methane

methane

Applied Thermal Engineering 119 (2017) 373–386 Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier...

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Applied Thermal Engineering 119 (2017) 373–386

Contents lists available at ScienceDirect

Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Kinetic modeling study of homogeneous ignition of dimethyl ether/hydrogen and dimethyl ether/methane Ying Wang a,⇑, Hong Liu b, Xichun Ke a, Zhenxing Shen a a b

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, PR China

h i g h l i g h t s  Kinetic effects on the homogeneous ignitions of DME/H2 and DME/CH4 were studied.  Ignition delays with different DME blending ratio were determined and analyzed.  Different DME ignition combustion trends are found for H2 and CH4 addition.  Key elementary reactions are identified at different DME ratios and temperatures.

a r t i c l e

i n f o

Article history: Received 31 May 2016 Revised 9 February 2017 Accepted 14 March 2017 Available online 16 March 2017 Keywords: Ignition combustion Dimethyl ether/hydrogen Dimethyl ether/methane Kinetic

a b s t r a c t Homogeneous ignition combustion of different proportion DME/H2 and DME/CH4 blend fuels in the air is investigated through the numerical simulation with the detailed chemistry at the low and high temperatures in this paper. The emphasis is the assessment of the kinetic effects involved in the ignition combustion of DME/H2 and DME/CH4 dual-fuel. It is found that the homogeneous ignition process has a clear distinction at the different temperatures. At the low temperature (900 K), the ignition delay times of DME/H2 blends and DME/CH4 blends both show an increase with a decrease of the DME blending ratio; furthermore, it is observed that CH4 addition is more effective than H2 addition in terms of delaying the DME homogeneous ignition due to the stable molecular structure of CH4. At the high temperature (1400 K), with the decrease of DME blending ratio, the ignition delay time of DME/CH4 blends is still increased, whereas, the ignition delay time of DME/H2 blends is shortened. Sensitivity analysis, reaction path analysis and main pollutant species calculation are carried out and key elementary reactions involved in homogeneous ignition of DME/H2 and DME/CH4 dual fuel are also identified in this paper. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction From the standpoint of both climate change protection and energy diversification, it is imperative to develop high-efficiency, low-emission combustion technology and adopt various alternative fuels. Recently DME as an alternative fuel for petroleum in the engines has received extensive attentions in many countries. DME can be produced from natural gas, coal or biomass, thus reducing dependence on petroleum fuels. DME becomes one of the most promising alternative fuels due to its favorable chemical characteristics, such as high cetane number (above 55), 34.8% oxygen content (wt%), no CAC bond in DME molecule and excellent ignition characteristic [1,2]. It has been reported that when DME fuel partially substitutes diesel fuel in the engines, the low emis⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.applthermaleng.2017.03.065 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.

sions as well as the high brake thermal efficiency still can be maintained in the engines [3,4]. DME is not only used as a substitute fuel for ordinary compression ignition (CI) engines, but also has a high potential as the main fuel for the homogeneous charge compression ignition (HCCI) combustion engines [5–8]. It is expected that the adoption of HCCI combustion mode in the internal combustion engines can lead to a higher thermal efficiency and lower NOx and PM emissions compared with those in the conventional combustion systems. However, DME’s ignition timing and combustion phase in HCCI combustion process are hard to control because DME fuel is very easy to be self-ignited. Now, an adjustment of the proportion of two kinds of fuels having the different reactivities has been proposed as a common method for controlling the ignition timing in the HCCI combustion [9,10]. Hydrogen (H2) and methane (CH4) are also two kinds of being extensively studied alternative fuels. Both of them have been used in DME HCCI engine to control the ignition timing and expand the

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operating range of the engines [11,12]. Therefore, a thorough understanding of the combustion properties of CH4/DME and H2/ DME is crucial for developing advanced DME-based combustion engines and corresponding operating strategies. In the literatures, there have been numerous studies on the fundamental combustion properties of CH4/DME or H2/DME binary fuel blends, including their ignition delay time [13–15], laminar flame speed [16–18], performance of the internal combustion engine [11,12] and so on. For example, Chen et al. [13] studied the effects of DME addition on the high temperature ignition and reported that in a homogeneous system, adding a small amount of DME into CH4 could lead to a significant decrease in the ignition delay, which can be attributed to that the chain propagation reaction via CH3 and HO2 substituted the slow reactions via CH3 and O2 in the CH4 ignition and thus accelerated the ignition. Tang et al. [14] measured the ignition delay of CH4/DME over a wide range of temperature in the shock-tube experiment and found the ignition delay of CH4/ DME decreased obviously with only 1% DME at a given temperature. Furthermore, as the DME blending ratio increased, the ignition delay was correspondingly decreased. Pan et al. [15] conducted shock-tube experiments to measure the ignition delay of DME/H2/air mixtures and they found that the ignition delay time of DME/H2 binary fuel blends has complex dependence on the temperature. Lowry et al. [16] measured and calculated the laminar flame speed, Markstein length, and Lewis number for binary blends of CH4/DME, with volumetric fractions of 60% CH4/40% DME and 80% CH4/20% DME performed at various initial pressures. They found a small amount of DME addition caused the great change in Markstein lengths, but for the 80/20 blend of CH4/DME, the Lewis number remained close to unity as the equivalence ratio increased. Yu et al. [17] measured the laminar flame speeds of DME/H2/air flames in a constant volume vessel at different temperatures, equivalence ratios, and fuel blending ratios. Their results denoted that the laminar flame speeds increased with an increase in H2 fraction and initial temperature. The chemical kinetic effect induced by H2 addition played a dominant role in an increase of the laminar flame speed compared with thermal and diffusive effects. Liu et al. [18] simulated the effects of H2 addition to the DME base flame. The results indicated that the reduction of CH3OCH3 mole fraction in the blend is the dominant factor for the reduction of CH3OCH3 and CO mole fractions in the flame. The H, O and OH radicals increased when H2 was added, and these radicals promoted the combustion process. Liu [19] also studied the chemical effects of H2 addition on premixed laminar low-pressure DME flames. The results showed that the chemical effects of H2 addition could suppress the formation of C2H2 and C2H4. He also reported that the dominant effects of H2 addition on H, OH and O radicals were the chemical effects which made mole fractions of these radicals increase. According to previous studies, both H2 and CH4 addition can affect the ignition process of DME, especially the ignition delay time. However, because H2 oxidation mechanism is different from that of CH4, it is expected that there is some difference between the ignition of H2/DME and CH4/DME blend. Moreover, the kinetics involved in the ignition behavior should be different for H2/DME and CH4/DME blend. But, there are few literatures reported above-mentioned difference between the ignition of the H2/DME and CH4/DME blend. In addition, exhaust gas recirculation (EGR) is one kind of main control technologies for the HCCI engine. EGR includes some active gases such as carbon monoxide (CO) and nitrogen oxide (NO), carrying out the chemical effect. The chemical kinetic mechanism in HCCI combustion can be changed by NO, which is also one compo-

