Chemical Effects of Carbon Dioxide Addition on Dimethyl Ether and

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Chemical Effects of Carbon Dioxide Addition on Dimethyl Ether and Ethanol Flames: A Comparative Study Dong Liu* School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, People’s Republic of China S Supporting Information *

ABSTRACT: The chemical effects of CO2 addition on premixed laminar low-pressure dimethyl ether and ethanol flames were studied by comprehensive numerical analysis from fuel-lean to fuel-rich conditions. Added CO2 is assumed as normal reactive CO2 and fictitious inert CO2 to assess the chemical effects of CO2. The dilution and thermal effects of CO2 addition decrease C2H2 mole fractions in ethanol flames instead of DME flames, but the chemical effects can reduce C2H2 mole fractions in both DME and ethanol flames at all equivalence ratios, which reveals that C2H2 formation can be suppressed chemically by CO2 addition. The chemical effects have a weak influence on formaldehyde formation in both DME and ethanol flames. The CO2 chemical effects only result in a slight decrease of acetaldehyde peak mole fractions in DME flames but not in ethanol flames at all equivalence ratios. Mole fractions of the H radical decrease because of the chemical effects of CO2 addition by shifting the equilibrium of CO + OH = CO2 + H in both DME and ethanol flames at all equivalence ratios, and mole fractions of OH and O radicals also decrease for equivalence ratios of 0.8, 1.0, and 1.2, whereas the chemical effects of added CO2 enhance the productions of OH and O radicals for rich conditions at an equivalence ratio of 1.5.

1. INTRODUCTION Because of tightened fossil fuel resources and rigorous environmental laws, bio-derived oxygenated fuels and fuel additives have attracted increasing interest. Two isomeric oxygenated fuels, dimethyl ether (DME) and ethanol, have received considerable attention because both of them can potentially replace the conventional hydrocarbon fuels, such as gasoline and diesel fuels.1 However, they exhibit different combustion characteristics, properties, etc. because of their different chemical structures. It is of great interest to obtain a better understanding of the combustion characteristics of these two oxygenated fuels under the same flame conditions. McEnally et al.2 measured the gas temperature, hydrocarbon concentrations, and soot volume fractions in ethylene non-premixed flames when DME or ethanol was added. They found that the maximum soot volume fractions increased with the addition of both DME and ethanol, which suggested that CH3 produced from the decomposition of DME and ethanol promotes the formation of C3H3 through C1 + C2 addition reactions and then benzene formed by C3H3 self-reaction. Later, Bennett et al.3 investigated the ethylene non-premixed laminar co-flow flames with DME or ethanol added to the fuel stream, and their results supported the conclusions given by McEnally et al.2 Wang et al.1 analyzed the combustion intermediates in DME/propene and ethanol/propene mixture flames with emphasis on the formation of harmful emissions using molecular-beam mass spectrometry (MBMS) employing electron- or photoionization (EI or PI), and Frassoldati et al.4 proposed a comprehensive kinetic model for a series of propene flames with the addition of DME or ethanol from ref 1. Xu et al.5 measured intermediate mole fractions in the low-pressure premixed stoichiometric DME and ethanol flames using PI-MBMS and compared several oxidation mechanisms to a new mechanism proposed. Very recently, Herrmann et al.6 © 2015 American Chemical Society

investigated the DME and ethanol oxidation at low temperature under the same conditions in the laminar flow reactor at 1 atm using an EI time-of-flight mass spectrometer. Exhaust gas recirculation (EGR) is often used in modern engines to reduce NOx emissions.7,8 It is particularly interesting to perform a comparative and comprehensive study to obtain a better understanding of chemical influences of CO2 addition on the flames of DME and ethanol under identical conditions from fuel-lean to fuel-rich. Earlier related work mainly focused on the effects of CO2 dilution on flames of conventional fuels. Du et al.9 analyzed the effects of CO2 additions on soot formation in ethylene and propane diffusion flames and found that soot inception can be suppressed by the chemical effects of CO2 addition. Yossefi et al.10 carried out methane combustion simulations in a synthetic system where CO2 replaced N2 and found in the beginning stage of the combustion that much CO2 could have large chemical effects. Liu et al.11 added CO2 on the fuel and the oxidizer side in the diffusion flame of ethylene and performed numerical analysis of chemical effects of CO2 addition, and it was found that CO2 addition can chemically suppress soot formation. Renard et al.12,13 used MBMS to investigate the effects of CO2 addition on species profiles in several low-pressure premixed ethylene/oxygen/argon-rich flames and found that CO2 addition makes the flame front shift downstream. Le Cong et al.14−16 investigated the effects of CO2 addition on the oxidation of hydrogen, methane, ethylene, and propene fuels in a fused silica jet-stirred reactor (JSR) from fuel-lean to fuel-rich conditions and found that the effects of CO2 addition mainly result from the equilibrium shifting of the Received: September 1, 2014 Revised: April 15, 2015 Published: April 15, 2015 3385

