Applications of Biochar in Fuel and Feedstock Substitution: A Review

Abstract

With the continuous growth of global energy consumption and the advancement of carbon reduction targets, the development of low-carbon and renewable energy resources has become a central focus in energy science research. As the only renewable carbon source, biomass exhibits significant application potential in future energy and resource systems due to its widespread availability, carbon neutrality, and environmental friendliness. Biochar, the primary solid product generated during biomass pyrolysis, is characterized by its high energy density, excellent thermal stability, and abundant porous structure. It has been increasingly regarded as a promising substitute for conventional fossil-based fuels and feedstocks. In this study, VOSviewer was employed to identify representative applications of biochar in energy systems. Particular attention is given to its roles in fossil fuel substitution and raw material replacement. By summarizing recent research progress, this review aims to provide theoretical support and technical references for the large-scale and efficient utilization of biochar.

1. Introduction

With the development of the global economy and societal progress, energy demand worldwide continues to rise. In the context of fossil fuel depletion and environmental degradation, many countries have placed energy transition and technological innovation high on their policy agendas. International agreements such as the Kyoto Protocol and subsequent climate initiatives emphasize the urgency of reducing greenhouse gas emissions. Despite these efforts, global warming continues to accelerate, with atmospheric CO₂ concentrations surpassing previously anticipated thresholds. Recognizing this challenge, governments have set ambitious long-term emission reduction targets—for instance, aiming for substantial decreases relative to 1990 levels by the mid-century—thereby strengthening the demand for sustainable alternatives to fossil-derived products. Against the backdrop of global efforts to combat climate change and achieve carbon reduction targets, the challenge of ensuring energy supply while simultaneously reducing carbon emissions has become a critical concern for nations across the world [1]. In recent years, renewable energy sources have attracted increasing attention for their green and sustainable characteristics, with global consumption steadily rising at an average annual growth rate of approximately 3% from 2018 to 2050 [2]. This trend is further illustrated in Figure 1, which presents the global installed capacity of renewable energy, CO₂ emissions, and the share of fossil fuels in energy consumption from 2015 to 2024. Furthermore, the current global energy mix is shown in Figure 2, where fossil fuels such as oil, coal, and natural gas still dominate the supply structure, although renewable sources are gradually increasing their share.

Among various renewable energy sources, biomass stands out as the only renewable source of organic carbon in nature. It is derived from a wide array of sources, including agricultural, forestry residues (e.g., sawdust, bark, straw, bagasse), and municipal solid waste. It is estimated that nature produces approximately 170 billion tons of biomass annually through photosynthesis, with a theoretical energy potential that far exceeds current global energy consumption [4]. Furthermore, the CO₂ emissions generated during biomass utilization are offset by the equivalent amount of CO₂ fixed during biomass growth, rendering it carbon neutral. This unique feature positions biomass as a critical component in achieving carbon reduction goals and promoting sustainable development.

Currently, the primary routes for biomass energy conversion include thermochemical and biochemical methods. Due to low energy efficiency and limited scalability, biochemical conversion is less favored. In contrast, thermochemical processes—owing to their rapid reaction rates, high conversion efficiencies, and strong industrial potential—have attracted significant attention [5,6]. These include combustion, pyrolysis, liquefaction, and gasification. Biomass combustion typically occurs in the presence of air, but suffers from relatively low thermal efficiency [7]. Although progress has been made in gasification technology, its broader application in biofuel production remains constrained by challenges such as tar formation, feedstock variability, reactor performance under diverse environmental conditions, and high initial capital costs with uncertain returns on investment [8,9].

In the pyrolysis and carbonization process, biomass is converted into the solid product known as “biochar,” along with bio-oil and syngas as byproducts. The term “biochar” was first introduced by Lehmann in 1999 to describe carbon-rich solid materials with properties similar to activated carbon [10]. In 2013, the International Biochar Initiative (IBI) formally defined biochar as “a carbon-rich solid material obtained through thermochemical conversion under limited oxygen conditions” [11]. The historical development and milestones of biochar are illustrated in Figure 3. Owing to its high carbon content, low ash content, and porous structure, biochar exhibits diverse functionalities in various fields, as summarized in Figure 4, including applications such as solid fuel, metallurgical reductants, and adsorbents [12,13,14,15].

In recent years, research on biochar has gradually expanded from agriculture and environmental remediation to energy system integration and fuel substitution. However, comprehensive analyses of its functional mechanisms, preparation technologies, and cross-sectoral application pathways in the energy sector remain relatively scarce. Therefore, this review employs VOSviewer (version 1.6.20) to identify the representative applications of biochar in energy systems, with a particular focus on its roles in fossil fuel substitution and feedstock replacement. Typical application scenarios, such as cement kilns, boilers, and blast furnaces, are analyzed in detail. Additionally, the evolution of keywords and technological trends are discussed to explore potential future development pathways, providing theoretical support and a reference framework for the efficient utilization of biochar in energy systems. A comparison of existing review articles and their limitations is summarized in

Table 1. Comparison between various review articles on utilization of biochar in energy applications and their literature gaps.

Inferences	Literature Gaps	Source
Reviewed the preparation of biochar by thermochemical methods and its application mechanisms and reaction types as a catalyst in algae-based fuel production	Did not involve the comprehensive application of industrial raw materials and the replacement of multiple fuels	[16]
Discussed the power generation mechanisms and improvement strategies of plant microbial fuel cells	Had low relevance to the fuel replacement field	[17]
Summarized the sources, processing methods, and properties of solid biomass fuels	Did not involve the replacement potential of high-temperature carbon–thermal reactions	[18]
Focused on the processes, properties, and applications of preparing slurry fuels from fast pyrolysis products	Did not involve solid fuel replacement and carbon reactivity	[19]
Summarized the thermochemical conversion processes and product characteristics of leaf-based biomass	Lacked exploration of fuel/raw material replacement applications	[20]
Summarized the potential of making biochar slurry fuel from Indonesian agricultural wastes	Was highly regional and did not involve cross-domain performance comparison	[21]
Reviewed the roles and product properties of catalysts in the HTC process	Did not involve the application of dry-state high-temperature carbon–thermal reduction	[22]
Analyzed the research hotspots of biochar applications from 2022 to 2023	Lacked comparison of specific fuel/raw material replacement technologies	[23]
Reviewed the technology of biochar-catalyzed plastic pyrolysis for liquid fuel production	Had limited association with traditional industrial replacement applications	[24]

, which highlights the research gaps that this review seeks to address.

2. Analysis of Biochar Applications

2.1. Research Trends in Biochar

Figure 5 illustrates the annual publication trend of biochar-related studies indexed in the Web of Science from 1998 to 2024. The year 2025 is excluded from the analysis as the data are incomplete and not suitable for comparison. The results show that, after the concept of biochar was introduced in 1999, a few initial studies emerged, followed by a steady increase in publications beginning in 2010. This trend indicates that biochar has become a key research focus across multiple fields such as energy, environment, and catalysis, with growing academic attention and strategic research significance.

To highlight the interdisciplinary nature of biochar research, Figure 6 presents a treemap based on Web of Science subject categories, covering the period from the inception of research to 29 April 2025. The publications are mainly concentrated in Environmental Sciences and Ecology (35,920 articles), Chemistry (32,724 articles), Agriculture (26,805 articles), and Engineering (26,211 articles). Other significant fields include Energy and Fuels (19,247 articles), Materials Science (16,274 articles), Public and Occupational Health (15,753 articles), Business and Economics (13,088 articles), Water Resources (12,631 articles), and Multidisciplinary Sciences (12,925 articles). This broad disciplinary distribution clearly demonstrates the high adaptability of biochar across research domains and provides a solid academic foundation for the energy-focused perspective adopted in this review.

