argon premixed flames

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Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/argon premixed flames Yacine Rezgui*, Miloud Guemini Laboratory of Applied Chemistry and Technology of Materials, University of Oum El Bouaghi, P.O.B. 358, Constantine Road, Oum El Bouaghi 04000, Algeria

article info

abstract

Article history:

A modified one-dimensional model (Premix) in conjunction with Chemkin II and a detailed

Received 29 July 2017

kinetic scheme combining the chemistries of hydrogen, dimethyl ether and iso-octane

Received in revised form

combustion were used to investigate the effect of hydrogen addition on the chemical

6 October 2017

composition of laminar premixed dimethyl ether/iso-octane/oxygen/argon flames. In the

Accepted 12 October 2017

current modeling study, the key reaction mechanisms responsible for the observed vari-

Available online xxx

ation in mole fractions of some major species, including radicals H, O and OH, species CO, CO2 and CH4 as well as oxygenated products CH2O and CH3CHO, were defined. The nu-

Keywords:

merical modeling was focused on both chemical and dilution and thermal effects of

Hydrogen addition

hydrogen addition. It was found that chemical effect of hydrogen addition induced a

Laminar flames

boosting in the oxidation process whereas dilution effect led to its inhibition. The higher

Dimethyl ether

the hydrogen level the higher was the magnitude of the phenomenon. Besides, the

Iso-octane

chemical effect of hydrogen addition induced an enhancement in the OH, O, CO, CO2 and

Fuel blend

CH4 mole fractions, whereas the dilution and thermal effect led to a lowering in the amount of these species. This latter phenomenon was more important than the first effect yielding a net decrease in the concentrations of the four species. In addition, it was found that CH2O and CH3CHO levels showed a dramatic decrease in peak height with hydrogen doping and that the contribution of the thermal effect of hydrogen addition to this lowering was more important than that of its chemical effect. Thus it can be concluded that the synergism between chemical and dilution effects played a paramount role in the dimethyl ether/iso-octane/oxygen/argon premixed flames oxidation process as well as in the rise or the decrease in the pool radicals and the major species concentrations. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction With increasing concern about depletion of crude oil resources and environmental problems, the search of using alternative fuels and developing high-efficiency combustion technology become the important issues for engine and

combustion researchers [1,2]. In the first context, it was found that renewable biofuels which can be derived through biochemical processes from biomass have the potential to provide a path towards carbon-neutral fuels that are renewable and clean burning [3e5], whereas in the second context, it was reported that homogeneous charge compression ignition (HCCI) would be a potential solution technology. Dimethyl

* Corresponding author. E-mail address: [email protected] (Y. Rezgui). https://doi.org/10.1016/j.ijhydene.2017.10.063 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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ether (DME), considered as a second generation biofuel [6] which can be produced from biomass as well as from natural gas and coal [7], is widely discussed as alternative fuel or additive in diesel and homogeneous charge compression ignition (HCCI) engines [8,9], gas turbines [8,10], or fuel cells [8,11]. The key properties that make the simplest aliphatic ether, DME, so attractive for compression ignition engines are its physical and combustion properties including no carboncarbon bonds, high oxygen fraction, low carbon to hydrogen ratio, high cetane number, low boiling point, good cold-start characteristics and no air or ground-water pollution effects [1,12e14]. All these properties make dimethyl ether an excellent alternative fuel or fuel additive candidate for reducing polycyclic aromatic hydrocarbons (PAHs), NOx, CO2, combustion noise and emissions of particles [7,15]. Due to these promising features, extensive studies of DME pyrolysis, oxidation and combustion have been carried out in a variety of laboratory facilities including static and flow reactors [16e18], jet-stirred reactors [19e22], flow reactors [8,23e32], shock tubes [2,6,20,33e39], rapid compression machines [6,40,41] and low- and high-pressure flames [7,14,42e47]. Homogeneous charge compression ignition is a dual fuel combustion strategy leading to a potential major advance in high efficiency and low-emission engines [48]. This new strategy relies on the use of combinations of fuels with high octane and high cetane numbers [41,49,50]. In this context isooctane, which is a gasoline primary reference fuel (PRF) that is highly knock resistant, with an assigned octane number of 100 [51], can be an attractive fuel candidate for use in HCCI engines. On the other hand, due to the availability of various hydrogen production systems, good combustion and favorable physicochemical properties of H2 [52e55], the use of hydrogen as a second fuel is a widely accepted option of reducing emissions. In this context, co-combustion of hydrogen with hydrocarbons such as methane [56e60], ethylene [61,62], propane [63,64], n-heptane [65,66], iso-octane [55,67e69], and with oxygenated products such as methanol [70e73], ethanol [74e77], dimethylether [77e81], as well as with classical fuels, gasoline [82e84] and diesel [85e87], was investigated by several research groups. In all these studies, it was reported that the combustion of hydrogen/hydrocarbons mixtures exhibited several benefits as compared to that of the neat hydrocarbons. In view of these discussions, the main purpose of this work is to numerically investigate the effect of hydrogen enrichment on the combustion of an equimolar dimethyl ether/isooctane premixed flame. Chemical and dilution effects of hydrogen addition on the studied mixture behavior are highlighted.

