Elsevier

Combustion and Flame

Volume 162, Issue 2, February 2015, Pages 315-330
Combustion and Flame

An ignition delay and kinetic modeling study of methane, dimethyl ether, and their mixtures at high pressures

https://doi.org/10.1016/j.combustflame.2014.08.014Get rights and content

Abstract

The development of accurate chemical kinetic models capable of predicting the combustion of methane and dimethyl ether in common combustion environments such as compression ignition engines and gas turbines is important as it provides valuable data and understanding of these fuels under conditions that are difficult and expensive to study in the real combustors. In this work, both experimental and chemical kinetic model-predicted ignition delay time data are provided covering a range of conditions relevant to gas turbine environments (T = 600–1600 K, p = 7–41 atm, ϕ = 0.3, 0.5, 1.0, and 2.0 in ‘air’ mixtures). The detailed chemical kinetic model (Mech_56.54) is capable of accurately predicting this wide range of data, and it is the first mechanism to incorporate high-level rate constant measurements and calculations where available for the reactions of DME. This mechanism is also the first to apply a pressure-dependent treatment to the low-temperature reactions of DME. It has been validated using available literature data including flow reactor, jet-stirred reactor, shock-tube ignition delay times, shock-tube speciation, flame speed, and flame speciation data. New ignition delay time measurements are presented for methane, dimethyl ether, and their mixtures; these data were obtained using three different shock tubes and a rapid compression machine. In addition to the DME/CH4 blends, high-pressure data for pure DME and pure methane were also obtained. Where possible, the new data were compared with existing data from the literature, with good agreement.

Introduction

The depletion of crude oil resources has motivated the search for alternative energy sources. Currently, the combustion of hydrocarbons remains the biggest producer of energy throughout the world, and in the short- to medium-term will remain so. However, the combustion of fossil fuels has contributed to global warming and increased levels of pollution. Biofuels can be produced from renewable sources and can reduce undesirable emissions associated with conventional fossil fuels. Methane (CH4), which is the predominant component of natural gas, is a relatively clean-burning fossil fuel and can be considered a renewable energy source when produced as biomethane via anaerobic digestion of biomass [1]. Dimethyl ether (DME) is also considered a second generation biofuel as it can be produced from biomass. It is more commonly produced in a two-step process, where syngas is converted to methanol which can then be used to generate DME through a dehydration reaction [2]. Semelsburger et al. [3] found that DME ranks highly as an alternative fuel for the future.

It is for these reasons that a large number of studies have been performed on the combustion of these fuels. Due to its high cetane number (55), DME is considered a good alternative to diesel. DME has been studied experimentally in diesel engines [4], [5], [6], [7] showing its advantages in terms of emissions and engine efficiency. Particulate matter (PM) emissions were found to be greatly reduced, as were NOx and SOx, while there was a slight increase in carbon monoxide (CO) and hydrocarbon (HC) emissions.

Methane is the main component of natural gas and is commonly burned in gas turbines. Due to DME’s excellent autoignition characteristics, it has been used as an additive or alternative to natural gas in gas turbines [8], [9], leading to interest in the combustion kinetics of mixtures of these two fuels. Mixtures of methane and DME have also been studied within homogeneous charge compression ignition (HCCI) engines [10], [11], where DME was found to be an excellent ignition improver. Several flame speed studies have been conducted on DME [12], [13], [14], [15], [16], [17] using a variety of different methods such as constant-volume bomb using optical observation of the flame and counterflow flames; in some devices, particle image velocimetry has been utilized to determine the laminar flame speed from the observed gas velocities.

Species profiles were first measured by Dagaut et al. [18] in a jet-stirred reactor (JSR) using fuel mixtures highly diluted in argon, for equivalence ratios from 0.2 to 1.0, at a pressure of 10 atm, and in the temperature range 550–1100 K. Subsequently, flow-reactor data were taken by Fischer et al. [19] (1118 K, 3.5 atm, and 1085 K, 1 atm, ϕ = 0.32–3.40) and Curran et al. [20] (550–850 K, 12–18 atm, ϕ = 0.7–4.2). These studies [19], [20] also developed a detailed chemical kinetic mechanism to simulate their experimental data, using it to identify the important reaction pathways controlling DME fuel oxidation. This mechanism was also used to simulate JSR data [18] and shock-tube ignition delay times.

Zhao et al. [21] used Rice–Ramsperger–Kassel–Marcus (RRKM)/master equation calculations to calculate rate constants for the unimolecular decomposition of DME as a function of temperature and pressure. Their study also reported flow reactor data at 980 K and 10 atm as a function of residence time. A chemical kinetic model was validated using experimental data which included flow reactor, JSR, shock tube ignition delays, laminar flame speciation and flame speed measurements. Wang et al. [22] and Cool et al. [23] used electron–ionization molecular-beam mass spectrometry and photoionization molecular-beam mass spectrometry performed using synchrotron radiation for the analysis of a stabilized flat flame to provide species profiles within DME flames.

