Left - a nutrient-poor oxisol; right - an oxisol transformed into fertile terra preta using biochar
Biochar is a name for charcoal when it is used for particular purposes, especially as a soil amendment. Like all charcoal, biochar is created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration to produce negative carbon dioxide emissions. Biochar thus has the potential to help mitigate climate change, via carbon sequestration. Independently, biochar can increase soil fertility, increase agricultural productivity, and provide protection against some foliar and soil-borne diseases. Furthermore, biochar reduces pressure on forests. Biochar is a stable solid, rich in carbon and can endure in soil for thousands of years.
Pre-ColumbianAmazonians are believed to have used biochar to enhance soil productivity. They produced it by smoldering agricultural waste (i.e., covering burning biomass with soil) in pits or trenches. European settlers called it terra preta de Indio. Following observations and experiments, a research team working in French Guiana hypothesized that the Amazonian earthwormPontoscolex corethrurus was the main agent of fine powdering and incorporation of charcoal debris to the mineral soil.
The term “biochar” was coined by Peter Read to describe charcoal used as a soil improvement.
Pyrolysis produces biochar, liquids, and gases from biomass by heating the biomass in a low/no oxygen environment. The absence of oxygen prevents combustion. The relative yield of products from pyrolysis varies with temperature. Temperatures of 400–500 °C (752–932 °F) produce more char, while temperatures above 700 °C(1,292 °F) favor the yield of liquid and gas fuel components. Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds instead of hours. High temperature pyrolysis is also known as gasification, and produces primarily syngas. Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis can produce substantially more char (~50%). 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. Modern pyrolysis plants can use the syngas created by the pyrolysis process and output 3–9 times the amount of energy required to run.
The Amazonian pit/trench method harvests neither bio-oil nor syngas, and releases a large amount of CO2, black carbon, and other greenhouse gases (GHG)s (and potentially, toxins) into the air. 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.
Centralized, decentralized, and mobile systems
In a centralized system, all biomass in a region is brought to a central plant for processing. 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.
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, with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally are burned in the field. 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.
Pyrolysis technologies for processing loose and leafy biomass produce both biochar and syngas.
Alternatively, thermo-catalytic depolymerization using microwaves has recently been used to efficiently convert organic matter to biochar on an industrial scale, producing ~50% char.
The burning and natural decomposition of biomass and in particular agricultural waste adds large amounts of CO 2 to the atmosphere. Biochar that is stable, fixed, and 'recalcitrant' carbon can store large amounts of greenhouse gases in the ground for centuries, potentially reducing or stalling the growth in atmospheric greenhouse gas levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.
Researchers have estimated that sustainable use of biocharring could reduce the global net emissions of carbon dioxide (CO 2), methane, and nitrous oxide by up to 1.8 Pg CO 2-C equivalent (CO 2-Ce) per year (12% of current anthropogenic CO 2-Ce emissions; 1 Pg=1 Gt), and total net emissions over the course of the next century by 130 Pg CO 2-Ce, without endangering food security, habitat, or soil conservation.
Biochar is a high-carbon, fine-grained residue which today is produced through modern pyrolysis processes. Pyrolysis is the direct thermal decomposition of biomass in the absence of oxygen to obtain an array of solid (biochar), liquid (bio-oil), and gas (syngas) products. The specific yield from the pyrolysis is dependent on process conditions, and can be optimized to produce either energy or biochar.
These positive qualities are dependent on the properties of the biochar, and may depend on regional conditions including soil type, soil condition (depleted or healthy), temperature, and humidity. Modest additions of biochar to soil reduce nitrous oxideN 2O emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO2.
Pollutants such as metals and pesticides seep into soil and contaminate food supplies, reducing the amount of land suitable for agricultural production. Studies have reported positive effects from biochar on crop production in degraded and nutrient–poor soils. Biochar can be designed with specific qualities to target distinct properties of soils. Biochar reduces leaching of critical nutrients, creates a higher crop uptake of nutrients, and provides greater soil availability of nutrients. 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.
Slash and char
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.
Slash and char can sequester up to 50% of the carbon in a highly stable form. 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. Additionally, by improving the soil tilth, fertility, and productivity, biochar–enhanced soils can indefinitely sustain agricultural production, whereas non-amended 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 tillage machinery or equipment used to apply fertilizer.
Biochar 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 surface area. As a result, nutrients, phosphorus, and agrochemicals are retained for the plants benefit. Plants therefore, are healthier and fertilizers leach less into surface or groundwater.
