Investigation of wet ammonia combustion characteristics using LES with finite-rate chemistry

Abstract

The interest in climate action and building sustainable energy system, has highlighted electricity derived fuels (eFuels) and has promoted studies considering ammonia as a eFuel for sustainable transportation and as a mean of energy storage. Ammonia combustion, however, poses several challenges due to the low reactivity and potential high NOx emissions. A promising solution is to add hydrogen and steam into the fuel/air mixture. Steam contributes to the enhancement of specific power output and efficiency in humidified gas turbine cycle, and hydrogen promotes reactivity of the eFuel and can be obtained by ammonia decomposition on-site. In the present study, wet combustion of ammonia/hydrogen blends is systematic studied using one-dimensional flame analysis and Large eddy simulation (LES) to understand the flame stabilisation and NOx formation mechanisms. When hydrogen content is low (H2%<30%vol) and Tad = 1750 K, flame speed is nearly independent while flame thickness is significantly dependent on the equivalence ratio Φ due to the steam dilution. At high hydrogen content, the reverse applies. The impact of the H2% on the emission is non-monotonic, peaking between 60%∼70% at various Φ. Rich and stoichiometric combustion benefits NO reduction at a balance of H2% and steam dilution. High fidelity LES demonstrates that the steam diluted hydrogen/ammonia blends can be stably burnt in a swirl burner in a MILD regime. The wet combustion distributes the flame at low H2% and reduces local NO emission. In rich conditions, low NO emission attributes to the activation of NO reburning progress. Compared to dry condition, low steam addition (Ω=0.1) in the reactant lifts the flame and enlarges the opening angle of the swirling flow. When flame speed is low, POD analysis shows two PVC related structures - a single and a double helix, which dominate the flow and flame dynamics. While when flame speed is high, quick combustion process damps the precessing motion.