Experimental study of pollutants formation in laminar premixed flames of tetrahydrofuran family fuels

Transcription

1 Paper RK- Topic: Reaction kinetics th U. S. National Combustion Meeting rganized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 9-, Experimental study of pollutants formation in laminar premixed flames of tetrahydrofuran family fuels Luc-Sy Tran a, Pierre-Alexandre Glaude a,*, Frédérique Battin-Leclerc a a Laboratoire Réactions et Génie des Procédés (LRGP), CNRS, Université de Lorraine, Nancy, France * pierre-alexandre.glaude@univ-lorraine.fr Abstract To better understand the combustion chemistry of tetrahydrofuran (TF) and -methyltetrahydrofuran (MTF) which has been considered as promising bio-fuel, the structure of their stoichiometric (φ=.) low-pressure laminar premixed flame has been investigated. The flames have been stabilized on a burner at a pressure of 5 Torr (. kpa) using argon as dilutant ( %), with a gas velocity at the burner of 59 cm s - at 9 K. The results consist of mole fraction profiles of about species for the and species for the identified and quantified as a function of the height above the burner by probe sampling followed by online gas chromatography analyses. An analysis and comparison of the combustion of these fuels was performed regarding the mole fraction of the products. Keywords Tetrahydrofuran, -methyltetrahydrofuran, laminar premixed flame, bio-fuels, combustion chemistry, gas chromatography.

2 . Introduction At present an increasing interest is noted to shift from fossil fuels to bio-fuels. The use of bio-fuels allows a reduction of the dependence on petroleum-based fuels. Moreover burning bio-fuel should not lead to an increase of the total amount of greenhouse gases in the atmosphere. Cyclic ethers of tetrahydrofuran (TF) family, e.g. -methyltetrahydrofuran (MTF) and,5-dimethyltetrahydrofuran (DMTF) have been considered as promising bio-fuel compounds for adding to gasoline fuel. They have a high lower heating value (~ MJ/L), which is close to that of furan family fuels (~.-. MJ/L) and of gasoline (~.MJ/L), and higher than that of ethanol (~. MJ/L) (see Table ). MTF has good antiknock characteristics, and satisfactory performance when mixed in a % blend with gasoline in conventional internal combustion engine (Wallington et al., 99). The TF family fuels can be produced from non-edible biomass (Yang and Sen, ), (Lange et al., ), (Tran et al., ). Some tests for MTF as fuel on engine have been reported. Rudolph and Thomas (in 9) have comparatively analyzed pollutant emissions from a spark-ignition engine performed on mixtures of gasoline with % potential liquid fuels, including ethanol, methanol, methyl t-butyl ether (MTBE), and MTF. The result has shown that the fuel blend containing % MTF has power outputs and carbon monoxide, nitrogen oxides, and non-methane hydrocarbons emissions that most closely resemble unleaded gasoline. MTF has been approved by the USDE as a component of P-series fuels for spark-ignition engine, which were firstly developed by Dr. Stephen Paul of Princeton University and awarded Patent number 599 by the United States Patent and Trademark ffice on December, 99. The P-series fuels are blends of ethanol (5- % by vol.), MTF (-5 %), and pentanes plus (C 5; 5- %), with butane (- %) added to blends that would be used in severe cold-weather conditions to meet engine cold start requirements (Paul, 99), (Paul, 99), (DE, 999). Furthermore, TF, MTF, DMTF and other saturated cyclic ethers have been also identified among the emissions produced during the combustion and auto-ignition of alkanes and alkenes by isomerization of alkylhydroperoxy radicals (Leppard, 9), (Leppard, 99), (erbinet et al., a), (erbinet et al., b). The subsequent reactions of these cyclic ethers can then influence the overall chemical kinetics mechanisms of alkanes and alkenes combustion. Table : Properties of TF family and other selected fuels (Thewes et al., ), (Tran et al., ), (Simmie, ). Fuels Chemical Formula Density Boiling Point LV a AIT a RN/MN a CN a (kg/l) (K) (MJ/L) (MJ/Kg) (K) TF C MTF C / ~5 DMTF C /-- -- Ethanol C /99 5- n-butanol C / -- MF b C /. -- DMF b C /-- -- Diesel c C -C 5 C. -.. ~ Gasoline c C -C C. -.. ~ 95/5 -- a LV: Lower eating Value (LCV- Lower Calorific Value); RN: Research ctane Number; MN: Motor ctane Number; CN: Cetane Number; AIT: Autoignition Temperature. b MF: -methylfuran; DMF:,5-dimethylfuran c Fuel standard.

