Effect of ammonia reaction kinetics on the two-stage ignition mechanism of dimethyl ether

Abstract

This paper investigates the impact of ammonia (NH₃) kinetics on the ignition mechanism of dimethyl ether (DME), a topic minimally addressed in existing literature, by utilizing a hypothetical NH₃ representative species with identical thermodynamic properties and atomic mass to actual NH₃, yet remaining inert during reactions, thereby distinguishing the kinetic effects from thermal and dilution influences. Kinetic analysis via zero-dimensional (0D) idealized reactor calculations shows that DME ignition in the ammonia-air atmosphere is still primarily governed by peroxy kinetics, yet ammonia kinetics significantly modify the ignition reaction pathways of DME. Specifically, during the low-temperature oxidation preparation stage, ammonia oxidation yields nitrogen-containing species that (e.g., NO₂, NO, NH₂), through CN reactions, reduce the flux in the keto-hydroperoxides (KET) formation pathway in DME. The NH₃ oxidation pathway also competes for OH radicals, which disfavors DME ignition. The rapid decomposition of KET during the low-temperature heat release (LTHR) stage emits a substantial amount of OH radicals, increasing temperature and causing the shift from chain branching to chain propagation pathways in DME oxidation, leading to significant CH₂O production and decreased reaction reactivity. This shift also promotes the hydrogen‑oxygen reaction mechanism, transitioning the controlling mechanism from the KET mechanism to the hydrogen peroxide (H₂O₂)-loop mechanism. The LTHR stage further enhances CN reactions in the CH₃ pathway, favoring NO production and increasing the flux of NO and HO₂ reactions releasing OH radicals. Moreover, the ammonia oxidation pathway, characterized by HO₂ radical consumption and concurrent OH radical and H₂O₂ generation, significantly influences the H₂O₂-loop system, resulting in a diminished reaction flux in the H → HO₂ → H₂O₂ mechanism during the thermal ignition preparation stage. In summary, these findings underscore the significance of CN interactions in the NH₃/DME ignition process and highlight the necessity of considering CN interactions in mixed fuels between ammonia and other high-reactivity fuels (e.g., diesel with higher carbon atoms), for accurate ignition prediction.