Coal pyrolysis : chemistry and kinetics in a flow reactor

Coal conversion technologies offer the potential to supplement and improve the supply of liquid fuels. Coal pyrolysis involves the thermal degradation of coal in an inert atmosphere to generate liquids (tar), gases and char. The ultimate commercial viability of such a conversion process must examine all of the quantitative (yield) and qualitative aspects of the products. A theoretical and experimental program was undertaken to study the variables affecting the product yield and selectivities from coal pyrolysis. A review was made of a variety of pyrolysis reactions of pure compounds possessing the same chemical functional groups as those identified in coal organic matter. A generalized hydrocarbon reaction flow diagram was developed to consolidate the reaction possibilities in coal pyrolysis and to attempt to derive a priori kinetic model. This led to rationalizations of observed trends and predictions which guided the experimental work undertaken. Experimentally, a complete lab scale pyrolysis system was designed and constructed. Investigations were directed at the effects of temperature, vapour residence time, coal loading, reactor area/volume ratios and oxygen on the product yields from the pyrolysis of Balraer coal from southeastern British Columbia. To assess the value of each product a number of analytical procedures were developed. Solvent partitioning, solubility, preparative and high performance liquid chromatography and infrared spectroscopy were used on the tars obtained. Chars were characterized by thermal gravimetric analysis. Pyrolysis off-gases were analyzed by gas chromatography. Highest tar yields were observed when dilute phase loading, short residence times (< 1 s) and intemediate temperatures (600-800°C) were employed. In all cases, however, improvements in liquid total tar yield were entirely attributable to the greater yield of undesirable asphaltenes. The yields of desirable tar components, aromatics and saturates, remained essentially unchanged over the entire temperature, time and loading range. Increasing the reactor area/volume ratio had a detrimental effect on tar yield but a slightly advantageous impact on the selectivity of aromatics. Char quality, as measured by the content of residual volatiles, showed a constant decrease with increasing pyrolysis temperature. In terms of overall product value, total tar yield is not the most reliable indicator of commercial viability. Rather, some considerations of product quality is required. Based on the combined experimental and theoretical consideration a number of recommendations are made for improved reactor design and future pyrolysis research.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat