Inhibiting and promoting effects of NO on dimethyl ether and dimethoxymethane oxidation in a plug-flow reactor

Abstract

The effects of NO addition (1000, 2000 ppm) on the low-temperature oxidation of dimethyl ether (DME) and dimethoxymethane (DMM), as particular cases of oxymethylene ethers (OME_n) with n = 0 and 1, have been investigated in a plug-flow reactor at near-atmospheric pressure in a temperature range of 400–1000 K. An in-situ electron ionization molecular-beam mass spectrometer (EI-MBMS) was used to measure the reactants, intermediates, and products, with particular attention on nitrogenous species that were scarcely detected previously. Explorative modeling with published mechanisms was performed, indicating the necessity of further model development. Potential kinetic fuel/NO interactions are discussed based on the experimental observations. The results reveal an overall inhibiting effect of NO addition on DME reactivity in the low-temperature regime, but a pronounced promoting effect at higher temperatures. For DMM, a similar temperature-dependent effect of NO was observed, but only for high NO concentration (2000 ppm). NO addition significantly suppresses the formation of hydrocarbon intermediates for both DME and DMM, but remarkably promotes the formation of methyl formate and methanol for DME. Several nitrogenous species were detected upon NO addition. The interactions of NO + HO₂ and NO + OH, together with the regeneration routes of NO, are thought to be influential for both DME and DMM oxidation, while the significance of the NO + RO₂ (R, fuel radical) reaction depends on the reactivity of the respective RO₂ radical of DME and DMM. These results contribute to the understanding of OME_n/NO interactions and serve as a basis for further model development by providing new and detailed speciation data for DME/NO and DMM/NO oxidation.

Introduction

Environmental, climate, and health considerations demand a transition to sustainable, carbon-reduced power generation and transportation. While renewable energy technologies are being introduced, it is highly likely, especially in view of the magnitude of the necessary changes, that internal combustion engines will continue to play an important role for the next decades. Accordingly, the global energy crisis and increasingly stringent emission regulations motivate the search for more environmentally friendly fuels and combustion techniques. Simulation methods of combustion processes with appropriate physics-based models are increasingly used to transfer molecular-level understanding of the combustion process to the engineering environment. For the optimization of combustion processes towards the dual goal of more efficient performance and reduced harmful emissions, significant work has been performed regarding the combustion chemistry of hydrocarbon fuels up to the level of real, distillate fuels [1]. Exhaust gas recirculation (EGR) is increasingly used as an effective approach to reduce important pollutants such as NO_x and soot by diluting the reactants with exhaust burnt gases, allowing operation under fuel-lean conditions and at lower temperatures. However, introducing thermally and chemically active species such as H₂O, CO₂, and NO_x into the engine may affect the combustion process [2,3]. Among these species, NO is probably the most active one capable of altering the ignition process that is highly sensitive to low-temperature kinetics [4,5]. Therefore, the effect of NO addition on fuel oxidation characteristics was widely studied [6], [7], [8], [9], [10], [11], [12], [13].

Numerous studies have investigated the interactions between NO and fuels such as hydrocarbons, alcohols, and ethers in different experiments including flow reactors, jet-stirred reactors (JSRs), and homogeneous charge compression ignition (HCCI) engines, showing sensitization effects of NO on fuel oxidation [4,[6], [7], [8], [9], [10],[12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Nevertheless, the complex fuel-specific chemical interactions between fuel and NO are still not fully understood and questions remain how NO affects different types of fuels [24]. Most experimental investigations have been devoted to the interactions between hydrocarbons and NO, while studies on emerging renewable alternative fuels, particularly regarding promising oxygenated fuels, are still limited [25].

