The effect of process conditions on coal pyrolysis and char reactivity

Abstract

The effect of process conditions on coal pyrolysis and char reactivity is systematically studied using the thermogravimetric reactor equipped with in-situ video microscopy imaging (TGA/VMI). This system provides a complete time-registered record of the history of single coal particles as they are sequentially pyrolyzed and combusted in the reactor. Three coals are investigated: Illinois #6, Utah Blind Canyon, and Wyodak-Anderson seam. An image analysis procedure was developed to monitor the transient swelling pattern of the two plastic coals (Illinois and Utah). Particle swelling increases with increasing pyrolysis heating rate and increasing particle size. When pyrolyzed in reactive atmospheres, these coals exhibit less plasticity and less "bubbling", but they swell more. The maximum devolatilization rate occurs during the stage of vigorous "bubbling." Since the Wyodak coal is not plastic, it does not swell during pyrolysis. Process conditions affect char ignition behavior. Particles will ignite more easily when the observed reaction rates are high and heat removal rates are low. During combustion in the presence of intraparticle diffusional limitations, we observed that large char particles and chars produced at high pyrolysis heating rates ignite more easily. Increased particle sizes slow the heat removal rates, while high pyrolysis heating rates produce chars with larger macroporosities and macropore surface areas thus enhancing the reaction rates. Chars produced in reactive pyrolysis atmospheres appear to be more reactive and ignite more easily. Experiments at low temperatures show that the pyrolysis heating rate, particle size, and pyrolysis atmosphere do not affect the reactivity pattern in the kinetic control regime. We also developed a steady-state mathematical model for char particle ignition. Model parameters were obtained from structural measurements for the Illinois #6 chars. The model predicts that the ignition temperature decreases with decreasing gas flow rate and increasing pyrolysis heating rate, increasing oxygen concentration and increasing particle size. Model predictions are in excellent agreement with our experimental data