Modeling approach Kinetic model The detailed chemical mechanism for DME/iso-octane oxidation and combustion developed by Zeng et al. [50] was used here. The mechanism containing 379 species evolved in 1931 reactions, was a combination of an iso-octane mechanism with diethyl ether and dimethyl ether submechanisms. The

core of the reaction mechanism, used to describe oxidation and combustion of iso-octane, was taken from the work of Pitsch and coworkers dealing with gasoline surrogate and including submechanisms of iso-octane, n-heptane, toluene and polycyclic aromatic hydrocarbons [88]. This kinetic model has been validated against a large array of experimental data including shock tube ignition delays [89], laminar flames speeds [90] and premixed iso-octane flames [91]. The submechanism for diethyl ether was gathered from the kinetic scheme of Tran and collaborators [92] which had been validated by using several datasets, including shock tube ignition delay times [93,94], flame speeds [95], pyrolysis experiments [94] and non-premixed flames [96]. Finally, AramcoMech2.0 sub-mechanism [97] was used for describing the dimethyl ether oxidation and combustion. It is noteworthy that the combined iso-octane/diethyl ether/dimethyl ether has been examined against different experiments including laminar premixed flames of iso-octane [91], diethyl ether [92] and dimethyl ether [98]; iso-octane species shock tube [99,100], ignition delay times for iso-octane [101,102], diethyl ether [93] and dimethyl ether [39]; laminar flame speed for iso-octane [103e110], diethyl ether [95] and dimethyl ether [111,112]. In addition to the examination against pure hydrocarbons, the combined mechanism was validated against premixed lowpressure flames of iso-octane with different amounts of dimethyl ether and diethyl ether [50].

Computational method Calculations were performed with a modified Chemkin II/ Premix code [113] with implementation of high pressure PLOG function for one-dimensional flames. Adiabatic equilibrium was kept by broadening the calculation domain from
2.0 cm at the upstream to 8.0 cm at the downstream. Averaged transport and withdraw differencing method were used for computing the iso-octane-DME-hydrogen-argon-O2 freely propagated laminar premixed flames and solving the steadystate species and energy conservation equations. Computations were performed for the conditions of Zeng et al. [50]: cold gas temperature of 298 K, a pressure of 40 mbar, a mass flux of 0.003463 g/cm2 s, an equivalence ratio of 1.53 and an equimolar composition of iso-octane and dimethyl ether (50%, 50%). The percentage of the relative hydrogen fraction in the H x100 where XM is the fuel mixture, defined as RH ð%Þ ¼ XHXþX M sum of dimethyl ether and iso-octane mole fraction in dual fuel, and XH the mole fraction of hydrogen, has been varied from 0 to 60 (0, 30 and 60) under fuel rich conditions. To numerically isolate the chemical effect of hydrogen addition on the investigated flames from its dilution and thermal effects, the strategy mentioned in Refs. [77,79,114e117] was employed. The added H2 is assumed as normal reactive H2 and fictitious inert H2 (referred as FH2). Normal H2 is allowed to participate in chemical reactions in flames, whereas the fictitious H2 has identical thermal and transport properties as well as third-body collision efficiencies as reactive H2 but cannot be involved in chemical reactions in flames. Thus, the observed differences between the results from the reactive and inert hydrogen additions are ascribed to the H2 chemical effects. In order to keep the same equivalence ratio for all flames, total oxygen is divided in reactive (O2) and

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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fictitious inert (FO2) oxygen. As for FH2, FO2 exhibits the same thermal and transport properties as well as third-body collision efficiencies as reactive O2 but it is not allowed to participate in chemical reactions in flames. It is noteworthy that pathway analysis was performed with the appropriate subroutines in the Chemkin package (CKQYP, CKCONT), which systematically compute the rate of production and consumption of each species [113]. Only the main reactions that have an important role in chemicals belonging to the studied system will be presented.

