Biochar Production Technology as a Negative Emission Strategy: A Review

Abstract

The urgent need to reduce greenhouse gas emissions and shift towards renewable energy has increased attention on biochar as a viable negative emission strategy. This review assesses the potential of biochar produced from organic and waste biomass via thermochemical processes—including pyrolysis, gasification, and hydrothermal carbonization—to address climate and energy challenges. Recent advances in biochar production are critically examined, highlighting how process design controls improve key properties such as carbon stability, atomic ratios, porosity, and energy density. These factors influence biochar’s performance in carbon sequestration and its utility across industrial sectors, ranging from agriculture and construction to energy generation and carbon capture systems. Results indicate that large-scale adoption of biochar could lower carbon emissions, enhance soil fertility, and produce renewable fuels like hydrogen, while also benefiting circular economy initiatives. However, obstacles remain, including economic costs, feedstock logistics, process optimization, and potential environmental or social impacts. This review underscores that unlocking biochar’s full promise will require interdisciplinary research, robust quality standards, and supportive policies. With integrated efforts across science, industry, and policy, biochar can serve as an effective and sustainable technology for emission reduction and contribute significantly to global carbon neutrality goals.

1. Introduction

The increasing demand for energy and the phenomenon of global warming motivate steps to process alternative energy sources and address greenhouse gases (GHG) worldwide [1]. GHG emissions are projected to reach 60 gigatons of CO₂ (GtCO₂)-eq/year. So maintaining global temperature stability below 1.5 °C becomes difficult. Efforts that can be made to prevent global temperature increases of more than 1.5 °C include reducing GHG by 15 GtCO₂-eq. Biochar can utilize 4.5 tons of CO₂ from waste materials, so it is categorized as an effective negative emission technology [2].

Challenges across various aspects, including economic, environmental, and technological aspects, pose challenges for biochar to meet the requirements of a negative emissions strategy. Economically, the cost challenges for biochar production from biomass as a negative emission technology (NET) are identified in the cost of raw materials, production technology, labor and maintenance, logistics costs, storage costs, and the selling price range of biochar [3]. High costs also need to be addressed regarding production technology which can be reduced by increasing the number of production technology units [4]. Furthermore, biomass transportation and handling costs are high due to its low bulk density, low energy density, and high water content [5]. Furthermore, a government recommendation for a gradual transition from fossil fuel-based power generation to biochar has become a future target. This transition is not only an economic challenge but also drives changes in the workforce and all aspects related to fossil fuel production [6].

Environmentally, the challenges of climate change can impact the potential of biomass energy, leading to more adverse environmental conditions for crop yields, including increased temperatures, flooding or drought, changing rainfall patterns, and extreme winds [7]. Agricultural waste also needs to be managed through a number of methods to produce good biochar. The method chosen depends on the volume and size of the residue, soil type and quality, land location, and slope [8].

The use of biochar as a renewable energy alternative to utilize biomass as a substitute for fossil fuels is the most effective strategy for reducing GHG. As an abundant renewable resource, biomass residues can be easily converted into biofuels through various technologies [9]. The potential for biomass resources reaches 200 million tons of coal equivalent, with a potential increase of 600 million tons of coal equivalent by 2050. However, more than 90% of these biomass resources remain untapped. Utilizing biomass feedstock to produce alternative energy products could be a sustainable solution to meet future commodity demand [10].

Large-scale carbon removal, also known as negative emissions, is considered an important and highly promising scenario for climate mitigation. The implication of this approach is to sequester carbon as effectively as possible to limit global temperature rise [11]. The technologies and approaches used for this carbon removal are known as negative emissions practices/technologies (NEPs or NETs) [2]. Appropriate methods for carbon removal include addressing alternative energy sources, including bio-based sources [12]. Biomass-derived materials, such as biochar, show significant potential for accelerating the development of energy production technologies [13].

Biochar, as a carbon-negative source, can be used to remove emissions (e.g., sulfur) during its production through carbon capture. Carbon capture mechanisms, which determine fixed-carbon formation, are widely used in various industries. CO₂ Capture and Storage (CCS) technology is the primary means of achieving CO₂ emission reductions in today’s industry. One potential approach within CCS technology is the use of CO₂ adsorbents to enhance carbon sequestration [14]. Biochar has the potential to be an energy source, for water and air purification, and as charcoal in home cooking. Other potential benefits include nitrate leaching, absorption of inorganic and organic contaminants and reduction in trace gas emissions from soil and the atmosphere [3].

Charcoal can be categorized into two classes, conventional biochar and hydrochar, based on their production processes, reaction mechanisms, and characteristics. Conventional biochar is produced from dry raw materials (containing up to 10% moisture) through various thermochemical methods. However, these thermochemical processes are very energy-intensive for converting wet biomass such as municipal waste into biochar, as they require preliminary drying, which is economically unfeasible [15]. Charcoal is the largest component of wood decomposition products. Typical wood charcoal contains approximately 80% carbon, 1–3% ash, and 12–15% volatile components [16]. Biomass is rich in organic carbon, oxygen, and hydrogen [17]. The conversion of biomass into liquid, solid, and gaseous products can be achieved through thermochemical processes such as hydrothermal conversion, gasification, and pyrolysis [18]. Biochar can also act as a promising low-cost adsorbent for capturing CO₂ due to its highly porous structure and absorption capacity [19].

Biochar’s strategy to become a negative emission technology is discussed in various topics, namely how biochar is sourced from organic waste with biochar characteristics and its carbon neutrality. Then, biochar production technology through thermochemical processes and the role of biochar in carbon management with various carbon capture mechanisms such as CCS and others. Biochar also has various environmentally friendly and useful applications in various fields such as its contribution in producing hydrogen, in the construction sector, and others. Discussions from techno-economic, socio-environmental perspectives and future directions regarding biochar as a negative emission strategy are also further examined to address challenges in society. The scope and limitations of this review include not delving into non-thermochemical routes and bioenergy products such as bio-oil and syngas. References were selected based on the potential of biochar as a strategy for achieving negative emissions.

2. Biochar from Organic Waste

Char or charcoal is produced by heating organic waste (carbonization) in an oven indoors with various gases, or in a furnace supplied with a limited and controlled amount of air [16]. Energy from biomass can be relied upon as a substitute for fossil fuels due to its low carbon emissions during use and the availability of abundant biomass sources. Energy from biomass reduces CO₂ emissions because it comes from most plants that capture carbon for their growth [7]. Converting biomass to fossil fuels is a phenomenon that has the dual benefit of obtaining more energy and further reducing carbon emissions. Therefore, modification and purification of biochar are proposed to increase its affinity for CO₂. One-step pyrolysis at 800 °C in an ammonia-rich atmosphere as one of the methods is able to produce several benefits, namely carbonization of biomass for the formation of biochar structures, simultaneous nitrogen doping (ammonialization) as an increase in CO₂ capture capacity, high carbon content (~67–80%) is maintained, forming a stable carbon matrix with high microporosity [20].

The quality of biochar is influenced by several aspects, and the application of biochar can also have an impact on the environment. Biochar produced from organic waste has good environmental quality [13]. Thermochemical conversion processes, such as pyrolysis and gasification, play an important role in the production of biochar with high carbon stability and nutrient content [21]. Organic waste that has the potential to be used as biochar is very diverse in terms of feedstock, which also impacts its physical output. Converting various types of biomass feedstock into biochar offers a sustainable mitigation pathway by demonstrating a remarkable CO₂ sequestration capacity. For example, food waste and agricultural residues, thus expanding the scope of biomass resources. Then there is woody biomass, which has the most suitable physicochemical properties for biochar production, with a high surface area and carbon content, as well as low ash and nitrogen content so it can be a raw material that can maximize carbon absorption [3]. There are also municipal solid waste (MSW) and plastic waste (PW) consisting of various organic materials [21] and plantation waste such as spent coffee grounds (SCG) whose production process utilizes exhaust gas with good efficiency and sustainability of biochar production [22]. There is also marine organic waste as a source of carbon-rich raw materials commonly found in various geographic areas along the coastline. If not utilized, this organic material can cause increased CO₂ emissions [23]. In fact, peat soil dried through the pyrolysis process shows extraordinary CO₂ absorption capacity. Analysis of Sumatra peat soil (SPS) decomposition and pyrolysis product distribution found that the organic content in SPS can be decomposed almost completely at temperatures above 500 °C, leaving about 18% inorganic residue [24].

2.1. Biochar Characteristics

There are various ways to produce good-quality biochar. Increasing the pyrolysis temperature increases the carbon content of the biochar, enhancing its stability and carbon sequestration potential. Furthermore, higher carbon concentrations are also associated with lower hydrogen and oxygen contents. For example, increasing the production temperature in the biochar pyrolysis process tends to increase the carbon content. This is important for long-term stability and carbon sequestration [3]. Another example, in the valorization of waste cotton (WC) through direct pyrolysis, elemental analysis showed that biochar stability and carbon retention from the impact of increasing pyrolysis temperature (300–500 °C) showed an increase in carbon content from 53.13% to 73.62% and a reduction in oxygen and hydrogen [25]. Furthermore, the composition of biochar pyrolyzed at 400 °C showed a carbon content of approximately 70%, hydrogen content of slightly less than 4%, and oxygen content ranging from 24% to 25%. Increasing carbon tends to decrease oxygen and hydrogen [26].

The characteristics of the Oxygen-to-Carbon (O/C) and Hydrogen-to-Organic Carbon (H/C_org) ratios also affect biochar. For example, decreasing the O/C and H/C_org ratios can result in a char structure with stronger aromaticity and resistance to microbial and chemical degradation. The O/C ratio lower than 0.32 indicates a charcoal half-life of more than 500 years. This low H/C_org and O/C ratio can be considered high quality in terms of the stability of the biochar’s durability structure in the soil which is a key factor in negative emissions [27]. Meanwhile, biochar with an O/C ratio < 0.2 is predicted to have a half-life of more than 1000 years, supporting its role in long-term carbon sequestration. In estimating the stability and durability of biochar in the soil, the atomic ratio is observed as a Quality Indicator. The H/C_org and O/C mole ratios, which are used for biochar classification according to international standards and carbon sequestration potential, are best correlated with volatile matter (VM)/fixed-carbon (FC) through the expression (1) and (2) [28]:

H/C_org = 0.397 × (VM/FC) + 0.251

(1)

O/C = 0.188 × (VM/FC) + 0.035

(2)

The H/C_org and O/C atomic ratios serve as indicators for measuring biochar stability and carbon sequestration potential. These ratios are related to the volatile matter/fixed-carbon ratio, which standardizes biochar classification, thus providing a basis for assessing biochar’s resilience in soil and its role in negative emissions.

