Combustion of Single Coal Char Particles under Fluidized Bed Oxyfiring Conditions

The fluidized bed combustion of single coal char particles was investigated at high CO2 concentrations, typical of oxyfiring conditions, at different bed temperatures and oxygen concentrations. The conversion rate of the char particles was followed as a function of time by continuously measuring the outlet CO and O2 concentrations. Char gasification tests were also carried out under 100% CO2 at different temperatures to quantify the importance of this reaction and to extract a suitable kinetic expression. This expression was then combined with a correlation for the mass transfer controlled particle burning rate to simulate the experimental conversion rate data. The calculated carbon consumption rate was an excellent fit to the experimental data for all the operating conditions. Results showed that carbon combustion dominates particle conversion at high oxygen concentrations and low temperatures, while carbon gasification contributes to a comparable extent at high temperatures and low oxygen concentrations. Even under fluidized bed oxyfiring conditions oxygen boundary layer diffusion controls the combustion rate, and the main combustion product is CO2 rather than CO.