False deformation temperatures for ash fusibility associated with the conditions for ash preparation

A study was made to investigate the fusibility behaviour of coal ashes of h igh ash fusion temperatures. Coals and ashes formed in the boiler were samp led in several Australian power stations, with laboratory ashes being prepa red from the coals. The laboratory ashes gave lower values for the deformat ion temperature (DT) than the combustion ashes when the ash had low levels of basic oxide components. Thermo-mechanical analysis quantitative X-ray di ffraction and scanning electron microscopy were used to establish the mecha nisms responsible for the difference. Laboratory ash is finer than combusti on ash and it includes unreacted minerals (such as quartz, kaolinite and il lite) and anhydrite (CaSO4). Fusion events which appear to be characteristi c of reacting illite, at temperatures from 900 to 1200 degrees C, were obse rved for the laboratory ashes, these being associated with the formation of melt phase and substantial shrinkage. The combustion ashes did not contain this mineral and their fusion events were observed at temperatures exceedi ng 1300 degrees C. The low DTs of coal ashes with low levels of basic oxide s are therefore a characteristic of laboratory ash rather than that found i n practical combustion systems. These low temperatures are not expected to be associated with slagging in pulverised coal fired systems. (C) 1999 Else vier Science Ltd. All rights reserved.