2.2. Analysis of Biochar Research Themes

To further explore the development trajectory and thematic distribution of biochar-related research, Figure 7 presents a knowledge map constructed via keyword co-occurrence analysis. Different color clusters in the figure represent the major research directions within the biochar field, including agriculture and soil remediation (e.g., “soil”, “plant”), pollutant adsorption and wastewater treatment (e.g., “adsorption”, “removal”), as well as biomass conversion and process optimization (e.g., “biomass”, “production”). Notably, the concentrated presence of high-frequency keywords such as “biomass production,” “carbon sequestration,” and “material functionalization” highlights the substantial expansion potential of biochar within the domain of energy science.

2.3. Analysis of Biochar Applications in the Energy Sector

To analyze the trends in biochar applications within the energy sector, a keyword co-occurrence analysis was performed using VOSviewer, based on publications indexed in the Web of Science from 2021 to 2025, under the theme of ‘biochar and fuel’. The analysis identified six major clusters representing research hotspots in soil and agriculture (green), pollution control (red), production processes (blue), environmental impacts (purple), material mechanisms and functionalization (orange), and electrochemical processes (light blue), as shown in Figure 8. However, it is noteworthy that energy-related topics—such as fuel, energy recovery, biomass gasification, and biofuel—are scattered across these clusters as secondary themes, rather than forming an independent and dominant cluster. This indicates that while energy-related applications of biochar are emerging, they remain less mature and less central compared to traditional applications such as agriculture and environmental remediation.

Overall, the keywords “biochar” and “pyrolysis” form the core of the network, radiating out to a series of energy-related terms such as “biodiesel,” “hydrochar,” “syngas,” “gasification,” “biofuel,” and “renewable energy,” underscoring biochar’s research value in fuel substitution and resource recovery.

Figure 9 presents a keyword overlay visualization showing the average publication year in the energy-related subdomain. Early-stage research focused primarily on conventional conversion technologies such as “pyrolysis,” “gasification,” and “combustion,” represented by cooler colors (blue), indicating that these topics became established hotspots prior to 2021. In contrast, newer keywords such as “microwave torrefaction,” “techno-economic assessment,” “co-pyrolysis,” and “bio-oil upgrading” appear in warmer tones (yellow), reflecting a recent shift in research interest toward more efficient, precisely controlled, and integrative energy systems. This trend reveals the field’s transition from conventional pyrolytic energy conversion to multi-energy coupling and integrated utilization systems.

Figure 10 displays the keyword density map for biochar-related energy applications. The distribution exhibits a pronounced high-frequency core structure, with keywords such as “biochar,” “pyrolysis,” and “gasification” at the center of the map in the brightest tones, indicating extremely high occurrence and foundational importance in this research area. Secondary terms like “syngas,” “bio-oil,” and “combustion” also display notable density, signifying their critical role in energy conversion pathways. In contrast, peripheral keywords such as “techno-economic assessment,” “direct carbon fuel cell,” and “co-firing” appear less frequently but form localized clusters, suggesting emerging research frontiers. The density map highlights a characteristic pattern in biochar energy research: pyrolysis-dominated, with multi-energy crosslinking.

Taken together, these findings show that literature on “biochar and fuel” from 2021 to 2025 spans a range of topics from conventional pyrolysis techniques to advanced energy systems. Biochar plays a critical role not only in traditional pathways like pyrolysis and gasification, but also shows substantial potential in emerging areas such as biofuel production and energy substitution. The average publication year and density visualizations further reveal a clear shift from fundamental energy conversion technologies toward system-level integration and multi-energy synergy applications, forming an evolutionary trajectory characterized by pyrolysis-dominated research with increasing cross-sectoral integration.

With the continued push for energy transition and carbon reduction, the multifunctionality of biochar as a renewable carbonaceous material in energy systems has garnered growing attention. Therefore, this review builds upon existing research progress to systematically examine two promising directions—fuel substitution and feedstock replacement—to analyze the advancements and prospects of biochar in the emerging energy sector. The aim is to provide theoretical guidance and technical references for the efficient utilization of biochar in future multi-scenario energy systems.

3. Application of Biochar in the Energy and Resource Sector

3.1. Fuel Substitution

Biochar is a carbon-rich solid material produced by the thermochemical conversion of biomass under limited oxygen. It has gained significant attention for its applications in soil amendment, carbon sequestration, pollution control, and as a sustainable alternative in energy and materials [25].

Hydrothermal carbonization (HTC), suitable for wet biomass, yields hydrochar with lignite-like characteristics—high HHV (25–36 MJ/kg), low ash, and elevated carbon content—making it ideal for direct or co-combustion [26]. Jamal-Uddin et al. reported hydrochar from greenhouse residues reaching 26.0 MJ/kg with low slagging risk, enabling its use as a renewable fuel in greenhouse heating systems [27].

Pyrolysis is another key route. Laird et al. showed that pyrolytic biochar, with ~18 MJ/kg HHV, can replace pulverized coal in industrial boilers [28]. Jahirul et al. further analyzed slow, fast, and flash pyrolysis, confirming biochar’s suitability as a low-emission solid fuel [29].

In low-income regions, biochar from urban biowaste is used as cooking fuel. Lohri et al. highlighted briquetted biochar as both a clean energy solution and a tool for waste management and forest conservation [30].

Overall, biochar shows strong potential for large-scale fossil fuel substitution across industrial and domestic applications due to its favorable energy performance and environmental benefits.

3.1.1. Cement Industry

With the rapid evolution and transformation of modern industry, cement—as a fundamental construction material—has experienced significant growth to meet increasing infrastructure demands. However, its production process is associated with substantial CO₂ emissions, accounting for approximately 5–8% of global industrial carbon emissions [31], placing the industry under intense decarbonization pressure. Against this backdrop, biochar—a carbon-rich material derived from biomass pyrolysis—has attracted growing attention as a potential alternative to conventional fossil fuels in cement production. In particular, in rotary kilns, biochar is considered a sustainable substitute for coal due to its high calorific value, good combustibility, and capacity for carbon emission reduction.

Biochar is mainly composed of light elements such as carbon, oxygen, hydrogen, and nitrogen, along with inorganic components including potassium, sodium, calcium, and magnesium. The carbon primarily exists as aromatic carbon in a fixed carbon form, while the inorganic components typically appear in ash as carbonates, phosphates, or oxides [32]. Cellulose-based biochars generally exhibit higher carbon content compared to non-cellulose-based biochars, with woody and bamboo-derived biochars possessing especially high carbon content. Regarding other elements, cellulose-based biochars tend to have higher potassium content, whereas non-cellulose-based biochars are richer in calcium, magnesium, nitrogen, and phosphorus [33]. According to He Zhijun, Sheng Ruyuan, and colleagues, who conducted industrial and elemental comparisons between typical biochars and metallurgical coals (see

Table 2. Proximate analysis and elemental composition comparison between typical biochar and metallurgical coal [34].

