Biochars are the carbonaceous solid materials collected from the thermochemical
conversions of biomass by processes including torrefaction, pyrolysis, and gasification
(Spokas et al. 2012). Torrefaction typically involves the anaerobic conversion of biomass
at temperatures ranging from 200 to 300 °C with yields reaching 70 to 90% (Ciolkosz
and Wallace 2011; van der Stelt et al. 2011). Higher temperatures used in fast pyrolysis
(ca. 500 °C) give fuel-quality liquids and gases, with biochar accounting for 15 to 25% of
the biomass as a byproduct (Boateng et al. 2007; Jahirul et al. 2012). In a gasifier, the
biomass is converted at even higher temperatures (ca. 900 °C) in the presence of limited
O2, with the target being the maximum generation of producer gas; yields of biochar can
be as low as 1% for very efficient conversions of biomass to producer gas (Pan and
Eberhardt 2011).