Synthetic gas production from dry black liquor gasification process using direct causticization with CO2 capture

Abstract

Synthetic natural gas (SNG) production from dry black liquor gasification (DBLG) system is an attractive option to reduce CO2 emissions replacing natural gas. This article evaluates the energy conversion performance of SNG production from oxygen blown circulating fluidized bed (CFB) black liquor gasification process with direct causticization by investigating system integration with a reference pulp mill producing 1000 air dried tonnes (ADt) of pulp per day. The direct causticization process eliminates use of energy intensive lime kiln that is a main component required in the conventional black liquor recovery cycle with the recovery boiler. The paper has estimated SNG production potential, the process energy ratio of black liquor (BL) conversion to SNG, and quantified the potential CO2 abatement. Based on reference pulp mill capacity, the results indicate a large potential of SNG production (about 162MW) from black liquor but at a cost of additional biomass import (36.7MW) to compensate the total energy deficit. The process shows cold gas energy efficiency of about 58% considering black liquor and biomass import as major energy inputs. About 700ktonnes per year of CO2 abatement i.e. both possible CO2 capture and CO2 offset from bio-fuel use replacing natural gas, is estimated. Moreover, the SNG production offers a significant fuel replacement in transport sector especially in countries with large pulp and paper industry e.g. in Sweden, about 72% of motor gasoline and 40% of total motor fuel could be replaced.