Lightweight black residue, made of carbon and ashes, after pyrolysis of biomass

Biochar is charcoal that is produced by pyrolysis of biomass in the absence of oxygen; it is used as a soil ameliorant for both carbon sequestration and soil health benefits. Biochar is a stable solid that is rich in carbon and can endure in soil for thousands of years.[1] Biochar is being investigated as a means of carbon sequestration,[1] and it may be a means to mitigate global warming and climate change.[2][3][4] It results from processes related to pyrogenic carbon capture and storage (PyCCS).[5]

Biochar may increase the soil fertility of acidic soils, increase agricultural productivity, and provide protection against some foliar and soil-borne diseases.[6] Biochar is defined by the International Biochar Initiative as "The solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment".[7]

The word "biochar" is a late 20th century English neologism derived from the Greek word βίος, bios, "life" and "char" (charcoal produced by carbonisation of biomass).[8] It is recognised as charcoal which performs a number of functions in biological processes found in soil, aquatic habitats or in the digestive systems of animals.

Pre-Columbian Amazonians produced biochar by smoldering agricultural waste (i.e., covering burning biomass with soil)[9] in pits or trenches.[10] It is not known if they intentionally used biochar to enhance soil productivity.[10] European settlers called it terra preta de Indio.[11] Following observations and experiments, a research team working in French Guiana hypothesized that the Amazonian earthworm Pontoscolex corethrurus was the main agent of fine powdering and incorporation of charcoal debris in the mineral soil.[12]

Biochar is a high-carbon, fine-grained residue that is currently produced through modern pyrolysis processes; it is the direct thermal decomposition of biomass in the absence of oxygen (preventing combustion), which produces a mixture of solids (the biochar proper), liquid (bio-oil), and gas (syngas) products. The specific yield from pyrolysis is dependent on process condition, such as temperature, residence time, and heating rate.[13] These parameters can be optimized to produce either energy or biochar.[14] Temperatures of 400–500 °C (673–773 K) produce more char, whereas temperatures above 700 °C (973 K) favor the yield of liquid and gas fuel components.[15] Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds rather than hours. The increasing heating rate will also lead to a decrease of pyrolysis biochar yield, while the temperature is in the range of 350–600 °C (623–873 K).[16] Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%);[15] this contributes to the observed soil fertility of terra preta. Once initialized, both processes produce net energy. For typical inputs, the energy required to run a "fast" pyrolyzer is approximately 15% of the energy that it outputs.[17] Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run.[10]

Besides pyrolysis, torrefaction and hydrothermal carbonization processes can also thermally decompose biomass to the solid material. However, these products cannot be strictly defined as biochar. The carbon product from the torrefaction process still contains some volatile organic components, thus its properties are between that of biomass feedstock and biochar.[18] Furthermore, even the hydrothermal carbonization could produce a carbon-rich solid product, the hydrothermal carbonization is evidently different from the conventional thermal conversion process.[19] Therefore, the solid product from hydrothermal carbonization is defined as "hydrochar" rather than "biochar".

The Amazonian pit/trench method[10] harvests neither bio-oil nor syngas, and releases a large amount of CO
, black carbon, and other greenhouse gases (GHGs) (and potentially, toxins) into the air, though less greenhouse gasses than captured during the growth of the biomass. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products. The production of biochar as an output is not a priority in most cases.

In a centralized system, all biomass in a region is brought to a central plant (i.e. biomass-fueled thermal power station[20]) for processing into biochar. Alternatively, each farmer or group of farmers can operate a lower-tech kiln. Finally, a truck equipped with a pyrolyzer can move from place to place to pyrolyze biomass. Vehicle power comes from the syngas stream, while the biochar remains on the farm. The biofuel is sent to a refinery or storage site. Factors that influence the choice of system type include the cost of transportation of the liquid and solid byproducts, the amount of material to be processed, and the ability to feed directly into the power grid.

