Effect of Mineral Matters in Coal on Formation Behaviors of Particulate Matter and Alkali Metal Compounds During Coal Combustion

Combustion tests for 2 types of coal (SC and BT coals), which have similar combustion performance and main fraction in the ash compositions, were conducted, using an electrically heated drop tube furnace. The burning particles were sampled by the Low Pressure Impactor (LPI) and analyzed concentration of alkali metal in the collected particles. Furthermore, they were observed by a scanning electron microscope (SEM) to discuss formation behaviors of the fine particles during combustion. In order to elucidate the relationship between formation characteristics of fine particulates during combustion and coal types, analysis of included and excluded mineral particles in the raw coals was conducted by a Computer Controlled SEM (CCSEM). Additionally, the composition analysis of the mineral particles in the raw coal was also carried out, using an energy dispersive X-ray spectrometer (EDS). As a result, the particle size distribution of ash particulates formed was different each other for two types of coal. Especially for the fine particulates with the size of less than 1 μm, the result for SC coal showed much more fine particulates formation than those for BT coal. Sodium compounds were enriched in the fine particulates for both the coals. Shapes of the fine particles with the size less than 1 μm for both the coals were spherical. For the coarse particles, however, spherical particles were observed for only SC coal. This difference was due to difference of the characteristics of mineral particles in the raw coals. The CCSEM analysis indicated that the excluded mineral matters, which tended to fragment during combustion, were contained more in SC coal than those in BT coal. This is the reason why SC coal forms more fine particulates during combustion than BT coal. From the EDS analysis, moreover, content of sodium in the excluded minerals of SC coal was higher than that of BT coal. This result suggests that SC coal tends to form the spherical fine particulates, in which the sodium compounds are enriched, during combustion.