Use of methanol as a promoter for ammonia combustion

Abstract

This work aims to study the oxidation of ammonia and methanol mixtures (NH₃/CH₃OH). For this purpose, laboratory experiments were conducted using a quartz flow reactor at atmospheric pressure, in a temperature range of 875–1425 K. The oxygen excess ratio (λ) and the NH₃/CH₃OH ratio were modified during the experiments. The experimental results have been simulated with a literature-based kinetic mechanism. The results show that the presence of CH₃OH and the oxygen excess ratio affect the conversion of NH₃, shifting its oxidation to lower temperatures as these variables increase. The oxidation of both fuels was slightly boosted with increasing CH₃OH concentration. The λ study showed that the fuel-lean conditions accelerate NH₃ oxidation at lower temperatures whereas do not have the same effect on CH₃OH oxidation. The H radical concentration significantly influences fuel consumption, especially in reactions involving CH₃OH and NH₂, and it is also key for inhibition processes. CH₃OH was found to initiate NH₃ reactions, with strong competition for OH radicals between the two fuels. Nevertheless, methanol helps reduce ammonia's oxidation temperature. CH₂OH was identified as the predominant species following H-abstraction from CH₃OH. In the NH₃/CH₃OH ratio studies, increasing methanol concentration lowered the oxidation temperature of both fuels, with a temperature difference of up to 150 K observed for NH₃/CH₃OH ratios from 0.6 to 10. Increasing methanol concentration for a given NH₃ value also shifted the prominence of secondary reaction pathways, further influencing the overall oxidation process.