Biochar insights: Sustainable production, market evolution, and climate change mitigation

Introduction

Rapid industrial growth and increasing global population have significantly intensified environmental pollution, making it one of the most serious threats to Earth's ecosystems and living organisms (Briffa et al., 2020) [13] . Human-driven climate change, along with unsustainable agricultural practices, has further worsened environmental stress, leading to frequent droughts, nutrient losses through fertilizer leaching, and growing concerns over global food security (Glaser et al., 2012) [34] . In the face of urgent and emerging global challenges, biochar has gained attention as a promising tool for building a more sustainable future and creating value within a circular economy framework. Researchers increasingly view biochar as an effective soil amendment capable of improving soil quality and enhancing the long-term sustainability of agricultural systems (Alling et al., 2014) [5] . Key indicators of healthy soil include its texture, ability to support microbial activity, and capacity to retain nutrients and moisture, all of which play a crucial role in crop productivity (Novotny et al., 2015) [83] . Traditionally, biochar has been applied in its raw form; however, recent studies suggest that physical and chemical modifications can further enhance its effectiveness in soil applications (Kim et al., 2007) [59] . Numerous studies have demonstrated that biochar improves soil health and crop yields while maintaining the natural balance of the soil ecosystem (Bruun et al., 2012) [15] . These benefits are largely attributed to biochar's high surface area, porous structure, strong adsorption capacity, water-holding ability, and cation exchange capacity. In addition, biochar is highly stable and resistant to physical, chemical, and biological degradation, making it a long-lasting amendment for both agricultural and environmental applications. Because of its distinctive physicochemical characteristics, biochar has increasingly been applied to improve soil fertility and promote healthy plant growth (Yadav et al., 2023) [116] . Beyond its role in agriculture, biochar also contributes to climate change mitigation by enabling long-term carbon sequestration, lowering greenhouse gas emissions, and removing both organic and inorganic pollutants from soil, water, and air through processes such as adsorption and degradation (Shoudho et al., 2024) [97] . The Intergovernmental Panel on Climate Change (IPCC), in its 2014 assessment, highlighted serious concerns over the rising concentration of greenhouse gases in the Earth's atmosphere and emphasized the urgent need for mitigation actions. According to the report, global greenhouse gas emissions must be reduced by 40-70% by the middle of this century compared to 2010 levels. In response, a wide range of mitigation strategies and policy measures have been introduced worldwide to address climate change (Adnan et [2,4,42,80,88,119] . Given its wide-ranging benefits and longterm stability, biochar has attracted growing scientific interest as a viable and effective strategy for climate change mitigation both now and in the future.

