Reducing NOx Emission of Swirl-Stabilized Ammonia/Methane Tubular Flames through a Fuel-Oxidizer Mixing Strategy

Ammonia as a fuel can be a carbon-free and long-duration scalable hydrogen energy carrier. In the carbon-neutral power system, co-firing ammonia with natural gas is a promising technique for flexible operations of gas turbines and gas-fired boilers. However, the NOx propensity of ammonia/methane flames is a severe problem under global fuel-lean conditions. This work has investigated the NOx performance of various fuel-oxidizer mixing strategies on a tangential swirl burner. We propose a new ammonia co-combustion technology with a central ammonia jet surrounded by a group of separately tangential swirl methane jets. The NOx emissions can be reduced to about 1000 ppm under lean combustion conditions (global Phi = 0.5-0.9) when the ammonia blending ratio (E-NH3, in terms of input energy) is 20% (corresponding to the ammonia molar fraction x(NH3) as 0.41). Regarding the NOx emissions and ammonia slip at the tube exit, such a strategy outperforms other feed modes for a wide range of ammonia blending ratios (E-NH3 up to 0.3 and x(NH3) up to 0.52) and equivalence ratios (Phi ranging from 0.5 to 1.1). The variations of emitted NOx with the total flow rate and the axial position of ammonia injection were also studied. Combining measurements of gaseous temperature, composition, and NH2* chemiluminescence, we demonstrated the massive formation of NH2 radicals in the central high-temperature ammonia-rich zone. A model involving the chemical reaction network was developed and validated against the experiments. It provides further evidence for the role of NH2 radicals in reducing NOx emission.