Energy production: bio-oil and syngas
Bio-oil can be used as a replacement for numerous applications where fuel oil is used, including fueling space heaters, furnaces, and boilers. Additionally, these biofuels can be used to fuel some combustion turbines and reciprocating engines, and as a source to create several chemicals. If bio-oil is used without modification, care must be taken to prevent emissions of black carbon and other particulates. Syngas and bio-oil can also be “upgraded” to transportation fuels such as biodiesel and gasoline substitutes. 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 producing electrical energy from biomaterial. Syngas can be burned directly, used as a fuel for gas engines and gas turbines, converted to clean diesel fuel through the Fischer–Tropsch process, or potentially used in the production of methanol and hydrogen.
Bio-oil has a much higher energy density than the raw biomass material. 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. 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. The greatest potential for bio-oil seems to be its use in a bio-refinery, where compounds that are valuable chemicals, pesticides, pharmaceuticals, or food additives are first extracted, and the remainder is either upgraded to fuel or reformed to syngas.
Direct and indirect benefits
The pyrolysis of forest- or agriculture-derived biomass residue generates a biofuel without competition with crop production.
Biochar is a pyrolysis byproduct that may be ploughed into soils in crop fields to enhance their fertility and stability, and for medium- to long-term carbon sequestration in these soils.
Biochar enhances the natural process: the biosphere captures CO 2, especially through plant production, but only a small portion is stably sequestered for a relatively long time (soil, wood, etc.).
Biomass production to obtain biofuels and biochar for carbon sequestration in the soil is a carbon-negative process, i.e. more CO 2 is removed from the atmosphere than released, thus enabling long-term sequestration.
Calculations suggest that emissions reductions can be 12–84% greater if biochar is put back into the soil instead of being burned to offset fossil-fuel use. Thus Biochar sequestration offers the chance to turn bioenergy into a carbon-negative industry.
Johannes Lehmann, of Cornell University, estimates that pyrolysis can be cost-effective for a combination of sequestration and energy production when the cost of a CO2 ton reaches $37. As of mid-February 2010, CO2 is trading at $16.82/ton on the European Climate Exchange (ECX), so using pyrolysis for bioenergy production may be feasible even if it is more expensive than fossil fuels.
Current biochar projects are small scale and make no significant impact on the overall global carbon budget, although expansion of this technique has been advocated as a geoengineering approach. The approach which favors applications that benefit the poorest is gaining traction: in May 2009, the Biochar Fund received a grant from the Congo Basin Forest Fund to implement its concept in Central Africa. In this concept, biochar is a tool used to simultaneously slow down deforestation, increase the food security of rural communities, provide renewable energy to them, and sequester carbon.[dead link]
The 2009 International Biochar Conference in Boulder, Colorado saw the launch of a mobile pyrolysis unit with a specified intake of 1,000 pounds per hour (450 kg per hour). The unit, with a length of 12 feet and height of 7 feet (3.6 m by 2.1m), was intended for agricultural applications.
^Solomon, Dawit, Johannes Lehmann, Janice Thies, Thorsten Schafer, Biqing Liang, James Kinyangi, Eduardo Neves, James Petersen, Flavio Luizao, and Jan Skjemstad, Molecular signature and sources of biochemical recalcitrance of organic carbone in Amazonian Dark Earths, 71 Geochemica et cosmochemica ACTA 2285, 2286 (2007) (“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.”) (internal citations omitted)
^ abcLehmann 2007a, pp. 381–387 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 biocahr for soil treatment, but really for transportable fuel charcoal. Abandoned or forgotten charcoal pits left for centuries were eventually reclaimed by the forest. In that time the harsh negative effects of the char (high pH, extreme ash content, salinity) had worn off and turned to positive as the forest soil ecosystem saturated the charcoals with nutrients. supra note 2 at 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.”) (internal citations omitted).
^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)
^ abWinsley, Peter (2007). "Biochar and bioenergy production for climate change mitigation". New Zealand Science Review64. (See Table 1 for differences in output for Fast, Intermediate, Slow, and Gasification).
^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.”
^Perera, K.K.C.K., P.G. Rathnasiri, S.A.S. Senarath, A.G.T. Sugathapala, S.C. Bhattacharya, and P. Abdul Salam, Assessment of sustainable energy potential of non-plantation biomass resources in Sri Lanka, 29 Biomass & Bioenergy 199, 204 (2005) (showing RPRs for numerous plants, describing method for determining available agricultural waste for energy and char production).
^Laird 2008, pp. 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.”
^Jorapur, Rajeev; Rajvanshi, Anil K. (1997). "Sugarcane leaf-bagasse gasifier for industrial heating applications.". Biomass and Bioenergy13 (3). doi:10.1016/S0961-9534(97)00014-7.>
^Lehmann, Johannes. "Terra Preta de Indio". Soil Biochemistry (internal citations omitted). 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.
