(545c) Process Considerations When Selecting Black Liquor Gasifiers

Gasification of black liquor offers several distinct advantages. First, the gasifier product gas can be used to produce more electrical power than can be obtained from conventional recovery boilers and steam power cycles, or to produce higher value products via conversion of the syngas produced. Second, the split of sulfur between the gas and condensed phases opens the possibility for alternative pulping processes with higher pulp yields.

The cost of a gasification and chemical recovery island, the yield of power and/or higher value products from syngas, the efficiency of chemical recovery, gas clean-up requirements, and process operability depend largely on the conditions at which black liquor is gasified. For example, the residence time for near-complete conversion of char carbon to gases decreases rapidly with increasing temperature, but increases with increasing pressure, thus offsetting the expected decrease in gasifier volume with increasing pressure. Also, the fraction of sulfur recovered in the gas phase increases substantially with pressure.

In this paper, we examine the impact of process conditions ? temperature, pressure, etc. - on

?carbon conversion: residence time requirements vs T, P

?tar production: yields and composition vs T; toxicity of tars; options for reducing or disposing of them

?sulfur speciation and split between gas and condensed phase

?the advantages of converting to polysulfide pulping

?control and removal of K and Cl from spent pulping liquors

?benefits of reduction of NOX emissions

We then evaluate the impact of these on the selection of gasifier conditions for operability and to meet specific product demands ? power, fuels or chemicals.