nent of environmental pollution. The active chemical property of NO has some influence on HCCI combustion and the emission process. Moréac et al. [20] investigated the effects of NO on the oxidation of n-heptane, iso-octane, toluene, and methanol in a jet-stirred reactor. The NO effect was found related to temperature. Risberg et al. [21] carried out experiments on the effects of various NO concentrations on an HCCI engine fueled with primary reference fuel (PRF) and toluene reference fuel (TRF). The NO concentration with maximum influence on ignition delay was obtained under two engine conditions, one at 0.2 MPa intake pressure and 40 °C intake temperature and the other at 100 °C intake temperature and atmospheric intake pressure. Frassoldati et al. [22] used the chemical kinetic model to analyze the interaction between NO and C1–C4. Dayma et al. [23] investigated the interaction between NO and PRF using the models and experiments. Above researches prove that NO can affect the oxidation of a hydrocarbon and hydrocarbons can also affect the conversion of NO to NO2 or to N2 and HCN. Moreover, NO is one of the main pollutants from the HCCI engine. However, in recent literatures, most of DME kinetic calculations were conducted using LLNL DME Mech [24], Zhao DME Mech [25] and NUIG Mech_56.54 [26], all of which do not include NO mechanism. Therefore, the objective of the study is to compare different ignition combustion behavior and identify the kinetics involved in ignition variation caused by the different proportion H2/DME and CH4/DME blend with new DME mechanism including NO. The chemical kinetics effects of H2 and CH4 addition on the DME ignition combustion are also analyzed. The research results are expected to provide a further understanding of fundamental combustion properties of CH4/DME and H2/DME blends for contributing to the development of the advanced DME-based combustion engines.

2. Chemical mechanism and computational numerical models The most recent available DME/CH4 mechanism, NUIG Mech_56.54, consists of 111 species and 784 reactions, including the H2/CO sub-mechanism, the C1–C2 base sub-mechanism, and the propene mechanism. But, NO as one of main products from DME HCCI combustion can’t be obtained in NUIG Mech_56.54. There is also an increasing body of data that shows chemistry is important in the homogeneous charge combustion engines and especially EGR compounds such as NO. Therefore, NO mechanism from GRI-Mech 3.0 is incorporated into NUIG Mech_56.54 to form the new DME/CH4/NO mechanism for a more complete study of DME/H2 and DME/CH4 combustion. This new mechanism used in this paper includes 130 species and 816 reactions. Because smokeless combustion is always achieved in DME engine, soot mechanism is not considered in this paper. NUIG Mech_56.54 mechanism has been extensively tested against experimental data from jet-stirred reactor, shock tube and rapid compression machine. It was uncertain, however, whether the chemical kinetics described by the NUIG Mech_56.54 mechanisms will be significantly altered by combining the NUIG Mech_56.54 and NO mechanisms together. To investigate this possibility, model runs were conducted to compare ignition delay times predicted by the new DME/CH4/NO mechanism to those determined with the NUIG Mech_56.54 mechanism and experimental data. The ignition delay times of CH4/DME and H2/DME blends were measured by Tang et al. [14] and Pan et al. [15]. In Fig. 1 we compare the experimental results with the numerical prediction with the new DME/CH4/NO mechanism and NUIG Mech_56.54 mechanism. It can be seen that the calculation results from the new DME/CH4/NO mechanism agree well the results from NUIG Mech_56.54 mechanism. Furthermore, using the new DME/

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Fig. 2. Cylinder pressure in HCCI engine of CH4/DME and H2/DME blend fuel predicted by the NUIG-56.54 mechanism (solid lines) and DME/CH4/NO mechanism (dot lines).

