CO-COMBUSTION OF COAL AND OIL SHALE BLENDS IN CIRCULATING FLUIDIZED BED BOILERS

Coal co-firing experiments were conducted in a 250 MW oil shale fired circulating fluidized bed combustion (CFBC) boiler. The objective of the experiments was to test whether adding coal to oil shale would allow the use of the latter with lower heating value. Bituminous coal was mixed with oil shale and fed into the boiler via existing fuel feeding ports. Two test series were accomplished: 11-29% thermal input of coal mixed with 8.4 MJ/kg oil shale (standard fuel), and 12-32% thermal input of coal mixed with 7.5 MJ/kg oil shale. During the experiments, which lasted in total for 15 days, ash samples were collected and flue gas analysis was performed. The boilers were able to continue work with all the fuel mixtures, but a significant increase of nitrogen oxides (NOx) emissions and heat losses due to unburnt carbon in the bottom and fly ashes were observed. The heat losses can be reduced by upgrading the fuel preparation system, but NOx emissions limit can be reached only with installation of an additional DeNO(x) system. The ash chemical composition remained similar. Sulphur emissions stayed minimal, but a slight increase of carbon monoxide concentration was noticed. Coal co-firing is possible in oil shale CFBC boilers, but the coal must have low fuel nitrogen content and extra attention to the fuel preparation system has to be paid.