Hybrid rich- and lean-premixed ammonia-hydrogen combustion for mitigation of NOx emissions and thermoacoustic instabilities

The large-scale direct utilization of two carbon-free fuels, ammonia and hydrogen, is currently attracting significant interest in the context of the development of new gas turbine combustion technologies, with paying close attention to the reduction of nitrogen oxides and unburned ammonia emissions. Motivated by recent observations that rich-premixed conditions tend to mitigate excessive NOx emissions from ammonia combustion, and that uniformly blended ammonia-hydrogen fuel/air mixtures tend to increase NOx production exponentially, here we propose a hybrid configuration of hydrogen -doped rich-premixed ammonia-air flames (inner stage) and lean-premixed pure hydrogen -air flames (outer stage) in a radially stratified primary reaction zone. This fuel staging scheme offers a possible mechanism for chemical kinetics-controlled NOx abatement and asymmetryinduced thermoacoustic instability suppression. The relevance and feasibility of the unconventional premixing approach are rigorously evaluated based on detailed measurements of exhaust gas concentrations and selfexcited pressure fluctuations, in conjunction with OH*/NH2*/NH* chemiluminescence and OH PLIF imaging measurements. We observe that drastically different non-homogeneous reaction regions can be stably established in a comparatively compact combustion volume without producing negative flame stabilization effects - such as blowoff of less reactive ammonia flames and flashback of more reactive hydrogen flames. As compared with the uniform-blend baseline condition, the hybrid staging method is shown to significantly mitigate total nitrogen oxides emissions, from 7764 to 310 ppmvd (96 % reduction), and achieving an approximately two-fold reduction in dynamic pressure amplitude. Interestingly, hydrogen-enriched rich-premixed ammonia flames are revealed to exhibit anomalous oscillatory states originating from the preferential diffusion of hydrogen molecules and reaction rate-dependent separation of reactive layers, enabling interacting non-homogeneous reaction zones with markedly different characteristic time scales to resist the growth of intense pressure perturbations.