Investigation into fly ash production during co-firing of biomass with pulverized coal

Muanza, Nkongolo
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This dissertation considers the behaviour of particulate fly ash produced during cocombustion experiments of biomass materials with pulverized coal in a 1 MWth pilotscale combustion test facility (PSCTF). Particular attention is generally given to fly ash particles of diameters less than 10 and 2.5 microns (namely PM10 and PM2.5). These small particles have the potential for affecting human health and forcing climate change because of their ability to scatter and absorb light and also to act as cloud condensation nuclei. South African coal has high ash content that consequently affects the ash burden and the efficiency of ash removal system. Previous research work reports increase of the concentration of fine particles during the co-firing of biomass with coal, thus limiting the amount of biomass co-fired. Coal and two types of biomass, grass and sawdust, were used in this study. The coal chosen was representative on the basis of the annual average calorific value of coals burned at ESKOM’S coal-fired power stations. For each biomass, the ratios of biomass to coal used on an energy basis were 10%:90%, 15%:85% and 20%:80%, resulting in a total of seven different tests including coal alone. Seven tests with similar fuels were also carried out using a drop tube furnace (DTF) to determine their reaction kinetics for the combustion simulation. The experimental results revealed that the grass and sawdust blends showed decreases of PM10, and PM2.5 particles percentages compared to the coal baseline. The grass because of its high content of alumina-silicate showed considerable agglomeration whereas sawdust blends gave minor increase of PM10 under high pressure condition inside the combustion chamber. The pulverised-coal fineness, flue gas temperature and excess air were found to affect the particulate matter behaviour. The fly ash samples collected were also analysed by scanning electron microscope and spectrometry; alkali metals released were observed to react with the alumino-silicate phase. The fine sulphate enriched particles formation during combustion process was 4 modelled based on the Glarborg-Marshall mechanism using CFD tool. The simulation results were validated by the experimental data from the pilot-scale combustion test facility.