Biochar classification standards are also enforced by international bodies such as the International Biochar Initiative (IBI) and the European Biochar Foundation (EBF). International standards by the International Biochar Initiative (IBI) and the European Biochar Foundation (EBF) classify biochar based on its carbon content and H/C_org ratio, ensuring its suitability for safe use and long-term carbon sequestration. According to IBI, biochar with a C mass fraction ≥ 60% is Class 1, 30–60% is Class 2, and 10–30% is Class 3. Materials with <10% carbon or an H/C_org ratio > 0.7 are not considered biochar. Furthermore, the EBF also requires a carbon mass fraction >50% and a molar ratio H/C_org < 0.7 [28]. Biochar derived from pyrolysis meets international standards for safe environmental and sequestration applications. The organic carbon content certification in organic materials is higher than the minimum values required by IBI [29]. Carbon stabilization can be seen in fixed carbon if it is higher than that found in biomass and its C content meets the European Biochar Certificate standards for long-term carbon sequestration [30]. International validation of pyrolysis-based biochar production for carbon sequestration efforts is supported by Global Climate Agencies such as the Intergovernmental Panel on Climate Change (IPCC) which has recognized biochar as a carbon dioxide removal (CDR) technology. CDR quantification for various chars can be calculated in g CO₂ e/g biochar [31]. Higher carbon content and thermal stability increase the carbon storage life, which is important for NET effectiveness. Biochar obtained at high temperatures shows an increase in Higher Heating Value (HHV), making it more thermally stable [32]. Biochar with a higher fixed-carbon content burns more efficiently and has a higher calorific value. Therefore, it is important to focus on maximizing fixed-carbon yield [30]. The influence of raw materials also affects the total carbon content. Biochar from wood and sugarcane has high carbon but low nutrient concentrations, in contrast, poultry litter and sewage sludge have high micronutrient content such as Zn, Cu, and Fe [33]. Woody materials showed the highest surface area (126.2 m²/g) and carbon content (73.8%) at relatively moderate pyrolysis temperatures (around 440 °C) [3].

Observations on the calorific value were also conducted to demonstrate biochar’s role as an energy carrier. This aims to increase the energy content of biomass sources. Biochar can have a high energy density, making it suitable as an alternative to fossil fuels. Biochar has the potential to be a solid fuel substitute for carbon-rich coal, with improved stability and energy value. Compared to coal, the carbon and calorific value of biochar can increase through processing, while its volatile matter can decrease [34]. Various factors can be improved in biochar as an energy product, such as energy density, hydrophobicity, and in terms of water, nitrogen, sulfur, and bulk volume reduction [30]. In terms of HHV, its value is almost similar to coal, around 30.5–31.8 MJ kg⁻¹. In decarbonizing the industrial sector, biochar can be used as a sustainable substitute for fossil fuels. The HHV of biochar at 500 °C reaches 27.33 MJ kg⁻¹, comparable to anthracite [34]. Another example, palm kernel shell (PKS) and empty fruit bunch (EFB) have higher biochar potential comparable to coal with higher HHV (26.18–27.50 MJ kg⁻¹) and fixed-carbon (53.78–59.92%) [35]. Improved biochar elemental quality also occurs in sugarcane residues. High carbon content and low nitrogen in biochar where the C content is up to 80.93%, the H/C_org and O/C atomic ratios are consistent with condensed aromatic structures, low N content (<1%) making it suitable for energy use, carbon sequestration, and soil amendments [36]. Its improved stability, reduced volatile matter, and favorable elemental composition further support its role in decarbonizing the industrial sector.

Identification of porosity is also an important feature for carbon storage and filtration. Pore sizes less than 2 nm are microporous, which are similar to the pore size of activated carbon. Production methods with better porosity and surface area can affect the carbon adsorption capacity. Microwave pyrolysis is considered a promising method to produce porous biochar with a higher specific surface area. Valuable for soil fertility and nutrient cycling if biochar is enriched with surface functional groups and minerals such as nitrogen, phosphorus, calcium, magnesium, and potassium [37]. Nitrogen and functional groups in pyrolytic biochar can be activated and modified for use as adsorbents [38].

2.2. Carbon Neutrality

Utilizing waste as a sustainable energy source can reduce CO₂ emissions [39]. Waste can produce stable carbon products and act as energy carriers that can contribute to carbon neutrality [40]. The waste pyrolysis process can produce energy, increase methane yield, reduce toxicity, and support carbon mitigation. This is evidenced by aqueous pyrolysis condensate (APC), a co-product formed during pyrolysis. Its use as a raw material for biogas production is facilitated by the presence of biochar [41]. Biochar stabilizes carbon in the long term, contributing to carbon neutrality. Biochar’s resistance to microbial degradation increases with increasing pyrolysis temperature due to the increased concentration of aromatic carbon [42]. The dual role of biochar in carbon sequestration and greenhouse gas reduction is evident in CO₂, CH₄, and N₂O emissions in soil and compost, which affect the abundance of functional bacteria [29].

Other efforts are also being made to reduce emissions that contaminate soil, such as polycyclic aromatic hydrocarbons (PAHs). As a remediation effort for organically contaminated soil, biochar supported by nanoscale zerovalent iron (nZVI/BC) has successfully activated persulfate, significantly increasing the rate of contaminant degradation by up to 322%. These findings collectively highlight the dual role of biochar in immobilizing PAHs [43]. Caution is needed as the levels of PAHs, a type of toxic organic pollutant, in biochar are influenced by pyrolysis temperature and feedstock type. Raw materials containing high amounts of cellulose and pectin cause more PAHs than raw materials containing lignin [44]. Land treatment efforts such as PAH leaching from treated soil have values 99.9% lower than untreated soil (piled) for 100 years. Heavy metals such as Cu, Hg, Ni, and Zn are also better retained in the soil, although some elements such as Mo and Ba are leached more depending on conditions [45]. Efforts to increase soil organic carbon are also carried out by using pig manure compost pellets (BCP). Long-term carbon storage occurs due to the stable aromatic structure of biochar which produces the highest carbon sequestration of 2.94 tons C/ha and 10.76 tons CO₂-eq/ha greenhouse gas mitigation (compared to the control 4.7 tons CO₂-eq/ha) [46].

The output benefits of biochar sustainability and the efficient mechanism of biomass utilization through processing that produces reliable products can reduce greenhouse gas emissions and other environmental impacts. There is multifunctional biochar for sustainability such as soil remediation, waste management, greenhouse gas reduction, and energy production [37]. The potential of biochar to minimize negative impacts on the environment and offer co-benefits such as increased soil fertility, reduced N₂O emissions, and increased water retention [13]. For example, biochar from coffee pulp waste exhibits hydrophobic properties, making it more suitable for long-term environmental storage and applications, including carbon sequestration [22]. Then, biochar from pine wood, encourages waste value enhancement resulting in lower flame spread, smoke, and toxic emissions. CO and CO₂ emissions during combustion, making the composite safer and more environmentally friendly. Unlike traditional flame retardants (e.g., ammonium polyphosphate), biochar does not release toxic substances during combustion. Increases thermal stability and reduces smoke emissions [47]. Then there is a resource-efficient biomass utilization (RBU) system that produces high value-added products from pyrolysis and rapid rectification technologies with lower environmental impacts. The greenhouse gas emissions of the RBU system were reduced by 29%, and impact factors such as abiotic depletion, acidification, eutrophication, ozone layer depletion, and human toxicity were reduced by 64.3%, 69.6%, 26.8%, 10.8%, and 63.3%, respectively, compared to the Typical Biomass Utilization (TBU) system [10].

The increase in carbon in biochar indicates good carbon capture capacity, supporting carbon neutrality. For example, SCG showed an increase in carbon content from 45.91% by weight to 54.55% by weight, and the HHV could be increased by approximately 23.41%. A controlled heating process applied to biomass can convert it into a solid fuel (biochar) with higher energy density and superior combustion characteristics [22]. Biochar produced by carbonization and chemical activation (using KOH) of biomass develops an ultra-microporous structure with a pore size of approximately 0.65 nm and very high microporosity (up to 91.7%). This structure provides a large specific surface area (up to 1027 m²/g), ideal for adsorbing CO₂ molecules at room temperature and atmospheric pressure [48].

Countries such as Norway, Greece, and the Netherlands have found biomass energy to have a statistically significant long-term negative effect on CO₂ emissions. These results were obtained from an analysis of the statistical relationship between CO₂ emissions, biomass energy consumption, technological progress, and GDP. CO₂ emissions initially increase along with GDP but decrease after reaching a certain economic threshold. Biomass energy consumption is negatively correlated with CO₂ emissions, but the impact is not statistically significant due to its small share in total energy use [49].

3. Biochar Production as a Negative Emission Technology

Biochar absorption is gaining increasing attention as a negative emissions technology to mitigate climate change. Biochar systems can be both carbon-negative and energy-positive. Life Cycle Assessment (LCA) results show that slow pyrolysis of lignocellulosic biomass results in positive energy production and negative global warming potential (GWP) emissions (see Section 4 for BECCS) [50]. Fast pyrolysis of lignocellulosic household waste provides a sustainable pathway for energy and material recovery. Biochar in this process contributes through long-term soil stability [51]. Biochar has a positive effect on the environment by helping reduce greenhouse gas production [29]. Approximately 3 Gt/year of biomass conversion to biochar can reduce approximately 2.75 Gt of CO₂ per year until 2050 [2]. The basic details come from the calculation where 3/11 (27.3%) of the mass of a molecule of CO₂ consists of carbon, with the remainder the mass of the oxygen atoms. 1 gigaton of carbon (GtC) is therefore equivalent to 3.7 GtCO₂ in the atmosphere [52]. This also provides a surplus energy impact of approximately 15.6 gigajoule (GJ) for reducing 1 ton of CO₂ [2]. This is supported by the availability of biomass, which accounts for approximately 10% of the global energy supply, equivalent to 50 exajoule (EJ) of energy. The potential energy generation from biomass is estimated at 140–270 EJ [53].

The key to negative emissions also lies in biochar’s resistance to degradation and stability indicators, such as lower H/C_org and O/C ratios. The H/C_org and O/C molar ratios decrease with increasing pyrolysis temperature, indicating increased carbonization and aromaticity [3]. The resulting carbon content can exceed 70–84 wt%, depending on the raw material and conditions. The resulting biochar can be used for applications such as soil amendment, carbon sequestration, pollutant absorption, microbial fuel cells, and catalyst support [54]. For example, the carbon content of biochar derived from olive cake is 85% with an O/C ratio of 0.15. Biochar with an O/C ratio < 0.4 is better for soil amendment and carbon sequestration [55]. Various thermochemical methods for realizing negative emission strategies are shown in Table 1.

3.1. Pyrolysis

The thermochemical process of pyrolysis produces bioenergy in the form of biochar, syngas, and bio-oil, making it a sustainable and environmentally friendly energy production technique without oxygen. For example for biochar, its heating value reaches 27.31 MJ kg⁻¹ at 400 °C, making it a good solid biofuel [25,38,56,57]. The carbon pathway and biochar formation through pyrolysis where organic matter is heated until it decomposes under anaerobic or anoxic conditions, and is categorized into three phases, namely drying, pyrolysis, and carbonization. In the drying stage, the water in the raw material evaporates, in the pyrolysis stage, the organic matter begins to decompose to produce volatile gases, liquid products, and solid residues (charcoal), in the carbonization phase, the remaining solids are then carbonized to form biochar [58]. Pyrolysis also produces biogas products as a high-value by-product from dolomite catalyst with 2.27 wt% bio-oil with the highest hydrocarbon content (86.35%) [59]. Alternative energy produced from pyrolysis is also in the form of gas and bio-oil which is used as a substitute for fossil fuels by replacing coal to produce electricity equivalent to 678 kWh/ton of residue and also offsets approximately 725 kg CO₂ e/ton of plant residue [60].

