Article pubs.acs.org/EF
Experimental and Kinetic Modeling Study of CH3OCH3 Ignition Sensitized by NO2 W. Ye,†,‡ J. C. Shi,‡,§ R. T. Zhang,‡ X. J. Wu,‡ X. Zhang,‡,§ M. L. Qi,*,† and S. N. Luo*,‡,§ †
School of Science, Wuhan University of Technology, Wuhan, Hubei 430070, People’s Republic of China The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610031, People’s Republic of China § Key Laboratory of Advanced Technologies of Materials, Ministry of Education, Southwest Jiaotong University, Chengdu, Sichuan 610031, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: We investigate the role of NO2 in dimethyl ether (DME) ignition with a combustion shock tube. Ignition delay times are measured at 987−1517 K and 4 and 10 atm. Different equivalence ratios (0.5, 1.0, and 2.0) and NO2 and DME concentrations are explored. NO2 promotes DME ignition, and the promoting effect becomes more pronounced at high NO2 concentrations or low temperatures. NO2 addition also augments the influence of the equivalence ratio on ignition delay times. Four detailed reaction mechanisms from the literature are examined against the measurements, and an updated kinetic model is proposed and validated in comparison to experiments. On the basis of the updated model, sensitivity analysis, reaction flux analysis, and rate of production analysis are conducted to provide details on the kinetic effect of NO2 on DME ignition.
1. INTRODUCTION The concern of increasing pollutant emission from fossil fuel combustion has driven the search for clean alternatives and advanced combustion strategies. As an alternative to diesel, CH3OCH3 or dimethyl ether (DME) is of particular interest for its high cetane number (55−60) and soot-free combustion.1,2 Several investigations with DME-fueled engines demonstrate that DME is capable of lowering NOx emission,3−5 but the level of NOx emission is still a problem.6,7 Song et al.8 reported that exhaust gas recirculation can reduce the NOx level in DMEfueled engines. Moreover, nitric oxide has been found to impact ignition timing in homogeneous charge compression ignition (HCCI) engines, which depends upon the operating condition and fuel type.9−12 Therefore, the effect of NOx on the combustion of DME should be fully investigated, and a wellvalidated DME/NOx kinetic model is desirable for the development of promising DME-fueled engines. Many kinetics studies have been conducted on DME combustion. Pfahl et al. 13 studied the self-ignition of stoichiometric DME/air mixtures using a shock tube (650− 1250 K and 13 and 40 bar). It is found that self-ignition of DME is a two-step process at lower temperatures, and there exists a negative temperature coefficient regime in ignition delay times. Dagaut et al.14 extended their previous work15 on DME oxidation, obtained low-temperature data in a jet-stirred reactor (550−1100 K, 10 atm, and equivalence ratio of 0.2−1.0) and high-temperature data in a shock tube (1200−1600 K, 3.5 atm, and equivalence ratio of 0.5−2.0), and then proposed a kinetic model. Fischer et al.16 and Curran et al.17 also developed a detailed kinetic mechanism to reproduce their flow reactor data and literature data.13,14 More recently, Aramco Mech 1.3 was developed18 to describe the oxidation of small fuel molecules (including the DME subset) and validated against a wide range of DME oxidation experiments. © XXXX American Chemical Society
However, the DME/NOx ignition kinetics is still underexplored. Alzueta et al.19 studied the effect of NOx (both NO and NO2) on DME oxidation at atmospheric pressure and a wide temperature range of 600−1500 K using a quartz flow reactor. They found that DME oxidation is largely affected by the equivalence ratio, and NOx facilitates DME oxidation only at fuel-lean conditions. In atmospheric pressure flow reactor experiments, Liu et al.20 detected several formate species during low-temperature oxidation of DME (513−973 K) with Fourier transform infrared spectroscopy and observed the formation of CH3OCHO in the presence of NO (no formation if without NO), which had been noted previously.21 Dagaut et