Evolution of biochar

Biochar is a charcoal-like material produced by heating biomass at very high temperatures, and it also occurs naturally in soils as a result of vegetation fires. Humans have intentionally used biochar in agriculture for over 2,500 years, most notably in the Amazon Basin of South America. There, ancient farming communities created dark, charcoal-rich soils known as terra preta (black earth), which transformed previously poor and even toxic soils into highly productive farmland. These remarkable soils were first scientifically documented in the 1950s by Dutch soil scientist Wim Sombroek and are estimated to still cover nearly 10% of the Amazon Basin. Similar biochar-enriched soils have also been identified in regions of Ecuador, Peru, and parts of West Africa, including Benin and Liberia. Unlike typical tropical forest soils, which are generally low in fertility, the incorporation of biochar enabled soils to retain organic matter, essential plant nutrients, and moisture required for sustained crop growth. This innovation made it possible to cultivate the same land year after year, reducing reliance on slash-and-burn farming practices. The resulting improvement in soil fertility supported stable food production systems and enabled the development of large, permanent human settlements (Wayne, 2012) [112] . [35] Since the beginning of the twenty-first century, biochar has emerged as a major focus of multidisciplinary research due to its unique properties, wide range of applications, and strong potential for future development. In response to growing concerns over food security, environmental pollution, and energy shortages, both fundamental and applied research on the use of biochar in agriculture, environmental management, and energy systems has increased substantially (Chen et al., 2019) [18] . Although interest in biochar has surged in recent decades, its use dates back at least 2,000 years (Hunt et al., 2010) [49] . Biochar was first formally recognized in Western agricultural studies during the midnineteenth century, though its practical use likely extends much further into history. The earliest known applications of biochar are closely associated with indigenous communities of the Amazon Basin, who intentionally modified soils to improve fertility (Lehmann et al., 2009; Han et al., 2020) [44,65] . These practices led to the formation of highly fertile soils known as terra preta, meaning "black soils of the Indians," which serve as clear evidence of extensive biochar use by ancient civilizations (O'Neill et al., 2009; Tenenbaum, 2009) [84,102] . These soils are characterized by exceptionally high concentrations of essential nutrients such as potassium, phosphorus, and nitrogen (Chen et al., 2019) [18] . Owing to their dark, organic-rich nature, terra preta soils remain highly productive even after centuries of nutrient leaching under intense tropical rainfall (Hunt et al., 2010) [49] . Biochar should not be confused with terra preta, even though the two are closely related. Terra preta, also referred to as Amazonian dark earth, occurs naturally and can also be recreated through the deliberate addition of low-temperature charcoal mixed with organic materials such as compost, manure, plant residues, animal waste, and bones (Graham et al., 2006) [39] . In comparison, biochar differs from terra preta in both its overall composition and carbon structure (Igalavithana et al., 2018). Importantly, scientific interest in biochar was largely stimulated by the discovery of the exceptional fertility of terra preta soils (Glaser et al., 2002) [36] . Biochar is present in soils across the globe due to natural processes such as forest and grassland fires (Krull et al., 2008) [63] . Regions with naturally high biochar content, including the North American prairies, are recognized among the most productive agricultural soils worldwide (Hunt et al., 2010) [49] . Historically, biochar has also been widely used in agricultural practices across various parts of Asia, particularly in Japan and Korea. Insights gained from terra preta research during the mid-1990s revealed biochar's strong potential for carbon dioxide sequestration and emission reduction, positioning it as a promising tool for addressing climate change (Han et al., 2020). Following the first International Biochar Initiative meeting held in Australia in 2007, many countries established national biochar societies to promote research, field demonstrations, and knowledge exchange, leading to a steady growth in global biochar research efforts (Han et al., 2020) [44] .

Biochar production

Charcoal can form naturally through vegetation fires or be deliberately produced by humans using controlled burning methods such as fire pits and simple handmade structures. Biochar, in contrast, is produced under anaerobic or oxygenlimited conditions, which allow biomass to undergo controlled thermal conversion. Based on the level of technological advancement, biochar production methods are broadly categorized into traditional and modern techniques, reflecting differences in efficiency, control, and degree of modernization (Gabhane et al., 2020) [29] .

Traditional approaches

Archaeological records indicate that humans began producing and using biochar more than 2,500 years ago, with the earliest evidence originating from the Amazon Basin in South America (Glaser, 2007) [33] . These practices led to the formation of terra preta soils, which contain nearly three times higher levels of soil organic matter and nutrients compared to surrounding soils (Glaser et al., 2001) [35] . Ancient communities typically produced biochar by slowly burning wood in soil-covered pits under little or no oxygen supply (Thines et al., 2017; Barrow et al., 2012) [8 104] . In another approach, biomass was burned in open areas and the partially charred material was quickly covered with soil to limit oxygen exposure (Emrich, 1985; Reilly, 1925) [90] . To improve control over the process, early societies developed simple, handmade reactors such as clay burners, firebrick pits, brick kilns, and iron retorts. Despite differences in materials, these systems shared a common design element-a pit enclosed by clay, bricks, or metal. Such modified production techniques allowed for the recovery and utilization of volatile compounds released during pyrolysis (Reilly, 1925) [90] . These traditional biochar-making methods remained widely used until the late nineteenth century, when more efficient, labor-saving steel ovens were introduced (Emrich, 1985; Reilly, 1925) [25,90] .