Results and discussion Flame temperature Fig. 1 presents the evolution of the temperature profile as well as the adiabatic flame temperature of the dimethyl ether-isooctane-hydrogen-oxygen-argon mixtures with variation in the percentage of hydrogen (0, 30 and 60%). Whatever the hydrogen amount, the flame temperature profiles displayed the same shape. They increased upon increasing the height above the burner to reach a plateau at about 0.8 cm without any further observable trend to change (Fig. 1a). Besides, regardless of the distance from the burner, increasing the hydrogen level in the fuel mixture led to a lowering in the flame temperature. These findings are in accordance with the results reported by Chen et al. [118] in the case of hydrogen blended dimethyl ether premixed laminar flames, where the temperature decrease was

ascribed to two facts. The first one was the low volume calorific value of H2 as compared to DME, which induced a lowering in the heat release of the unit volume mixture upon increasing the hydrogen fraction in the fuel; and the second one was the higher values of the burning velocities of DME/ H2 blended fuels as compared to the neat DME flames, resulting in a more rapid heat transfer to the burner and consequently yielding a lower mixture temperature upon raising the hydrogen proportion in the fuel. On the other hand, flame adiabatic temperatures were dependent on the hydrogen level as well as on the nature of its effect, thermal (dilution) or chemical. The higher the hydrogen amount the lower the adiabatic flame temperature. The decrease due to the hydrogen chemical effect did not exceed 7 K, whereas the dilution effect induced a lowering of 87 K for mixtures with 60% FH2 (Fig. 1b). These facts suggest that the observed decrease in the adiabatic flame temperature by hydrogen addition was mainly due to thermal effects. Similar trends were observed by Liu during his investigation on the chemical effects of hydrogen addition on dimethyl ether flames [79] and by Luo and Liu during their kinetic study on the effect of hydrogen addition on ethanol and dimethyl ether flames [77].

Radicals pool Because small radicals such as H, O, and OH can control the decay of any combustible by activating the reacting process and accelerating the oxidation and heat release process [119e121], the effects of the hydrogen addition on the

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Fig. 1 e Effect of hydrogen addition on the temperature profiles as well as on the flame adiabatic temperatures of the CH3OCH3/iC8H18 mixtures. Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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dilution and thermal effects. As earlier mentioned, the H2 chemical effect induced a rise in the OH mole fraction, whereas the dilution and thermal effect led to a lowering in the OH amounts. This latter phenomenon was more important such as the net effect (combined effect ¼ chemical effect þ thermal and dilution effects) was in favor of the decrease in OH mole fraction (Fig. 3b). Besides, our flux analysis data indicated that, whatever the hydrogen percentage in the fuel mixture, OH production was governed by the reaction H þ O2 ¼ O þ OH (R2), whereas its main depletion pathway was found to be the reaction H2 þ OH ¼ H2O þ H (R1). OH formation rate was found to be boosted by a rise in the hydrogen proportion, whereas the opposite trend was observed for the consumption one. However the lowering in the consumption rate was more pronounced than the boosting in the formation one, consequently the net OH formation decreased upon increasing H2 amounts in the fuel mixture. On the other hand, both OH formation and consumption rates were decreased by dilution effect leading to a net lowering in OH mole fraction upon increasing FH2 percentage in the fuel mixture. Regardless of the hydrogen percentage in the fuel mixture, O radical mole fraction increased with increasing height above the burner and reached a maximum, then decreased to remain constant for distances above 1.7 cm (Fig. 4a). As in the case of OH radical, O atoms decreased upon raising H2 and FH2 amounts. The effect of dilution by hydrogen addition was found to be more important than its chemical effect (Fig. 4b).