3 Therefore, the fundamental understanding of their combustion chemistry is really wishful, and need to be done before using MTF and DMTF as a fuel in practice. TF is well suited as a model fuel to study more about the combustion chemistry of saturated cyclic ethers, especially of its derivatives. Very early, the pyrolysis of TF has been studied by Klute and Walters (in 9), Mcdonald et al. (in 95), and Lifshitz et al. (in 9). The later suggested two main possible decomposition pathways of TF which give: (i) ethylene and (C ) bi-radical, and (ii) propene and formaldehyde with a rate constant of four times lower than the first one. The low-temperature (9 K) oxidation of TF has been investigated by Molera et al. (in9), in a static reactor. A motored engine study on auto-ignition chemistry of acyclic and cyclic ethers, including MTBE, ethyl t-butyl ether-etbe, methyl t-amyl ether-tame, TF, MTF, and tetrahydropyran, has been performed by Leppard (in 99). The chemical mechanisms responsible for auto-ignition of both ether classes are detailed, compared, and used to explain the differences in antiknock characteristics of the two ether classes. The author explained that, due to a dominance of very reactive alkoxy-carbonyl radicals produced by lowtemperature oxygen-addition chemistry in the cyclic ethers auto-ignition, octane number of the cyclic ethers is significantly lower than that of acyclic ethers. Later, the ignition delay times and oxidation of TF have been studied by Dagaut et al. (in 99), in a single-pulse shock tube and in a jet-stirred reactor, respectively, under a range of conditions of - kpa,.5 φ., - K. n the basis of these experimental data, a detailed reaction mechanism with species and reaction was proposed and used to describe the ignition and the oxidation of TF. This study has shown that a large amount of aldehydes (formaldehyde, acetaldehyde, and propanal) were produced during the TF oxidation. Recently, Kasper et al. (in ) investigated the structure of laminar premixed low-pressure s using photoionization (PI) and electron-ionization (EI) molecular-beam mass spectrometry (MBMS). About intermediates were measured and analyzed, but several assumptions have been given for species identification. Large uncertainties in mole fraction values were considered, especially for minor species, and species assignment at some mass (m/z) remained ambiguous and uncertain. Very recently, the kinetics and thermochemistry of TF, MTF and DMTF have been theoretically investigated by Simmie (in ). From bond dissociation energies (BDE) calculated in this study and their previous paper (Simmie and Curran, 9), we present in Fig. the structure of TF, MTF, and DMTF, compared to that of furan, MF, and DMF. This figure shows that ring C- and C-C bonds of TF family fuels are much weaker than those of furan family fuels, while C- bonds (in methyl group) have an inverse trend. In MTF and DMTF molecules, C-C bond has the lowest BDE. In the saturated cyclic ethers, the remoteness from the heterocyclic atom strengthens the C- bond and the presence of methyl group slightly weakens the C- bond, while it is not significantly affected in the unsaturated cyclic ethers (TF) C C 5. C (MTF) (DMTF) C C...5. (Furan) (MF) (DMF) Figure. Structure of TF, MTF, and DMTF, compared with furan, MF, and DMF. Italic number near the atom: atom label; bold numbers: bond energy (in kcal mol - ) (Simmie and Curran, 9), (Simmie, ) C 5

