Biochar

Biochar is a form of

Beyond soil application, biochar can be used for slash-and-char farming, for water retention in soil, and as an additive for animal fodder. There is an increasing focus on the potential role of biochar application in global climate change mitigation. Due to its refractory stability, biochar can stay in soils or other environments for thousands of years.^[7] This has given rise to the concept of biochar carbon removal, a process of carbon 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 English

Biochar is a high-carbon, fine-grained residue that is produced via pyrolysis. It is the direct thermal decomposition of biomass in the absence of oxygen, which prevents combustion, 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, and reduction.^[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, and residence 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 more char, 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,

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 a kiln.^{[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. 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 supply the power grid.^{[citation needed]}

Various companies in North America, Australia, and England 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 various

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 instance, Brazil harvests approximately 460 million tons (MT) of sugarcane 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 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.^[35]

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 contains some volatile 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 utilizes microwaves. It has been used to efficiently convert organic matter to biochar on an industrial scale, producing about 50% char.^[38][39]

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 the proximate and elemental composition, pH value, and porosity. The atomic ratios of biochar, including H/C and O/C, correlate with the properties that are relevant to organic content, such as polarity and aromaticity.^[40] A van-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]

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 an amorphous carbon structure. Temperatures above this range will result in the progressive thermochemical conversion of amorphous carbon into turbostratic graphene sheets. Biochar conductivity also increases with production temperature.^[43][44][45] Important to carbon capture, aromaticity and intrinsic recalcitrance increases with temperature.^[46]

The

Biochar can sequester carbon in the soil for hundreds to thousands of years, like coal.^{[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 for carbon dioxide removal to create a long-term stable carbon sink were published in the early 2000s.^[58][59][60] This technique is advocated by scientists including James Hansen^[61] and James Lovelock.^[62]

A 2010 report estimated that sustainable use of biochar could reduce the global net emissions of carbon dioxide (CO
₂),

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]

In 2021 the cost of biochar ranged around European carbon prices,^[67] but was not yet included in the EU or UK Emissions Trading Scheme.^[68]

Biochar adsorption of CO
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 from

Biochar offers multiple soil 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 biologist Elaine Ingham highlighted its suitability as a habitat for beneficial soil microorganisms. She pointed out that when pre-charged with these beneficial organisms, biochar promotes good soil and plant health.^[73]

Biochar reduces 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.^[74][75][76]

For plants that require high potash and elevated pH,^[77] biochar can improve yield.^[78]

Biochar can improve water quality, reduce soil emissions of

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 reduce nitrous oxide (N
₂O)^[89] emissions by up to 80% and eliminate methane 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.

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 taking individual 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 from

Biochar has been used in animal feed for centuries.^[109]

Doug Pow, a

Ordinary Portland 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-gate

Biochar mixed with liquid media such as water or organic liquids (such as ethanol) is an emerging fuel type known as biochar-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 a coal 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]}

Volcani Center,^[129] and the Swedish University of Agricultural Sciences.^[130]

Research is also ongoing on the application of biochar to coarse soils in semi-arid and degraded ecosystems. In Namibia, biochar is under exploration as a climate change adaptation effort, strengthening local communities' drought resilience and food security through the local production and application of biochar from abundant encroacher biomass.^{[131][132][133]} Similar solutions for rangeland affected by woody plant encroachment have been explored in Australia.^[134]

In recent years, biochar has attracted interest as a wastewater filtration medium as well as for its adsorbing capacity for wastewater pollutants, such as pharmaceuticals,

per- and polyfluoroalkyl substances.^{[136][137][138]}

In some areas, citizen interest and support for biochar motivates government research into the uses of biochar.^[139][140]

Studies

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 using ¹³
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]

See also

• Activated carbon
• Charring
• Dark earth
• Pellet fuel
• Soil carbon
• Soil ecology

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External links

Scholia has a topic profile for Biochar.

• Practical Guidelines for Biochar Producers, Southern Africa Archived 25 October 2020 at the Wayback Machine
• Biochar Production in Namibia (Video)
• International Biochar Initiative
• Biochar-us.org