concentrations of these radicals was investigated, and the results are depicted in Figs. 2e4. Regardless of the amount of hydrogen in the mixture, hydrogen atoms mole fraction displayed increasing trends at the beginning of the flame to reach a plateau at distances beyond 1.2 cm (Fig. 2a). H radical mole fraction was boosted with a rise in the hydrogen (H2) proportion in the fuel mixture, hydrogen radical amounts were the lowest in the 0% hydrogen flame and the highest in the 60% hydrogen flame. Opposite trends were observed when increasing the fictitious hydrogen concentration in the fuel mixture. These findings led to the fact that while the chemical effect of hydrogen promoted the formation of hydrogen radicals, the dilution and thermal effects inhibited this formation (Fig. 2b). Sensitivity analysis indicated that, under our conditions, hydrogen abstraction with OH (H2 þ OH ¼ H2O þ H (R1)) was the dominant H radical formation pathway. Thus any addition of H2 will induce a shift in the chemical equilibrium toward H production. Although the chemical effect of hydrogen addition induced a rise in OH mole fraction (Fig. 3b), a lowering in the hydroxyl radical concentration was observed when raising the hydrogen percentage in the fuel mixture especially at the post-flame zone (height above the burner >1.25 cm) (Fig. 3a). OH radical mole fractions displayed the highest value in the case of the neat CH3OCH3-iC8H18 flame and the lowest one in the case of 60% hydrogen blended fuels. This finding could be ascribed to the synergism between the H2 chemical and

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Fig. 2 e Effect of hydrogen addition on the mole fraction of H atoms.

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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Fig. 3 e Effect of hydrogen addition on the mole fraction of OH radicals.

Reactants Fig. 5a and b depict the variation of the reactants (DME and iC8H18) mole fraction with the height above the burner for different hydrogen amounts. Regardless of the hydrogen percentage in the fuel mixture, dimethyl ether was completely consumed at about 0.4 cm from the surface of the burner, whereas the complete iso-octane consumption was found to be at about 0.3 cm. Which means that, under our condition, iC8H8 is more reactive than CH3OCH3. Besides, profiles shapes in all flames were globally very similar, indicating that oxidation of dimethyl ether-isooctane/hydrogen mixtures, under the studied conditions, was largely determined by dimethyl ether and iso-octane oxidation rather than hydrogen. Similar observations have been reported by Mze Ahmed et al. [122] during their study on the effect of hydrogen addition to methane/hydrogen/air laminar premixed flames on soot gaseous precursors' formation. On the other hand, mole fractions of the reactants where found to be dependent on the hydrogen enrichment as well as on its effect nature (chemical or transport and dilution). An increase in dilution, by FH2 addition, shifted the reactants mole fractions toward the post-flame zone, whereas the opposite trend was observed when raising the percentage of hydrogen in the fuel mixture. These observations led to the fact that dilution inhibited the reactants oxidation process, whereas hydrogen chemical effect promoted it. These findings may lead to wrong conclusions on the influence of hydrogen addition on the oxidation

process, because we cannot assess this effect except when the initial amounts of carbonated reactants are the same. In this context, the mole fractions of the reactants in all the hydrogen blended fuels were normalized to their concentrations in the flame without hydrogen. The new mole fractions were 0 jY j Norm ¼ Yi Yi;10 , where Yi is the mole calculated using the formula Yi;j i;j fraction of the reactant “i” (dimethyl ether or iso-octane) in the Norm 0 its corresponding normalized value, Yi;j the flame “j”, Yi;j 0 initial mole fraction of the reactant “i” in the flame “j” and Yi;1 its corresponding value in the neat flame (flame without hydrogen). Using the new calculated values (Fig. 5c and d), it was found that dilution inhibited the oxidation process of both dimethyl ether and iso-octane, the phenomenon was more noticeable for iC8H18, whereas a very little effect was induced by the chemical effect of hydrogen due to H2 addition. Based on these results it could be said that the reduction of the initial reactants (dimethyl ether and iso-octane) due to their replacement by hydrogen play a paramount role in the reduction of CH3OCH3 and iso-C8H18 mol fractions in the blended flames. This finding is in accordance with the results of Liu et al. [78] who reported that the reduction of CH3OCH3 mole fraction in the hydrogen blended dimethyl ether mixture was the dominant factor for the reduction of CH3OCH3 mole fractions in the flame. In order to elucidate the main chemical pathways of DME and iC8H18 oxidation and combustion in the neat and hydrogen blended fuels, a flux analysis was performed. Conditions were chosen to have adequate fuel conversion so that

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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Fig. 4 e Effect of hydrogen addition on the mole fraction of O radicals.

