Emissions analyses of humidified cracked ammonia swirling flames

Using renewably produced ammonia as a zero-carbon fuel is gaining momentum due to its ease of transportation and storage as a hydrogen vector. This is particularly true for partially cracking ammonia immediately prior to use, injecting a blend of NH3, H2 and N2. Challenges with this fuel combination relate to the emissions of NOx and unburned NH3, as well as understanding flame stability for practical applications. In this study, a 20 %(vol.) cracked ammonia blend was investigated using a fully premixed swirl burner, operating at a thermal power of 10 kW with steam injection of 30 %(vol.) of the fuel and preheating inlet temperatures of up to 390 K, for a range of equivalence ratios from lean to rich. Emissions of NO, NO2, N2O, NH3, H2, O2 and H2O were recorded, along with OH*, NH* and NH2* chemiluminescence. Additionally, a numerical investigation was conducted using CHEMKIN-PRO to elucidate the main reactions responsible for reducing emissions by providing a rate of production analysis. The 20 %(vol.) cracked ammonia blend was found to reduce NO, NO2 and N2O significantly, with an increase in NH3 emissions at rich conditions and instabilities at both lean and rich extremes, compared to the widely investigated 70/30(vol %) ammonia/hydrogen blend. Humidification reduced NO and NO2 emissions due to a reduction in HNO production via OH and NH but caused an increase in N2O by reducing the flame temperature and unburned NH3 emissions at rich, low power conditions due to combustion instabilities. Unburned H2 emissions however were reduced, likely relating to a reduction in exhaust temperature thermally cracking less unburned NH3 into H2 and N2.