Lightweight black residue, made of carbon and ashes, after pyrolysis of biomass
This article is about charcoal which goes into soil. For more general information, seeCharcoal.
A pile of biocharBiochar mixture ready for soil application
Biochar is a form ofcharcoal, sometimes modified, that is intended for organic use, as in soil. It is the lightweight black remnants remaining after thepyrolysis ofbiomass, consisting ofcarbon andashes.[1] Despite its name, biochar is sterile immediately after production and only gains biological life following assisted or incidental exposure to biota. Biochar is defined by the International Biochar Initiative as the "solid material obtained from thethermochemical conversion of biomass in anoxygen-limited environment".[2]
Biochar is mainly used in soils to increase soil aeration, reduce soil emissions of greenhouse gases,[3] reduce nutrient leaching, reducesoil acidity,[4] and potentially increase the water content of coarse soils.[5] Biochar application may increasesoil fertility andagricultural productivity.[4] However, when applied excessively or made fromfeedstock unsuitable for the soil type, biochar soil amendments also have the potential for negative effects, including harming soil biota, reducing available water content, altering soil pH, and increasing salinity.[6]
Beyond soil application, biochar can be used for slash-and-char farming, forwater retention in soil, and as an additive foranimal fodder. There is an increasing focus on the potential role of biochar application in global climate change mitigation. Due to itsrefractory stability, biochar can stay in soils or other environments for thousands of years.[7] This has given rise to the concept ofbiochar carbon removal, a process ofcarbon sequestration in the form of biochar.[7] Carbon removal can be achieved when high-quality biochar is applied to soils, or added as a substitute material to construction materials such as concrete and tar.
The word "biochar" is a late-20th century Englishneologism derived from theGreek word 'βίος' (bios, 'life') and 'char' (charcoal produced bycarbonization of biomass).[8] It is recognized as charcoal that participates in biological processes found in soil, aquatic habitats, and animal digestive systems.[citation needed]
Pre-ColumbianAmazonians produced biochar bysmoldering 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 itterra preta de Indio.[11] Following observations and experiments, one research team working inFrench Guiana hypothesized that the Amazonian earthwormPontoscolex 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 produced viapyrolysis. It is the directthermal decomposition of biomass in the absence ofoxygen, which preventscombustion, and produces a mixture of solids (biochar), liquid (bio-oil), and gas (syngas) products.[13]
Gasifiers produce most of the biochar sold in the United States.[14] The gasification process consists of four main stages: oxidation, drying,pyrolysis, andreduction.[15] Temperature during pyrolysis in gasifiers is 250–550 °C (523–823 K), 600–800 °C (873–1,073 K) in the reduction zone, and 800–1,000 °C (1,070–1,270 K) in the combustion zone.[16]
The specific yield from pyrolysis (the step of gasification that produces biochar) is dependent on process conditions such as temperature, heating rate, andresidence time.[17] These parameters can be tuned to produce either more energy or more biochar.[18] Temperatures of 400–500 °C (673–773 K) produce morechar, whereas temperatures above 700 °C (973 K) favor the yield of liquid and gas fuel components.[19] Pyrolysis occurs more quickly at higher temperatures, typically requiring seconds rather than hours. The increasing heating rate leads to a decrease in biochar yield, while the temperature is in the range of 350–600 °C (623–873 K).[20] Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%);[19] this contributes to soil fertility. 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.[21] Pyrolysis plants can use the syngas output and yield 3–9 times the amount of energy required to run.[10]
The Amazonian pit/trench method,[10] in contrast, harvests neither bio-oil nor syngas, and releases CO2,black carbon, and othergreenhouse gases (GHGs) (and potentially,toxicants) into the air, though less greenhouse gasses than captured during the growth of the biomass.[citation needed] 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.[22][23] The 2018 winner of theX Prize Foundation foratmospheric water generators harvests potable water from the drying stage of the gasification process.[24][25] The production of biochar as an output is not a priority in most cases.[citation needed]
Smallholder biochar production with fruit-orchard prunings
Smallholder farmers in developing countries easily produce their own biochar without special equipment. They make piles of crop waste (e.g., maize stalks, rice straw, or wheat straw), light the piles on the top, and quench the embers with dirt or water to make biochar. This method greatly reduces smoke compared to traditional methods of burning crop waste. This method is known as the top-down burn or conservation burn.[26][27][28]
Alternatively, more industrial methods can be used on small scales. While in a centralized system, unused biomass is brought to a central plant for processing into biochar,[29] it is also possible for each farmer or group of farmers to operate akiln.[citation needed] In this scenario, a truck equipped with a pyrolyzer moves from place to place to pyrolyze biomass. Vehicle power comes from the syngas stream, while the biochar remains on the farm. Thebiofuel 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 supply the power grid.[citation needed]
Various companies inNorth America,Australia, andEngland also sell biochar or biochar production units. In Sweden, the 'Stockholm Solution' is an urban tree planting system that uses 30% biochar to support urban forest growth.[30] 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.[31]
Common crops used for making biochar include various tree species, as well as variousenergy crops. Some of these energy crops (i.e.Napier grass) can store much more carbon on a shorter timespan than trees do.[32]
For crops that are not exclusively for biochar production, theresidue-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 instance,Brazil harvests approximately 460 million tons (MT) ofsugarcane annually,[33] with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field.[34] This translates into approximately 100 MT of residue annually, which could be pyrolyzed to create energy and soil additives. Adding in thebagasse (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.[35]
Besides pyrolysis,torrefaction andhydrothermal 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 contains somevolatile organic components; thus its properties are between that of biomass feedstock and biochar.[36] And although hydrothermal carbonization can produce a carbon-rich solid product, the process is evidently different from the conventional thermal conversion process,[37] so the product is therefore defined as "hydrochar" rather than "biochar".