The properties of biochar are influenced by the type of biomass and pyrolysis conditions. Higher treatment temperatures lead to aromatization, micropore formation, and enhanced stability for long-term carbon sequestration. Biochar is an effective CO₂ absorber product due to its microporous structure and composition, including carbon and minerals [56]. The main stages of pyrolysis are starting from the evaporation of water as Zone 1 (25–125 °C), then the degradation of hemicellulose, cellulose, and lignin components at different temperature ranges in the pyrolysis process, namely hemicellulose at 210–370 °C (low thermal stability), cellulose at 260–410 °C (main devolatilization phase), lignin at 350–650 °C (thermally stable, decomposes slowly over a wide range). These thermal decomposition steps produce Biochar (carbon-rich solid residue), Bio-oil (condensable volatiles), and non-condensable gases (e.g., CO, CO₂, CH₄, H₂). The mechanism involves depolymerization of long-chain polysaccharides, decarboxylation and dehydration leading to the loss of functional groups (O-H, COOH) and the formation of a thermally stable aromatic carbon structure (biochar) [36]. Thermal cracking of polymers (in plastics) and lignin in biomass helps to form a dense aromatic carbon network, increasing the stability of biochar and its potential for carbon sequestration. As the pyrolysis temperature increases, the carbon structure undergoes aromatization and shrinkage, resulting in the formation of micropores and graphitic domains [40]. These characteristics make wood-derived biochar especially valuable for soil improvement, nutrient recovery, and water purification, while controlled heat transfer offers a pathway to optimize sustainable production. Wood biochar is a very suitable soil amendment to increase its retention capacity (even from a long-term perspective). Wood biochar is a promising agent for P and N recovery. It is an appropriate biofilter for (waste) water purification (during which time various modifications, such as surface activation, are often required) [61]. For the same feedstock, with increasing Highest Treatment Temperature (HTT) during pyrolysis, higher organic matter destruction occurs. To refine and control the heat transfer rate, which can then be used to optimize the process for sustainable biochar production [62].

Pyrolysis technology features fast reaction speed, diverse products, and high energy conversion efficiency, with reactor design and operating conditions strongly influencing product distribution and quality. For example, rotary reactors function to promote uniform heating, improve heat and mass transfer, and minimize local overheating [58]. Then, continuous flow pyrolysis by utilizing screw-type pyrolysis furnaces characterized by ease of operation and maintenance as a smart solution for waste utilization operations [63]. For slow pyrolysis, it is most suitable to maximize biochar yield and absorption potential. Factors that affect the composition and yield of pyrolysis products, including feedstock characteristics, pyrolysis temperature, heating rate, and residence duration [58]. Slow Pyrolysis is an optimal process that produces more biochar than other methods, and about 35% of biochar is obtained from slow pyrolysis of biomass. Slow pyrolysis stands out as a highly effective NET due to its stable carbon absorption capability, forming biochar from accumulated solid carbon as a climate change mitigation measure [64]. The optimal temperature stabilizes energy content, allowing biochar to function as both a carbon sink and a solid fuel comparable to low-rank coal. Pyrolysis increases the long-term absorption potential of biochar by optimizing the temperature. Medium temperatures (300–500 °C) optimize stability and surface area for carbon adsorption and higher temperatures (>500 °C) increase carbon content and reduce volatiles [21]. Biochar yield increases due to the possibility of secondary reactions and can decrease due to higher biomass volatilization. Loss of aliphatic fractions leads to higher yields at low temperatures. Low biochar yields are caused by the loss of hydroxyl groups due to dehydration, thermal decomposition of cellulose and hemicellulose [65]. For example, the formation of biochar carbon from macroalgae showed good quality for calorific value in the range of 23.12–25.89 MJ kg⁻¹, which indicates greater potential for use as a solid fuel compared to low-rank coal [66]. In terms of GWP values to measure emission levels, slow pyrolysis of plant residues has a GWP value of −1050 kg CO₂ eq/t to −770 kg CO₂ eq/t [50].

In fast pyrolysis of biomass, there is research on the intensification process using autothermal operation in a fluidized bed reactor. Autothermal operation uses partial oxidation of pyrolysis products (mainly char and light volatiles) to provide internal heat, eliminating external heat carriers such as sand or inert gases [67]. This reduces energy costs and system complexity. Autothermal pyrolysis increases the porosity of biochar and provides better control over product distribution. Meanwhile, due to the reduction in liquid, the process increases gas output as a hydrogen production pathway. Providing more efficient conversion at higher O₂ [68]. High-temperature vertical fast pyrolysis reactors drop tube reactors have a mechanism where biomass is supplied at the top and falls rapidly through a heated zone under the influence of gravity. The simulation was carried out using Computational Fluid Dynamics (CFD) with a multiphase Euler–Lagrange model to simulate the reactor as an evaluation of good char quality at a particle size of 500 µm. The best quality of biochar with carbon content found was wheat straw biochar 70.4% C (experiment) and 73.3% (simulation), while corn straw biochar 65.6% C (experiment) and 64.2% (simulation) [69]. Overall, these innovations strengthen the case for fast pyrolysis as a dual pathway for clean energy and carbon sequestration, but further optimization is needed to balance product distribution and ensure economic feasibility.

Efficient and low-emission microwave pyrolysis of waste biomass can produce high-value biochar. In achieving carbon stability, the carbon content of biochar increased after microwave pyrolysis at 550 °C, increasing from 19.6% to 70.7%, while the volatile content decreased [34]. Microwave pyrolysis as a modern thermochemical pathway with a heating process promotes more efficient, fast, and productive chemical reactions. The pore structure of the biochar is also improved, making it an ideal carbon-negative material for long-term land applications and climate change mitigation [70]. Microwave-assisted Pyrolysis (MAP) pyrolysis can also produce biochar as an alternative to coal [59]. Its superior technology has a shorter processing time, higher heating rate, and lower energy consumption than traditional heating sources [30]. MAP is ideal for waste conversion systems into local resources, especially in small towns or urban areas that do not have large incineration plants. It produces biochar with nutrient retention, useful for soil improvement, and adsorption properties for pollution control [71]. Biochar formation mostly occurs in slow pyrolysis and MAP at lower temperatures with long residence time. The yield decreases with increasing temperature, for example, rice husk produces 43.3% biochar at 300 °C [65]. Microwave pyrolysis can be useful as an emission remover (e.g., sulfur) during its production as a carbon negative. Its content is relatively reduced from five sulfur species and also the conversion of some organic sulfur to inorganic sulfate which is retained in the biochar [34]. Microwave pyrolysis reduces the formation of hazardous products and minimizes pollutant emissions, making this technique environmentally friendly [43]. This positions MAP not only as a sustainable biochar production route but also as a practical strategy for carbon sequestration and environmental remediation.

Pyrolysis improves carbon stability and nutrient retention, supporting its role in agricultural systems. For co-pyrolysis, higher biochar yields can be produced which increases P and K recovery. The thermochemical efficiency of co-pyrolysis is more economical when combined with nutrient- and carbon-rich feedstocks [72]. Co-pyrolysis techniques also release less CO₂ than fossil fuels [73]. The effectiveness of co-pyrolysis biochar can be demonstrated by evaluating three types of waste: poultry litter (PL), banana stalks (BP), and phosphogypsum (PG), which produce biochars rich in potassium (K), phosphorus (P), and sulfur (S). The highest content found in biochar was K (5.1%), S (11.35%), and P (4.48%) [74]. Integration of plastic residues in co-pyrolysis systems can also be performed in line with negative emission strategies when biochar is produced. Temperature and catalyst loading are the two most important features that significantly affect the yield of biochar and syngas from co-pyrolysis of torrefied biomass and plastic waste [42]. Therefore, co-pyrolysis is a versatile negative-emission pathway that couples waste valorization with soil fertility and energy recovery. The low calorific value (LHV) of plastic components reached 29.93 MJ kg⁻¹ as a good result of pyrolysis raw materials compared to coal with a calorific value of 29.31 MJ kg⁻¹ [75].

Pyrolysis has also been integrated with other technologies, such as anaerobic digestion (AD) for lignocellulosic biomass valorization. Pyrolysis of the resulting solid digestate yielded biochar from 28.81 to 35.96%, while bio-oil and pyrolysis gas gradually decreased. The highest energy efficiency was 71.9% with a net energy yield of 2.0 MJ kg⁻¹ [76]. Production of nitrogen-containing biochar from AD digestate using urea from pyrolysis at 600 °C with a digestate-to-urea ratio of 1:1 yielded carbon-rich biochar and a nitrogen-enriched biochar with a high CO₂ adsorption capacity of 1.22 mmol/g. Its strong nitrogen content significantly enhanced the chemical adsorption of CO₂, highlighting its dual function as an effective carbon storage and adsorbent material for negative emission applications [77].

Pyrolysis has many potentials, for example, as a heat and electricity cogeneration that supports energy recovery systems beyond biomass electricity generation, thus increasing the efficiency of biochar production [78]. Then the processing of organic residues of food waste as a carbon-negative sustainable strategy. Compared with landfilling and incineration, pyrolysis is generally considered a more energy-efficient, environmentally friendly, and economical method [75]. Then it was also proposed that the use of pyrolysis to process biomass RBU systems is a synthetic way to save significant energy. Compared with conventional systems, RBU systems produce various high value-added products based on pyrolysis [10]. Unlike incineration, pyrolysis has low carbon consumption and CO₂ emissions. To achieve carbon neutrality, it is indicated that the production of 100 and 300 Gt equivalent of biochar through pyrolysis is necessary to prevent global warming below 1.5 °C [79]. Biochar from pyrolysis has several benefits compared to raw biomass, including increased hydrophobicity and friability, which increase processing and storage capacity [73].

3.2. Gasification

Gasification converts biomass into a gaseous mixture through partial oxidation at high temperatures. Gasification can mitigate climate change in landfills and convert carbonaceous waste into H₂, potentially replacing fossil fuels. Controlled gasification can transform biochar into a carbon-negative solid product [80]. Gasification can produce porous biochar, which is accompanied by significant increases in carbon gasification efficiency (CGE) and hydrogen gasification efficiency (HGE) [81]. One such method is supercritical water gasification (SCWG), which converts wet waste without the need for drying [80].

A system design combines pyrolysis, gasification, and combustion processes in a single device for optimal hydrogen and biochar production. This system also utilizes biochar in tar cracking, where a biochar catalyst at the gasifier outlet reduces the tar content from 3.24 to 1.02 g/Nm³ (a 68.6% reduction). This system consists of a Cyclone Pyrolysis Zone at the top with airflow forming a vortex. In the middle, there is a fast pyrolysis system that produces volatile gases from solid biochar due to centrifugal force and gasification that reacts biochar with CO₂ and/or H₂O to produce H₂ with a maximum range of 3.91% [82]. Biochar produced from gasification has the potential to be recycled as a fuel, because it contains carbon and high calorific value. The biochar can be used as a gasification feedstock to convert the remaining carbon “left behind” in the previous gasification process into additional gas fuel [83]. Biochar products from the gasifier system can generate electricity and replace electricity from the grid. This process also has negative values for potential damage to ecosystems and human health with values ranging from −1 × 10⁻⁷ to −2 × 10⁻⁸ species x year and from −1 × 10⁻⁵ to −5 × 10⁻⁶ DALY per kg of biowaste processed. Potential damage has a negative score, indicating environmental benefits. The life cycle impact assessment methodology used to measure this score is ReCiPe 2016 [84]. These advances signal a promising pathway for sustainable energy and waste management, transforming biowaste into clean fuels while yielding net-positive environmental impact scores.

There is also a combination of other technologies such as solar power and gasification of oil palm empty fruit bunches (OPEFB) which successfully captures carbon and utilizes CO₂ as a gasification agent to produce syngas. This process achieves a net carbon consumption rate of 0.7 g/min and a carbon conversion efficiency of 94.9%. Syngas (including H₂) has an energy enhancement factor of 1.4. The presence of hydrogen improves the energy quality of syngas, especially in applications requiring clean combustion or synthesis pathways [85].