al.22 investigated mutual sensitization of the oxidation of DME and NO at low temperatures (550−800 K) and atmospheric pressure using a jet-stirred reactor, and drastically different effects of NO were observed in different temperature regimes. Above 620 K, NO enhances the oxidation of DME, whereas it is inhibited by NO below 616 K. A detailed mechanism, including lowtemperature chemistry of DME/NOx interactions, was also proposed by Dagaut et al.22 to interpret their observations. Recently, El-Asrag and Ju23 performed numerical simulations to study the effect of NO on DME autoignition in the negative temperature coefficient regime. They used a kinetic model assembled from the NOx mechanism by Miller and Bowman24 and the DME mechanism by Zhao et al.25 Exhaust gas recirculation of NO was found to increase the heat release rate at low temperatures and accelerate the ignition process at both low and intermediate temperatures.23 Hwang et al.26 and Cung et al.27 developed different mechanisms to study the NO2 emission characteristics of DME. Received: September 23, 2016 Revised: November 6, 2016 Published: November 8, 2016 A
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
kinetics and the finite inner diameter of a shock tube. For highly diluted reactions as in the current study, the first-stage ignition can be ignored and the boundary layer effect results in an approximately 8%/ms increase in pressure before main ignition. Ignition delay time, τ, is also defined in Figure 1. It is the time interval between the arrival of the reflected shock wave (the second pressure jump) and the intersection of the baseline with the steepest rising slope of the OH* emission curve. The uncertainty in T5, estimated through the root-sum-square method,34,35 is less than 18.8 K in all experiments. The overall uncertainty in ignition delay times is ∼18.8%. Details of uncertainty analysis were presented elsewhere.29 To confirm the reliability of the shock tube, we measure ignition delay times of CH4/NO2/O2/Ar and compare them to previous measurements by Mathieu et al.36 The agreement is excellent, as seen from Figure 2.
As an important combustion property, ignition delay time is widely used to help validate chemical reaction mechanisms. However, the effect of NO2 on DME ignition is still unexplored. In this study, ignition delay times for DME/NO2 mixtures are measured over a wide range of temperatures, pressures, and equivalence ratios with a combustion shock tube. A kinetic model based on the recent work on CH3NO2 ignition28 is proposed with updated reaction rate constants. Kinetic analyses are performed to gain further insight into DME/NO2 interactions.
2. SHOCK TUBE EXPERIMENTS All experiments are performed on a stainless-steel, single-diaphragm, shock tube with a 50 mm inner diameter. A detailed description of our shock tube facility was presented previously.29 The shock tube is separated into two parts: a 3.26 m long driver section and a 4.52 m long driven section. Before each experiment, the shock tube is evacuated to below 10 Pa by a mechanical vacuum pump, and then He and a gas mixture to be tested are injected into the driver section and driven section, respectively. Upon firing, the shock wave develops quickly and propagates into the driven section. Incident shock wave speeds are measured with four axially piezoelectric pressure transducers, which are located in the last 1.2 m segment of the driven section. The incident shock speed at the end wall is determined by linearly extrapolating the measured incident shock wave speeds to the end wall. The shock temperature T5 and pressure P5 upon the first reflection at the end wall can be calculated using the Gaseq software package.30 Gas mixtures for ignition are prepared manometrically in a 15 L stainless-steel mixing tank, and the fractions of constituent gases are determined with Dalton’s law of partial pressures. The mixtures are allowed to mix overnight, and the concentration of each constituent gas is further confirmed by a gas chromatograph (7890B GC System, Agilent). The presence of N2O4 in the gas