Investigations into the Air Heater Ash Deposit Formation in Large Scale Pulverised Coal Fired Boiler

Abstract

A mineralogical study was undertaken of air heater deposits in a 300 MWe pf boiler located in Western Australia to understand the deposit formation in air heater sections of boiler as an aid to implement possible remediation actions. Several air heater deposit samples were collected in the selected regions of the air heater along with samples of the feed coal, bottom ash and fly ash for comparison of ash chemistry and mineralogy. The deposit samples were examined using a combination of ash chemistry and quantitative X-ray diffraction analysis; the ash samples by bulk chemical analysis to determine the major element chemistry and mineralogy by quantitative X-ray diffraction. Samples of the deposits were also analysed using optical microscopy and QEMSCAN, an automated electron beam image analysis system. Chemical and mineralogical analysis showed that the deposits are unusual in containing high amounts of sulphate, particularly of aluminium and, to a lesser extent, iron. From the analyses it appears that the formation of the deposits is due to the high sulphate content which is acting as a cementing agent. There is an indication that temperature could be another factor in formation of the deposits, with a decrease in temperature leading to the formation of sulphurous acid which then reacts with the reactive glassy amorphous fly ash phase to form the aluminium and iron sulphates. Dew point calculations indicated that this is a possible deposit formation mechanism based upon air heater temperature data obtained from the utility. There was no evidence that unburnt carbon has played a significant role in deposit formation. Large temperature fluctuations resulting from the inherent nature of the operation of the air heater are a significant factor in deposit formation and a practical solution to consider would be the use of an SO2 absorbent placed prior to the air heater.