Article pubs.acs.org/EF
Dimethyl Ether Autoignition at Engine-Relevant Conditions Zhenhua Li,†,‡ Weijing Wang,‡ Zhen Huang,† and Matthew A. Oehlschlaeger*,‡ †
Key Laboratory for Power Machinery and Engineering of M.O.E., Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China ‡ Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, United States S Supporting Information *
ABSTRACT: The autoignition of dimethyl ether (DME), an alternative diesel engine fuel, has been studied at elevated pressures. Ignition delay times were measured in reflected shock experiments at temperatures from 697 to 1239 K and at a nominal pressure of 22−23 bar for DME/air/N2 mixtures at equivalence ratios of 0.5, 1.0, and 1.5 and with 0−40% N2 dilution. DME ignition delay times were observed to display three regimes of reactivity (high-temperature, negative-temperaturecoefficient (NTC), and low-temperature) characteristic of paraffinic hydrocarbons and were shown to decrease with increasing equivalence ratio and increase with increasing dilution at the conditions studied. Ignition delay time measurements are compared to the detailed kinetic model of Zhao et al. (Zhao et al. Int. J. Chem. Kinet. 2008, 40, 1−18) with remarkable agreement; experiment−model deviations are mostly within the experimental uncertainties. Reaction flux and sensitivity analysis performed with the Zhao et al. model illustrates the importance of H abstraction from DME in controlling high-temperature autoignition (∼1200 K). At NTC and low-temperature conditions, the competition between the addition of molecular oxygen to and βscission of the methoxymethyl (CH3OCH2) and hydroperoxy-methoxymethyl (CH2OCH2O2H) radicals plays an important role in governing autoignition and its temperature dependence.
1. INTRODUCTION Dimethyl ether (DME, CH3OCH3) can be produced from synthesis gas derived from biomass, natural gas, or coal and used in combustion applications ranging from diesel engines and gas turbines to domestic heating and cooking. At present, the largest consumption of DME occurs in developing countries where DME is blended into liquefied petroleum gas (LPG), due to its similar properties, and used for domestic heating and cooking. In the future, given its ease of production and advantageous properties as a diesel alternative, DME may see increasing use in diesel or other compression−ignition engine applications. DME has a high cetane number of 55, desirable for diesel engine operation, and has been shown to severely reduce diesel engine particulate emissions as well as NOx, SOx, and CO emissions.1 In addition to studies aimed at ascertaining the performance of compression−ignition internal combustion engines fuel with DME,2,3 there have been a number of studies focused on experimental characterization of fundamental combustion properties for DME. DME laminar flame speed measurements have been reported by a number of authors.4−10 Premixed and nonpremixed flame extinction has been investigated by Wang et al.,9,11 and diffusive flame ignition has been investigated by Zheng et al.12 Several groups have reported speciation measurements made during both DME oxidation and pyrolysis in jet-stirred reactors (JSRs),13−15 flow reactors,16−20 and premixed flames.21−24 DME autoignition is of particular interest, owing to the potential for DME use in compression−ignition engines; hence, previous ignition delay measurements (see Table 1) have been carried out in shock tubes14,25−27 and rapid compression machines28 (RCMs). See Table 1 for the condition space © 2013 American Chemical Society
examined in previous DME autoignition studies; most have considered dilute reactant mixtures at either high-temperature shock tube conditions (Dagaut et al.,14 Cook et al.,26 and Tang et al.27) or at low-temperature RCM conditions (Mittal et al.28). Only Pfahl et al.25 have previously investigated DME autoignition at conditions consistent with those found in compression−ignition engines: DME/air mixtures at high pressures and a range of temperatures spanning the lowtemperature, negative-temperature-coefficient (NTC), and high-temperature reactivity regimes. Here, we report DME autoignition measurements at engine-relevant conditions that substantially extend the conditions investigated in the Pfahl et al.25 study. Detailed kinetic modeling schemes describing DME oxidation,13,14,18,19,29,30 outlined in Table 2, have been reported in the literature starting in 1996 with Dagaut et al.13 These modeling studies have considered an increasing database of experimental targets as measurements have become available, including JSR, flow reactor, and flame speciation; laminar flame speeds; and ignition delay. DME kinetic modeling has become progressively more predictive as mechanistic understanding has improved and kinetic and thermochemical parameters have been refined based on theoretical and experimental work. The most recent comprehensive kinetic model focused on DME was published in 2008 by Zhao et al.30 and incorporates significant improvements to the previous models of Fischer et al.18 and Curran et al.,19 notably improvement of the DME chemistry, specifically rate parameters for DME thermal decomposition Received: February 19, 2013 Revised: March 26, 2013 Published: April 1, 2013 2811
