Comparison of options for debottlenecking the recovery boiler at kraft pulp mills – Economic performance and CO2 emissions.

The trend in the European pulp and paper industry is toward fewer mills with larger capacity. As a result, a number of existing mills will increase their production capacity. For increased production capacity in kraft pulp mills, the recovery boiler is often a bottleneck. This paper compares three different options for debottlenecking the recovery boiler and utilizing a potential mill steam surplus at a typical Scandinavian kraft pulp mill, when increasing the production capacity by 25%: 1) Upgrading the recovery boiler, 2) Lignin extraction and 3) Black liquor gasification (as a booster). For black liquor gasification (BLG) two options for using the product gas are considered: production of electricity or DME motor fuel. Furthermore, both BLG and upgrading of the recovery boiler are assumed to be possible to combine with carbon capture and storage (CCS). The extracted lignin is assumed to either be valued as wood fuel or as oil. The different options are evaluated and compared with respect to annual net profit and global CO2 emissions for four different future energy market scenarios. The results show that, generally, BLG with motor fuels production and lignin extraction with lignin valued as oil achieve the best economic performance. Upgrading the recovery boiler and capture and store CO2 from the boiler flue gases gives the highest CO2 emissions reduction but is only an economically attractive option in scenarios with a high CO2 emissions charge