Among oxygenated fuel candidates, the family of oxymethylene ethers (OME_n) with molecular structure of CH₃O(CH₂O)_nCH₃ is particularly interesting. With generally high cetane number, high oxygen content and an absence of C–C bonds allowing for practically soot-free and low-NO_x combustion, OME_n, particularly OME_3–5, have attracted increasing attention as a fuel additive [26], [27], [28], [29], [30], [31], [32]. Moreover, OME_n are considered as potential electric fuels (e-fuels) that can be produced sustainably [27,30,31]. Although a relatively clean combustion process with high efficiency has been observed for OME_n blends in a diesel engine using the EGR technique [33], experimental investigations on the nature of the interaction between NO and OME_n with n ≥ 1 are not yet available. Very recently, Shrestha et al. [25] proposed a comprehensive model to describe the chemistry of two particular cases of OME_n, i.e., OME₀ (dimethyl ether, DME, CH₃OCH₃) and OME₁ (dimethoxymethane, DMM, CH₃OCH₂OCH₃), as well as their chemical interactions with NO; their model could not be validated for DMM/NO oxidation, however, due to the lack of experimental data. The other few studies on this topic are only on DME [12,[16], [17], [18] as a promising fuel that has attracted considerable attention [34]. In view of the common moiety (CH₃O-), the knowledge of DME/NO interactions is a key prerequisite to understand the combustion process of larger OME_n compounds.

Hydrocarbon/NO interactions have been reported in previous studies including both inhibiting and promoting effects of NO on fuel oxidation, depending on fuel type, oxidation conditions, temperature regions, and NO concentrations. Radical propagation and termination reactions, including NO + HO₂ = NO₂ + OH and NO + RO₂ = NO₂ + RO (R represents the fuel radical), are considered important for the sensitization effect of NO on the fuel oxidation [2,9,10,23,35]. The former reaction functions always as promoter since it converts unreactive HO₂ into reactive OH, while the impact of the latter can be quite different, depending on the temperature- and fuel-dependent reactivity of RO₂.

Regarding DME oxidation, Alzueta et al. found an accelerating effect of NO at temperatures above 800 K in an atmospheric flow reactor, albeit only under fuel-lean conditions [16]. In their recent study on high-pressure (20–60 bar) DME oxidation [12], both retarding and promoting effects of NO at low and high temperatures were observed that are in accordance with the JSR results by Dagaut et al. [17]. Model predictions show that the reaction NO + RO₂ = NO₂ + RO inhibits the oxidation at low temperatures, since RO₂ is responsible for the low-temperature DME reactivity arising from the involved chain-branching pathways [12,17]. However, the complex chemical interactions between DME and NO are highly sensitive to the NO concentration and temperature ranges, and thus remain incompletely understood [36]. Moreover, detailed quantitative speciation measurements in a DME/NO system, that would include both hydrocarbon and oxygenated intermediates that were detected in pure DME oxidation [37], [38], [39], are not available. In addition, the reported NO_x deficit in some fuel/NO studies indicates the formation of undetected nitrogenous species [5,6,14,40,41]. These species, which are model-predicted to be likely HONO, CH₃NO₂, etc. [4,5,40,42], may contribute significantly to the sensitization effect of NO on fuel oxidation. They were rather scarcely detected experimentally in the oxidation of alkane fuels doped with NO_x [13,40,41,43], but not in the fuel/NO oxidation systems relevant for the present investigations.

In this study, we have chosen DME (OME₀) and DMM (OME₁) as representative molecules of the OME_n family since we wished to examine the influence of the common (CH₃O-) and (-OCH₂O-) moieties in the fuel/NO interactions, which can then benefit the understanding for the interactions of larger OME_n compounds with NO. The oxidation of DME and DMM in a temperature range of 400–1000 K with different amounts of NO addition (0, 1000, 2000 ppm) was studied in a laminar plug-flow reactor (PFR) at near-atmospheric pressure (970 mbar). As previously reported, DME exhibits high low-temperature reactivity characterized by a negative temperature coefficient (NTC) behavior [37], while DMM has insignificant low-temperature reactivity [28,29]. It is therefore interesting to unravel the fuel-specific effects of NO on DME and DMM oxidation. Using electron ionization molecular-beam mass spectrometry (EI-MBMS), temperature-dependent profiles of reactants, intermediates, and products were simultaneously detected with attention to hydrocarbon, oxygenated, and nitrogenous species. This work is majorly focused on providing detailed and new speciation data for DME and DMM oxidation in the absence and presence of NO and provides a consistent experimental basis for analyzing the effect of NO on DME and DMM oxidation. Experiments on DMM/NO interactions have, to the best of our knowledge, not been reported previously. These results are believed to be helpful towards a better understanding of the OME_n/NO interactions and to serve as a basis for further model development.