White liquor production: A comparison of the causticizing stages using gasifier and recovery boiler produced green liquor

摘要

Green liquor, starting with sludge removal and ending with white liquor separation, has been investigated from a recovery boiler and a state-of-the-art gasification unit using the same black liquor for the two processes. This has been possible due to the co-location of a demonstration-sized pressurized gasification unit and a recovery boiler. Tests were performed under varying conditions in the gasifier while the conditions in the recovery boiler were assumed constant. Our investigation shows that the separation properties of green liquor sludge after a gasifier are almost similar to the separation properties after a recovery boiler. It is clear that the total load of reburned lime mud increases in the case of a gasifier, but the total concentration of the produced white liquor equals mill demand (i.e. is the same as recovery boiler produced white liquor). It was also found that causticizing efficiency was not affected by the increased load of reburned lime mud and the changed composition in the gasifier produced green liquor. Therefore, it can be concluded that the formation of sodium hydroxide in the white liquor produced from an entrained flow gasifier will not be limited by the existing process technology.