3.3. Hydrothermal Carbonization (HTC)

This technique is a thermochemical process that uses water as a reaction medium at high temperature and pressure, without the need for extensive pre-drying. The reaction takes place in a closed reactor with batch pressure (e.g., 2 L capacity, up to 300 °C and 15 MPa). The reactor is vented with nitrogen to keep the system oxygen-free. Heating and reaction occur when the reactor is heated to 180–250 °C and maintained for 30 min. Water acts as a solvent and catalyst, accelerating the hydrolysis, dehydration, condensation, and polymerization reactions to form hydrochar [86].

Hydrochar (a solid product of HTC) offers significant environmental benefits, including up to an 81.5% reduction in GWP and a 96% reduction in human toxicity. Hydrochar can replace fossil fuels as a primary objective in negative emissions strategies and sequesters stable carbon in solid form, reducing life-cycle greenhouse gas emissions. It is carbon-rich and combustible, with properties similar to subbituminous coal [87]. Hydrochar is characterized by a relatively high carbon content (approximately 48% on a dry, ash-free basis) and a low ash content (4.8% on a dry basis). Its use as a fuel is also supported by its relatively low ash content compared to other biological wastes [88]. This technology can be combined with gasification, which converts biomass with 90% water into high-quality solid fuels (hydrocarbons) [89]. Its low ash content and compatibility with gasification processes further underscore its utility for clean energy and environmental benefits.

Table 1. Biochar Production Method as Negative Emission Technology.

Method	Feedstock	Product and Application	NET Strategy	Typical Operating	Char Quality	References
Simple kiln	Crop residues	Biochar as soil conditioner in agriculture	LCA, Socio-economic impact	T: 450–650 °C, Biochar Yield: 31.38%, Heating Rate 30 °C/min	HHV: 13–17 MJ kg−1 O/C: 0.14 H/Corg: 0.696	[84,90]
Pelletization + pyrolysis	Swine manure pellets	Biochar as an alternative energy	Soil C sequestration, Carbon Credit	T: 270–500 °C, Biochar Yield: 44.02%	HHV: 28.3 MJ kg−1 H/Corg: 0.045	[46,91,92]
Pyrolysis + Ammoniation	Leucaena leucocephala	N-doped Biochar as a material for capturing CO2	Carbon Capture by modifying and refining biochar	T: 500–900 °C, Heating Rate 10 °C/min	22.6 MJ kg−1, O/C: 0.09 H/Corg: 0.23	[20,93]
Slow Pyrolysis	Lignocellulosic biomass	Biochar as a solid fuel and in soil amendment, Bio-oil, Gas	Carbon Capture, LCA	T: 500 °C, Biochar Yield: 45.9%, Heating Rate 10 °C/min	HHV: 31.1 MJ kg−1, O/C: 0.92 H/Corg: 1.47	[50,65,94,95]
Fast Pyrolysis	Lignocellulosic waste, Wheat/corn straw	Biochar as a solid fuel and in soil amendment, Bio-oil, Syngas	Carbon Capture, Soil Amendment	T: 500–1000 °C, Biochar Yield: 26%, Heating Rate 1000 °C/min	HHV: 25.1 MJ kg−1, O/C: 0.55 H/Corg: 1.719	[51,69,96,97]
Microwave Pyrolysis	General biomass	Biochar as a solid fuel and in soil amendment, Bio-oil, Syngas	Techno-economic, Environmental	T: 600 °C, Biochar Yield: 31.1%, Heating Rate 163 °C/min	HHV: 13.7 MJ kg−1, O/C: 0.1 H/Corg: 0.92	[34,55,98]
Supercritical Water Gasification	Biomass, Palm Oil EFB	Improve the quality of pore structure of Biochar, H2, Syngas	Hydrogen production Carbon Capture	T: 500–800 °C, Product Yield: 21.71 mol/kg, Heating Rate 10 °C/min	LHV: 4506 kJ/Nm3, O/C: 0.05 H/Corg: 0.16	[80,99,100,101]
Hydrothermal Carbonization (HTC)	Food Waste	Hydrochar as a solid fuel and in soil amendment,	Applications as a carbon material, catalyst, as a filter aid for water purification, in fuel cells, and as an absorbent	T: 200–230 °C, Solid Yield: 49%, Heating Rate 3.25 °C/min	HHV: 22.39 MJ kg−1 O/C: 0.62 H/Corg: 0.15	[86,96,102]

Table 1. Biochar Production Method as Negative Emission Technology.

Method	Feedstock	Product and Application	NET Strategy	Typical Operating	Char Quality	References
Simple kiln	Crop residues	Biochar as soil conditioner in agriculture	LCA, Socio-economic impact	T: 450–650 °C, Biochar Yield: 31.38%, Heating Rate 30 °C/min	HHV: 13–17 MJ kg−1 O/C: 0.14 H/Corg: 0.696	[84,90]
Pelletization + pyrolysis	Swine manure pellets	Biochar as an alternative energy	Soil C sequestration, Carbon Credit	T: 270–500 °C, Biochar Yield: 44.02%	HHV: 28.3 MJ kg−1 H/Corg: 0.045	[46,91,92]
Pyrolysis + Ammoniation	Leucaena leucocephala	N-doped Biochar as a material for capturing CO2	Carbon Capture by modifying and refining biochar	T: 500–900 °C, Heating Rate 10 °C/min	22.6 MJ kg−1, O/C: 0.09 H/Corg: 0.23	[20,93]
Slow Pyrolysis	Lignocellulosic biomass	Biochar as a solid fuel and in soil amendment, Bio-oil, Gas	Carbon Capture, LCA	T: 500 °C, Biochar Yield: 45.9%, Heating Rate 10 °C/min	HHV: 31.1 MJ kg−1, O/C: 0.92 H/Corg: 1.47	[50,65,94,95]
Fast Pyrolysis	Lignocellulosic waste, Wheat/corn straw	Biochar as a solid fuel and in soil amendment, Bio-oil, Syngas	Carbon Capture, Soil Amendment	T: 500–1000 °C, Biochar Yield: 26%, Heating Rate 1000 °C/min	HHV: 25.1 MJ kg−1, O/C: 0.55 H/Corg: 1.719	[51,69,96,97]
Microwave Pyrolysis	General biomass	Biochar as a solid fuel and in soil amendment, Bio-oil, Syngas	Techno-economic, Environmental	T: 600 °C, Biochar Yield: 31.1%, Heating Rate 163 °C/min	HHV: 13.7 MJ kg−1, O/C: 0.1 H/Corg: 0.92	[34,55,98]
Supercritical Water Gasification	Biomass, Palm Oil EFB	Improve the quality of pore structure of Biochar, H2, Syngas	Hydrogen production Carbon Capture	T: 500–800 °C, Product Yield: 21.71 mol/kg, Heating Rate 10 °C/min	LHV: 4506 kJ/Nm3, O/C: 0.05 H/Corg: 0.16	[80,99,100,101]
Hydrothermal Carbonization (HTC)	Food Waste	Hydrochar as a solid fuel and in soil amendment,	Applications as a carbon material, catalyst, as a filter aid for water purification, in fuel cells, and as an absorbent	T: 200–230 °C, Solid Yield: 49%, Heating Rate 3.25 °C/min	HHV: 22.39 MJ kg−1 O/C: 0.62 H/Corg: 0.15	[86,96,102]

4. Carbon Management Systems from Carbon Capture and Storage

An effective alternative to achieve CO₂ emission reduction in realizing the current negative emission strategy is a CCS technology. One potential approach in CCS technology is the use of CO₂ adsorbents to increase carbon absorption [14]. The CCS strategy has emerged as a technology to reduce and capture CO₂ emissions globally, which, according to the International Energy Agency (IEA) scenario, can reduce CO₂ emissions by 19% by 2050. Currently, countries with the highest percentage of global CO₂ emissions from fossil fuel combustion are China (28.0%), the United States (15.0%), the European Union (9.9%), India (6.4%), and Russia (4.5%). The current priority for CCS development is for carbon capture from coal-fired power plants which produce 27% of global energy and 44% of anthropogenic energy-related CO₂ emissions. It is worth mentioning that power plant exhaust gases generally contain around 15% CO₂ with varying percentages of N₂, H₂O, O₂, and Ar [103]. This thermochemical process with CCS offers a cost-effective and sustainable method of carbon sequestration with additional environmental benefits.

Biochar production from biomass is considered a CCS strategy to reduce environmental pollution from atmospheric CO₂ accumulation [104]. Biochar can be applied to soil, thus acting as a long-term carbon storage [13]. The mechanism of biochar carbon sequestration is by converting atmospheric CO₂ through plant photosynthesis into biomass, which undergoes a thermochemical process to become a stable solid product that is then buried for long-term storage [50]. Solid carbon, as a negative emission approach, can be produced from the pyrolysis process of CH₄, which can also be permanently stored underground at negligible cost and environmental impact [105]. Biochar’s role in sustainable applications is as a soil conditioner and to improve soil health [106].

Biochar obtained at pyrolysis temperatures above 600 °C can reach a carbon content exceeding 80 wt%, suitable for carbon sequestration in soil, which is in line with the IBI threshold for effective carbon sinks [51]. Low H/C_org and O/C ratios indicate increased aromaticity and chemical stability, key features for long-term carbon sequestration and environmental applications [54]. Biochar content has higher stability in the soil and carbon sequestration potential. Some indicators of the carbon sequestration potential of biochar, such as fixed carbon, correspond to the efficiency of carbon conversion in biomass to carbon in biochar and the percentage of stable carbon content [107]. Based on elemental analysis, carbon storage capacity can be explained from the condition of biochar containing 42–43% carbon, which is equivalent to 1230–1267 kg of CO₂ captured and stored per ton of biochar. Approximately 80% of the carbon is considered stable, thus qualifying for permanent CO₂ removal [108]. There are two ways of carbon capture, namely before and after combustion. In post-combustion systems, CO₂ is captured from the flue gas after biomass is burned either alone or mixed with fossil fuels. In pre-combustion systems, CO₂ is collected earlier in the conversion process, such as when biomass is converted to syngas or fermented into bioethanol [13]. Carbon capture can occur pre- or post-combustion, with biochar offering a robust method for permanent CO₂ removal through biomass conversion processes.

Various efforts to increase CO₂ absorption such as ammonialization which significantly increases the specific surface area, pore volume, and ultra-microporosity of biochar and introducing nitrogen functional groups increases the CO₂ capture capacity. Ammonialization introduces reactive N functionality that is able to form chemical bonds with CO₂, thereby increasing absorption [20]. There is also a model as a climate mitigation measure, such as proposing Integrated Assessment Models (IAM) to limit the increase in global average temperature to 1.5–2 °C through the widespread deployment of NET, especially bioenergy with carbon capture and storage (BECCS). For example, a 100% biomass-CCS power plant (with a 90% capture rate) is able to achieve negative emissions [109].

4.1. BECCS Is an Effective Negative Emissions Strategy

Bioenergy with carbon capture and storage (BECCS) is a climate solution that uses plants to remove carbon dioxide (CO₂) from the atmosphere and then converts the plants into biomass to generate energy through combustion. However, BECCS systems capture and lock CO₂ deep underground, preventing it from contributing to climate change, instead allowing it to return to the atmosphere [13]. BECCS is a NET that integrates waste-based gasification and CO₂ capture. It provides a net energy gain of 18.08 GJ/ton of CO₂ by generating revenue through syngas and electricity and is also capable of capturing and storing CO₂ emissions at $408/t CO₂ (USD 2019 or $514.07/t CO₂ in USD 2025, as product selling value) [2]. Various BECCS systems such as landfill gas combusted in a gas turbine (LFG-GT), bagasse with Carbon Capture Storage (BG-CCS), Forest Residue with Carbon capture Storage (FR-CCS), and municipal solid waste with carbon capture storage (MSW-CCS) have a net negative CO₂ emission range of −0.89 to −1.35 t/MWh and therefore this technology reduces CO₂ emissions better than Coal-CCS with 0.11 t/MWh. All BECCS systems offer CO₂ emission reduction costs of up to 30% compared to Coal-CCS (

Table 2. CO₂ emission and avoided emission [110].