Pyrolysis: Pyrolysis is a thermochemical process in which biomass is heated under anaerobic or oxygen-limited conditions, resulting in the formation of gaseous products, liquid by-products, and solid char residues commonly referred to as biochar (Basu, 2010) [9] . Agricultural biomass mainly consists of cellulose, hemicellulose, lignin, and silica, each of which decomposes at different temperatures during pyrolysis. Cellulose typically degrades around 350 °C, while lignin breaks down over a broader temperature range and begins decomposing above 350 °C (Czernik and Bridgwater, 2004) [21] . As a result, an effective temperature window for pyrolysis generally lies between 300 and 700 °C (Suarez-Abelenda et al., 2017) [101] . Although pyrolysis occurs in the absence of oxygen, external heat sources are required to sustain the process, including hot gases, heated solids, liquid heat-transfer media, or heat generated through controlled oxidation and partial oxidation reactions (Li et al., 2016; Xie et al., 2015) [60,67,115] . Over time, pyrolysis technologies have evolved significantly. Based on operational parameters such as temperature, heating rate, and residence time, pyrolysis can be categorized into different modes, most commonly slow and fast pyrolysis, which are associated with traditional production methods. In contrast, advanced techniques such as flash pyrolysis, vacuum pyrolysis, and microwave-assisted pyrolysis incorporate modern technological innovations. Accordingly, slow and fast pyrolysis are classified under traditional approaches, while flash, vacuum, and microwave pyrolysis are grouped under modern biochar production methods.

Slow pyrolysis:

As its name suggests, slow pyrolysis is a timeintensive process that typically takes several hours to complete and results in biochar as the primary product. Also referred to as conventional pyrolysis, this method involves heating biomass at temperatures ranging from 300 to 600 °C using a relatively low heating rate of about 5-7 °C min⁻¹ (Lai et al., 2013) [64] . Slow pyrolysis generally produces a higher proportion of biochar, accounting for approximately 35-45% of the total output, along with by-products such as bio-oil (25-35%) and syngas (20-30%) ( [64,76,110] . A wide variety of feedstocks have been successfully utilized for biochar production through slow pyrolysis, including Conocarpus wood waste (Moussavi et al., 2012) [78] , cotton stalks (Shen et al., 2015) [95] , and agricultural residues such as coconut shells, palmyra nutshells, and rice husks (Jegajeevagan et al., 2016) [53] .

Fast pyrolysis: Fast pyrolysis is ahighly efficient thermochemical process designed primarily for the conversion of biomass into biofuels (Dai et al., 2017) [22] . This process operates at temperatures exceeding 500 °C, with very high heating rates of more than 300 °C min⁻¹ under oxygen-free conditions. Unlike slow pyrolysis, fast pyrolysis occurs rapidly and is typically completed within a few seconds. The product distribution from fast pyrolysis generally consists of about 60% bio-oil, 20% biochar, and 20% syngas (Dai et al., 2017; Huang et al., 2016) [22,48] . Due to its high efficiency and rapid processing capability, fast pyrolysis is commonly applied in large-scale biochar and biofuel production systems (Liu et al., 2016) [67,70] . [118] 7 Hydrothermal carbonization Less than 200 Less than 1 1 to 16 h 35 30 35 Brown et al. (2020) [14] Fig 4: Different methods of thermochemical conversion process

Modern approaches

By the end of the twentieth century, significant advancements and modifications in biochar production methods emerged. Over time, a variety of modern approaches have been developed, including advanced pyrolysis techniques such as flash, vacuum, and microwave pyrolysis, as well as gasification, torrefaction, hydrothermal carbonization, and electro-modified methods (Gabhane et al., 2020) [29] .

Gasification: Gasification is a thermochemical process in which biomass is converted into biochar, bio-oil, and syngas by applying high temperatures above 750 °C in the presence of oxygen, air, or steam, using the heat generated from biomass decomposition (Ravindiran et al., 2024) [89] . This method is widely employed for producing syngas from various solid fuel sources. Compared to conventional techniques such as pyrolysis, combustion, and fermentation, gasification offers higher syngas yields and lower levelized emissions. Hydrogen is a major component of the syngas produced through gasification (Gonzalez et al., 2008) [38] . Overall, gasification serves as an efficient thermochemical approach to converting biomass into energy while generating biochar as a valuable byproduct (Guan et al., 2016) [41] .

