A Mechanistic and Experimental Study of Ammonia Flames

Major properties of lean to stoichiometric, ammonia-oxygen-nitrogen. atmospheric pressure flames are investigated to identify the reactions and conditions which control the con-version of the nitrogen in ammonia to NO or N-2. These free (as opposed to porous plug burner-stabilized) ammonia flames arc investigated over the combustible range of initial dilutions (0.0-0.5 initial N-2 mole fraction) and the equivalence ratios (phi=0.5-1.1). These flames are studied both experimentally and theoretically; the flame properties investigated arc the flame speeds (ranging between 25 and 130 cm/scc), temperature (2000-2700 K), major species profiles and the postflame NO concentrations (1000-70,000 ppm). The measured NO concentrations are 2-3 times greater than the equilibrium values. These free ammonia flames arc calculated with detailed kinetics from the unsteady conservation equations using the recently developed GMR flame program. A flame mechanism is developed which describes the properties of these flames. The mechanism predicts the measured flame speeds, temperature profiles and postflame NO concentrations. Analysis of the flame mechanism leads to simplifying concepts for the prediction of such quantities as flame produced NO over this range of conditions for these nitrogen-diluted ammonia flames and suggests that similar concepts will apply to hydrocarbon flames with fuel-bound nitrogen.