Thermo-catalytic depolymerization is another method to produce biochar, which utilizesmicrowaves. It has been used to efficiently convert organic matter to biochar on an industrial scale, producing about 50% char.[38][39]
Smaller pellets of biocharBiochar produced from residual wood
The physical and chemical properties of biochars as determined by feedstocks and technologies are crucial. Characterization data explain their performance in a specific use. For example, guidelines published by the International Biochar Initiative provide standardized evaluation methods.[13] Properties can be categorized in several respects, including theproximate and elemental composition, pH value, and porosity. Theatomic ratios of biochar, includingH/C andO/C, correlate with the properties that are relevant to organic content, such aspolarity andaromaticity.[40] Avan-Krevelen diagram can show the evolution of biochar atomic ratios in the production process.[41] In the carbonization process, both the H/C and O/C atomic ratios decrease due to the release of functional groups that contain hydrogen and oxygen.[42]
Scanning electron image of biochar shows detailed morphology
Production temperatures influence biochar properties in several ways. The molecular carbon structure of the solid biochar matrix is particularly affected. Initial pyrolysis at 450–550 °C leaves anamorphous carbon structure. Temperatures above this range will result in the progressive thermochemical conversion of amorphous carbon into turbostraticgraphene sheets. Biocharconductivity also increases with production temperature.[43][44][45] Important to carbon capture, aromaticity and intrinsic recalcitrance increases with temperature.[46]
Biochar can sequester carbon in the soil for hundreds to thousands of years, likecoal.[52][53][54][55][56] According to the World Bank, "biochar retains between 10 percent and 70 percent (on average about 50 percent) of the carbon present in the original biomass and slows down the rate of carbon decomposition by one or two orders of magnitude, that is, in the scale of centuries or millennia".[57] Early works proposing the use of biochar forcarbon dioxide removal to create a long-term stablecarbon sink were published in the early 2000s.[58][59][60] This technique is advocated by scientists includingJames Hansen[61] andJames Lovelock.[62]
A 2010 report estimated that sustainable use of biochar could reduce the global net emissions of carbon dioxide (CO 2),methane, andnitrous oxide by up to 1.8 billion tonnescarbon dioxide equivalent (CO 2e) per year (compared to the about 50 billion tonnes emitted in 2021), without endangeringfood security,habitats, orsoil conservation.[50] However a 2018 study doubted enough biomass would be available to achieve significant carbon sequestration.[63] A 2021 review estimated potential CO2 removal from 1.6 to 3.2 billion tonnes per year,[64] and by 2023 it had become a lucrative business renovated by carbon credits.[65]
As of 2023, the significance of biochar's potential as a carbon sink is widely accepted. Biochar was found to have the technical potential to sequester 7% of carbon dioxide on average across all countries, with twelve nations able to sequester over 20% of their greenhouse gas emissions—Bhutan leads this proportion (68%), followed by India (53%).[66]
Biochar adsorption ofCO 2 can be limited by the surface area of the material, which can be improved by using resonant acoustic mixing.[69]
In developing countries, biochar derived fromimproved cookstoves for home-use can reduce carbon emissions (when the traditional cookstove is discontinued), as well as achieve other benefits for sustainable development.[70]
Biochar offers multiplesoil health benefits in degraded tropical soils but is less beneficial in temperate regions.[71][72] Its porous nature is effective at retaining both water and water-soluble nutrients. Soil biologistElaine Ingham highlighted its suitability as a habitat for beneficial soilmicroorganisms. She pointed out that when pre-charged with these beneficial organisms, biochar promotes good soil and plant health.[73]
Biochar reduces leaching ofE-coli through sandy soils depending on application rate, feedstock, pyrolysis temperature,soil moisture content,soil texture, and surface properties of the bacteria.[74][75][76]
For plants that require highpotash and elevatedpH,[77] biochar can improve yield.[78]
Biochar can improve water quality, reduce soil emissions ofgreenhouse gases, reducenutrient leaching, reducesoil acidity,[79] and reduceirrigation andfertilizer requirements.[80][81] Due to its porosity, the small holes in biochar can keep water and dissolved minerals in the upper layers of soil, assisting plant growth and reducing the need for and expense of fertilizer.[81] Under certain circumstances biochar induces plant systemic responses tofoliar fungal diseases and improves plant responses to diseases caused by soilborne pathogens.[82][83][84] Biochar can remove heavy metals from the soil.[85]
