Advancing Biochar Applications: A Review of Production Processes, Analytical Methods, Decision Criteria, and Pathways for Scalability and Certification

Abstract

This article reviews biochar production and its potential applications across various sectors, including agriculture, environmental remediation, and energy storage. It emphasizes the critical role of feedstock source and process parameters, such as residence time, heating rate, and temperature, in determining biochar’s properties. Although extensive research has been conducted on the physical and chemical properties of biochar, there remains a significant gap in evaluating its practical applications. This paper emphasizes the role of Multi-Criteria Decision Analysis (MCDA) as a valuable tool for optimizing both biochar production and its application strategies. By exploring scholarly articles and patents, it offers insights into biochar production techniques, characterization methods, and the importance of process optimization for sustainable and efficient biochar use in real-world scenarios.

1. Introduction

Carbonaceous materials have long played a critical role in various human activities. Defined by their high carbon content (typically exceeding 60% by weight), these materials are produced through the thermal processing of organic substances [1]. Their origins predominantly lie in vegetal sources, but they can also be derived from animal byproducts, agricultural residues, and even urban waste streams, such as sludge and municipal solid waste [1,2].

Commonly known as coal, carbonaceous materials have been extensively studied and have reached a crucial stage of development, serving as precursors for widely used materials such as activated carbon. And with exceptional adsorption capabilities in both liquid and gas phases, these materials are highly versatile and can be tailored for specific applications, similar to traditional coal [3,4].

Biochar has garnered substantial attention in recent research. The escalating technology readiness level (TRL) of thermochemical conversion routes has significantly contributed to the increasing interest in biochar, primarily for its remarkable versatility across various applications [5]. The multifaceted potential of biochar in different domains underscores its significance in contemporary studies, and ongoing research continues to unveil more opportunities for its sustainable and effective utilization in various applications.

Biochar can be produced through various thermochemical processes under controlled atmospheres (inert or oxygen-lean conditions) and operational parameters (temperature, heating rate, and treatment time). Each process yields specific outcome products (

Table 1. Thermal process temperature and its products.

Thermal Process	Temperature (°C)	Product
Torrefaction	200–300	Low-temperature biochar
Pyrolysis	300–800	Biochar + Bio-oil + Biogas
Gasification	600–1200	Synthesis gas + Biochar

). For instance, torrefaction (200–300 °C) primarily focuses on low-temperature biochar production [6,7,8,9], pyrolysis (300–800 °C) generates biochar along with gaseous and liquid phases [10,11,12], while gasification (600–1200 °C) is designed to produce syngas as the main output [13,14,15,16].

In addition, a crucial aspect of biochar production and related applications is its feedstock source, where its constituents will significantly influence the efficiency of the thermochemical process (given different degradation rates from each constituent) [17]. Therefore, different processes and related operation conditions will result in biochars with numerous physical–chemical characteristics, which must be assessed and assigned to a specific application.

One key application of biochar is as a soil amendment [18,19,20], where its high adsorption capacity [21] enhances soil fertility and removes heavy metal contaminants, thereby improving overall soil quality [22]. Additionally, biochar can serve as an energy source, such as biofuel [23,24,25], and the gaseous by-products can be utilized as a hydrogen source, liquid products as substitutes for fossil fuels, and solid products as a replacement for coal [26,27].

Biochar supports various catalysts based on noble, non-noble, and bimetallic metals. It is widely applied in biorefinery processes such as esterification, hydrogenation, catalytic reform, pyrolysis, Fisher–Tropsch synthesis, and electrocatalysis [28,29]. Additionally, biochar’s adsorption properties [30,31,32], especially when activated or functionalized [33,34], make it useful for wastewater treatment [35,36,37]. Lignocellulosic biomass, with its high lignin content, is particularly suited for biochar production, supporting carbon sequestration. Lignin contributes to biochar’s stability, making it highly suitable for carbon sequestration [38]. Beyond these uses, biochar is also effective in pollutant removal, catalysis, and energy storage, making it a candidate for electrodes in supercapacitors and batteries due to its porosity and conductivity. Furthermore, biochar can be incorporated into melamine foams for efficient oil absorption, aiding environmental cleanup efforts [39]. Its adsorption capacity and electrochemical properties are valuable for applications in environmental monitoring, biosensing, and electrochemical energy storage [40].

The ongoing industrial focus has been integrating sustainable production processes, whether in conventional industrial environments (readapting parts of already established production lines) or in raw materials production, such as the agricultural sector (where the relationship with the food, materials, and energy is intrinsic). Therefore, searching for new biomaterials designed around environmental impact mitigation issues attracts research development. Biochar is an example since it can be produced from renewable raw materials (biomass) and residues, for example, agricultural (straw, rusk, bark, bagasse, among others) and urban solids (municipal solid waste, pruning tree, sludge).

In the agro-industrial context, integrating its residue into a new, economically viable conversion route is desirable. It aligns with biorefinery and circular thinking, pursuing minimizing of the disposal of non-productive renewable residue and maximizing conversion process efficiency and product possibilities. An entire production chain can benefit from the ‘relocation’ of its residues. Figure 1 exemplifies such a concept, depicting possibilities and interactions between the correct application of recovered agro-industrial residues into new routes.

Figure 1 presents the coffee processing case. Its leading destination is the production of soluble coffee (food industry). Therefore, the spent coffee grounds are a by-product of this process (here, exemplifying the various other agricultural processes that generate residues). As lignocellulosic biomass has numerous possibilities, the pyrolysis process is highlighted as one of the possible conversion routes.

Therefore, it is essential to acknowledge that biochar finds utility in various contexts and applications, making it crucial to assess production process conditions and the viability of specific biomass for this purpose. Hence, specific questions arise concerning understanding the biochar formation process and its application. A fundamental aspect revolves around identifying the necessary criteria to evaluate the essential biochar properties. Additionally, one may wonder if standardized parameters apply to all biochar types. Another significant aspect to consider is the impact of biomass constituents on biochar formation and whether the properties of biochar undergo significant changes depending on the biomass origin. These questions raise a critical discussion intricately linked to investigating biochar properties, especially concerning its stability in the face of different processes to which it may be applied. Addressing these queries can significantly optimize biochar production and utilization while ensuring its efficacy and environmental benefits.

This work introduces the critical parameters for evaluating biochar production and explores how its destination/application can significantly influence these aspects. The study also discusses the evolution of biochar research over the past years through a prospective approach. Moreover, the study’s originality relies on discussing the linkage between experimental assessments and MCDA to optimize the pyrolysis process and/or set the desirable application based on process conditions and biochar properties.

Prospective Study on Biochar Research

The prospective study of a subject related to a specific technology helps understand the maturity and development level of a particular product or research segment [43,44]. Technological prospecting is usually carried out in three segments: a search for scientific articles/publications, patents, and/or products (focused on the technology already consolidated in the market). Therefore, the present study’s focus was to evaluate the tendency of research related to biochar in the last 20 years. By analyzing the wealth of information gathered from the scientific literature, patents, and existing biochar products, a comprehensive understanding of the advancements and potential future directions in this field was sought. This investigation sheds light on the current state of biochar research and provides valuable insights for future innovation and development.

Web of Science was used for searches on the topic and associated with some related terms. The search period was defined between 2000 and 2022 with the terms shown in

Table 2. Keywords used for prospective study based on publications.

Search	Keywords	Boolean Operator a	Keywords	Boolean Operator	Keywords	Results
1	biochar	AND	-	-	-	29.645
2	biochar	AND	pyrolys *	-	-	11.417
3	biochar	AND	gasificat *	-	-	1.570
4	biochar	AND	Soil	-	-	13.954
5	biochar	AND	catalys *	-	-	2.504
6	biochar	AND	Water	-	-	12.264
7 *	biochar	AND	Water	AND	treatment	4.783
8	biochar	AND	wastewater	-	-	3.486
9	#7	OR	#8	-	-	6.901

^a Boolean operator: AND—restricts the searches to documents that necessarily have both terms; OR—searches for terms, but without the need to appear together (in this case, it was used to join searches 7 and 8, removing the repetitions). * Biochar AND (water AND treatment).

, searched in the title, abstract, and keywords of these documents. The terms were associated with biochar, given its relation to the primary conversion processes (pyrolysis and gasification) and two possible applications (soils and catalysis) chosen as examples to be addressed herein.

Figure 2 illustrates the number of publications per year. In addition, further refinement of the results can be accomplished through the Web of Science platform, enabling classification based on research fields and the countries with the highest publication rates, alongside various other classifications. The research field most closely associated with biochar is Environmental Sciences/Ecology, boasting 4430 articles. It is closely followed by Agriculture, with 2982 publications, and Engineering, which has contributed 2746 articles to the corpus. Among the countries at the forefront of biochar research, China leads the way with a remarkable 4470 articles, firmly establishing itself as a prominent contributor in this domain.

The United States secures the second position with 2264 publications, demonstrating its significant engagement in biochar-related studies. Australia also makes a substantial impact, with 807 articles showcasing its dedication to this area of research. Biochar is associated with research in environmental sciences, ecology, and agriculture precisely because of its widespread application and greater impact in current discussions on soil sustainability. China is emerging as one of the top publishing countries in the area, mainly due to its excellent agricultural capabilities (especially for rice, corn, and wheat) and new technologies associated with reducing industrial environmental impacts [45].

Established in 2010, the Chinese Biochar Network (CNB) has played a key role in advancing discussions on biochar within both academia and industry in China [45]. Alongside other organizations, CNB has also been instrumental in defining classification parameters for raw materials and biochar products deemed suitable for agricultural applications. Notably, the International Biochar Initiative (IBI) and the European Biochar Foundation (EBC) have contributed significantly to these efforts [46].

Similar to China, the United States has a strong agricultural sector and actively seeks innovative technologies to boost productivity while reducing environmental impacts, particularly greenhouse gas emissions. The US Biochar Initiative (USBI) has been pivotal in this context, expanding its activities and effectively coordinating the efforts of biochar producers nationwide. Additionally, the US has attracted investments in pilot plants dedicated to biochar production, focusing on three key objectives: soil enhancement, carbon sequestration, and greenhouse gas mitigation. These collective efforts represent a significant step toward sustainable agriculture and environmental conservation [47].

An analysis of the results presented in Table 2 reveals that when biochar is linked to the term “soil”, it yields a total of 6.319 articles, with nearly 56% of all articles solely focusing on biochar. This number reiterates the significance of academic production in the field of soil amendment while also shedding light on research opportunities and gaps for carbon materials of higher value when applied to fine chemistry. Utilizing these materials as supports and platforms for catalyst production is particularly noteworthy in this context [48].

When comparing the number of articles, it becomes evident that technology development remains under constant research owing to its extensive applications. This question is primarily because ground applications are well established and adhere to highly advanced European standards. At the national level in Brazil, there seems to be no promising outlook for innovation projects, as they generally exhibit low success rates, with biobased innovations showing no significant improvement. As for product development, the literature consistently indicates a broad spectrum of failure rates, ranging from 30% to 80% [49].

2. Biochar Production

Biochar, also called pyrogenic carbon [50], black carbon [51], or charcoal, is a carbon-rich solid material typically associated with soil applications. It is produced through the thermochemical conversion of biomass under oxygen-lean or inert conditions, resulting in a stable material with unique physicochemical properties [52,53]. The atmospheric conditions facilitate the formation of a material with unique characteristics, driven by the absence of an oxidizing agent, which triggers the condensation of biomass components.

