Textural properties of chars as determined by petrographic analysis: comparison between air-blown, oxygen-blown and oxygen-enriched gasification

Abstract

In this study, the textural properties of chars generated from a vitrinite, high ash coal in a fluidised bed gasifier under air-blown, oxygen-blown and oxygen-enriched conditions were determined by detailed petrographic analysis. The char samples were assessed in terms of their microscopic characteristics (reflectance properties, carbon rich forms (char types) and basic forms of visible minerals). The chars formed under oxygen-blown conditions exhibited the lowest in overall mean reflectance value and also possessed the lowest proportion of chars with reflectance values above 4% RoVr relative to those in the air-blown and oxygen-enriched samples. The oxygen-blown sample also possessed the lowest total carbon forms (53%) whereas the oxygen-enriched and air-blown chars had higher proportions of organic matter (58% and 60% respectively). Melted ‘‘slag’’ minerals were more common in the oxygen-blown char, totalling 31%, compared with 24% in the other two samples. These results suggest that a greater degree of consumption of carbon took place under the oxygen-blown conditions, possibly with higher particle temperatures, and that under these conditions, the highly reflecting porous thin-walled chars would have been consumed first and fastest. This would have left the higher proportion of partially consumed chars and inertinites behind which, in turn, would have lead to the occurrence of lower reflectance readings. These conditions also resulted in the presence of higher proportions of melted ‘‘slag’’ minerals in the oxygen-blown sample. The oxygen-enriched and air-blown chars appear to have undergone reduced levels of carbon consumption and at lower particle temperatures relative to the oxygen-blown sample. Of these two samples, the air-blown sample had the lowest rate of consumption of all three conditions. This, in turn, implies that the temperatures operating under these conditions were the lowest of all. In summary, it would appear that the conditions under which the most rapid rates of consumption would occur, in decreasing order of reactivity and particle temperature, would be (i) oxygen-blown, (ii) oxygen-enriched and (iii) air-blown. These results have important implications for the future, namely, (i) to ensure the highest optimum efficiencies in future gasification processes by selecting the most appropriate gaseous environments and (ii) to prevent or minimise the occurrence of slagging in future gasification processes. This is specifically applicable to operations involving fluidised bed technologies and may well be of value in countries in which high ash coals will be the primary energy source in such technologies.