Cost-benefit assessment of black liquor gasifier/combined cycle technology integrated into a kraft pulp mill

Advances in black liquor gasification/combined cycle processes are bringing this technology closer to commercialization. Although the benefits of gasification have been described qualitatively, the forest products industry is understandably reluctant to commit the substantial resources required for commercialization without a more global and quantitative picture of the true costs and benefits of implementation. In this paper, detailed thermodynamic modeling of high-temperature gasification-combined cycle powerhouses are combined with mass and energy balance process models of the pulping and chemical recovery area. Coupled with capital and operating cost estimates, the result is an economic assessment of the impact of gasification on total mill operations under different operating scenarios. Capital and operating costs are estimated for new installations of a gasifier/combined cycle system with oxygen- or air-blown gasification and with different H₂S recovery systems. One scenario examines the potential to exploit the splitting of sodium and sulfur (inherent in gasification) in white liquor preparation. For a 1327 odmtpd installation, capital costs are roughly equivalent (±12%) to those for a comparable Tomlinson furnace. However, the 100% to 175% greater electric power generation with the combined cycle results in substantial annual operating credits. The lowest cost gasifier/combined cycle option considered here, air-blown gasification with recycle of H₂S to the gasifier, has a $20 million capital cost advantage over the Tomlinson furnace, while generating an additional $11.6 million per year in operating credits.