4 From the above bibliography, we find that information about the combustion properties of TF is very little in flame condition, and there is no report on MTF and DMTF combustion in laminar premixed low-pressure flame as well as in other fundamental devices (excluding engine configuration), which provide a stringent test for kinetic reaction models. In addition, to our knowledge, an experimental analysis of the two fuels TF and MTF under the same laminar premixed flame conditions, which allows to compare between them, has not been published prior to the present study. In the present work, as part of a continuing effort to enrich experimental data and to improve the knowledge on the combustion chemistry of TF family fuels, we report experimental data of low-pressure premixed TF and MTF flames with Ar dilution of %, pressure of 5 Torr (. kpa), and an equivalence ratio φ=., using on-line gas chromatography (GC) analyses.. Methods The experimental setup has been developed in the LRGP to study stable species profiles in a laminar premixed flat flame at low-pressure and has been described previously (Pousse et al., 9). Briefly, all flames were stabilized on a McKenna burner (diameter mm, water-cooled) housed in a vacuum chamber which is maintained at 5 Torr (. kpa). The burner is cooled with water at a constant temperature of K. xygen (99.5% pure) and argon (99.995% pure) were provided by Messer. Liquid fuels: TF (>99. % pure) and MTF ( >99. % pure), were supplied by VWR and Sigma-Aldrich, respectively. Liquid fuel was contained in a metallic vessel pressurized with argon. After each load of liquid fuel, argon bubbling and vacuum pumping were performed in order to remove traces of dissolved oxygen. Liquid fuel was mixed with argon and then evaporated by passing through a CEM (Controlled Evaporator and Mixer). The temperature of this CEM was set at K. Liquid and gas flow rates were measured by using mass flow controllers provided by BronKhorst, with a mass flow accuracy of ±.5%. Analyses were made by GC with a heated (at K) online connection to a quartz probe. The quartz probe had an upper diameter of mm and was tipped by a small cone with a µm diameter orifice at the tip and an angle to the vertical of. Three types of columns were used: Carbosphere, P-Molsieve, and P-Plot Q, and two types of detectors: flame ionization detection (FID) coupled with a methanizer and thermal conductivity detection (TCD). The Carbosphere column with argon as carrier gas was used to analyze and by TCD. The P-Molsieve column with helium as carrier gas was used to analyze C and C by FID and Ar by TCD. The P-Plot Q column with helium as carrier gas was used to analyze all hydrocarbon species from C and oxygenated species by FID. Additionally, this column was used also to analyze by TCD. In usual gas chromatography, C and C can only be detected by TCD, and formaldehyde cannot be measured by FID. ere, C, C, as well as formaldehyde were passed through the methanizer, were converted to methane, and could then be detected by FID which is more sensitive (by a factor of ) than TCD. Stable species were identified by the determination of their individual retention times and by mass spectrometry (GC/MS). Calibrations were made directly using cold-gas mixtures. The calibration factors were estimated using the effective carbon number (ECN) method for species for which a direct calibration procedure was not possible. The calculated uncertainties of the mole fraction measurements of the quantified species were ~5% for the major compounds and ~% for minor products (< ppm). The FID detection threshold was about.5 ppm, while the TCD

5 detection limit was about 5 ppm for, and. Note that in the present experiments, furan and acrolein peaks were not separated by GC. owever their individual mole fraction values were deduced using additional aid of their signals ratio in GC-MS. This process could give an uncertainty up to % in their absolute mole fraction values. The uncertainty for these two species is, however, identical in all measured flames and a relative comparison of trends between the flames can thus be performed with a significantly higher precision. Flames of TF and MTF were investigated under the same conditions, pressure of 5 Torr (. kpa) with a dilution of %, a gas velocity of 9 cm s - at K, an equivalence ratio φ=, a C/ and C/ ratios of. and.5, respectively. The initial operating conditions of these flames are presented in Table. Table. Flame conditions Flame name φ* *Dilution=Ar/(Ar+ ); φ-equivalence ratio.. Results and Discussion Gas flow (NL/min) Fuel Ar The carbon (C), hydrogen (), and oxygen () balances were checked in all two flames. The quantified mole fraction of argon allowed taking into account the change in the total mole number along the flame profiles. The difference between inlet and outlet is ~- % for C, ~5-9 % for and. About and species were identified and quantified in the TF and s, respectively. owever, only some selected important species will be presented and discussed in the following paragraphs. Figures - present the mole fraction profiles of chemical species (major and intermediate) as a function of distance above the burner h. The reaction zone peaks at ~- mm above the burner. Figure shows the major species profiles, including reactants (TF, MTF and ), diluent (Ar), and main products (C, C,, and ) in both TF and s. This figure shows that TF is completely consumed at height. mm, but at height.5 mm for MTF. A significant mole fraction of (~ - ) remains in the post flame region. The main final products are, to a large extent, C and. The profiles of C display a marked maximum at height.5 mm (~9.5 - ) in the and at height. mm (. - ) in the. It can be seen that there is also a remaining mole fraction of C (~5 - ) and (~-.5 - ) in the post flame region. The maximum mole fraction of the main products is quite comparable between the TF and s. This observation is logical with the similar C/ and C/ ratios in both flames conditions (see Table ). The major as well as intermediate species profiles show that the front is closer to the burner compared to the, reflecting a higher adiabatic burning velocities of TF (~ cm s - at 9 K, φ=., compared to ~ cm s - for MTF (Monge et al., )). C/ C/ Pressure (Torr) Dilution* (%) Flow velocity at T=9 K (cm s - )

