Gasifying black liquor enables it to be used as fuel in a gas turbine combined cycle, a much more efficient electricity generating option than the Tomlinson boiler steam turbine technology. In a typical “mill scale” black liquor gasification combined cycle (BLGCC), shown in Figure 9.19, the black liquor is gasified, and the syngas product is cooled, cleaned, stripped of H2S (using a Selexol® unit), and then burned in the gas turbine. The turbine exhaust passes through a “duct burner” to the heat recovery steam generator (HRSG), where steam is raised to drive a steam turbine. A small amount of natural gas is burned in the duct burner to enable production of the requisite process steam needed to run the mill. Steam is extracted at two different pressures from the steam turbine. The HRSG steam production is augmented by steam delivered from hog fuel boilers assumed to be pre‐existing at the reference mill. One design constraint in the BLGCC analysis was the size of the existing hog fuel boilers, which limited the available steam delivered from these boilers (This limitation is the reason a small quantity of natural gas is used in the duct burner (Black Liquor, Wikipedia, 2016).
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