Carbon sink potential
Biochar can sequester carbon in the soil for hundreds to thousands of years. Modern biochar 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 exothermic (energy producing)—producing an output of 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 scientists such as James Hansen, an internationally-renowned climate scientist and head of NASA's Goddard Institute for Space Studies, and 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 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. Even when optimized to produce char rather than energy, the energy produced per unit energy input is higher than for corn ethanol.
Use 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).
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 ECX) – so using pyrolysis for bioenergy production is feasible, even though it may be more expensive than fossil fuels at the moment.
Co-benefits for soil 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.
Slash and char
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.
Energy production: bio-oil
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.