mixtures should be taken into account, owing to the positive pressure dependence of the reaction 2NO2 ⇌ N2O4. Therefore, the partial pressure of nitrogen oxides (NO2 and N2O4) should be adjusted properly to eliminate the N2O4 effects during gas mixture preparation. However, given the low initial pressure (below 50 kPa) in the driven section, the effects of N2O4 in our experiment are negligible. A photomultiplier (CR131, Hamamatsu), installed at the same plane with the last pressure transducer along the shock tube axis, is used to acquire the OH* emission through a narrowband filter (307FS10-25, Andover) centered at 307 ± 10 nm. When the last transducer signal triggers digital oscilloscopes (HDO6104, Teledyne LeCroy), the pressure and OH* emission histories are recorded simultaneously. Figure 1 shows representative raw signals. A slight pressure rise occurs prior to the main ignition event, and such an increase is possibly due to the first-stage ignition31,32 or the boundary layer effect33 caused by
Figure 2. Comparison of the current and literature ignition data36 for the CH4 mixture with 0.0831% NO2 addition. Compositions of the tested gas mixtures in terms of molar fractions (percentages), and corresponding pressure (p) and temperature (T) conditions are listed in Table 1. Three equivalence ratios16 (ϕ = 0.5, 1.0, and 2.0) and NO2 concentrations (0, 30, and 70% DME molar concentrations) are investigated. In this work, all of the gases, i.e., He (>99.999%), NO2 (>99.9%), O2 (>99.99%), Ar (>99.99%), and DME (>99.9%), are provided by Chengdu Xiyuan Chemical Co., Ltd. The numbers in the parentheses indicate purities.
3. RESULTS Ignition delay times of DME/NO2/O2/Ar mixtures are measured at different pressures, equivalence ratios, and NO2 concentrations, and the results are collected in the Supporting Information. Figure 3 shows the effect of NO2 addition on DME ignition at certain fixed pressures and equivalence ratios. NO2 reduces the ignition delay time of DME remarkably, and such an effect becomes more pronounced as the NO2 concentration increases. A similar phenomenon was reported for hydrocarbon oxidation sensitized by NO2.36−38 Quantitatively, at a pressure of around 4 atm and 1250 K (Figure 3a), the fuel-lean N30 and N70 mixtures demonstrate 52.4 and 73.3% reductions in τ compared to N0, respectively. At ∼10 atm and 1250 K (Figure 3d), reductions of 62.9% (N 30 ) and 77.5% (N 70 ) are incurred. For the stoichiometric conditions, Figure 3b shows 61.2 and 79.2% reductions at 4 atm and T = 1250 K for N30 and N70, respectively. At 10 atm (Figure 3e), these values become 66.8% (N30) and 83.2% (N70). For the fuel-rich cases, reductions of 67.9% (N30) and 84.5% (N70), shown in Figure 3c, are observed at 4 atm and 1250 K. At higher pressure (10 atm; Figure 3f), the corresponding reductions become 70.2 and 85.7%. Furthermore,
Figure 1. Typical pressure and OH* emission profiles at 10 atm and 1084 K. Ignition delay time (τ) is indicated. B
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Mixture Compositions and Experimental Conditions Investigated in the Present Study mixture
ϕ
DME (%)
O2 (%)
Ar (%)
NO2 (%)
p (atm)
T (K)
N0
0.5
0.67
4.02
95.31
0.000
N0
1.0
1.31
3.93
94.76
0.000
N0
2.0
2.46
3.69
93.85
0.000
N30
0.5
0.67
4.02
95.109
0.201
N30
1.0
1.31
3.93
94.367
0.393
N30
2.0
2.46
3.69
93.112
0.738
N70
0.5
0.67
4.02
94.841
0.469
N70
1.0
1.31
3.93
93.843
0.917
N70
2.0
2.46
3.69
92.128
1.722
3.98 9.81 3.94 9.86 4.10 10.40 4.21 9.77 4.13 9.96 4.23 10.20 4.35 9.74 4.15 9.88 4.30 10.20
1263−1461 1203−1423 1216−1499 1171−1451 1211−1471 1176−1443 1152−1517 1114−1384 1153−1497 1112−1421 1105−1462 1061−1354 1153−1467 1028−1353 1099−1395 987−1348 1089−1370 1016−1256
Figure 3. Effect of the NO2 concentration on the ignition delay time (τ) of DME at different pressures and equivalence ratios: (a) ϕ = 0.5 and p = 4.0 atm, (b) ϕ = 1.0 and p = 4.0 atm, (c) ϕ = 2.0 and p = 4.0 atm, (d) ϕ = 0.5 and p = 10.0 atm, (e) ϕ = 1.0 and p = 10.0 atm, and (f) ϕ = 2.0 and p = 10.0 atm. Symbols are measurements, and solid lines are the predictions from the current model.
