Characteristics of co-combustion of strongly caking and non-caking coals in a pilot circulating fluidized bed combustor (CFBC)

Abstract

Co-firing of Russian caking coal with non-caking coals (mixtures of Australian bituminous and Korean anthracite coals) was performed on a pilot scale using a circulating fluidized bed combustor (CFBC). The free swelling index (FSI) of caking coal was very high and the alkali index of non-caking coals was relatively high. However, stable fluidization and combustion were attained using various coal blends (containing up to 20 wt% of strongly caking coal). The average particle size in bottom ash increased slightly as the proportion of caking coal increased, triggering a slight temperature rise in the lower part of the combustor. Scanning electron microscopy/energy dispersive spectrometry and X-ray diffraction analyses of coarse particles showed that Al-rich bottom ash captured problematic K to form kalsilite (KAlSiO4), with a melting point much higher than the operating temperature of a typical CFBC. Such data strongly suggest that stable CFBC co-combustion is possible using mixtures featuring strongly caking coals. The unburnt carbon content of fly ash and the O2 concentration in flue gas at the steady state increased linearly as the proportion of caking coal increased, indicating reduced co-combustion of caking coal. Morphological analyses of unburnt carbon in bottom ash revealed that pore clogging progressed on addition of caking coal, decreasing char reactivity by blocking oxygen access during co-combustion. In terms of pollutants, NO emission from fuel N in flue gas increased slightly on addition of caking coal, but N2O emission decreased slightly, attributable to a temperature rise in the dense combustor bed. However, the conversion rate of fuel S to SO2 did not significantly change when the proportion of caking coal increased.