System	CO2 Emission (t/MWh)	Avoided CO2 Emission (t/MW h)	Cost of Avoided CO2 Emission (USD/t) (USD 2018)	Cost in USD 2025
LFG-CCS	−1.35	1.35	70	89.85
MSW-CCS	−0.89	0.94	78	100.12
BG-CCS	−1.32	1.32	76	97.55
FR-CCS	−1.29	1.29	69	88.57
Coal-CCS	0.11	0.7	103	132.21

) [110].

If managed carefully, BECCS can remove more CO₂ than it emits. While energy production from biomass is a mature technology, the CCS component (Figure 1) is still in its infancy. Currently, there are only five BECCS facilities operating worldwide, capturing approximately 1.5 million tons of CO₂ annually [13].

Full conversion of biomass from BECCS as a CO₂ capture system can be diverted with a specific output. The biogenic carbon in the feedstock can be retained and then diverted, such as for permanent storage (BECCS route) or the production of char residue in combustion, which can be applied to soil carbon or material reinforcement [110].

4.2. Carbon Storage in Soil

Solid carbon storage can be permanently stored underground, offering long-lasting durability, low cost, and no environmental impact as a negative emission technology [105]. Carbon flow and capture mechanisms can occur during biomass combustion, which is then captured and compressed (up to 150 bar) and then passed through a pipeline to permanent underground storage [110]. The soil carbon storage capacity of biochar can reach 599 kg CO₂ e/t. This can reduce N₂O emissions, soil acidification, and water eutrophication. Increased soil microbial activity also occurs during soil organic carbon stabilization [60].

Biochar can be used for both in-soil storage and as a durable carbon sink at various temperatures. Biochar obtained at 400 °C can be used for fertilizer or soil applications, and biochar produced at 600 and 900 °C can be used for carbon-based applications [32]. Fixed-carbon formation occurs from the conversion of the original biomass into a stable aromatic carbon structure. This process locks atmospheric carbon into a solid form (biochar) that is resistant to decomposition and can persist in soil or engineered systems for hundreds to thousands of years. The optimized biochar achieved a CO₂ sequestration capacity of 6.05 mmol/g at 1 bar, 273 K and 3.96 mmol/g at 1 bar, 298 K, demonstrating its effectiveness for carbon capture under practical conditions [48].

Superior CO₂ capture and long-lasting carbon sequestration also occur in nitrogen-doped and chemically activated biochars, combining high porosity performance, surface functionality, and stability. The results showed excellent selectivity for CO₂ over N₂ (selectivity [CO₂/N₂] of 74.2), making it very effective for separating CO₂ from flue gas [111]. Biochar can improve carbon storage stability, porosity, and functionality through chemical processes (KOH, H₂SO₄), and doping (N, B), or metal addition (Fe, Cu, Ni) [112]. For key transformations at high temperatures, carbon content increased to 81.5 wt% at 600 °C, the formation of aromatic carbon from FTIR, XRD, and 13C NMR observations confirmed the transition from aliphatic to aromatic/graphitic structures which are stable, and the carbon content still increased from 14.6% to 64.3% [113]. Significant increases in carbon adsorption and capture capacity were also carried out on sewage sludge biochar through washing with HCl. This was achieved through increased microporosity, surface area, and the development of functional groups such as carbonyl and hydroxyl that interact with CO₂ and organic molecules. Increases the formation of carbonyl functional groups (C=O) and –OH, –NH, increasing the binding of CO₂ and pollutants [114].

4.3. Carbon Capture and Process in Various Sectors

Several mechanisms of carbon capture with biochar are chemical, physical, and combined. Chemically, biochar enables CO₂ carbon capture when combined with amine-based solvents (such as monoethanolamine, MEA). Nitrogen-doped biochar from coconut shell significantly enhances carbon capture by providing basic N functional groups and a high surface area of 825 m²/g. This catalyzes the CO₂ absorption reaction in amine solutions and increases the CO₂ capture rate and capacity. Capture performance improvement up to 43.4% faster absorption and 33% more desorption compared to MEA alone [115]. Then, there is also CO₂ capture from chemical absorption processes caused by mineralogical reactions originating from raw materials such as pig manure, sewage sludge, and wheat straw. High absorption capabilities are shown for CO₂, with a maximum capacity reaching 18.2–34.4 mg g⁻¹ at 25 °C. Increasing absorption temperature and water content encourages the transition of CO₂ absorption from physical to chemical processes. Mineral components such as Mg, Ca, Fe, K, in biochar induce chemical CO₂ uptake through mineralogical reactions which account for 17.7–50.9% of the total uptake [116]. Nitrogen-doped (N-doped) highly porous carbon prepared through a combination of KOH activation and urea treatment exhibited a remarkable CO₂ uptake of 4.46 mmol/g. This cost-effective carbon adsorbent for effective CO₂ capture, for example, uses anthracite as a precursor [117].

The effectiveness of KOH activation was also carried out on livestock manure, where biochar showed the highest CO₂ absorption capacity (2.92 mol/kg) with the highest surface area (1408 m²/g), micropore area (690.18 m²/g), and micropore volume (0.36 cm³/g). Biochar activation, either with KOH or KOH + CO₂, increased the CO₂ absorption capacity due to increased surface area and microporosity, regardless of the type of raw material. Hydrophobicity, aromaticity and the presence of hetero atoms (N and S) also positively affected the CO₂ absorption capacity of biochar [118]. The formation of narrow micropores (0.30–0.86 nm) and high micropore volume also contributed to showing CO₂ capture performance through KOH activation. This is due to the simultaneous fixed-carbon formation process that converts volatile organic matter into a stable carbon structure. Pomegranate peel-based carbon showed the highest CO₂ adsorption (4.11 mmol/g at 298 K) and selectivity (15.1) [119]. Carbon fibers derived from poly-acrylonitrile (PAN) through KOH treatment can also be activated using hydrofluoric acid oxidation to significantly enhance CO₂ capture by 112%. This surface-modified carbon exhibits strong semi-ionic interactions with CO₂, making it highly effective for post-combustion carbon capture applications [120].

For physical adsorption, biochar captures CO₂ through van der Waals interactions in its microporous structure (<2 nm) and chemical mineralization by reacting with CO₂. When combined with municipal solid waste incineration bottom ash (MSWIBA) rich in calcium and magnesium oxides, composite artificial aggregates (CCAA) blends of sawdust biochar (1–5%), cement and slag binder showed enhanced carbonation and CO₂ adsorption. The highest performing blend (5% biochar) captured up to 26.67 kg CO₂/ton, demonstrating the effective use of biochar and waste as carbon-negative materials [14]. The addition of incineration bottom ash to anaerobic digestion of waste activated sludge also allows in situ CO₂ adsorption through Ca²⁺-driven carbonation, forming stable bicarbonate and calcium carbonate for wastewater treatment plant (WWTP) sludge treatment. Simultaneously, this process increases fixed-carbon formation through enhanced hydrolysis, acidogenesis, and methanogenesis, leading to a 26.6% increase in methane production and a significant increase in the CH₄ content of biogas [121]. Another property that enables CO₂ physisorption, especially at lower temperatures (30 °C) for flue gas treatment which has the potential to enhance carbon capture by providing a high fixed-carbon content, extensive microporosity, and basic surface chemistry. With an adsorption capacity, raw sawdust biochar shows strong potential as a carbon-negative material. Biochar produced at 850 °C exhibits a high surface area of up to 182.04 m²/g, a pore volume of up to 0.016 cm³/g, and a micropore and mesopore size range of 1.5–1.7 nm for raw samples [122].

While the combination of physical and chemical, the CO₂ capture mechanism occurs through physical adsorption dominated by micropore filling due to the high surface area and chemical absorption through interactions with surface functional groups [123]. Adsorption mechanisms of CO₂ capture through physisorption and chemisorption can also be carried out by setting up a Fluidized Bed Reactor (FBR) with a gas mixture of 10% CO₂ + 90% air at a flow rate of 1.6 L/min. Fluidization allows efficient gas-biochar mixing, thereby reducing operational costs [124]. Biochar activated through physical or chemical techniques shows excellent performance using cyclic adsorption methods. Physical activation uses steam or CO₂ at a temperature of 600–900 °C to create microporosity. Chemical activation using KOH, H₃PO₄, ZnCl₂, etc., at lower temperatures (~600 °C), which results in a larger surface area, better micropore distribution, and higher carbon yield (no carbon combustion). With high microporosity, specific surface area, and functional group modification, biochar can achieve CO₂ adsorption of more than 6 mmol/g, making it competitive or superior to traditional commercial adsorbents. Its novelty, low cost, and strong stability make it ideal for negative emission technologies [103].

5. The Role of Biochar in Various Sectors

Biochar can be a technology that can realize negative emission strategies. Biochar can convert waste into hydrogen as a catalyst [125]. Various waste treatment methods, such as wood and plastic, can be obtained from the combustion chamber of a gasification or pyrolysis unit.

Furthermore, in the construction and industrial sectors, examples include carbonization of cement to strengthen construction structures and carbon capture from industrial waste. Biochar derived from biomass waste can absorb CO₂ and achieve good carbon stability.

5.1. Waste to Hydrogen Conversion Technology

Hydrogen-rich gas can be produced from biomass gasification. For example, methane gasification and integration of waste-to-hydrogen technology can produce 0.5 kg-H₂/kg-CH₄ with an efficiency of up to 30% [126]. Supercritical water gasification (SCWG) is a technology that converts organic matter into valuable gaseous products consisting mainly of hydrogen [127]. For example, it can produce hydrogen up to 38.29 mol/kg when 14 wt% K₂CO₃ is used [39]. This technology is one of the green hydrogen production methods because biomass is CO₂ neutral. The highest hydrogen yield (3.64 mmol/g dry waste) and other gaseous products produced consist of CO₂ and CH₄ [89]. Another benefit of SCWG is the cogeneration capability that simultaneously produces H₂-rich gas and porous biochar, increasing overall carbon utilization. The liquid residue is reduced, where >90% of the carbon ends up as gas and char, where H₂ production is up to 14.95 mol/kg at 700 °C, indicating a highly efficient system [81]. SCWG is thermodynamically advantageous for converting high-moisture biomass into energy-dense gas. Catalytic enhancement significantly improves H₂ selectivity, while reducing undesirable carbon and tar residues [80]. For example, hydrogen production from algae waste biomass using catalytic SCWG technology (with alkali or noble metal catalysts) can increase H₂ by up to 40% [128]. Then, there is the selection of catalysts in the SCWG process can also produce high-quality hydrogen from waste cooking oil processing which requires careful control of operating parameters. To produce high-quality hydrogen (10.16 mol/kg, ~97% selectivity), the optimal conditions are at 675 °C, 25 wt% feed, 60 min. This setting maximizes conversion, minimizes coke formation, and increases H₂ output [99]. Optimized operating conditions further maximize hydrogen production while reducing unwanted byproducts, supporting sustainable hydrogen energy development.