Biomass Type	Industrial Analysis (Mass Fraction)/%				Elementary Analysis (Mass Fraction)/%					Lower Heating Value (MJ·kg−1)
	M	A	V	CF	C	H	N	S	O
Wheat straw	9.02	1.85	72.48	16.65	44.65	5.24	0.28	0.08	40.72	15.83
Cotton straw	6.93	3.18	73.00	16.89	44.90	7.50	1.20	—	35.47	18.40
Maize stalk	8.52	7.09	68.09	16.30	42.47	3.27	1.18	0.26	52.82	15.50
Rice husk	6.00	16.92	51.98	25.10	35.34	5.43	1.77	0.09	35.36	13.38
Peanut shell	8.84	4.69	68.48	17.99	43.53	6.54	2.24	0.12	34.04	16.28
Pine sawdust	6.11	3.47	74.60	15.82	45.76	6.74	0.07	—	37.85	15.41
Rice straw	4.13	13.56	77.77	14.54	38.09	6.15	0.70	0.06	37.31	13.67
Bamboo	5.71	4.10	71.49	18.70	52.00	5.10	0.40	—	42.50	19.90
Birch bark	5.88	4.20	72.72	17.20	57.00	6.70	0.50	0.10	35.70	25.90
Coconut shell	7.51	1.10	79.89	11.50	51.10	5.60	0.10	0.10	43.10	16.40
Corncobs	8.87	0.50	73.60	17.03	49.00	5.40	0.40	—	44.20	17.90
Poplar	6.56	1.10	78.04	14.30	48.50	5.90	0.50		43.70	16.50
Sugarcane bagasse	4.21	5.20	73.09	17.50	49.80	6.00	0.20	0.10	43.90	19.50
Waste tea leaves	5.45	2.40	80.15	12.00	48.00	5.50	0.50	0.10	44.00	17.60
Corn straw biochar	—	18.75	20.36	60.89	62.05	1.91	0.69	0.13	16.47	23.57
Peanut shell biochar	—	16.68	24.54	58.78	59.66	1.88	0.93	0.21	20.64	22.35
Fig biochar	—	2.37	21.04	76.59	88.81	5.34	0.62	0.10	5.12	36.00
Sycamore leaf biochar	—	18.39	19.98	56.38	68.76	1.34	1.16	1.10	26.74	22.30
Bituminous coal	8.85	21.37	38.48	31.30	57.42	3.81	0.93	0.46	7.16	24.30
Anthracite coal	8.00	19.02	7.85	65.13	65.65	2.64	0.99	0.51	3.19	24.42

“—” indicates that the data are not available or not reported in the referenced source.

), pyrolysis treatment significantly enhances the calorific value and fixed carbon content of biomass, supporting its feasibility for pulverized coal substitution [34]. Moreover, biomass and biochar typically exhibit low sulfur and nitrogen content, enabling reduced SO_x and NO_x emissions when used as fuels.

Based on the analysis of biomass and biochar ash compositions, the metal content in aqueous extracts of the tested biomass ashes can be ranked in the following order [35]:

Oat straw: K>P>Ca>Na>Mg>Mn>Fe>Zn>Cu>Pb>As>Ni>Cr>Co>Cd.

Triticale straw: K>P>Ca>Mg>Na>Fe>Mn>Zn>Cu>As>Pb>Ni>Cr>Co>Cd.

Horbeam bark: K>Na>Mg>Ca>Zn>Cu>Fe>Ni>Cr.

Alder bark: K>Mg>Na>Ca>Zn>Cu>Fe>Ni.

Pine bark: K>Na>Ca>Mg>Cu>Zn>Fe>Ni>Cr.

Oak bark: K>Mg>Ca>Na>Zn>Fe>Cu>Ni.

Municipal waste biochar, composting: K>Na>Ca>Mg>Cr>Zn>Cu>Fe>Ni.

Biochar: Ca>K>Mg>Na>Zn>Fe.

Pellet biochar: K>Na>Ca>Mg>Zn>Fe>Cu.

In terms of combustion stability, biochar possesses a high calorific value (25–30 MJ/kg), moderate particle size, and porous structure, making it suitable for stable, long-duration combustion in rotary kilns. Co-combustion tests have shown that blending biochar with coal can achieve synergistic combustion, improving thermal efficiency and reducing NO_x and SO_x emissions [36]. In recent years, researchers have systematically evaluated the application of biochars produced at different pyrolysis temperatures in the cement industry.

Pyrolysis temperature is one of the most critical parameters affecting product distribution and structural properties of biochar [37,38]. Experimental studies have shown that, as pyrolysis temperature increases, biochar yield tends to decrease, primarily due to the intensified cracking of heavy hydrocarbons under high temperatures [39], which leads to increased yields of gaseous and liquid products [40]. However, despite the yield reduction, fixed carbon content and aromaticity typically increase with temperature [41], resulting in improved structural stability, surface area, and porosity, and enhancing both adsorption performance and thermal stability. At temperatures exceeding 800 °C, however, pore structure collapse may occur due to thermal shrinkage, while molten inorganic salts can block the pores, thereby reducing surface area and functional performance [33]. In cement-related industries, co-pyrolysis of agricultural residues and scrap tires at 500–600 °C has been reported to yield biochar with high fixed carbon and low volatile content, leading to an enhanced calorific value and suitability for high-temperature rotary kiln applications [42]. Similarly, biochar derived from spent mushroom substrate (SMS) under pyrolysis conditions of 700 °C and 60 min residence time demonstrated the highest heating value and combustion index, indicating excellent fuel quality [43].

In addition to feedstock type and pyrolysis temperature, the composition of the reaction atmosphere also plays a crucial role in determining the subsequent combustion performance of pyrolysis products. Studies have shown that different atmospheres (e.g., N₂, CO₂, CH₄, and CO) significantly affect the yields of liquid products and the characteristics of biochar. Under a CH₄ atmosphere, pyrolysis tends to produce lower yields of biochar and higher liquid fractions, while a CO atmosphere results in relatively higher biochar yields [44]. Moreover, although the presence of CO₂ may reduce overall yields, the reactivity of the resulting biochar is comparable to that obtained under N₂ conditions [45], and the physicochemical properties of biochar produced under different atmospheres exhibit certain similarities [46].

Beyond acting as a primary fuel, biochar can also be incorporated into cementitious materials as an additive to enhance functional properties. Studies have shown [47,48] that moderate biochar incorporation can improve the compressive and flexural strength of cement mortars (Figure 11), while also modifying pore structure and thermal properties to enhance insulation and structural stability. Under controlled pyrolysis temperatures and appropriate dosing, the porous structure and surface reactivity of biochar can reduce thermal conductivity and enhance mechanical performance. Furthermore, when processed into cold-bonded lightweight aggregates [49], biochar can undergo curing treatments to improve structural integrity, reduce water absorption and powdering, and provide potential carbon sequestration benefits, offering a material solution that balances mechanical performance and environmental sustainability. These composite materials have shown the potential to maintain mechanical strength while avoiding carbon emissions associated with traditional aggregate calcination, indicating broad application prospects in building energy efficiency and carbon reduction.

From a lifecycle and environmental perspective, Wang et al. [50] reported that accelerated carbonation treatment of biochar–cement composites can further enhance both carbon sequestration and mechanical strength, providing an integrated solution combining resource recycling, decarbonization, and material enhancement for the construction industry. As illustrated in Figure 12, the resource circulation pathway involving biochar and cement-based materials demonstrates the synergistic role of hydration and carbonation reactions in enhancing composite performance.