Biochar is primarily produced through pyrolysis, typically conducted at temperatures ranging from 300 to 700 °C, though higher temperatures may be employed depending on the reactor type [50]. In pyrolysis, three fractions are formed: a gas (non-condensable compounds), a liquid (bio-oil), and a solid (biochar). The proportion between these three fractions is influenced by experimental conditions such as temperature, heating rate, residence time, and pyrolysis type. The pyrolysis process can be classified into three main types:

Slow pyrolysis: Occurs at temperatures between 300–650 °C, with a residence time ranging from 5 min to 12 h (longer duration) and a heating rate of 10–30 °C min⁻¹. This process primarily yields a significant fraction of solids (biochar), accounting for 20–40% of the total products [50,53,54];

Fast pyrolysis: Takes place at higher temperatures (above 500 °C) with an extremely short residence time (1–2 s) and a rapid heating rate (>1000 °C min⁻¹). This method maximizes bio-oil production, with the gaseous phase comprising 50–70% of the output, while the liquid and solid residues account for 10–30% and 15–20%, respectively [46,54];

Flash pyrolysis: Operates at even higher temperatures (above 800 °C) with an ultra-short residence time (<0.5 s) and a heating rate exceeding 1000 °C min⁻¹ [46].

Understanding these different pyrolysis systems is essential for analyzing how process parameters influence the distribution of products. Figure 3 illustrates the three possible fractions resulting from biomass pyrolysis: (i) a gas phase consisting of non-condensable gases (H₂, CO, CO₂, CH₄, and other light hydrocarbons); (ii) a liquid phase composed of organic acids, water, and lower molecular weight tar; and (iii) a solid phase consisting of biochar [46]. As the process evolves and thermal exchange occurs with the biomass, understanding the mechanisms driving the formation of these fractions is crucial for evaluating their stability and viability [55]. The focus on biochar formation underscores the objective of creating a stable carbonaceous material with a precisely defined composition and well-established physicochemical properties, ensuring its suitability for targeted applications.

Figure 3 depicts the mechanisms that drive the formation of each fraction, highlighting their connection to the intrinsic composition of biomass, as detailed by Collard and Blin [17]. During the thermal degradation process, the inter- and intramolecular interactions within cellulose, hemicelluloses, and lignin undergo rearrangement and decomposition, leading to volatile compounds and alterations in the solid structure. This stage corresponds to the primary mechanisms of decomposition. Subsequently, some volatile compounds and primary products may undergo recombination, a process referred to as secondary mechanisms.

In the primary mechanism, three main segments are highlighted during pyrolysis: char formation, depolymerization, and fragmentation. It is worth pointing out and highlighting that, just as monomers of the polymeric structure can break (depolymerization), chain fragmentation can occur, forming non-condensable gases (fragmentation). The resulting solid structure will be rearranged, favoring the formation of a more stable aromatic polycyclic structure with the concomitant release of low-molecular-weight compounds and water [17,55]. The reaction temperature will have a significant effect in this sense. Some authors suggest that the optimum temperature of the process is around 450 °C, being flexible in an area that goes from 300–500 °C to reach good biochar yields [46].

Steiner [57] points out that temperature plays a central role in the final characteristics of biochar. While residence time and heating rate are important complementary factors in the process, numerous studies in the literature evaluate biochar’s stability and physicochemical properties under varying temperatures, often standardizing other variables to maintain consistent conditions [58,59]. Keiluweit et al. [51] proposed a diagram illustrating the transformation of biomass and its components into four types of char as temperature increases, as shown in Figure 4. Additionally, a gravimetric analysis categorizes the char fractions into five distinct regions, each representing unique characterization properties and potential applications.

The physicochemical properties relevant for Region I (unaltered plant materials) include moisture content and functional groups, information relevant only for torrefaction products. Region II is associated with transition char and may include data similar to those obtained from Region I, applicable for material with desirable functional properties, such as adsorption-inactivated char. Regions III and IV char materials present properties relevant for surface and pore sizes, as well as fixed carbon contents, and may be relevant for applications related to physical or chemical adsorption. Region V (Figure 4) presents a material with unstable resistance, which does not present interesting properties for char application [51].

3. Physicochemical Characterization of Biochar

The physicochemical characterization of a material involves investigating its fundamental properties related to its application or intended use. Standard protocols do not exist for all material types, making it necessary to establish some form of standardization to compare results and demonstrate effectiveness, as is the case with lignocellulosic biomasses. Although their primary constituents are known (cellulose, hemicelluloses and lignin), their chemical composition is variable since the proportion between these will change from species to species and depend on the part of the plant (leaves, branches, trunk, bark, bagasse, etc.) [60]. Groups like the National Renewable Energy Laboratory (NREL) [61] have established standard biomass analysis procedures that are a reference for several research groups, in addition to complementing some existing adapted procedures described by ASTM-certified institutions.

Similarly, biochar lacks universally standardized procedures for analysis and application across different countries. However, key organizations have taken the lead in promoting these standards. For example, the International Biochar Initiative (IBI) [62] has proposed essential physicochemical parameters that should be evaluated to ensure biochar’s suitability for soil applications. Similar organizations partner with IBI and share several analysis procedures [62]. Meanwhile, IBI is a signatory to the procedures described by organizations such as the International Standards Organization (ISO), ASTM International (ASTM), and the Institute of Electrical and Electronics Engineers (IEEE). Not all the characteristics of biochars are exploited by these procedures since they determine a specific application. Thus, other characterization techniques will be described to cover other studies contemplating biochars destined to different areas, such as wastewater treatment.

It is essential to highlight that the varied composition of biomasses, which depends on their type and origin, influences the composition of the derived biochar (logically if there is a comparison based on experimental conditions). It is, therefore, vital that the characterization process is conducted for any type of biochar due to the diversity of possible biomasses that have not yet been exploited for this purpose [60]. Thorough characterization of biochar is crucial for understanding its suitability in various applications. Ultimately, the specific characteristics of interest will depend on the intended use, emphasizing the need for tailored characterization.

3.1. Main Standard Biochar Characterization

The EBC described the leading techniques and requirements for biochar characterization in a document comparing its procedures with the points required by the IBI [62]. Both have a vision for applications directed to use on the ground, as they define it in their design, while concisely discussed in

Table 3. Overview of key biochar properties and their implications (modified from Xie et al. [4]).

Properties	Implications
Specific surface area (SSA)	High values of SSA are indicators of adsorption capabilities
Porosity	High values of porosity are indicators of adsorption capabilities
Pore volume/distribution	Pore volume and distribution are significant criteria for the adsorption/desorption capability and selectivity
Surface functional group	The superficial composition of biochar delimitates the application and adsorption capabilities
Water-holding capability	High water-holding capability values indicate water retention and delimit the applications of plant stress remediation
Ion exchange capability	Indicative of efficiency in remediation of soil nutrient leaching
Elemental composition	The elemental composition is an indicator of stability and possible remediation applications
High heating value (HHV)	Indication of combustion energy production. The greater the value, the better

.

The characterizations for biochar are also commonly required for their original raw materials. As with the immediate analyses where moisture, volatiles, ash, and fixed carbon are evaluated [63], these will be essential parameters in the processing of pyrolysis (also associated with the issue of forecasts regarding yield and process efficiency) and are also present among the analyzes performed with the biochar.

Biochar parameters include pH, organic carbon content, volatile compound content, ash content, elemental composition, nutrient content, plant-available nutrients, bulk density, pore volume, porosity, specific surface area, water-holding capacity, cation exchange capacity (CEC), iodine number, surface functional groups, sorption properties, nutrient release dynamics, and stability. Some of these parameters are challenging to assess.

Some characteristics, such as surface area, are mainly influenced by production conditions. The density and initial pore-size distribution of feedstock are also critical. Huge surface areas characterize activated carbon, and the pore-size distribution depends on the lignin or cellulose content of the feedstock. A superior lignin content is generally responsible for a macroporous structure, whereas a higher cellulose content yields a microporous structure [57].

3.2. Atomic H/C and O/C Ratios

The atomic hydrogen/carbon (H/C) and oxygen/carbon (O/C) ratios are indicators of biochar’s chemical stability and degree of aromaticity. A low H/C ratio suggests a material with high stability and fixed carbon content suitable for long-term carbon sequestration [4,64]. The O/C ratio is equally important for inferring surface reactivity and polarity, impacting biochar’s capacity to adsorb polar compounds in remediation applications [64].

According to the IBI and EBC, H/C and O/C can classify biochar samples. The classification based on H/C is preferable due to H’s direct experimental determination, whereas O is indirectly determined. IBI classifies biochar samples based on organic carbon contents (OC), being Class 1: OC ≥ 60%, Class 2: 30% ≤ OC < 60%, Class 3: 10% ≤ OC < 30%, and non-biochars OC < 10% [65]. The atomic H/C ratios might be used to predict pyrolytic temperatures, aromatic cluster sizes, and sorption properties [66]. However, O/C can be used to estimate carbon stability in biochar samples. Spokas [67] stated that the half-life of an O/C < 0.2 biochar would be >1000 years, for an O/C 0.2–0.6 biochar sample, the half-life would be 100–1000 years and for O/C > 0.6 the half-life would be <100 years. Table 4 shows the % C, H and O, the H/C and O/C.

Figure 5 presents the Van Krevelen diagram illustrating the standard regions of biomass and derived biochars based on Table 4. This diagram proves valuable in assessing the coalification and reaction pathways of the modified products [68]. It offers valuable insights into the reduction of polarity (O/C) and aromaticity (H/C), as well as the extent of reactivity (degradation of carboxyl and hydroxyl groups) [69,70]. The conversion of hemicelluloses (at temperatures ranging from 200 to 350 °C) involves a process of dehydration combined with the cleavage of weak linkages between small substituents and the leading polymer chains [71]. In the case of cellulose, dehydration reactions start at approximately 200 °C, releasing water as the primary product.

Table 4. Pyrolytic CHO elementary data for different biomass sources.