equivalence ratio becomes much more pronounced; the reduction factor becomes ∼2.4 at the same conditions (panels c and f of Figure 4).
the NO2-promoting effect strengthens gradually with an increasing equivalence ratio. For N30 at ∼4 atm, reductions of 52.4, 61.2, and 67.9% are induced for ϕ = 0.5, 1.0, and 2.0, respectively. Similar trends are also demonstrated in other cases. The promoting effect on DME ignition by NO2 also shows a strong temperature dependence; the reduction in τ is more pronounced at low temperatures than at high temperatures. The influence of the equivalence ratio on ignition delay times is shown in Figure 4. DME mixtures with and without NO2 are investigated. At low temperatures, ignition delay time decreases with an increasing equivalence ratio. The promoting effect of the equivalence ratio depends strongly upon the NO2 concentration. For neat DME mixtures, fuel ignition is insensitive to the equivalence ratio. τ is reduced by a factor of 1.3 when ϕ increases from 0.5 to 2.0 at 4 and 10 atm (1175 K; panels a and d of Figure 4). For N30 (1175 K and 4 and 10 atm; panels b and e of Figure 4), τ is reduced by a factor of ∼1.8 between ϕ = 0.5 and 2.0, while the reduction from ϕ = 0.5 to 1.0 is much less than that from 1.0 to 2.0 at the two tested pressures. For N70, the effect of the
4. KINETICS MODEL DEVELOPMENT Chemical reaction kinetics in the post-reflected shock region are simulated with Senkin39 in the Chemkin II package40 assuming a constant volume adiabatic model. To consider the volume changes caused by the boundary layer effects, the VTIM method (i.e., volume as a function of time t)41 is applied to the cases where ignition delay time is longer than 0.7 ms and a pressure rise rate of 8%/ms is employed. A calculated ignition delay time is the interval between time zero and the instant defined at the temperature inflection point (maximum dT/dt), consistent with the experimental definition. The ignition delay times obtained in this study are compared to the predictions from the following four kinetic models: (I) a model by Deng et al.,38 (II) a model by Mathieu et al.,28 (III) Aramco-G model, which is assembled from the detailed C
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 4. Effect of the equivalence ratio on the ignition delay time (τ) of DME at different pressures and NO2 concentrations: (a) N0 and p = 4.0 atm, (b) N30 and p = 4.0 atm, (c) N70 and p = 4.0 atm, (d) N0 and p = 10.0 atm, (e) N30 and p = 10.0 atm, and (f) N70 and p = 10.0 atm. Symbols are measurements, and solid lines are predictions from the current model.
Figure 5. Comparison between experiments and predictions (current updated model and four literature models) for fuel-lean mixtures (ϕ = 0.5) at different NO2 concentrations and pressures.
