Experimental and numerical validation of ammonia combustion in oxygen at high pressures and temperatures
Session chaired by Dr. Agustin Valera-Madina
The combustion of fossil fuels produces large quantities of carbon dioxide which participates in the global warming. Due to alarming concerns, initiatives are taken to promote the use of ‘zero-carbon fuel’. Ammonia stands as a potential fuel as it is carbon-free and relatively safe to store and transport. The well-known Haber-Bosch process is used for its production. The use of ammonia as an alternative fuel in gas turbines and spark-ignition engines has already been tested. Ammonia flames are characterized by a low combustion intensity, low laminar burning velocities, and narrow flammability limits. The low speed leads to an early blow-off and a difficulty in ignition. Nevertheless, ammonia combustion has anti-knock characteristics because of its high-octane number which makes it favorable to use in the spark-ignition engine. Studies on ammonia flame speeds for different equivalence ratios have been done for a narrow range of pressures and temperatures. The maximum flame speed data available in the literature is for a pressure and temperature of 5 bar and 473 K respectively. As high pressures and temperatures are encountered in spark-ignition engines and gas turbines, it is essential to obtain accurate flame speed data for these conditions. Moreover, the existing kinetic mechanisms for ammonia combustion have not been assessed for elevated conditions. It is important to identify the key reactions and to check if pressure dependency is accounted well at high pressures. The present study affords new flame speed results at elevated temperatures and pressures conditions using the constant volume method. The tests were performed at an initial temperature of 300 K for 3 different equivalence ratios (0.8,1.1 and 1.3) and an initial pressure ranging from 1 to 4 bar which allows to obtain laminar flame speeds upto a pressure of 20 bar and temperature of 485 K. At these conditions, hydrodynamic and thermo-diffusive instabilities are favored. In order to stabilize the flame, a mixture of argon and helium was used. The ratio of argon and helium was chosen based on the ability to easily ignite the mixture. Helium has a high thermal diffusivity. Therefore, larger quantities of helium were used only for the high-pressure conditions. A mixture containing a mole fraction of 30% of oxygen and 70% of diluents was used. These experiments were conducted in the OPTIPRIME facility of ICARE, CNRS. This experimental device is a perfectly spherical chamber with full optical access and allows the simultaneous recording of pressure and flame radius inside the chamber during the combustion process. A literature survey was conducted to choose and evaluate the most recent kinetic mechanisms in ammonia combustion. It was seen that most of the chosen mechanisms could reproduce the experimental flame speed trend for any given test condition: flame speeds increase with the pressure and its corresponding isentropic temperature. Sensitivity analyses for a set of mechanisms were performed to further understand the working of these mechanisms. From the sensitivity analyses, it was seen that the driving reaction of all the mechanisms is the well-known reaction. It is the most dominant reaction and its rate constant is pressure independent. It was observed that the recombination reaction of ammonia flames showed very low pressure dependency unlike the methane flames’ reaction , which is highly pressure dependent playing a major role at high pressures. The absence of this sensitive recombination reaction and the domination of the OH reaction could explain why most of the mechanisms can predict a consistent trend for elevated conditions. However, only a few mechanisms can predict the flame speed values that lie within the experimental range. It is interesting to note that for the mechanisms of Nakamura [1] and Stagni [2], the flame speeds for all conditions are quite close to each other and within the experimental range even if the sensitivity tests result in two different sets of the top 10 reactions with only 3 reactions in common for all the given initial conditions. It was seen that the reactions are dependent on the equivalence ratio rather than the initial pressure. References: [1] Nakamura H, Hasegawa S. Combustion and ignition characteristics of ammonia/air mixtures in a micro flow reactor with a controlled temperature profile. Proc Combust Inst 2017;36:4217–26. https://doi.org/10.1016/j.proci.2016.06.153. [2] Stagni A, Cavallotti C, Arunthanayothin S, Song Y, Herbinet O, Battin-Leclerc F, et al. An experimental, theoretical and kinetic-modeling study of the gas-phase oxidation of ammonia. React Chem Eng 2020;5:696–711. https://doi.org/10.1039/C9RE00429G.
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