Biochar's impacts are dependent on its properties[86] as well as the amount applied,[84] although knowledge about the important mechanisms and properties is limited.[87] Biochar impact may depend on regional conditions including soil type, soil condition (depleted or healthy), temperature, and humidity.[88] Modest additions of biochar reducenitrous oxide (N 2O)[89] emissions by up to 80% and eliminatemethane emissions, which are both more potent greenhouse gases than CO2.[90]
Studies reported positive effects from biochar on crop production in degraded and nutrient–poor soils.[91] The application ofcompost and biochar underFP7 project FERTIPLUS had positive effects on soil humidity, crop productivity and quality in multiple countries.[92] Biochar can be adapted with specific qualities to target distinct soil properties.[93] In Colombian savanna soil, biochar reduced leaching of critical nutrients, created a higher nutrient uptake, and provided greater nutrient availability.[94] At 10% levels, biochar reduced contaminant levels in plants by up to 80%, while reducingchlordane andDDX content in the plants by 68 and 79%, respectively.[95] However, because of its high adsorption capacity, biochar may reduce pesticide efficacy.[96][97] High-surface-area biochars may be particularly problematic.[96]
Biochar may be plowed into soils in crop fields or added to gardens to enhance their fertility and stability[98] and for medium- to long-term carbon sequestration in these soils. It even shows good results when top-dressed. It has shown positive effects in increasing soil fertility and improving disease resistance in West European soils.[92] Gardeners takingindividual action on climate change add biochar to soil,[99] increasing plant yield and thereby drawing down more carbon.[100] The use of biochar as a feed additive is a way to apply biochar to pastures and to reduce methane emissions.[101][102]
Application rates of 2.5–20 tonnes per hectare (1.0–8.1 t/acre) appear required to improve plant yields significantly. Biochar costs in developed countries vary from $300–$7,000/tonne, which is generally impractical 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. A compromise is to use small amounts of biochar in lower-cost biochar-fertilizer complexes.[103]
Biochar soil amendments, when applied at excessive rates or with unsuitable soil type and biochar feedstock combinations, also have the potential for negative effects, including harming soil biota, reducing available water content, altering soil pH, and increasing salinity.[6]
Switching fromslash-and-burn toslash-and-char farming techniques in Brazil can decrease both deforestation of theAmazon basin andcarbon dioxide emission, as well as increase crop yields. Slash-and-burn leaves only 3% of the carbon from the organic material in the soil.[104] Slash-and-char can retain up to 50%.[105] Biochar reduces the need for nitrogen fertilizers, thereby reducing cost and emissions from fertilizer production and transport.[106] Additionally, by improving soil's till-ability, fertility, and productivity, biochar-enhanced soils can indefinitely sustain agricultural production. This is unlike slash-and-burn soils, which quickly become depleted of nutrients, forcing farmers to abandon fields, producing a continuous slash-and-burn cycle. Using pyrolysis to produce bio-energy does not require infrastructure changes the way, for example, processing biomass forcellulosic ethanol does. Additionally, biochar can be applied by the widely used machinery.[107]
Biochar has been used in animal feed for centuries.[109]
Doug Pow, aWestern Australian farmer, explored the use of biochar mixed withmolasses as stockfodder. He asserted that inruminants, biochar can assist digestion and reducemethane production. He also useddung beetles to work the resulting biochar-infused dung into the soil without using machinery. The nitrogen and carbon in the dung were both incorporated into the soil rather than staying on the soil surface, reducing the production ofnitrous oxide andcarbon dioxide. The nitrogen and carbon added to soil fertility. On-farm evidence indicates that the fodder led to improvements of liveweight gain inAngus-cross cattle.[110] Doug Pow won the Australian Government Innovation in Agriculture Land Management Award at the 2019 Western AustralianLandcare Awards for this innovation.[111][110] Pow's work led to two further trials on dairy cattle, yielding reduced odour and increased milk production.[112]