Torrefaction: Torrefaction is a thermal treatment process in which biomass is heated at moderate temperatures ranging from 300 to 550 °C, with a heating rate of approximately 50 °C min⁻¹ and residence times of 20 to 40 minutes, producing biochar along with bio-oil and syngas (Manyà et al., 2014). This process is commonly used as a pre-treatment to reduce biomass moisture content, thereby enhancing its subsequent thermal conversion efficiency. Biochar yields from torrefaction typically range between 30% and 70% (Enaime et al., 2020) [26] . During torrefaction, the hemicellulose fraction of the biomass decomposes, resulting in carbon-rich, stable solid products and volatile compounds that are more energy-dense and suitable for further applications (Prins et al., 2006) [87] . [23,68] . This combination of extremely high temperature, rapid heating, and short vapor residence time favors the production of bio-oil in high yields. However, the rapid conversion conditions result in comparatively lower biochar yields (Tripathi et al., 2015) [108] .

Microwave assisted pyrolysis: Microwave-assisted pyrolysis (MAP) is a modern advancement in thermochemical conversion technologies. In this method, microwaves are used to efficiently convert uniform biomass waste into energy-rich feedstocks (Ge et al., 2021) [32] . Unlike conventional heating, where energy transfers from the surface inward, MAP delivers energy directly within the biomass, enabling rapid and uniform pyrolysis throughout the material (Liu et al., 2021b) [71] .

Hydrothermal Carbonization (HTC):

Hydrothermal carbonization (HTC) is a method used to convert high-moisture feedstocks, such as sewage sludge, animal manure, and compost, into biochar-like products called hydrochar (Titirici et al., 2007) [106] . Unlike conventional biochar, which is produced through dry pyrolysis, hydrochar is formed under wet conditions, where biomass undergoes thermal treatment in a mixture of solid and liquid phases. Consequently, the properties of hydrochar differ from those of traditional biochar, reflecting the differences between dry and hydrothermal carbonization processes (Kambo & Dutta, 2015) [55] .

Biochar applications

Biochar contains a higher carbon content compared to commercially available activated carbon, making it a valuable tool for environmental management and various pollution control applications. Its versatility extends across multiple sectors, including biofuel production, soil enhancement, construction materials, carbon sequestration, and the improvement of composting processes. Biochar for soil structure and enrichment: In agriculture, biochar is often referred to as "black gold" due to its remarkable benefits for soil health and crop productivity. Intensive land use and modern farming practices have altered soil properties, leading to numerous ecological challenges (Ravindiran et al., 2024) [89] .

Incorporating biochar into soils can enhance their physical structure by increasing porosity, surface area, water retention, oxygen availability, and adsorption capacity (Janus et al., 2015; Chan et al., 2007) [17,52] . The expanded surface area and pore network also provide habitats for beneficial bacteria and fungi, which facilitate nutrient absorption from the soil (Chan et al., 2007) [17] . Higher soil porosity helps maintain moisture and aeration, supporting microbial activity and stimulating nitrification processes (Zhang et al., 2013; Mukherjee et al., 2013) [79,120] .

In addition to physical improvements, biochar positively influences soil chemical properties, including organic matter, nitrogen, carbon, and pH levels. It also contains essential micronutrients such as nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur, which, when added to soil, enhance nutrient availability and plant growth (Glaser et al., 2001) [35] .

Several studies have shown that enriched biochar, compared to raw biochar, further improves soil fertility by maintaining higher levels of organic matter and releasing greater amounts of micronutrients (Kizito et al., 2019) [60] .

Biochar as a source of nutrients: Biochar can serve as an important source of essential micronutrients such as boron, molybdenum, potassium, phosphorus, and calcium, all of which play a key role in the nodulation process of Rhizobium bacteria (Randon et al., 2007). When applied together with compost, biochar markedly improves nutrient availability in the soil, leading to enhanced crop growth and higher productivity (Fischer and Glaser, 2012) [28,34] . Numerous studies have consistently reported that biochar application has a positive effect on soil fertility and supports yield improvement across a wide range of crops (Agegnehu et al., 2015; Fischer and Glaser, 2012; Liang et al., 2014) [3,28,34,69] .