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Table 1. Previous Experimental Studies of DME Autoignition study (year)
facility
reactant mixture
P (bar)
T (K)
Pfahl et al.25 (1996) Dagaut et al.14 (1998) Mittal et al.28 (2008) Cook et al.26 (2009) Tang et al.27 (2012) current study
ST ST RCM ST ST ST
DME/air; Φ = 1.0 dilute 1% DME/O2/Ar; Φ = 0.5, 1.0, 2.0 dilute 2.86% DME/O2/N2; Φ = 0.47, 0.78, 1.40 dilute 1% DME/O2/Ar; Φ = 0.5, 1.0, 2.0 dilute DME/CH4/O2/Ar, ∼94% Ar; Φ = 1.0 DME/air and DME/air/N2; Φ = 0.5, 1.0, 1.5
13, 40 3.5 10, 20 1.6−6.6 1, 5, 10 22
650−1250 1200−1600 615−735 1175−1900 1134−2105 700−1270
Table 2. Detailed Kinetic Models Found in the Literature for DME Oxidation study (year) 13
no. of species/reaction
Dagaut et al. (1996) Dagaut et al.14 (1998)
43/286 55/331
Curran et al.29 (1998)
78/336
Fischer et al.18 (2000) and Curran et al.19 (2000)
82/351
Zhao et al.30 (2008)
55/290
validation 13
Dagaut et al. JSR speciation (800−1300 K, 1−10 bar) Dagaut et al.14 JSR speciation (550−1100 K, 10 bar) Dagaut et al.14 shock tube ignition delay (1200−1600 K, 3.5 bar) Pfahl et al.25 shock tube ignition delay (650−1250 K, 13 and 40 bar) Dagaut et al.13 JSR speciation (800−1300 K, 1−10 bar) Pfahl et al.25 shock tube ignition delay (650−1250 K, 13 and 40 bar) Amano and Dryer16 flow reactor speciation (800−1060 K, 10−18 atm) Fischer et al.18 flow reactor speciation (1060−1118 K, 1−2.5 atm) Dagaut et al.13 JSR speciation (800−1300 K, 1−10 bar) Alzueta et al.17 flow reactor speciation (875−1275 K, 1 bar) Dagaut et al.14 shock tube ignition delay (1200−1600 K, 3.5 bar) Curran et al.19 flow reactor speciation (550−850 K, 12−18 atm) Dagaut et al.14 JSR speciation (550−1100 K, 10 bar) Pfahl et al.25 shock tube ignition delay (650−1250 K, 13 and 40 bar) Alzueta et al.17 flow reactor speciation (875−1275 K, 1 bar) Fischer et al.18 flow reactor speciation (1060−1118 K, 1−2.5 atm) Curran et al.19 flow reactor speciation (550−850 K, 12−18 atm) Dagaut et al.13,14 JSR speciation (550−1300 K, 1−10 bar) Dagaut et al.14 shock tube ignition delay (1200−1600 K, 3.5 bar) Cook et al.26 shock tube ignition delay (1250−1550 K, 1.8 bar) Pfahl et al.25 shock tube ignition delay (650−1250 K, 13 and 40 bar) Kaiser et al.21 flame speciation (4 kPa) McIlroy et al.22 flame speciation (1 atm) laminar flame speeds4−6 (1−10 atm)
and DME and radical reactions, and the base H2/C1−C2 chemistry. The present experimental study provides a partial validation of the Zhao et al. model at high-pressure conditions similar to those found in internal combustion engines. In addition to the detailed kinetic modeling schemes outlined in Table 2, Toulson et al.31 have reported a multistep ignition model, based on the modified version32 of the Shell model originally proposed by Halstead et al.,33 optimized against the RCM ignition measurements of Mittal et al.28 The Toulson et al. multistep model is useful for modeling autoignition in computational fluid dynamic simulations of internal combustion engines, due to the dramatic reduction in computational time the multistep ignition model (7 species, 8 reactions, and 26 constants) provides relative to detailed kinetic models.
Table 3. Experimental Conditions for DME Ignition Delay Time Measurements
2. EXPERIMENTAL METHOD
mechanically mix for at least 20 min prior to their introduction into the shock tube for ignition experiments. Dimethyl ether was from Aldrich at 99+% purity, and O2 and N2 were from Noble Gas Solutions at 99.995% purity. Polycarbonate diaphragms were burst to generate shock waves by filling the driver with either helium or tailored helium−nitrogen mixtures, for extended reflected shock test times (greater than ∼1.5 ms). The postshock conditions were calculated using the normal shock relations with input measured end wall incident shock velocities and specified initial conditions, where thermodynamic properties for reactant species were taken from Goos et al.38 Incident shock velocity profiles were measured using five piezoelectric pressure transducers spaced over the last meter of the driven section. The calculated initial reflected shock temperature and pressure are reported as the experimental conditions in the Supporting Information and have uncertainties of ±1−1.5% and ±1.5−2%, respectively, primarily due to uncertainties in the measured incident shock velocity but also due to contributions to uncertainty from the initial preshock conditions.
