Pressure Effects on Black Liquor Gasification

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Young, Christopher Michael
Iisa, Kristiina
Frederick, William J.
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Gasification of black liquor is an alternative to the combustion of black liquor, which is currently the dominant form of chemical recovery in the paper industry. Gasification of black liquor offers the possibility of higher thermal efficiencies than combustion, reducing manufacturing costs and creating new revenue streams through a forest biorefinery. Pressurizing the gasification reactor further enhances the efficiency advantage of gasification over combustion. This study uses a pressurized entrained flow reactor (PEFR) to study black liquor gasification behavior under pressures, temperatures, and heating rates similar to those of next-generation high-temperature black liquor gasifiers. The effects of pressure on black liquor char morphology, gasification rates, pyrolysis carbon yields, and sulfur phase distribution were studied. These characteristics were investigated in three main groups of experiments at 900oC: pyrolysis (100% N2), gasification with constant partial pressure (0.25 bar H2O and 0.50 bar CO2), and gasification with constant mole fraction (10% CO2, 2% H2O, 1.7% CO, 0.3% H2), under five, ten, and fifteen bar total pressure. It was found that pressure had an impact on the char physical characteristics immediately after the char entered the reactor. Increasing pressure had the effect of decreasing the porosity of the chars. Pressure also affected particle destruction and reagglomeration mechanisms. Surface areas of gasification chars decreased with increasing pressures, but only at low carbon conversions. The rate of carbon conversion in gasification was shown to be a function of the gas composition near the particle, with higher levels of inhibiting gases slowing carbon conversion. The same phenomenon of product gas inhibition observed in gasification was used to explain carbon conversions in pyrolysis reactions. Sulfur distribution between condensed and gas phases was unaffected by increasing total pressure in the residence times investigated. Significant amounts of sulfur are lost during initial devolatilization. With water present this gas phase sulfur forms H2S and did not return to the condensed phase.
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