6 5x - Fuel x - 9x - Ar x - 5 x - C x - C Figure. Major species Figure displays the mole fraction profiles of C -C hydrocarbon intermediates, including methane (C ), acetylene (C ), ethylene (C ), and ethane (C ). In the, the maximum mole fractions of these C intermediates are quite comparable to those in the. Ethylene is the most abundant one of all intermediates in both flames, with maximum mole fractions of ~ - and ~ - in the TF and s, respectively. Note that for unsaturated cyclic ethers such as furan family fuels (Tian et al., ), acetylene is the most abundant one. As discussed by Kasper et al. () and very recently by Simmie (), ethylene can be directly formed from fuels (TF and MTF) through -abstractions followed by β-scission steps (see reaction pathways rp, rpa, rpb, and rpc). -abstractions C C C C + C (rp) (TF) C

7 -abstractions C C C C C C C C + C (rpa) C C (MTF) -abstractions C C C + C (C -) C C C + C C (rpb) C C C C + C C (rpc) x - Figure. C -C hydrocarbon intermediates Under the same conditions, the ethylene and acetylene mole fractions reach higher levels in the TF and MTF flames than those in the ethanol flame reported by Tran et al. () (by a factor of ~.). In the published by Kasper et al. () (φ=., 5 Torr, and dilution ~ %), the mole fractions of ethylene, acetylene, and methane are ~- times higher than those in the present. Figure presents the profiles of C hydrocarbon intermediates, including propyne (pc ), allene (ac ), propene (C ), and propane (C ). Unsaturated C species (ac, pc, and C ) were detected with a much higher mole fraction in the than those in the, but saturated C (C ) has an inverse trend, its mole fraction is higher in the. Among these C intermediates, C is the most abundant one, with maximum mole fractions of - (at h=. mm) and 5 - (at h=. mm) in the TF and the s, respectively. Kasper et al. () and Dagaut et al. (99) found also this trend in their studies. owever, in the stoichiometric TF flame of Kasper et al. (), the mole fraction of all these C intermediates were measured with higher absolute values than those in the present (by a factor of -.5 for C and C, and of 9- for ac and pc ). This too large difference for ac and pc could be caused by the low signal-to-noise ratios of the PI-MBMS or by the difference of dilution ratio. C x - C : C :.x C

8 .x Propyne: Allene: C x C x - C Figure. C hydrocarbon intermediates As discussed by Simmie (), propene can be yielded by a combination of an -atom and an allyl radical (C 5 -Y) which can be produced in a few -abstraction and β-scission steps from TF and MTF (see reaction pathways rp and rpa). In addition, propene can be formed directly from MTF through reaction pathway (rpb) which should be a more important source of this C alkene. Probably due to this reason, C mole fraction reaches a higher level in the than that in the (Fig. ). (TF) -abstractions C C C C + C (C 5 -Y) C C (rp) -abstractions C C C C C C C C C + C (C 5 -Y) C (rpa) (MTF) -abstractions C C C C C C C + C (rpb) C Figures 5- presents the C -C hydrocarbon intermediates, including three isomers of C -,-butadiene (,-C ),,-butadiene (,-C ), and -butyne (-C ), three isomers of C - -butene (-C ), -butene (-C ) and i-butene (i-c ), three isomers of C 5 - -pentene (-C 5 ), -pentene (-C 5 ), and -methyl--butene (Fig. 5), and two cyclic species -,-cyclopentadiene (,-C 5 ) and benzene (C ) (Fig. ). Among C isomers, the mole fraction of,-butadiene is the largest one with maximum value of ~5. - in the, and with a larger value (~. - ) in the. Its mole fraction is much larger than those of,-butadiene and -butyne. For C isomers, -butene is the most abundant one (contribution of ~95% to the three C isomers), with a