Ignition delay time measurements were made in reflected-shockheated gases for DME/air/N2 mixtures, at conditions specified in Table 3, in the Rensselaer heated high-pressure shock tube facility previously described by Oehlschlaeger and co-workers.34−36 Exhaust gas recirculation (EGR) was simulated through the dilution of DME/ air mixtures with added N2. In a previous study of methyl decanoate ignition,37 we have observed that, at moderate to high temperatures (>900 K) and high pressures (20−50 bar), N2 dilution is suitable to simulate EGR containing CO2 and water vapor (i.e., at these conditions differences in ignition delay times for fuel/air/CO2/H2O and fuel/air/N2 mixtures are indiscernible). Reactant mixtures were prepared in a mixing vessel, with an internal rotating vane assembly for mechanical gas mixing, via partial pressures prior to shock tube experiments. DME, N2, and O2 were added in the gas phase and in that order to the mixing vessel at fractions defined for the various mixtures in Table 1. The mixtures were allowed to 2812
DME (%)
O2 (%)
N2 (%)
dilution (%)
ϕ
P (bar)
T (K)
3.38 6.54 9.51 2.71 5.24 7.60 2.03 3.93 5.70
20.30 19.63 19.01 16.24 15.71 15.21 12.18 11.78 11.41
76.32 73.82 71.48 81.06 79.06 77.19 85.79 84.29 82.89
0 0 0 20 20 20 40 40 40
0.5 1.0 1.5 0.5 1.0 1.5 0.5 1.0 1.5
23 23 23 22 23 23 22 22 22
± ± ± ± ± ± ± ± ±
697−1224 698−1221 800−1239 1084−1202 1068−1180 1056−1163 1102−1183 1044−1154 1035−1119
2 2 2 2 2 2 2 2 2
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Figure 1. Example reflected shock ignition delay time measurements: single-stage ignition (left, observed for T > ∼800 K) and two-stage ignition (right, observed for T < ∼800 K). Profiles are side wall pressure (2 cm from shock tube end wall) and OH* emission viewed through the end wall.
Figure 2. Ignition delay times for DME/air mixtures. Left, equivalence ratio dependence: present study and Zhao et al.30 model at 23 bar and ϕ = 0.5, 1.0, and 1.5. Right, pressure dependence: present study (23 bar), Pfhal et al.25 (13 and 40 bar), and Zhao et al. model at ϕ = 1.0.
Figure 3. High-temperature ignition delay time dependence on equivalence ratio at 23 bar. Left: DME/air/20% N2. Right: DME/air/40% N2. Symbols are current data, and lines are Zhao et al.30 model.
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Figure 4. High-temperature ignition delay time dependence on N2 dilution at 22−23 bar. Left: ϕ = 0.5 DME/air/N2. Right: ϕ = 1.5 DME/air/N2. Symbols are current data, and lines are Zhao et al.30 model. Results at ϕ = 1.0 show similar trends. Following the passage of the reflected shock, the pressure at the driven section end wall location is observed to rise slowly due to viscous gasdynamics. The rate of pressure rise is measured to be (dP/dt)(1/ P0) = 2−3% ms−1, which can be incorporated into kinetic modeling calculations to more accurately simulate the reflected-shock reacting environment than the standard constant volume assumption. Ignition times were determined from measured pressure profiles, monitored using a piezoelectric pressure transducer flush-mounted in the shock tube side wall at a location 2 cm from the shock tube end wall (Kistler transducer model 603B1 and amplifier model 1050B), and electronically excited OH radical chemiluminescence profiles, monitored through an UV fused silica optic mounted in the shock tube end wall using a filtered silicon photodetector (Thorlabs model PDA36A, filtered with UG-5 Schott glass for OH* detection around 300−320 nm). Example experiments are shown in Figure 1. Ignition times were defined as the time between shock reflection at the end wall, determined from time of shock passage 2 cm upstream from the end wall and the measured shock velocity, and the onset of ignition at the end wall, defined by extrapolating the maximum slope in the excited OH chemiluminescence signal to the baseline. Measured ignition times (see the Supporting Information) have estimated uncertainties of ±20%, calculated based on contributions to uncertainty from reactant mixture composition (∼2% uncertainty in all reactant concentrations), reflected shock temperature and pressure (1−1.5% and 1.5−2% uncertainty, respectively), and uncertainty in determining ignition delay from recorded pressure and chemiluminescence signals (5−20 μs, depending on length of ignition delay time). As is typically the case in shock tube autoignition experiments, the primary contributor to ignition delay uncertainty is reflected shock temperature uncertainty. For example, for an apparent activation energy of 30 kcal/mol, representative of the high-temperature reactivity region for DME and many hydrocarbon fuels, uncertainty of 1% in reflected shock temperature yields a contribution to ignition delay uncertainty at 1000 K of 14%, prior to the consideration of uncertainty from other sources. While the onset of a strong ignition event is clearly defined from the excited OH chemiluminescence signals for all experimental conditions, at temperatures less than approximately 800 K, the strong ignition event is preceded by a mild but pronounced first-stage ignition event, see Figure 1 (graph on right). The observation of two-stage ignition, a mild first-stage ignition followed by a strong second-stage ignition, is consistent with DME ignition time measurements reported by Pfhal et al.25 and is due to the negative-temperature-coefficient behavior characteristic of intermediate- to low-temperature DME oxidation.
