Characterization of Ash Particles from Co-combustion with a Zhundong Coal for Understanding Ash Deposition Behavior

Abstract

Co-combustion is the most attractive option for extending the utilization of Zhundong coals from the newly discovered and the largest intact coalfield in China. However, operational practices have shown that power plants frequently encounter ash deposition problems during co-combustion with Zhundong coals. To address such an issue, in the present work, coal blends of a bituminous coal and a Zhundong sub-bituminous coal, with blending ratios of 90:10, 70:30, 50:50, 30:70, and 10:90 on a weight basis, were burned on a laboratory drop tube furnace at 1350 °C. For comparison, combustion experiments of the component coals were also carried out under the same conditions. The resulting ash samples were thoroughly characterized by using a Malvern particle size analyzer and a computer controlled scanning electron microscope. The obtained data were correlated to the ash deposition behavior in co-combustion with Zhundong coals in power plant boilers. The results show that particle mass size distributions of the ash samples from combustion of low-ZD-loaded fuels (with the proportion of the Zhundong coal investigated ≤50 wt %) are similar. The basic and acidic elements are partitioned similarly into ash particles. Ash deposition propensities, evaluated as the ratio of basic to acidic oxides (B/A) of the ash, are all low and show insignificant differences. These are consistent with the similarities in ash deposition behavior during co-combustion with Zhundong coals with low proportions in practical coal-fired boilers. In contrast, the ash properties are apparently different for the fuels with the proportion of the Zhundong coal higher than 50 wt % (denoted as high-ZD-loaded fuels). Small particles of <10 μm are more abundant in the ash samples and are more enriched in basic elements (especially Ca, Fe, and Mg) for the high-ZD-loaded fuels than for the low-ZD-loaded fuels. These data could well explain the more serious ash deposition problems arising during co-combustion with higher proportions of the Zhundong coals in practical boilers. It is also found that, for the ashes from combustion of the high-ZD-loaded fuels, both the amount of the small ash particles and the contents of basic elements in them increase with increasing the proportion of the Zhundong coal in the fuel. These data agree well with field observations of higher ash deposition propensities for coal blends with higher proportions of the Zhundong coals.