Fuel Processing Technology (Oct 2024)
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.