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Energy & Fuels reaction CO + OH = CO2 + H. Matynia et al.17 compared the effects of CO2 addition on the structure of rich and lean methane/air flames, and CO2 chemical inhibition is found. The numerical studies on the chemical effects of CO2 addition in hydrogen and methane counterflow diffusion flames have also been extensively reported by Park et al.18−21 Mancarella et al.22,23 studied the effects of CO2 addition in premixed ethylene flames with fuel-rich conditions and found that added CO2 can chemically suppress the polycyclic aromatic hydrocarbon (PAH) and soot formation. More recently, the effects of CO2 addition on the kinetics and structure of DME flames at low and high pressures were investigated by Liu et al.24 However, to the author’s best knowledge, until now, no available work was carried out to comparatively investigate the chemical effects of CO2 addition on flames of the isomeric fuels, DME and ethanol, under identical flame conditions from fuel lean to fuel rich. The present study is therefore devoted to a comparative study for the chemical influences of CO2 addition on the flames of the isomeric fuels, DME and ethanol, from fuel-lean to fuel-rich conditions by the comprehensive numerical analysis.

addition from other effects, including dilution effects and thermal effects. Following the analysis method proposed in refs 11 and 28, added CO2 is assumed as normal reactive CO2 and fictitious inert CO2 (written as FCO2) to obtain the actual CO2 chemical effects. Normal reactive CO2 is allowed to participate in chemical reactions in flames. Fictitious CO2 has exactly the same transport, thermochemical data, and third-body collision efficiency as normal CO2 but cannot be involved in chemical reactions in flames.

3. RESULTS AND DISCUSSION 3.1. Flame Temperature Profiles. Flame temperatures with and without CO2 addition are shown in Figure 1 for both

2. KINETIC MODELING AND ANALYSIS METHOD The detailed chemical mechanism for DME oxidation and combustion used here was originally developed by Zhao et al.25 (called the Zhao mechanism here) and was recently updated by Liu et al.24 (called the updated mechanism here). The updated mechanism (available in the Supporting Information of ref 24) improved the high-pressure flame speed prediction for DME flames with fuel-lean conditions. The Zhao mechanism and the updated mechanism can also be used for modeling ethanol oxidation and combustion5,6 because the detailed ethanol submechanism26 included in the Zhao mechanism and the updated mechanism was developed in a hierarchical way and validated against comprehensive experimental results from laminar flames, shock tubes, flow reactors, etc. Model predictions for freely propagated premixed laminar low-pressure DME/O2/Ar or ethanol/O2/Ar flames with and without CO2 addition are obtained using a modified ChemkinII/Premix code,27 and the calculations include multi-component molecular diffusion and thermal diffusion. The computational domain was set from −1.0 cm at the upstream to 10.0 cm at the downstream for long enough to achieve the adiabatic equilibrium at the downstream. The upstream initial temperature is set to 300 K, and the pressure is 40 mbar. The flame temperatures are obtained by solving the energy equation. The equivalence ratios are set to 0.8, 1.0, 1.2, and 1.5, which cover from fuel-lean to fuel-rich conditions. The detailed flame conditions are shown in Table 1. It was pointed out in ref 9 that the effects of additives can be mainly divided into three categories: (1) dilution effects resulting from the decrease of reactive species mole fractions and collision frequencies, (2) thermal effects as a result of flame temperature variation, and (3) chemical effects as a result of the participation of additives in chemical reactions. Generally, it is difficult to isolate chemical effects of CO2

Figure 1. Flame temperatures of DME and ethanol flames for different equivalence ratios with and without CO2 additions.