9 s maximum mole fraction of ~ - in the being times lower than that in the (~. - ). owever, the mole fraction of butane (n-c, not shown in Fig. 5) reaches a higher level in the than that in the, with mole fractions of.5 - and. -, respectively. ther C (not shown in Fig. 5), e.g. diacetylene (C ) and iso-butane (ic ) were detected with a mole fraction lower than ppm in both flames. Among all these C species, -butene is the most abundant one followed by butane in the, but by,-butadiene in the. Kasper et al. () have detected butane, -butene, -butene, and,-butadiene as well in their TF flames using photoionization MBMS. -Butene,,-butadiene and diacetylene were also identified in the TF thermal decomposition performed by Lifshitz et al. (9). nly -butene has been identified and quantified in the TF oxidation study of Dagaut et al. (99), using GC analysis. In the, -butene can be formed by a combination of methyl radical (C ) and allyl radical (C 5 -Y) which can be produced through a few -abstraction and β-scission steps from TF as shown in reaction pathway rp (Simmie, ). In the case of the, a similar pattern of -butene formation can be considered (via the formation of C 5 -Y in reaction pathway rpa), but other sources of -butene can be also considered: combinations of a -atom with a but--en--yl radical (C -) or with a resonance stabilized but--en--yl radical (C -Y). These two C - and C -Y radicals can be yielded directly from the MTF decompositions (see rpb and rp5, respectively) (Simmie, ). C C C -abstractions C C (MTF) C C C C+ C (C -Y) (rp5) x -,-C : (x),-c :(x5) -C :(x) C x - -Butene: -Butene:(x) i-butene:(x5) C.x - Figure 5. C -C 5 hydrocarbon intermediates..... C 5 -Pentene: -Pentene: -Methyl- --butene: For the three C 5 isomers, the contributions of -methyl--butene, -pentene, and -pentene are ~5%, ~%, and ~%, respectively, in the, and are %, %, and %, respectively, in the. We can see the same isomers identification, but their contributions and mole fraction values are different between the TF and s. In the, -methyl--butene is the most abundant species (mole fraction of. - ) in the three C 5 isomers, while in the, the mole fractions of -methyl--butene (~. - ) and -pentene (. - ) are quite comparable. In the of Kasper et al. (), -pentene was also detected, but -pentene has been omitted and -methyl--butene was not seen. These C 5 species have not been presented in the TF oxidation study of Dagaut et al. (99). 9

10 x - 9,-Cyclopentadiene 5x - Benzene Figure. Cyclic hydrocarbon intermediates The cyclic hydrocarbon intermediates (Fig. ) were detected with small mole fractions, only ppm for,- cyclopentadiene (,-C 5 ) in the,.5 and. ppm for benzene (C ) in the TF and s, respectively. Several oxygenated intermediates were detected in both TF and s, feature of the combustion of oxygenated fuels. nly some important selected species are presented in the present paper as seen in Figs.-. Figure displays the mole fraction profiles of C -C oxygenated intermediates, including formaldehyde (C), two C isomers - acetaldehyde (C C) and ethylene oxide (cy-c ), ketene (C ), and dimethyl ether (C ; DME). Formaldehyde, toxic and cancerogenic, is measured with the largest mole fraction and quite comparable between the TF and s (maximum of ~.9 - and ~. -, respectively). Acetaldehyde is also found in large amounts in the with a maximum mole fraction of ~. -, but with a much lower mole fraction (~. - ) in the. Under the similar conditions, the ethanol flame reported by Tran et al. () emitted a much larger mole fraction of acetaldehyde (5.5 - at φ=.) compared to the present flames, while the mole fraction of formaldehyde in the ethanol flame (. - at φ=.) is quite comparable to that in both TF and s. Note that acetaldehyde and formaldehyde mole fractions reach higher levels in the stoichiometric reported by Kasper et al. () than those in the present (by a factor of ). owever, the ratio of formaldehyde to acetaldehyde ( ) is similar for the two studies at φ=. x - Formaldehyde.x C Acetaldehyde: Ethylene xide: (x5).5x Dimethyl ether: Ketene:. Figure. C -C oxygenated intermediates.