Fig. 1. Homogeneous ignition delay time of (a) H2/DME and (b) CH4/DME blend fuel predicted by the DME oxidation mechanism (lines) and measured in shock tube experiments (symbols); (a) Black – 100% DME, Red – 5% DME + 95% H2, Green – 10% DME + 90% H2, Blue – 20% DME + 80% H2, Cyan – 50% DME + 50% H2, Magenta – 100% H2. (b) Black – 100% DME, Red – 5% DME + 95% CH4, Green – 10% DME + 90% CH4, Blue – 20% DME + 80% CH4, Cyan – 50% DME + 50% CH4, Magenta – 100% CH4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

CH4/NO mechanism can also accurately predict the ignition delay time of both H2/DME and CH4/DME blends with the various DME blending ratio. We also calculated and compared the cylinder combustion pressure of a single-cylinder HCCI engine using DME/CH4 or DME/H2 as fuel with the different mechanisms respectively. The main parameters of the modified Volvo HCCI engine [27] are in Table 1. Fig. 2 shows the comparison between cylinder pressure results from the NUIG Mech_56.54 mechanism (solid line) and results obtained from the new DME/CH4/NO mechanism (dot line).

Table 1 Engine specifications. Parameter

Setting

Bore/mm Stroke/mm Displacement/cm3 Compression ratio Engine speed/rmin1

120.65 140 1600 17 1000

The fuels are a mixture of 40% H2 and 60% DME, a mixture of 40% CH4 and 60% DME and 100% DME. The fuel/air equivalence ratio is 0.3. Again, the new DME/CH4/NO mechanism shows excellent agreement with the numerical results of the NUIG Mech_56.54 mechanism. This further proves the new DME/CH4/NO mechanism can be used in the following calculation. Because there is no mass or heat transport exists in the homogenous ignition process and homogenous ignition is entirely controlled by chemical kinetics, the adiabatic homogeneous ignition process at a constant volume is studied in this paper in order to reveal the chemical kinetics involved in CH4/DME and H2/DME ignition. Stoichiometric DME/CH4/air and DME/H2/air mixtures initially at T0 = 900 K, p0 = 10 bar and T0 = 1400 K, p0 = 10 bar are simulated with detailed chemistry mechanism. The DME blending ratio, defined as the molar fraction of DME in the fuel blends, changes from 0 (pure H2 or CH4) to 100% (pure DME).

3. The homogenous ignition process 3.1. Ignition delay times To investigate the ignition characteristics of DME/H2 blends and DME/CH4 blends, the effects of low-temperature and hightemperature on the ignition delay times of DME/H2 and DME/CH4 blends using NUIG Mech_56.54 are respectively presented in Fig. 3. The ignition delay time in this paper is defined as the time when the temperature exceeds its original value by 400 K [28]. The other definition of the time corresponding to the maximum temperature changing rate is checked, indicating that both different definitions of ignition delay times have almost the same results. It can be observed in Fig. 3 that DME exhibits notable NTC (Negative Temperature Coefficient) behaviors. It is generally accepted that during DME combustion CH2OCH2OOH radical undergoes b-scission or a second oxygen addition, which can lead to a low-temperature radical branching pathway including a number of isomerization and decomposition steps yielding reactive products [29]. Such a low-temperature radical branching pathway is the remarkable characteristic of DME ignition, being largely responsible for the DME NTC behavior. However, in Fig. 3(a) the hydrogen combustion shows shorter ignition delay times at a high temperature, but longer ignition delay times and no observable NTC behavior at a low temperature compared with those of pure

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Fig. 3. Change of the homogeneous ignition delay time with the temperature for stoichiometric DME/H2/air and DME/CH4/air mixtures initially at p = 10 bar.

DME combustion. Therefore, it is observed that ignition delay times of DME/H2 at the low temperature are increased with a decrease of DME blending ratio. This phenomenon is exact contrary to that at a high temperature. In Fig. 3(b), CH4 shows longer ignition delay times and no observable NTC behavior at all temperatures compared with those of pure DME combustion. Therefore, the ignition delay times of DME/CH4 increase with a decrease of DME blending ratio. To quantitatively display the effect of H2 and CH4 addition on the ignition of pure DME at the low and high temperature, the ignition delay times at various DME blending ratios are presented at T = 900 K, p = 10 bar and T = 1400 K, p = 10 bar, as shown in Fig. 4. It is found in Fig. 4 that H2 or CH4 addition shows the non-linear impact on the ignition delay times of both kinds of mixture at the low and high temperature. With the increase of DME blending ratio, the ignition delay times of DME/H2 blends and DME/CH4 blends at a low temperature both show an initially steep decrease when DME blending ratio is lower than 40%, and then a gradual decrease. At a high temperature, the ignition delay time of DME/CH4 blends also shows a similar varying trend compared with that at a low temperature, but, the ignition delay time of DME/H2 blends shows an opposite varying trend compared with that at a low temperature. H2 addition demonstrates an opposite effect: inhibiting and promoting the ignition of DME/H2/air mixtures at a low temperature and a high temperature, respectively. Pan et al. [29] ascribed it to the competition between reactions of H + O2 ? O + OH and CH3OCH3 + H ? CH3OCH2 + H2. Shudo et al. [30] assumpted that retarding DME auto-ignition was owing

to that hydrogen consumed OH radicals and retarded Habstraction from DME by OH radicals at a low temperature. Furthermore, it can be observed in Fig. 3 that CH4 addition is more effective than H2 addition in terms of delaying the homogeneous ignition of DME/air mixture due to DME’s stable molecular structure. 3.2. Chemical kinetic analysis on the ignition of DME/H2 and DME/CH4 blend