DME and ethanol flames. The dilution and thermal effects of CO2 addition result in temperature differences between 0% CO2 and 20% FCO2 addition, and the differences between 20% CO2 and 20% FCO2 additions are due to added CO2 chemical effects. The combined influences of dilution, thermal, and chemical effects lead to temperature differences between 0% CO2 and 20% CO2 additions. The differences on the main features of temperature profiles between DME and ethanol flames at all equivalence ratios are not obvious. Temperatures for all flames at different equivalence ratios decrease as CO2 is added. The dilution and thermal effects of added CO2 lead to a decrease of temperatures, whereas CO2 chemical effects only slightly reduce temperatures. 3.2. Major Species and Radicals. Figures 2 and 3 and Figure S1 of the Supporting Information give mole fraction profiles of selected major species, such as DME, ethanol, CO, and H2. As shown in Figure 2, the dilution and thermal effects of CO2 addition on DME/ethanol profiles in both DME and ethanol flames are not so obvious, except for the larger equivalence ratio ϕ = 1.5, whereas DME/ethanol profiles shift to the downstream side mainly because of chemical effects of CO2 addition for all equivalence ratios in both flames. Similarly, with profiles of DME/ethanol, in Figure 3 and Figure S1 of the Supporting Information, CO and H2 profiles in both DME and ethanol flames at different equivalence ratios are influenced very weakly by CO2 dilution and thermal effects but CO2 chemical effects can

Table 1. Flame Conditions for DME and Ethanol Flames at 40 mbara

a

ϕ

DME/ethanol

O2

Ar

CO2

0.8 0.8 1.0 1.0 1.2 1.2 1.5 1.5

0.1053 0.1053 0.1250 0.1250 0.1429 0.1429 0.1667 0.1667

0.3947 0.3947 0.3750 0.3750 0.3571 0.3571 0.3333 0.3333

0.5000 0.3000 0.5000 0.3000 0.5000 0.3000 0.5000 0.3000

0.0000 0.2000 0.0000 0.2000 0.0000 0.2000 0.0000 0.2000

Values are mole fraction, and ϕ is the equivalence ratio. 3386

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Figure 2. Mole fraction profiles of DME and ethanol in DME and ethanol flames for different equivalence ratios with and without CO2 additions.

C2H5OH + OH = CH2CH2OH + H2O, C2H5OH + OH = CH3CH2O + H2O, C2H5OH + H = CH3CHOH + H2, C2H5OH + OH = CH3CHOH + H2O, and C2H5OH + H = C2H4OH + H2. Similar to DME oxidation, all of the dilution, thermal, and chemical effects decrease the rates of ethanol consumption and also make the rate profile position shift to the downstream side. Moreover, noted from reactions above, the H, O, and OH radicals could play an important role in the DME and ethanol oxidation process. Mole fraction profiles of H, O, and OH radicals with different levels of CO2 addition are illustrated in Figure 5. H radical mole fractions are very slightly affected by the dilution and thermal effects but decrease significantly as a result of CO2 chemical effects through equilibrium shifting of reaction CO + OH = CO2 + H in both DME and ethanol flames at all equivalence ratios. Because of the decrease of the H radical, the importance of branching reactions is reduced, so that mole fractions of OH and O radicals decrease. The influence of CO2 on the production and consumption rates of the OH radical is also analyzed in detail, as shown in Figure 6 for equivalence ratio ϕ = 1.2 (results for other equivalence ratios ϕ = 0.8, 1.0, and 1.5

lead to a decrease of H2 mole fractions and an increase of CO mole fractions at all equivalence ratios in DME and ethanol flames. The reaction CO + OH = CO2 + H plays a major role in the increase of CO mole fractions.24 To analyze the detailed oxidation process of DME and ethanol, the rates of consumptions of DME and ethanol are given in Figure 4. Concerning DME consumption, three main reactions are identified: CH3OCH3 + O = CH3OCH2 + OH, CH3OCH3 + OH = CH3OCH2 + H2O, and CH3OCH3 + H = CH3OCH2 + H2. The CO2 dilution and thermal effects weakly affect the rates of these three reactions, but the chemical effects of added CO2 lead to a decrease of consumption rates and also shift the rate profile positions to the downstream sides, which causes the oxidation profile of the DME mole fraction to move toward downstream sides, as shown in Figure 2. Panels b−e of Figure 4 give the dominant reactions responsible for the ethanol oxidation. The reaction C2H5OH = C2H4 + H2O is more dominant for the ethanol oxidation than other reactions only at distances larger than about 2 mm. At the distances smaller than 2 mm, the reactions responsible for the ethanol consumptions are 3387

