Converting a kraft pulp mill into a multi-product biorefinery: techno-economic analysis of a case mill

Abstract

In this case study, we investigated the conversion of an existing Swedish kraft pulp mill to the production of dissolving pulp, with export of electricity, lignin, and a hemicellulose stream suitable for upgrading. By increasing the level of heat integration of the mill, it was possible to achieve self-sufficiency in terms of steam and to produce significant amounts of excess steam. The excess steam could facilitate the integration of a lignin separation plant or be used for power generation. The production of dissolving pulp requires a higher input of wood that is required for the same level of pulp production as is achieved with kraft pulp. For the studied mill, the batch digester was the main limitation for pulp production. Nevertheless, if the digester capacity was increased, then the level of pulp production could be maintained. In addition, the recovery boiler, causticization plant, and evaporation plant had sufficient capacities for preserving the same production level upon conversion, and could easily be upgraded to a certain degree through relatively simple measures for an increase in pulp production. However, increasing pulp production beyond that limit required extensive upgrades or investments in new equipment, which negatively affected annual earnings. Annual earnings were found to be also dependent upon the level of heat integration, type of by-product, and the costs for lignin and electricity. However, our results suggest that the optimal process configuration is more dependent upon other factors, such as the long-term vision of the company and policy instruments.