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energy invested, even with the proposed use of biochar as a carbon sink instead of an energy source (Gaunt and Lehmann unpublished data).