Comparative Evaluation of Hydrogen Addition and Ammonia Cracking on the Combustion Characteristics of Ammonia/Methanol Blends

This investigation examined the combustion characteristics of ammonia/methanol blends under varying conditions of hydrogen enrichment, comparing direct hydrogen additon against ammonia cracking. The analysis encompassed key parameters including laminar burning velocity, adiabatic temperature, pool radical (H/O/OH/HO₂) concentrations, ammonia and methanol reaction pathways, and NO_x emissions. The research was conducted under premixed combustion conditions with air as the oxidizer, across a wide range of equivalence ratios (0.6 to 1.2 with a step of 0.1) and hydrogen fractions (from 0 to 60%). A modified one-dimensional model (Premix) integrated with Chemkin II and a detailed kinetic mechanism combining the chemistries of hydrogen, ammonia, methanol, syngas, and methane was employed. The neat laminar premixed flame consisted of 60% ammonia and 40% methanol at 1 atm pressure. Hydrogen was incrementally incorporated to this mixture, either through direct addition or via ammonia cracking, in 10 wt % steps, while maintaining constant equivalence ratios. Particular focus was given to the concentration-dependent effects of these blends on the formation of NO, NO₂, and N₂O. The modified fictitious diluent gas method was utilized to isolate thermal contributions from other effects in enhancement of laminar burning velocities of NH₃/CH₃OH mixtures. The findings revealed that both hydrogen incorporation methods substantially enhanced the combustion intensity of NH₃/CH₃OH mixtures, with direct addition showing superior performance. In the case of ammonia cracking, the effects of H₂ and N₂ on laminar burning velocity became more pronounced with increasing NH₃ cracking. Notably, the H₂-promoting effect consistently outweighed the N₂-inhibiting effect. For a fixed hydrogen percentage (whether from direct addition or ammonia cracking), NO emissions peaked at an equivalence ratio of 0.9 before declining. Furthermore, the relationship between hydrogen content and NO formation exhibited two distinct zones: 0–40 and 40–60% hydrogen. These findings were explained through a comprehensive analysis of radical species dynamics and reaction pathways.