Effects of biochar on microbial diversity and soil enzyme:

Biochar is known to supply several essential micronutrients, including boron, molybdenum, potassium, phosphorus, and calcium, which are crucial for effective Rhizobium nodulation in leguminous crops (Randon et al., 2007). Its combined application with compost has been shown to improve soil nutrient availability, thereby promoting better crop growth and increased yields (Fischer and Glaser, 2012) [28,34] . A growing body of research confirms that biochar enhances soil fertility and contributes to productivity gains in a wide range of agricultural crops (Agegnehu et al., 2015; Fischer and Glaser, 2012; Liang et al., 2014) [3,28,34,69] .

Biochar amendment suppresses plant diseases: Beyond its well-recognized role in carbon sequestration, nutrient enrichment, and improvement of soil physical and biological properties, biochar has also been shown to reduce the severity of plant diseases and enhance resistance against a range of pathogens. [10,24,47,75,100,111] .

Biochar for wastewater treatment: Growing population pressure and rapid industrialization have led to the discharge of large volumes of wastewater into natural ecosystems. Among the most frequently encountered and hazardous contaminants in these effluents are dyes and heavy metals, which are persistent in nature and can cause serious environmental damage even at very low concentrations, such as 1 mg L⁻¹. A range of treatment methods is therefore used to remove these pollutants, including coagulation, reverse osmosis, ion exchange, membrane filtration, and biosorption (Gabhane et al., 2020) [29] . In this context, biochar has attracted increasing attention due to its properties that closely resemble those of activated carbon. Its high carbon content, porous structure, large surface area, and abundance of surface functional groups make biochar an effective and promising material for wastewater treatment applications (Oliveira et al., 2017) [85] . [74,82,122] . Between 2011 and 2015, research in this area expanded significantly, providing detailed insights into biochar stability. Factors such as higher hydrothermal treatment (HTT), longer reaction times, larger particle size, and higher lignin content in the biomass were found to produce more stable biochar ( [27,72,98,113] . Mineral additions, including kaolin, calcite, and calcium dihydrogen phosphate, were also shown to enhance biochar stability, suggesting new strategies for producing biochar with high carbon sequestration potential (Li et al., 2014) [66] . Beyond carbon storage, biochar influences soil biochemical processes. For example, it can increase methane (CH₄) uptake (Karhu et al., 2011) [56] , reduce the abundance of nitrifying microbes, enhance denitrification and N₂ fixation, and adsorb ammonium (NH₄⁺), leading to lower N₂O emissions (Anderson et al., 2011) [7] . These findings help explain how biochar contributes to the reduction of climate-relevant greenhouse gases through changes in soil microbial communities. Fig. 2. A schematic representation illustrates how organic materials, using cellulose as an example, transform during pyrolysis as temperature increases. At lower temperatures, dehydration reactions lead to the formation of compounds such as levoglucosan and anhydrides (Mok and Antal, 1983) [77] . As the temperature rises, poly-aromatic and heterocyclic carbon structures rich in oxygen are formed, while significant amounts of carbon are released in gaseous and liquid forms (Knicker, 2007) [61] . At even higher temperatures, these intermediate structures break down into amorphous carbon (Kercher and Nagle, 2003) [58] , which can then reorganize into highly ordered structures containing reactive radicals and heterocycles (Harris, 2005) [45] . These transformations summarize the progressive chemical changes during pyrolysis ( [43,57,62] .

Cornelissen et al. (2013) [20] suggested that the sorption of N₂O to anhydrous biochars could be a key mechanism for reducing N₂O emissions. Similarly, Van Zwieten et al. (2014) [109] , using ^15N stable isotope techniques, showed that biochar amendment in Tenosol soils reduced N₂O emissions by creating favourable conditions for the enzyme nitrous oxide reductase (nosZ).

Overall, the reduction of greenhouse gas emissions (CO₂, CH₄, N₂O) in biochar-amended soils occurs through a combination of abiotic and biotic mechanisms, with the extent of mitigation influenced by the specific properties of both the soil and the biochar.

Market analysis of biochar

A word on this market research: Industry composition (Global Biochar Market Report, 2023) (Precedence Research, 2024) [86] . Its expanding use in organic farming, along with its proven benefits for soil fertility and crop productivity, are key drivers of this growth. Earlier projections indicated the global market would reach USD 541.8 million in 2023, with a CAGR of 13.9% between 2024 and 2030 (Regional Analysis, 2024).