Feedstock/Biochar	%C	%H	%O	H/C *	O/C *	HHV	Ref.
Pinewood							[67]
PW	48.90	6.20	42.50	1.51	0.65	18.10 a/18.66 b
PW450	75.50	3.70	17.00	0.58	0.17	27.80 a/27.94 b
Timothy grass
TG	43.40	6.10	45.40	1.68	0.79	15.90 a/16.23 b
TG450	63.70	3.60	23.10	0.67	0.27	22.30 a/22.90 b
Wheat straw
WS	44.10	6.00	45.00	1.62	0.77	15.60 a/16.38 b
WS450	64.80	3.10	23.00	0.57	0.27	22.00 a/22.58 b
Pinewood-2							[68]
PW-2	48.50	5.92	45.16	1.45	0.70	21.18 b
PW450-2	71.80	3.94	22.66	0.65	0.24	28.08 b
PW600-2	84.66	2.81	10.25	0.40	0.09	31.80 b
PW800-2	89.70	1.24	3.61	0.16	0.03	31.81 b
Switchgrass
SW	45.58	5.45	45.65	1.43	0.75	20.67 b
SW450	66.54	3.43	15.31	0.61	0.17	26.34 b
SW600	71.52	2.53	5.39	0.42	0.06	26.81 b
SW800	71.62	1.16	4.85	0.19	0.05	25.64 b
Wheat straw-2							[58]
WS-2	45.53	3.56	42.53	0.93	0.70	17.30 b
WS300-2	61.48	2.73	19.61	0.53	0.24	22.34 b
WS400-2	64.18	1.78	13.93	0.33	0.16	22.90 b
WS500-2	67.39	1.01	7.35	0.18	0.08	22.35 b
WS600-2	65.34	0.52	10.77	0.09	0.12	22.63 b
Corn straw
CS	44.53	5.31	41.18	1.42	0.69	19.80 b
CS300	61.2	3.68	17.39	0.72	0.21	23.20 b
CS400	63.36	1.96	16.46	0.37	0.20	22.27 b
CS500	65.08	0.77	11.36	0.14	0.13	21.51 b
CS600	67.48	0.18	8.98	0.03	0.10	16.34 b
Rape straw	44.63	4.89	42.34	1.31	0.71	21.91 b
RP
RP300	61.8	3.54	17.95	0.68	0.22	23.66 b
RP400	63.74	1.91	13.48	0.36	0.16	22.61 b
RP500	66.96	0.87	9.46	0.15	0.11	22.45 b
RP600	67.85	0.18	7.89	0.03	0.09	22.99 b
Rice straw
RS	42.12	4.16	41.22	1.18	0.73	17.30 b
RS300	56.49	2.95	17.73	0.62	0.24	21.01 b
RS400	56.42	1.35	13.71	0.29	0.18	19.55 b
RS500	59.59	0.47	8.27	0.09	0.1	19.75 b
RS600	61.3	0.12	5.71	0.02	0.07	20.71 b
Brewers’ spent grain							[41]
BSG	44.72	6.86	44.66	1.83	0.75	17.06 a/17.87 b
BSG300	63.28	5.03	31.2	0.95	0.37	~24 a/23.55 b
BSG500	71.67	4.11	23.84	0.68	0.25	~27 a/26.19 b
BSG700	83.17	3.66	12.62	0.52	0.11	31.23 a/31.13 b
Grape seeds							[72]
GSs	45.00	6.99	44.40	0.16	0.99	18.18 b
GSs300_3h	51.00	5.57	39.20	0.11	0.77	18.97 b
GSs300_24h	61.70	3.10	29.80	0.05	0.48	20.49 b
Defatted grape seeds
DGSs	47.2	6.72	38.9	0.14	0.82	19.38 b
DGSs300_3h	57.8	5.08	28.8	0.09	0.5	22.16 b
DGSs300_24h	63.1	3.62	23.4	0.06	0.37	22.69 b
WSs300_3h	51.00	5.57	39.20	0.11	0.77	18.97
Wood stems
WGSs	51.5	6.01	38.1	0.12	0.77	19.94 b
WSs300_3h	63.9	5.98	24.7	0.09	0.52	26.11 b
WSs300_24h	68.8	5.62	19.5	0.06	0.39	28.04 b
Whole grape seeds
DGSs	51.2	6.08	39.2	0.12	0.77	19.77 b
DSs300_3h	58.9	5.28	30.6	0.09	0.52	22.53 b
DSs300_24h	64.9	3.72	25.00	0.06	0.39	23.19 b
Oil palm trunk							[73]
OPT	45.79	6.15	46.33	0.13	1.01	16.96 b
OPT500	77.53	3.45	18.63	0.04	0.24	28.01 b
OPT550	79.35	1.87	16.65	0.02	0.21	26.68 b
OPT600	82.02	1.59	14.22	0.02	0.17	27.55 b
Oil palm fronds
OPF	44.95	5.89	48.71	0.13	1.08	15.94 b
OPF500	75.07	2.41	22.00	0.03	0.29	25.19 b
OPF550	76.41	1.8	21.31	0.02	0.28	24.88 b
OPF600	78.34	1.91	19.19	0.02	0.24	26.01 b
Rubberwood awdust
RWS	47.55	6.22	45.91	0.13	0.97	17.71 b
RWS500	76.65	2.71	20.21	0.04	0.26	26.42 b
RWS550	78.59	3.3	17.63	0.04	0.22	28.31 b
RWS600	80.19	2.09	17.35	0.03	0.22	27.17 b

* The ratios were recalculated from the CHN values of the references using the following molar mass values: C—12.0107 g mol⁻¹; H—1.0078 g mol⁻¹; O—15.999 g mol⁻¹. ^a Experimental; ^b theoretical (calculated by modified Dulong’s formula [74]).

Table 4. Pyrolytic CHO elementary data for different biomass sources.

Feedstock/Biochar	%C	%H	%O	H/C *	O/C *	HHV	Ref.
Pinewood							[67]
PW	48.90	6.20	42.50	1.51	0.65	18.10 a/18.66 b
PW450	75.50	3.70	17.00	0.58	0.17	27.80 a/27.94 b
Timothy grass
TG	43.40	6.10	45.40	1.68	0.79	15.90 a/16.23 b
TG450	63.70	3.60	23.10	0.67	0.27	22.30 a/22.90 b
Wheat straw
WS	44.10	6.00	45.00	1.62	0.77	15.60 a/16.38 b
WS450	64.80	3.10	23.00	0.57	0.27	22.00 a/22.58 b
Pinewood-2							[68]
PW-2	48.50	5.92	45.16	1.45	0.70	21.18 b
PW450-2	71.80	3.94	22.66	0.65	0.24	28.08 b
PW600-2	84.66	2.81	10.25	0.40	0.09	31.80 b
PW800-2	89.70	1.24	3.61	0.16	0.03	31.81 b
Switchgrass
SW	45.58	5.45	45.65	1.43	0.75	20.67 b
SW450	66.54	3.43	15.31	0.61	0.17	26.34 b
SW600	71.52	2.53	5.39	0.42	0.06	26.81 b
SW800	71.62	1.16	4.85	0.19	0.05	25.64 b
Wheat straw-2							[58]
WS-2	45.53	3.56	42.53	0.93	0.70	17.30 b
WS300-2	61.48	2.73	19.61	0.53	0.24	22.34 b
WS400-2	64.18	1.78	13.93	0.33	0.16	22.90 b
WS500-2	67.39	1.01	7.35	0.18	0.08	22.35 b
WS600-2	65.34	0.52	10.77	0.09	0.12	22.63 b
Corn straw
CS	44.53	5.31	41.18	1.42	0.69	19.80 b
CS300	61.2	3.68	17.39	0.72	0.21	23.20 b
CS400	63.36	1.96	16.46	0.37	0.20	22.27 b
CS500	65.08	0.77	11.36	0.14	0.13	21.51 b
CS600	67.48	0.18	8.98	0.03	0.10	16.34 b
Rape straw	44.63	4.89	42.34	1.31	0.71	21.91 b
RP
RP300	61.8	3.54	17.95	0.68	0.22	23.66 b
RP400	63.74	1.91	13.48	0.36	0.16	22.61 b
RP500	66.96	0.87	9.46	0.15	0.11	22.45 b
RP600	67.85	0.18	7.89	0.03	0.09	22.99 b
Rice straw
RS	42.12	4.16	41.22	1.18	0.73	17.30 b
RS300	56.49	2.95	17.73	0.62	0.24	21.01 b
RS400	56.42	1.35	13.71	0.29	0.18	19.55 b
RS500	59.59	0.47	8.27	0.09	0.1	19.75 b
RS600	61.3	0.12	5.71	0.02	0.07	20.71 b
Brewers’ spent grain							[41]
BSG	44.72	6.86	44.66	1.83	0.75	17.06 a/17.87 b
BSG300	63.28	5.03	31.2	0.95	0.37	~24 a/23.55 b
BSG500	71.67	4.11	23.84	0.68	0.25	~27 a/26.19 b
BSG700	83.17	3.66	12.62	0.52	0.11	31.23 a/31.13 b
Grape seeds							[72]
GSs	45.00	6.99	44.40	0.16	0.99	18.18 b
GSs300_3h	51.00	5.57	39.20	0.11	0.77	18.97 b
GSs300_24h	61.70	3.10	29.80	0.05	0.48	20.49 b
Defatted grape seeds
DGSs	47.2	6.72	38.9	0.14	0.82	19.38 b
DGSs300_3h	57.8	5.08	28.8	0.09	0.5	22.16 b
DGSs300_24h	63.1	3.62	23.4	0.06	0.37	22.69 b
WSs300_3h	51.00	5.57	39.20	0.11	0.77	18.97
Wood stems
WGSs	51.5	6.01	38.1	0.12	0.77	19.94 b
WSs300_3h	63.9	5.98	24.7	0.09	0.52	26.11 b
WSs300_24h	68.8	5.62	19.5	0.06	0.39	28.04 b
Whole grape seeds
DGSs	51.2	6.08	39.2	0.12	0.77	19.77 b
DSs300_3h	58.9	5.28	30.6	0.09	0.52	22.53 b
DSs300_24h	64.9	3.72	25.00	0.06	0.39	23.19 b
Oil palm trunk							[73]
OPT	45.79	6.15	46.33	0.13	1.01	16.96 b
OPT500	77.53	3.45	18.63	0.04	0.24	28.01 b
OPT550	79.35	1.87	16.65	0.02	0.21	26.68 b
OPT600	82.02	1.59	14.22	0.02	0.17	27.55 b
Oil palm fronds
OPF	44.95	5.89	48.71	0.13	1.08	15.94 b
OPF500	75.07	2.41	22.00	0.03	0.29	25.19 b
OPF550	76.41	1.8	21.31	0.02	0.28	24.88 b
OPF600	78.34	1.91	19.19	0.02	0.24	26.01 b
Rubberwood awdust
RWS	47.55	6.22	45.91	0.13	0.97	17.71 b
RWS500	76.65	2.71	20.21	0.04	0.26	26.42 b
RWS550	78.59	3.3	17.63	0.04	0.22	28.31 b
RWS600	80.19	2.09	17.35	0.03	0.22	27.17 b

* The ratios were recalculated from the CHN values of the references using the following molar mass values: C—12.0107 g mol⁻¹; H—1.0078 g mol⁻¹; O—15.999 g mol⁻¹. ^a Experimental; ^b theoretical (calculated by modified Dulong’s formula [74]).

As the temperature rises to approximately 280 °C, the reactions release CO, CO₂, and small organic compounds [75]. On the other hand, lignin undergoes a gradual and partial degradation during torrefaction, commencing from around 200 °C. Its dehydration reactions involving the hydroxyl substituents of the phenolic rings take place at lower temperatures [76]. Understanding these thermal conversion processes and their corresponding temperature ranges is crucial for optimizing pyrolysis and tailoring the properties of the resulting biochar.

3.3. Calorific Value

Determining the calorific value is essential when biochar is considered for energy applications, such as a solid fuel. This value indicates the amount of energy released during combustion and reflects the energy efficiency of biochar [4,64].

The increase in temperature in biochar production decreases the H/C ratio due to the condensation of carbonaceous structures and increases the calorific value of this material. The high heating value (HHV) indicates an upper limit for the available thermal energy produced on a given combustion and stands as the main comparison parameter for the combustion of biochar samples [4]. Padilla et al. [77] characterized the biochar produced from eucalyptus bark at temperatures of 300–500 °C. Their study reported increased calorific value from 20.88–22.94 MJ kg⁻¹, respectively.

The use of coal as a source of increasing the energy density of a product is a resource that has also been explored for biochar. Since the material has a calorific value greater than its original biomass, its use for energy will be much more profitable with the concomitant possibility of generating other derivatives that may also integrate the fuel chain, as is the case of bio-oil production.

In conclusion, the production method of biochar, particularly the pyrolysis temperature, plays a crucial role in determining its elemental composition, specifically in terms of hydrogen, carbon, nitrogen, and oxygen content, directly influencing the higher heating value (HHV) [78]. As the pyrolysis temperature increases, there is a notable rise in the carbon content of biochar, while hydrogen and oxygen contents decrease, reflecting the development of more stable aromatic carbon structures. This transformation is reflected in the reduction of the O/C and H/C ratios, which are strongly correlated with an increase in the higher heating value (HHV) [59,78]. As a result, the biochar becomes more suitable as an energy source. Studies have shown that biochars produced at higher pyrolysis temperatures, typically within the range of 600–800 °C, exhibit a greater HHVs due to increased carbonization and enhanced thermal stability. These characteristics make it ideal for energy applications requiring high fuel efficiency [59]. Thus, optimizing the pyrolysis temperature to around 600–800 °C appears to be the most effective condition for maximizing the HHV of biochar, offering a renewable and efficient alternative to conventional energy sources [59,78].

