Prediction of minimum achievable NOx levels for fuel-staged combustors

Abstract

While lean-premixed (LPM) combustors are used widely in combined cycle power plants, their NOx emissions rise drastically at the high firing temperatures required for increased combined cycle efficiencies, e.g., >60 ppm at 1975 K, the target for 65% thermal efficiency. The axially staged combustor architecture is a promising approach to increase temperature without a thermal NOx penalty. A chemical reactor network model in a design optimization framework is used to investigate the minimum theoretical NOx achievable by a high-pressure, staged combustor under CO emission constraints, as well as the corresponding optimum configuration, e.g., fuel splits and post-secondary injection residence times. Under ideal conditions, minimum NOx favors the most fuel-lean main burner that can autoignite the secondary stage, and the shortest secondary stage residence time that allows sufficient oxidation to achieve the CO constraint. Compared to the theoretical minimum emissions of standard LPM approaches, the staged combustor architecture is capable of achieving dramatically lower NOx levels, e.g., ∼1 ppm corrected to 15% excess oxygen at 1975 K, with greater relative NOx reductions as firing temperature increases. In fact, the minimum NOx levels are relatively insensitive to combustor firing temperature and overall residence time. As an added advantage, the main LPM burner can be optimized for a single operating condition. The results also demonstrate that the low NOx entitlement levels achievable for single-point designs are also valid for a fixed architecture, staged combustor required to operate over a range of firing temperatures.