Influence of coal particle size on particulate matter emission and its chemical species produced during coal combustion

Pulverized coals were burnt in a laboratory-scale drop tube furnace to study emission of particulate matter (PM) in this process. Reaction temperature of 1200 °C, residence time of 2.4 s and an atmosphere, 20% O2 and N2 being balance, were used as the combustion condition. Three coals were divided into three sizes, 125-250, 63-125 and <63 μm, and subjected to combustion under above-mentioned conditions. PM was size-segregated by low-pressure-impactor (LPI) into different sizes ranging from 10.0 to 0.03 μm. Besides its properties including concentration, particle size distribution and elemental composition, PM having three sizes was also subjected to CCSEM analysis to quantify the chemical species within them. The results indicate that, coal particle size affects PM emission significantly in the combustion. Decreasing coal particle size leads to formation of much PM. For coals of 125-250 μm, a single mode distribution of PM was formed, which has the central mode at approximately 4.0 μm with an edge of 10.0 μm. For coals of 63-125 μm, the resultant PM had the similar distribution to that for coals of 125-250 μm. In contrast, with coal particle size down to < 63 μm, a bimodal mode distribution of PM was formed. The large mode at 4.0 μm, mainly of aluminosilicate, was formed by the direct transferring of inherent excluded minerals into PM. Decreasing coal leads to prevalence of excluded fine mineral particles within it, and accordingly, more the fine mineral directly transferred into PM. Another mode was formed at about 0.5 μm, which was caused by both fragmentation of inherent minerals and vaporization of heavy metals within coal. Aluminosilicate salts are relatively abundant in this size, and their surface was coated with condensed heavy metallic vapors. In addition, the amount of PM having the size around 0.13 μm was also found relatively great in the case of combustion of coal < 63 μm, it was mainly composed of vaporized alkali, heavy metals and their sulfate, chloride and phosphates.