The most common crops used for making biochar include various tree species, as well as various energy crops. Some of these energy crops (i.e. Napier grass) can also store much more carbon on a shorter timespan than trees do.[21]

For crops that are not exclusively for biochar production, the Residue-to-Product Ratio (RPR) and the collection factor (CF) the percent of the residue not used for other things, measure the approximate amount of feedstock that can be obtained for pyrolysis after harvesting the primary product. For instance, Brazil harvests approximately 460 million tons (MT) of sugarcane annually,[22] with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field.[23] This translates into approximately 100 MT of residue annually, which could be pyrolyzed to create energy and soil additives. Adding in the bagasse (sugarcane waste) (RPR=0.29 CF=1.0), which is otherwise burned (inefficiently) in boilers, raises the total to 230 MT of pyrolysis feedstock. Some plant residue, however, must remain on the soil to avoid increased costs and emissions from nitrogen fertilizers.[24]

Pyrolysis technologies for processing loose and leafy biomass produce both biochar and syngas.[25]

Alternatively, "thermo-catalytic depolymerization", which utilizes microwaves, has recently been used to efficiently convert organic matter to biochar on an industrial scale, producing ≈50% char.[26][27]

The physical and chemical properties of biochars as determined by feedstocks and technologies are crucial for the application of biochars in the industry and environment. Different characterization data are applied to biochars and determine their performance in a specific use. For example, the guidelines published by the International Biochar Initiative provide standardized methods in evaluating the product quality of biochar for soil application.[28] The properties of biochar can be characterized in several respects, including the proximate and elemental composition, pH value, and porosity, which correlate with different biochar properties. The atomic ratios of biochar, including H/C and O/C, correlate with the biochar properties that are relevant to the organic content, such as polarity and aromaticity.[29] The van-Krevelen diagram can be used to show the evolution of biochar atomic ratios in the production process.[30] In the carbonization process, both the H/C and O/C ratio decrease due to the release of functional groups which contain hydrogen and oxygen.[31]

The burning and natural decomposition of biomass releases large amounts of carbon dioxide and methane to the Earth's atmosphere. The biochar production process also releases CO2 (up to 50% of the biomass), however, the remaining carbon content is stable indefinitely.[32] Biochar presents a stable means of storing carbon in the ground for centuries, potentially reducing or stalling the growth in atmospheric greenhouse gas levels. Simultaneously, its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.[33]

Biochar can sequester carbon in the soil for hundreds to thousands of years, like coal.[34][35][36][37][38] Such a carbon negative technology would lead to a net withdrawal of CO2 from the atmosphere, while producing consumable energy. This technique is advocated by prominent scientists such as James Hansen, previously head of the NASA Goddard Institute for Space Studies,[39] and James Lovelock, creator of the Gaia hypothesis, for mitigation of global warming by greenhouse gas remediation.[40]

Researchers have estimated that sustainable use of biochar could reduce the global net emissions of carbon dioxide (CO
), methane, and nitrous oxide by up to 1.8 Pg (CO
-Ce) per year (12% of current anthropogenic CO
-Ce emissions), and total net emissions over the course of the next century by 130 Pg CO
-Ce, without endangering food security, habitats, or soil conservation.[41]

Biochar is recognized as offering a number of soil health benefits. The extremely porous nature of biochar is found to be effective at retaining both water and water-soluble nutrients. Soil biologist Elaine Ingham indicates[42] the extreme suitability of biochar as a habitat for many beneficial soil micro organisms. She points out that when pre-charged with these beneficial organisms biochar becomes an extremely effective soil amendment promoting good soil and, in turn, plant health.

Biochar has also been shown to reduce leaching of E-coli through sandy soils depending on application rate, feedstock, pyrolysis temperature, soil moisture content, soil texture, and surface properties of the bacteria.[43][44][45]

For plants that require high potash and elevated pH,[46] biochar can be used as a soil amendment to improve yield. [47]

Biochar can improve water quality, reduce soil emissions of greenhouse gases, reduce nutrient leaching, reduce soil acidity, and reduce irrigation and fertilizer requirements.[48] Biochar was also found under certain circumstances to induce plant systemic responses to foliar fungal diseases and to improve plant responses to diseases caused by soilborne pathogens.[49][50][51]

The various impacts of biochar can be dependent on the properties of the biochar,[52] as well as the amount applied,[51] and there is still a lack of knowledge about the important mechanisms and properties.[53] Biochar impact may depend on regional conditions including soil type, soil condition (depleted or healthy), temperature, and humidity.[54] Modest additions of biochar to soil reduce nitrous oxide N
[55] emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO

Studies have reported positive effects from biochar on crop production in degraded and nutrient–poor soils.[57] The application of compost and biochar under FP7 project FERTIPLUS has had positive effects in soil humidity, and crop productivity and quality in different countries. Biochar can be designed with specific qualities to target distinct properties of soils.[59] In a Colombian savanna soil, biochar reduced leaching of critical nutrients, created a higher crop uptake of nutrients, and provided greater soil availability of nutrients.[60] At 10% levels biochar reduced contaminant levels in plants by up to 80%, while reducing total chlordane and DDX content in the plants by 68 and 79%, respectively.[61] On the other hand, because of its high adsorption capacity, biochar may reduce the efficacy of soil applied pesticides that are used for weed and pest control.[62][63] High-surface-area biochars may be particularly problematic in this regard; more research into the long-term effects of biochar addition to soil is needed.[62]