major pathways for primary products depletion could be observed. Snapshots of the two reactants' depletion paths, as given by means of the pathway analysis, are depicted in Fig. 6a and b. For dimethyl ether (Fig. 6a), it can be seen that, regardless of the hydrogen amount in the fuel mixture, the dominant consumption channels were the H-abstraction reactions by H atoms (R3), OH and methyl radicals (R4 and R5), and O atoms (R6) to form CH3OCH2: CH3OCH3 þ H ¼ CH3OCH2 þ H2 (R3) CH3OCH3 þ OH ¼ CH3OCH2 þ H2O (R4) CH3OCH3 þ CH3 ¼ CH3OCH2 þ CH4 (R5)

Once formed, the methoxymethyl radical (CH3OCH2) underwent a unimolecular decomposition yielding methyl (CH3) and formaldehyde (CH2O). Methyl radical underwent several reactions producing ethane, singlet methylene (SCH 2), methane, acetaldehyde (CH3CHO) and methoxide (CH3O), whereas formaldehyde produced HCO via H-abstraction. All these species yielded finally CO which was subsequently oxidized to CO2. It is noteworthy that the magnitude of all these routes was dependent on the hydrogen level in the fuel mixture. From the scheme given in Fig. 6a, it can be concluded that the oxidation process of dimethyl ether, in the investigated mixture, was largely determined by DME oxidation rather than hydrogen. The abovementioned findings indicate that CO2 formation from DME oxidation, under the investigated conditions, could be resumed by the following scheme:

CH3OCH3 þ O ¼ CH3OCH2 þ OH (R6)

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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Fig. 5 e Effect of hydrogen addition on the depletion of CH3OCH3 and iC8H18.

Iso-octane oxidation process was found to be more complex than that of dimethyl ether (Fig. 6b). H-atom abstraction accounted for most of the iC8H18 depletion, whereas the unimolecular decomposition route was found to be of minor importance (less than 9% of the iC8H18 consumption). Four distinct iso-octyl radicals were formed, the 2,2,4-trimethyl- pentan-4-yl radical (CXC8H17) was the most abundant radical due to the easiness of the H-abstraction reaction at the tertiary CeH site, followed by the 2,2,4trimethyl-pentan-3-yl radical (BXC8H17) resulting from the H-abstraction at the secondary CeH site. The less abundant iso-octyl radical was found to be the 2,2,4-trimethyl-pentan5-yl radical (DXC8H17) due to the high bond dissociation energy of the primary site. Then the produced iso-octyl radicals underwent monomolecular decomposition reactions giving alkenes and smaller alkyl radicals. As in the case of dimethyl ether oxidation, the major iC8H18 oxidation pathways were not strongly affected by H2, however, the magnitudes of radical production and depletion changed. This finding suggests that the iso-octane oxidation process, in the investigated mixture, was largely determined by iC8H18 oxidation rather than H2.

Major species In this section special attention is paid to carbon monoxide which is a common intermediate produced during the combustion of fossil fuels [118,123], to carbon dioxide which is the dominant contributor to global warming [124], and to methane which is found to be one of the most abundant species during the combustion of the investigated mixture. CO mole fraction profiles displayed similar shapes whatever the hydrogen percentage in the fuel mixture. Their values were dependent on both hydrogen level and H2 dilution effect. The higher the hydrogen amount in the mixture and the H2 dilution effect the lower the CO concentration (Fig. 7a). Similar trends were reported by Liu during his study on the chemical effects of hydrogen addition on dimethyl ether premixed flames [78]. It is noteworthy that the overall hydrogen addition effect is a combination of chemical and dilution effect. In our case, the chemical effect led to a rise in the CO mole fraction whereas the dilution (addition of FH2) had a significant CO depressing effect. This latter phenomenon was more important than the first effect yielding a net decrease in CO amounts (Fig. 7b). To get more insights on the role of chemical and

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Fig. 6a e Sequence reactions for the consumption of dimethyl ether and the formation of CO, CO2, CH4, CH2O and CH3CHO for a DME conversion of 90%. Black and red values are for the mixtures with 0% and 60% hydrogen, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

dilution effects in CO production, a flux analysis was performed. It was found that for all flames with and without hydrogen, the importantly contributing reactions to CO production were: HCO þ O2 ¼ CO þ HO2 (R7)

HCO þ M ¼ CO þ H þ M (R8)