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OH = CH3 + H2O, CH4 + H = CH3 + H2, and CH4 + O = CH3 + OH. When CO2 or FCO2 is added in DME and ethanol flames, the rates of these reactions are all reduced. It is noted from Figure 7 that C2H2 mole fractions decrease when FCO2 is added in the ethanol flame because of CO2 dilution and thermal effects, whereas this effect is not obvious in DME flames. It is clear that CO2 chemical effects decrease C2H2 mole fractions in both DME and ethanol flames for all equivalence ratios. The lower C2H2 concentration can lead to a lower soot nucleation rate,11 which reveals that CO2 addition might inhibit soot formation. The dilution, thermal, and chemical effects of CO2 addition affect C2H2 profile peak positions in ethanol flames, whereas in DME flames, only chemical effects have a large influence on peak positions of C2H2 profiles. The rate of production analysis is performed for C2H2 in both flames, as shown in Figure 8. In DME flames, the main reactions responsible for C2H2 formation are H + C2H2 (+M) = C2H3 (+M) and C2H3 + H = C2H2 + H2 and the reactions responsible for C2H2 consumption are C2H2 + O = HCCO + H, C2H2 + O = CH2 + CO, and C2H2 + OH = C2H + H2O. In ethanol flames, the reactions responsible for C2H2 productions are the same as those in DME flames. It can be clearly seen that the chemical effects of CO2 addition on these reactions are larger than dilution and thermal effects. It can be concluded that C2H2 formation could be suppressed chemically by CO2 addition. For small equivalence ratios, chemical effects of added CO2 are mainly responsible for the shifts of C2H4 and C2H6 profile peak positions, as shown in Figures S4 and S5 of the Supporting Information. CO2 dilution and thermal effects can reduce the mole fractions of C2H4 in ethanol flames, instead of DME flames, and also increase the mole fractions of C2H6 in both DME and ethanol flames at different equivalence ratios. For large equivalence ratios, CO2 dilution and thermal effects could also cause C2H4 and C2H6 profile peak positions to shift toward the downstream sides. Mole fractions of C2H6 increase with CO2 additions in both DME and ethanol flames for all equivalence ratios. Moreover, the peak mole fraction of C2H6 in the DME flame is larger than that in the ethanol flame as a result of the direct combination reaction CH3 + CH3 (+M) = C2H6 (+M) and abundant CH3 radicals formed through reactions CH3OCH3 + R = CH3OCH2 + RH and CH3OCH2 = CH3 + CH2O (R are radicals) in DME flame, whereas in the ethanol flame, the main reaction responsible for the ethanol oxidation is the reaction CH3CH2OH = C2H4 + H2O, which leads to higher C2H4 concentrations in the ethanol flame than that in the DME flame. The third-body collision efficiency of CO2 is larger than that of argon, which might promote the formations of C2H6. With CO2 addition, the mole fractions of C2H4 decreased in both DME and ethanol flames. For the ethanol flame, it is easy to understand this result because C2H4 can be directly formed from the reaction CH3CH2OH = C2H4 + H2O, which is one of the main reactions responsible for ethanol oxidation from ROP analysis. From Figure 4e, as CO2 is added, the consumption rate of ethanol in the reaction CH3CH2OH = C2H4 + H2O is decreased, which induces the decrease of the C2H4 concentration. For the DME flame, because radicals, such as H, O, and OH, decrease with the addition of CO2, the C2H5 formation is expected to be reduced from the reaction C2H6 + R = C2H5 + RH, and thus, the C2H4 mole fraction reduces accordingly. It is also found from ROP analyses of C2H2 that the formation of C2H2 is closely related with C2H3, which can be directly formed from C2H4. Therefore, C2H2 mole fractions are always lower in both DME and ethanol flames.

