LARGE-PARTICLE COAL DEVOLATILIZATION AND TAR PRODUCTION FROM LUMP COAL. (VOLUMES I AND II)

Theoretical and experimental studies of coal devolatilization and tar production were carried out.; Reduction or elimination of tars from the fixed-bed gasification of lump coal could improve process efficiency. The limited literature on tar production in the devolatilization zone of fixed-bed gasifiers provided neither a clear understanding of nor adequate models for lump-size coal devolatilization.; The fundamental chemical and physical processes which occur during coal devolatilization were examined in the theoretical study. The study includes all "large-particle" conditions (i.e. those for which physical effects are important) as well as lump coal devolatilization. Heat transfer, chemical reaction, volatiles release and volatiles transport processes were identified and the literature was examined for data and models in each process class. Characteristic times and their ratios ("eigenratios") were useful in identifying the controlling process or processes at different stages during the devolatilization and as a basis for devolatilization modelling.; Existing decomposition and devolatilization models were examined, and their utility in fixed-bed gasifier modelling was discussed. Other topics examined included: the fundamental roles of pressure, particle size and heating condition, validity criteria for lumped solid decomposition models, the role of physicochemical competition in determining product composition, and the nature of the initial softening of coal during devolatilization.; An apparatus was designed, built and operated for the captive-sample devolatilization of lump coal. Single lumps or a small number of lumps of coal were lowered into a pre-heated 2-inch diameter stainless steel pipe through which hot nitrogen was flowing. Heating of the coal was by convection from the gas flowing past the sample and by radiation from the walls.; Tar yields were obtained for the devolatilization of a caking Pittsburgh #8 high-volatile A bituminous coal in the range of 15 to 50 psia. Tar yields were generally higher than those in a fixed-bed gasifier; the differences between the experimental devolatilizer and the fixed-bed gasifier were discussed. Tar yield increased from 9.1% to 13.3% when the superficial gas velocity was increased from 0.3 to 2.4 m/s. Tar yield underwent a broad maximum at around 500(DEGREES)C and coke decreased with temperature over the range of 430 to 550(DEGREES)C. There appears to be a reduction in tar yield with an increase in lump size from 3 to 15 g (1.7 to 2.8 cm), although the difference over the range employed was comparable to the data variability. No effect of pressure on tar yield was observed because of the limited range employed in this study. Tar production peaked earlier than gas production, and the products of tar decomposition were coke and gas in the ratio of 4 to 1. There was no evidence of tar cracking to gas either at or downstream from the coal. Maximum values of the weight loss rate, surface efflux velocity and Peclet number (Pe(h)) were 2.2 wt.%/min, 0.7 cm/s and 0.2, respectively. The volatiles efflux had minimal influence on the rate of heat transfer. The general reaction pattern and the degree of swelling varied with process conditions in a manner consistent with the theoretical analysis. Recommendations were made for an improved lump captive-sample devolatilizer and for extension of this work.