hydrocarbon oxidation subset18 (Aramco Mech 1.3) and the NOx submodel by Gersen et al.,42 and (IV) Aramco-D model, which is assembled from Aramco Mech 1.3 and the NOx submodel by Dagaut et al.22 The thermodynamic properties are adopted from the same sources as the corresponding model. The predicted ignition delay times are compared against our experiments in Figures 5−7 for different experimental conditions. Model I predicts the measurements accurately, except the case of 10 atm. At high temperatures, the predictions of model II and Aramco-G model show reasonable agreement with the measurements. However, when the temperature is below 1200 K, these two models overestimate ignition delay time overall and the discrepancy becomes more remarkable with a decreasing temperature. The Aramco-D model reproduces
ignition delay times under the fuel-rich condition, but it shows a relatively poor performance for the fuel-lean and stoichiometric cases. To better predict ignition delay times for DME/NO2 mixtures, we choose model II as the base model to develop a more accurate mechanism. The reaction set of DME/NOx interactions from Dagaut et al.22 is added to model II, with a few modifications. For DME/NO2 reactions, we estimate the rate constants from similar CH3OH/NO2 reactions,43 where a NO2 molecule abstracts the H atom from a methyl radical. The rate constant of the reaction CH3 + NO2 = CH3O + NO used here is a factor of 1.5 smaller than that by Glaborg et al.44 in the range of 1000−1500 K. Table 2 lists the parameters of rate coefficients36 for selected reactions added or altered in this work. D
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 6. Comparison between experiments and predictions (current updated model and four literature models) for stoichiometric mixtures (ϕ = 1.0) at different NO2 concentrations and pressures.
Figure 7. Comparison between experiments and predictions (current updated model and four literature models) for fuel-rich mixtures (ϕ = 2.0) at different NO2 concentrations and pressures.
Table 2. Parameters of Rate Coefficients for Selected Reactions in the Updated Model reaction
A (cm3 mol−1 s−1)
n
Ea (cal mol−1)
reference
DME + NO = CH3OCH2 + HNO CH3OCH2 + NO2 = CH3OCH2O + NO DME + NO2 = CH3OCH2 + HONO DME + NO2 = CH3OCH2 + HNO2 CH3 + NO2 = CH3O + NO
1.00 × 10 3.00 × 1013 1.45 × 102 2.41 × 103 2.20 × 1014
0.00 0.00 3.32 2.90 −0.50
43400.0 0.0 20035.0 27470.0 0.0
22 22 43 43 this work
14
also validated against atmospheric flow reactor data from Alzueta et al.,19 and the comparison is provided in the Supporting
As seen from Figures 3−7, the current updated model reproduces our measurements well. Furthermore, this model is E
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 8. Sensitivity analysis for stoichiometric mixtures (ϕ = 1.0) at T = 1150 K and p = 4 atm using the current model: (a) neat DME mixture and (b) mixtures containing NO2.
Figure 9. Sensitivity analysis for the equivalence ratio from 0.5 to 2.0 at T = 1150 K and p = 4 atm using the current model: (a) N0 mixtures and (b) N70 mixtures.
For the cases with finite NO2 concentrations (Figure 8b), several NOx-related reactions are introduced into the most important reactions and the chemical process during induction time is perturbed significantly. The sensitivity coefficient of reaction R29 (HCO + O2 = CO + HO2) increases remarkably when NO2 is seeded and becomes the most sensitive one as a result of abundant HCO radicals in NO2-added cases. Reaction R1 is the second most promoting reaction for the mixtures containing NO2, with its sensitivity greater than that shown in Figure 8a for neat DME. Reaction R947 (CH3 + NO2 = CH3O + NO) is another important promoting reaction for DME/NO2 mixtures. Essentially, once NO2 is added, CH3 radicals can react with NO2 preferentially via reaction R947, downgrading the roles of H atom abstraction via reactions R74 and R436 and selfrecombination via reaction R188. Therefore, CH3 radicals are largely converted to reactive CH3O radicals instead of stable alkalies, assisting faster formation of H atoms through reaction R90 [CH3O (+M) = CH2O + H (+M)]. Similar results were reported in previous studies on NO2/hydrocarbon interactions.19,38 Furthermore, reaction R991 (NO2 + H = NO + OH) is the most important reaction to restrain fuel autoignition at a lower NO2 concentration (competing with promoting reaction R1). However, for N70, the most-sensitive inhibiting reaction becomes reaction R1180 (HCO + NO = HNO + CO) as a result of its competition for the HCO radical with reaction R29. With an increasing NO2 concentration, reactions R29 and R1180 become
Information. The major uncertainties in the updated model may stem from the DME/NOx subset, and future work on this subset is desirable to develop a more accurate DME/NOx mechanism.
