Formation of SO3 under oxy-fired fluidised beds - POSSIBLE RESTRICTED THESIS

Abstract

In oxy-fuel combustion, fuel is burned in the presence of higher concentrations of oxygen diluted with the recycled flue gas (RFG). The nitrogen fraction in the air is replaced by the recycled flue gas to moderate the combustion temperature in the furnace. Nitrogen is removed from the mainstream by the air separation unit (ASU) to provide nearly pure oxygen. However, combustion environment of fuel with RFG differs significantly from conventional air combustion due to the high heating capacity of CO2. In typical air combustion process, sulfur in fuel is converted to sulfur dioxide (SO2), while a smaller fraction is converted to the sulfur trioxide (SO3). However, during the oxy-fuel combustion with RFG tends to increase the SO2 concentrations in the combustion furnace. Thus, higher concentrations of SO3 along with SO2 and steam may increase risks associated with the corrosion of metal surfaces in the boiler. This occurs as the fraction of SO3 reacts with steam to form sulfuric acid at low temperatures. The aim of this PhD thesis is to evaluate the formation of SO3 concentrations during the oxy-fuel combustion. Thus, SO3 concentrations were measured experimentally under air and oxy-fuel environments. To reach the goals of this work, bench-scale bubbling fluidised bed and fixed bed rigs were used to measure SO3 concentrations. At a larger scale, a pilot-scale 50 kW circulating fluidised bed rig (CFB) was employed. The PhD thesis provides a literature survey on SO3 formation in air and oxy-fuel combustion environments, the development of SO3 sampling train and an experimental description of the tests. Finally, the results obtained from each rig have been discussed. As supporting case, fly ashes were analysed in TGA to investigate the amount of unburnt carbon content. The collected fly ash after the tests was scanned by using X-ray fluorescence (XRF) to evaluate the change of metal oxides.