Figure 3. Mole fraction profiles of CO in DME and ethanol flames for different equivalence ratios with and without CO2 addition.

can be found in the Supporting Information). As CO2 is added in DME and ethanol flames, the rate of OH consumption for reaction CO + OH = CO2 + H is decreased mainly because of CO2 chemical effects by shifting reaction equilibrium. However, reaction CO + OH = CO2 + H is not the dominant reaction responsible for OH production. The dominant reactions responsible for OH formation are H + O2 = O + OH in both DME and ethanol flames. When CO2 is added, the rates of reactions for OH radical formation are all decreased because of CO2 dilution, thermal, and chemical effects. This overall effect might lead to a decrease of the OH radical mole fraction. Moreover, in comparison of OH and O radical profiles of 20% CO2 and FCO2 additions for rich conditions with ϕ = 1.5, the profiles with 20% CO2 addition are higher than the profiles with FCO2 addition. From this point of view, although the dilution and thermal effects of CO2 addition reduce the mole fractions of OH and O radicals, CO2 chemical effects can enhance the production of OH and O radicals for rich conditions with ϕ = 1.5. However, the overall influences of dilution, thermal, and chemical effects lead to a decrease of the OH radical mole fraction seen from profiles of 0% CO2 and 20% CO2 additions noted from Figure 5. 3.3. Intermediate Hydrocarbon Species. For stable intermediate hydrocarbon species, the mole fraction profiles of CH4, C2H2, C2H4, and C2H6 for different CO2 additions are shown in Figure 7 and Figures S3−S5 of the Supporting Information. From Figure S3 of the Supporting Information, CO2 dilution and thermal effects reduce CH4 mole fractions in DME flames but not in ethanol flames at all equivalence ratios and chemical effects of CO2 addition have little influence on CH4 concentrations in DME and ethanol flames. The shift of peak positions to downstream sides is mainly due to chemical effects of CO2 addition in both flames. The mole fractions of CH4 in DME flames are higher than those in ethanol flames, which is consistent with previous results.5 By rate of production (ROP) analysis in both DME and ethanol flames, the main reaction responsible for CH4 formation is HCO + CH3 = CO + CH4 and the main reactions responsible for CH4 consumption are CH4 + 3388

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Figure 4. (a) DME and (b−e) ethanol (equivalence ratio of 1.2) rate of consumption in DME and ethanol flames with different CO2 additions. In panel a, the line numbers 1−3 represent the same reactions. In panels b−d, the line numbers 1−6 represent the same reactions.

3.4. Intermediate Oxygenated Species. For intermediate oxygenated species, the mole fraction profiles of formaldehyde (CH2O), methanol (CH3OH), and acetaldehyde (CH3CHO) with different CO2 additions are illustrated in Figure 9 and Figures S6 and S7 of the Supporting Information. The dilution

and thermal effects of CO2 addition are small on CH2O in ethanol flames with all equivalence ratios in Figure 9, whereas in DME flames, CO2 dilution and thermal effects can slightly reduce the mole fractions of CH2O. The chemical effects shift CH2O peak positions toward the downstream side in both DME and 3389

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Figure 5. Mole fraction profiles of (a) H, (b) OH, and (c) O radicals in DME and ethanol flames for different equivalence ratios with different CO2 additions.

Figure 7. Mole fraction profiles of C2H2 in DME and ethanol flames for different equivalence ratios with and without CO2 addition.

ratios, except for slightly shifting of peak positions, which can be seen from Figure S6 of the Supporting Information. As the CO2 is added, methanol mole fractions decrease in DME flames but increase in ethanol flames for all equivalence ratios, which is due to dilution and thermal effects of CO2 addition. For acetaldehyde in Figure S7 of the Supporting Information, CO2 chemical effects only result in a small decrease of acetaldehyde peak mole fractions in DME flames but not in ethanol flames for all equivalence ratios. The dilution, thermal, and chemical effects all make the peak positions of acetaldehyde move to the downstream side for different equivalence ratios.