Nevertheless, large-scale application of biochar as a fuel substitute in the cement industry still faces several challenges. First, significant variability in the physicochemical properties of biochar from different sources, such as ash content and alkali metal concentrations, can affect the thermodynamic stability of cement kiln systems. Additionally, heterogeneity in particle size and moisture content may pose technical difficulties in combustion control and powder feeding. Moreover, producing large quantities of stable, low-impurity, and high-energy-value biochar requires the systematic optimization of feedstock pretreatment and pyrolysis processes.

In summary, the use of biochar as an alternative fuel in the cement industry not only offers significant benefits in terms of carbon reduction and energy efficiency, but also contributes to material performance enhancement and environmental friendliness. Future development should focus on improving pyrolysis efficiency, standardizing combustion characteristics, and developing scalable adaptation technologies to enable their broader adoption in green construction and clean production.

3.1.2. Boiler Systems

In recent years, biochar has shown considerable application potential as a coal substitute or additive in industrial boiler systems. Compared to raw biomass, biochar exhibits superior combustion characteristics, lower pollutant emissions, and greater process adaptability, making it particularly well-suited for use in existing fixed-bed, fluidized-bed, and suspension boiler systems.

A number of empirical studies have systematically evaluated the performance of biochar as a fuel substitute in boilers. Zhang et al. [51] prepared various pyrolysis chars from agricultural and forestry residues at 200–300 °C and assessed their grindability, particle size distribution, and burnout behavior. The results indicated that pyrolysis significantly enhanced the Hardgrove Grindability Index (HGI [52]) of the biochar, facilitating efficient pulverization and stable combustion in fluidized-bed boilers. Sunarno et al. [53] used empty fruit bunches (EFB) from oil palm as feedstock and employed peat water pretreatment followed by co-pyrolysis to produce biochar with a high heating value of 34.36 MJ/kg and an energy yield of 91.16%, demonstrating its potential as a coal substitute in boiler systems.

The preparation method of biochar directly influences its phase structure, surface functional groups, and application scope. Currently, two primary techniques are widely used: pyrolytic carbonization and hydrothermal carbonization [54,55]. Pyrolytic carbonization is typically conducted under oxygen-limited or inert atmospheres at elevated temperatures, yielding biochar with a high degree of aromaticity, calorific value, and structural stability, and making it suitable for high-temperature combustion or use as a reducing agent in metallurgical systems. In contrast, hydrothermal carbonization is performed in aqueous environments at 130–250 °C under milder conditions and lower energy requirements. The resulting hydrochars are spherical, porous, low in ash, and rich in oxygen-containing functional groups, making them suitable for applications in environmental remediation, catalyst supports, or low-grade fuel substitution [56,57].

Biochars derived from different thermal treatments exhibit markedly different performance profiles in boiler systems (Figure 13). Studies show that increasing hydrothermal carbonization temperature can enhance thermal stability and critical heat flux (CHF), reduce ignition delay, and improve burnout efficiency, making hydrochars particularly suitable for replacing low-rank coal [58]. Low-temperature pyrolysis chars, on the other hand, retain a high energy conversion efficiency while reducing nitrogen and sulfur contents, offering favorable environmental benefits and stable combustion behavior [59]. Moreover, combining pretreatment techniques such as water washing–roasting or two-step thermal treatments can further optimize ash content and alkali metal composition, effectively reducing PM₁₀ emissions and enhancing compatibility with small- to medium-scale boiler systems [60].

Each type of biochar offers unique structural and functional advantages, allowing differentiated deployment in boiler systems based on specific operational requirements. Pyrolytic biochar is better suited for high-temperature, high-intensity environments, while hydrothermal biochar offers low-emission and environmentally friendly properties, making it ideal for clean combustion scenarios where emission control is critical. Future research should focus on elucidating the coupled mechanisms among carbon structures, reactivity, and emission behavior, and on exploring hybrid processing pathways to enhance the engineering adaptability and carbon reduction potential of biochar in coal-fired boiler substitution.

In terms of environmental performance, Sztancs et al. [61] conducted a life cycle assessment to evaluate the greenhouse gas mitigation potential of co-firing catalytic hydrochar with coal. The results indicated that, at a 25 wt.% blending ratio, net CO₂ emissions per unit electricity generation decreased by up to 1.54 kgCO₂-eq/kWh, underscoring biochar’s strong support for decarbonizing boiler systems (Figure 14).

However, the presence of alkali metals in biochar (e.g., K, Cl, S) may lead to slagging, fouling, and corrosion, presenting a key barrier to its large-scale deployment. To address this, researchers have proposed a range of feedstock pretreatment and process modification strategies to improve biochar quality [62,63,64]. For example, acid pretreatment combined with co-pyrolysis using waste cooking oil can significantly reduce K₂O content while increasing the higher heating value (HHV) to 31.97 MJ/kg, thereby enhancing thermal stability and substitution potential in small- to medium-scale boiler applications [65]. Similarly, a two-stage thermal treatment followed by water washing has been shown to effectively remove undesirable elements such as K, S, and Cl from raw biomass, reducing the corrosivity and slagging tendency of the resulting biochar and improving its operational stability as a primary fuel [66].

Biochar is also frequently co-fired as a supplementary fuel with coal dust, gasification slag, or sewage sludge, showing significant synergistic effects. On the one hand, its high volatile matter content and reactivity contribute to improved burnout efficiency and reduced emissions of CO and NO_x (Figure 15) [67,68]. On the other hand, the mineral elements present in biochar promote the transformation of sulfur and chlorine into stable mineral forms, thereby inhibiting SO₂ release and facilitating the formation of sulfur-fixing compounds such as K₂SO₄ (Figure 16) [69]. During co-firing, heavy metals such as Cr and Cd are more likely to transform into stable forms and become immobilized in ash residues, offering environmental safety and the potential for resource recovery [67]. Studies have also shown that optimizing biochar blending ratios (typically 10–40%) can achieve a favorable balance between thermal efficiency, pollutant reduction, and fuel synergy [68,69].

As a co-firing substitute in boiler systems, biochar demonstrates favorable fuel properties, including moderate calorific value, high volatile content, and strong combustion reactivity. It also significantly reduces emissions of gaseous pollutants such as SO₂ and NO_x, providing strong environmental co-benefits. Furthermore, as a carbon-neutral fuel, biochar contributes to the decarbonization of boiler systems. While large-scale co-firing still faces challenges related to dust emissions, burnout control, and combustion stability, the inherent adaptability and tunability of biochar make it a promising bridge for transitioning traditional “high-carbon, high-pollution” boiler systems toward “low-carbon, high-efficiency” operations, laying a practical foundation for its future application under high-temperature and complex operational conditions.

3.1.3. Combined Heat and Power (CHP) Systems

Combined heat and power (CHP) systems have become a key pathway for the integrated utilization of biomass resources due to their high energy efficiency and strong adaptability. With the advancement of biomass gasification and pyrolysis technologies, biochar—as a solid-phase byproduct—has demonstrated increasing potential for synergistic energy production and environmental co-benefits in CHP systems. In recent years, researchers have begun to evaluate the application value of biochar-based CHP systems from multiple dimensions, including energy performance, environmental impact, and economic feasibility. This is particularly evident in integrated practices within the agricultural and food processing sectors, where such systems provide typical scenarios for implementation.

