Biochar is a form of charcoal, sometimes modified, that is intended for organic use, as in soil. It is the lightweight black remnants remaining after the pyrolysis of biomass, consisting of carbon and ashes. 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 the thermochemical conversion of biomass in an oxygen-limited environment".
Biochar is mainly used in soils to increase soil aeration, reduce soil emissions of greenhouse gases, improve soil fertility,Ding, Y., Liu, Y., Liu, S. et al. Biochar to improve soil fertility. A review. Agron. Sustain. Dev. 36, 36 (2016). https://doi-org.ezproxy.rice.edu/10.1007/s13593-016-0372-z reduce nutrient leaching, reduce soil acidity, and potentially increase the water content of coarse soils. Biochar application may increase soil fertility and agricultural productivity. However, when applied excessively or made from feedstock 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.
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. This has given rise to the concept of biochar carbon removal, a process of carbon sequestration in the form of biochar. 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.
Etymology The word "biochar" is a late-20th century English neologism derived from the Greek word '' (bios, 'life') and 'char' (charcoal produced by carbonization of biomass). It is recognized as charcoal that participates in biological processes found in soil, aquatic habitats, and animal digestive systems.
History Pre-Columbian Amazonians produced biochar by smoldering agricultural waste (i.e., covering burning biomass with soil) in pits or trenches.: "Similar soils are found, more scarcely, elsewhere in the world. To date, scientists have been unable to completely reproduce the beneficial growth properties of terra preta. It is hypothesized that part of the alleged benefits of terra 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." It is not known if they intentionally used biochar to enhance soil productivity. European settlers called it terra preta de Indio.: "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) Following observations and experiments, one 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.
Production
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.
Gasification Gasifiers produce most of the biochar sold in the United States. The gasification process consists of four main stages: oxidation, drying, pyrolysis, and reduction. Temperature during pyrolysis in gasifiers is , in the reduction zone, and in the combustion zone.
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. These parameters can be tuned to produce either more energy or more biochar.: "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 MWn−1 of electricity generated where the pyrolysis process is optimized for energy and 120 to 360 kg MWn−1 where biochar is applied to land. This compares to emissions of 600–900 kg MWh−1 for fossil-fuel-based technologies." Temperatures of produce more char, whereas temperatures above favor the yield of liquid and gas fuel components. (See Table 1 for differences in output for Fast, Intermediate, Slow, and Gasification). 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 . Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (≈35%); 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.: "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." Pyrolysis plants can use the syngas output and yield 3–9 times the amount of energy required to run.
The Amazonian pit/trench method, in contrast, harvests neither bio-oil nor syngas, and releases , black carbon, and other greenhouse gases (GHGs) (and potentially, toxicants) 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 2018 winner of the X Prize Foundation for atmospheric water generators harvests potable water from the drying stage of the gasification process. The production of biochar as an output is not a priority in most cases.
Small-scale methods
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.
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, it is also possible for each farmer or group of farmers to operate a kiln. 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.
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. At the 2009 International Biochar Conference, a mobile pyrolysis unit with a specified intake of was introduced for agricultural applications.
Crops used 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 store much more carbon on a shorter timespan than trees do.
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, with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field. (showing RPRs for numerous plants, describing method for determining available agricultural waste for energy and char production). 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.: "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."
Hydrochar 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. And although hydrothermal carbonization can produce a carbon-rich solid product, the process is evidently different from the conventional thermal conversion process, so the product is therefore defined as "hydrochar" rather than "biochar".
Thermo-catalytic depolymerization 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.
Properties
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. 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. A van-Krevelen diagram can show the evolution of biochar atomic ratios in the production process. 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.