Analysis of high flue gas recirculation for small energy conversion systems

Restrictions of energy production emissions set new challenges to combustion facilities, and new methods, such as High Temperature Air Combustion (HiTAC) are considered to meet these challenges. In HiTAC, the flue gas is recirculated to the combustion region while preheating the combustion air. The HiTAC combustion is an environmentally friendly and energy-efficient method, but it requires special burner arrangements and additional equipment for air preheating. This work investigates the feasibility to obtain low emissions without preheating the combustion air. Experimental work showed that in this case the applicable flue gas recirculation rates were lower than with conventional HiTAC. Numerical analysis was performed to analyze flow behavior in the combustion chamber. The main contributing factor for combustion stability was found to be pronounced internal recirculation. The flame was forced aside towards the side walls by a back flow in the chamber centerline, which kept the flame stable and attached to the burner. The results suggest that the advantages of HiTAC can be partly achieved without the preheating of combustion air and with moderate flue gas recirculation. This enables a simplified and more economical construction, applicable for instance in small-scale boilers.