Improved fast pyrolysis bio-oils from straw and miscanthus by fractionated condensation
Fast pyrolysis is one of the most promising conversion processes for producing advanced biofuels, which can be used as substitute for fuel oils or chemicals. Herbaceous biomass due to its comparatively high amount of cellulose and catalytically active inorganic content results in pyrolysis oil with high water content even if the original biomass was dry. Such bio-oil separates into two phases: an upper aqueous phase and a bottom tarry phase. The phase separation by decantation is not an efficient method to reduce the water content and total acid number because a part of valuable components will be lost to the aqueous phase. To improve the quality of bio-oils producing single-phase liquids by staged condensation seems to be the better method. The minimum condensation temperature which leads to single-phase liquid bio-oils was determined to be around 66°C for wheat/barley straw and 62°C for miscanthus. A further increase in temperature will result in bio-oils with lower water content and total acid number, but goes along with higher losses of organics to the second condensation stage.