Effect of ammonia reaction kinetics on the two-stage ignition mechanism of dimethyl ether
Juan Ou, Zunhua Zhang, Zhentao Liu, Jinlong Liu
Abstract
This paper investigates the impact of ammonia (NH 3 ) kinetics on the ignition mechanism of dimethyl ether (DME), a topic minimally addressed in existing literature, by utilizing a hypothetical NH 3 representative species with identical thermodynamic properties and atomic mass to actual NH 3 , 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., NO 2 , NO, NH 2 ), through C N reactions, reduce the flux in the keto-hydroperoxides (KET) formation pathway in DME. The NH 3 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 CH 2 O 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 (H 2 O 2 )-loop mechanism. The LTHR stage further enhances C N reactions in the CH 3 pathway, favoring NO production and increasing the flux of NO and HO 2 reactions releasing OH radicals. Moreover, the ammonia oxidation pathway, characterized by HO 2 radical consumption and concurrent OH radical and H 2 O 2 generation, significantly influences the H 2 O 2 -loop system, resulting in a diminished reaction flux in the H → HO 2 → H 2 O 2 mechanism during the thermal ignition preparation stage. In summary, these findings underscore the significance of C N interactions in the NH 3 /DME ignition process and highlight the necessity of considering C N interactions in mixed fuels between ammonia and other high-reactivity fuels (e.g., diesel with higher carbon atoms), for accurate ignition prediction. • The effect of NH 3 kinetics on DME ignition is investigated. • NH 3 affects the KET kinetics and the H 2 O 2 loop sub-mechanism. • C-N interactions decrease KET pathway flux in DME oxidation. • NH 3 oxidation alters the HO 2 distribution within the H 2 O 2 loop. • C-N interactions gradually activate the NO/NO 2 conversion process.