A numerical investigation on flame stability of oxy-coal combustion: Effects of blockage ratio, swirl number, recycle ratio and partial pressure ratio of oxygen

Keeping the flame stable and having the appropriate flame shape is essential in operating an oxyfuel combustion coal fired power plant with CO2 capture. This is more critical if the boiler is required to operate in full and partial thermal load with varying volume of flue gas recycled. Therefore, in designing the oxyfuel combustion burner, it is important to evaluate the burner stability, shape structure and flame type. This work presents numerical investigation of a burner designed for oxyfuel combustion coal fired boiler. The study evaluated the effects of the blockage ratio, swirl number, flue gas recycle ratio, and oxygen partial pressure in the primary RFG stream on the flame stability, type, shape and structure. The model results were validated against experimental data obtained from a novel oxyfuel combustion burner designed and operated in a 2.5MWt test pilot facility. The model developed for this study incorporated a modified chemical reaction mechanism that allows the addition of CO and Boudouard reaction that is considered significant in an oxyfuel combustion flame. The results show that the stability of an oxyfuel combustion flame is greatly improved by having a moderate to strong internal recirculation zone to produce a Type-II flame; and could also be easily destabilized by a high velocity jet of unburned carbon/char as illustrated by a dark central region emanating from the centre of the burner. Additionally, it could also be illustrated that the swirl number and flue gas recycle ratio have a strong influence to the formation of the central dark region along the centerline of the burner. It could be concluded that maintaining the flame Type-II over the whole range of thermal load should maintain the necessary flame stability appropriate to an oxyfuel combustion boiler.