Effect analysis on energy conversion enhancement and NOx emission reduction of ammonia/hydrogen fuelled wavy micro-combustor for micro-thermophotovoltaic application

Ammonia (NH₃) is regarded as an alternative fuel not only as a carbon-free fuel but also as a renewable hydrogen-carrier. It is possible that the safety in micro-combustor can be improved through partial NH₃ substitution for hydrogen. However, knowledge of the thermal performance and nitrogen oxides (NOx) emission of ammonia/hydrogen combustion, especially in the micro-combustor, has been insufficient. In order to enhance thermal performance, reduce NOx emission and improve flame stabilization of ammonia/hydrogen fuelled micro-combustors for thermophotovoltaic (TPV) application, three types of micro-combustors with a wavy profile are designed and evaluated. For this, a three-dimensional (3D) numerical model with a detailed chemical reaction mechanism has been verified and applied to assess the thermal performance of the modified micro combustors in terms of the outer wall temperature distributions. The average temperature of these wavy combustors is found to be much higher than that of the conventional smooth combustor, regardless of the hydrogen/ammonia mixture flow velocity. Moreover, the wavy is a more effective measure to improve temperature uniformity when the mixture velocity is greater than 12 m/s. Comparing the flame stability behaviours of hydrogen/ammonia/air blended combustion in both the conventional and the proposed wavy combustors reveals that the blowout limit is effectively broadened. Finally, the effects of 1) hydrogen/ammonia blended ratio and 2) fuel-air equivalence ratio on NO_x emissions are examined in detail. It is found that approximately 21.2% of NOx emission reduction could be achieved in the ARC wavy micro-combustor. NOx emission reduction can be gradually improved, as the nitrogen fuel mass ratio is increased. This present research sheds lights on an effective design of a micro-combustor with enhanced thermal performance and reduced NOx emission.