madsen, J.; Bach Knudsen, K.E. Fiber and Lignin Analysis in Concentrate, Forage, and Feces: Detergent Versus Enzymatic-Chemical Method. J. Dairy Sci. 2006, 89, 2168–2176. [Google Scholar] [CrossRef]
  43. Lin, Y.; Ma, X.; Peng, X.; Yu, Z.; Fang, S.; Lin, Y.; Fan, Y. Combustion, Pyrolysis and Char CO2-Gasification Characteristics of Hydrothermal Carbonization Solid Fuel from Municipal Solid Wastes. Fuel 2016, 181, 905–915. [Google Scholar] [CrossRef]
  44. Chen, X.; Ma, X.; Peng, X.; Lin, Y.; Yao, Z. Conversion of Sweet Potato Waste to Solid Fuel via Hydrothermal Carbonization. Bioresour. Technol. 2018, 249, 900–907. [Google Scholar] [CrossRef] [PubMed]
  45. Li, Y.; Li, J.; Zhou, S.; Meng, B.; Wu, T. A Review on Thermogravimetric Analysis-Based Analyses of the Pyrolysis Kinetics of Oil Shale and Coal. Energy Sci. Eng. 2024, 12, 329–355. [Google Scholar] [CrossRef]
  46. Ma, B.G.; Li, X.G.; Xu, L.; Wang, K.; Wang, X.G. Investigation on Catalyzed Combustion of High Ash Coal by Thermogravimetric Analysis. Thermochim. Acta 2006, 445, 19–22. [Google Scholar] [CrossRef]
  47. Naqvi, S.R.; Tariq, R.; Hameed, Z.; Ali, I.; Naqvi, M.; Chen, W.H.; Ceylan, S.; Rashid, H.; Ahmad, J.; Taqvi, S.A.; et al. Pyrolysis of High Ash Sewage Sludge: Kinetics and Thermodynamic Analysis Using Coats-Redfern Method. Renew. Energy 2019, 131, 854–860. [Google Scholar] [CrossRef]
  48. Tang, L.; Xiao, J.; Mao, Q.; Zhang, Z.; Yao, Z.; Zhu, X.; Ye, S.; Zhong, Q. Thermogravimetric Analysis of the Combustion Characteristics and Combustion Kinetics of Coals Subjected to Different Chemical Demineralization Processes. ACS Omega 2022, 7, 13998–14008. [Google Scholar] [CrossRef]
  49. Tortosa Masiá, A.A.; Buhre, B.J.P.; Gupta, R.P.; Wall, T.F. Characterising Ash of Biomass and Waste. Fuel Process. Technol. 2007, 88, 1071–1081. [Google Scholar] [CrossRef]
  50. Cao, Z.; Hülsemann, B.; Wüst, D.; Oechsner, H.; Lautenbach, A.; Kruse, A. Effect of Residence Time during Hydrothermal Carbonization of Biogas Digestate on the Combustion Characteristics of Hydrochar and the Biogas Production of Process Water. Bioresour. Technol. 2021, 333, 125110. [Google Scholar] [CrossRef] [PubMed]
  51. Yoo, J.; Alavi, S.; Vadlani, P.; Amanor-Boadu, V. Thermo-Mechanical Extrusion Pretreatment for Conversion of Soybean Hulls to Fermentable Sugars. Bioresour. Technol. 2011, 102, 7583–7590. [Google Scholar] [CrossRef] [PubMed]
  52. Kerr, B.J.; Trabue, S.L.; van Weelden, M.B.; Andersen, D.S.; Pepple, L.M. Impact of Narasin on Manure Composition, Microbial Ecology, and Gas Emissions from Finishing Pigs Fed Either a Corn-Soybean Meal or a Corn-Soybean Meal-Dried Distillers Grains with Solubles Diets. J. Anim. Sci. 2018, 96, 1317–1329. [Google Scholar] [CrossRef]
  53. Wang, R.; Lin, Z.; Meng, S.; Liu, S.; Zhao, Z.; Wang, C.; Yin, Q. Effect of Lignocellulosic Components on the Hydrothermal Carbonization Reaction Pathway and Product Properties of Protein. Energy 2022, 259, 125063. [Google Scholar] [CrossRef]
  54. Hu, J.; Shen, D.; Wu, S.; Zhang, H.; Xiao, R. Effect of Temperature on Structure Evolution in Char from Hydrothermal Degradation of Lignin. J. Anal. Appl. Pyrolysis 2014, 106, 118–124. [Google Scholar] [CrossRef]
  55. Pauline, A.L.; Joseph, K. Hydrothermal Carbonization of Organic Wastes to Carbonaceous Solid Fuel—A Review of Mechanisms and Process Parameters. Fuel 2020, 279, 118472. [Google Scholar] [CrossRef]
