Lignite hydrogasification kinetics

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Title: Lignite hydrogasification kinetics
Author: Baker, Daniel C
Abstract: 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 -100°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 -1000°C . Comparisons with pyrolysis data show that no hydrogasification occurs below 500°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 .
URI: http : / /hdl .handle .net /2346 /8593
Date: 1984-05

Citation

Baker, Daniel C Lignite hydrogasification kinetics. Master's thesis, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /8593 .

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