Formation and behavior of submicron fly ash in pulverized coal combustion furnace

Abstract

In this study, the formation mechanism and the behavior of submicron fly ash (SFA) (larger than 0.1 μm) in the industrial type pulverized-coal combustion furnace was examined experimentally by using two different models of furnace: a turbulent furnace, which was designed to realize practical combustion conditions, and a small, laminar flow furnace. Observations were made by an electron micrograph, x-ray microanalysis, and infrared spectroscopic analysis of the submicron particles sampled from the furnace, and measurements of the concentration distribution of the suspended and coagulated SFA were carried out. Both the electron micrographs and the infrared spectroscopic analysis showed that the submicron particles sampled near the concentration peak in the furnace closely resembled raw coal. The major components (aluminum, silicon, calcium, and iron) in the ash contained in the submicron raw coal fragment was compared with the SFA sampled at the flue for six different coals. The composition of the SFA was mostly compared to that of the raw coal fragment. There was no silicon condensation in these coals, with one exception. The measured concentration distribution of the SFA in the furnace showed that its peak concentration level in the furnace was two or three times higher than that calculated from the SFA content in coal, assuming that no formation or extinction of the SFA occurs in the region of coal combustion. From these results, we concluded that approximately half of the SFA in the furnace is simply carried over from the submicron coal fragment originally contained (3.5–9.5 wt%) in the pulverized raw coal, and that the remaining SFA is newly formed in the furnace through the breakup of larger coal particles in the devolatilization region. However, the breakup ratio was dependent on the primary air velocity at the burner exit. The percentage of the submicron coal fragment originally contained in the pulverized raw coal that became suspended in the furnace was estimated to be less than 10 wt% under actual flow conditions. The effect of the submicron coal fragment in the pulverized raw coal on the emission of suspended SFA was clearly shown using a small laminar furnace.