Switching from slash-and-burn to slash-and-char farming techniques in Brazil can decrease both deforestation of the Amazon basin and carbon dioxide emission, as well as increase crop yields. Slash-and-burn leaves only 3% of the carbon from the organic material in the soil.[64] Slash-and-char can keep up to 50% of the carbon in a highly stable form.[65] Returning the biochar into the soil rather than removing it all for energy production reduces the need for nitrogen fertilizers, thereby reducing cost and emissions from fertilizer production and transport.[66] Additionally, by improving the soil's ability to be tilled, its fertility and its productivity, biochar-enhanced soils can indefinitely sustain agricultural production, whereas non-enriched soils quickly become depleted of nutrients, forcing farmers to abandon the fields, producing a continuous slash and burn cycle and the continued loss of tropical rainforest. Using pyrolysis to produce bio-energy also has the added benefit of not requiring infrastructure changes the way processing biomass for cellulosic ethanol does. Additionally, the biochar produced can be applied by the currently used machinery for tilling the soil or equipment used to apply fertilizer.[67]

Biochar is hygroscopic. Thus it is a desirable soil material in many locations due to its ability to attract and retain water. This is possible because of its porous structure and high specific surface area.[68] As a result, nutrients such as phosphate, and agrochemicals are retained for the plants benefit. Plants are therefore healthier, and less fertilizer leaches into surface or groundwater.

Mobile pyrolysis units can be used to lower the costs of transportation of the biomass if the biochar is returned to the soil and the syngas stream is used to power the process.[69][70] Bio-oil contains organic acids that are corrosive to steel containers, has a high water vapor content that is detrimental to ignition, and, unless carefully cleaned, contains some biochar particles which can block injectors.[71] Currently, it is less suitable for use as a kind of biodiesel than other sources.

If biochar is used for the production of energy rather than as a soil amendment, it can be directly substituted for any application that uses coal. Pyrolysis also may be the most cost-effective way of electricity generation from biomaterial.[72]

A Western Australian farmer has explored the use of biochar mixed with molasses as stock fodder. He asserts that in ruminants, biochar can assist digestion and reduce methane production. The farmer also uses dung beetles to work the biochar infused dung down into the soil without using machinery. It is proposed that the nitrogen and carbon in the dung are both incorporated into the soil rather than staying on the soil surface, reducing the production of nitrous oxide and carbon dioxide, which are both greenhouse gasses. The nitrogen and carbon then both add to soil fertility. There is also on-farm evidence that the fodder has led to improvements of liveweight gain in Angus-cross cattle.[73]

Doug Pow won the Australian Government Innovation in Agriculture Land Management Award at the 2019 Western Australian Landcare Awards for this innovation.[74][73] Mr Pow's work led to two further trials on dairy cattle, with the results of odour reduction and increased milk production.[75]

Research into aspects involving pyrolysis/biochar is underway around the world. From 2005 to 2012, there were 1,038 articles that included the word "biochar" or "bio-char" in the topic that had been indexed in the ISI Web of Science.[77] Further research is in progress by such diverse institutions around the world as Cornell University, the University of Edinburgh (which has a dedicated research unit),[78] the University of Georgia,[79][80] the ,[81] and the University of Delaware.

Long-term effect of biochar on soil C sequestration of recent carbon inputs has been examined using soil from arable fields in Belgium with charcoal-enriched black spots dating >150 years ago from historical charcoal production mound kilns. Topsoils from these 'black spots' had a higher organic C concentration [3.6 ± 0.9% organic carbon (OC)] than adjacent soils outside these black spots (2.1 ± 0.2% OC). The soils had been cropped with maize for at least 12 years which provided a continuous input of C with a C isotope signature (δ13C) −13.1, distinct from the δ13C of soil organic carbon (−27.4 ‰) and charcoal (−25.7 ‰) collected in the surrounding area. The isotope signatures in the soil revealed that maize-derived C concentration was significantly higher in charcoal-amended samples ('black spots') than in adjacent unamended ones (0.44% vs. 0.31%; P = 0.02). Topsoils were subsequently collected as a gradient across two 'black spots' along with corresponding adjacent soils outside these black spots and soil respiration, and physical soil fractionation was conducted. Total soil respiration (130 days) was unaffected by charcoal, but the maize-derived C respiration per unit maize-derived OC in soil significantly decreased about half (P < 0.02) with increasing charcoal-derived C in soil. Maize-derived C was proportionally present more in protected soil aggregates in the presence of charcoal. The lower specific mineralization and increased C sequestration of recent C with charcoal are attributed to a combination of physical protection, C saturation of microbial communities and, potentially, slightly higher annual primary production. Overall, this study provides evidence of the capacity of biochar to enhance C sequestration in soils through reduced C turnover on the long term. ().