3.2.1. Small radical pool analysis To clarify the dependence of ignition delay time on DME blending ratio, the effects of DME blending ratio on the evolution profiles of the total radical pool (sum of H, O, OH, HO2, CH3 and C2H5 radicals) for all blends are depicted in Fig. 5 (T = 900 K, p = 10 bar) and Fig. 6 (T = 1400 K, p = 10 bar). Fig. 5(a) and (b) reveal that DME addition has an obvious boosting effect on the radical build-up with a low proportion of DME addition while such an effect weakens rapidly as the DME blending ratio further increases for both DME/H2 blend and DME/CH4 blend at the relatively low temperature (900 K). It can also be observed that the delay of the radical build-up for the ignition of DME/CH4 blend is relatively obvious than the ignition of DME/H2 blend. Fig. 6(a) shows that DME addition has a magnificant depressing effect on the radical build-up with a low DME blending ratio, and such an effect weakens rapidly when the DME blending ratio further increases at the relatively high temperature (1400 K) for the

Fig. 4. Ignition delay times of stoichiometric H2/DME/air and CH4/DME/air mixtures as a function of the DME blending ratio.

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Fig. 5. Temporal evolution of molar fraction of the radical pool (sum of H, O, OH, CH3, HO2 and C2H5) during the homogeneous ignition of stoichiometric H2/DME/air and CH4/ DME/air mixtures at low temperature.

Fig. 6. Temporal evolution of molar fraction of the radical pool (sum of H, O, OH, CH3, HO2 and C2H5) during the homogeneous ignition of stoichiometric H2/DME/air and CH4/ DME/air mixtures at high temperature.

homogeneous ignition of stoichiometric DME/H2/air mixture. However, Fig. 6(b) shows that the homogeneous ignition of stoichiometric DME/CH4 blend at the high temperature still exhibits the same varying trend as that at the low temperature with the variation of DME proportions. Because radical accumulation and run-away can determine the ignition process, the difference between ignition process by DME/ H2 and DME/CH4 observed in Fig. 6 can be attributed to the different impact of DME/H2 and DME/CH4 on the radical pool development. 3.3. Reaction pathway and sensitivity analysis 3.3.1. Sensitivity analysis of ignition delay The sensitivity analysis is carried out to determine the key elementary reactions involved in DME ignition influenced by the addition of H2 or CH4. The sensitivity coefficient of the ignition delay time with the reaction rate of the ith elementary reaction is defined as:

si ¼

sð2:0ki Þ  sð0:5ki Þ sðki Þ

where s (2.0ki) and s (0.5ki) denote the ignition delay time with the rate constant of the ith reaction being artificially adjusted to be 2.0 times and 0.5 times of its original value, while s (ki) represents the original ignition delay time under the condition of the unchanged reaction rates. Negative value of sensitivity coefficient means that

an increase of the reaction rate constant results in a reduction in the ignition delay time and thus increases the overall reactivity. Based on above study, the temperature has a significant impact on the ignition combustion of DME blends. Thus, the sensitivity analysis is conducted at 900 K and 1400 K respectively and the calculation results are shown in Figs. 7 and 8. Fig. 7 exhibits the sensitivity coefficients of the homogenous ignition of DME/H2 and DME/CH4 blend at 900 K. From this figure, it can be concluded that for pure DME ignition, the ignition chemistry is mainly characterized by reaction CH3OCH3 + HO2 = CH3OCH2 + H2O2 (R436) and CH3OCH3 + OH = CH3OCH2 + H2O (R433). Above reactions yield methoxymethyl (CH3OCH2) to feed the low temperature pathway, responsible for the NTC behavior. In the case of the homogeneous ignition of pure H2 in the air at 900 K, based on kinetic calculation, the most promoting reaction is H + O2 , O + OH (R1), which has the highest negative sensitivity coefficient, and the reaction OH + H2 , H + H2O (R3) is also some promoting effect in the homogeneous ignition of pure H2. The most inhibiting reaction is H consumption reaction H2O2 + H = H2 + HO2 (R21). HO2 consumption reaction 2HO2 = H2O2 + O2 (R17) also has some inhibiting effect on the pure H2 ignition process. It can be obtained that the H-related chain reactions controlled the H2 ignition at this condition. But, the sensitivity analysis (in Fig. 7A) indicates that once a amount of DME is blended with H2, the system is strongly driven by the H-abstraction from DME through reaction R433 and R436. These reactions are the major initial source of radicals and continue to contribute to radical production thereafter.

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Fig. 7. Sensitivity coefficients of key reactions versus DME blending ratios for DME/ H2 and DME/CH4 at 900 K.

In a case of the homogenous ignition of pure CH4 in the air at 900 K, the hydrogen abstraction from CH4 through reaction CH4 + HO2 = CH3 + H2O2 (R131) has the maximum negative value of sensitivity coefficient. The ignition delay time is also sensitive to several chain reactions involving with CH3, including CH4 + OH = CH3 + H2O (reverse R129) and CH3 + HO2 = CH3O + OH (R145). CH3 + HO2 = CH4 + O2 (R146) and 2CH3(+M) = C2H6(+M) (R189) remove the methyl radical from the radical pool and they have comparably high positive sensitivity values. Furthermore, the CH3 radical also participates in reaction CH3 + HO2 = CH3O + OH (R145) to yield major initial OH radical and reaction CH2O + CH3 = HCO + CH4 (R75) to yield HCO radical. R145 and R75 have negative sensitivity coefficients and promote CH4 ignition. The main ignition inhibition reaction includes the OH radical consumption reaction CH2O + OH = HCO + H2O (R72), HO2 recombination reaction 2HO2 = H2O2 + O2 (R17) and CH3 consumption reaction CH3 + HO2 = CH4 + O2 (R146). Fig. 8B reveals the fact that at a low temperature most of the high sensitivity reactions involve the

