Experimental and Kinetic Modelling Studies of Flammability Limits of NH3/H2/N2 Mixtures

NH3 has been proposed as a clean and carbon free transport fuel for combustion in spark ignition engines. This paper presents experimental and kinetic modelling studies of the flammability limits of NH3/H2/N2 mixtures in air at ambient temperature and atmospheric pressure. The experiments were carried out using a Hartmann bomb apparatus and the methodology used was in accordance with the EN 1839 (T) standard. The kinetic modelling was performed using CHEMKIN software with opposed-flow premixed flame model to predict the lean and rich flammability limits. The H2 concentration varied from 0 to 20 %vol. It was found that the lower (LFL) and upper (UFL) flammabilty limits of pure NH3 in air were 15 %vol and 30 %vol, respectively, which are consistent with the literature data. The experimental results showed that the flammability limits of NH3 widened significantly with increasing the H2 concentration. With 20 %vol H2 in the fuel mixture, the LFL was reduced to 9.2 %vol and the UFL increased to 35.6 %vol. The kinetic modelling predictions shows the LFL and UFL of pure NH3 were 14.7 %vol and 30.7 %vol, whereas, for the fuel mixture with 20 %vol H2, the LFL and UFL were 10.9 %vol and 37.1 %vol, which were in good agreements with the experimental results. Sensitivity analyses were performed to identify the key elementary reactions and radicals during NH3 combustion and it was discovered that the productions of key radicals including OH, H, O, NH2 were enhanced in the presence of H2 which widened the flammability limits of the mixtures.