Biomass utilization can use biochar as a catalyst [129]. This material is effective for increasing the production of hydrogen-rich gas [130]. High pyrolysis temperature (700 °C) produces more gas, including hydrogen. Gas yields increased in various biomasses such as 73.03% for wheat straw and 65.35% for corn straw at 700 °C [69]. For the production of H₂-rich syngas, tire char was also used as a catalyst during the pyrolysis-reforming of biomass using a two-stage fixed bed reactor. Increasing the steam injection rate and reforming temperature resulted in increased H₂ production because the steam reforming and char gasification reactions were enhanced. The maximum H₂ content in the syngas product of 56% was obtained at a reforming temperature of 900 °C [131]. Biochar is also an effective catalyst for the production of turquoise hydrogen and methane. Biochar showed a stability coefficient of 0.83, higher than conventional carbon catalysts. The biochar is produced through high-temperature pyrolysis of wood pellets, which are also used as carbon catalysts for LPG decomposition [125]. Then, there is the integration of hydrogen production from autothermal reforming (ATR) of methanol and biochar generation from SCG through waste heat utilization, which is a new contribution to renewable energy research [22]. Integrating biochar generation with hydrogen production from waste sources like spent coffee grounds presents innovative pathways for renewable energy.

Another method of using biochar is as a carbon feedstock in direct carbon fuel cells (DCFCs) to produce large amounts of valuable gases (CO, H₂, and CH₄). DCFC offers superior electrical efficiency, scalability, and overall reliability compared to conventional technologies such as steam and gas turbines. The high carbon and carbon-oxygen group content of biochar facilitates its application [83]. The addition of biochar has shown positive results in hydrogen production, increasing the maximum hydrogen production rate by up to 107% and the hydrogen production rate by up to 54%. Steam-treated char contains high amounts of K and Ca, which enhance the gas-water shift reaction. This char has high reducibility, moderate surface area, and acts as an efficient catalyst [39].

Other processes for hydrogen production, such as anaerobic digestion, also offer a sustainable way to convert organic waste, such as food waste, into biohydrogen while producing valuable by-products such as biogas and biofertilizer [132]. The addition of biochar to mesophilic anaerobic digestion of food waste increased H₂ yield by 31.0%, increased the H₂ production rate by 32.5%, and shortened the lag phase by 36.0% [133]. Hydrogen production through anaerobic digestion of food waste can be achieved by adding biochar as a cavitation-based technology. The results were positive, with an increase in maximum hydrogen production of 107% and a hydrogen production rate of 54% [39].

Hydrogen can be a significant alternative energy source to fossil fuel, thereby reducing CO₂ emissions. Substitution with hydrogen can replace 5.16 million tons of LPG, 4.13 million tons of LNG, and 9.89 million tons of coal [126]. It can also be used as an alternative energy source for electricity generation. Converting food waste into hydrogen gas and then producing electricity can replace more than 15 billion liters of diesel fuel and reduce global warming by more than 42 million tons of CO₂ equivalent. The use of hydrogen as a substitute for diesel in this amount has a positive impact on the environment by reducing global warming by 42,041.90 kilotons of CO₂ emissions [39].

5.2. Biochar in the Construction, Industrial, and Agricultural Sectors

Carbon utilization is also carried out in the construction sector through cement carbonation. When added to cement paste, absorbed CO₂ participates in the carbonation reaction that converts Ca(OH)₂ to CaCO₃, forming a denser microstructure. The CO₂ absorbed by fine-particle biochar (BCF) is utilized and stored in the composite paste, ultimately producing CaCO₃ [108]. This makes biochar a potential material for producing low-carbon cement building materials, which can transform future cities into “carbon sinks”. CO₂ is captured through the carbonation of minerals in the cement composite. Increased pore connectivity between biochar and the cement matrix promotes CO₂ diffusion into the internal cement structure, to create nucleation sites for calcium carbonate (CaCO₃) formation. CO₂ uptake increased by up to 148% compared to the control (without biochar), especially with a higher water/cement (W/C) ratio (0.4), larger biochar particle size, and higher biochar dosage [134]. Biochar acts as an absorber of atmospheric CO₂ into the porous structure of artificial lightweight coarse aggregates (ALCAs). Chemical fixation prevents the re-release of CO₂, improving long-term storage stability compared to physical adsorption alone. Biochar-ALCAs achieved a total carbon sequestration of 30.58–33.06 kg/ton of aggregate, an apparent density increase of 0.9–2.8%, and a strength increase of 4.3–7.0% post-carbonation [135]. Biochar made from wood waste enhanced carbon sequestration in foamed mortars through an accelerated carbonation technique (2% CO₂ concentration). Based on thermogravimetric analysis, carbon dioxide sequestration of 26.80% and 30% by mass of cement was achieved in foamed mortars containing biochar. The addition of biochar to replace 3% of fly ash mass (density 1150 kg/m³) and 20% of silica fume mass (density 1450 kg/m³) improved the workability of foamed mortar mixtures, as evidenced by a 32% and 44% reduction in yield stress up to 35 min after wet mixing. The addition of biochar reduced water-accessible porosity by 2–3%, resulting in 17% and 23% higher compressive strengths [136]. Lightweight concrete (LC) from Sewage sludge incineration ash (SSIA) mixtures of Portland cement and peanut shell biochar were produced at 500 °C and 700 °C pyrolysis (BC500, BC700) with addition levels of 1–5%. Both BC500 and BC700 improved the mechanical properties of LC at their optimal addition levels, i.e., 5% BC500 and 3% BC700, respectively, achieving compressive strengths of 10.8 MPa and 11.1 MPa, flexural strengths of 2.1 MPa and 2.8 MPa, which were significantly higher than those of the control (compressive strength of 6.2 MPa and flexural strength of 1.35 MPa). Biochar also significantly improved the water resistance of LC by 56.3% (BC500) and 60.4% (BC700) [137]. By acting as both a chemical fixative and a workability enhancer, biochar transforms cementitious materials into effective “carbon sinks” that improve the sustainability and performance of future infrastructure.

Biochar-enhanced cement accelerates hydration and mineral carbonation, yielding stronger, denser composites and significantly reducing CO₂ emissions compared to conventional cement blends. The addition of biochar to cement can densify the matrix by accelerating hydration and increasing its total. This results in higher carbonate mineralization due to calcium hydroxide carbonation compared to silica fume cement, where more carbonate mineralization is contributed by carbonation of the binder gel. This results in 8–10% higher compressive strength and lower strength loss (4–5% loss) in biochar compared to silica fume cement (15% loss) [138]. Even small biochar additions dramatically boost CO₂ uptake and mechanical properties, while higher dosages enable substantial greenhouse gas reductions and improve insulation, confirming biochar’s role as a durable carbon sink in sustainable construction. The use of biochar to replace 3% of cement can reduce CO₂ equivalent emissions. The addition of biochar can accelerate strength development in fly ash cement, which is promising for the development of high early strength concrete with reduced cement requirements [139]. Biochar-enriched cement systems enhance carbon capture by providing microporous pathways for CO₂ diffusion and mineralization to CaCO₃. 1% replacement of cement with biochar increases CO₂ uptake by up to 42%, while synergy with fly ash increases it to 92%. Biochar also improves flexural strength, fracture resistance, and matrix compaction. These findings confirm biochar’s dual role as a carbon sink and performance enhancer, positioning it as a viable negative emissions technology in sustainable construction [140]. Biochar enhances carbon capture in cement composites by increasing the formation of hydration products and promoting carbonation reactions. Its high porosity and surface area provide ideal sites for CO₂ diffusion and CaCO₃ formation. When incorporated at 2–3%, biochar increases CO₂ sequestration by up to 3%, which is equivalent to 9.4 kg/m³ of CO₂-absorbing concrete [141]. Biochar-cement composites effectively sequester carbon while maintaining functional durability at replacements of ≤5 wt%. Higher dosages (20 wt%) sacrifice mechanical strength but improve insulation and environmental durability, positioning biochar as a viable carbon-negative additive for sustainable construction. Emission reductions occur when replacing Portland cement with biochar at 20 wt%, reducing greenhouse gas emissions by 62–66% due to avoided cement production. Thermogravimetric analysis (TGA) confirmed 92% of the residual mass in nitrogen, indicating high thermal stability [142]. A synergistic process combines concrete wash water (CW) and biochar to enhance carbon capture through chemical precipitation and mineralization, forming stable carbon compounds. CW provides high alkalinity (pH 11.9) and calcium ions, while biochar offers a porous substrate for CO₂ absorption and carbonate nucleation. The CO₂ mineralization reaction is accelerated by the porosity of biochar and the alkalinity of CW. Carbon-rich biochar (32.86% C) becomes a permanent carbon sink when embedded in concrete. The carbon sequestration efficiency captured 22.85 wt% CO₂, converting gaseous CO₂ into solid carbonates embedded in the biochar. Cement paste with 30% biochar undergoing CO₂ weathering resulted in net negative emissions (−0.0128 kg C per kg of material) [143]. These innovations demonstrate biochar’s dual function: improving material performance and enabling scalable negative emissions for climate-resilient building solutions.

In the industrial sector, carbon capture also occurs in waste. Biochar derived from pine sawdust exhibits superior CO₂ absorption and carbon stability due to its high surface area, optimal micropore size reaching 0.5 nm, and strong aromatic structure. The CO₂ absorption capacity reaches 1.76 mol/kg. Steam activation further improves selectivity by increasing microporosity and oxygenated functional groups. Meanwhile, paper mill sludge biochar shows potential for fixed-carbon formation at higher pyrolysis temperatures where it has more aromatic carbon, which is more durable and thus better for long-term carbon storage [144]. Steam-activated porous carbon derived from cellulose fibers exhibits superior carbon capture and fixed-carbon formation performance. Steam activation at 800 °C produces ultra-micro pores (<0.7 nm) and a high surface area (1018 m²/g), which allows CO₂ absorption up to 3.78 mmol/g with a CO₂/N₂ selectivity of 47.1. Physically captured CO₂ is readily released under mild regeneration conditions, making this process efficient and industrially viable [145].

In the agricultural sector, biochar production through pyrolytic stoves captures carbon in a stable form, acting as a long-term carbon sink with observed improvements in soil fertility. This is evident in increased crop yields (reportedly 95%), better moisture retention (25%), reduced pest infestation (12%), and reduced soil degradation, a major problem across Sub-Saharan Africa [146]. This carbon storage has the effect of reducing the need for synthetic fertilizers for agriculture, as it contains 0.10 kg of N, 0.02 kg of P, and 0.01 kg of K/ton of crop residue. Biochar systems can create industrial biochar production facilities and field applications [60].

There are also several common scenarios for agricultural waste management, namely as an alternative energy source and soil amendment. The definition of system boundaries is an important step in LCA, where the production of biochar (or activated carbon) from agricultural residues through pyrolysis is generally classified as cradle-to-grave or cradle-to-gate, as shown in Figure 2 [147].

6. The Challenge of Biochar as a Negative Emission Strategy

Biochar, as a negative emission (NET) technology, faces economic, socio-environmental, and future development challenges. From an economic perspective, it aims to address issues related to climate change mitigation, such as supporting food security and contributing to a circular economy [23]. This phenomenon represents a challenge that requires further development, also related to techno-economics.

Biochar also plays an active role in effective carbon sequestration for environmental restoration. Sustainability and soil improvement are derived from several factors, such as its high surface area, porosity, and nutrient retention capacity, making it an attractive candidate for increasing soil fertility, enhancing water retention, and reducing environmental pollution [72]. Further study is needed to determine the environmental and social relationships associated with biochar development.

The future direction of biochar also addresses aspects that need to be addressed to ensure its sustainability in society. Challenges faced in biochar development relate to raw materials, technology, and other factors.