In Europe, countries such as Austria and Switzerland have recently adopted the European Biochar Certificate, which allows for the direct incorporation of biochar into agricultural soils (Abu-Rumman et al., 2016) [1] . Factors such as growing environmental awareness, lower raw material costs, and supportive government waste management policies are expected to create further opportunities for market expansion (Zorpas, et al., 2020) [123] . The biochar industry comprises both organized and unorganized sectors, reflecting the presence of large-scale manufacturers alongside a rising number of small and medium-sized producers, particularly in North America and Europe (Growth Analysis Report, 2024). However, adoption remains slower in the Middle East and Asia-Pacific due to limited awareness of biochar and its long-term benefits (Biochar Market Growth, Size, Share, Trends, Forecast 2033 | Brainy Insights, n.d.). In countries such as rural China, Japan, Brazil, and Mexico, significant quantities of biochar are produced in collaboration with research institutions and organizations (Shi et al., 2021) [96] . Beyond improving soil properties, biochar contributes to carbon sequestration (He et al., 2024; Li et al., 2014) [66] , further strengthening its market potential across multiple sectors. With the increasing global emphasis on carbon neutrality and circular economy initiatives, the demand and application of biochar are expected to expand significantly in the coming years. The biochar industry remains in its early development stage and is highly fragmented, comprising a small number of producers with production capacities ranging from small to large scale. Market participants often compete by offering high-quality biochar products at competitive prices in the global marketplace (Garcia et al., 2022) [30] . North America hosts over 80% of the world's large and medium-scale biochar manufacturers, surpassing the numbers in Europe and the Asia-Pacific region. By 2023, agriculture emerged as the dominant application segment, accounting for more than 77% of total revenue. Meanwhile, applications such as water treatment and waste management are experiencing growing demand, particularly in emerging economies like China and India. [103,121] . In the US, prices have historically varied widely, from $270/t to $13,480/t, with an average of $2,740/t in 2013 (Cheng et al., 2020) [19] , and a slightly lower value of $2,512/t in 2014 (Campbell et al., 2018) [16] . According to IBI reports, the price remained around $2,062/t in subsequent years (Jirka & Tomlinson, 2015) [54] . The market began experiencing a notable decline starting in 2017, with surveys reporting prices as low as $661/t, an average of $1,137/t, and a most frequently cited price of $1,764/t (Groot et al., 2018) [40] .

Recent studies recorded biochar prices of $ [103,121] . [107] Year Actual Prices ($/t)

The biochar prices reported in Table 3 align closely with those presented by Campion et al. (2023). Despite this relative price stabilization, biochar remains considerably more expensive than conventional charcoal, which limits its competitiveness in the market. As noted by Mardoyan and Braun (2015) [73] , economic incentives and supportive policies are essential for encouraging biochar adoption, especially in regions where cheaper charcoal alternatives dominate. In the absence of targeted subsidies or carbon credit schemes, the high production costs of biochar continue to constrain its large-scale commercial deployment. Although still in its early stages, the biochar market is experiencing growing interest and adoption across various industries. Market research suggests that the Indian biochar sector is poised for strong growth, with a projected compound annual growth rate (CAGR) exceeding 15% during the forecast period. The emerging technologies employed for biochar production in India are summarized in Table 1. Beyond its established use in agriculture, biochar is increasingly being investigated for potential applications in water purification, remediation of contaminated soils, enhancement of concrete properties as an additive, and as a supplement in animal feed.

Conclusion

Biochar has emerged as a well-established and versatile material with the capacity to tackle several interconnected issues, including climate change, declining soil quality, environmental contamination, and sustainable use of natural resources. Its role in improving soil health, storing carbon for extended periods, lowering greenhouse gas emissions, and removing pollutants demonstrates its wide applicability in both agricultural and environmental contexts. Ongoing improvements in production methods, along with increasing market interest, have further reinforced its importance within the circular bioeconomy. However, broader adoption-especially in countries such as Indiawill require coordinated research, economically viable large-scale production, and strong policy support with appropriate incentives. With such integrated efforts, biochar has the potential to contribute significantly to long-term climate resilience and sustainable development.

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