Liftoff heights and blowoff limits of coflow nonpremixed ammonia/methane flames

Presentation given at the 13th US National Combustion Meeting on 20 March 2023 in College Station TX.

Ammonia (NH3) is a carbon-free fuel that can be burned to produce energy without carbon dioxide emissions.  Unlike hydrogen – the other major carbon-free fuel – it condenses easily and can be transported as a liquid.  However, it has a low flame speed and is difficult to burn stably.  For example, if ammonia is added to the fuel of a coflow nonpremixed methane/air flame, the flame will blow off when the ammonia mole fraction xNH3 reaches about 0.4.  The goal of this work is to understand how ammonia compromises flame stabilization mechanisms and to develop interventions to improve the stability of ammonia flames.  Laminar coflow nonpremixed flames are well-suited to this purpose since they are simple enough to allow high quality in-situ measurements and detailed computational simulations, but they are also complicated enough to include stabilization by thermal and mixing mechanisms.  We have developed procedures to preheat the fuel and measure the liftoff height HL of methane/ammonia flames as a function of the initial reactant temperature T0 with high data acquisition rates.  The results show that preheating stabilizes the flame; for example, at T0 = 165 °C, the maximum xNH3 before blowoff increases to 0.54.  Interestingly, at a given HL, there is a linear relationship between T0 and xNH3.  Future work will consider other interventions, such as hydrogen addition to the fuel, and include high spatial resolution measurements of temperature and mixing in the stabilization region.