NOx reduction strategies for high speed hydrogen fuelled vehicles

The growing need to drastically reduce aircrafts CO2 emissions has led engineers and scientists in the last years to develop a clean, renewable and sustainable energy system Hydrogen as a green fuel in aviation is a good choice since does not emit any particulate, CO and CO2. However, its high combustion temperature is disadvantageous in terms of NOx production. Further, the higher altitudes associated with hyper/supersonic flight make the emission of NOx critical for the ozone layer. In fact, at current subsonic flight altitudes in the troposphere and lower stratosphere, NOx emissions are associated with ozone production whereas, at altitude of 20,000e30,000 m, which corresponds closely to the maximum ozone density, NOx can catalyze ozone destruction. Reduction of the NOx Emission Index (grams of emission produced per kilogram of fuel consumed) remains therefore a primary concern. This paper has the goal to investigate possible strategies to reduce NOx emissions from H2 combustion. Since these increase when burning fuel near stoichiometric air-to-fuel ratios, a strategy simulated in this study to reduce NOx production consists in operating at lean or very lean equivalence ratios (thanks to the wider flammability limits of the hydrogen-air flames compared to kerosene-air flames), or in reducing the combustor length (thanks to the higher flame speed of hydrogen compared to other fuels. In this paper, the RQL (Rich-Quench-Lean) strategy for the NOx abatement is proposed for a high speed hydrogen fuelled vehicle. This strategy has shown that the equivalence ratio in the rich strategy is a key parameter to reduce the nitrogen oxide emissions to ICAO acceptable values.