A gasification-based biorefinery for the pulp and paperindustry

This paper is drawn from a 2-year study of integrated pulpmill biorefineries based on black liquor (the lignin-rich byproduct of fiber extraction from wood) and wood residue gasification at a large kraft mill representative of those in the Southeast United States. The study included detailed mass-energy balance simulations, financial analyses, and energy and environmental benefits estimates for seven pulpmill biorefinery process configurations. All seven configurations include an oxygen-blown, high-temperature black liquor gasifier, syngas cooling, clean-up by a Rectisol (methanol) system, and a catalytic gas-to-liquid process; six of them also include a fluidized-bed, oxygen-blown biomass gasifier and a gas turbine combined cycle fully integrated with the gasification and syngas cooling section. Three biofuels were examined: dimethyl ether (DME), Fischer-Tropsch liquids, and ethanol-rich mixed-alcohols. For the integrated biorefineries analyzed here, the ratio of useful energy outputs (steam, electricity and fuels) to total energy inputs (black liquor, wood residuals and fuel oil) ranges from 66 to 74%; these values compare with about 57% for conventional systems based on Tomlinson boilers and 65% for gasification combined cycles that produce only electricity. Because of the integration of the biorefinery with the pulp and paper mill, the adjusted liquid fuel yield per unit of biomass – a measure of the effectiveness of biomass conversion to liquids – is far higher than for “standalone” gasification-based biorefineries or for ethanol production via biochemical conversion (based on enzymatic hydrolysis). Besides better energy performance, the integration between the biorefinery and the pulp mill effectively limits the specific capital investment associated with liquid fuels production to a surprisingly modest $60,000–150,000 per barrel of diesel equivalent per day—specific capital costs comparable to those for much larger coal-to-liquids facilities. Gasification-based pulp mill biorefinery technologies, once fully commercialized, offer the potential for attractive investment returns and, if implemented widely, significant energy and environmental benefits to the United States.