Insight into the regulation effect of steam dilution on oxygen-enriched ammonia combustion characteristics

Abstract

NH3/O2/H2O combustion technology combines oxygen enrichment to enhance NH3 combustion and steam dilution to control flame temperature and emissions, which can achieve clean and efficient power generation. Steam dilution strategies necessitate a balance between flame speed, temperature, and emissions. The key lies in kinetic investigation into the regulation effects of steam dilution on NH3/O2/H2O flames. An adaptive kinetic model is developed and validated against the measured flame speed and NO distribution. Experimental and numerical investigations are conducted to analyze the steam dilution effect on NH3/O2/H2O flame at varying Φs and Tus. Fuel-lean NH3/O2/H2O flames (Φ = 0.8–0.9) possess the highest flame speeds and a 25 % steam dilution results in a 39–45 % reduction (67.63–82.09 cm/s) in SL. H/O radical pools and NH3 primary decomposition (NH3 → NH2 + H) are the keys to the SL, and steam dilution significantly decreases the key radical accumulation. As ZH2O increases from 0 to 0.25, the stoichiometric flame temperature decreases to approximately 2600 K (a reduction of about 200 K). The staged dehydrogenation of NH3 always dominates the heat release, while the conversions of the H/O radical pool are the main endothermic reactions ·H2O dilution reduces the reactant concentration, and the high specific heat capacity of H2O also further reduces the flame temperature. ZH2O of 0.25 can reduce NO emissions to the order of magnitude of 100, while steam dilution may turn to promote NO emissions at Φ < 0.75 because steam dilution accelerates the generation of H and OH and promotes NO formation at fuel-lean conditions. ZH2O needs to be increased to more than 30 %, controlling the flame temperature not to exceed 2000 K and improving the fluid work efficiency. To further reduce NO emissions to 50 ppmv@15 %O2, it is necessary to adopt staged combustion nozzle/chamber structure designs, such as co-flow or secondary injection.