Radiative Properties and Combustion Chemistry in Oxy-Fuel Flames – Experiments and Modelling Elements

Doktorsavhandling, 2011

The current understanding of global warming translates into an urgent need to reduce anthropogenic CO2 emissions. Carbon capture and storage is a measure to reduce CO2 emissions and oxy-fuel combustion is emerging as one possible CO2 capture process. In oxy-fuel combustion the fuel is burnt in O2 and recycled flue gas (mainly CO2 and H2O), instead of air as in conventional combustion. To take advantage of existing knowledge on air-firing, the conditions in the first generation oxy-fuel furnaces will likely resemble those in air-fired furnaces, but the replacement of air as oxidizer will affect the combustion characteristics. The higher molar heat capacities of CO2 and H2O compared to N2, require an increased O2 fraction in the feed gas to match air-fired thermal conditions. CO2 and H2O also actively participate in the combustion chemistry, and, their radiative properties enhance the exchange of thermal radiation. In this thesis the characteristics of oxy-fuel and air flames are investigated and compared through experiments and modelling to deepen the understanding of oxy-fuel flame behaviour, mainly with respect to combustion chemistry and radiative heat transfer. The experiments on oxy-coal combustion focus on temperature, gas composition, ignition and radiative properties. The modelling part concerns gas-fired oxy-fuel flames, in which reaction chemistry schemes and gaseous radiative properties models are thoroughly evaluated using CFD. In addition, the impact of soot radiation and turbulence models are addressed. From the experimental work it is concluded that the flue gas recycling conditions in oxy-fuel combustion is a critical process parameter with respect to temperature and heat transfer conditions as well as combustion chemistry and ignition properties. Thus, the recycle rate can be used as a tuning parameter to control the main combustion process characteristics. Global reaction mechanisms for air and oxy-fuel combustion are investigated and compared by modelling of a propane-fired oxy-fuel flame. The reaction mechanisms show large deviations in the predicted temperature and gas composition. Surprisingly, a global reference mechanism yields the most convincing results, even though peak temperatures are overestimated. Furthermore, inclusion of gray/non-gray gas radiation and soot radiation in CFD modelling of an oxy-fired and an air-fired propane flame is investigated. The non-gray gas approach is shown to be preferable, since the gray model fails in predicting the radiative source term, which affects the temperature field in the CFD calculations. Inclusion of soot radiation, when modelling soot containing flames, is however more critical than the use of a more rigorous description of the radiative properties of the gaseous components.