OCR Text |
Show combustion of the char is believed to be a further delaying action as the petroleum coke closes off the coal char pores. Thermograms for bituminous and lignitic COMs appearing in Figures 18 and 19 indicate COMs containing coal in excess of 30 percent take longer to burn out than the raw coal sample. As the porosity is increased to that of lignite, the burnout time is greatly increased for all mixtures with coal concentrations as low as 10 percent. An examination of the residual char frozen at various stages of combustion indicated the vanadium was retained in the unspent particles of fly ash and char. Thermal analysis implies that combustion of COMs rich in coal may be delayed, lengthening the flame and subjecting the upper reaches of the furnace to slagging by coal ash. Coals rich in iron may be particularly troublesome because of their inherently low melting temperatures under reducing conditions, compounded by the presence of oxides of vanadium. Washable coals in which pyrites exist as independent species of iron would appear to be the most troublesome. These conclusions are speculative and require confirmation by field trials. CONCLUSIONS Conversion of oil-fired steam generators to direct firing of coal requires steam generator derating to accommodate the slower burning coal without encouraging excessive furnace exit temperatures, furnace slagging, convection pass fouling, or carbon loss. The degree of derating will depend upon the fuels and ash chemistry for which the steam generator was originally designed (i.e., oil firing or coal firing with the ability to convert to oil) and the fuels and ash chemistry of the coal to which the conversion is being made. 1-58 |