Limits of inflammability

Abstract

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH3 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 NH3 dissociation was varied from 0 to 25% (0 to 20%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH3 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 NH3 dissociation. At 25% NH3 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 NH3 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 NH3. The production of key radicals including OH, H, O, and NH2 was enhanced in the presence of H2 and thus the conversion of NH to NO and then NO to N2 near LFL and the conversion of NH2 and NO to N2 near UFL leading to wider flammability limits.