6.1. The Economic Benefits Obtained from the Role of Biochar

Economic benefits can be derived from the effectiveness of biochar production energy sources. The economic feasibility of biochar is influenced by the type of raw material used, conversion technologies such as pyrolysis, gasification, and others, as well as the inclusion of carbon credits or carbon sequestration subsidies. Biochar is a cost-effective alternative to conventional adsorbents used in wastewater and soil remediation [148]. Regarding raw material costs, efforts to minimize them can be made from the pyrolysis/gasification process using low-cost biomass such as agricultural waste and wood residues [125]. Production processes such as biorefineries, green hydrogen, and side-stream valorization using renewable resources (e.g., biomass, agricultural waste, algae, and manure) are circular bioeconomy innovations. Circular economy strategies are also carried out by maximizing the reuse, recycling, and valorization of resources to minimize waste and emissions [6]. Biochar production costs fall into several categories, including raw material costs, transportation costs (biomass collection and delivery), production costs (equipment, energy, labor), labor and maintenance, storage costs, and regeneration costs (for reuse as an adsorbent). Global biochar market prices range from $0.08–$13.48/kg (USD 2013 or $0.10–$16.60/kg in USD 2025, as the mean price for blended biochars) depending on the region and whether the biochar is pure or a mixture [3]. Biochar depend on the scale of the process and its handling, for example, biochar through microwave pyrolysis and chemical activation is up to $27,800/ton (USD 2018 or $35,682.76 in USD 2025, as production cost), then the steam-activated wood biochar (for adsorption) is $3587–$3737/ton (USD 2018 or $4604.10–$4796.64/ton in USD 2025, as the production cost of woody biochar and steam activated woody biochar). Costs are influenced by pre-processing, post-treatment for surface enhancement, pyrolysis method (e.g., microwave, hydrothermal, plasma), and production scale (lab vs. industrial) [9].

Techno-economic assessment (TEA) combines direct production costs and potential revenues from biochar sales, energy recovery (syngas, bio-oil), and environmental credits (e.g., carbon sequestration). Profitability is sensitive to market prices for biochar, scale of operation, feedstock costs, and the potential for by-product valorization. Product yield and quality such as biochar yield and physicochemical properties (e.g., surface area, porosity, functional groups, ash content) are highly dependent on production parameters (temperature, residence time, heating rate). Soil amendments, adsorbents, catalyst support, and others determine target quality attributes and thus influence process selection and economics [15]. Techno-economic assessment of a portable system for producing solid biofuel (briquettes) and biochar from forest residues was evaluated using the discounted cash flow rate of return method to estimate the minimum selling price (MSP) for each product. The MSP for woodchips briquettes (WCB), torrefied woodchips briquettes (TWCB), and biochar are $162, $274, and $1044 per ton (USD 2019 or $204.19, $345.37, and $1315.92 per ton in USD 2025, as the MSP considering tax 16.5% and transport distance of 200 km), respectively. Capital investment (16–30%), labor costs (23–28%), and raw material costs (10–13%) excluding wood costs are the main factors influencing the MSP. However, the MSP of WCB, TWCB, and biochar can be reduced to $65, $145, and $470 per oven-dry metric, respectively, with an improved portable technology system [5]. The technical-economic advantage of producing porous biochar from wood is that it does not require an external energy source for the pyrolysis process because it takes place at temperatures above 500 °C (lignin decomposition is a completely exothermic process) [61]. Life cycle assessment and techno-economic assessment show the benefits of replacing conventional activated carbon with biochar. Associated with low raw material costs, biochar can have lower production costs than commercial activated carbon (AC). In magnetic biochar produced from used tires, the production cost of biochar is estimated at $299/ton (USD 2020 or $375.54/ton in USD 2025, as the production cost of biochar), much lower than coal-derived powdered activated carbon (PAC) ($1100–$1700/ton or $1381.58–$2135.17/ton in USD 2025) [9].

Solid carbon co-products can be sold in addition to being permanently stored, significantly offsetting the cost of H₂ production and supporting carbon mitigation. The carbon strategy is to be used as a commodity product in the material market. Carbon black and graphite are valuable carbon commodities. The mass ratio between H₂ and solid carbon co-products from CH₄ pyrolysis is one to three. Even if only 1% of global H₂ production comes from CH₄ pyrolysis, 2.82 Mt of carbon would be produced [105]. At a commercial scale, a pyrolysis and co-pyrolysis plants with a capacity of 20 tons/hour were investigated for its effect on the feedstock ratio of rice straw (RS) and waste tire (WT). Initial costs ranged from $17.0–$19.9 million (USD 2022 or $18.72 million to $21.91 million in USD 2025, as the plant capital investment), with Plant A (RS only) having the lowest value and Plant E (20% RS and 80% WT) having the highest value. Plant E is the most economical alternative with the highest gross margin, highest net present value, and lowest payback period of 7.06%, IRR of 14.28%, $5.63 million, and 6.23 years, respectively [149]. The revenue source is from the sale of biochar, some of which is sold commercially (e.g., horticultural markets). The market price of biochar fluctuates which directly affects the revenue from sales. Product distribution generates higher profits when biochar is sold externally (e.g., horticultural) rather than used on-site. Cost Savings are achieved by fertilization, which results in savings of up to $90/ha (USD 2015 or $122.92 in USD 2025, at biochar application rates of ≥3 tons/ha). The cellular pyrolysis production capability processes 4 t/batch, producing approximately 1 ton of biochar in 4 h (3 batches/day) [8].

The sale of integrated biochar and BECCS products shows promising revenue potential with significant net energy output and water savings per tons of CO₂ processed, but economic viability depends heavily on feedstock costs. It could be worth $525 million (USD 2024 or $549.91 million USD in 2025, selling price of the integrated product of 53% Biochar and 47% BECCS) which then produces a net energy of 16.95 GJ/t CO₂ and water consumption of 2.71 m³/t CO₂ [2]. Then, there is a char with a HHV of 26.25 MJ kg⁻¹ and a yield of 34.5% produced. This char is comparable to sub-bituminous coal A. The high energy efficiency is around 82% and the economic feasibility of the plant is very sensitive to the cost of Corn Stover (CS) feedstock. The process has a net present value (NPV) of −$1.17 million on the CS base assumption of $20/ton (USD 2018 or $25.67/ton in USD 2025, as product price). A cost sensitivity analysis shows that when the CS cost is reduced to $3/ton, the NPV is zero [150]. As a measure to mitigate climate change and improve soil quality, carbon sequestration is carried out with a value of $2.93–$90.83 per metric ton (USD 2008 or $4.39–$135.97/ton in 2025, the cost of carbon offset). Compared to activated carbon, biochar costs approximately $91–329/ton, while activated carbon costs approximately $1500/ton and zeolite costs $6000/ton [148]. The minimum selling prices for biochar in water and ethanol mixtures are $0.07 and $0.052 per MJ, respectively (AUD 2024 or $0.05 USD and $0.04 USD in 2025, as the levelized production cost). The advantage of converting waste material into energy is that it offers an affordable and clean energy economy solution through new slurry fuels [7].

The growing popularity of biochar is a sustainable and cost-effective solution with wide applications across industries and regions. The biochar market is growing, particularly in Asia, Europe, and the Americas, with India and China leading the way in production. Pyrolysis (especially MAP) produces by-products (bio-oil and syngas) from MAP. In Tasmania, a positive net present value (NPV) of biochar production was demonstrated, reflecting both environmental and financial returns. The return on revenue includes savings on waste management, income from biochar sales, and reduced greenhouse gas emissions [71]. The economic feasibility of a pyrolysis plant processing biochar-based slurry fuel (CS) from rice straw (RS) at production rates of 10 and 40 tons/h measured the environmental and economic life cycle impacts of biochar production and agriculture in various countries. Net economic benefits were achieved only when simple, low-cost kilns were used in countries with low labor costs, such as Ethiopia, Kenya, and Vietnam [84]. In Korea, the economic benefits and returns to farmers were seen in equivalent gains from CO₂ reductions through the carbon credit market of $145.59/ha (KAU 2019 or $3541.75/ha in USD 2025, its profit analysis) for pig manure compost (BCP) pellets and significantly higher than for pig manure pellets ($61.22/ha) [46]. In Kenya, the benefits of biochar users in the community include economic assistance where 98% of households use less fuel, lower household energy costs, reduced dependence on chemical fertilizers, thanks to the nutrient retention properties of biochar, increased net household income through lower input costs and higher crop yields [141].

The potential for system integration and economic feasibility is reviewed from the development of technological facilities. For example, in biochar production, fuel can be sourced from the gas produced for production that can be returned to the system so that it does not require external energy [125]. Also, a circular economy occurs from waste heat and agricultural by-products that can be reused in industrial applications [22]. Integration of CO₂ flue gas reuse into pyrolysis not only improves environmental performance but also reduces operational costs by eliminating the need for inert gases such as nitrogen which can increase biochar production in terms of economic feasibility [151]. Economic benefits are seen in composting plants due to the synergistic calorific value derived from the recycling process of abundant composting residues (lignin 69%, bone 18% and plastic 12%) [75]. Then the energy output and reactor design can be modified. The system consists of interlocking three cylinders that form three different zones and allow the separation of the partial oxidation and pyrolysis zones. The resulting biochar is highly porous with a specific surface area of 200 m²/g and a constant carbon content of approximately 90% on a dry basis due to the separation of the partial oxidation and pyrolysis zones [79]. Integrating a biorefinery with pyrolysis and anaerobic digestion facilities has also been developed, which produces no secondary waste, in line with circular economy principles. This occurs by recovering energy while minimizing the visible environmental impact of the spent anaerobic digestion (AD) inoculum. This is used to produce biochar, which is used to supplement the AD aqueous pyrolysis condensate (APC) [41].

A Carbon Removal Certificate (CORC) is a tradable and verifiable unit representing the permanent removal of 1 ton of CO₂ equivalent (tCO₂e) from the atmosphere. How CORC works is as follows: carbon removal activity where an entity (e.g., a biochar producer) removes CO₂ from the atmosphere, trading or retirement where companies purchase CORC to offset their emissions or to meet climate targets, and finally CORC issuance, where for every 1 ton of net CO₂ removed, 1 CORC is issued [152]. The net ecosystem carbon budget (NECB), carbon footprint (CF), nitrogen footprint (NF), and net ecosystem economic benefits (NEEB) were comprehensively evaluated using LCA and Z-score methods [153].

6.2. Effectiveness of Social Impact

Biochar facilities can create significant rural employment and stimulate local economies by utilizing forest and agricultural waste, while supporting resilient farming in water-scarce regions. In terms of rural employment and industrial development, a biochar facility with a plant production capacity of, for example, 45,000 Mg/year would employ approximately 27 people full-time. Rural economic development is realized through markets for forest waste. Agricultural productivity support is provided for dryland farmers (e.g., potatoes, alfalfa) to increase yields with fewer resources and help build resilient food systems in semi-arid regions with scarce water. Public health and improved air quality can also result from reduced air pollution from controlled fire burning of felled areas and cleaner combustion in pyrolysis plants resulting in lower NO_x and Volatile Organic Compound (VOC) emissions [154]. In South Africa, from a socio-economic perspective, bioenergy and biomass resources have the potential to create 3700 jobs per megawatt (MW) of energy generated from a total system capacity of 89,532 MW. The Department of Energy’s Integrated Energy Plan (IEP) estimates that 26% of the country’s energy mix will come from renewable resources by 2030 [150]. In Kenya, consumers benefit from lighter labor burdens such as less firewood collection, less time spent on fuel processing and cooking [141].

