Hi. Yes the burning part is our experiment with biochar. Pyrolysis (in our case a smothered fire) is essentially a decomposition of organic material at elevated temperatures in the absence of oxygen. It involves changing both the chemical composition and the physical state of the fuel. Because we struggled with oxygen holes appearing in our smoldering bed I'm not sure how much char we actually created versus just straight out ash.
There is quite a bit of debate about the subject, but it is thought to create fertile soil for possibly hundreds of years after construction, and be a GHG mitigation strategy. Below is the excerpt on Biochar from a report called "Greenhouse Gas Mitigation Potential of Agricultural Land Management in the United States."
My personal take on GHG are that Methane (CH4) and nitrous oxide (N20) are probably more pertinent to climate change but am fascinated by the whole field.
Biochar
Biochar is produced by pyrolysis, the incomplete combustion of biomass into charred organic matter. While the pyrolysis process can be designed to capture heat and co-generate electricity as biofuel, the end product can also be used for soil application, with potential to increase soil C via three mechanisms: (1) by storing recalcitrant C in biochar soil amendments, (2) by stabilizing existing C in the soil, and (3) by increasing biomass production aboveground, thereby increasing C inputs into soil (Gaunt and Driver 2010). It can also have impacts on other GHGs as described below. Research suggests that this black carbon, or terra preta, thought to be charcoal from burning of organic matter hundreds of years ago, is a key factor for organic matter persistence in the tropics (Glaser et al. 2001; Lehmann et al. 2004). While few field studies exist in the U.S., similar effects are anticipated for temperate regions. As a result, soil C sequestration potential of biochar has been estimated based on calculations of available feedstock and expected C stability in the biochar (Table 12). Due to high variability and uncertainty, GHG impacts of possible productivity gains are not included in these calculations.
Possible biomass sources include milling residues (e.g., rice husks, nut shells, sugar cane bagasse), crop residues, biofuel crops, urban municipal wastes, animal manure, and logging residues, although their suitability is dependent on lignin content (Lehmann et al. 2006; Verheijen et al. 2009). Most research into biochar has focused on wood feedstocks in (sub) tropical regions, and scientific understanding of the properties of biochar from other feedstocks and in other regions remains limited (Verheijen et al. 2009). Not all forms of biochar have equivalent rates of C storage or stabilization, which are dependent on factors such as feedstock source and temperature, and rate or residence time of the pyrolysis process (Gaunt and Driver 2010). Therefore, further research into biochar application in U.S. cropping systems is needed to examine whether the anticipated impacts can be realized in large-scale implementation.
Table 12 [not included because I couldn't get it to print well in the TSP format]
The response of soil to biochar amendments has to be biochar- and ecosystem-specific (Shneour 1966; Spokas and Reicosky 2009). When most plant biomass is decomposed, less than 10%–20% of the original C remains after 5–10 years (Lehmann et al. 2006). By comparison, research has found biochar to be highly stable, with mean residence time of hundreds to thousands of years (Lehmann et al. 2008; Roberts et al. 2010; Verheijen et al. 2009). Assuming that biochar application retains up to 50% of biomass C as a stable residue, Lehmann et al. (2006) estimated that up to 514 t CO2e ha–1 could be stored under typical soil and plant species conditions. However, the characteristics of the applied biochar and the method of storage will greatly influence biochar recalcitrance. As with other organic material, biochar decay is facilitated by decomposition, microbial co-metabolism, abiotic processes, and physical breakdown and influenced by temperature, depth of burial, and soil cultivation (De Gryze et al. 2010). The complex interactions among these factors have not been studied extensively, so biochar recalcitrance remains widely variable in the literature.
Beyond sequestration, biochar may have potential to mitigate GHGs by decreasing the need for fertilizer and other inputs, reducing upstream emissions (Gaunt and Driver 2010; Lehmann et al. 2006), and by reducing emissions of N2O and CH4 (Suddick et al. 2010), possibly due to production of ethylene, which inhibits microbial processes (Spokas et al. 2010). However, there are currently no peer-reviewed studies documenting suppression of N2O or CH4 emissions in the field (Sohi et al. 2010), even though a number of short-term studies and laboratory experiments have noted N2O emission reductions of 50%–80% and near complete suppression of CH4 with biochar additions (Fowles 2007; Lehmann et al. 2006; Renner 2007; Rogovska et al. 2008; Yanai et al. 2007). The potential seems particularly large in tropical soils, as shown by a study by Rondon et al. (2005) where 50% reduction of N2O emissions was reported in soybean plots and 80% in grass stands. Yet, in another laboratory experiment, Yanai (2007) found that the impact of biochar on N2O emissions was highly dependent on soil hydrology, where N2O emissions varied from an 89% reduction in very wet soil to a 51% increase in N2O emissions in drier soil. Another recent study found no reduction in N2O production after urine application to pasture soils (Clough et al. 2010). Residence time of biochar in the soil may also be important, as increases in N2O emissions have been noted when biochar is first applied to soil, with a shift to N2O emission reduction over time as sorption capacity of biochar was enhanced with aging (Singh et al. 2010).
With many potentially variable factors,26 full lifecycle assessments of biochar energy and GHG impacts are useful for comparing scenarios. For example, Roberts et al. (2010) calculate net GHG emission reductions of 0.86 t CO2e t–1 of feedstock for corn stover. McCarl et al. (2009) estimate a net mitigation potential of 0.823 t CO2e t–1 of feedstock for fast pyrolysis and 1.113 t CO2e t–1 of feedstock for slow pyrolysis, accounting for emissions from collection, hauling, pyrolysis, and replacing nutrients. Laird (2008) estimates a net potential of 0.33 t CO2e t–1 of feedstock through displacement of fossil fuel by bio-oil in a bioenergy pyrolysis platform, for a total national potential GHG mitigation of 822 Mt CO2e yr–1 combined with sequestration potential (510 Mt CO2e yr–1) from harvestable forest and crop residues. Additional process emissions reductions may result from decreased need for fertilizer or lime (e.g., 20% reduction estimate in Laird (2008)), and other inputs due to biochar’s soil-amending characteristics (Fowles 2007; Laird 2008; Lehmann et al. 2006; Roberts et al. 2010). A detailed comparison of feedstock alternatives, pyrolysis methods, and tradeoffs, and other costs of biochar production can be found in a report produced for the Climate Action Reserve (De Gryze et al. 2010).
