MILD combustion is a promising combustion technology for the future gas turbine combustor due to its high combustion efficiency, low exhaust emissions and enhanced combustion stability. It utilizes the concept of exhaust gas recirculation to achieve combustion at reduced temperature and flat thermal field. To examine the role of gas recirculation level on MILD combustion performance, a laboratory-scale axially staged combustor constituted of gas generation zone, mixing zone and MILD combustion zone is presented. To realize ultra-low NOx emissions for syngas characterized by high flame temperature, it is necessary to select an appropriate combustion mode for the gas generation zone. This study compared combustion performance and gas/fuel/air mixing feature between two configurations, gas generation zone of which are based on swirl diffusion combustion and coflow diffusion combustion, respectively. The results are compared on flow field with numerical simulation, and global flame signatures and exhaust emissions with experiment. Both numerical simulation and experiment are performed at equivalence ratio of 0.4, heat load of 24.4 kW, using 10 MJ/Nm3 syngas as the fuel at atmospheric pressure and normal temperature fuel and air. More uniform oxidizer, lower flame temperature and less NOx production are observed in coflow diffusion staged combustion. MILD combustion zone is beneficial for the reduction of NOx and oxidation of CO exit from the gas generation zone.

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