  56. Ipiales, R.P.; Lelli, G.; Diaz, E.; Diaz-Portuondo, E.; Mohedano, A.F.; de la Rubia, M.A. Study of Two Approaches for the Process Water Management from Hydrothermal Carbonization of Swine Manure: Anaerobic Treatment and Nutrient Recovery. Environ. Res. 2024, 246, 118098. [Google Scholar] [CrossRef] [PubMed]
  57. Alonso, M.; Ledesma, B.; Román, S.; Olivares-Marín, M. Insights about the Formation of Secondary Char during HTC Processes. Sustain. Chem. Pharm. 2024, 37, 101420. [Google Scholar] [CrossRef]
  58. Yu, S.; Xie, M.; Li, Q.; Zhang, Y.; Zhou, H. Evolution of Kraft Lignin during Hydrothermal Treatment under Different Reaction Conditions. J. Energy Inst. 2022, 103, 147–153. [Google Scholar] [CrossRef]
  59. Falco, C.; Baccile, N.; Titirici, M.M. Morphological and Structural Differences between Glucose, Cellulose and Lignocellulosic Biomass Derived Hydrothermal Carbons. Green Chem. 2011, 13, 3273–3281. [Google Scholar] [CrossRef]
  60. ISO 17225-8:2023; Solid Biofuels—Fuel Specifications and Classes—Part 8: Graded Firewood. International Organization for Standardization: Geneva, Switzerland, 2023.
  61. Liu, X.; Peng, L.; Deng, P.; Xu, Y.; Wang, P.; Tan, Q.; Zhang, C.; Dai, X. Co-Hydrothermal Carbonization of Sewage Sludge and Rice Straw to Improve Hydrochar Quality: Effects of Mixing Ratio and Hydrothermal Temperature. Bioresour. Technol. 2025, 415, 131665. [Google Scholar] [CrossRef] [PubMed]
  62. Lu, X.; Ma, X.; Qin, Z.; Chen, X.; Yue, W. Co-Hydrothermal Carbonization of Sewage Sludge and Swine Manure: Hydrochar Properties and Heavy Metal Chemical Speciation. Fuel 2022, 330, 125573. [Google Scholar] [CrossRef]
  63. Ghazidin, H.; Suyatno, S.; Prismantoko, A.; Ruhiyat, A.S.; Prabowo; Darmawan, A.; Soleh, M.; Aziz, A.; Asmanto, P.; Vuthaluru, H.; et al. Control of Slagging and Fouling during Co-Firing of Solid Recovered Fuel and High-Sodium Coal Using Aluminum and Magnesium-Based Additives. Energy 2025, 318, 134873. [Google Scholar] [CrossRef]
  64. Riaza, J.; Mason, P.; Jones, J.M.; Gibbins, J.; Chalmers, H. High Temperature Volatile Yield and Nitrogen Partitioning during Pyrolysis of Coal and Biomass Fuels. Fuel 2019, 248, 215–220. [Google Scholar] [CrossRef]
  65. Wang, Z.W.; Zhu, M.Q.; Li, M.F.; Wang, J.Q.; Wei, Q.; Sun, R.C. Comprehensive Evaluation of the Liquid Fraction During the Hydrothermal Treatment of Rapeseed Straw. Biotechnol. Biofuels 2016, 9, 142. [Google Scholar] [CrossRef]
  66. Ferrentino, R.; Sacchi, G.; Scrinzi, D.; Andreottola, G.; Fiori, L. Valorization of Swine Manure for a Circular Approach through Hydrothermal Carbonization. Biomass Bioenergy 2023, 168, 106689. [Google Scholar] [CrossRef]
  67. He, C.; Giannis, A.; Wang, J.Y. Conversion of Sewage Sludge to Clean Solid Fuel Using Hydrothermal Carbonization: Hydrochar Fuel Characteristics and Combustion Behavior. Appl. Energy 2013, 111, 257–266. [Google Scholar] [CrossRef]
  68. Khosravi, A.; Zheng, H.; Liu, Q.; Hashemi, M.; Tang, Y.; Xing, B. Production and Characterization of Hydrochars and Their Application in Soil Improvement and Environmental Remediation. Chem. Eng. J. 2022, 430, 133142. [Google Scholar] [CrossRef]