3.4. Spectroscopic Analysis

Spectroscopic analyses are commonly used to analyze the profile of the molecular bonding/interaction types in each compound. For the biochar, this analysis is combined with the comparison with its original biomass since, in the degradation process, some functional groups are disrupted, and chemical bonds of the C-C type associated with the formation of the carbonaceous material are formed more intensely. Such a principle will also be essential to corroborate a process temperature gradient (if addressed) since there is an increasingly intense degradation of the original compounds with increasing temperature. Techniques such as FTIR and Raman Spectroscopy are crucial for identifying functional groups and the structural order of carbon in biochar.

3.4.1. Fourier Transform Infrared (FTIR) Spectroscopy

FTIR is an absorption and vibrational technique in which biochar surface functional groups absorb the energy of a beam and emit the energy at a different frequency. After data processing (Fourier transformation), it is possible to identify characteristic bands for different functional groups, such as hydroxyl, carbonyl, esters, and aromatic rings, allowing the direct study of the biochar decomposition degree (Figure 6) [79,80].

Table 5. Main stretches for biochar FTIR.

Band Position (cm−1)	Component	Ref.
480, 592, 652	Aromatic deforming rings, C-C stretching	[74]
782, 840, 885	C-H, aromatic hydrogen
1097	C-O-C symmetric stretching
1618	Aromatic C-C ring stretching
1709	Phenyl ring substitution overtones
2950	Alkyl/aliphatic C-H stretching
3544	-OH stretching
3642	-OH stretching, alcohols, phenols

shows the leading characteristic of stretching.

The conventional FTIR analysis of biochar typically involves preparing potassium bromide (KBr) pastilles for examination with standard FTIR equipment. Alternatively, biochar can be processed into KBr pellets for Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), applied to a crystal for Attenuated Total Reflectance (ATR-FTIR), analyzed using photoacoustic cells in Fourier Transform Infrared Photoacoustic Spectroscopy (FTIR-PAS) or subjected to Synchrotron Radiation for Synchrotron-based Fourier Transform Infrared Spectroscopy (SR-FTIR) [64,81,82,83,84].

3.4.2. Raman Spectroscopy

Raman Spectroscopy is a scattering and vibrational technique on which a beam is applied over a sample and dispersed in frequencies related to the photon’s vibrational states post-exposition (Figure 7). The scattering is elastic (input and output frequencies are the same—Rayleigh scattering) and inelastic (output frequencies are higher or lower than input—Stokes and Anti-Stokes lines). The Stokes and Anti-Stokes lines (given in cm⁻¹) compose a quantifiable fingerprint pattern for the molecule, or in the case of biochar materials, the fingerprint of a specific point on the surface [64,85].

Therefore, Raman Spectroscopy might be used to identify functional groups and carbonaceous structure aspects, such as amorphous and crystalline, as described by Wu et al. [86]. Although FTIR and Raman analysis are complementary techniques for biochar study, Raman has higher sensitivity, suffers less interference from water, and requires minimal sample preparation, although at a higher cost, limiting its adoption [64,85,87].

The Raman spectroscopy as a characterization technique for crystalline and amorphous carbon materials is directly dependent on two modes, band D and band G. The D band over 1350 cm⁻¹ is commonly assigned to K-point phonons with A_1g symmetry. At the same time, the G band between 1580 and 1600 cm⁻¹ is usually attributed to E_2g symmetry zone center phonons. Amorphous carbon material comparisons using fixed values of $\lambda$ and integrated D ($I_d$) and G ($I_g$) bands allow for ordering identification. Consequently, for amorphous carbonaceous samples, $I_d/I_g$ increases with increasing ordering or crystallinity [87,88].

3.5. Thermogravimetric Analysis (TGA)

Biochar’s structural and stability properties are critically important for agricultural applications and carbon sequestration. Thermogravimetric Analysis (TGA) is applied to assess biochar’s thermal stability and fixed carbon content, indicating its resistance to degradation and potential for long-term soil retention [4,64].

TGA is a technique where a sample is heated from room temperature until 1000 °C at a steady flow of nitrogen, helium or air, and a mass loss dynamics is registered. The interpretation of the final TGA pattern allows for the assessment of structural stability, moisture content, inorganic content (ashes content), and loss of characteristic functional groups. It may also be used for kinetic studies and feedstock pyrolytic temperature assessment. Biochar materials prepared below 400 °C are less stable than those prepared above 400 °C due to the greater stability of cyclic carbon chains formed at higher temperatures. Degradation temperatures vary based on the material stability, but commonly, degradations between 100 and 200 °C are assigned to loss of water, in the range of 200–600 °C to volatile matter, and the residue past 600 °C to inorganic materials, if under air flow, or inorganic materials and graphite, if under He or N₂ flow [64,89]. Figure 8 presents the weight loss patterns for residual brewer spent grains biomass and biochar samples [41].

3.6. Scanning Electron Microscopy (SEM)

SEM is a morphological technique for the characterization of surfaces. It is another essential technique for analyzing biochar intended for soil improvement, as it allows for visualization of surface morphology and porous structure. When applied to biochar, it is possible to identify pore arrangement and distribution. The biochar texture and distribution observed through SEM are important parameters for evaluating biochar’s ability to retain substances [4,64]. In direct contrast, SEM coupled with Energy Dispersive X-Ray spectroscopy (EDX) allows for the localized estimation of surface elemental composition and, therefore, can be used for elemental mapping post-adsorption, as shown in Figure 9 [90,91,92].

3.7. Böehm Titration—Functional Group Identification and Acidity Analysis

The Böehm titration technique, first proposed by Böehm in 1994 [93], is a characterization method for quantifying and identifying oxygenated groups on the surface of carbonaceous materials, such as biochar. The method is based on the acidity difference between oxygenated groups. Carboxylic acids, phenols, and lactic groups are neutralized by sodium hydroxide, carboxylic acids and lactic groups are neutralized by sodium carbonate, and carboxylic acid groups are neutralized by sodium bicarbonate. The biochar samples undergo a pre-treatment by washing to remove superficial inorganic compounds to avoid interferences, while the system itself is degassed with Ar or N₂, reducing the effect of dissolved CO₂ [94].

3.8. Solid-State Nuclear Magnetic Resonance (ssNMR)

The ssNMR of ¹³C is a spectroscopic technique used to study the structural compositions of carbonaceous materials. On biochar, ¹³C ssNMR allows for the determination of carbon-containing compounds and groups, such as aliphatic and aromatic hydrocarbons, and phenolic and methoxyl groups, which is particularly relevant for understanding structural stability [64]. Moreover, ssNMR and FTIR are commonly and simultaneously used to identify structure and functional biochar groups [95,96].

3.9. X-Ray Photoelectron Spectroscopy (XPS)

XPS analysis allows for the identification of compounds and bonds on a particular point of the biochar surface at a depth of up to 10 nm. It is an essential technique for determining heavy metals and may also be used to determine elements on the biochar surface, such as Fe, Mn, Si, Al, Na, N, and O [97,98,99].

3.10. X-Ray Diffraction (XRD)

X-ray diffraction (XRD) has proven to be an invaluable tool in the structural characterization of biochar, particularly due to its ability to provide insight into the crystallinity of the organic and inorganic components present [100,101]. While biochars are generally considered largely amorphous materials, the presence of mineral phases and crystalline structures, especially after modification or in composite forms, can be effectively identified and characterized using XRD [102]. This ability to distinguish between amorphous carbon and crystalline impurities or additives makes XRD highly relevant for biochar studies, particularly in applications requiring specific structural properties, such as adsorption, catalysis, or soil amendment [103,104].

For instance, XRD was employed to analyze biochars derived from maize and pigeonpea stalks, demonstrating that higher pyrolysis temperatures reduced the amorphous organic phase and increased crystalline structures [103]. This structural transition is critical as it impacts the biochar’s physical properties, such as porosity and stability, which are essential for its application in soil carbon sequestration and environmental remediation.

Moreover, in studies focusing on biochar composites, such as the novel copper–biochar nanocomposite for lead adsorption, XRD was crucial for confirming the presence of zerovalent copper in the composite [100]. The identification of crystalline copper phases validated the composite’s successful synthesis and directly correlated with its enhanced adsorptive capabilities. Similarly, in biochar doped with iron, XRD was used to confirm the formation of iron oxide phases, which enhanced the material’s performance as a slow-release fertilizer [101].

Additionally, the incorporation of magnetic components into biochar, as seen in the synthesis of magnetic rice straw biochar, further underscores the importance of XRD in biochar research. Here, XRD was utilized to identify magnetic phases (e.g., Fe₃O₄), critical for the material’s magnetic properties and adsorption capabilities [104]. The crystalline nature of these phases directly impacts the biochar’s performance in applications such as pollutant removal, as the magnetic property allows for easy recovery from aqueous solutions.

Overall, XRD is helpful in the characterization of biochar, particularly when it is modified or functionalized for specific applications. It provides insights into the material’s crystallinity and can help correlate structural features with functional properties. Thus, using XRD in biochar studies is useful for comprehensive material characterization and optimization for environmental applications.

4. Biochar Standard Application

4.1. Soil Amendment

Figure 10 presents a schematic diagram illustrating biochar production and its detailed characterization, emphasizing its effectiveness as a soil amendment and its contribution to sustainable agriculture, pollutant removal, and carbon sequestration.

Effective soil amendments should exhibit strong binding capabilities, ensure environmental safety, and maintain soil structure, fertility, and overall ecosystem health [21]. Biochar, composed of a mix of labile aliphatic carbon, stable aromatic carbon, and mineral ash, has emerged as a sustainable and practical solution. Its application is widely regarded as a promising strategy for enhancing soil quality and facilitating the removal of heavy metal contaminants. Moreover, biochar has become a valuable soil amendment because it enhances soil properties, promotes sustainable agriculture, and mitigates environmental issues. Derived from the pyrolysis of biomass, higher temperatures (600–700 °C) generally produced biochar with lower hydrophobicity and greater recalcitrance, making it ideal for long-term soil enhancement [22], improving soil fertility, carbon sequestration, and overall agricultural productivity [50]. It may be argued that it is necessary for regions where naturally poor soils predominate, whose fertility and structure maintenance depend almost exclusively on the conservation of soil organic matter [22].

Various organic materials are suitable for biochar production for soil amendment, including agricultural residues, industrial byproducts, and municipal waste. In one study, the biomass used was the byproduct of acai fruit processing, specifically acai seeds, which were otherwise an environmental burden due to the large amount of waste generated in the Brazilian Amazon region [22]. Another commonly utilized biomass source is sugarcane bagasse and other agro-industrial wastes [50]. The addition of biochar to soils brings several agronomic benefits. First, it improves soil structure by increasing porosity, which enhances water retention and drainage, which is critical in tropical regions with sandy, nutrient-poor soils such as those in the Amazon.

A high surface area and abundant functional groups are favorable if high sorption potential (e.g., nutrient retention and adsorption of pesticides or toxic compounds) is needed. A larger particle size is advantageous if used as a bulking agent during organic waste composting. In contrast, powdery biochar might be better for other applications, such as the remediation of contaminated soils [50]. Biochar produced at higher temperatures has also shown an increased ability to retain nutrients like nitrogen and phosphorus, essential for crop growth [22,50]. Additionally, biochar application reduces soil acidity, particularly beneficial for the highly acidic soils typical of many tropical regions [22].

Moreover, ash-rich biochars may increase soil pH and EC and provide nutrients, whereas biochars rich in volatile matter may reduce nitrogen availability. Biochars with a high OC content and relatively low ash and volatile matter content (clean biochars) are versatile, ranging from potting media to bulking agents and soil amendment [50].