OrdinaryPortland cement (OPC), an essential component of concrete mix, is energy- and emissions-intensive to produce; cement production accounts for around 8% of global CO2 emissions.[113] The concrete industry has increasingly shifted to using supplementary cementitious materials (SCMs), additives that reduce the volume of OPC in a mix while maintaining or improving concrete properties.[114] Biochar has been shown to be an effective SCM, reducing concrete production emissions while maintaining required strength and ductility properties.[115][116]
Studies have found that a 1–2% weight concentration of biochar is optimal for use in concrete mixes, from both a cost and strength standpoint.[115] A 2 wt.% biochar solution has been shown to increase concrete flexural strength by 15% in a three-point bending test conducted after 7 days, compared to traditional OPC concrete.[116] Biochar concrete also shows promise in high-temperature resistance and permeability reduction.[117]
A cradle-to-gatelife cycle assessment of biochar concrete showed decreased production emissions with higher concentrations of biochar, which tracks with a reduction in OPC.[118] Compared to other SCMs from industrial waste streams (such asfly ash andsilica fume), biochar also showed decreased toxicity.[citation needed]
Biochar mixed with liquid media such as water or organic liquids (such as ethanol) is an emerging fuel type known asbiochar-based slurry.[119] Adapting slow pyrolysis in large biomass fields and installations enables the generation of biochar slurries with unique characteristics. These slurries are becoming promising fuels in countries with regional areas where biomass is abundant, and power supply relies heavily on diesel generators.[120] This type of fuel resembles acoal slurry, but with the advantage that it can be derived from biochar from renewable resources.
Biochar also has applications in water treatment.[121] Its properties, porosity in particular, can be modified using different methods to increase the efficiency of contaminant removal.[122] Biochar is reported to remove contaminants such as heavy metals, dyes, organic pollutants.[85][123][124]
Research is also ongoing on the application of biochar to coarse soils in semi-arid and degraded ecosystems. InNamibia, biochar is under exploration as aclimate change adaptation effort, strengthening local communities' drought resilience andfood security through the local production and application of biochar from abundantencroacher biomass.[131][132][133] Similar solutions for rangeland affected by woody plant encroachment have been explored inAustralia.[134]
Long-term effects of biochar on carbon sequestration have been examined using soil from arable fields in Belgium with charcoal-enriched black spots dating from before 1870 from charcoal production mound kilns. This study showed that soil treated over a long period with charcoal showed a higher proportion of maize-derived carbon and decreased respiration, attributed to physical protection, carbon saturation of microbial communities, and, potentially, slightly higher annual primary production. Overall, this study evidences the capacity of biochar to enhance carbon sequestration through reduced carbon turnover.[141]
Biochar sequesters carbon in soils because of its prolonged residence time, ranging from years to millennia. In addition, biochar can promote indirect carbon sequestration by increasing crop yield while potentially reducing carbon mineralization. Laboratory studies have evidenced effects of biochar on carbon mineralization using13 C signatures.[142]
Fluorescence analysis of organic matter dissolved in biochar-amended soil 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. Biochar application consistently reduced the co-localization of aromatic carbon and polysaccharides carbon. These findings suggested that reduced carbon metabolism is an important mechanism for carbon stabilization in biochar-amended soils.[143]
^Solomon, Dawit; Lehmann, Johannes; Thies, Janice; Schäfer, Thorsten; Liang, Biqing; Kinyangi, James; Neves, Eduardo; Petersen, James; Luizão, Flavio; Skjemstad, Jan (May 2007)."Molecular signature and sources of biochemical recalcitrance of organic C in Amazonian Dark Earths".Geochimica et Cosmochimica Acta.71 (9):2285–2298.Bibcode:2007GeCoA..71.2285S.doi:10.1016/j.gca.2007.02.014.ISSN0016-7037.Archived from the original on 22 November 2021. Retrieved9 August 2021.Amazonian Dark Earths (ADE) are a unique type of soils apparently developed between 500 and 9000 years B.P. through intense anthropogenic activities such as biomass-burning and high-intensity nutrient depositions on pre-Columbian Amerindian settlements that transformed the original soils into Fimic Anthrosols throughout the Brazilian Amazon Basin.