HCO þ H ¼ CO þ H2 (R9) As H2 is added, rates of the reverse reactions of (R8 and R9) were boosted due to the enhancement of H and H2 mole fractions, consequently R8 and R9 chemical equilibriums will be shifted toward the formation of HCO yielding less CO amounts. Besides, it can be seen, from Fig. 6a and Fig. 6b, that HCO was mainly produced from the initial carbonated reactants (CH3OCH3 and iC8H18) which means that the partial replacement of DME and iso-octane by H2 will induce a lowering in HCO mole fraction and consequently a decrease in CO concentration. Combination of these findings with the CO amount promotion due to the chemical effect of hydrogen

addition suggest that the CO formation was boosted with hydrogen addition, but less iC8H18 and CH3OCH3 participated in the reaction such that less CO was produced. These trends are in qualitative agreement with the data reported by Liu et al. [78], who observed a decrease in the CO mole fraction upon an increase in the hydrogen percentage in the fuel mixture. As in the case of carbon monoxide, carbon dioxide mole fraction exhibited similar shapes regardless of the hydrogen percentage in the fuel mixture. CO2 amounts displayed a decreasing trend upon increasing dimethyl ether and iso-octane replacement percentage by hydrogen. The higher the hydrogen amount the lower the CO2 mole fraction (Fig. 8a). This could be related to the lowering of the initial mole fraction of the reactants (DME and i-C8H18) as well as to the decrease in the C/H ratio of the mixture induced by hydrogen addition. A detailed inspection of the hydrogen addition effect indicated that the H2 chemical effect led to a rise in CO2 mole fraction, whereas the H2 dilution effect induced a lowering in this concentration with the latter effect being more noticeable. This implies that the net effect (summation of the chemical effect and dilution effect (Fig. 8b) was in favor of decreasing CO2 mole fraction upon increasing hydrogen addition. In addition, it was found that CO2 resulted directly

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Fig. 6b e Sequence reactions for the consumption of iso-octane and the formation of CO, CO2, CH4, CH2O and CH3CHO for iC8H18 conversion of 89%. Black and red values are for the mixtures with 0% and 60% hydrogen, respectively.

from the CO oxidation process (Fig. 6a and Fig. 6b), thus any decrease in CO concentrations will reduce the CO2 obtained amounts. Fig. 9a portrays the dependence of the methane (CH4) mole fractions on the hydrogen (H2 and FH2) percentage. It can clearly be seen that the same trends were observed for any level of the reactants' (dimethyl ether and iso-octane) replacement by hydrogen (H2 or FH2) additive. In all cases, the concentration of methane displayed increasing trends with distance above the burner, reached a maximum and decreased thereafter. Besides, CH4 levels decreased upon increasing the percentage of the hydrogen in the mixture. Methane amounts were the highest in the 0% H2 flame (8.04  10
3) and the lowest in the 60% H2 flame (6.96  10
3), which means a decrease of 13.9%. The observed decrease was essentially due to dilution and transport effect which contributed with 13.7% (Fig. 9b). On

the other hand, CH4 peak maxima were staying at their position when adding H2 (combined effect) and were shifting to higher distances above the burner with an increment in the fictitious hydrogen (FH2: thermal and transport effect) (Fig. 9c). These findings led to the fact that chemical effect of hydrogen addition, which is the result of the combined effect minus the thermal effect, induced a CH4 peak maxima shift toward lower distances above the burner with a magnitude equal to that induced by the thermal hydrogen effect but in the opposite direction. Thus it could be concluded that hydrogen chemical effect promoted methane production by enhancing the oxidation process whereas the opposite trend was obtained by the thermal effect of hydrogen addition. Pathway analysis indicated that methane formation, from the CH3OCH3/iC8H18/H2 mixture, was governed by the reactions set:

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12

0% 30% 30%-F 60% 60%-F

0.12

8

4

0.08

0.04

30%

0.0

0.5

1.0

1.5

2.0

2.5

60%

Decrease in CO mole fraction (%)

16

0

3.0

Height above the burner (cm)

H2 or FH2 (%)

Fig. 7 e Effect of hydrogen addition on CO mole fractions.

CH4 þ H ¼ CH3 þ H2 (R10)

CH3 þ H(þM) ¼ CH4(þM) (R11)

CH3 þ HCO ¼ CH4 þ CO (R12)

CH3 þ HO2 ¼ CH4 þ O2 (R13)

CH3 þ CH2O ¼ CH4 þ HCO (R14)

CH3OCH3 þ CH3 ¼ CH3OCH2 þ CH4 (R15) Whereas CH4 consumption was mainly due to the reaction:

CH4 þ OH ¼ CH3 þ H2O (R16) Hydrogen addition decreased the rates of both formation and consumption reactions. However, the first reactions were more influenced than the latter ones, which led to a lowering in the methane concentration upon raising the hydrogen amount.

