The effect of char kinetics on the combustion of pulverized coal under oxyfuel conditions

Oxyfuel combustion is a technical option to yield energy from carbon based fuels without releasing CO2 into the atmosphere. The main feature of oxyfuel combustion is replacing air with an oxidizer that mainly consists of CO2 and O2. This leads to high concentrations of CO2 in the gas phase, which influences heat release, temperature and chemical reactions. This thesis studies how high concentations of CO2 influence char combustion, the second stage of combustion of solid fuels such as coal and biomass. The first part of the thesis describes experimental investigations. Experiments show that in oxyfuel conditions char combustion is promoted, but only at high temperatures and low O2 concentrations. Under these conditions, local lack of O2 at the particle surface is partly compensated by the slower Boudouard reaction. This may have different – even opposing – effects on the overall combustion process. To account for the complex interaction of effects, the second part of the work uses numerical simulation with Computational Fluid Dynamics. Simulations include validation of sub-models, especially for pyrolysis, char combustion and thermal radiation, and evaluation of a pilot-size oxyfuel coal flame. Comparison with experimental data and various char combustion models shows that while char burnout is locally increased the overall burnout is almost unaffected