G. W. Bryant

Thermal conductivity of coal ash and slags and models used

Abstract A one-dimensional heat transfer method was used to determine the thermal conductivity for a range of coal ash and synthetic ash samples at elevated temperatures. The effect of parameters such as temperature, porosity, and sintering time were investigated. The thermal conductivity of the samples was generally observed to increase with increasing temperature. During heating of the samples, softening of minerals and sintering reactions resulted in changes in the physical structure of the ash, which then altered the observed thermal conductivity. The thermal conductivity of sintered ash samples was found to be higher than that of unsintered samples. The sintering temperature and sintering time were found to increase the observed thermal conductivity irreversibly. A decrease in sample porosity was also observed to increase the thermal conductivity. Chemical composition was found to have little effect on the thermal conductivity, apart from influencing the extent of sintering. Predictions of the thermal conductivity of ash samples based on Rayleighs model are also presented. The thermal conductivity of slag and particulate structures was modelled by considering spherical pores distributed in a continuous slag phase. A particulate layer structure was modelled by considering solid particles dispersed in a continuous gas phase. The Brailsford and Major model of random distribution for mixed phases gives results within 20% of the measured values for a partially sintered sample.

The effect of potassium on the fusibility of coal ashes with high silica and alumina levels

Abstract The poor repeatability and reproducibility of fusibility temperatures of the ashes with high (SiO 2 + Al 2 O 3 ) levels are well known. A study was, therefore, made of such ashes using thermo-mechanical analysis (TMA) to indicate the shrinkage of samples progressively heated at the same rate as the test for standard ash fusibility temperatures (AFT). The results confirm that the extent and rate of change of shrinkage of the samples with low K 2 O levels ( wt. %) is low in regions of the deformation temperature (DT), resulting in poor accuracy of its determination. It was found that DT is mainly related to substantial melting of minerals containing K 2 O (i.e. illite) in these ashes. The scanning electron microscope (SEM) analysis of quenched ash samples and thermodynamic calculations were used to illustrate the effect of K 2 O in terms of the extent of slag generation necessary for shrinkage. The TMA test is shown to provide a more sensitive alternative procedure for characterizing the ash fusibility.

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

Abstract A study was made to investigate the fusibility behaviour of coal ashes of high ash fusion temperatures. Coals and ashes formed in the boiler were sampled in several Australian power stations, with laboratory ashes being prepared from the coals. The laboratory ashes gave lower values for the deformation temperature (DT) than the combustion ashes when the ash had low levels of basic oxide components. Thermo-mechanical analysis quantitative X-ray diffraction and scanning electron microscopy were used to establish the mechanisms responsible for the difference. Laboratory ash is finer than combustion ash and it includes unreacted minerals (such as quartz, kaolinite and illite) and anhydrite (CaSO4). Fusion events which appear to be characteristic of reacting illite, at temperatures from 900 to 1200°C, were observed 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 exceeding 1300°C. The low DTs of coal ashes with low levels of basic oxides are therefore a characteristic of laboratory ash rather than that found in practical combustion systems. These low temperatures are not expected to be associated with slagging in pulverised coal fired systems.

THERMOMECHANICAL ANALYSIS AND ALTERNATIVE ASH FUSIBILITY TEMPERATURES

The traditional measurement of the temperatures of fusibility of coal ash, which are called ash fusibility temperatures (AFT), are shown in correspond to the existence of more than 60% of melt phase in the samples. These temperatures do not appear to correspond to the ash melting characteristics often associated with them. It was found that the deformation temperature is not the temperature at which initial melting begins as normally perceived and the hemisphere temperature is below the liquidus temperature.