Another commonly used substance for soil amendment is zeolites [105]. While biochar and zeolite improve nitrogen retention and mitigate nutrient loss, biochar provides additional environmental benefits. Biochar is notably more effective at reducing pathogenic microorganisms, such as coliforms, in soil and leachate, which may be attributed to its porous structure that facilitates microbial attachment and filtration [105]. Additionally, biochar enhances soil water-holding capacity, thereby reducing leachate volume. Both biochar and zeolite are effective soil amendments for nutrient retention and environmental management; however, biochar’s broader benefits, including improved microbial control and water retention, make it particularly advantageous for sustainable agricultural practices [105].

4.2. Catalysis

Applying biochar as catalysts (catalyst support or heterogenic catalyst) is directly dependent on the mass transport phenomena. Functional sites must be accessible for subtracts while successfully diffusing the products from the catalytic interface. The control of access and diffusion is accomplished by the pore size/volume control, surface activation/functionalization, and distribution of active sites. Thus, information about these characteristics is the most important properties related to biochar as a catalyst [29,48].

Biochar supports heterogeneous catalysts based on noble, non-noble, and bimetallic metals. In addition, it applies to biorefinery processes: esterification, transesterification, hydrogenation, catalytic reform, catalytic pyrolysis, hydrolysis, Fisher–Tropsch reactions, isomerization, electrocatalysis and others [28,29], as can be seen in Figure 11. Yan and coworkers [106] showed the effectivity of iron nanoparticles, supported over biochar, for converting syngas into liquid hydrocarbons by Fisher–Tropsch synthesis. Biochar has proven to be an effective solid acid catalyst for the transesterification of oils and esterification of oleic acid, owing to its high density of sulfonic acid groups, hydrophobicity, and strong active sites [107,108].

Biochar was also demonstrated to be an efficient and low-cost catalyst for removing tar in the catalytic reform process of pyrolytic biomass volatiles [109]. Biochar catalysts presented great catalytic activity in producing phenols and hydrogen-rich gas from catalytic pyrolysis of Sargassum brown macroalgae [110]. Biochar was studied by Zhao and coworkers [111] as an activator for peroxymonosulfate and applied to the degradation of organic contaminants in water. Therefore, biochar-based catalysts represent a cost effective and catalytic active possibility for various applications [29].

The characterization techniques of biochar as catalysts and their application are directly related. The results of the direct comparison are often limited due to non-standard methodologies. For instance, in biodiesel production using sulfonated biochar, Cao and collaborators demonstrated the application of sulfonated corncob biochar to the esterification of oleic acid [112]. The biochar catalyst was characterized by elemental composition, BET, XRD, FT-IR, XPS, TGA, and SEM + EDS, while the –SO₃H content was determined by Boehm tritation. Corrêa et al. demonstrated the activity of murumuru kernel shell sulfonated biochar catalyst on the esterification of oleic acid [113]. The catalyst was characterized by SEM, EDS, CHN, FT-IR, Raman, TGA, and XPS. The –SO₃H content was determined by NaOH tritation.

Azman and collaborators showed the application of activated woodchip biochar on waste oil esterification/transesterification [114]. The biochar catalyst was characterized by XRD, TGA, and FESEM (Field Emission Scanning Electron Microscopy). Daimary and co-workers presented the application of potato peel alkaline biochar on the cooking waste oil esterification/transesterification [115]. The catalyst characterization methods included XRD, FTIR, TEM, EDX, and FESEM, while the catalyst basicity was determined using the Hammet indicator test.

Biochar offers benefits over traditional catalysts like zeolites in various reactions, particularly in biomass conversion and biodiesel production. For example, biochar’s inherent stability at high temperatures and ability to host macromolecules and facilitate larger-scale reactions like Diels–Alder make it especially effective in pyrolysis processes involving complex substrates. In the study by Singh et al. on biomass co-pyrolysis, biochar-based catalysts demonstrated superior selectivity for producing longer-chain olefins (C11–C20) compared to zeolites, which were more selective towards lighter olefins (C1–C10) [116]. Furthermore, biochar offers a large surface area enriched with functional groups, which enhances key interactions in biodiesel synthesis. Notably, a biochar–zeolite composite produced a higher biodiesel yield than zeolite alone, primarily because biochar facilitates both esterification and transesterification reactions in a single step [117].

Unlike zeolites, which are limited by smaller pore sizes and lower thermal stability under extreme conditions, biochar’s mesoporous structure supports better mass transfer and can accommodate a wider range of reaction types and substrates. This adaptability makes biochar an effective catalyst for sustainable fuel production, especially in reactions that require high durability and lower deactivation rates [117,118].

4.3. Biofuel

The biomass pyrolytic process, fast or slow, produces three main portions with energetic application. The gas phase might be used as a hydrogen source, the liquid phase as a fossil fuel substitute, and the solid phase (biochar) can be used as a substitute for coal [26,27]. Therefore, solid fuel sources for bioenergetics applications must be energetically dense, with proper fuel ratio, combustibility index, and high heating values (HHV) [4,6,119,120].

Thermo-chemical processes might enhance the quality of biochar as an energetic source. The main objective of such is to increase density while decreasing O/C and H/C ratios [26,121,122]. Some applications for biochar as a solid fuel are shown in Figure 12; here it is possible to identify the correlation between biochar preparation conditions and HHV.

Table 6. Biochar applications as fuel along source biomass and preparation conditions.

Source Biomass	Biochar Preparation Conditions	Optimum Biochar Characteristics and Combustibility Properties	Ref.
Orange peel	Pyrolysis Atmosphere: N2 Temp.: 500 °C HR: 5 °C min−1 Time: 60 min	Biochar O/C—0.12; biochar H/C—0.66; HHV—25.73 MJ kg−1; energy yield—47.52%; fuel ratio—5.87; thermal stability—0.85.	[126]
Sugarcane bagasse	Pyrolysis Atmosphere: N2 Temp.: 600 °C HR: 10 °C min−1 Time: 30 min	Biochar O/C—0.3; H/C—0.2; HHV—29.99 MJ kg−1; biochar yield—21.75%; fuel ratio—3.21; energy yield—36.32%.	[127]
Palm fiber	Pyrolysis Atmosphere: N2 Temp.: 700 °C HR: 5 °C min−1 Time: 120 min	Biochar O/C—0.23; biochar SSA—0.272 m2 g−1; HHV—26.77 MJ kg−1; biochar yield—28.37%; energy yield—45.72%.	[128]
Spent coffee grounds	Torrefaction Atmosphere: N2 Temp.: 300 °C Time: 30 min	Biochar O/C—0.39; biochar H/C—0.10; biochar SSA—0.524 m2 g−1; HHV—30.32 MJ kg−1; biochar yield—62%.	[129]
Poplar wood	Pyrolysis Atmosphere: N2 Temp.: 600 °C HR: 5–20 °C min−1	Biochar O/C—0.06; biochar H/C—0.025; HHV—32.73 MJ kg−1; biochar yield—24.3%; fuel ratio—6.86; combustibility index—1.9 s−1 °C−2; combustion characteristic index—3.3 s−2 °C−3	[130]
Sewage sludge	Pyrolysis Atmosphere: N2 Temp.: 450 °C HR: 10 °C min−1 Time: 30 min	Biochar O/C—0.19; biochar H/C—0.74; HHV—13.58 MJ kg−1; biochar yield—57.90%; fuel ratio—0.86; energy yield—42.04%; combustion index—0.59 106%−2 s−2 °C−3.	[2]
Sesame stalks	Torrefaction Atmosphere: N2 Temp.: 275 °C Time: 30 min	Biochar O/C—0.64; biochar H/C—1.25; HHV—20.5 MJ kg−1; biochar yield—76.25%; fuel ratio—0.52; energy yield—86.16%; bulk density—290.01 kg m−3.	[131]
Microalgae—Chlorella pyrenoidosa	Pyrolysis Atmosphere: N2 Temp.: 400–600 °C HR: 10 °C min−1 Time: 30 min	Biochar O/C—0.23; biochar H/C—0.055; HHV—17.15 MJ kg−1; biochar yield—51.23%; fuel ratio—1.72.	[132]
Camellia shell	Steam-torrefaction Atmosphere: N2 Temp.: 280 °C Time: 30 min	Biochar O/C—0.36; biochar H/C—0.07; biochar SSA—28.66 m2 g−1; HHV—24.76 MJ kg−1; biochar yield—50.45%; fuel ratio—1.04.	[133]

relates several biomass-derived biochar with combustibility properties. In other fuel-related applications, biochar as an electrode has been studied in the composition of solid oxide fuel cells with promising results [123,124]. One such method is the composition of anodes for lithium-ion batteries, where extensive surface area, porosity, and ion storage capabilities are desired [125].

4.4. Wastewater Treatment

An additional application is in wastewater treatment [35,36,37], made possible as a result of its adsorbing capabilities [30,31,32] when the biochar is activated and functionalized [33,34]. Wastewater treatment solutions based on biochar adsorption depend on the material’s capabilities. On the other hand, the biochar adsorption capabilities depend on parameters such as superficial area, pore volume, activation, and functionalization processes [33,134]. Therefore, a functionalized high surface area is desirable for higher adsorption values, while the pore volume can be tuned to assign selectivity to the process [33,34].

Figure 13 shows a schematic example of the use of biochar in wastewater treatment. For example, biochar derived from walnut shells has shown high efficiency in removing dyes and heavy metals from wastewater, highlighting its role in environmental cleanup. Similarly, tuning the pore structure of biochar enhances its adsorption properties, making it a recyclable and sustainable solution for ecological remediation [135,136,137,138].

One more application due to its activation is as a support for functionalized materials applicable in chemical synthesis [139,140], bringing its applicability as a catalyst as described by Chin et al. [141]. Niju et al. [142] also addressed the production of biochar derived from sugarcane bagasse and its application in the transesterification of waste cooking oil to produce biodiesel. Thus, two integrated biofuel production chains can be contemplated, corroborating the feasibility of integrated biorefinery building and connecting different and more sustainable technologies.

Different contaminants require different biochar parameters for efficient adsorption and possible desorption for biochar recycling/reuse [143,144]. The publications on wastewater treatment with biochar presented adsorptions with synthetic solutions of contaminants, varying between antibiotics, dyes, and toxic metals [135,136,145]. Some studies with possible water contaminants, such as tetracycline (antibiotic), methylene blue (dye), and Cr (III)/Cr(IV) (toxic metals), are compiled in Table 7, which identifies the direct relation between adsorption capabilities, superficial area, pore volume, and temperature/activation conditions.

Figure 13. Schematic diagram illustrating biochar production from various biomass feedstocks via pyrolysis and its application in wastewater treatment through adsorption. The diagram emphasizes biochar’s efficiency in removing a wide range of target contaminants [141,142,143,144,145,146,147,148,149,150,151].

Figure 13. Schematic diagram illustrating biochar production from various biomass feedstocks via pyrolysis and its application in wastewater treatment through adsorption. The diagram emphasizes biochar’s efficiency in removing a wide range of target contaminants [141,142,143,144,145,146,147,148,149,150,151].

Biochar presents notable advantages in wastewater treatment due to its high adsorption capacity and surface functionality, which facilitate the efficient removal of various contaminants, including heavy metals and organic pollutants [146]. According to Li et al., biochar’s porous structure and functional groups enhance its capacity to immobilize microorganisms, creating a stable environment that supports microbial degradation processes. This capability allows biochar to outperform conventional biological treatments, such as activated sludge, by providing repeated microbial utilization and a higher metabolic capacity essential for sustained pollutant removal in wastewater settings [146].

Compared to advanced oxidation processes and membrane technologies, biochar offers a more energy-efficient approach due to its passive adsorption mechanisms. Calderón-Franco et al. highlighted that biochar from sewage sludge achieved higher adsorption efficiencies for contaminants such as extracellular DNA and antibiotic resistance genes, outperforming materials like iron-oxide-coated sands [147]. These results suggest that biochar provides a sustainable, low-cost alternative and targets emerging contaminants more effectively than traditional treatments, which often entail high operational costs and complex maintenance requirements [147].