5. KINETIC ANALYSIS Given the updated model, we perform sensitivity analysis to identify the important reactions controlling DME ignition under specific pressure, temperature, equivalence ratio, and concentration conditions, in terms of the sensitivity coefficient.29 A negative sensitivity coefficient indicates that the corresponding elementary reaction promotes the ignition process, while a positive value points to an inhibiting effect. Figure 8 presents the sensitivity analysis for N0, N30, and N70 mixtures with ϕ = 1.0 at 1150 K and 4 atm on the basis of the updated model. Figure 8a reveals that the most important reaction for neat DME (N0) is the thermal dissociation reaction R430 [DME (+M) = CH3O + CH3 (+M)], followed by the H atom abstraction reaction R74 (CH2O + CH3 = HCO + CH4) and reaction R436 (DME + CH3 = CH3OCH2 + CH4). It appears that CH3 radicals play an essential role in H atom abstraction for neat DME during induction time. Another important reaction is the chain-branching reaction R1 (H + O2 = O + OH), which can generate reactive O atoms and OH radicals. Reaction R188 [CH3 + CH3 (+M) = C2H6 (+M)] exhibits the most inhibiting effect because of the consumption of CH3 radicals to form C2H6 and competes with reactions R74 and R436. F
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 10. Reaction flux analysis for the stoichiometric mixtures at T = 1150 K and p = 4 atm using the current model. Black font, N0; blue font, N30; and red font, N70.
presence of NO2, unlike neat DME mixtures, almost all CH3 radicals (81.2 and 90.4% for the N30 and N70 mixtures, respectively) are consumed via the disproportionation reaction R947 to form CH3O radicals. These can decompose thermally to create abundant H atoms (R90), accelerating the production of OH radicals. Moreover, with an increasing NO2 concentration, the consumption of CH3 radicals via the termination reactions (the products are mainly CH4 and C2H6) decreases to 11.5 and 2.52% for N30 and N70, respectively. Therefore, most DME undergoes H atom abstraction via OH radicals (73.6 and 86.4% for N30 and N70, respectively) instead of CH3 radicals initially, reducing the fluxes of reactions R430 and R436. Similarly, H atom abstraction by OH radicals and H atoms controls the consumption of CH2O for mixtures containing NO2, promoting faster formation of reactive HCO radicals. As discussed above, the production of OH radicals is important in the chemical reaction process, and the main reaction pathways forming OH radicals are analyzed and shown in Figure 11. The consuming pathways are not considered here, and the normalized time is calculated on the basis of corresponding ignition delay times in each case. Reaction R1 largely contributes to the production of OH radicals for neat
more sensitive, while there is only a small change in the sensitivity coefficient for most other key reactions, indicating that HCOrelated reactions have a significant influence on the ignition delay time for DME/NO2 mixtures. It is worth noting that reaction R903 (DME + NO2 = CH3OCH2 + HONO) shows a considerable promoting effect on DME ignition (Figure 8b). The rate constant of reaction R903 is greater by orders of magnitude than that of the initiation reaction DME + O2 = CH3OCH2 + HO2, suggesting that reaction R903 is an important initiation step in the DME/NO2/O2/Ar system. Some previous studies45,46 also observed that H atom abstraction from fuel by NO2 plays an important role in low-temperature initiation of fuel oxidation. It can be thus inferred that the reactivity for NO2 mixtures would be underpredicted if the kinetic model lacked the DME/NOx subset, and this may be the reason why models I−III perform poorly at low temperatures. Figure 9 shows sensitivity coefficients of several key reactions in ignition of DME/NO2/O2/Ar mixtures (N0 and N70) for different equivalence ratios (ϕ = 0.5, 1.0, and 2.0) at 1150 K and 4 atm. For neat DME in Figure 9a, the reactions R74, R430, and R436 involving fuel-derived species dominate ignition, and as a result, the fuel-rich mixture (N0) has higher reactivity and, thus, ignites faster below 1150 K. In the case of N70 shown in Figure 