Flame structure and NO production on laminar non-premixed flames of ammonia with highly preheated air

Numerical simulations of one-dimensional counterflow non-premixed ammonia flames were conducted to investigate the effects of highly preheated air on flame structure and NO production mechanisms. The oxidizer stream was modeled as an O 2 /N 2 mixture with oxygen concentrations varying from 21% to 1% at a temperature of 1400 K. The simulation results for an oxidizer composition equivalent to air (oxygen concentration of 21%) showed that hydrogen was locally produced, reaching up to 10% in mole fraction under lower strain rate conditions. As the strain rate increased, the peak temperature and hydrogen concentration decreased monotonically. As the oxygen concentration decreased, the flame temperature decreased, ammonia pyrolysis was suppressed. The abrupt extinction disappeared at an oxygen concentration of 3%. Analyses of NO x production and maximum concentration were performed. At oxygen concentration of 21%, at high strain rates induced NO production through incomplete combustion due to short residence times, while low strain rates resulted in thermal NO production from hydrogen produced via ammonia pyrolysis. Reducing oxygen concentration decreased maximum NO concentration due to lower temperatures and the suppression of ammonia pyrolysis. In addition to NO, N 2 O was produced at concentrations of up to 5000 ppm, whereas NO 2 remained in the tens of ppm range. An increase in strain rate and a decrease in oxygen concentration result in higher N 2 O concentrations. A path-flux analysis clearly explained the changes in NO and N 2 O concentrations. As the oxygen concentration decreases, NO production via the Extended Zel’dovich reactions is greatly reduced, whereas the DeNOx reaction producing N 2 O is enhanced. • NO formation mechanism in ammonia combustion affected by highly preheated air. • Hydrogen from ammonia pyrolysis increase NO via the thermal NO pathway. • Lower oxygen concentrations suppress NO production by reducing pyrolysis effects. • DeNOx reactions producing N 2 O are also promoted under low oxygen concentrations.