(513dq) Optimum Equivalence Ratio for the Minimum NO and NH3 Emissions of CH4-NH3-Air Flames in Swirling Flows

As for the candidates of hydrogen energy carrier and carbon free fuel, ammonia is anticipated. In order to achieve ammonia fueled gas turbine combustor, there are two challenges to be solved, i.e., flame stability and NOx production. Even though the laminar burning velocity of ammonia/air flame is almost 1/5 of that of methane/air flame, ammonia flame can be stabilized in a swirling flow. In addition, NOx and unburnt ammonia can be reduced simultaneously using the rich-lean two-stage combustion concept. In the rich-lean two-stage combustion concept, control of equivalence ratio in the primary zone is important to achieve simultaneous reductions of NO and unburnt ammonia. Our previous study [1] indicated that there is an optimum equivalence ratio window at which NO and unburnt ammonia emission reaches the minimum value simultaneously, and the suitable equivalence ratio for simultaneous reductions of NO and unburnt ammonia using the rich-lean two-stage combustion concept is close to the optimum equivalence ratio in the case of single-stage combustion. Therefore, it is important to elucidate the optimum equivalence ratio in ammonia combustion. In this study, the product gas characteristics of methane-ammonia-air flames were experimentally investigated using a radial type gas-turbine model swirl burner. In the experiments, product gas characteristics were analyzed for various ammonia concentrations in the methane-ammonia binary fuel and equivalence ratio. An FTIR gas analyzer was employed for the gas analysis. As for the results, the optimum equivalence ratio in the case of single-stage combustion appeared in the fuel rich condition, and the optimum equivalence ratio decreased with an increase in ammonia concentration in the binary fuel of ammonia-methane. In order to understand this variation of the optimum equivalence ratio, numerical simulations with detailed reaction kinetics were carried out. CHEMKIN PRO laminar burning velocity calculator model was used for the simulation and Okafor's detailed reaction mechanism [2] which is optimized for methane-ammonia combustion were employed. Numerical simulation also showed a decrease in the optimum equivalence ratio with an increase in ammonia concentration in the fuel. OH is one of the most important radicals in the ammonia flame [3]. Therefore, production and consumption rates of OH were investigated. It was clarified that the production rate of OH decreased with an increased in the ammonia fraction in the binary fuel. It was considered that the decrease in OH radical corresponds to the decrease in fuel NO production and NH3 consumption. Therefore, the change in OH production rate plays an important role in the variation of the optimum equivalence ratio for simultaneous reduction of NO and unburnt ammonia in ammonia-methane flames.

Acknowledgment
This study is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

References
[1] K.D.K.A. Somarathne et al., Int. J. Hydrogen Energy, 42 (2017) 27388-27399.
[2] E.C. Okafor et al., Combust. Flame, 187 (2018) 185-198.
[3] S. Colson et al., J. Therm. Sci. Technol., 11 (2016) 16-00384.