Black Liquor Fractionation for the Production of Biobutanol

Woody biomass consists of lignin (20-30% of dry weight), cellulose (30-45%), hemicellulose (20-35%) and extractives (2-5%). Cellulose is the main raw material for chemical pulp production, while the three other components could be considered as by-products in the pulp industry. Today lignin and part of hemicellulose end up in the black liquor and are burned in a recovery boiler and the energy is recovered as steam, supplying a large fraction of the mills process steam requirements. Most of the energy is provided by lignin (27 MJ/kg), since hemicellulose has a very low heating value (13.6 MJ/kg). The hemicelluloses fraction is an underutilized renewable resource in many mills and a more attractive alternative would be to extract the hemicellulose from the black liquor and use it for biochemical conversion into fuels and chemicals. Precipitation of lignin from black liquor, would decrease the load on the recovery-boiler which might be a bottleneck for increased pulp production. Fractionation of black liquor represents, therefore, a very interesting business opportunity for many pulp mills. In this study, conditions for black liquor fractionation for optimal utilization of each component (hemicellulose, lignin, and chemicals) were established. Lignin precipitation and recovery was integrated with xylan separation and hydrolysis. The xylose rich hydrolyzate was subsequent used for Clostridium acetobutylicum (ATCC 824) aceton-butanol-ethanol (ABE) butanol fermentation, demonstrating the feasibility of using black liquor as an alternative renewable source for biobutanol production.