Fig. 8. Sensitivity coefficients of key reactions versus DME blending ratios for DME/ H2 and DME/CH4 at 1400 K.

consumption of CH3 radical, indicating its importance in radical pool growth and hence the CH4 ignition process. But, like DME/ H2 ignition, the sensitivity analysis shows that once amount of DME is blended with CH4, the system is strongly driven by the H-abstraction from DME through reaction R433 and R436. These reactions are the major initial source of radicals and continue to contribute to radical production thereafter. In a case of the homogeneous ignition of pure DME in the air at 1400 K, CH3OCH3 decomposition reaction CH3OCH3(+M) = CH3 + CH3O(+M) (R432) has the maximum negative value of sensitivity coefficient. The ignition delay time is also sensitive to several chain reactions including CH3 + HO2 = CH3O + OH (R145), H + O2 = O + OH (R1), CH2O + CH3 = HCO + CH4 (R75), CH3OCH3 + CH3 = CH3OCH2 + CH4 (R439) and CH2O + HO2 = HCO + H2O2 (R76). Meantime, 2CH3(+M) = C2H6(+M) (R189) and CH3 + HO2 = CH4 + O2 (R146) remove the methyl radical from the radical pool. In the case of the homogeneous ignition of the pure H2 in the air at 1400 K, Fig. 8A shows that H + O2 = O + OH (R1) has the maximum negative value of sensitivity coefficient and is much higher than that of other reactions. The ignition delay time is secondary

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Fig. 9. The temporal change of CH3 molar fraction at 900 K.

Table 2 List of reactions presented in Fig. 9. Reaction number

Reaction

R129 R145 R146 R150 R189 R432 R439 R444

CH4 + OH , CH3 + H2O CH3 + HO2 , CH3O + OH CH3 + HO2 , CH4 + O2 CH3 + O2(+M) , CH3O2(+M) CH3 + CH3(+M) , C2H6(+M) CH3OCH3(+M) , CH3 + CH3O(+M) CH3OCH3 + CH3 , CH3OCH2 + CH4 CH3OCH2 , CH3 + CH2O

sensitive to the reaction OH + H2 = H + H2O (R3). Above H-related chain reactions controlled the pure H2 ignition. Furthermore, pure H2 ignition is much faster than pure DME ignition. When DME is blended with H2, the ignition is dominated by the combined chemistry of DME and H2. Meantime, the sensitivity coefficients of R1 and R3 decrease, but the sensitivity coefficients of reactions involving with CH3 radical (from DME) such as R145, R432 and R439 increase. For a CH4/air mixture at 1400 K, the chain-branching reaction CH3 + O2 = CH2O + OH (R149) has the maximum negative value of sensitivity coefficient. The ignition delay time is also sensitive to several chain reactions including CH3 + HO2 = CH3O + OH (R145), H + O2 = O + OH (R1) and CH2O + CH3 = HCO + CH4 (R75). Methyl radical recombination reaction 2CH3(+M) = C2H6(+M) (R189), the most important ignition inhibition reaction, removes the methyl radical from the radical pool. The hydrogen abstraction reaction CH4 + H = CH3 + H2 (R128), competing with reaction R1 for H

radical, also has a comparably high positive value of sensitivity coefficient. When DME is blended with CH4, the sensitivity coefficients of R149, R1 and R128 decrease, but the sensitivity coefficients of R75, R432 and R439 increase. DME benefits the ignition of CH4 mainly by the unimolecular decomposition reaction CH3OCH3(+M) = CH3 + CH3O(+M) (R432) and the CH3 chain reaction CH2O + CH3 = HCO + CH4 (R75). The chemical path analysis indicates that once amount of DME is present, the system is strongly driven by the reaction R432, which is the major initial source of radicals and continues to contribute to radical production thereafter [28]. After DME is decomposed into CH3 and CH3O through reaction R432, subsequent H-abstraction reaction CH3OCH3 + CH3 = CH3OCH2 + CH4 (R439) produces CH3OCH2 which in turn yields additional radical growth through the decomposition reaction CH3OCH2 = CH2O + CH3 (R444). Therefore, when DME is blended with CH4, the radical pool grows faster compared with that of pure CH4. Above analysis of the ignition delay time sensitivity coefficient indicates that H-related reactions control the pure H2 ignition and CH3-related reactions control the pure CH4 ignition both at the low and high temperature; but, the key reaction appears certain different between low and high temperature when DME is blended with H2 or CH4. At 900 K the ignition chemistry is mainly characterized by CH3OCH3 + HO2 = CH3OCH2 + H2O2 (R436) and CH3OCH3 + OH = CH3OCH2 + H2O (R433) whether DME is mixed with H2 or CH4. At 1400 K the ignition is dominated by the combined chemistries of DME (R145 and R432) and hydrogen (R1) when DME is mixed with H2. Whereas, the ignition of CH4 is accelerated mainly by DME decomposition reaction CH3OCH3(+M) = CH3 + CH3O(+M)

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Fig. 10. The temporal change of H molar fraction at 900 K.

