Techno-economic evaluation of butanol production via black liquor fractionation

The hemicelluloses fraction of black liquor is an underutilized resource in many chemical pulp mills. It is possible to extract and separate the lignin and hemicellulose from the black liquor and use the hemicellulose for biochemical conversion into biofuels and chemicals. Precipitation of the lignin from the black liquor would consequently decrease the load on the heat recovery boiler, which might be a bottleneck for increased pulp production. Fractionation of black liquor therefore represents an interesting business opportunity for pulp mills. The main objective of this work has been to techno-economically evaluate an integrated process for black liquor fractionation and the subsequent processing and recovery of the hemicelluloses and lignin-derived products. The considered final products are sodium-free lignin as a solid fuel and butanol to be used as fossil gasoline replacement in Otto engines. The hydrolysis and fermentation processes are modeled in Aspen Plus to analyze energy and material balances. A mathematical process integration model of an existing pulp and paper mill is used to analyze the effects on the energy performance of the mill subprocesses as well as to evaluate the plant economics. Based on the composition and amount of the extracted stream and the model developed in this work, about 13.8 ktonne-lignin/year (on dry basis) and about 1 ktonne-butanol/year are produced from the integrated process. In addition, about 0.5 and 0.02 ktonne/year of acetone and ethanol are co-produced, respectively. Under the assumed economy, the cost of production of n-butanol is estimated to be kUSD 3.95/tonne-butanol and this corresponds to kUSD 2.5/tonne-ABE on ABE (acetone-butanol-ethanol) basis. An assessment on the impact of the extracted hemicelluloses and lignin on the recovery boiler has resulted in about 30% reduction in the thermal energy output of the unit.