Table 7. Biochar applications as adsorbent along source biomass and preparation conditions.

Biomass	Process	Contam.	Biochar Properties and Adsorption Data	Ref.
Microalgae (Spirulina sp.)	Pyrolysis Atmosphere: N2 Temp.: 750 °C Time: 120 min	Tetracycline (TC)	O/C—0.138; H/C—1.38; SSA—2.63 m2 g−1; desorption efficiency—61%; highest TG adsorption at 147.9 mg g−1 (TC 100 mg L−1; dosage 0,1 g L−1; pH 6).	[148]
Wheat straw	Pyrolysis Atmosphere: N2 Temp.: 500 °C Time: 120 min Activation: KMnO4/KOH		O/C—0.225; H/C—0.007; SSA—1524.6 m2 g−1; pore volume—0.85 cm3 g−1; Raman ID/IG before adsorption–2.58; desorption efficiency (NaOH solution)—7%; highest TG adsorption at 584.19 mg g−1 at 318 K (TC 10–200 mg L−1; pH 3–10); no co-existing ion effect	[149]
Sunflower seed husk	Pyrolysis Atmosphere: N2 Temp.: 700 °C Time: 120 min Activation: KMnO4/KOH/ZnCl2		O/C—0.1; H/C—0.014; SSA—1578.3 m2 g−1; pore volume—1.138 cm3 g−1; Raman ID/IG before adsorption–0.585; desorption efficiency–97.61%; highest TG adsorption at 673.0 mg g−1 at 298 K for 24 h (TC 1–20 mg L−1; pH 3.0–11.0); highest TG adsorption with ions at 583.1 (K+), 539.8 (Mg2+) and 555.9 (Ca2+).	[150]
Microalgae—Chlorella sp. GD	Wet Torrefaction (water vapor) and Microwave Torref. (2450 MHz, 800 W) Temp.: 160–170 °C Time: 5–10 min	Methylene Blue (MB)	O/C between (0.462–0.506); SSA—2.66 m2 g−1; pore volume 0.00043 cm3 g−1; maximum removal of 85.47% MB; highest MB adsorption at 113 mg g−1 (optimum pH 2–8).	[151,152]
Penicillin mycelial residues	Torrefaction Atmosphere: N2 Temp.: 260 °C HR.: 5 °C min−1 Time: 45 min Impregnation: KOH		O/C—0.08; H/C—0.021; SSA—1809.74 m2 g−1; pore volume 1.02 cm3 g−1; Raman ID/IG before adsorption—1.21; highest MB adsorption at 620 mg g−1.	[153]
Bamboo particles	Pyrolysis Atmosphere: N2 Temp.: 700 °C HR.: 10 °C min−1 Time: 120 min Impregnation: KHCO3/Urea		O/C—0.081; SSA—1693 m2 g−1; biochar total pore volume 0.90 cm3 g−1; Raman ID/IG before adsorption—1.10; highest MB adsorption at 499 mg g−1.	[154]
Orange peel	Pyrolysis Atmosphere: N2 Temp.: 400 °C HR.: 5 °C min−1 Time: 180 min Activation: KOH	Cr(III)/Cr(IV)	Cr(IV) synthetic solution; O/C—0.100; SSA—998 m2 g−1; biochar total pore volume—1.24 cm3 g−1; Raman ID/IG before adsorption–1.03; highest Cr(IV) adsorption at 285.5 mg g−1 (pH—2, dosage 0.2 g L−1, C0 100 mg L−1, T = 25 °C, contact time = 40 h).	[155]
Vegetal waste—rice husk/polyethylene	Co-pyrolysis Atmosphere: N2 Temp.: 390 °C Time: 35 min		Cr(III) synthetic solution; O/C 0.089; Biochar SSA < 5.0 m2 g−1; adsorption capacity—9.23 mg g−1 (final pH 4–5).	[156]
Algae—Potamogeton crispus	Pyrolysis Atmosphere: N2 Temp.: 300 °C HR.: 5 °C min−1 Time: 120 min		Cr(IV) synthetic solution; O/C—0.53; H/C—0.09; SSA—0.42 m2 g−1; Pore volume—0.002 cm3 g−1; Raman ID/IG before adsorption—1.83; highest Cr(IV) adsorption at 34.37 mg g−1 (pH–2, dosage 2 g L−1, C0 100 mg L−1 and T = 25 °C.).	[157]
Paper sludge waste	Pyrolysis Atmosphere: N2 Temp.: 350–550 °C HR.: 5 °C min−1 Time: 120 min Activation (KOH) Temp.: 105 °C Time: 150 min/ Pyrolysis Atmosphere: N2 Temp.: 800 °C Time: 120 min Impregnation: NH4Cl		Cr(IV) synthetic solution; O/C—0.383; H/C—0.059; SSA—3336.7 m2 g−1; pore volume—2.10 cm3 g−1; Raman ID/IG before adsorption—0.95; highest Cr(IV) adsorption at 356.25 mg g−1 (99% removal under 30 min).	[158]

Table 7. Biochar applications as adsorbent along source biomass and preparation conditions.

Biomass	Process	Contam.	Biochar Properties and Adsorption Data	Ref.
Microalgae (Spirulina sp.)	Pyrolysis Atmosphere: N2 Temp.: 750 °C Time: 120 min	Tetracycline (TC)	O/C—0.138; H/C—1.38; SSA—2.63 m2 g−1; desorption efficiency—61%; highest TG adsorption at 147.9 mg g−1 (TC 100 mg L−1; dosage 0,1 g L−1; pH 6).	[148]
Wheat straw	Pyrolysis Atmosphere: N2 Temp.: 500 °C Time: 120 min Activation: KMnO4/KOH		O/C—0.225; H/C—0.007; SSA—1524.6 m2 g−1; pore volume—0.85 cm3 g−1; Raman ID/IG before adsorption–2.58; desorption efficiency (NaOH solution)—7%; highest TG adsorption at 584.19 mg g−1 at 318 K (TC 10–200 mg L−1; pH 3–10); no co-existing ion effect	[149]
Sunflower seed husk	Pyrolysis Atmosphere: N2 Temp.: 700 °C Time: 120 min Activation: KMnO4/KOH/ZnCl2		O/C—0.1; H/C—0.014; SSA—1578.3 m2 g−1; pore volume—1.138 cm3 g−1; Raman ID/IG before adsorption–0.585; desorption efficiency–97.61%; highest TG adsorption at 673.0 mg g−1 at 298 K for 24 h (TC 1–20 mg L−1; pH 3.0–11.0); highest TG adsorption with ions at 583.1 (K+), 539.8 (Mg2+) and 555.9 (Ca2+).	[150]
Microalgae—Chlorella sp. GD	Wet Torrefaction (water vapor) and Microwave Torref. (2450 MHz, 800 W) Temp.: 160–170 °C Time: 5–10 min	Methylene Blue (MB)	O/C between (0.462–0.506); SSA—2.66 m2 g−1; pore volume 0.00043 cm3 g−1; maximum removal of 85.47% MB; highest MB adsorption at 113 mg g−1 (optimum pH 2–8).	[151,152]
Penicillin mycelial residues	Torrefaction Atmosphere: N2 Temp.: 260 °C HR.: 5 °C min−1 Time: 45 min Impregnation: KOH		O/C—0.08; H/C—0.021; SSA—1809.74 m2 g−1; pore volume 1.02 cm3 g−1; Raman ID/IG before adsorption—1.21; highest MB adsorption at 620 mg g−1.	[153]
Bamboo particles	Pyrolysis Atmosphere: N2 Temp.: 700 °C HR.: 10 °C min−1 Time: 120 min Impregnation: KHCO3/Urea		O/C—0.081; SSA—1693 m2 g−1; biochar total pore volume 0.90 cm3 g−1; Raman ID/IG before adsorption—1.10; highest MB adsorption at 499 mg g−1.	[154]
Orange peel	Pyrolysis Atmosphere: N2 Temp.: 400 °C HR.: 5 °C min−1 Time: 180 min Activation: KOH	Cr(III)/Cr(IV)	Cr(IV) synthetic solution; O/C—0.100; SSA—998 m2 g−1; biochar total pore volume—1.24 cm3 g−1; Raman ID/IG before adsorption–1.03; highest Cr(IV) adsorption at 285.5 mg g−1 (pH—2, dosage 0.2 g L−1, C0 100 mg L−1, T = 25 °C, contact time = 40 h).	[155]
Vegetal waste—rice husk/polyethylene	Co-pyrolysis Atmosphere: N2 Temp.: 390 °C Time: 35 min		Cr(III) synthetic solution; O/C 0.089; Biochar SSA < 5.0 m2 g−1; adsorption capacity—9.23 mg g−1 (final pH 4–5).	[156]
Algae—Potamogeton crispus	Pyrolysis Atmosphere: N2 Temp.: 300 °C HR.: 5 °C min−1 Time: 120 min		Cr(IV) synthetic solution; O/C—0.53; H/C—0.09; SSA—0.42 m2 g−1; Pore volume—0.002 cm3 g−1; Raman ID/IG before adsorption—1.83; highest Cr(IV) adsorption at 34.37 mg g−1 (pH–2, dosage 2 g L−1, C0 100 mg L−1 and T = 25 °C.).	[157]
Paper sludge waste	Pyrolysis Atmosphere: N2 Temp.: 350–550 °C HR.: 5 °C min−1 Time: 120 min Activation (KOH) Temp.: 105 °C Time: 150 min/ Pyrolysis Atmosphere: N2 Temp.: 800 °C Time: 120 min Impregnation: NH4Cl		Cr(IV) synthetic solution; O/C—0.383; H/C—0.059; SSA—3336.7 m2 g−1; pore volume—2.10 cm3 g−1; Raman ID/IG before adsorption—0.95; highest Cr(IV) adsorption at 356.25 mg g−1 (99% removal under 30 min).	[158]

4.5. Application Trends of Biochar

4.5.1. Carbon Sequestration

Lignocellulosic biomass, such as wood residues and agricultural by-products, is a promising feedstock for biochar production due to its high lignin content, which enhances the stability of biochar and its effectiveness in carbon sequestration [38]. The pyrolysis process encourages the formation of stable, aromatic carbon structures, which are key to biochar’s effectiveness in sequestering carbon. Not to mention, biochar produced from lignocellulosic biomass retains over 50% of the carbon from its original feedstock, with wood residues yielding biochar that contains up to 85% carbon [38]. This results in biochar production, use, and storage potential to sequester an estimated 0.3–2 Gt of CO₂ per year by 2050, making biochar a significant tool in global efforts to mitigate climate change [159].

Biochar stands out as a highly effective material for carbon sequestration due to its chemical stability and resistance to decomposition, a feature that differentiates it from traditional carbon storage techniques in soils or forests. Studies show that biochar derived from lignocellulosic waste has a porous structure and high carbon content, ensuring stability and enabling carbon sequestration in soil for hundreds to thousands of years [38,160]. In comparison, techniques such as directly using plant residues in soil have lower stability and are prone to rapid decomposition, quickly releasing carbon back into the atmosphere [161].

4.5.2. Surface Coatings

Another promising application of biochar is developing advanced materials for surface coatings. Biochar-based superhydrophobic surfaces have been successfully fabricated for various industrial purposes, offering excellent chemical stability, corrosion resistance, and anti-scaling properties. These coatings are particularly valuable for protecting metal surfaces in harsh environments, such as marine or outdoor industrial applications [162]. For example, superhydrophobic coatings made from biochar derived from rice straw demonstrate superior resistance to UV radiation, mechanical abrasion, and corrosion, making them ideal for long-term use in exposed conditions [163]. Moreover, these biochar-based coatings can repel aqueous deep eutectic solvent (DES) mixtures, further broadening their applicability in environments requiring chemical stability and low surface adhesion [163].