3. RESULTS AND DISCUSSION Ignition delay time measurements for DME/air/N2 mixtures are compared on Arrhenius axes in Figures 2 and 3 with the predictions of the comprehensively validated DME kinetic modeling scheme from Zhao et al.30 and the only previous DME shock tube ignition delay measurements made at elevated pressures by Pfahl et al.25 (ϕ = 1.0 DME/air at 13 and 40 bar). Experimental results show three regions of reactivity: a hightemperature region of Arrhenius-like temperature dependence for temperatures greater than ∼1000 K at 23 bar, a region of NTC behavior where ignition delay mildly decreases with decreasing temperature (∼800−1000 K at 23 bar), and a lowtemperature region with positive apparent activation energy for temperatures lower than ∼800 K at the 23 bar experimental pressure. The results also illustrate decreasing ignition delay with increasing pressure and equivalence ratio and increasing ignition delay with increasing dilution, typical of fuel/air/ diluent mixtures at elevated pressures and temperatures less than ∼1300 K. Kinetic simulations with the Zhao et al.30 model, shown in Figures 2−4, were carried out with CHEMKIN-PRO39 using the closed adiabatic homogeneous reactor model with a variable volume history to account for a measured pressure gradient in the reflected shock region of (dP/dt)(1/P0) = +3% ms−1 due to viscous gasdynamics. Modeled ignition delay times were determined from the simulated ground-state OH profiles using the maximum gradient method used to interpret the experimental excited OH chemiluminescence profiles. The agreement between the Zhao et al. model and the present data is generally very good in terms of temperature dependence, equivalence ratio dependence, and absolute ignition delay. For the most part, experiment−model deviations for ignition delay are within the ±20% experimental uncertainties and at most within ±50%, with the larger deviations occurring on the rich side of stoichiometric. Comparison of the Zhao et al. model and the Pfhal et al.25 data at 13 and 40 bar shows similar agreement. While there is no overlap in the conditions of the present study (23 bar) and Pfhal et al. (13 and 40 bar), Figure 2 (graph on right) illustrates consistency between the combination of the two experimental studies and the Zhao et al. model for stoichiometric DME/air mixtures, in terms of both pressure 2814
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and temperature dependence of ignition delay times and absolute ignition delay times. In Figures 3 and 4, ignition delay measurements are illustrated at dilute conditions (DME/air/N2 mixtures with 20% and 40% N2 dilution) with comparison to the Zhao et al. model. Again, the deviation between model and experiment are mostly within or close to the ±20% experimental uncertainties. The agreement with the present study provides further validation of the Zhao et al. model at high-pressure conditions encountered in internal combustion engines. As described in Table 2, the Zhao et al. model was previously validated using a wide range of flow reactor, JSR, shock tube, and flame experiments, many of which were performed at dilute and lower pressure conditions, the primary exception being the Pfhal et al. shock tube study. For temperatures in excess of 1000 K, the present ignition delay times can be correlated using power law-dependence of ignition delay on equivalence ratio and N2 dilution, and when the present study is combined with the 13 and 40 bar results from Pfhal et al.,25 the combined data sets provide hightemperature (>1000 K) power-law dependence of ignition delay on pressure. A regression analysis applied to the combination of the present study and the Pfhal et al. study provides the following ignition delay scaling for temperatures in excess of 1000 K: −1 ⎡ N dilution (%) ⎤ τ ∝ P−0.85ϕ−0.65⎢1 − 2 ⎥ ⎣ ⎦ 100
