Assessment of carbon-free fuel ammonia combustion with low methanol blends in reducing GHG emissions including N2O

Ammonia is presenting an ideal pathway towards reducing greenhouse gas (GHG) emissions by substituting traditional fuels to achieve carbon neutrality. However, the generation of N2O during ammonia combustion weakens ammonia’s carbon-free nature due to its GHG effect being 298 times greater than that of CO2. To enhance the combustion efficiency of ammonia and preserve its carbon-free characteristics, the N2O generation mechanism during ammonia combustion with low methanol blending ratios was investigated. A hybrid Perfectly Stirred Reactor-Plug Flow Reactor chemical reactor network was utilized, and the results indicate that N2O emissions are much higher under lean ammonia conditions (especially for Ф = 0.7–0.8) than under ammonia rich conditions (Ф = 1.1–1.2). Moreover, the equivalence ratio region achieving low emissions of NO, unburned NH3, and N2O simultaneously is found at Ф = 1.1–1.2. This region enables a reduction in GHG emissions by over 70% compared to pure methanol combustion. The production and consumption rates of N2O are controlled by the reaction NH + NON2O + H. At low equivalence ratios, the mole fraction of H decreases, causing a separation between production and consumption rate curves and resulting in significant emissions of N2O. Compared to pure ammonia combustion, the N2O emissions are reduced by low methanol blends of 2%–10% through different mechanisms during lean and rich combustion conditions, which are controlled by H and NH, respectively. Under lean conditions, the presence of methanol enhances the production of H, which promotes the decomposition of N2O. Under rich conditions, methanol reduces the formation of NH, thereby suppressing the generation of N2O. The obtained low GHG emission control region of ammonia/methanol combustion is expected to support the ammonia fuel application in the future. •NH3 combustion with low methanol blends are studied using PSR-PFR network.•It indicates low NH3 and low N2O at lean (0.7–0.8) versus rich (1.1–1.2) conditions, respectively.•Low methanol blends found different N2O reduction mechanisms under lean and rich conditions.•Ammonia/methanol combustion achieves the lowest GHG (CO2 and N2O) at Ф = 1.2.