GASIFICATION OF BLACK LIQUOR CHAR WITH STEAM AT ELEVATED PRESSURES

The kinetics of the gasification of black liquor char by steam were investigated by means of a pressurized thermogravimetric reactor over the range 600-675 degrees C, 1-30 bar (0.1-3.0 MPa) total pressure. The experiments were performed either with or without one of the reaction products (H-2, CO) present and under conditions such that film mass transfer and pore diffusion did not affect the overall rate. The rate of gasification was found to decrease with increasing pressure, by a factor of 3.5 over the pressure range 2-30 bar (0.2-3.0 MPa). This decrease is presumably due to the increase of inhibiting effects of the products, hydrogen and carbon monoxide, with increasing pressure. The reaction was found to be of order 0.56 in steam partial pressure. The presence of either hydrogen or carbon monoxide slowed the reaction, with carbon monoxide being the stronger inhibitor The reaction is very temperature sensitive. The apparent activation energy was found to be 230 kJ/mol, which corresponds to an increase in the rate by a factor of roughly 30 over the temperature range 600-700 degrees C. A comparison with carbon dioxide gasification of black liquor char shows that the rate of steam gasification is 3.8 times higher at equivalent CO2 versus H2O partial pressures, with no H-2 or CO present in the reacting gas. The rate of steam gasification could be predicted by means of a model developed for catalytic gasification of carbon.