Rate and mechanism of black liquor char gasification with carbon dioxide at elevated pressures

Abstract

The kinetics of sodium-catalyzed CO[sub 2] gasification of carbon at elevated pressures were investigated using a char, produced from spent kraft pulping liquor, in which the catalyst is distributed more uniformly in the carbon matrix than in alkali-metal-impregnated carbons. Gasification measurements were carried out in a pressurized thermobalance at 700 C, 0.1--25 bar CO[sub 2] partial pressures, and 0--6 bar CO partial pressures. The rate of gasification was an order of magnitude or more higher than the rate of gasification of potassium-impregnated activated carbon and several orders of magnitude greater than for alkali-metal-impregnated coal chars. It was less than first order in CO[sub 2], strongly inhibited by CO, and strongly temperature dependent. The gasification kinetics followed the same mechanism as reported previously for potassium-catalyzed CO[sub 2] gasification of activated carbon in spite of large differences in the carbon source, catalyst type, and loading. The rate of gasification initially increased with increasing carbon conversion, but decreased with increasing conversion over most of the range of conversion. The change in rate with carbon conversion is apparently related to changes in the number of active carbon sites and catalyst sites. There was no loss of sodium from the char during gasification.