Experimental and numerical low-temperature oxidation study of ethanol and dimethyl ether

Abstract

Low-temperature combustion (LTC) receives increasing attention because of its potential to reduce NO_x and soot emissions. For the application of this strategy in practical systems such as internal combustion engines and gas turbines, the fundamental chemical reactions involved must be understood in detail. To this end, reliable experimental data are needed including quantitative speciation to assist further development of reaction mechanisms and their reduction for practical applications.

The present study focuses on the investigation of low-temperature oxidation of ethanol and dimethyl ether (DME) under identical conditions in an atmospheric-pressure laminar flow reactor. The gas composition was analyzed by time-of-flight (TOF) mass spectrometry. This technique allows detection of all species simultaneously within the investigated temperature regime. Three different equivalence ratios of ϕ = 0.8, 1.0, and 1.2 were studied in a wide, highly-resolved temperature range from 400 to 1200 K, and quantitative species mole fraction profiles have been determined.

The experiments were accompanied by numerical simulations. Their results clearly show the expected different low-temperature oxidation behavior of both fuels, with a distinct negative temperature coefficient (NTC) region only observable for DME. With detailed species information including intermediates, differences of the kinetics for both fuels are discussed. Small modifications of the mechanisms served to identify sensitivities in the model. The experimental results may assist in the improvement of kinetic schemes and their reduction.

Introduction

For a transformation of the global energy system towards increasing sustainability, it is desirable to improve conventional or to design novel combustion processes, while at the same time, environmentally-friendly fuels and routes to their production should be identified. Future combustion processes must feature substantially reduced pollutant emissions while maintaining high efficiency. A promising concept in that respect is low-temperature combustion (LTC). The LTC regime can be realized, for example, by high dilution of the fuel-oxidizer mixture through exhaust gas recirculation [1]. The low-temperature behavior is important for homogeneous charge compression ignition (HCCI) engines [2], [3], [4], [5], characterized by low soot and low NO_x emissions [6], [7], [8], [9], [10]. Combustion at low temperatures may feature different reaction pathways from those occurring in the common high-temperature combustion regime. Low-temperature kinetics could lead to combustion instabilities, and appropriate control strategies may be needed [11].

The reaction schemes for many fuels in the low-temperature regime have recently been reviewed by Battin-Leclerc [12] and Zádor et al. [13]. However, detailed kinetic studies for the low-temperature regime are scarce, and the examination of reaction mechanisms for combustion in a highly diluted regime is of particular importance. Ethanol as an established alternative fuel and its isomer dimethyl ether (DME) are interesting targets for an in-depth analysis under such conditions.

A significant number of studies exists on both ethanol and DME combustion. Ethanol combustion reactions have been investigated experimentally and in mechanistic studies [14], [15], [16], [17], [18], [19], with the mechanism of Cancino et al. [17] as the only ethanol oxidation mechanism in the intermediate-temperature regime. The thermal decomposition of ethanol in a flow reactor was investigated in an early study by Rotzoll [20] and, more recently, by Li et al. [21]. Ethanol combustion was studied in low-pressure flames [18], [22], [23], [24], [25], [26], and ethanol oxidation was investigated at 1100 K in a flow reactor by Norton and Dryer [27]. Haas et al. [28] have investigated the low- and intermediate-temperature oxidation of ethanol in a flow reactor under knock-relevant conditions, while Alzueta and Hernández [29] have addressed the influence of NO on ethanol oxidation. More recently, Leplat et al. [23] and Dagaut and Togbé [30] have investigated the oxidation of ethanol in a jet-stirred reactor (JSR) at atmospheric and high pressure in the ranges of 890–1250 K and 770–1220 K, respectively. Although Frassoldati et al. [19] have modeled low-pressure propene–oxygen–argon flames blended with DME or ethanol at higher temperatures, a comparative study of the low-temperature chemistry of the isomeric fuels ethanol and DME, especially in the regime below ∼800 K, is still lacking.

Mechanisms for oxidation and pyrolysis of DME in the low- and high-temperature regimes have been reported in [31], [32], [33], [34]. Hidaka et al. [35] and Sivaramakrishan et al. [36] investigated the pyrolysis of DME, while Pfahl et al. [37] studied its autoignition. A wide range of combustion conditions for DME was addressed in several flame experiments [26], [38], [39], [40], [41], [42]. Also, DME oxidation was investigated by Dagaut et al. [43], [44] in a JSR, and its thermal decomposition and oxidation in a flow reactor were studied by Fischer, Curran and the group of Dryer et al. [31], [32], [33]. Moreover, Liu et al. [45] and Alzueta et al. [46] have investigated the influence of NO on DME oxidation. Species concentrations in previous work have commonly been determined by Fourier-transform infrared (FT-IR) spectroscopy or online gas chromatography (GC). Mass spectrometric detection of species in DME oxidation in a flow reactor has only been used by Guo et al. [47] and in our own work [48].

Here, we report quantitative mole fractions of reactants, products and intermediates for both DME and ethanol oxidation, using simultaneous mass spectrometric detection for all species. A laminar flow reactor was coupled to an electron ionization (EI) time-of-flight (TOF) mass spectrometer and used for a systematic study of the low-temperature oxidation for the two C₂H₆O-isomers over a wide temperature range of 400–1180 K at equivalence ratios of ϕ = 0.8, 1.0, and 1.2. With online detection of the complete stable species pool, a superior temperature resolution was achieved through continuous ramping of the reactor temperature. Identical conditions were chosen for both fuels so that a direct comparison is feasible. A major aspect of our present paper is to report the experimental data for the interesting low-temperature oxidation regime. Modeling was performed in addition for a better understanding of the chemistry in this system, but not with an aim to develop a new model. An analysis of the low-temperature combustion chemistry with mechanisms available in the literature was performed to elucidate details of the reaction pathways. Minor changes in the kinetic models served to emphasize sensitive reactions in the established mechanisms.