9b, the most sensitive promoting reaction is reaction R29, which includes fuel-derived species HCO. Therefore, shorter ignition delay times can also be seen at fuel-rich conditions. The sensitivity of reaction R29 varies considerably with different equivalence ratios, leading to a marked effect on ignition delay times, as shown in panels c and f of Figure 4. To further explain how NO2 promotes DME ignition, reaction flux analysis taken at 20% DME consumption is conducted at ϕ = 1.0, T = 1150 K, and p = 4 atm, shown in Figure 10. For the neat DME, DME is mainly consumed via H atom abstraction by small radicals, such as CH3 (44.0%), OH (21.9%), H atoms (16.9%), and HO2 (4.1%), yielding CH3OCH2 radicals, which can further undergo β scission to form CH3 radicals and CH2O. Another initial channel is the thermal dissociation of DME (11.2%), in which DME directly decomposes to CH3 radicals and CH3O radicals. Subsequently, CH3 radicals are converted into stable alkalies (87.6%) and CH3O radicals (7.22%). CH3O radicals, formed by DME dissociation and CH3 oxidation, are rapidly consumed to generate CH2O (R90), and these can be further oxidized through HCO radicals to yield CO and CO2. In the
Figure 11. Rate of production (ROP) for OH radicals during induction time at T = 1150 K, p = 4 atm, and ϕ = 1.0 using the current model. G
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
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DME. When NO2 is added, OH radicals can form via reaction R987 (NO + HO2 = NO2 + OH), reaction R988 [HONO (+M) = OH + NO (+M)], and reaction R991 (NO2 + H = NO + OH) at the early stage, while reaction R1 (H + O2 = O + OH) is still the most important reaction near the main ignition event, indicating that DME ignition is pre-disturbed at the initial stage by NO2 addition. With an increasing NO2 addition, the rate of OH production via reactions R987, R988, and R991 increases dramatically, thus forming more OH radicals. In comparison to reactions R987 and R991, the decomposition of HONO (a net consumption of HONO) has a minor contribution to OH formation in both N30 and N70 cases.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02457.
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REFERENCES
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6. CONCLUSION Ignition delay times of DME/NO2/O2/Ar mixtures are measured behind reflected shock waves at 986−1517 K, 4 and 10 atm, and equivalence ratios of 0.5−2.0. Different NO2 concentrations (0, 30, and 70% fuel concentrations) are explored. NO2 can accelerate DME ignition considerably, and the reduction in ignition delay time increases with an increasing NO2 concentration. The promotion effect of NO2 at low temperatures is much stronger than that at high temperatures. The influence of the equivalence ratio on ignition delay times become stronger with an increasing NO2 concentration. A kinetic model with updated reaction rates is proposed to predict the measured ignition delay times and captures the ignition feature accurately in comparison to our experiments. On the basis of the updated mechanism, kinetic analyses are conducted to explain the promoting effect of NO2. In the presence of NO2, CH3 radicals are mainly consumed via reaction R947 (CH3 + NO2 = CH3O + NO), followed by reaction R90 [CH3O (+M) = CH2O + H (+M)], which produces a large amount of H atoms at the initial ignition stage. Meanwhile, the interconversion of NO and NO2 via reaction R987 (NO + HO2 = NO2 + OH) and reaction R991 (NO2 + H = NO + OH) leads to faster formation of OH radicals before main ignition compared to that for neat DME. The highly reactive H atoms and OH radicals can perturb the initial reaction considerably, thus increasing the reactivity of DME mixtures.
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Article
Data obtained in the present study and model validation (PDF)
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support by the 973 Project of China (2014CB845904), NSAF (U1330111), and the Scientific Challenges Project of China. H
DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.energyfuels.6b02457 Energy Fuels XXXX, XXX, XXX−XXX