For example, Aguado et al. [70] developed a fixed-bed gasification-based micro-CHP system utilizing olive and almond residues as feedstock in the agro-food industry. The resulting biochar not only served as a soil amendment, but also contributed to carbon sequestration, thereby achieving synergistic benefits across energy production, agriculture, and environmental protection. A by-product flow diagram of the olive oil industry is illustrated in Figure 17.

In another study, Fernández-Lobato et al. [71] conducted a comparative environmental assessment of integrating a gasification-based CHP system into the olive oil production chain using the life cycle assessment (LCA) methodology. The results showed that, compared to conventional treatment pathways, the implementation of the gasification system reduced greenhouse gas emissions by 21%. Moreover, the net carbon flux during the industrial phase became negative (−0.51 kgCO₂-eq./kg olive oil), indicating a significant carbon sink effect.

This study highlights the potential of biochar as a carbon sequestration medium and demonstrates that gasification-based CHP systems can provide a viable pathway toward carbon neutrality, or even net-negative emissions. The integrated system is illustrated in Figure 18, where dried olive pomace pellets are used as the primary feedstock for a downdraft gasifier that produces combustible gas to power an internal combustion generator for electricity production. Simultaneously, thermal energy and biochar are generated as byproducts. An auxiliary burner and a hot water recovery system are employed to close the energy loop. The system integrates three key functions: multi-energy co-generation, waste valorization, and carbon emission reduction.

Beyond its carbon reduction potential, biochar has also demonstrated promising capabilities in agricultural co-utilization. When CHP systems are deployed in controlled agricultural environments, such as indoor cultivation facilities, they enable the co-production of heat, electricity, and biochar. The returned application of biochar to soil has been shown to enhance crop yields by 7.7–33.9%, while the system exhibits a short investment payback period, indicating a strong economic feasibility [72]. In terms of gasification pathways, supercritical water gasification (SCWG) has been identified as an effective approach to improve both gas yields and synergy with char byproducts. Studies have reported a hydrogen yield of 14.95 mol/kg at 700 °C, along with the simultaneous production of solid char, thereby achieving a “low-temperature, high-efficiency, dual-product” mode of energy conversion [73].

In anaerobic digestion systems, the addition of biochar as a co-digestion agent has been shown to enhance system stability, optimize microbial community structures, and significantly increase methane production. This strengthens both gas output performance and process controllability [74]. More advanced integration efforts include the development of multi-generation systems based on biochar composite materials that simultaneously harness solar, thermal, and electrical energy. Research indicates that such systems, through the effective coupling of photothermal conversion and power generation, can achieve an overall efficiency of up to 97.2%, demonstrating biochar’s broad adaptability and potential for integration in multi-path renewable energy technologies (Figure 19) [75].

The role of biochar in CHP systems has evolved from a mere byproduct of fuel processing to a core functional component within the system, encompassing carbon sequestration, soil enhancement, pollutant adsorption, and energy integration. Its applications within CHP pathways continue to expand, covering energy production, environmental remediation, agricultural productivity enhancement, and carbon neutrality strategies. Future research should focus on system integration optimization, comparative analysis of different biochar materials, mechanisms of thermo-electric coupling, and the expansion of economic boundaries. These directions will further drive the industrialization and regional deployment of biochar-based CHP systems.

3.2. Feedstock Substitution

Compared to traditional reductants such as coke, biochar not only provides an effective carbon source at high temperatures, but also demonstrates a high reactivity, enabling efficient carbothermic reduction reactions with metal oxides. This enhances the reduction efficiency while improving environmental performance. In recent years, driven by carbon reduction targets and the push for solid waste valorization, biochar—due to its high carbon content, strong reductive capability, and environmental friendliness—has emerged as a promising candidate to replace fossil-derived carbon sources as a metallurgical reductant.

3.2.1. Direct Reduction

With the metallurgical industry increasingly pursuing low-carbon emission goals, the use of biochar—a green, renewable carbon source—in direct reduction (DR) processes has gained growing attention. Conventional DR methods typically rely on coal-based reductants, which are associated with high energy consumption and substantial CO₂ emissions. In contrast, biochar possesses a high fixed carbon content, low sulfur and ash levels, good reactivity, and inherent carbon neutrality, making it a highly promising alternative reductant.

Attah-Kyei et al. [76] employed various types of biochar to reduce copper smelting slag at 1250–1350 °C, achieving the recovery of metals such as copper, nickel, and iron. The results showed that biochar outperformed conventional coke in reduction efficiency at lower temperatures, and also promoted the formation of metal alloy phases. Ardados et al. [77] developed a novel high-temperature plasma smelting process in which olive stone-derived biochar was used as a substitute for anthracite to reduce copper smelting dust, enabling the effective enrichment and recovery of Zn and Pb. Ge et al. [78] utilized straw-derived biochar as a reductant to convert iron from potassium jarosite residue into metallic iron through high-temperature roasting. As shown in Figure 20, roasting temperature, biochar type, dosage, and roasting time significantly affected the metallic iron content in the reduction product. Under optimal conditions—950 °C for 60 min—the iron recovery rate reached 83.20%, with a metallic iron purity of up to 93.54%. These studies collectively validate the technical feasibility of biochar as a carbon source for metal recovery from solid waste, offering a low-carbon pathway for advancing sustainable metallurgical practices.

The application of biochar as a reductant also presents a novel and sustainable strategy for the green regeneration of spent lithium-ion batteries. Mei et al. [79] demonstrated that, under high-temperature conditions, biochar produced via rapid pyrolysis could directly function as a carbothermic reductant. Through a synergistic effect between the solid biochar and the pyrolysis gases, the efficient recovery of Co and Li from LiCoO₂ was achieved, with lithium recovery reaching as high as 98.18%.

In metallurgical processes, the role of biochar is increasingly significant. Chuanchai et al. [80] used mushroom cultivation residue-derived biochar as a substitute for coke in the reduction of Fe₂O₃. The study found that, at a C/O ratio of 0.8, the metallization rate of biochar reached 31.99%, with fixed carbon and volatile matter playing key roles in the reduction process. This confirmed the applicability of agricultural residue-based biochar for the direct reduction of iron ore and provided experimental support for optimizing biochar formulations. Xu et al. [81] employed gasification-residue biochar as a replacement for charcoal in the carbothermic reduction process for metallurgical-grade silicon. The study systematically investigated the influence of particle size on mechanical strength, porosity, and reactivity. Results indicated that biochar briquettes composed of fine particles met the requirements for both strength and reductive activity in polysilicon production, positioning biochar as a viable novel carbon source in this field. Dornig et al. [82] validated the applicability of woody biochar in a solid–gas ZnO reduction through thermogravimetric analysis and Waelz kiln simulation experiments. Ye et al. [83] utilized waste woody biochar for the microwave-assisted carbothermic prereduction of chromite. The study confirmed that biochar can act as a direct solid-phase reductant, achieving high metallization degrees of Fe and Cr (98.1% and 86.8%, respectively) at 1100 °C. The fixed carbon in biochar reacted with ore phases to form (Fe, Cr)₇C₃ alloy, significantly reducing both smelting temperature and energy consumption (Figure 21).

