Fast pyrolysis of biomass thermally pretreated by torrefaction

Torrefied biomass samples were produced from hardwood and switchgrass pellets using the biochar experimenter's kit (BEK) reactor and analyzed for their utility as pretreated feedstocks for biofuels production via fast pyrolysis. The energy efficiency for the BEK torrefaction process with propane gas as fuel source was about 50% for hardwood pellets and over 67% for switchgrass, but the energy retained in the torrefied product ranged between 72 and 92% of the initial biomass energy content resulting from a high product yield of 70–88 wt%. The liquid pyrolysate by-product of the torrefaction process was an aqueous solution of mainly acetic acid and levoglucosan associated with dehydration of the carbohydrate fraction of the raw biomass. Subsequent fast pyrolysis in a fluidized bed led to production of pyrolysis oil with lower acidity and increased energy content compared with pyrolysis of non-torrefied biomass. However, the yield, carbon conversion and energy conversion to these pyrolysis oils were decreased with the torrefection pretreatment which predisposed the biomass for conversion to biochar rather than liquid products.

► Torrefied biomass was produced from hardwood and switchgrass pellets.
► The energy efficiency for the torrefaction process was 50–67%.
► Torrefied biomass was used as feedstock for bio-oil production via fast pyrolysis.
► Higher quality bio-oil was produced from torrefied biomass than from raw biomass.
► However, the yield and energy conversion to these bio-oils were decreased.