Biochar sequesters carbon (C) in soils because of its prolonged residence time, ranging from several years to millennia. In addition, biochar can promote indirect C-sequestration by increasing crop yield while, potentially, reducing C-mineralization. Laboratory studies have evidenced effects of biochar on C-mineralization using 13C isotope signatures.

Fluorescence analysis of the dissolved organic matter from soil amended with biochar revealed that biochar application increased a humic-like fluorescent component, likely associated with biochar-carbon in solution. The combined spectroscopy-microscopy approach revealed the accumulation of aromatic-carbon in discrete spots in the solid-phase of microaggregates and its co-localization with clay minerals for soil amended with raw residue or biochar. The co-localization of aromatic-C:polysaccharides-C was consistently reduced upon biochar application. These finding suggested that reduced C metabolism is an important mechanism for C stabilization in biochar-amended soils

Students at Stevens Institute of Technology in New Jersey are developing supercapacitors that use electrodes made of biochar.[82] A process developed by University of Florida researchers that removes phosphate from water, also yields methane gas usable as fuel and phosphate-laden carbon suitable for enriching soil.[83] Researchers at the University of Auckland are also working on utilizing biochar in concrete applications to reduce carbon emissions during concrete production and to improve the strength considerably.[84] It has also demonstrated that the biochar can be used as a suitable filler in polymer matrix.[85] Recently, biochar-starch bio-composites were prepared[86] and its nano-mechanical behaviours were investigated using advanced dynamic atomic force microscopy.[87] Recently, the agglomeration behaviour of biochar in Polypropylene was investigated using micro-CT studies.[88] These studies give more insight into the biochar interaction with the polymer matrix.

Research and practical investigations into the potential of biochar for coarse soils in semi-arid and degraded ecosystems are ongoing. In the Southern African country Namibia biochar is explored as a measure under climate change adaptation efforts, strengthening local communities' drought resilience and food security through the local production and application of biochar from abundant encroacher biomass.[89]

If biomass is pyrolyzed to biochar and put back into the soil, rather than being completely burned, this may reduce carbon emissions. Potentially, the bioenergy industry might even be made to sequester net carbon.[90] Pyrolysis might be cost-effective for a combination of sequestration and energy production when the cost of a CO
ton reaches $37.[90] Carbon credits could help to make implementation easier as most large biomass power producers are neither equipped to create biochar nor are financially motivated for making it (because implementing biochar production would leave less energy for power production).[20][91]

Current biochar projects make no significant impact on the overall global carbon budget, although expansion of this technique has been advocated as a geoengineering approach.[92] In May 2009, the Biochar Fund, a small "social profit organization", received a grant from the Congo Basin Forest Fund for a project in Central Africa to simultaneously slow down deforestation, increase the food security of rural communities, provide renewable energy and sequester carbon. Though some farmers did report better maize crops, the project ended early without significant results and with promises to the farmers not kept.[93]

Application rates of 2.5–20 tonnes per hectare (1.0–8.1 t/acre) appear to be required to produce significant improvements in plant yields. Biochar costs in developed countries vary from $300–7000/tonne, generally too high for the farmer/horticulturalist and prohibitive for low-input field crops. In developing countries, constraints on agricultural biochar relate more to biomass availability and production time. An alternative is to use small amounts of biochar in lower cost biochar-fertilizer complexes.[94]

Various companies in North America, Australia, and England sell biochar or biochar production units. In Sweden the 'Stockholm Solution' is an urban tree planting system that uses 30% biochar to support healthy growth of the urban forest.[95] The Qatar Aspire Park now uses biochar to help trees cope with the intense heat of their summers[citation needed].

At the 2009 International Biochar Conference, a mobile pyrolysis unit with a specified intake of 1,000 pounds (450 kg) was introduced for agricultural applications. The unit had a length of 12 feet and height of 7 feet (3.6 m by 2.1m).[96]

A production unit in Dunlap, Tennessee by Mantria Corporation opened in August 2009 after testing and an initial run, was later shut down as part of a Ponzi scheme investigation.[97]