  69. Islam, M.A.; Sharif, M.; Limon, H.; Romić, M.; Islam, A. Hydrochar-Based Soil Amendments for Agriculture: A Review of Recent Progress. Arab. J. Geosci. 2021, 14, 102. [Google Scholar] [CrossRef]
  70. Yan, T.; Zhang, Z.; Zhang, Z.; Wang, W.; Li, D.; Zhang, T.; Zhu, Z. Applying Hydrochar Affects Soil Carbon Dynamics by Altering the Characteristics of Soil Aggregates and Microbes. Agronomy 2024, 14, 1015. [Google Scholar] [CrossRef]
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Figure 1. HTC/co-HTC products distributed across various mass ratios and temperature conditions. The diamond symbols show the calculated value (CV) of the hydrochar yield for the different mass ratios.
Figure 1. HTC/co-HTC products distributed across various mass ratios and temperature conditions. The diamond symbols show the calculated value (CV) of the hydrochar yield for the different mass ratios.
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Figure 2. Proximate (a) and ultimate (b) analyses of hydrochars. Symbols show the calculated values (CV) of the different variables in the HTC/co-HTC treatments.
Figure 2. Proximate (a) and ultimate (b) analyses of hydrochars. Symbols show the calculated values (CV) of the different variables in the HTC/co-HTC treatments.
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Figure 3. Dubinin–Astakhov surface area of the hydrochars. Black symbols show the calculated value (CV) of the surface area in the co-HTC treatment.
Figure 3. Dubinin–Astakhov surface area of the hydrochars. Black symbols show the calculated value (CV) of the surface area in the co-HTC treatment.
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Figure 4. Van Krevelen diagram for swine manure, soy hull, and hydrochars.
Figure 4. Van Krevelen diagram for swine manure, soy hull, and hydrochars.
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Figure 5. HHV and energy recovery efficiency of hydrochars and feedstocks. The left y-axis represents the HHV values of the hydrochars (bars), while the right y-axis shows the energy yield (black diamond symbols) of HTC/co-HTC process. Yellow and blue diamond symbols represent the calculated value (CV) for HHV and EY, respectively.
Figure 5. HHV and energy recovery efficiency of hydrochars and feedstocks. The left y-axis represents the HHV values of the hydrochars (bars), while the right y-axis shows the energy yield (black diamond symbols) of HTC/co-HTC process. Yellow and blue diamond symbols represent the calculated value (CV) for HHV and EY, respectively.
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Table 1. Proximal and ultimate analysis; minerals and heavy metals composition on a dry basis.
Table 1. Proximal and ultimate analysis; minerals and heavy metals composition on a dry basis.
SMSH SMSH
FC (%)6.95.7Al (g/kg)0.40.3
VM (%)80.291.0Ca (g/kg)19.75.5
Ash (%)12.93.4Fe (g/kg)1.30.6
C (%)46.042.2K (g/kg)4.213.7
H (%)5.66.5Mg (g/kg)2.91.9
N (%)2.22.9Na (g/kg)1.3<0.01
S (%)0.80.1P (g/kg)12.61.3
O * (%)32.544.9Cd (mg/kg)0.20.1
HHV (MJ/kg)19.117.7Co (mg/kg)0.90.7
Hemicellulose (%)12.411.6Cu (mg/kg)126.316.1
Cellulose (%)16.944.5Ni (mg/kg)6.12.5
Lignin (%)33.26.1Pb (mg/kg)1.20.2
Lipids (%)2.31.5Cr (mg/kg)12.73.4
Proteins (%)13.211.1Zn (mg/kg)418.148.1
* Calculated by difference.
Table 2. Process water characteristics.
Table 2. Process water characteristics.
SM:SH210 °C240 °C
1:01:11:30:11:01:11:30:1
