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Biochar
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Biochar is charcoal created by pyrolysis of biomass. The resulting charcoal-like material can be used as a soil improver to create terra preta, and is a form of carbon capture and storage. Charcoal is a stable solid and rich in carbon content, and thus, can be used to lock carbon in the soil. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide and other greenhouse gases.
Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste).
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Biochar is charcoal created by pyrolysis of biomass. The resulting charcoal-like material can be used as a soil improver to create terra preta, and is a form of carbon capture and storage. Charcoal is a stable solid and rich in carbon content, and thus, can be used to lock carbon in the soil. Biochar is of increasing interest because of concerns about climate change caused by emissions of carbon dioxide and other greenhouse gases.
Biochar is a way for carbon to be drawn from the atmosphere and is a solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste). Since biochar can sequester carbon in the soil for hundreds to thousands of years, it has received considerable interest as a potential tool to slow global warming. The burning and natural decomposition of trees and agricultural matter contributes a large amount of CO2 released to the atmosphere. Biochar can store this carbon in the ground, potentially making a significant reduction in atmospheric GHG 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.
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.
Background
In specific locations - e.g. at the tropical forest frontier in Central Africa amongst ultra-poor communities of slash-and-burn farmers - biochar may become the kernel of a highly integrated sustainable development and poverty alleviation concept that can tackle key issues simultaneously: hunger and food insecurity, low agricultural productivity and soil depletion, deforestation and biodiversity loss, energy poverty (and related health problems such as indoor air pollution), and climate change. In this concept, multiple carbon reductions would come from the char in the soil (carbon sink), the avoided deforestation and its associated emissions, and the emissions avoided by making a switch from cooking with unsustainably harvested fuel wood on inefficient open fires to more efficient biochar-generating, small-scale energy production.
As a result of significant product variations due to varying technology, process conditions, and feedstock compositions, biochar cannot be considered a commodity product.
Current biochar projects are small scale, though many developments show that organic matter can be efficiently turned into biochar, potentially making a significant impact on the overall global carbon budget.
Carbon sink potential and soil co-benefits
Biochar can sequester carbon in the soil for hundreds to thousands of years. Pre-Columbian Amazonian Natives used it to enhance soil productivity and made it by smoldering agricultural waste. European settlers called it Terra Preta de Indio. Its modern equivalent is being developed using pyrolysis to heat biomass in the absence of oxygen in kilns.
Modern biochar production can be combined with biofuel production in a process that is energy-positive(exothermic)—producing 3-9 times more energy than invested, is carbon-negative—withdrawing CO2 from the atmosphere and rebuilds geological carbon sinks. This technique is advocated by prominent scientist James Lovelock, creator of the Gaia hypothesis, for mitigation of global warming by greenhouse gas remediation.
Biochar is a high-carbon, fine-grained residue which can be produced either by smoldering biomass utilizing centuries-old techniques (i.e., covering burning biomass with soil and letting it smolder) or 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. Even when optimized to produce char rather than energy, the energy produced per unit energy input is higher than for corn ethanol.
In addition to its potential for carbon sequestration, biochar has numerous co-benefits when added to soil. It can prevent the leaching of nutrients out of the soil, increase the available nutrients for plant growth, increase water retention, and reduce the amount of fertilizer required. Additionally, it has been shown to decrease N2O (Nitrous oxide) and CH4 (methane) emissions from soil, thus further reducing GHG emissions. Biochar can be utilized in many applications as a replacement for or co-terminous strategy with other bioenergy production strategies. One of its most immediate uses is in switching from "slash-and-burn” to “slash-and-char” to prevent the rapid deforestation and subsequent degradation of soils.
“Biochar sequestration does not require a fundamental scientific advance and the underlying production technology is robust and simple, making it appropriate for many regions of the world.” Johannes Lehmann, of Cornell University, estimates that pyrolysis will be cost feasible when the cost of a CO2 ton reaches $37, (as of the end of June 2008, CO2 is trading at $45/ton on the [https://www.theice.com/marketdata/emissionsView/emissionsIndexView.jsp ECX]) – so using pyrolysis for bioenergy production is feasible, even though it may be more expensive than fossil fuels at the moment.
Pyrolysis of biomass as a carbon sink
Biochar can be used to sequester carbon on centurial or even millennial time scales. Plant matter absorbs CO2 from the atmosphere while growing. In the natural carbon cycle, plant matter decomposes rapidly after the plant dies, which emits CO2; the overall natural cycle is carbon neutral. Instead of allowing the plant matter to decompose, pyrolysis can be used to sequester the carbon in a much more stable form. Biochar thus removes circulating CO2 from the atmosphere and stores it in virtually permanent soil carbon pools, making it a carbon-negative process. In places like the Rocky Mountains, where beetles have been killing off vast swathes of pine trees, the utilization of pyrolysis to char the trees instead of letting them decompose into the atmosphere would offset substantial amounts of CO2 emissions. Although some organic matter is necessary for agricultural soil to maintain its productivity, much of the agricultural waste can be turned directly into biochar, bio-oil, and syngas. The use of pyrolysis also provides an opportunity for the processing of municipal waste into useful clean energy rather than increased problems with land space for storage.
