Ammonia blends can potentially become a breakthrough chemical for power generation, cooling storage and distribution of energy. Gas turbines and internal combustion engines are potential candidates for the use of the resource in an efficient way that will enable commissioning of combined cycles to power communities around Europe and around the world while serving as sources of heat and chemical storage. Therefore, development of these systems will bring to the market a safer, zero carbon fuel that can be used for multiple purposes, thus decentralizing power generation and increasing sustainability in the communities of the future whilst positioning the developing and manufacturing companies as global leaders of a new generation of energy devices.
This work summarises results from various analytical, numerical and experimental campaigns carried out at Cardiff University, Wales, UK. Each of these is related to combustion of ammonia blended with hydrogen at different concentrations and conditions.
Initially, computational work used to determine laminar flame speed, combustion products and ignition delay time is described. Software such as Gaseq, CHEMKIN-PRO and CANTERA were used to find the best mechanism suitable for NH3/H2 blend combustion. Next, experimental and numerical campaigns are described on the work carried out on a generic swirl burner employing NH3/H2 blend (50:50 by volume) at different equivalence ratios. Results showed that the flame stabilised above equivalence ratios of Ø=0.43. It was also observed that hydrogen seemed to be burning first, up to equivalence ratios <Ø=0.52, point at which the flame became unstable. Therefore, the results threw evidence of the narrow stability for high hydrogenated ammonia blends under lean conditions, steering the research to rich fuel combustion conditions. Production of NOX was also measured, showing the high influence of OH radicals in the formation of the contaminant. Further works are presented for numerical, experimental and analytical studies where higher equivalence ratios were analysed at various conditions (pressure, inlet temperature and humidity) to determine the “sweet” spot where ammonia / hydrogen can be burned with the lowest emissions and highest stability. It was found that equivalence ratios between 1.2 and 1.3 provide the best point of operation, with relatively high humidity needed to bring down NOx emissions and unburned ammonia, consequence of the reactivity of intermediate radicals and flame compactness. Analytical studies determined the efficiencies of gas turbines operating with these blends, showing values that ranged from ~9% to 28% depending on the employed humidity. It is emphasized that further work is needed to determine equations and equipment that better characterise these efficiency values, as comparison needs to be carried out using feasible equipment that can withstand ammonia/hydrogen combustion under humidified conditions, equipment that does not exist at the moment.
Finally, results from numerical analyses and experimental studies using internal combustion engines are presented. Ammonia and hydrogen blends were evaluated in a methane-run Spark Ignition engine. A reduced chemical kinetic model is used with the closest-to-methane blend to determine cycle characteristics via CHEMKIN-PRO. Experiments demonstrate the potential of using these blends with moderate emissions and power outputs >15kW. However, it was also found that the use of ammonia, due to its low energy content, needs to be carefully assessed to ensure that components, feeding lines and flowrates are precisely calculated before starting operation of these systems. Nevertheless, the results are encouraging, opening further interest in the development of more advanced injection strategies for these blends.
Overall, the motivation of the present study is to develop a “Hydrogen through Carbon-Free Ammonia Economy.”
