The temperature dependence of the laminar burning velocity and superadiabatic flame temperature phenomenon for NH3/air flames

Combustion of ammonia (NH3) as a carbon-free alternative fuel has been recently widely studied, with vast majority of the burning velocity data obtained at room temperature. In the present study, the laminar burning velocity S-L of NH3/air mixtures has been measured at unburnt gas temperature T-u from 298 K to 448 K, covering equivalence ratios from 0.85 to 1.25 and at 1 atm using the heat flux method. Kinetic simulations were made with five literature mechanisms developed for NH3 combustion, i.e., Nakamura et al., Otomo et al., San Diego, Okafor et al., and Mei et al. mechanisms, and the influence of radiation heat losses was considered. Using the obtained burning velocity data at different temperatures, the temperature dependence coefficients alpha in S-L/S-L(0) = (T-u/T-u(0))(alpha) were derived, and compared with different models' predictions. Further analyses of the temperature dependence of S-L were carried out through examination of the overall activation energy, temperature and species profiles as well as the reaction paths, and a unique flame structure at the rich side of adiabatic NH3/air flames was found, which resembles 'over-rich' phenomena in hydrocarbon flames. At equivalence ratio larger than 1.1 +/- 0.05, the NH3/air flames become so rich that (1) the NH2 radical overwhelms the H and OH radicals in maximum mole fraction; (2) after the flame front, H2O converts back to H-2 with NO formed at the same time, causing the superadiabatic flame temperature phenomena, i.e. adiabatic flame temperature being lower than the maximum achieved in the flame. Moreover, local minimum NO concentration is found right after the over-rich NH3/air flame front, which may be helpful in reducing NO emissions from NH3 flames in practical applications. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.