Experimental and kinetic modelling studies of flammability limits of partially dissociated NH 3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH 3 in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH 3 dissociation was varied from 0 to 25% (0 to 20%v/v H 2 in the fuel mixture with a fixed H 2 /N 2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH 3 in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH 3 dissociation. At 25% NH 3 dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH 3 in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH 3 . The production of key radicals including OH, H, O, and NH 2 was enhanced in the presence of H 2 and thus the conversion of NH to NO and then NO to N 2 near LFL and the conversion of NH 2 and NO to N 2 near UFL leading to wider flammability limits. (c) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.