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.
© 2016 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
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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.
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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
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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.
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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
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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.
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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.
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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
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02457.
■
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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.
■
Article
Data obtained in the present study and model validation
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: [email protected].
*E-mail: [email protected] and/or [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors acknowledge the support by the 973 Project of
China (2014CB845904), NSAF (U1330111), and the Scientific
Challenges Project of China.
10907
DOI: 10.1021/acs.energyfuels.6b02457
Energy Fuels 2016, 30, 10900−10908
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Energy Fuels 2016, 30, 10900−10908