A Comparison of Superheat and Reheat Steam Cycles with Black Liquor Gasification for Pulp Mill Power Generation, 2010 TAPPI/PAPTAC International Chemical Recovery Conference

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Pulp mills have used recovery boilers to burn the waste products of the pulping process (black liquor), and reconstitute the chemicals used in the pulping process since the 1930s. Black liquor is a biomass-derived fuel produced from the lignin that is in the feedstock to the pulping process. The steam generated from the boiler is primarily used to supply the pulping process and also to generate electricity. Rising energy costs and incentives for energy generated from renewable sources have motivated pulp mills to improve the electrical power generating potential from the black liquor.

The traditional Tomlinson recovery boiler efficiency has been improved by increasing the liquor’s percent firing solids, and the generating efficiency has been improved by increasing superheater steam pressures and temperatures.

Black liquor gasification is a process that replaces the Tomlinson recovery boiler by gasifying the liquor and delivering the gas to a boiler, combustion turbine, or some other process. Some studies have shown that burning the gases in a gas turbine that exhausts into a heat recovery steam generator (HRSG) would generate more power and would be financially more attractive than traditional mill steam cycles. This process is known as black liquor gasification combined cycle (BLGCC). BLGCC has yet to be successfully implemented. Gasification projects have been abandoned for varying reasons, and no project has been able to burn the product gas in a gas turbine.