Energy Consequences When Integrating a Black Liquor Gasifier Into a Pulp Mill, 1999 Pulping Conference Proceedings

Anna Isaksson, Martin Andrén, Mikael Ahlroth, Jinyue Yan & Gunnar Svedberg--Spent liquor, or so-called black liquor, is a residual of the kraft cooking process. Black liquor is one of the most important energy sources from biomass in Sweden, and additionally supplies the majority of the energy used in the production of chemical kraft pulp. The black liquor contains the cooking chemicals in an oxidized state and also biomass as lignin and carbohydrates. In the recovery boiler, the cooking chemicals in the black liquor (sulfur and sodium compounds) are recovered and the biomass is combusted. The heat released is used to generate high pressure steam (HP steam) in the recovery boiler. The recovery boiler, which is one of the most expensive units in a pulp mill, is often the bottleneck when an increase in production is desired.

Black liquor gasification is a promising technology to complement or replace the recovery boiler. The important feature of this technology is the simultaneous improvement of both chemical and energy recovery. For instance, there is a split between the cooking chemicals. Half of the sulfur is obtained in the product gas and can be used to produce polysulfides. The use of polysulfides in the cooking process increases the yield of pulp. However, increasing the yield of pulp results in a lower content of biomass in the black liquor, at the same time the steam demand for drying the pulp increases. The product gas from the gasification can be used in a combined cycle for the production of electricity and process heat.

In this study, a pressurized oxygen-blown entrained flow gasifier has been considered, based on a planned demonstration plant in Piteå, Sweden. The total capacity of this plant will, after an increase in production, be 2000 t DS/d of which 550 t DS/d is gasified. This paper includes two parts. In Part 1 of the study, the increased power production has been simulated for five different cases, when integrating a gasifer into a pulp mill. ASPEN PLUSä has been used as the process simulator. The power efficiency is around 35% and the power production 20 MW. In Part 2, the changes of the energy content in black liquor have been calculated and discussed, when introducing polysulfides in the pulping process. It is shown that the heat value of the black liquor decreases with about 3% using polysulfides in the cooking process.