Biomass fuels such as wood, herbaceous materials and agricultural by-products are the world’s third largest primary energy resource, behind coal and oil. At best, conventional biomass to energy is considered to be carbon neutral. Harvesting biomass to produce energy may not be sustainable because it can result in reduced soil productivity by depletion of carbon and nutrients. Biomass pyrolysis addresses this dilemma, because it can utilize waste products and about half of the original carbon can be returned to the soil (Lehmann, 2007).
In a recent paper published by the Ecological Society of America, Johnannes Lehmann of Cornell University discussed the basics of biomass pyrolysis (excerpts from: Lehmann, 20007, Bioenergy in the Black, available as a PDF).
“Pyrolysis is one of many technologies to produce energy from biomass (Bridgwater,2003). What distinguishes pyrolysis from alternative ways of converting biomass to energy is that pyrolysis produces a carbon-rich, solid byproduct, biochar (Figure 1). Under complete or partial exclusion of oxygen, biomass is heated to moderate temperatures, between about 400 and 500°C (giving the process the name “low-temperature pyrolysis”), using a variety of different reactor configurations. At these temperatures, biomass undergoes exothermic processes and releases a multitude of gaseous components in addition to heat (Czernik and Bridgwater 2004). Both heat and gases can be captured to produce energy carriers such as electricity, bio-oil, or hydrogen for household use or powering cars. In addition to energy, certain valuable co-products can be obtained, including wood preservative, meat browning, food flavoring, adhesives, or specific chemical compounds (Czernik and Bridgwater 2004)."
Dr. Lehmann also explains that
“Several carbon costs are associated with the land-based production of biomass, transport to the bio-energy plant, pyrolysis itself, and land application of biochar (the latter is much less costly for biochar than for biomass, due to the fact that the mass per unit carbon of biochar is about 60% that of biomass). Our preliminary calculations take all of these carbon costs into account and suggest that the energy balance for various feedstocks, such as corn or switchgrass, is very favorable, with approximately 3–9 kg C energy yield for every kg C energy invested, even with the proposed use of biochar as a carbon sink instead of an energy source (Gaunt and Lehmann unpublished data).
"Comparable ratios for ethanol currently amount to 0.7–2.2 kg C (kg C)-1 (Pimentel and Patzek 2005; Metzger 2006) and, for biomass burning, to 10–13 kg C (kgC)-1 (willow; Keoleian and Volk 2005), with the caveat that the latter produces only heat, not liquid fuel."
Therefore Dr. Lehmann concludes that:
“This means that 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. Such a carbon-negative technology would lead to a net withdrawal of CO2 from the atmosphere, while producing and consuming energy.”
(Adapted from: Day, Et. Al. 2004. The Utilization of CO2 For the Creation of a Valuable and Stable Carbon Co-product from Fossil Fuel Exhaust Scrubbing )