where τ is the ignition time, P is the pressure, ϕ is the equivalence ratio, and N2 dilution is the percent N2 dilution as given in Table 3. Similar ignition delay scaling with pressure, equivalence ratio, and dilution have been measured for hydrocarbon35,40,41 and oxygenated fuels37 at high-pressure (>10 bar) fuel/air/diluent conditions. At temperatures less than 1000 K, where low-temperature oxidation chemistry plays a role, the above power-law scaling does not hold, and the influence of pressure, equivalence ratio, and dilution on ignition delay increases. The Zhao et al. model is also in good agreement with the high-temperature pressure, equivalence ratio, and dilution power-law scaling determined via regression analysis of the experimental data and also clearly shows deviation from these power-laws scalings in the NTC window (∼800−1000 K at 23 bar, Figure 2). Reaction flux and sensitivity analysis was carried out using the Zhao et al. model to illuminate the reaction pathways controlling DME reactivity and its temperature dependence. Figure 5 illustrates the primary reaction fluxes for stoichiometric DME/air oxidation at 23 bar and 700, 900, and 1200 K at a time equivalent to 10% DME consumption, and Figure 6 shows ignition delay sensitivity coefficients for the 10 most sensitive reactions at the same conditions. At temperatures of ∼1200 K and less, DME thermal decomposition to methyl (CH3) and methoxy (CH3O) radicals is relatively slow, and DME is almost wholly consumed by Hatom abstraction by radicals (primarily CH3, OH, HO2, and H) or at initiation by molecular oxygen to produce methoxymethyl radicals (CH3OCH2). At high temperatures (1200 K in Figure 5), the methoxymethyl radicals quickly decompose via βscission to form methyl radicals and formaldehyde that then can react through a number of channels. The sensitivity calculation at 1200 K shows that the reactions involving the products of methoxymethyl radicals (methyl radicals and formaldehyde) are very important in controlling ignition
Figure 5. Reaction flux analysis for DME oxidation (at 10% DME consumption) using the Zhao et al.30 kinetic model. Conditions: DME/air, ϕ = 1.0 and 23 bar and 700, 900, and 1200 K. The reaction fluxes are given at each temperature as a percent of total reaction flux and are rounded to the nearest percent.
delay as are the H-abstraction reactions from DME. At 1200 K, it is notable that the fate of the methyl radical is crucial in controlling ignition. Methyl radicals formed by methoxymethyl β-scission can recombine to form stable ethane, inhibiting reactivity, or attack DME producing another methoxymethyl radical and methane, which despite the formation of stable methane promotes reactivity. At high temperatures, each methoxymethyl radical yields formaldehyde (CH2O), which leads to radical production through the generation of hydrogen peroxide and its subsequent decomposition: CH2O + HO2 → HCO + H2O2 and H2O2 + M → OH + OH + M. Hydrogen peroxide production is also enhanced by HCO + O2 → CO + HO2 where HCO can be produced by reaction of formaldehyde with any radical, CH2O + X → HCO + XH. HO2 then leads to hydrogen peroxide mostly through CH2O + HO2 → HCO + H2O2 and CH3OCH3 + HO2 → CH3OCH2 + H2O2. At intermediate temperatures (900 K in Figure 5), methoxymethyl radical β-scission competes with the addition of molecular oxygen to form the methoxymethyl-peroxy radical (CH3OCH2O2). The CH3OCH2O2 radical then can isomerize to the hydroperoxy-methoxymethyl (CH2OCH2O2H) radical. At 900 K, the CH2OCH2O2H radical is unstable and mostly undergoes β-scission, CH2OCH2O2H → CH2O + CH2O + OH, an inhibitive radical propagation pathway. Sensitivity analysis performed at 900 K (Figure 6) shows that the β2815
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chemistry, important at 900 and 1200 K, has very small ignition delay sensitivity.
4. SUMMARY Ignition delay time measurements are reported for DME, an alternative fuel that can be used in a number of applications including diesel engines. Measurements were made in a shock tube at pressures near 22−23 bar and temperatures from 697 to 1239 K for ϕ = 0.5, 1.0, and 1.5 DME/air/N2 mixtures containing 0−40% N2 dilution. The measurements reveal the three distinct regimes of reactivity typical of paraffinic hydrocarbons: a high-temperature regime (>1000 K at 23 bar) with large positive apparent activation energy, a negativetemperature coefficient regime (800−1000 K at 23 bar) exhibiting a slight reduction in ignition delay with decreasing temperature, and a low-temperature regime (