Biochar is believed to have long mean residence times in the soil. While the methods by which biochar mineralizes (turns into CO2) are not completely known, evidence from soil samples in the Amazon shows large concentrations of black carbon (biochar) remaining after they were abandoned thousands of years ago. The amount of time the biochar will remain in the soil depends on the feedstock material, how charred the material is, the surface:volume ratio of the particles, and the conditions of the soil the biochar is placed in. Estimates for the residence time range from 100 to 10,000 yrs, with 5,000 being a common estimate. Lab experiments confirm a decrease in carbon mineralization with increasing temperature, so carefully controlled charring of plant matter can increase the soil residence time of the biochar C.
Under some circumstances, the addition of biochar to the soil has been found to accelerate the mineralization of the existing soil organic matter, but this would only reduce and not suppress the net benefit gained by sequestering carbon in the soil by this method. Furthermore, the suggested soil conditions for the integration of biochar are in heavily degraded tropical soils used for agriculture, not organic matter-rich boreal forest soils (as tested in the above reference).
Production of biochar
The yield of products from pyrolysis varies heavily with temperature. The lower the temperature, the more char is created per unit biomass. High temperature pyrolysis is also known as gasification, and produces primarily syngas from the biomass. The two main methods of pyrolysis are “fast” pyrolysis and “slow” pyrolysis. Fast pyrolysis yields 60% bio-oil, 20% biochar, and 20% syngas, and can be done in seconds, whereas slow pyrolysis can be optimized to produce substantially more char (~50%), but takes on the order of hours to complete. For typical inputs, the energy required to run a “fast” pyrolyzer is approximately 15% of the energy that it outputs. Modern pyrolysis plants can be run entirely off of the syngas created by the pyrolysis process and thus output 3-9 times the amount of energy required to run. Alternatively, microwave technology has recently been used to efficiently convert organic matter to biochar on an industrial scale, producing ~50% char.
The ancient method for producing biochar as a soil additive was the “pit” or “trench” method, which created terra preta, or dark soil. While this method is still a potential to produce biochar in rural areas, it does not allow the harvest of either the bio-oil or syngas, and releases a large amount of CO2, black carbon, and other GHGs (and potentially, toxins) into the air. Modern companies are producing commercial-scale systems to process agricultural waste, paper byproducts, and even municipal waste.
There are three primary methods for deploying a pyrolysis system. The first is a centralized system where all biomass in the region would be brought to a pyrolysis plant for processing. A second system would effectively mean a lower-tech pyrolysis kiln for each farmer or small group of farmers. A third system is a mobile system where a truck equipped with a pyrolyzer would be driven around to pyrolyze biomass. It would be powered using the syngas stream, return the biochar to the earth, and transport the bio-oil to a refinery or storage site. Whether a centralized system, a distributed system, or a mobile system is preferred is heavily dependent on the specific region. The cost of transportation of the liquid and solid byproducts, the amount of material to be processed in a region, and the ability to feed directly into the power grid are all factors to be considered when deciding on a specific implementation.
Unless crops are going to be dedicated to biochar production, the residue-to-product ratio (RPR) for the feedstock material is a useful gauge of the approximate amount of feedstock that can be obtained for pyrolysis after the primary product is harvested and the waste remains. The amount of crop residue available to be used for pyrolysis can be determined by using the RPR, and the collection factor (the percent of the residue not used for other things). For instance, Brazil harvests approximately 460Mt of sugar cane annually, with an RPR of 0.30, and a collection factor (CF) of 0.70 for the sugar cane tops, which are normally burned on the field. This translates into approximately 100Mt of residue which can be pyrolyzed to create energy and soil additives annually. Adding in the bagasse (sugar cane waste) (RPR=0.29 CF=1.0) which is currently burned inefficiently in boilers, raises the total to 230 Mt of pyrolysis feedstock just from sugar cane residues. Some plant residue, however, must remain on the soil to avoid heavily increased costs and emissions from nitrogen fertilizers.
Co-benefits of pyrolysis
Biochar can be used as a soil amendment to increase plant growth yield, improve water quality, reduce soil emissions of GHGs, reduce leaching of nutrients, reduce soil acidity, and reduce irrigation and fertilizer requirements. These properties are very dependent on the properties of the biochar, and may depend on regional conditions including soil type, condition (depleted or healthy), temperature, and humidity. Modest additions of biochar to soil were found to reduce N2O emissions by up to 80% and completely suppress methane emissions.
Switching from slash-and-burn to slash-and-char techniques in Brazil can both decrease deforestation of the Amazon and increase the crop yield. Under the current method of slash-and-burn, only 3% of the carbon from the organic material is left in the soil.
Switching to slash-and-char can sequester up to 50% of the carbon in a highly stable form. Adding the biochar back into the soil rather than removing it all for energy production is necessary to avoid heavy increases in the cost and emissions from more required nitrogen fertilizers. Additionally, by improving the soil tilth, fertility, and productivity, the biochar enhanced soils can sustain agricultural production, whereas non-amended soils quickly become depleted of nutrients, and the fields are abandoned, leading to 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.
Pyrolysis for the production of energy
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 like 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 Fischer Tropsch 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 itself if the biochar is returned to the soil and the syngas stream is used to power the process. Bio-oil contains organic acids which are corrosive to steel containers, has a high water vapor content which 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.
See also
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