Table 3 List of reactions presented in Figs. 10 and 12. Reaction number

Reaction

R1 R3 R9 R13 R30 R73 R78 R91 R128 R434

H + O2 , O + OH OH + H2 , H + H2O H + O2(+M) , HO2(+M) H2 + O2 , H + HO2 HCO + M , H + CO + M CH2O + H , HCO + H HOCH2O , HOCHO + H CH3O(+M) , CH2O + H(+M) CH4 + H , CH3 + H2 CH3OCH3 + H , CH3OCH2 + H2

(R432) and the CH3 chain reaction CH2O + CH3 = HCO + CH4 (R75) as DME is mixed with CH4. 3.3.2. Reaction path analysis of CH3 and H In order to further compare and analyze the ignition variation when H2 and CH4 are blended with DME, the temporal evolution of CH3 and H radicals at the same DME blending ratio is computed. Furthermore, in order to further discuss the kinetics and key elementary reactions involved in the ignition variation by H2 and CH4 addition, reaction path analysis for radicals such as H and CH3 is conducted. The contribution of the ith reaction to the concentration change of a certain intermediate species M is defined as

C M;i ¼

f m;i n  X  f  M;i i¼1

where fM,i indicates the rate of change in the concentration of species M due to the ith reaction, and N is the total number of elementary reactions [27]. A positive value of CM,i indicates that M is produced by the ith reaction, whereas negative value of CM,i indicates that M is consumed by the ith reaction. First the reaction path analysis of CH3 and H for the homogenous ignition of pure DME, 40% DME + 60% H2 blend and 40% DME + 60% CH4 blend is conducted at a low temperature. Fig. 9 gives the reaction path analysis of CH3 for the homogenous ignition of pure DME, 40% DME + 60% H2 blend and 40% DME + 60% CH4 blend at 900 K, as well as the temporal change of CH3 molar fraction. Reactions indicated by the reaction numbers in this figure are exhibited in Table 2. It can be seen from Fig. 9(a) and (b) that 60% H2 addition does not cause dramatic change of CH3 production/consumption pathways compared with that for the ignition of pure DME. CH3 productions are mainly from CH3OCH3 decomposition reaction CH3OCH3 (+M) , CH3 + CH3O(+M) (R432) and CH3OCH2 decomposition reaction CH3OCH2 , CH3 + CH2O (R444). CH3 consumptions are mainly from reactions CH3 + O2(+M) , CH3O2(+M) (R150) and CH3OCH3 + CH3 , CH3OCH2 + CH4 (R439). In Fig. 9(c), when 60% CH4 is blended with DME, CH3 consumption pathway almost has no dramatic change compared with that of pure DME. However, CH3 production pathway changes obviously due to CH4 addition. Besides R432 and R444, CH3 + HO2 , CH4 + O2 (reverse R146) and CH4 + OH , CH3 + H2O (R129) are promoted due to the presence of CH4, thus resulting in a little more CH3 pool built-up compared with those for pure DME and 40% DME + 60% H2 combustion.

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Fig. 11. The temporal change of CH3 molar fraction at 1400 K.

Fig. 10 gives the reaction path analysis of H for pure DME, 40% DME + 60% H2 blend and 40% DME + 60% CH4 blend at 900 K as well as the temporal change of H molar fraction. Reactions represented by the reaction numbers in this figure are exhibited in Table 3. It can be seen from Fig. 9(a) and (c) show that 60% CH4 addition does not cause dramatic change of H production/consumption pathways compared with those of the ignition of pure DME. H production is mainly from HOCH2O decomposition reaction HOCH2O , HOCHO + H (R78) and CH3O decomposition reaction CH3O(+M) , CH2O + H(+M) (R91). H consumption is chiefly from reactions H + O2(+m) , HO2(+m) (R9), CH3OCH3 + H , CH3OCH2 + H2 (R434) and CH2O + H , HCO + H2 (R73). In Fig. 10(b) when H2 is blended with DME, H consumption pathway has no dramatic change compared with that of pure DME, but H production pathway changes. Besides reactions R78 and R91, reactions OH + H2 , H + H2O (R3) and H2 + O2 , H + HO2 (R13) are promoted mainly due to the presence of H2, thus resulting in relatively more amount of H production compared with that of 100% DME. The reaction path analysis of CH3 and H for the homogenous ignition of pure DME, 40% DME + 60% H2 blend and 40% DME + 60% CH4 blend conducted at a high temperature is shown in Figs. 11 and 12. Fig. 11 gives the results of CH3 reaction path analysis for the homogenous ignition of three kinds of fuel as well as CH3 molar fraction. Reactions indicated by the reaction numbers in this figure are exhibited in Table 4. It is seen from Fig. 11(a), for pure DME ignition, CH3 is mainly produced from CH3OCH3 decomposition reaction CH3OCH3(+M) , CH3 + CH3O(+M) (R432) and CH3OCH2 decomposition reaction CH3OCH2 , CH3 + CH2O (R444); while CH3 is mainly consumpted by methyl radical

