Abstract

Ammonia (NH3) is a promising carbon-free fuel and hydrogen energy carrier for decarbonizing power generation because it can be produced from renewable energy, has relatively high energy density, and is safer to store, especially when compared to hydrogen. Nonetheless, there are limitations to using NH3 as a fuel, such as limited reactivity, low flame speed, restricted flammability limits, and a tendency to produce significant levels of nitrogen oxides (NOx). This body of work examines the combustion of NH3-based fuel mixes to power a micro gas turbine and evaluates exhaust emissions and flame stability results. First, the stability and exhaust emissions of a commercial micro gas turbine fueled with NH3-based fuel blends were investigated. By substituting methane (CH4) with NH3 up to 63% in volume, stable functioning was obtained. This limit was increased to 75% replacement by adding hydrogen (H2) to a fuel combination of 90% NH3 - 10% H2. However, hardware modifications will be required to meet current NOx limits and ensure suitably low N2O emissions. The excessively high nitrous oxide (N2O) content in the exhaust gases was caused by the far lean equivalence ratios near the lean blowout. Next, an optically-accessible reduced-scale burner inspired by the micro gas turbine combustor was employed as an experimental platform. Experiments at the relevant elevated pressure demonstrated that stable far lean NH3-CH4-air combustion was attainable due to the improved flame stability afforded by the pilot flame. Although far lean NH3-based flames exhibit nitric oxide (NO) emissions that are much reduced compared to that found for lean equivalence ratios typically associated with lean premixed combustors, the NO concentration is still too high to satisfy current regulations. In contrast, N2O emissions are negligible for lean equivalence ratios, except for far lean equivalence ratios where N2O reaches unacceptably high values, as observed during the experiments in the micro gas turbine. The impact of the pilot flame characteristics on exhaust emissions and flame morphology was also investigated. It was found that the pilot power and pilot fuel composition must be tuned for different NH3-based fuel blends to ensure satisfactory flame stability and low emissions. For example, the fuel composition of the pilot flame has a complicated effect on exhaust emissions. This is because it changes the flame shape and the OH concentration in the inner recirculation zone, which has a positive correlation with NO concentration. Finally, a strategy to reduce NOx emissions, called two-stage rich-lean combustion, was investigated using the reduced-scale burner as an experimental platform. This strategy satisfactorily reduces the NO emissions by an order of magnitude. Since the reduced-scale burner is a smaller-scale replica of the actual AE-T100 mGT burner, further improvement might be expected at the elevated pressure and temperature conditions while running the mGT. On the other hand, high secondary airflow rates induce flame instabilities at low frequencies, resulting in flame blowout. This phenomenon is reduced by increasing the number of holes. However, it still occurs at high enough secondary airflow rates, implying that not only the high velocity of the jets in the secondary combustion zone causes the onset of unstable combustion but also cooling.

Bio

Current Ph.D. candidate in Mechanical Engineering under the supervision of Prof. William L. Roberts and Prof. Thibault F. Guiberti. B.Sc and M.Sc in Mechanical Engineering from Universidad de Antioquia, Colombia. My research focuses on integrating ammonia (NH3) and NH3-based fuel bends combustion into real-scale micro gas turbines for electrical power generation to reduce CO2 emissions.