Oxygenated products In this section special attention is paid to the potential oxygenated pollutants including formaldehyde (CH2O) and acetaldehyde (CH3CHO). The modeling results, depicted in Fig. 10a, showed that, regardless of the hydrogen (H2 and FH2) amount in the fuel mixture, the shape of the formaldehyde mole fraction profile was similar in all flames: this parameter increased at the beginning of the flame up to its peak value and then decreased in the post-flame zone. Besides, CH2O levels showed a dramatic change in peak height with doping, they decreased with increasing the percentage of hydrogen in the fuel mixture. Formaldehyde amounts were the highest in the 0% hydrogen flame (4.707  10
3) and the lowest in the flame with 60% hydrogen (3.884  10
3), which means a decrease of 17.5%. The observed lowering in CH2O mole fraction was the result of the chemical and dilution effect combination (Fig. 10b). It is noteworthy that the contribution of the thermal effect of hydrogen addition was more important than that of its chemical effect. On the other hand, an increment in the fictitious hydrogen (FH2: thermal and dilution effect) induced a shift in the formaldehyde peak maxima toward higher distances above the burner, whereas the opposite trend was observed with a rise in the hydrogen (H2: chemical effect) level in the fuel mixture (Fig. 10c). These findings suggest that

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

0.075

12

CO2 mole fraction

0.060 0% 30% 30%-F 60% 60%-F

0.045

8

0.030 4

Decrease in CO2 mole fraction (%)

16

Combined effect Thermal effect Chemical effect

0.015

30% 0.000

0.0

0.5

1.0

1.5

2.0

2.5

Height above the burner (cm)

60%

0

3.0

H2 or FH2 (%)

Fig. 8 e Effect of hydrogen addition on CO2 mole fractions.

Fig. 9 e Effect of hydrogen addition on CH4 mole fractions.

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Fig. 10 e Effect of hydrogen addition on CH2O mole fractions.

dilution effect induced a slowing in the oxidation process whereas chemical effect led to its promotion. The higher the hydrogen (H2 or FH2) level the higher is the magnitude of the phenomenon. Pathway analysis revealed that, regardless of the hydrogen amount in the fuel mixture, the predominant channels for formaldehyde formation were: CH3OCH2 ¼ CH3 þ CH2O (R17)

CH2O þ H(þM) ¼ CH3O(þM) (Rev R18)

CH3 þ O ¼ CH2O þ H (R19) Contribution of each reaction to the CH2O production was dependent on the H2 concentration. In the case of the neat flame, contributions were 63.2%, 11.6% and 25.2%, whereas for the 60% hydrogen blended fuel, the contributions were 78.5%, 10.75% and 10.75% for R17, Rev R18 and R19, respectively. This means that the monomolecular decomposition of the methoxymethyl radical (CH3OCH2) yielding methyl radical and formaldehyde was the most important route for formaldehyde formation in the two cases. On the other hand, the modeling results indicated that the majority of formaldehyde consumption occurred through hydrogen atoms and hydroxyl radical attack, leading to HCO, H2 and H2O: CH2O þ H ¼ HCO þ H2 (R20)

CH2O þ OH ¼ HCO þ H2O (R21) Contribution of these two reactions was independent on the hydrogen level. In addition, it was found that hydrogen addition induced a decrease in both formaldehyde formation and consumption rates. The effect being more pronounced for the first ones, resulting in a net lowering in CH2O concentration upon increasing the hydrogen level in the fuel mixture. The dependence of acetaldehyde (CH3CHO) mole fractions on hydrogen (H2 and FH2) proportion in the fuel mixture is depicted in Fig. 11a. It can be seen that regardless of the hydrogen percentage, the concentration of acetaldehyde increased to reach a maximum and then decreased with a further rise in the H2 (FH2) amount. Besides, the mole fraction of CH3CHO decreased upon increasing the hydrogen level in the mixture. Acetaldehyde amounts were the highest in the 0% H2 flame (6.41  10
4) and the lowest in the 60% H2 flame (5.56  10
4), which means a decrease of 13.3%. Flux analysis results indicated that, whatever the hydrogen amount in the fuel mixture, acetaldehyde was exclusively produced via the recombination reaction of methyl and formyl radicals: CH3 þ HCO ¼ CH3CHO (R22) and consumed via its reactions with H atoms and OH radical leading to CH3, CO, H2, CH2CHO and H2O: CH3CHO þ H ¼ CH3 þ CO þ H2 (R23)

CH3CHO þ H ¼ CH2CHO þ H2 (R24)

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

13

Fig. 11 e Effect of hydrogen addition on CH3CHO mole fractions.