In the field of non-ferrous metallurgy, Tan et al. [84] investigated the reduction behavior of pine sawdust-derived biochar on antimony trioxide (Sb₂O₃). The study revealed that the biomass pyrolysis-based reduction process can be divided into four distinct stages (Figure 22): dehydration, pyrolysis, pyrolytic reduction, and biochar reduction. By optimizing reduction parameters, the average reduction rate of metallic antimony reached 88.20%. Compared with conventional coal-based reduction processes, the CO₂ emissions were reduced by 63.28%, further demonstrating the feasibility of biochar in achieving low-carbon metallurgy for the direct reduction of non-ferrous metal oxides.

In addition, the content of mineral elements (e.g., K, Ca, Mg, Cl) in different biomass feedstocks can significantly affect pyrolysis behavior and the structural stability of the resulting biochar. Gan et al. [85] developed a combined water-leaching and pyrolysis process (shown in Figure 23) to produce high-quality corn stalk biochar. This treatment effectively removed impurity elements such as K, Na, and Cl from the biomass, thereby mitigating issues such as furnace wall slagging and slag formation during metallurgical operations. The water-leached biochar exhibited high heating values (26–29 MJ/kg) and excellent combustion performance, with combustion activation energy approaching that of anthracite. It was found to be suitable for high-temperature metallurgical applications, such as direct reduced iron (DRI) pellet sintering, highlighting the resource utilization potential of agricultural waste in the iron and steel industry.

Overall, the application of biochar in direct reduction processes demonstrates high reactivity, excellent metal recovery efficiency, and significant carbon mitigation potential. Especially in systems involving iron ores, complex polymetallic ores, and non-ferrous metal oxides, biochar not only serves as a viable substitute for conventional coal-based reductants, but also helps reduce reduction temperatures and improve overall energy efficiency. In the future, further advancements in biochar preparation technologies (e.g., impurity removal by water-leaching, co-pyrolysis), along with the integration of auxiliary energy methods such as microwave heating, are expected to expand the application scope of biochar in green and low-carbon metallurgy.

3.2.2. Blast Furnace Industry

In recent years, with the introduction of carbon neutrality goals in the steel industry, biochar has garnered increasing attention as a carbon-neutral substitute fuel in blast furnace (BF) operations. Conventional BF ironmaking processes rely heavily on coke and pulverized coal injection (PCI), leading to substantial CO₂ emissions. Using biochar as an injection or blended fuel offers a technically feasible and environmentally friendly pathway, leveraging its carbon-neutral nature to reduce emissions while maintaining or enhancing calorific value and reactivity.

Liu et al. [86] experimentally assessed the ignition temperatures (250–300 °C), combustion rates, and low-pollution emission characteristics of various agricultural and forestry waste-derived biochars, confirming their practical potential to replace pulverized coal. Meng et al. [87], using material flow analysis and carbon accounting models, demonstrated that biochar application in the BF route could reduce CO₂ emissions by up to 66.9%, with the BF stage alone contributing 73.7% of the total reduction, highlighting it as a key emission mitigation node.

Residence time, as another critical process parameter [55], has limited influence on biochar yield [88], but significantly affects the structural evolution of the char. Longer residence times promote further carbon chain rearrangement and condensation reactions, increasing the fixed carbon content and stabilizing the pore structure [89]. However, under lower pyrolysis temperature ranges, extended residence time may induce the re-polymerization or volatilization of certain components, resulting in reduced biochar yield [90]. Hence, pyrolysis temperature and residence time should be optimized in a coordinated manner based on the target application. Solar et al. [91] investigated the injection performance of pine-derived biochar prepared at different pyrolysis temperatures (300–900 °C) (Figure 24). The results showed that, although high-temperature pyrolysis (≥700 °C) improved carbon content and thermal stability, it also reduced volatile matter and reactivity. Under typical industrial O/C ratios (≥1.4), coarse-grained biochar (45–200 μm) produced at 700 °C achieved a higher burnout rate than coal within 20 ms, demonstrating superior reactivity in short-residence-time, high-temperature injection environments and providing experimental support for biochar as a substitute for PCI.

Regarding preparation methods and particle morphology, Dang et al. [92] proposed a hybrid “hydrothermal carbonization + pyrolysis” upgrading strategy to produce high-quality injection-grade fuels with enhanced graphitization, reduced ash, and lower alkali metal content. This significantly improved char stability and gasification reactivity, making it suitable for high-temperature BF injection. Wang et al. [93] used three-dimensional CFD simulations to evaluate the effects of particle shape and pyrolysis temperature on injection behavior, finding that spherical particles and mid-temperature pyrolysis char offered the best overall performance (Figure 25).

As a potential substitute for pulverized coal (PC), biochar needs to exhibit a comparable proximate analysis, elemental composition, and heating value, while offering superior combustibility and gasification reactivity [94]. However, many biochars are rich in ash and alkali metals, which can shorten BF campaign life [95,96,97]. To mitigate this issue, various desalination approaches using deionized water, acidic, or alkaline solutions have been explored to pretreat biomass or biochar [98,99], though these add cost and pose wastewater treatment challenges. Selecting low-ash biomass feedstocks [100] for pyrolysis is considered a promising strategy to minimize ash and alkali metal issues. In a study by Yang et al. [101], low-ash mango pit (MP) biomass was used to produce biochar, and its fuel properties were further evaluated via principal component analysis (PCA). Biochar prepared at 600 °C for 60 min (MPB-600-60) exhibited the fuel characteristics most similar to PCI, while containing lower ash and alkali metal levels. With a heating value of 26–33 MJ/kg, its use in BF operations could reduce emissions by approximately 220 kgCO₂-eq/tHM, offering both environmental and economic advantages. The detailed fuel property evaluation of mango pit biochars prepared under different pyrolysis conditions, including mass and energy yields, energy densification, and Van Krevelen diagrams, is shown in Figure 26.

Biochar exhibits notable burnout reactivity, carbon-neutral emission potential, and environmental benefits when applied in BF injection. By selecting appropriate feedstocks, optimizing carbonization processes (e.g., mid-temperature pyrolysis or hydrothermal– pyrolysis coupling), improving particle morphology, and adopting coal–biochar co-injection strategies, the effectiveness of biochar in BF ironmaking can be further enhanced. These strategies offer practical and scalable pathways for promoting low-carbon transformation in the steel industry.

3.2.3. Application in Electric Arc Furnace (EAF)

In steelmaking, biochar—being a renewable carbon source—has been widely investigated as a partial substitute for conventional fossil fuels (e.g., coke, coal) to support carbon reduction objectives. With the increasing use of recycled scrap and the advancement of green steelmaking technologies, the electric arc furnace (EAF) has emerged as one of the key production routes in the steel industry. Although EAF processes emit less direct CO₂ than blast furnaces (approximately 0.4 tCO₂/t steel), they still rely on fossil carbon sources (e.g., graphite, coke, anthracite) for functions such as slag foaming, fluxing, and carburization, thus remaining a significant source of emissions. Consequently, replacing traditional carbon with renewable biochar has become a key research direction for decarbonization.

Yunos et al. [102] produced biochar from agricultural residues such as grape seeds and pumpkin seeds. Upon testing its interaction with EAF slag, the biochar exhibited a good thermal stability and reducing capacity, low ash content, and clean gasification products. It showed minimal impact on metal purity and arc stability, and no significant spattering or structural collapse was observed during melting simulations. These findings suggest strong compatibility for applications in carburization and slag foaming control. Han et al. [103] co-pyrolyzed straw and EAF dust to produce high-density, high-reactivity modified biochar. They confirmed that the metal oxides in the dust catalyzed carbon structure evolution, significantly enhancing the reduction rate and gas release during reaction with slag, thereby improving injection efficiency. These studies demonstrate that biochar, as a renewable resource, holds substantial carbon mitigation potential and can support the goal of “near-zero-carbon steelmaking” via the EAF route [104].

