Pulp Mill Heat Intergration of Black Liquor Gasifiers, 1998 International Chemical Recovery Conference Proceedings

Pressurized black liquor gasification has a potential to considerably improve the efficiency of energy recovery in the kraft process. Gasification systems will include new low-to-medium temperature heat sources, and it is attractive to integrate these sources well with heat sinks in the mill to further reduce steam and cooling water use. In the present study, the potential for process integration in the chemical recovery loop, including the digester, was examined. Using pinch analysis, two opportunities were identified which could increase the amount of steam available for operations outside this loop. In the evaporator plant, a relatively simple arrangement with five flash tanks to pm-evaporate black liquor-using heat from the gasification plant--could reduce steam use by about 25% or 0.8-0.9 GJ/ADt in a design suitable for retrofit. The capacity could possibly increase, by 7-8%, but it is likely that this will be counteracted by reduced heat transfer coefficients. In the digester area, heating of white liquor with heat from the gasification plant, could reduce steam use by about 0.6 GJ/ADt. Either of the two alternatives could be designed to use all of the excess low-temperature heat from the gasification plant studied, and they are therefore mutually exclusive.