CH3CHO þ OH ¼ CH3 þ CO þ H2O (R25) The rate of the formation reaction (R22) was found to be reduced by hydrogen addition, whereas rates of consumption reactions were found to be independent on the hydrogen level. This explains the observed decrease in the acetaldehyde mole fraction upon raising the hydrogen amount in the fuel mixture. A more detailed inspection of the effect of hydrogen addition indicated that the H2 chemical effect induced a very little change in the acetaldehyde amounts, whereas the contribution of the hydrogen thermal (dilution) effect was very important. The higher the value of the FH2 concentration in the fuel mixture the lower the value of the acetaldehyde peak (Fig. 11b). On the other hand, an increment in the FH2 percentage in the fuel mixture led to a shift in the peak maxima toward the post flame zone indicating a lowering in the oxidation process. Opposite phenomenon was induced by the chemical Effect of hydrogen addition (Fig. 11c) suggesting that hydrogen boosted the oxidation process. However the chemical boosting was less noticeable than the lowering caused by the dilution effect, such that the net result was in favor of lowering the oxidation process and shifting the acetaldehyde peak maxima toward the post flame zone. A close inspection of acetaldehyde formation from the carbonaceous reactants (CH3OCH3 and iC8H18), in the investigated mixture, revealed that, in the case of dimethyl ether as reactant (Fig. 6a), this species was produced via a simple sequence CH3OCH3/CH3OCH2/CH3/CH3CHO, acetaldehyde was mainly produced from methyl radical (CH3) which

was generated via the monomolecular decomposition of methoxymethyl radical (CH3OCH2) issued from the Habstraction reactions from dimethyl ether. In contrast to the simple sequence of acetaldehyde formation from DME, CH3CHO followed a complex sequence formation from isooctane (Fig. 6b). Acetaldehyde was mainly issued from formyl radical (HCO) which was produced from vinyl radical (C2H3), ethylene (C2H4) and formaldehyde (CH2O). All these species need several steps to be generated from the decomposition of iC8H18.

Conclusion In this paper, we report the effect of hydrogen addition on the oxidation properties of an equimolar dimethyl ether/iso-octane flame. The most outstanding observations are:  Regardless of the hydrogen level in the fuel mixture, the flame temperature profiles increased upon increasing the height above the burner to reach a plateau at about 0.8 cm without any further observable trend to change. Besides, hydrogen addition induced a decrease in the flame temperature. This finding was ascribed to hydrogen thermal effects rather than chemical effects.  Synergism between chemical and dilution effect of hydrogen addition was found to be the most important parameter governing the rise or the decrease in the H, OH and O mole fractions upon increasing the hydrogen level in the fuel mixture.

Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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 Under the studied conditions, the oxidation process of dimethyl ether-isooctane/hydrogen mixtures was largely determined by dimethyl ether and iso-octane oxidation rather than hydrogen. Besides, concentrations of the reactants where found to be largely influenced by the hydrogen enrichment as well as by the nature of its effect (chemical or dilution).  Whatever the hydrogen amount in the fuel mixture, the main consumption routes for dimethyl ether were the Habstraction reactions by H atoms, OH and methyl radicals, and O atoms producing methoxymethyl radical. Iso-octane oxidation process exhibited more complexity than that of dimethyl ether. H-atom abstraction accounted for most of the iC8H18 consumption, whereas the unimolecular decomposition route was found to be of minor importance.  Chemical effect induced an enhancement in both CO and CO2 mole fractions whereas dilution led to a significant lowering in the two species concentrations, with the latter effect being more pronounced such that the combined effect yielded a decrease in both CO and CO2 amounts.  Formaldehyde and acetaldehyde levels showed a dramatic change in peak height with doping; they decreased with increasing the percentage of hydrogen in the fuel mixture. Besides, dilution effect was found to be a slowing parameter in the oxidation process whereas the opposite trend was observed for hydrogen addition chemical effect.

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Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063

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Please cite this article in press as: Rezgui Y, Guemini M, Effect of hydrogen addition on equimolar dimethyl ether/iso-octane/oxygen/ argon premixed flames, International Journal of Hydrogen Energy (2017), https://doi.org/10.1016/j.ijhydene.2017.10.063