Public acceptance occurs as cost savings mean better profit margins for farmers. Opportunities for biochar production to become a value-added activity in rural areas. Biochar production replaces open burning, preventing air pollution and local health hazards. Increased efficiency of agricultural processes is characterized by a 50% reduction in fertilizer use with stable yields. Avoidance of open burning of rice straw produces high amounts of CH₄, N₂O, and particulate matter [155]. Social outputs include cleaner air and public health. Reducing open burning of crop residues, which emit CO, NO_x, CH₄, SO₂, PM_2.5, VOCs, which are at risk of causing cardiovascular problems. Biochar application reduces exposure to toxic emissions, especially in rural agricultural areas [60]. Reduced air pollution by indoor biochar use leads to fewer respiratory problems (sneezing, eye irritation, coughing) [141]. These benefits, combined with cost savings and reduced labor, enhance public acceptance and position biochar as a socially and environmentally valuable rural development tool.

As a benefit to society, the advantages of biochar can also play a role for the environment by removing organic and inorganic contaminants from the environment. Its use as an adsorbent for the immobilization of toxic elements such as heavy metals (HM), and as a catalyst in advanced oxidation processes (AOP) to degrade toxic pollutants such as complex organic compounds [104]. On the other hand, biochar can pose serious ecological and health risks without strict feedstock control, pyrolysis optimization, and environmental monitoring. The release of hazardous heavy metal compounds during the leaching process of Cu, Zn, Pb, Cr, and Cd from biochar—especially from sewage sludge or metal-rich biomass—can contaminate soil and groundwater. Toxic polycyclic aromatic hydrocarbons (PAHs) and dioxins can arise from biochar with certain feedstocks or under low-temperature pyrolysis, which migrate into soil and plants and pose mutagenic and carcinogenic risks. Biochar can also generate Environmentally Persistent Free Radicals (EPFRs) and then form Reactive Oxygen Species (ROS), which cause soil and aquatic toxicity and oxidative stress in organisms. Smaller particles migrate through soil and water systems, namely micro/nano-biochar (MB/NB), which can transport nutrients (phosphorus leaching) and contaminants (heavy metal co-transport) [156]. Practical mitigation of PAHs can be achieved through post-treatment and filtering of raw materials. Post-treatment processes, such as combining biochar with myco-remediation (Pleurotus ostreatus), can increase remediation efficiency. This relies on effective enzymes produced by fungi to decompose various types of substrates and contaminants [43]. High-lignin biomass feedstocks produce biochar with significantly lower PAHs than biomass with low lignin content. Biochar from horticultural, forestry, and plantation waste production exhibits the lowest PAH levels. Meanwhile, feedstocks high in cellulose and pectin produce more PAHs than lignin-enriched feedstocks [44].

In Sweden, the utilization and local benefits of urban wood waste can reduce reliance on incineration and landfilling and support circular economy goals by converting waste into remediation materials and carbon sinks. Public health benefits include reducing the leaching of hazardous compounds into the environment, thereby protecting ecosystems and public health. However, due to performance variability, site-specific risk assessments are required for reuse in urban or agricultural areas. Policy implications need to be addressed by encouraging the transition from landfill-based remediation to more sustainable methods, thus supporting environmental policy targets for carbon neutrality and controlling ambient pollution [45]. In Northern Europe, the role of government stakeholders in mining industry engineering projects can reduce regulatory uncertainty and expedite permitting, while ensuring balanced economic, social, and environmental outcomes. Collaborative practices not only enhance sustainable design solutions but also introduce the concept of intermediary stakeholders. This makes the early engagement framework a strategic tool in aligning industrial projects with sustainability goals [157]. In the Kingdom of Saudi Arabia (KSA), an estimated 400 million tons of agricultural waste biomass are produced annually. The resulting biomass primarily consists of date palm trees and fruit waste. There are over 22 million trees and 450 varieties of date palms, making KSA the third largest date-producing country in the world. The resulting biochar exhibits diverse mineralogical composition, high porosity and thermal stability, and an alkaline pH, making it suitable for enhancing energy recovery processes such as anaerobic digestion (AD), transesterification for biodiesel production, and pyrolysis [19]. In Kenya, technology adoption by all participating households indicated a willingness to continue producing and using biochar. 87% intended to use biochar as a soil amendment, valued the agronomic benefits over reuse as fuel, 98% would purchase a stove if available at a reasonable cost, and many expressed interest in pre-prepared feedstock to save labor [141].

6.3. Future Directions

Biochar’s prospects for sustainable production include the need to address high production costs due to low supply, although technological advances have begun to reduce production costs [71]. Energy-efficient raw materials can be good candidates for biochar production, as evidenced by the lower average apparent activation energy (Ea) values for palm kernel shell (PKS) and empty fruit bunch (EFB) compared to palm oil sludge (POS) [35]. Analysis of specific thermal kinetic parameters of the raw materials is also necessary, which is an important aspect in understanding the mechanisms and development of pyrolysis systems [106]. There are also gaps that need to be addressed, such as controlled process parameters, such as control of surface area and pore volume, which will affect pore diameter, where further increases cause pore damage and reduced surface area [70]. Research into feedstock optimization is needed to understand the yield and characteristics of biochar derived from various carbon-rich and nutrient-rich biomass materials. This is crucial for producing biochar that meets the needs of its application [72]. Feedstock-specific studies are needed to determine the behavior of differences in elemental composition that arise due to differences in biomass composition, where the atomic ratios (O/C) and (H/C_org) are indicators of biochar aromaticity and polarity [65]. Future research should develop standardized stability metrics. The use of molar ratios as indicators based on O/C and H/C_org is used to distinguish biochar from biomass that has not yet been fully carbonized. This is also considered an indicator of biochar stability in soil [107]. Research is needed on the synergy of co-pyrolysis feedstocks, scaling up biochar pellet production, and conducting techno-economic and life cycle assessments [73]. Addressing these gaps will support scale-up, improve biochar performance, and enhance its techno-economic and environmental viability.

Further feedstock-specific analysis and pilot-scale integration of biochar production using CO₂-rich industrial gas streams have also not been studied in depth, as CO₂ behavior depends on the type of feedstock and the production process. The potential for engineered biochar derived from slow pyrolysis of CO₂ to produce high-value products is evident [151]. The interaction of various components needs to be addressed, their classification such as organic (lignin), inorganic (bone), and synthetic (plastic) during pyrolysis, has not been widely explored in the dynamics of fixed-carbon formation with implications for optimal reactor design and biochar quality. Pyrolytic byproducts (such as char and inorganic) from the thermal decomposition of lignin/bone can settle on the plastic surface and further inhibit the plastic decomposition process [75]. It is also important to study the application of biochar to prevent misapplication and maximize environmental benefits. The use of biochar beyond its intended purpose can have negative impacts. Characterizing biochar and utilizing it according to its intended purpose will minimize its negative impacts and provide benefits [23].

There is a need for standard criteria to guide pyrolysis towards enhanced fixed-carbon formation and structural stability. Despite favorable pyrolysis temperatures and residence times, degradation of the carbon structure occurs beyond the optimal limits, indicating an upper threshold for porosity [79]. The relatively higher surface area of unmodified biochar in carbon structure formation can be attributed to thermal cracking, which causes carbon atoms and NH₃ molecules to interact with each other [30]. From a carbonization perspective, a research gap exists in the design of sustainable pyrolysis systems and their unit control, as no studies have investigated the impact of continuous flow pyrolysis on the carbonization process of palm oil waste [63]. It is also necessary to examine how nitrogen-functionalized biochar can form during co-pyrolysis, and how thermal parameters and composition affect the development of the carbon framework, porosity, and stability. This is because N-containing feedstocks lead to the formation of nitrogen-containing functional groups on the biochar surface, significantly enhancing the value and application of this product. Micropores are favored by the shrinkage of the carbon framework associated with a higher surface area at elevated temperatures [40].

Furthermore, from a technological perspective, further improvements are expected, and techno-economic studies are needed to assess the overall effectiveness of this technique. Research on the simultaneous thermal decomposition of various types of agricultural biomass is still lacking [73]. Research gaps also exist in the design of small-scale systems and the integration of waste heat recovery. Integrating thermoelectric power generation could contribute substantially to meeting the power requirements of the carbonization process [78]. Then, microwave absorbers affect product yield and quality, where the addition of graphite and activated carbon increases biochar yield from biomass while increasing carbon content and influencing pore formation and surface morphology [70].

Reactor designs (Figure 3) focused on biochar remain largely unexplored. Conical spouted bed reactors, often used for bio-oil production, show untapped potential for char optimization. Future research should tailor pyrolysis conditions in these reactors to improve carbon retention and biochar structural integrity [51].

In terms of feedstock, it is necessary to understand the interaction mechanisms between different types of biomasses, such as household waste, such as wood, plastic, and others. This prevents the opposing effects between plastic and lignin/bone, making it slightly more difficult to decompose the plastic components [75]. Furthermore, it is necessary to specify the individual gas species (H₂, CO, CH₄, etc.) numerically, where hydrogen is part of the gas phase [69]. This helps detect each species in the output for measuring greenhouse gas emissions. Furthermore, fixed-carbon formation is detected in each biochar conversion technology, and reactor modeling is needed to optimize carbon yield and process efficiency. Then the classification of chemical, thermal, and physical processes also needs to be followed up in modeling approaches as a powerful tool for further investigation [158]. From an emissions perspective, there is a need for better emission factor (EF) modeling, expanded life cycle assessment, and inclusive global research. This is in the form of a process-based GHG accounting model with a universally applicable methodology for accurate emission estimation from biochar production systems [64].

7. Conclusions

Biochar production technology has been proven to offer a significant contribution as a negative emissions strategy in responding to the challenges of climate change and the need for a global energy transition. By converting biomass waste into biochar, a number of solutions are achieved, ranging from reducing greenhouse gas emissions and increasing long-term carbon sequestration to providing renewable energy and value-added products such as green hydrogen.

Each stage of the process—from raw material selection, organic waste sorting, thermochemical conversion through pyrolysis, gasification, and hydrothermal carbonization—contributes to optimizing the physical and chemical properties of biochar as a CO₂ adsorbent and energy product. Research shows that carbon stability, atomic ratios (O/C and H/C_org), and porosity are key indicators in determining the quality and potential for environmental applications. International standards have recognized biochar as a critical component of carbon dioxide removal (CDR) technology, with the potential for carbon storage lasting hundreds or even thousands of years.

In the industrial and construction sectors, biochar can be utilized to increase material strength and as a carbon capture in the production of building materials. The use of biochar in agriculture also brings positive agronomic impacts in the form of increased soil fertility and water retention, while reducing the need for chemical fertilizer inputs. Meanwhile, the integration of biochar into hydrogen and electricity production systems demonstrates its ability to support the transition to cleaner energy sources, helping to reduce fossil fuel use and significantly reduce CO₂ emissions.

However, the widespread adoption of biochar technology still faces challenges related to economics, technological efficiency, raw material distribution, and socio-environmental aspects, such as the potential risk of heavy metal or hazardous compound contamination if processes and quality controls are not optimally implemented. Furthermore, the development of biochar quality standards and stability metrics, process optimization, and multi-sector synergy studies still needs to be expanded to ensure large-scale implementation and sustainability.

Moving forward, cross-disciplinary collaboration, increased innovative research, and policy approaches that support biochar development are crucial to accelerate the implementation of biochar-based negative emission technologies. With comprehensive and sustainable management, biochar will not only be a solution for reducing emissions but also a driver of a circular economy and the achievement of carbon-neutral targets in the future.