The demand for sustainable alternative fuels is ever-increasing in the power generation, transportation, and energy sectors due to the inherent non-sustainable characteristics and political constraints of current energy resources. A number of alternative fuels derived from cellulosic biomass, algae, or waste are being considered, along with the conversion of electricity to non-carbon fuels such as hydrogen or ammonia (NH₃). The latter is receiving attention recently because it is a non-carbon fuel that is readily produced in large quantities, stored and transported with current infrastructure, and is often a byproduct of biomass or waste conversion processes. However, pure or anhydrous ammonia combustion is severely challenging due to its high auto-ignition temperature (650 °C), low reactivity, and tendency to promote NO_x formation.

As such, the present study focuses on two major aspects of the ammonia combustion. The first is an applied investigation of the potential to achieve pure NH₃ combustion with low levels of emissions in flames of practical interest. In this study, a swirl-stabilized flame typically used in fuel-oil home-heating systems is optimized for NH₃ combustion, and measurements of NO and NH₃ are collected for a wide range of operating conditions. The second major focus of this work is on fundamental investigation of NO_x formation mechanisms in flames with high levels of NH₃ in H₂. For laminar premixed and diffusion jet flames, experimental measurements of flame speeds, exhaust-gas sampling, and in-situ NO measurements (NO PLIF) are compared with numerically predicted flames using complex chemical kinetics within CHEMKIN and reacting CFD codes i.e., UNICORN.

From the preliminary testing of the NO_x formation mechanisms, (1) Tian (2) Konnov and (3) GRI-Mech3.0 in laminar premixed H₂/NH₃ flames, the Tian and Konnov mechanisms are found to capture the reduction in measured flame speeds with increasing NH₃ in the fuel mixture, both qualitatively and quantitatively. The NO_x predictions by all the three chemical mechanisms are observed to be in fairly good agreement with the measured NO_x, qualitatively, however predictions are found to be 3 to 4 times higher than the measurements for both lean and rich H₂/NH₃ premixed flames.

For laminar H₂/NH₃ diffusion flames, detailed 2-D comparisons of in-situ NO measurements with the 2-D simulated NO using the Tian, GRI-Mech3.0 and modified GRI-Mech chemical mechanisms are performed and found to differ from the measured NO by approximately an order of magnitude. For NH₃ seeded H₂/air diffusion flames, GRI-Mech3.0 seemed to overpredict NO by more than an order of magnitude and failed to capture the fundamental flame characteristics, such as the flame length variation with increasing NH₃ in the fuel mixture. On the other hand, the predicted NO profiles by the Tian mechanism were not only found to be in better agreement with the measured NO, but they also captured the in-flame NO distribution as well, both qualitatively and quantitatively.

Overall, the Tian mechanism is found to be the superior chemical mechanism to capture the NO_x formation chemistry in NH₃ seeded flames.