An example of this application is the development of superhydrophobic coatings using biomass-derived carbon composites with polyaniline (PANI), inspired by the biomimetic structure of goose feathers, for enhanced anticorrosion and antibiofilm performance. The study describes how biochar–PANI composite coatings mimic the micro–nano hierarchical structures of feather surfaces to achieve superhydrophobicity with a high water contact angle that effectively prevents water infiltration. This structural design provides a dual function: increased corrosion resistance and antibiofilm efficacy, limiting bacterial adhesion from species such as Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa. The superhydrophobicity of the biochar–PANI coating offers improved electrochemical impedance and corrosion potential compared to traditional PANI coatings, demonstrating superior protective properties against corrosive environments. This innovative approach highlights the potential of biochar-based biomimetic coatings as effective, sustainable alternatives for surface protection applications, particularly in challenging environmental conditions [164].

4.5.3. Oil–Water Separation

In the context of oil–water separation, biochar has also shown great potential. Fabricating superhydrophobic–superoleophilic foams using biochar grafted onto melamine foam allows high-efficiency oil absorption. These foams exhibit excellent recyclability, pressure resistance, and performance across a wide pH range, making them suitable for addressing oil spills and other environmental challenges. The high oil absorption capacity of biochar-based foams underscores their utility in environmental management [39].

An example of this application is using superhydrophobic biochar modified with nickel–cobalt-layered double hydroxides (NiCo-LDH) as an efficient marine oil contamination mitigation approach. This advanced biochar achieves a high water contact angle of 156°, markedly enhancing its hydrophobicity over unmodified biochar. The NiCo-LDH modification introduces a hierarchical structure that facilitates strong hydrophobic interactions and π-π electron donor–acceptor interactions, thereby increasing its affinity for oil contaminants. Additionally, the NiCo-LDH coating improves the biochar’s environmental recalcitrance, providing greater resistance to chemical oxidation and thermal degradation, which enhances its durability and reusability in marine environments. Compared to conventional sorbent materials, such as activated carbon and graphene oxides, NiCo-LDH biochar offers superior oil removal efficiency, cost effectiveness, and ecological sustainability, making it a promising option for large-scale marine oil spill remediation [165].

4.5.4. Other Applications

Biochar has demonstrated significant potential not only in pollutant removal and catalysis but also in energy storage applications. Due to its inherent porosity and conductivity, biochar has been investigated for producing electrodes in supercapacitors and batteries. Modifying electrodes with biochar has shown considerable advancements in detecting heavy metals, such as lead, cadmium, and other pollutants. In addition, biochar has proven to be effective in non-enzymatic glucose sensing and electrochemical immunoassays for detecting viral proteins. Its adsorption capacity enables efficient pollutant removal, while its electrochemical activity enhances sensor performance. These characteristics make biochar a valuable material for applications in environmental monitoring, biosensing, and electrochemical energy storage [40]. Studies show that biochar enhances such devices’ energy density, power density, and cycling stability [166].

Additionally, recent research emphasizes that different preparation methods and biomass sources can significantly influence the electrochemical properties of biochar, making it a promising alternative to traditional carbon-based materials in energy storage technologies [166]. A further example of this application is its use as a renewable substitute for carbon black in lithium-ion batteries, offering comparable electrical conductivity and cycle stability while reducing the environmental impact associated with battery production. Lignin-derived biochar achieves conductivity levels similar to those of commercial carbon black (e.g., Super P), and the process yields lower greenhouse gas emissions due to its renewable biomass origin. Specifically, biochar’s structure can support the electrochemical demands of battery anodes with minimal compromise in performance, matching carbon black in conductivity and even surpassing it in high-rate charge capacity under certain conditions. This biochar alternative decreases reliance on fossil-fuel-derived additives, aligning with sustainable battery production goals [166,167].

5. Environmental Impacts of Biochar Production and Applications

The multifaceted applications of biochar in environmental remediation, energy storage, and catalysis reflect its potential to contribute positively to sustainability, but they also highlight specific environmental considerations across its lifecycle. In wastewater treatment, biochar effectively absorbs contaminants such as heavy metals and organic pollutants, thus potentially reducing the release of these substances into water bodies and minimizing pollution [125,168]. This capability is attributed to its high surface area and porous structure, which enhance its adsorption properties [169]. However, the environmental impact of producing biochar for this purpose depends on the feedstock choice and pyrolysis conditions, as emissions of volatile organic compounds (VOCs) and particulate matter during production can offset some benefits if not carefully managed [170,171]. For instance, the choice of feedstock significantly influences the emissions profile and the sustainability of biochar production [170].

For energy storage applications, using biochar as a component in batteries and supercapacitors reduces the reliance on non-renewable materials like traditional activated carbons, aligning with circular economy principles and potentially lowering the carbon footprint associated with battery production [172,173]. The incorporation of biochar in energy storage systems not only enhances performance but also promotes the use of renewable resources [173]. However, the need for specific activation processes to enhance electrochemical properties could lead to additional energy use and emissions, depending on the activation method and scale [169]. This requirement highlights the importance of optimizing production methods to minimize environmental impacts while maximizing the benefits of biochar in energy applications [170].

In catalysis, biochar serves as a sustainable support or active catalyst in various reactions, including biodiesel production and pollutant degradation, thus promoting greener chemical processes by replacing traditional catalysts with a renewable alternative [174,175]. This application reduces the environmental burden of metal-based catalysts, as biochar’s high surface area and functional groups can be optimized for efficiency without needing metal catalysts that pose disposal and toxicity issues [125,174]. Nevertheless, managing emissions during biochar production, particularly of polycyclic aromatic hydrocarbons (PAHs) and VOCs, remains essential to avoid compromising its environmental benefits [170,171].

Biochar’s diverse applications underscore its potential as a green technology, but responsible production practices are crucial to realize its environmental benefits while fully minimizing any adverse impacts. Research on biochar materials should target essential areas to unlock their full potential, with environmental impact evaluations and lifecycle analyses playing a crucial role in thoroughly assessing biochar’s sustainability across applications.

6. Scalability and Practical Large-Scale Implementation of Biochar

Transitioning from laboratory to industrial-scale processes faces several challenges, including product quality consistency and economic viability. This topic has been widely discussed in recent studies due to the growing interest in biochar as a sustainable alternative for carbon sequestration and soil enhancement [176]. One of the main challenges is maintaining consistency in its physicochemical properties. The characteristics of biochar, such as surface area, adsorption capacity, and chemical stability, are strongly influenced by pyrolysis temperature, residence time, and the type of biomass used [177]. In laboratory-scale production, these parameters can be rigorously controlled, but small variations in operational conditions in large-scale production may lead to significant changes in biochar quality. This critical limitation must be handled to ensure the consistent quality of biochar produced on an industrial scale. Figure 14 provides aspects for scaling up biochar production, including challenges, control of pyrolysis conditions, biomass optimization, economic considerations, and certifications required for ensuring quality, sustainability, and market viability.

6.1. Control of Pyrolysis Conditions, Energy Efficiency, and Co-Product Utilization

Slow pyrolysis is the most used method for large-scale biochar production, as it optimizes fixed carbon yield. However, in industrial plants, precise control over pyrolysis conditions, such as temperature and residence time, becomes more complex and requires a sophisticated structure capable of handling large volumes of biomass while maintaining operational stability [176]. Such control is essential to ensure that large-scale biochar production retains the desired properties. This process must be energy efficient at an industrial scale to ensure the economic viability of biochar production.

In this context, using pyrolysis co-products, such as bio-oil and syngas, for energy generation can significantly reduce operational costs and improve process efficiency to make the plant self-sufficient, essential for large-scale process sustainability [176].

6.2. Biomass Optimization

Biomass selection is another critical factor in scale-up. While it is possible to test various types of biomasses on a small scale, an industrial plant must use biomass sources that are economically viable and available in large quantities. Lignocellulosic residues, such as agricultural and forestry waste, have shown promise due to their high carbon content and low toxicity [177]. However, variability in biomass composition and the need for specific pretreatments present additional challenges, as these factors can impact both process efficiency and final biochar quality [176,177].

6.3. Economic Viability and Incentives

The economic viability of scaling up biochar production depends on the demand for agricultural and water treatment applications and comparisons with alternatives such as activated carbon. Government incentives, such as carbon credits, can also be crucial in offsetting industrial biochar plants’ high installation and operational costs [176]. Policies that encourage using biochar and the development of markets for pyrolysis co-products are essential to support large-scale production. Integrating biochar into biorefineries represents an opportunity to reduce costs and diversify products, promoting sustainability and process efficiency [178].

6.4. Companies Producing Biochar and Certifications Used

In recent years, biochar production has moved beyond research and pilot projects, with numerous companies scaling up commercial production processes. This growing industry leverages advancements in pyrolysis and carbon capture technologies to meet the increasing demand for biochar as both a carbon sequestration tool and a soil amendment. Key players in this sector have adopted internationally recognized certification standards to ensure their products meet stringent quality, safety, and environmental criteria. Below is an examination of prominent companies engaged in biochar production, their certifications, and the standards they follow to produce high-quality, sustainable biochar for various applications.

6.4.1. NetZero

NetZero, a company headquartered in France and Brazil, is a startup producing biochar from agricultural residues. It has The Puro Standard certification that is aimed at carbon removal activities, ensuring that CO₂ removed from the atmosphere is stored durably. This certification involves independent audits and rigorous quality and sustainability control. Puro Earth developed it. The certificates are independently audited, and the amount of carbon sequestered is recorded. They require a life cycle assessment (LCA), which includes evaluating emissions generated at each stage of biochar production and use [179,180].

The Puro Standard provides a transparent mechanism for companies to prove the effectiveness of their carbon sequestration practices and participate in the carbon credit market, promoting sustainability and credibility for customers and investors [179].

6.4.2. Biochar Works

Situated in the United States of America, Biochar Works adheres to the VM0044 methodology, which is applied to certify biochar production projects that contribute to greenhouse gas reductions, especially when applied in soil. This methodology requires producing high environmental control standards of biochar, focusing on long-term carbon sequestration benefits [181].

6.4.3. Carbon Gold

From the United Kingdom, Carbon Gold follows the International Biochar Initiative (IBI) and European Biochar Certificate (EBC) standards certifications that establish guidelines for safe and sustainable biochar production, ensuring that biochar is applied to the soil in a way that enhances fertility and provides long-term carbon sequestration [181,182].

The IBI establishes global standards for biochar production, particularly for soil application, ensuring the material is safe and free from contaminants. Tests include biochar composition and stability analyses to ensure its long-term effect on carbon sequestration [181,182]. The IBI guidelines is essential for companies wishing to demonstrate the safety and quality of their products, especially in the agricultural sector, where biochar is used to improve soil properties [181,182].

The EBC establishes criteria for sustainable European biochar production, including rigorous product quality controls and monitoring emissions generated during production. Certified companies must prove that the biochar is stable enough to store carbon long term and complies with local environmental regulations [181,182]. This certification ensures that the biochar produced follows best sustainability practices and is suitable for European and international markets that demand products with high environmental control.

In summary, companies such as NetZero, Biochar Works, and Carbon Gold demonstrate a commitment to sustainability and product quality through internationally recognized certifications, validating the role of biochar in combating climate change and encouraging the expansion of sustainable agricultural practices.

7. Multi-Criteria Decision Analysis

Different technologies characterize the thermochemical conversion route, which refers to various biomass feedstock and operational conditions, reflecting different applications that often conflict with each other, thus requiring multidisciplinary decision support. Therefore, for an optimized and comprehensive decision-making process, it is critical to include multiple criteria, allowing the selection of feedstocks, technologies, and operational conditions for a specific biochar application. MCDA is a tool that can effectively contribute to answering that challenge.