pH4.274.304.334.254.174.304.334.29
Conductivity (mS/cm)6.16.46.67.95.26.16.67.3
TS (g/L)17.519.121.224.715.620.422.225.5
VS (g/L)4.05.34.95.92.83.54.86.1
TOC (g/L)12.314.315.016.013.014.712.017.3
TKN (mg/L)1171.4937.41084.51121.21003.4867.0943.0931.6
NH4-N (mg/L)198.9160.7161.0162.0159.0139.8128.3116.0
Table 3. Combustion behavior and slagging/fouling analysis of hydrochars.
Table 3. Combustion behavior and slagging/fouling analysis of hydrochars.
SM:SH RatioTi (°C)Tb (°C)CCI·107AIRb/aSIFI
210 °C1:02905551.30.1072.650.815.8
1:12805127.30.2224.07.211.3
1:32885161.80.1419.17.65.8
0:12774736.20.198.30.83.2
240 °C1:02925470.70.0829.631.77.6
1:12615124.30.1119.99.95.4
1:32835261.00.1919.87.98.8
0:12685003.90.188.81.83.7
Table 4. Mineral composition of the hydrochars.
Table 4. Mineral composition of the hydrochars.
SM:SH RatioMineral Species (g/kg)
AlCaFeKMgNaP
210 °C1:00.7 (0.1)29.2 (0.2)2.0 (0.0)1.2 (0.1)2.0 (0.0)0.5 (0.0)18.7 (0.2)
1:10.7 (0.1)16.8 (0.1) 1.1 (0.0)2.3 (0.1)1.5 (0.0)0.2 (0.0)7.7 (0.1)
1:30.7 (0.2)9.1 (0.2)1.1 (0.0)3.8 (0.2)1.8 (0.1)0.1 (0.0)6.5 (0.1)
0:10.5 (0.1)4.4 (0.1)0.8 (0.1)3.2 (0.0)0.7 (0.0)0.1 (0.0)1.6 (0.1)
240 °C1:00.9 (0.0)33.4 (0.1)2.3 (0.1)1.0 (0.0)2.5 (0.2)0.6 (0.0)16.5 (0.1)
1:10.9 (0.1)20.2 (0.1)1.6 (0.1)2.1 (0.1)1.7 (0.0)0.2 (0.0)9.7 (0.1)
1:30.9 (0.1)10.7 (0.2)1.4 (0.0)3.5 (0.1)2.0 (0.1)0.2 (0.0)6.5 (0.1)
0:10.8 (0.1)5.9 (0.2)1.1 (0.1)3.5 (0.1)1.0 (0.1)0.1 (0.0)2.6 (0.0)
Table 5. Heavy metals composition of hydrochars.
Table 5. Heavy metals composition of hydrochars.
SM:SH Ratio Heavy Metals (mg/kg)
CdCoCrCuNiPbSiTiZn
210 °C1:00.2 (0.0)1.1 (0.1)52.9 (1.3)175.5 (3.2)12.9 (1.2)2.6 (0.1)240.1 (5.3)50.1 (5.3)583.3 (3.1)
1:10.2 (0.0)0.6 (0.0)9.9 (0.6)93.4 (2.5)4.3 (0.6)1.3 (0.0)365.5 (6.1)33.3 (2.6)261.9 (1.1)
1:30.1 (0.0)0.6 (0.0)8.9 (0.3)46.6 (1.7)3.9 (0.7)1.0 (0.0)392.7 (4.2)28.6 (2.5)213.3 (1.3)
0:10.1 (0.0)0.6 (0.0)7.2 (0.4)16.5 (1.9)4.5 (0.4)0.7 (0.0)627.1 (7.5)48.9 (4.3)60.6 (0.5)
240 °C1:00.3 (0.0)1.6 (0.1)94.9 (2.4)205.8 (4.2)19.7 (1.4)1.6 (0.0)760.4 (8.8)33.7 (2.1)665.1 (2.3)
1:10.3 (0.0)1.0 (0.1)18.2 (0.3)134.9 (2.8)8.2 (0.7)1.0 (0.0)611.0 (6.9)45.8 (2.1)425.4 (1.6)
1:30.1 (0.0)0.7 (0.0)13.3 (0.3)58.2 (2.4)8.1 (0.7)1.2 (0.1)489.1 (4.7)35.3 (1.7)256.2 (2.2)
0:10.2 (0.0)0.7 (0.0)12.5 (0.2)19.1 (1.3)8.1 (0.8)0.9 (0.0)713.5 (7.9)62.7 (3.1)79.4 (1.2)
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Chiguano-Tapia, B.; Diaz, E.; de la Rubia, M.A.; Mohedano, A.F. Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver. Sustainability 2025, 17, 5022. https://doi.org/10.3390/su17115022

AMA Style

Chiguano-Tapia B, Diaz E, de la Rubia MA, Mohedano AF. Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver. Sustainability. 2025; 17(11):5022. https://doi.org/10.3390/su17115022

Chicago/Turabian Style

Chiguano-Tapia, Bryan, Elena Diaz, M. Angeles de la Rubia, and Angel F. Mohedano. 2025. "Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver" Sustainability 17, no. 11: 5022. https://doi.org/10.3390/su17115022

APA Style

Chiguano-Tapia, B., Diaz, E., de la Rubia, M. A., & Mohedano, A. F. (2025). Co-Hydrothermal Carbonization of Swine Manure and Soybean Hulls: Synergistic Effects on the Potential Use of Hydrochar as a Biofuel and Soil Improver. Sustainability, 17(11), 5022. https://doi.org/10.3390/su17115022

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