recombination reaction CH3 + CH3(+M) , C2H6(+M) (R189) and chain reaction CH3OCH3 + CH3 , CH3OCH2 + CH4 (R439). For the homogenous ignition of the 60% H2 + 40% DME blend in the air, CH3 production pathway has no change compared with that of pure DME ignition, but CH4+H , CH3 + H2 (reverse R128) is added into the CH3 consumption pathways due to the presence of H2. For the homogenous ignition of 60% CH4 + 40% DME blend in the air, CH3 consumption pathway has no change compared with that of pure DME ignition, but chain reaction CH4 + H , CH3 + H2 (R128) and CH4 + OH , CH3 + H2O (R129) both become as the main CH3 production pathways due to the presence of CH4, ultimately leading to a little more CH3 pool build-up. Fig. 12 shows the results of H reaction path analysis for the homogenous ignition of pure DME, 40% DME + 60% H2 blend and 40% DME + 60% CH4 blend at 1400 K as well as the temporal change of H molar fraction. Reactions indicated by the reaction numbers in this figure are exhibited in Table 3. It can be seen from Fig. 12 (a) and (c) that 60% CH4 addition does not result in obvious change of H production pathway compared with that of pure DME ignition. H is produced mainly from CH3O(+M) , CH2O + H(+M) (R91) and HCO + M , H + CO + M (R30). But, due to the addition of CH4, CH4 + H , CH3 + H2 (R128) is promoted to become one of the main H consumption pathways during the 40% DME + 60% CH4 blend combustion. When 60% H2 is blended with DME, there is no obvious change in H consumption pathways between pure DME combustion and 40% DME + 60% H2 blend combustion, in addition that CH4 + H , CH3 + H2 (reverse R128) is promoted to be as one main pathways of H production due to the abundance of H2, ultimately leading to more H pool build-up.

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Fig. 12. The temporal change of H molar fraction at 1400 K.

Table 4 List of reactions presented in Fig. 11. Reaction number

Reaction

R128 R129 R145 R146 R189 R432 R439 R444

CH4 + H , CH3 + H2 CH4 + OH , CH3 + H2O CH3 + HO2 , CH3O + OH CH3 + HO2 , CH4 + O2 CH3 + CH3(+M) , C2H6(+M) CH3OCH3(+M) , CH3 + CH3O(+M) CH3OCH3 + CH3 , CH3OCH2 + CH4 CH3OCH2 , CH3 + CH2O

The results of reaction path analysis for H radical during pure DME, DME/H2 and DME/CH4 fuel combustion at low and high temperatures both indicate that CH4 blending does not cause dramatic change in H production/consumption pathways compared with that of pure DME combustion. H2 addition also does not cause dramatic change in H consumption pathway; however, H production is promoted due to the presence of H2, thus resulting in relatively more H pool built-up compared with those of pure DME and DME/ CH4 combustion. The results of reaction path analysis for CH3 radical for pure DME and DME/H2 at low and high temperatures indicates H2 addition does not cause dramatic change in CH3 production pathways compared with pure DME combustion at both low and high temperatures, and there is also no change perceptible change in CH3 consumption pathways between pure DME and DME/H2 blend

combustion at 900 K. But, CH4 + H , CH3 + H2 (reverse R128) is promoted to be one of CH3 consumption pathways due to the presence of H2 at 1400 K. For the homogenous ignition of DME/CH4 fuel, CH4 addition also does not cause dramatic change in CH3 consumption pathway compared with that of pure DME combustion at both 900 K and 1400 K. However, CH3 production is promoted due to the presence of CH4 at both 900 K and 1400 K, thus resulting in relatively more CH3 pool built-up compared with those of pure DME and DME/H2 combustion. 3.3.3. Major pollutant species It is generally known that CO, NO and formaldehyde (CH2O) are major pollutant species in DME combustion. Therefore, it is necessary to analyze the effect of H2 and CH4 addition on the above pollutant species during the DME combustion. The main CO reaction pathways in the combustion of the DME/H2 and DME/CH4 blend are as follows:

CO þ OH () CO þ H

ðR27Þ

HCO þ M () H þ CO þ M

ðR30Þ

HCO þ O2 () CO þ HO2

ðR31Þ

HCO þ H () CO þ H2

ðR32Þ

HCO þ OH () CO þ H2 O

ðR35Þ

Y. Wang et al. / Applied Thermal Engineering 119 (2017) 373–386

Fig. 13. Temporal evolution of HCO molar fraction of stoichiometric H2/DME/air and CH4/DME/air mixtures.

Fig. 14. Temporal evolution of CO molar fraction of stoichiometric H2/DME/air and CH4/DME/air mixtures.

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Fig. 15. Temporal evolution of NO molar fraction and the NO peak mole fraction of stoichiometric H2/DME/air and CH4/DME/air mixtures as a function of the DME blending ratio.

It is obvious that most of CO species is formed by the H abstraction reactions from HCO and consumed by OH radical to form CO2. Fig. 13 gives the temporal evolution of HCO molar fraction of the stoichiometric H2/DME/air and CH4/DME/air mixtures. It can be observed that HCO molar fraction falls with a reduction of DME blending ratio in the blend. Additionally, HCO molar concentration for H2/DME blend fuel combustion is much lower than that for CH4/DME blend fuel combustion. Consequently, we can find in Fig. 14 that CO molar fraction falls with a reduction of DME blending ratio in the blend, and CO molar concentration for H2/DME blend fuel combustion is much lower than that for CH4/DME blend fuel combustion due to no C atom in H2 molecule and lower HCO for H2/DME blend. While NO and NO2 are often grouped together as NOx emission, NO is the predominant oxide of nitrogen produced in the condition like engine. Fig. 15 exhibits the temporal evolution of NO molar

fraction of the stoichiometric H2/DME/air and CH4/DME/air mixtures. It is obvious NO increases significantly when the temperature reaches over a certain value. Without considering the fuel nitrogen, NO can mainly be contributed from three distinct chemical mechanisms; Zeldovich thermal NO mechanism, Fenimore prompt NO mechanism, and the nitrous oxide mechanism (Malte and Pratt, 1974). Based on the reaction path analysis, thermal NOx reactions (N + NO , N2 + O (R711), N + OH , NO + H (R713)), which are highly temperature dependent, are the most relevant NO sources during the combustion of stoichiometric H2/DME/air and CH4/DME/air mixtures. Accordingly, maximum NO molar fraction shows almost no obvious change when H2 and CH4 are added into DME respectively both at the low and high temperatures. Formaldehyde (CH2O) is a colorless and highly toxic gas. It is an intermediate combustion product, which can decrease sharply when the temperature achieves over a certain value. Fig. 16 shows