11 of other C species (Fig. ) - ethylene oxide, ketene, and dimethyl ether- reach lower levels (less than 5 ppm). Since ethylene oxide and ketene are very reactive, there is possibly a larger uncertainty in their mole fractions. Note that ethylene oxide and dimethyl ether were not found in both earlier studies of Kasper et al. () and Dagaut et al. (99). In the study of Dagaut et al. (99), only three oxygenated intermediates (formaldehyde, acetaldehyde, and propanal) were measured; others such as dihydrofurans were detected at trace levels. Simmie () has shown that formaldehyde can be produced directly from the TF and MTF decompositions via -abstractions followed by few β-scission reactions as seen in (rp) for TF, and in (rpb), (rp5), and (rp) for MTF. owever, formaldehyde formation can also be usually dominated by other different reactions in the flames. In the TF and s, acetaldehyde can be formed via the formation of formylmethyl radical (- oxomethyl, C C ) through the reaction pathways (rp) and (rpb), followed by a termination reaction. owever, in the case of MTF, acetaldehyde can be formed directly in the fuel decomposition via other route (see rpa). C (MTF) -abstractions C C C C C+ C (C -) (rp) ther aldehydes, e.g. propanal (C 5 C), acrolein (C C; Fig. ), -butenal (C 5 C; Fig. 9) and cyclopropanecarboxaldehyde (cy-c ; Fig. ) were also detected in the two TF and s. The structure of some C -C oxygenated intermediates is given in Table. The mole fraction of propanal (~- -5 ) and acrolein (~- -5 ) reach higher levels than those of -butenal and cyclopropanecarboxaldehyde, and are quite comparable between the TF and s. The mole fraction of - butenal (Fig. 9) in the (~. -5 ) are much lower than that in the (~.9-5 ), while cyclopropanecarboxaldehyde (Fig.) has an inverse trend, is much higher in the (5. -5 compared to ~. -5 in the ). In comparison with the studies of Kasper et al. () and Dagaut et al. (99), the acetaldehyde/propanal ratio - approximately ~-.5 - is quite comparable at φ=, despite the individual propanal and acetaldehyde mole fractions are different between the three studies. -Butenal was also identified in the reported by Kasper et al. (). The presence of cyclopropanecarboxaldehyde, as mentioned by Kasper et al. () can neither be confirmed nor excluded Table. Formulae of some C -C oxygenated intermediates Species Formula Propanal C C 5 C Acrolein C C C Acetone -Butenal Cyclopropanecarboxaldehyde Furan,-Dihydrofuran,5-Dihydrofuran C C C CC C C 5 C cy-c C,-DF; C,5-DF; C

12 based on the flame-sampled PIE curves because a complicated convolution of signals at m/z =. owever, this species is well identified and quantified in the present TF and s (Fig. ). The possible formation of this species and of -butenal from,-dihydrofuran has been reported by Lifshitz et al. (99) and (Dubnikova and Lifshitz, ), and will be presented below. The same picture with acetaldehyde and -butenal, acetone was measured with a much larger mole fraction in the than in the (Fig. ); e.g. MTF: X(acetone)= -5 and TF: X(acetone)=. -5, reflecting the effect of methyl group in MTF molecule on its combustion chemistry. 5x Propanal x -5 Acetone x -5 Acrolein Figure. C oxygenated intermediates Several cyclic ethers, such as,-dihydrofuran (,-DF),,5-dihydrofuran (,5-DF), furan (Fig. 9), TF (intermediate in the ) and MTF (intermediate in the ; Fig. ), were identified and quantified in the TF and s. Figure 9 shows that,-dihydrofuran,,5-dihydrofuran, and furan were found in much higher mole fractions in the than in the. The,-dihydrofuran is 5- times more abundant than,5-dihydrofuran, and its mole fraction reaches maximum. -5 at h=. mm in the and. -5 at h=. mm in the. These two dihydrofurans can be formed directly from fuel in the via the formation of cyclic radicals C followed by an elimination of a -atom (Simmie, ), (Kasper et al., ). As discussed by Lifshitz et al. (99) and Dubnikova and Lifshitz (), the isomerization of,-dihydrofuran can produce -butenal and cyclopropanecarboxaldehyde which subsequently isomerizes to -butenal, as seen in reaction pathway rp. x -5,-DF:,5-DF: 5x -5 Furan x -5 -Butenal Figure 9. C oxygenated intermediates

