SO3 formation and the effect of fly ash in a bubbling fluidised bed under oxy-fuel combustion conditions

•There is evidence of catalysis to form SO3 even at the low temperature of 400 °C.•SO3 formation was greater with increased temperature and SO2 concentration under tests not catalysed by fly ash.•Oxy-fuel SO3 formation catalysed by fly ash exceeded that in air-fired combustion, especially at higher SO2.•SO3 formation is significantly influenced by fly ash composition.

This study investigates SO3 formation under air-fired and oxy-fuel combustion environments in a bench-scale bubbling fluidised bed. Flue gas compositions typical of air-fired and oxy-fuel environments were simulated with synthetic gas mixtures. Parameters such as bed temperature and SO2 and H2O concentrations were varied to determine their effect on the formation of SO3. Tests were also performed using three fly ashes produced from three different coals and combustion systems. Oxy-fuel combustion led to increased SO3 concentration as compared to typical air-fired environments. SO3 concentrations increased with combustion temperature and O2 concentrations. SO3 formation also depends on the chemical composition of the fly ash.