Previous studies have examined the methodologies, scope, and multi-criteria techniques applied thus far to Waste-to-Energy (WTE) management, encompassing processes such as anaerobic digestion, incineration, gasification, landfill-to-gas conversion, and pyrolysis [183]. The study reported the application of different multi-criteria decision methods (MCDM), mainly focusing on selecting WTE generation technologies based on environmental, economic, technical, and socio-political factors [184]. The reviewed literature evaluates the technologies that could efficiently convert biomass by applying WTE technologies to produce different energy carriers [184,185,186,187,188]. However, few studies in the literature applied MCDM, focusing on evaluating and optimizing specific WTE technologies based on the produced biochar properties and their applications.

The previous study applied MCDM (dominance analysis and metric distance based on the approach of compromise programming) to select optimum operational parameters for pyrolysis grape pomace [189]. The MCDM evaluated the pyrolysis operational parameters (temperature and heating rate) considering the criteria that most favor biochar for biofuel application (solid yield, carbon content, HHV, and activation energies from pyrolysis kinetics) [189]. In addition, soil amendment application was assessed using the following criteria: solid yield, carbon content, H/C, O/C, N, P, Mg, and K biochar contents. The results demonstrated that multi-criteria decision methods could help determine the biochar’s advantages for each application, depending on the associated production process characteristics [189].

Regarding wastes from food industries, the valuation route of pequi seed and its extractives was investigated using MCDM, and pyrolysis, gasification, and transesterification WTE routes were considered [14]. The obtained biochar at different temperatures was investigated for biofuel and soil amendment applications. Results provided insights into the energetic supply and demand in local/rural communities, showing that the decision support method helped to identify and compare the optimal conversion parameters.

Past work assessed spent coffee grounds and brewers’ spent grains considering biochar production via pyrolysis for biofuel (biochar criteria: H/C, O/C, HHV, EF, and EY) application [41]. The MCDM results recommended optimum operational conditions to valorize low-cost feedstock more economically through pyrolysis and gasification.

MCDM was applied to investigate and define optimum biomass blends from urban forest wastes (pruning waste) for biochar production as biofuel via mild pyrolysis (torrefaction) [68,190]. As indicated, the MCDM allowed for the identification of the optimum conditions for each application and the definition of an optimized blend.

In a recent and complete study, Petrova et al. [191] performed a multi-objective decision analysis of produced biochars (grape pomace, cherry, peach, colza, sunflower husks, and softwood). Utilizing the experimentally obtained physical and chemical characteristics of the produced biochar (500 °C), a decision-support tool was employed, concentrating on its potential functions as a biofuel, catalyst, CO₂ sequestration method, soil enhancement, and even as a supercapacitor [191]. The evaluation aimed at aiding decision making revealed that biochar derived from peach stones, cherry stones, and grape pomace exhibited the most advantageous results for various applications, encompassing biofuel. Furthermore, the CO₂ adsorption potential of biochar suggests its utility could go beyond just energy conversion techniques.

In this context, the scientific challenge of the topic under consideration can be categorized into three main categories: (i) knowing which MCDM provides a multifaceted analysis with less effort for a specific application, (ii) knowing which properties (criteria) must be assessed for each specific application, (iii) the applied analytical technique evolved to characterize the obtained biochar and obtain those properties, and (iv) conducting the process optimization for a specific thermochemical route.

MCDM Criteria Selection, Analytical Technique, and Optimization

Due to its versatile physicochemical properties, biochar is indicated to have various potential applications [189]. For example, biochar can be used as biofuel, fertilizer, catalyst, and absorbent depending on its different properties derived from the process operational conditions. In addition, specific biochar properties are beneficial for some applications. Therefore, the step of criteria selection is essential and depends mainly on biochar criteria that have the most influence for a determined application and less complex/expensive analytical technique.

Table 8. Summary of biochar studies indicating the assessed feedstock (lignocellulosic biomass), biochar properties for each application, required analytical techniques, and the MCDM (if applied).

App.	Biomass	Properties/Criteria	Analytical Technique	MCDM	Ref.
Biofuel	Grape pomace	Biochar yield, carbon (%), HHV, kinetics combustion parameters (Ea)	Proximate analysis Calorific analysis Ultimate analysis TG/DTG	Pareto dominance analysis/ metric distance based on compromise programming	[189]
	Grape pomace, cherry stones, peach stones, colza, sunflower husks, and softwood	Moisture %, ash, HHV, kinetics combustion parameters (Ea)	Proximate analysis Calorific analysis TG/DTG	Pareto dominance analysis/ metric distance based on compromise programming	[191]
	Fruit seeds	Carbon enhancement index (CEI), HHV and its enhancement factor (EF) and energy-mass coefficient index (EMCI)	Calorific analysis Ultimate analysis	Pareto dominance analysis/ metric distance based on compromise programming	[14]
	Spent coffee ground Brew spent grains	O/C and H/C, HHV and its enhancement factor (EF), energy yield (EY)	Calorific analysis Ultimate analysis	Pareto dominance analysis/ metric distance based on compromise programming	[41]
	Pruning trees	FC, ash, HHV, O/C and H/C	Proximate analysis Calorific analysis Ultimate analysis	Pareto dominance analysis/ metric distance based on compromise programming	[68]
Catalyst	Grape pomace, cherry stones, peach stones, colza, sunflower husks, and softwood	K, Ca, P (%), carbon (%), specific surface area (m2 g−1)	EDS Ultimate analysis Brunauer– Emmett–Teller (BET) method	Pareto dominance analysis/ metric distance based on compromise programming	[191]
Soil amendment	Fruit seeds	O/C and H/C, carbon enhancement index (CEI), K, N, P (%)	EDS Ultimate analysis	Pareto dominance analysis/ metric distance based on compromise programming	[14]
	Grape pomace	biochar yield, carbon content, O/C and H/C, N, P, Mg, K	EDS Ultimate analysis	Pareto dominance analysis/ metric distance based on compromise programming	[189]
Soil amendment/ CO2 sequestration/ Supercapacitor development	Grape pomace, cherry stones, peach stones, colza, sunflower husks, and softwood	K, Ca, P (%)/ carbon (%)/ specific surface area, bulk density, electric conductivity, pH	EDS Ultimate analysis Brunauer– Emmett–Teller (BET) method Biochar pH, electrical conductivity and liming potential	Pareto dominance analysis/ metric distance based on compromise programming	[191]
Wastewater treatment	Spirulina sp. Wheat straw Sunflower seed husk Chlorella sp. Penicillin mycelia Bamboo particles Orange peel Rice husk Paper sludge	O/C SSA Pore volume Raman ID/IG Surface composition Adsorption capability	Ultimate analysis Physicochemical analysis Surface analysis Molecular/structural analysis	-	[64,148,149,150,151,153,154,155,156,157,158]

summarizes the MCDM studies indicating the investigated lignocellulosic biomass, evolved criteria, required analytical techniques, and decision methodology for each biochar application. In addition, studies related to wastewater treatment were selected to allow for insight into future research.

Table 8 identifies the criteria for each application, and Figure 15 presents the framework for the application of MCDA methodologies (based on Table 8).

As can be seen in Table 8, previous studies focused on proximate, ultimate, and calorific analysis and TG/DTG for assessing combustion behavior. Therefore, MCDM criteria selection must focus on lower moisture content, higher fixed carbon, lower volatile matter and ash content, lower H/C and O/C, and higher HHV for biofuel applications. The activation energy (mainly in the devolatilization stage) should be evaluated considering combustion behavior. Biochar applications are described as follows:

Biofuel: Biochar may be suitable as an alternative solid biofuel compared to raw biomass and has a lower CO₂ emission than coal. Even though the concerns with yield capacity may not yet be settled for large-scale (industrial) applications, advantageous biochar characteristics show promise in management and energy recovery.

Catalyst: Carbon-based materials are commonly utilized in catalytic processes, with biochar serving as a potential candidate. As depicted by Table 8, only one study applied MCDM to evaluate six biochars produced at 500 °C for catalysis application. For instance, a higher specific surface area (SSA) and K, Ca, P, and C contents indicate a better biochar for this application.

Soil amendment: Biochar possesses considerable benefits for soil enhancement. Considering the three studies that assessed biochar for soil amendment application, the explored criteria contemplated a higher biochar yield with more excellent carbon content and lower O/C and H/C. Moreover, a biochar with superior SSA and K, N, P, Ca, and Mg (%) is beneficial. In addition, higher bulk density, electric conductivity, and pH are also desired. As stated by [154], the comparable characteristics of biochar have potential applications in other processes, such as carbon dioxide (CO₂) capture and the advancement of supercapacitors, owing to its significant specific surface area, porous framework, surface functional groups, and substantial mineral content.

Wastewater treatment: Biochar as an adsorbent for water treatment is considerably more efficient than non-treated biomass. Therefore, such materials’ adsorption capabilities and application are directly linked to the pyrolytic conditions, activation, and superficial characteristics. According to the literature review (Table 8), no study has applied MCDM to evaluate the biochar produced for wastewater treatment. Based on the assessed properties of the previous research [64,148,149,150,151,153,154,155,156,157,158], MCDM criteria selection must include higher SSA, higher pore volume, higher Raman I_D/I_G, and higher adsorption capability.

8. Conclusions and Perspectives

Biochar production is one of the most efficient possibilities for using agro-forest-industrial residues. Therefore, it is directly linked to the circular economy concepts, mainly on the WtE, while also aligning with the 17 goals for sustainable development established by the United Nations [192].

Biochar, at its most simple definition, is a carbon-based material used in soil remediation processes. This review contextualizes its definitions, source lignocellulosic biomass, production techniques, characterization methods, applications, and multi-criteria analysis.

The versatile physicochemical properties of biochar highlight its potential for diverse applications, ranging from biofuel production and catalysis to soil enhancement and wastewater treatment. The selection of criteria is a crucial step in determining the suitability of biochar for specific applications, and this choice is influenced by the dominant factors governing each use case. While primary studies have provided insights into biochar’s potential as biofuel, there remains a need for comprehensive multi-criteria decision-making (MCDM) assessments in several areas, including catalysis and wastewater treatment. Moreover, due to its unique physical attributes, biochar’s exceptional properties extend to emerging fields like carbon capture and supercapacitor development.

Future investigations should connect economic and environmental aspects, such as energy expenditures for biochar production and impact categories (such as Global Warming Potential) from life cycle assessment. These assessments would guide the selection of biochar properties that hold significance for each application, ultimately contributing to informed decision making in the sustainable utilization of biochar’s capabilities.

The number of studies related to biochar preparation, characterization, and usage is expected to increase in the following years, along with implementing more efficient processes and green energy plants. This prospect indicates a bright future for countries with abundant biomass and that more studies must be conducted to implement biochar materials in more complex applications, effectively promoting sustainable development.

Despite these benefits, economic challenges remain, primarily related to the high initial costs of biochar production and the availability of biomass feedstocks. However, integrating biochar into carbon credit markets, where sequestration services are monetized, can offset production costs and incentivize wider adoption [159]. Additionally, biochar can reduce the need for costly agricultural inputs, such as water and fertilizers, further enhancing its economic viability in sustainable farming systems [38].

In conclusion, the versatility of biochar offers a wide range of possibilities for environmental sustainability and industrial applications. However, further research is required to optimize its production processes, expand its applications into emerging fields, and assess its long-term economic viability. Integrating multi-criteria decision making, life cycle assessment, and techno-economic analysis will be essential for advancing biochar utilization in sustainable technologies. Addressing these research gaps will contribute to the development of high-performance biochar-based materials and facilitate their adoption in energy storage, carbon sequestration, and environmental remediation applications.