Significance of gasification during oxy-fuel combustion of a lignite char in a fluidised bed using a fast UEGO sensor

Abstract

In oxy-fuel combustion, fuel is combusted in a mixture of O2 and recycled flue gas, i.e. the N2 is replaced by CO2 with the O2 supplied from an air separation unit. The resulting gas consists largely of steam and CO2, which would be ready for sequestration when dried. In this work, the rate of reaction of particles of lignite char, typically 1200μm diameter, in a fluidised bed reactor was determined using mixtures of O2 with either CO2 (“oxy-fuel”) or N2. A universal exhaust-gas oxygen (UEGO) sensor enabled rapid measurements of the oxygen partial pressures in the off-gas, representing a novel application of this type of sensor. It was found that the rate of combustion of the particles in oxy-fuel is much more sensitive to temperature than in the equivalent O2 and N2 mixture. This is because for bed temperatures >∼1000K particle combustion in mixtures of N2 and O2 is rate controlled by external mass transfer, which does not increase significantly with temperature. In contrast, using oxy-fuel, as the temperature increases, gasification by the high concentrations of CO2 present becomes increasingly significant. At low temperatures, e.g. ∼1000K, rates of combustion in oxy-fuel were lower than those in mixtures of O2 and N2 containing the same mole fraction of O2 owing, primarily, to the lower diffusivities of O2 in CO2 compared to O2 in N2 under conditions at which external mass transfer is still a significant factor in controlling the rate of reaction. At higher temperatures, e.g. 1223K, oxy-fuel combustion rates were significantly higher than those in O2 and N2. The point at which oxy-fuel combustion becomes more rapid than in mixtures of O2 and N2 depends not only on temperature but also on the ratio of O2 to CO2 or N2, respectively. A numerical model was developed to account for external mass transfer, changes in the temperature of the particle and for the effect of gasification under oxy-fuel conditions. The model confirmed that, at high temperatures, the high concentration of CO2 at the surface of the burning particle in the oxy-fuel mixture led to an increase in the overall rate of carbon conversion via CO2+C→2CO, whilst the rate of reaction with O2 was limited by mass transfer. Good agreement was observed between the rates predicted by the numerical model and those observed experimentally.