Optimum primary equivalence ratio for rich-lean two-stage combustion of non-premixed ammonia/methane/air and ammonia/hydrogen/air flames in a swirling flow

Rich-lean two-stage combustion concept was proposed for reducing emissions from ammonia-fueled gas turbine combustors. The primary combustion zone should be maintained at an optimum primary equivalence ratio for the simultaneous reduction of NO and unburned NH3; however, the optimum primary equivalence ratio has been reported only for a limited number of fuel blends and ammonia mixing ratios. This study experimentally and numerically investigated the optimum primary equivalence ratio and studied its application in the rich-lean two-stage combustion of ammonia/methane/air and ammonia/hydrogen/air flames over a wide range of ammonia mixing ratios under atmospheric pressure. This study identifies factors determining the optimum primary equivalence ratio. The emissions were measured using a swirl combustor for single- and two-stage combustion. Unstretched freely propagating premixed flame simulations were performed using CHEMKIN-PRO for investigating the effects of chemical reactions on the optimum primary equivalence ratio. In both single- and two-stage combustion, the optimum primary equivalence ratios of non-premixed ammonia/methane/air and ammonia/hydrogen/air flames decreased linearly and exponentially with increasing ammonia mixing ratios, respectively. The rich-lean two-stage combustion eliminated unburned NH3 and minimized NO over a wide range of ammonia mixing ratios for non-premixed ammonia/methane/air and ammonia/hydrogen/air flames. The numerical results confirm that OH radical plays an important role in determining the optimum primary equivalence ratio.