CFD modeling of char conversion in oxygen enhanced combustion

Oxy-fuel combustion is a process where fuel is burned in a mixture of oxygen and recycled flue gas. The flue gas recirculation is applied to limit the flame temperatures, otherwise too high when burning only with pure oxygen, to levels comparable to conventional combustion. In this way the atmospheric nitrogen is eliminated from the process leading to a flue gas consisting mainly of carbon dioxide and water vapor. Further on, this stream is treated to obtain a high CO₂ concentration by condensing the water. This renders the oxyfuel process as a suitable candidate for the Carbon Capture and Storage technology. For realizing an oxy-fuel based CCS process, understanding the fundamental characteristics of fossil fuel combustion under these conditions plays a key role. CFD modeling is a powerful tool which can be regarded as an aid in designing new facilities or retrofitting the existing ones. However, modeling of coal combustion under oxy-fuel conditions is not straightforward. Adjustments of the constituting submodels which form the combustion model are necessary in order to account for the possible changes in particle reactivity and the surrounding gas composition, and properties which influence the heat and mass transfer mechanisms. All these factors reflect on the predictability of particle burnout. This paper presents, in a comparative manner, two existing submodels for predicting pulverized coal combustion under oxygen enhanced conditions. The performance of the submodels is tested against experimental data from literature in terms of particle burnout as a function of residence time. To correct the reactivity at low oxygen levels, gasification reactions of the char particle are included. In addition the oxygen diffusivity towards the particle external surface is adjusted accordingly.