The heating rate during biochar preparation plays a regulatory role in pyrolysis pathways and product distribution. Slow pyrolysis (<20 °C/min) typically yields higher biochar proportions with a stable carbon structure, suitable for solid fuel and adsorbent production. In contrast, fast pyrolysis (>100 °C/min) favors the generation of bio-oil and gases, while producing more porous but less aromatic chars better suited for short-lifecycle applications such as soil amendments or reductants [89,105]. Stepwise or staged pyrolysis methods have also gained attention for enabling hierarchical energy utilization and tailored char properties via controlled temperature ramps, heating rates, and residence times. Adetoyese et al. [106] found that a three-stage pyrolysis method for wood achieved 30% energy savings and reduced reaction time. Elyounssi et al. [107] reported that two-step pyrolysis of black locust yielded both high biochar yield and quality while shortening the pyrolysis duration.

Cardarelli et al. [108] used three-dimensional computational fluid dynamics (CFD) modeling to analyze the combustion behavior of biochars prepared via different pretreatment methods under EAF injection conditions (Figure 27). The study showed that biochars produced via hydrothermal carbonization (HTC) and slow pyrolysis exhibited comparable carbon reactivity, heat release characteristics, and thermal stability to conventional injection coal. These chars formed localized high-temperature zones in the arc region without disrupting slag–metal interactions, indicating good substitution potential.

Wei et al. [109] systematically investigated the foaming performance and mechanism of biochar prepared from pine sawdust via HTC for use as an EAF foaming agent. By optimizing HTC temperature, stirring speed, solid-to-liquid ratio, and residence time, the resulting biochar had a high fixed carbon content and porosity. This extended the effective foaming time of slag to 353 s, significantly outperforming fossil-based foaming agents (219 s). The slag viscosity remained within the optimal range (0.30–0.32 Pa·s), supporting stable foam formation, reducing power consumption, and enhancing steelmaking efficiency. Given its renewable origin and carbon-neutral characteristics, HTC biochar offers a viable demonstration of biomass utilization in energy-saving and low-carbon EAF steelmaking. The schematic of the HTC reactor and process flow is shown in Figure 28.

Beyond its direct use as a carbon source, biochar can also facilitate EAF slag valorization and functional modification. Zhang et al. [110] fabricated composite briquettes using biochar and EAF slag, then subjected them to CO₂ curing to promote carbon sequestration and develop high-strength construction materials. With 5% biochar addition, 25 MPa compaction pressure, and 6 h CO₂ curing, the resulting material achieved a compressive strength of 64.49 MPa and improved CO₂ absorption efficiency by 1.3%, indicating excellent mechanical and carbon storage performance, as shown in Figure 29.

Han et al. [111] further explored the mechanism by which biochar enhances the reactivity of EAF slag. During thermal treatment, carbon in the char was found to interact with Ca, Si, and Fe in the slag, promoting the formation of reactive silicate phases such as β-C₂S, which improved hydraulic reactivity. This suggests that biochar not only supports slag valorization, but can also partially substitute cement clinker in binders, thus combining carbon reduction with solid waste recycling.

In summary, biochar in EAF systems serves multiple roles, including carburization, slag foaming control, and metal reduction. It also contributes to slag valorization, microwave-assisted reduction, and carbon-sequestering construction materials. As process optimization advances and local biochar supply chains develop, biochar is poised to become a key enabler in building low-carbon EAF steelmaking systems.

3.2.4. Biochar-Mediated Reduction of Oxidative Pollutants and Metal Oxides

As a renewable carbonaceous material, biochar exhibits a rich electron-donating capability, active site regulation effects, and synergistic interactions with metal loadings in the reductive transformation of various oxidative pollutants and metal oxides. However, current studies are mostly limited to mechanistic verification and demonstrative applications, with insufficient systematic comparisons across different systems and integration between domains. To further elucidate the representative mechanisms and reaction features of biochar in diverse reductive processes, recent representative studies are summarized in

Table 3. Representative studies of biochar in typical reduction reactions.

Target Pollutant	Reduction Role of Biochar	Main Mechanism	Source
NO2	Direct reduction	Surface phenolic hydroxyl groups react with NO2 for chemical reduction (low-temperature and pyrolysis reduction)	[112]
Cr(VI)	Direct reduction	–OH functional groups provide electrons, reducing part of Cr(VI) to Cr(III) (with Ca2+ precipitation simultaneously)	[113]
NO	Direct reduction regulation	Nitrogen-active sites and –OH functional groups regulate NO generation and reduction (DFT mechanism)	[114]
TC (tetracycline)	Direct electron donor	B-doped sp2 carbon electrons accelerate Fe(III) → Fe(II) and O2 activation	[115]
NO3− → NH4+	Direct electron supply synergy	Phenolic hydroxyl groups on biochar surface synergistically photogenerate electrons to reduce NO3−	[116]
NO3− → NH4+ (DNRA)	Indirect electron donor (electron shuttle)	Π-electron structure/free radical-mediated microbial DNRA electron transfer	[117]
Fe2O3 → ZVI (preparation)	Direct pyrolysis reduction	CO and H2 released from biomass pyrolysis, together with the carbon skeleton, reduce Fe2O3 to ZVI; subsequently, ZVI reduces NO2	[118]
NOx(SCR)	Indirect (carrier/active sites)	CeOx-BC biochar functional groups inhibit metal–oxygen vacancy regulation	[119]
NOx(SCR)	Indirect (carrier/electron transfer)	Nitrogen doping enhances NO adsorption and O2 activation, awakening the synergy of rice husk	[120]
NOx(SCR)	Indirect (carrier/dispersion)	Mn/TiO2-BC carrier enhances dispersion of active components and oxygen vacancy regulation	[121]
BrO3−	Indirect (carrier/stabilization)	Co/BC composite structure regulates Co phase transformation, promoting BrO3− reduction	[122]
NO3−−	Indirect (loading metal active sites)	Sewage sludge biochar loads nZVI, accelerating electron transfer and inhibiting agglomeration	[123]
Nitroaromatics	Carrier/dispersion	Pd/Fe3O4@BC, NaBH4 provides electrons	[124]
Nitroaromatics	Carrier coordination regulation	Ni-Nx@NPC/B, formic acid provides hydrogen	[125]
Norfloxacin	Indirect (adsorption–reduction coupling)	Nitrogen-doped biochar provides porous adsorption sites and N-containing groups	[126]
CO2	Electrode carrier	NiPx/N-BMRC, external power supply provides electrons	[127]
4-NP	Carrier/electron transfer	nZVI@Fe3O4/Cu, NaBH4·H2O2	[128]
4-NP	Carrier/dispersion	Ag@Ca-BC, NaBH4	[129]
4-NP	Carrier-assisted catalysis	MoOx-BC, NaBH4	[130]
4-NP	Direct electron donor/active oxygen generation	NHPC-800 biochar itself coordinately regulates electron transfer and directly provides electrons for reduction	[131]
4-NP	Indirect (adsorption –reduction coupling)	Biochar skeleton acts as a carrier for AgNPs to improve dispersion and stability of metal nanoparticles	[132]

, aiming to provide a reference for optimizing its carbothermic reduction performance and enhancing its potential as an efficient substitute for fossil-based carbon sources.