Cook et al. [24] used laser absorption of ȮH radicals behind reflected shock waves to isolate and measure the rate constants of the decomposition of DME and the rate constant for H-atom abstraction from DME by ȮH radicals at high temperatures. These measurements were coupled with RRKM/master equation calculations which agreed well with the measurements. Recently, Pyun et al. [25] measured species profiles for CO, CH4, and C2H4, during the pyrolysis of DME within a shock tube using tunable laser absorption with a quantum cascade laser. This laser absorption study was done at a range of reflected-shock temperatures (1300–1600 K) and at a reflected-shock pressure of 1.5 atm.

Previous ignition delay time studies of these fuels and their mixtures are summarized in Table 1, Table 2. These studies cover a wide range of conditions including low-to-high temperatures and pressures. Of these previous ignition delay time studies [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], only the work of Tang et al. [37] included mixtures of CH4 and DME. It covered dilute mixtures within a pressure range of 1–10 atm. The current study covers a wider range of temperatures and pressures and includes fuel in ‘air’ mixtures.

Recent experimental studies of DME combustion also include a flame speed and flame speciation study by Liu et al. [38]. The effect of diluting DME flames with 20% CO2 was studied, and flame speeds over a range of pressures were reported. Flame speciation using electron–ionization molecular-beam mass spectrometry was measured. Guo et al. [39] measured low temperature species profiles in an atmospheric flow reactor with electron–ionization molecular-beam mass spectrometry used as the detection system. Herrmann et al. [40] used an atmospheric flow reactor to measure mole fractions of species related to DME oxidation at low temperatures (400–1200 K) by time-of-flight mass spectrometry. Both of these studies compared their measured data to models available in the literature.

In this study, we provide new ignition delay time data for these two important fuels over wide regimes of temperature and pressure at engine- and turbine-relevant conditions. We have developed a detailed chemical kinetic mechanism (Mech_56.54) based on the widely validated mechanism AramcoMech1.3 [41], which is capable of predicting these new ignition delay data and available literature data. Presented first is an overview of the experiments, including details on the mixtures studied, the facilities, the measurement techniques, and modeling approaches. The experimental section is followed by a summary of the chemical kinetic mechanism and the reaction rates that were modified for the present study. A results and discussion section comprises the bulk of this paper and presents all of the ignition delay time data as well as comparisons to the kinetic mechanism. Sensitivity analyses and relevant discussions on the observed trends are also provided.

Section snippets

Experimental

A common set of mixtures was selected for study in both the rapid compression machine (RCM) and in the shock tubes, Table 3.

For the experiments at NUIG, methane and DME gases were obtained from Sigma–Aldrich at 99.0% and 99.9% purity respectively, while all other gases were supplied by BOC Ireland; nitrogen (CP Grade) 99.95%, argon (Research Grade) 99.9995%, oxygen (Medical Grade) 99.5% and all were used without further purification. At TAMU, the DME was Grade 2.6 purity (99.6%), the

Computational modeling

ChemkinPRO [51] was used for all simulations. Two methods of simulation were used to model the shock tube and RCM data and are discussed here.

Chemical kinetic model

The present model (Mech_56.54) consists of the H2/CO sub-mechanism of Kéromnès et al. [54], the C1–C2 base sub-mechanism of Metcalfe et al. [41] and the recently published propene mechanism of Burke et al. [55]. The data measured in this study and available literature data (JSR, flow reactor, RCM, shock tube, shock-tube speciation, flame speed, and flame speciation) have been used to re-validate the DME kinetic mechanism, Mech_56.54, developed here. The experimental data and the ChemkinPRO [51]

Results and discussion

As mentioned above, the shock-tube results for 80/20 and 60/40 CH4/DME mixtures have been described in the thesis of Zinner [49], and the shock-tube results for both of the pure fuels were presented in a conference paper [50], but neither set has been published formally in an archival journal until now. The tailored shock-tube data (presented as half-filled symbols) and the RCM data are presented for the first time in the present study. This section first presents the pure CH4 data and the

Conclusions

A wide range of new ignition delay time data for pure methane, pure DME, 80/20, and 60/40 mixtures of both fuels in ‘air’ are presented. Pressures from 7 to 41 atm were studied in the temperature range 600–1600 K. These data allow for mechanism validation under conditions similar to those found in compression ignition engines and gas turbines, where methane and DME have been burned previously. A new detailed chemical kinetic model (Mech_56.54) for the combustion of DME is presented and validated

Acknowledgments

Peter O’Toole acknowledges the financial support of the Irish government under PRTLI Cycle 4. Ultan Burke acknowledges the financial support of the Irish Research Council. Kieran P. Somers acknowledges the support of Science Foundation Ireland under Grant No. [08/IN1./I2055] as part of their Principal Investigator Awards. The work conducted at TAMU was funded by Rolls-Royce Canda under the direction of Dr. Gilles Bourque. The help from Nicole Donato and John Pemelton on some of the TAMU

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