13 The peak mole fraction of furan (Fig. 9) is. -5 at h=. mm in the, and is. -5 at h=. mm in the. These peak position and maximum mole fraction value indicate a possible formation of furan from,-dihydrofuran, especially evident in the. s of TF and MTF (Fig. ) formed in the MTF and s, respectively, are quite low (only ~. -5 for TF and. -5 for MTF). Addition of a methyl radical to the C-position of the TF-ring seems the most likely process for the MTF formation. Kasper et al. () have also detected MTF in their s. x -5 5 (cyclopropanecarboxaldehyde) (-butenal) Cyclopropanecarboxaldehyde: TF: MTF: Figure. Cyclopropanecarboxaldehyde, TF and MTF intermediates (,-dihydrofuran) C (rp) Note that, a number of other oxygenated species were also detected in the, such as -butanone (mole fraction of ~ -5 ), -pentenal (~. -5 ), methyl furan (~.5-5 ), -butenone (~. -5 ), ethyl acetone (~. -5 ), pentanal (~.9-5 ), butanal (~. -5 ), and methyl propanal (~. -5 ), while in the, these species were either not detected or detected at trace levels only.. Conclusions This paper presents new experimental results about the chemical structure of low-pressure laminar premixed flame of TF and MTF, under the same conditions: φ=., Ar dilution of %, pressure of 5 Torr (. kpa), C/=., and C/=.5. n-line GC and GC-MS were used. About and stable species were identified and quantified as a function of burner distance in the TF and s, respectively. A number of selected important species have been presented, discussed and compared to those in previous studies reported in the literature. Among all intermediate species measured for the two tetrahydrofuranic fuels flames, the production of ethylene is the largest and comparable between the two fuels. Note that for unsaturated cyclic ethers as furan family fuels acetylene is the most abundant one as reported in the literature. Due to the different structure of these molecules, the combustion chemistry of TF and MTF present some differences. Indeed, the profile positions of the major and intermediate species show that the front is closer to the burner compared to the, reflecting a higher laminar flame speed of TF. The mole fractions of C isomers, C, C isomers, C 5 isomers,,-c,,-c 5, C 5 isomers, C, acetaldehyde, acetone, and - butenal reach higher levels in the than those in the, while the mole fraction of C, oxide

14 ethylene, ketene, DME,,- and,5-dihydrofuran, furan, and cyclopropanecarboxaldehyde have inverse trends. ther species were found in comparable mole fractions between the TF and s. The comparison of the present results and that obtained for the combustion of ethanol previously studied under similar conditions shows that the combustion of tetrahydrofuranic fuels emits a much lower mole fraction of acetaldehyde, compared to the ethanol combustion, while formaldehyde emission is quite comparable in both fuels classes. The obtained experimental results provide database for the analysis of intermediates in flames, which will be used to develop a new mechanism for tetrahydrofuranic fuels combustion. Despite the structure of laminar premixed low-pressure s investigated using MBMS has been reported in the literature, it should be noted that the information from another independent quantitative measurement such as GC was very valuable to confirm species identifications and to unambiguously identify the respective isomers. To our knowledge, the present study is the first work on the. This is also the first experimental analysis of the two tetrahydrofuranic fuels under the same conditions. Acknowledgements This work was funded by the European Commission through the Clean ICE Advanced Research Grant of the European Research Council. References Dagaut, P., M. McGuinness, J.M. Simmie, and M. Cathonnet, 99, The Ignition and xidation of Tetrahydrofuran: Experiments and Kinetic Modeling: Combustion Science and Technology, v. 5, no. -, p. 9. DE, 999, P-series Fuels Part II: Federal Register, v., no. 9, p. 9. Dubnikova, F., and A. Lifshitz,, Isomerization of,-dihydrofuran and 5-methyl-,-dihydrofuran: Quantum chemical and kinetics calculations: Journal of Physical Chemistry A, v., no., p.. erbinet,., Frederique Battin-Leclerc, et al., a, Detailed product analysis during the low temperature oxidation of n-butane: Physical Chemistry Chemical Physics, v., no., p. 9. erbinet,., S. Bax, P. A. Glaude, V. Carre, and Frederique Battin-Leclerc, b, Mass spectra of cyclic ethers formed in the low-temperature oxidation of a series of n-alkanes: Fuel, v. 9, no., p Kasper, T., A. Lucassen, A. W. Jasper, W. Li, P. R. Westmoreland, K. Kohse-öinghaus, B. Yang, J. Wang, T. A. Cool, and N. ansen,, Identification of Tetrahydrofuran Reaction Pathways in Premixed Flames: Zeitschrift für Physikalische Chemie, v. 5, no. -, p.. Klute, C.., and W. D. Walters, 9, The Thermal Decomposition of Tetrahydrofuran: Journal of the American Chemical Society, v., no., p Lange, J.-P., E. van der eide, J. van Buijtenen, and R. Price,, Furfural A Promising Platform for Lignocellulosic Biofuels: ChemSusChem, v. 5, no., p. 5. Leppard, W. R., 9, The autoignition chemistry of n-butane: an experimental study: SAE transactions, v. 9, no., p

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