Figure 6. OH radical rate of production in (a) DME and (b) ethanol flames for an equivalence ratio of 1.2 with different CO2 additions.

ethanol flames. CO2 chemical effects on CH2O mole fractions are not obvious in both flames. The rate of production analysis is also given for CH2O in Figure 10. It is found in DME and ethanol flames that chemical effects of CO2 addition are more obvious than CO2 dilution and thermal effects on CH2O production. CO2 chemical effects can lead to a decrease of production rates for main reactions responsible for CH2O oxidation. The chemical effects of CO2 addition on methanol are not noticeable in both DME and ethanol flames at all equivalence

4. CONCLUSION The comparative study for the chemical effects of CO2 addition on the flames of the isomeric fuels, DME and ethanol, from fuellean to fuel-rich conditions was performed by comprehensive 3390

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Figure 8. C2H2 rate of production in (a) DME and (b) ethanol flames for different equivalence ratios with different CO2 additions. In panel a, the line numbers 1−5 represent the same reactions. In panel b, the line numbers 1−5 represent the same reactions.

fractions of C2H2 in both DME and ethanol flames. Chemical effects of CO2 addition on main reactions responsible for C2H2 productions are larger than dilution and thermal effects. C2H2 formation could be suppressed chemically by CO2 addition. CO2 chemical effects shift CH2O peak positions toward the downstream side but have a weak influence on formaldehyde

numerical analysis. CO mole fractions increase because of chemical effects of added CO2 in DME and ethanol flames for all equivalence ratios, and reaction CO + OH = CO2 + H is the main reason for the increase of CO. C2H2 mole fractions decrease as a result of CO2 dilution and thermal effects only in ethanol flames, instead of DME flames. The CO2 chemical effects reduce mole 3391

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= 1.5, but the overall influences of dilution, thermal, and chemical effects lead to a decrease of the OH radical mole fraction.

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ASSOCIATED CONTENT

S Supporting Information *

Mole fraction profiles of H2 in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S1), OH radical rate of production in (a) DME and (b) ethanol flames for equivalence ratios of 0.8, 1.0, and 1.5 with different CO2 additions (Figure S2), mole fraction profiles of CH4 in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S3), mole fraction profiles of C2H4 in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S4), mole fraction profiles of C2H6 in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S5), mole fraction profiles of CH3OH in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S6), and mole fraction profiles of CH3CHO in DME and ethanol flames for different equivalence ratios with and without CO2 addition (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Mole fraction profiles of CH2O in DME and ethanol flames for different equivalence ratios with and without CO2 addition.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-18362962967. Fax: +86-25-84314960. E-mail: [email protected].

concentrations in both DME and ethanol flames. The chemical effects of added CO2 only result in a slight decrease of acetaldehyde peak mole fractions in DME flames but not in ethanol flames for all equivalence ratios. The chemical effects of CO2 addition cause the concentration of the H radical to decrease by shifting the equilibrium of the reaction CO + OH = CO2 + H in both DME and ethanol flames for all equivalence ratios, and also mole fractions of OH and O radicals decrease for ϕ = 0.8, 1.0, and 1.2. The chemical effects of added CO2 enhance the productions of OH and O radicals for rich conditions with ϕ

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51306091), the Jiangsu Provincial Natural Science Foundation of China (BK20140034 and BK20130758), the Fundamental Research Funds for the Central Universities

Figure 10. CH2O rate of production in (a) DME and (b) ethanol flames for different equivalence ratios with different CO2 additions. In panel a, the line numbers 1−4 represent (1) CH3OCH2 = CH2O + CH3, (2) CH3 + O = CH2O + H, (3) CH2O + OH = HCO + H2O, and (4) CH2O + H = HCO + H2. In panel b, the line numbers 1−5 represent (1) CH2OH + O2 = CH2O + HO2, (2) CH3 + O = CH2O + H, (3) CH3CH2O + M = CH3 + CH2O + M, (4) CH2O + OH = HCO + H2O, and (5) CH2O + H = HCO + H2. 3392

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Energy & Fuels (30920140111005), and the Jiangsu Provincial Project of “Six Talent Summit” (2014-XNY-002). The author thanks Dr. Xin Tu from the University of Liverpool for critical reading and checking of the manuscript. The author also thanks the valuable suggestions and comments of the reviewers for improving this paper.

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DOI: 10.1021/ef501945w Energy Fuels 2015, 29, 3385−3393