Y. Wang et al. / Applied Thermal Engineering 119 (2017) 373–386

385

Fig. 16. Temporal evolution of CH2O molar fraction of stoichiometric H2/DME/air and CH4/DME/air mixtures.

the temporal evolution of CH2O molar fraction with an increase in DME blending ratio. With the addition of CH4, CH2O mole fraction shows a slight decrease. However, the CH2O mole fraction is decreased clearly with the addition of H2 as expected. It is perhaps due to two aspects. One is that there is no CH2O formation during H2 combustion. The other is that the H2 addition can affect the dominant reactions related with CH2O. The dominant reactions associated with CH2O during the DME combustion are:

CH2 O þ OH () HCO þ H2 O

ðR72Þ

CH2 O þ H () HCO þ H2

ðR73Þ

CH3 þ O () CH2 O þ H

ðR147Þ

It is evident that most CH2O is consumed by the H and OH to form HCO. Due to higher H concentration in H2/DME blend combustion, much more CH2O is transformed to HCO compared with that for CH4/DME combustion. Thus, the decrease of CH2O through H2 addition indicates that the CH2O emission in DME combustion can be depressed. 4. Conclusion Kinetic modeling study of ignition of dimethyl ether with different fractions of hydrogen and methane addition is carried out in this paper. Radical analysis, main pollutant species, reaction pathway analysis and sensitivity analysis of ignition delay are examined at 900 K and 1400 K. The conclusions are as follows: (1) The ignition delay times of DME/H2 blends and DME/CH4 blends at 900 K both decrease as DME blending ratio

increases; however, with the increase of DME blending ratio, the ignition delay times of DME/CH4 blends decreases, while the ignition delay times of DME/H2 blends increase at 1400 K. Furthermore, it is observed that CH4 addition is more effective than H2 addition in terms of delaying the pure DME ignition at both temperatures. (2) Both H2 addition and CH4 addition can delay the radical pool development of the ignition of DME at the low temperature (900 K). However, due to the different kinetics is involved in the ignition process, the different delaying trends are observed for these two fuels. When H2 is added, highly active radicals such as H and OH generate abundantly through a series of chain reactions, leading to a relatively rapid radical build-up compared to that of the addition of CH4. At a high temperature (1400 K), CH4 addition also delays the radical pool development of DME ignition, but H2 addition promotes the radical pool development of DME ignition. This is also due to the different kinetics involved in the ignition process. (3) The analysis of the ignition delay time sensitivity coefficient shows that H-related reactions control the pure H2 ignition and CH3-related reactions control the pure CH4 ignition, but when DME is blend with H2 or CH4, the ignition chemistry is mainly characterized by reaction CH3OCH3 + HO2 = CH3OCH2 + H2O2 (R436) and CH3OCH3 + OH = CH3OCH2 + H2O (R433) at 900 K. At 1400 K, when DME is blended with H2, the ignition is dominated by the combined chemistries of DME and hydrogen; whereas, the ignition of CH4 is accelerated mainly by the decomposition reaction CH3OCH3(+M) = CH3 + CH3O(+M) (R432) and the CH3 chain reaction CH2O + CH3 = HCO + CH4 (R75).

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(4) H radical reaction path analysis for pure DME, DME/H2 and DME/CH4 fuel combustion at 900 K and 1400 K indicates that CH4 addition does not cause dramatic change in H production/consumption pathways compared with that of pure DME combustion. H2 addition also does not cause dramatic change in H consumption pathway. However, H production is promoted due to the presence of H2, thus resulting in relatively more H pool built-up compared with those of pure DME and DME/CH4 combustion. (5) For the homogenous ignition of DME/H2 fuel, H2 addition does not cause dramatic change in CH3 production/consumption pathways compared with pure DME at 900 K. At 1400 K, CH3 production pathway of DME/H2 blend also has no obvious change compared with that of pure DME fuel, but CH4 + H , CH3 + H2 (reverse R128) promotes CH3 consumption due to the presence of H2. For the homogenous ignition of DME/CH4 fuel, CH4 addition also does not cause dramatic change in CH3 consumption pathway compared with that of pure DME combustion both at 900 K and 1400 K. However, CH3 production is promoted due to the presence of CH4, thus resulting in relatively more CH3 pool built-up compared with those of pure DME and DME/H2. (6) Three kinds of pollutant species such as CO, NO and CH2O are calculated in this paper. Results show that CO and CH2O molar concentration for H2/DME blend fuel combustion is much lower than that for CH4/DME blend fuel combustion. However, NO formation is sensitive to temperature, and therefore H2 or CH4 addition has little impact on NO formation.

Acknowledgement The authors acknowledge the support of National Natural Science Foundation of China (91541118 and 51376038), Natural Science Basic Research Plan in Shaanxi Province of China (2016JM5012) and the Fundamental Research Funds for the Central Universities (XJJ2016051).

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