Extracting lignin from black liquor: Consequences for the evaporation plant
An increase in the production of pulp is of economic interest to many kraft pulp mills. However, significant increases in production generally require the recovery boiler to be debottlenecked: it must either be rebuilt or replaced. Both options require costly investments that can be avoided by extracting lignin from the black liquor before it is burnt in the boiler. This, in turn, requires changes being made in the evaporation plant. In the present study, simulations have been carried out based on a 25% increase in the production of pulp. The evaporation plants are simulated with and without an integrated lignin separation plant to investigate the consequences of extracting lignin from black liquor. The results show that the evaporation plants with lignin extraction have investment costs that are 5-30% higher than an equivalent plant without lignin extraction. Furthermore, the evaporation plants with lignin extraction require an evaporation capacity that is 7% higher, since filtrates from the lignin separation plant are also evaporated here. This increases the steam demand by 9-12% compared with a plant without lignin extraction. The span in the investment cost and steam demand depends on where the filtrates are recirculated in the evaporation plant.