Lignite hydrogasification kinetics
Baker, Daniel C.
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Intrinsic reaction rates for the hydrogasification of Texas lignite and prepyrolyzed lignite char have been measured using a batch, tubular-flow reactor. Data from experiments performed using moderate heating rates (50-100Â°C/S) and low operating pressure (0.9 atm) have been collected. These data indicate the existence of three distinct intervals of hydrogen-solid reaction that occur for both lignite and char over a temperature range of 500-1000Â°C. Comparisons with pyrolysis data show that no hydrogasification occurs below 500Â°C. Rate curves were obtained at various temperatures within each interval of hydrogasification. Ultimate analyses of residual solids from these intervals show the changes occurring in the composition of lignite and char as they are gradually gasified. Findings show that the portions of char gasified at lower temperatures have a higher hydrogen-to-carbon molar ratio than the remaining residual solid. Nitrogen compounds are preferentially gasified at all hydrogasification temperatures. Residual solids from the intermediate and high temperature reaction intervals have the same relative amounts of oxygen and hydrogen. Experiments in which hydrogen partial pressure is varied indicate that the apparent reaction order with respect to hydrogen partial pressure changes from 1.2 in the low temperature reaction interval to 1.6 in both of the higher temperature intervals. Various physical properties are measured and estimated for lignite and char at various stages of hydrogasification. Particle size is shown to remain constant throughout hydrogasification. This is attributed to the formation of a rigid, but porous, skeletal particle structure consisting of mineral ash and residual organic material. A comparison of several simple gas-solid reaction models is done using experimental rate data. These models include empirical, shrinking core, uniform conversion, random pore, and particle-pellet models. Results indicate that the uniform conversion model represents hydrogasification fairly well in the low and intermediate reaction temperature intervals. Behavior in the highest reaction temperature interval is best represented by the particle-pellet model for the case of nonporous cylindrical grains. Experimental data indicate that changes of internal pore surface area occur during all three intervals of hydrogasification and that these changes affect the reaction rate in each interval. Kinetic parameters are calculated and compared for all lignite and char reactions using best fitting models and the uniform conversion model with constant surface area. The comparison shows that surface effects do not greatly alter the kinetic parameters for the low and intermediate reaction intervals. Ignoring surface effects occurring during the high temperature reaction interval is shown to lead to significant differences in the kinetic parameters for lignite and char residues. Inclusion of these effects results in very similar kinetic parameters for lignite and char. Activation energies are 22.1 kcal/mole, 40.1 kcal/mole, and 53.1 kcal/mole for lignite and 34.3 kcal/mole, 48.0 kcal/mole, and 51.5 kcal/mole for char in the low, intermediate, and high temperature hydrogasification reaction intervals, respectively.