Experimental and numerical investigation of NH₃ flames under the influence of hot flue gases in a novel two stage porous burner

Ammonia presents itself as a high hydrogen dense and carbon-free alternative for industrial heating, power generation, and transportation. Nevertheless, the challenges of its low flame speed and elevated NO_x emissions pose notable challenges in combustor applications. The current investigation delves into the utilization of porous media burners (PMBs) to stabilize lean NH₃ flames in the hot combustion products from the porous liquid petroleum gas (LPG) burner. A two-stage porous media burner utilizing zirconia-based foams with a pore density of 20 PPI (pores per inch) was tested across various combinations of LPG (7.5, 10, 12.5, and 15 kW) and NH3 (8.45, 10.85, and 13.35 kW) under fuel-lean to fuel-rich conditions. Experimental and chemical kinetic study are undertaken to ascertain the stability limits of porous media combustion, effect of PM on reaction zone and flame temperature, flame morphology, major emissions and intermediate species like NO_x, CO and unburned NH₃ at the exit of the burner. Initially experiments with premixed LPG/air showed that introducing porous foams allowed for higher thermal inputs, stabilizing the flame and significantly reducing CO emissions, though NO emissions remained high at increased thermal inputs. Ammonia injection without porous foams resulted in elevated NH₃ emissions, while the use of porous foams reduced both NO and NH₃ emissions significantly. The flame colour shifted from blue to orange with increasing ammonia due to NH₂ radicals, and NO emissions rose with thermal intensity due to the Zeldovich mechanism. Chemical kinetics analysis identified key radicals like HNO and NH₂ as crucial for NO formation, with higher temperatures driving more complete combustion and influencing NO reduction pathways.