^Lehmann 2007b “this sequestration can be taken a step further by heating the plant biomass without oxygen (a process known as low-temperature pyrolysis).”
^Lehmann 2007a, pp. 381, 385 “pyrolysis produces 3–9 times more energy than is invested in generating the energy. At the same time, about half of the carbon can be sequestered in soil. The total carbon stored in these soils can be one order of magnitude higher than adjacent soils.
^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 (203 to 604 lb) CO2 MW-1 of electricity generated where the pyrolysis process is optimized for energy and 120 to 360 kilograms (790 lb) CO2MW-1 where biochar is applied to land. This compares to emissions of 600–900 kilograms (1,300–2,000 lb) CO 2MW-1 for fossil-fuel-based technologies.)
^Lehmann, Johannes, and Jose Pereira da Silva Jr., Christoph Steiner, Thomas Nehls, Wolfgang Zech, & Bruno Glaser, Nutrient availability and leaching in an archaeological Anthrosol and a Ferralsol of the Central Amazon basin: fertilizer, manure and charcoal amendments, 249 Plant & Soil 343, 355 (2003)
^Supra note 6; Day, Danny, Robert J. Evans, James W. Lee, and Don Reicosky, Economical CO 2, SO x, and NO x capture from fossil-fuel utilization with combined renewable hydrogen production and large-scale carbon sequestration, 30 Energy 2558, 2560
^Glaser, Lehmann & Zech 2002, pp. 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 plant growth has been observed in tropical (depleted) soils by referencing Lehmann, but that in the boreal (high native soil 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 carbone 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, pp. note 3 at 384 “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.”
^Novak, Jeff. Development of Designer Biochar to Remediate Specific Chemical and Physical Aspects of Degraded Soils. Proc. of North American Biochar Conference 2009, University of Colorado at Boulder. Florence: U.S. Department of Agriculture, 2009. 1-16. Print
^Julie, Major, Johannes Lehmann, Macro Rondon, and Susan J. Riha. Nutrient Leaching below the Rooting Zone Is Reduced by Biochar, the Hydrology of a Columbian Savanna Oxisol Is Unaffected. Proc. of North American Biochar Conference 2009, University of Colorado at Boulder. Ithaca: Cornell University Department of Crop and Soil Sciences, 2009. Print.
^Elmer, Wade, Jason C. White, and Joseph J. Pignatello. Impact of Biochar Addition to Soil on the Bioavailability of Chemicals Important in Agriculture. Rep. New Haven: University of Connecticut, 2009. Print.
^Biochar Sequestration In Terrestrial Ecosystems – A Review, by Johannes Lehmann, John Gaunt, and Marco Rondon. Mitigation and Adaptation Strategies for Global change 403, 404 (2006). supra note 11 at 407 (“If this woody aboveground biomass were converted into biochar by means of simple kiln techniques and applied to soil, more than 50% of this carbon would be sequestered in a highly stable form.”)
^Gaunt & Lehmann 2008, pp. 4152 note 3 (“This results in increased crop yields in low-input agriculture and increased crop yield per unit of fertilizer applied (fertilizer efficiency) in high-input agriculture as well as reductions in off-site effects such as runoff, erosion, and gaseous losses.”)
^Lehmann 2007b, pp. note 9 at 143 “It can be mixed with manures or fertilizers and included in no-tillage methods, without the need for additional equipment.”
^Bridgwater, A. V., A.J. Toft, and J.G. Brammer, A techno-economic comparison of power production by biomass fast pyrolysis with gasification and combustion, 6 Renewable & Sustainable Energy Rev. 181, 231 (“the fast pyrolysis and diesel engine system is clearly the most economic of the novel systems at scales up to 15 MWe”);
^McKendry, Peter, Energy production from biomass (part 2): conversion technologies, 83 BIORESOURCE TECH. 47, 48-49 (2002) (“can be burnt directly or used as a fuel for gas engines and gas turbines. . . . The production of syngas from biomass allows the production of methanol and hydrogen.”) (internal citations omitted).
^Lehmann 2007b, pp. 143, 144 “We calculate that biochar sequestration in conjunction with bioenergy from pyrolysis becomes economically attractive, under one specific scenario, when the value of avoided carbon dioxide emissions reaches $37 per tonne.”
Gaunt, John L.; Lehmann, Johannes (2008). "Energy Balance and Emissions Associated with Biochar Sequestration and pyrolysis Bioenergy Production". Environmental Sciences & Technologies42.
Glaser, Bruno; Lehmann, Johannes; Zech, Wolfgang (2002). "Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoal – a review". Biology and Fertility of Soils35.
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