Dimethyl Ether Oxidation at Elevated Temperatures (295−600 K)

Abstract

Dimethyl ether (DME) has been proposed for use as an alternative fuel or additive in diesel engines and as a potential fuel in solid oxide fuel cells. The oxidation chemistry of DME is a key element in understanding its role in these applications. The reaction between methoxymethyl radicals and O(2) has been examined over the temperature range 295-600 K and at pressures of 20-200 Torr. This reaction has two product pathways. The first produces methoxymethyl peroxy radicals, while the second produces OH radicals and formaldehyde molecules. Real-time kinetic measurements are made by transient infrared spectroscopy to monitor the yield of three main products-formaldehyde, methyl formate, and formic acid-to determine the branching ratio for the CH(3)OCH(2) + O(2) reaction pathways. The temperature and pressure dependence of this reaction is described by a Lindemann and Arrhenius mechanism. The branching ratio is described by f = 1/(1 + A(T)[M]), where A(T) = (1.6(+2.4)(-1.0) x 10(-20)) exp((1800 +/- 400)/T) cm(3) molecule(-1). The temperature dependent rate constant of the methoxymethyl peroxy radical self-reaction is calculated from the kinetics of the formaldehyde and methyl formate product yields, k(4) = (3.0 +/- 2.1) x 10(-13) exp((700 +/- 250)/T) cm(3) molecule(-1) s(-1). The experimental and kinetics modeling results support a strong preference for the thermal decomposition of alkoxy radicals versus their reaction with O(2) under our laboratory conditions. These characteristics of DME oxidation with respect to temperature and pressure might provide insight into optimizing solid oxide fuel cell operating conditions with DME in the presence of O(2) to maximize power outputs.

Analysis of the Kinetics and Yields of OH Radical Production from the CH3OCH2 + O2 Reaction in the Temperature Range 195–650 K: An Experimental and Computational study

“…However, relatively high DME concentrations were needed and so formation of OH radicals via reaction 2b complicated data interpretation because CH3OCH2 radicals are reformed in reaction 1, even though Cl + DME and F + DME are much faster than R1: k(Cl/F + DME) / k(OH + DME) ≈ 60 at 295 K. Another important problem, associated with the use of high DME concentrations to achieve fast CH3OCH2 radical formation for kinetic analysis, stems from the fact that DME undergoes thermal degradation in the presence of O2 at temperatures above ~500 K, as has been observed by Rosado-Reyes et al 30 Recently, in this laboratory, Eskola et al 31 measured the kinetics and yields of OH radical at 295 In this article the kinetics of the CH3OCH2 + O2 reaction has been investigated as a function of pressure (5 -500 Torr) over the temperature range 195 -650 K by monitoring hydroxyl radical formation using laser induced fluorescence (LIF). Relative determinations of OH yields from reaction 2 were performed, as a function of pressure and temperature, using several CH3OCH2 radical sources and employing a Stern Volmer analysis.…”

Experimental and kinetic modeling study of the low- and intermediate-temperature oxidation of dimethyl ether

“…Due to its promising properties and potential as an alternative fuel, DME has been investigated extensively in the past decade including experiments such as pyrolysis [5][6][7][8][9][10][11], oxidation [4,5,8,[12][13][14][15][16][17][18][19][20][21][22][23][24] and flames [25][26][27][28][29][30][31][32][33][34][35][36], and theoretical calculations [7][8][9][17][18][19][37][38][39], covering a wide range of temperatures and pressures. Based on these experimental and theoretical studies, detailed kinetic models for DME combustion have been developed [5,7,15,25,40]…”

“…They improved the Zhao model to reproduce the measured fuel conversion by adjusting the branching ratio of the second oxygen addition channel and its competitive channel of the important QOOH intermediate to CH 2 O. Moreover, some further advances of chemical kinetics and experimental diagnostics have offered useful insight regarding the key reactions [9,[17][18][19]38,41] that affect the oxidation reactivity of DME in the low-temperature regime.…”

Global Uncertainty Propagation and Sensitivity Analysis in the CH3OCH2 + O2 System: Combining Experiment and Theory To Constrain Key Rate Coefficients in DME Combustion

“…7 This reaction system is a key step in the combustion of dimethyl ether (DME) [8][9][10][11][12][13] and, with interest in the use of DME as a possible fuel 14 , the CH 3 OCH 2 + O 2 reaction has attracted substantial attention. [15][16][17][18][19] The experiments 6 probed the reaction system by observation of OH, formed by dissociation of CH 2 OCH 2 OOH. Both the time dependence of OH formation and its yield were observed.…”