4. Challenge and Future Prospects

With the global advancement of carbon emission reduction initiatives, biochar—as a carbon-neutral, renewable, and multifunctional solid carbon material—is rapidly transitioning from laboratory-scale research to engineering practice and industrial deployment. A comprehensive review of current research and application cases of biochar in the two major energy substitution pathways—fuel replacement and feedstock substitution—reveals its high adaptability and engineering compatibility across diverse energy scenarios such as cement kilns, boilers, blast furnaces, electric arc furnaces, and CHP systems.

Biochar demonstrates several common advantages:

• High fixed carbon content, porous structure, and good reactivity, which enhance burnout efficiency and reaction kinetics.
• Wide availability and low carbon footprint, contributing to reduced emissions of CO₂, SO₂, and NO_x.
• Carrier functionality, carbon sequestration capacity, and surface functionalization potential, enabling biochar to support multifunctional integration in energy systems.

In terms of practical industrial applications, several pioneering case studies have been reported. For example, in Ireland, the Arigna Group has developed a biochar-based fuel named “Harvest Flame,” produced from olive pits through pyrolysis/carbonization, achieving performance and cost competitiveness with anthracite in commercial markets. In Australia, a research team at RMIT University has converted spent coffee grounds into biochar, which can replace up to 15% of sand in concrete mixtures, improving compressive strength by 30% and reducing cement usage by 10%, with pilot implementation in municipal sidewalk projects. In the steel sector, biochar has demonstrated potential as a partial substitute for coke or coal in fuel injection and as a process reductant, though such applications remain largely in the technical evaluation phase. While these examples illustrate promising site-specific deployments, large-scale, continuous production routes for industrial biochar utilization are still scarce, and no mature technology pathway has yet achieved widespread adoption.

So, several key challenges must be addressed to fully realize the potential of biochar in energy applications:

• Feedstock heterogeneity leads to significant variability in the structure, ash composition, and calorific value of the resulting biochar. The lack of standardized classification and performance evaluation systems hampers its large-scale engineering use.
• High-temperature/high-pressure stability, mechanical strength, and interfacial mechanisms with burden materials and metal oxides remain poorly understood, especially in complex systems such as metallurgy and carbothermic reduction, where multiphase interactions at the char–gas–slag–metal interface are still inadequately characterized.
• In most current energy systems, biochar remains in an auxiliary or blended role, lacking system-level solutions that position it as the core functional carbon source, limiting its strategic value in mainstream energy pathways.
• It is worth noting that the applicability of biochar may vary significantly across regions due to differences in biomass resource availability, energy infrastructure, and economic conditions. A more detailed, region-specific assessment could further guide its large-scale implementation, which we identify as an important direction for future research.
• Comprehensive techno-economic assessments and life cycle analyses remain insufficiently integrated across biochar applications. For industrial adoption—particularly in fuel and feedstock substitution—considerations such as cost competitiveness, energy return on investment (EROI), and payback time should be incorporated to ensure both economic and environmental viability.

To overcome these challenges, future research should prioritize the following directions (Figure 30):

• Establishing Structure–Property Relationships: Strengthening fundamental research on feedstock–structure–performance correlations to develop predictive models and application evaluation frameworks, enabling “on-demand design and targeted preparation” of biochar.
• Optimizing Pyrolysis Techniques: Developing advanced pyrolysis methods (e.g., microwave-assisted, plasma-enhanced, and electric field-regulated processes) to enhance structural ordering and energy efficiency while reducing production costs.
• Elucidating Reaction Mechanisms: Advancing multiscale reaction kinetics and interfacial studies, particularly in high-complexity processes such as carbothermal reduction (CTR), metal reduction, and environmental catalysis, with a focus on reaction pathways and electron transfer mechanisms.
• Integrating Cross-Cutting Technologies: Promoting synergies between biochar and hydrogen energy, carbon capture, utilization, and storage (CCUS), as well as energy storage systems to develop multifunctional hybrid systems integrating fuel, carbon sequestration, catalysis, and energy storage.
• Enhancing Policy and Industrial Adoption: Creating policy and industrial incentives to integrate biochar into key energy and metallurgical supply chains could reinforce its role as a crucial low-carbon resource. This would help foster broader adoption and contribute to sustainable development goals.

Figure 30. Future research directions for biochar-based energy systems.

Figure 30. Future research directions for biochar-based energy systems.

Beyond conventional sectors like cement and steel, the feedstock substitution potential of biochar in other high-carbon industries also merits attention. In the sulfate industry, elemental sulfur is typically produced via thermochemical reduction using fossil carbon, hydrocarbons, or reducing gases [133,134]. However, given that the metallurgical industry has successfully explored biochar for reducing metal oxides such as Cu and Zn, it raises the following question: Could biochar offer an innovative route for sulfur production in sulfate processing under high-temperature conditions? On the one hand, its high fixed carbon content allows for thermodynamic and reactive environment control during reduction. On the other hand, its renewability and low net carbon emissions offer a novel decarbonization approach for such high-carbon processes. If biochar can demonstrate comparable reductive performance to traditional agents in sulfate carbothermic reduction, it would not only enable sulfate resource recovery, but also expand biochar’s use as a reductant for gaseous pollutant mitigation, broadening its engineering applicability.

Despite these advances, challenges remain in raw material variability, structural control, and interfacial reaction mechanisms, limiting its further promotion in complex high-temperature environments. Future research should focus on pyrolysis process optimization, char structure tuning, interfacial reaction analysis, and life-cycle environmental performance assessments to establish a robust feedstock–structure–application coupling framework, as illustrated in Figure 31. In addition, biochar’s reductive capabilities could be extended to new high-carbon applications (e.g., sulfates, SO₂), transitioning its role from a substitute to a functional carbon source with enhanced process integration.

In summary, as thermodynamic and kinetic mechanisms become better understood, the substitution potential of biochar in high-carbon processes is expected to be systematically developed. Future efforts should emphasize its application in high-temperature reduction systems and conduct thorough assessments of industrialization pathways, promoting its dual role as a feedstock and energy substitute in broader industrial contexts.

5. Conclusions

This review systematically examined the application of biochar as a substitute material in energy systems, with a specific focus on its roles in fuel substitution (cement and boiler sectors) and feedstock substitution (carbothermic reduction in metallurgical systems).

In fuel substitution, biochar’s high calorific value, low ash content, good combustibility, and carbon-neutral nature support its use in cement kilns and industrial boilers. Combustion tests and life cycle assessments confirm its ability to partially or fully replace coal, reduce CO₂, SO_x, and NO_x emissions, and enhance overall system safety and process compatibility. In feedstock substitution, biochar’s high fixed carbon content, good reactivity, and low impurity levels have enabled it to replace charcoal or coke in various metallurgical applications. Studies have demonstrated its effectiveness in direct reduction, blast furnace injection, EAF carburization, and slag foaming control, highlighting its broad potential as a renewable functional carbon source for low-carbon metallurgy.

In conclusion, biochar is not merely a low-carbon alternative, but a critical bridge connecting energy utilization, resource recovery, and environmental mitigation. Its deployment in energy systems holds strategic significance for achieving industrial decarbonization goals and building a clean, low-carbon energy future.