^abcdLehmann 2007a, pp. 381–387: "Similar soils are found, more scarcely, elsewhere in the world. To date, scientists have been unable to completely reproduce the beneficial growth properties ofterra preta. It is hypothesized that part of the alleged benefits ofterra preta require the biochar to be aged so that it increases the cation exchange capacity of the soil, among other possible effects. In fact, there is no evidence natives made biochar for soil treatment, but rather for transportable fuel charcoal; there is little evidence for any hypothesis accounting for the frequency and location of terra preta patches in Amazonia. Abandoned or forgotten charcoal pits left for centuries were eventually reclaimed by the forest. In that time, the initially harsh negative effects of the char (high pH, extreme ash content, salinity) wore off and turned positive as the forest soil ecosystem saturated the charcoals with nutrients." (internal citations omitted) supra note 2 at p. 386: "Only aged biochar shows high cation retention, as in Amazonian Dark Earths. At high temperatures (30–70 °C), cation retention occurs within a few months. The production method that would attain high CEC in soil in cold climates is not currently known."
^Glaser, Lehmann & Zech 2002, pp. 219–220: "These so-called Terra Preta do Indio (Terra Preta) characterize the settlements of pre-Columbian Indios. In Terra Preta soils large amounts of black C indicate a high and prolonged input of carbonized organic matter probably due to the production of charcoal in hearths, whereas only low amounts of charcoal are added to soils as a result of forest fires and slash-and-burn techniques." (internal citations omitted)
^Amonette, James E; Blanco-Canqui, Humberto; Hassebrook, Chuck; Laird, David A;Lal, Rattan; Lehmann, Johannes; Page-Dumroese, Deborah (January 2021)."Integrated biochar research: A roadmap".Journal of Soil and Water Conservation.76 (1):24A –29A.doi:10.2489/jswc.2021.1115A.OSTI1783242.S2CID231588371.Large-scale wood gasifiers used to generate bioenergy, however, are relatively common and currently provide the majority of the biochar sold in the United States. Consequently, one of these full-scale facilities would be used to produce a standard wood biochar made from the same feedstock to help calibrate results across the regional sites.
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^Gaunt & Lehmann 2008, pp. 4152, 4155: "Assuming that the energy in syngas is converted to electricity with an efficiency of 35%, the recovery in the life cycle energy balance ranges from 92 to 274 kg CO2 MWn−1 of electricity generated where the pyrolysis process is optimized for energy and 120 to 360 kg CO2 MWn−1 where biochar is applied to land. This compares to emissions of 600–900 kg CO2 MWh−1 for fossil-fuel-based technologies."
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^Laird 2008, pp. 100, 178–181: "The energy required to operate a fast pyrolyzer is ≈15% of the total energy that can be derived from the dry biomass. Modern systems are designed to use the syngas generated by the pyrolyzer to provide all the energy needs of the pyrolyzer."
^Laird 2008, p. 179: "Much of the current scientific debate on the harvesting of biomass for bioenergy is focused on how much can be harvested without doing too much damage."
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^Lehmann, Johannes."Terra Preta de Indio".Soil Biochemistry (Internal Citations Omitted).Archived from the original on 24 April 2013. Retrieved15 September 2009. Not only do biochar-enriched soils contain more carbon - 150gC/kg compared to 20-30gC/kg in surrounding soils - but biochar-enriched soils are, on average, more than twice as deep as surrounding soils.[citation needed]
^Lehmann 2007b, p. 143: "this sequestration can be taken a step further by heating the plant biomass without oxygen (a process known as low-temperature pyrolysis)."
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^Glaser, Lehmann & Zech 2002, p. 224, note 7: "Three main factors influence the properties of charcoal: (1) the type of organic matter used for charring, (2) the charring environment (e.g. temperature, air), and (3) additions during the charring process. The source of charcoal material strongly influences the direct effects of charcoal amendments on nutrient contents and availability."
^Dr. Wardle points out that improved plant growth has been observed in tropical (depleted) soils by referencing Lehmann, but that in the boreal (high nativesoil organic matter content) forest this experiment was run in, it accelerated the native soil organic matter loss. Wardle,supra note 18. ("Although several studies have recognized the potential of black C for enhancing ecosystem carbon sequestration, our results show that these effects can be partially offset by its capacity to stimulate loss of native soil C, at least for boreal forests.") (internal citations omitted) (emphasis added).
^Lehmann 2007a, p. 384, note 3: "In greenhouse experiments, NOx emissions were reduced by 80% and methane emissions were completely suppressed with biochar additions of 20 g kg-1 (2%) to a forage grass stand."
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^Lehmann 2007b, p. 143, note 9: "It can be mixed with manures or fertilizers and included in no-tillage methods, without the need for additional equipment."
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