argon flame

Fuel 89 (2010) 2633–2639

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Experimental and modeling study of a lean premixed iso-butene/hydrogen/oxygen/argon flame Véronique Dias a,*, Jacques Vandooren b a b

Institute of Mechanics, Materials and Civil engineering, Catholic University of Louvain, Place du Levant, 2, 1348 Louvain-la-Neuve, Belgium Institute of Condensed Matter and Nanosciences, Catholic University of Louvain, Place Louis Pasteur, 1, 1348 Louvain-la-Neuve, Belgium

a r t i c l e

i n f o

Article history: Received 27 April 2009 Received in revised form 22 April 2010 Accepted 4 May 2010 Available online 15 May 2010 Keywords: Iso-butene Flame structure Kinetic mechanism

a b s t r a c t The experimental structure of a lean iso-butene/hydrogen/oxygen/argon flame (2.7% iC4H8, 4.5% H2, 83.0% O2, 9.8% Ar, / = 0.225) has been determined by molecular beam mass spectrometry at low pressure (40 mbar). The detected species throughout the flame thickness were: H2, CH3, O, OH, H2O, C2H2, CO, C2H4, CH2O, O2, HO2, Ar, C3H4, C3H6, CO2, C2H4O, C4H6, iC4H8, C3H6O, C4H6O and C4H8O. An original model, validated against premixed rich C2H4, has been extended by building a sub-mechanism taking into account the formation and the consumption of species involved in iso-butene combustion. This mechanism contains 520 reactions and 99 chemical species. A good agreement appears between calculated mole fraction profiles predicted by this mechanism, compared to experimental results. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Iso-butene is an important intermediate species in pyrolysis and oxidation of iso-octane, also MTBE and ETBE, which are used worldwide as octane enhancer. To be able to model correctly the combustion of these fuels in engines, it is necessary to understand more precisely the oxidation mechanism of iso-butene. In 1975, Bradley and West [1] studied the thermal decomposition of iso-butene (mixtures of 0.1% and 0.5% in argon) in a single-pulse shock tube (1055–1325 K). Iso-butene oxidation has been investigated by Brezinsky and Dryer [2] in a turbulent flow reactor, at atmospheric pressure. Curran et al. [3] studied the ignition of iso-butene/oxygen/argon mixtures at equivalence ratios from 0.1 to 4 in a shock tube by measuring and modeling ignition delays behind reflected shock waves. In 1998, Dagaut and Cathonnet [4] investigated experimentally and numerically iso-butene oxidation and ignition in a jet stirred reactor in the temperature and pressure ranges of 800 to 1230 K and at 1 to 10 atm, respectively. Several iso-butene studies in shock tube were performed in the last decade. Particularly, Bauge et al. [5] measured ignition delays of iso-butene-oxygen-argon mixtures (/ = 1–3) behind reflected shock waves at temperatures from 1230 to 1930 K and pressures from 9.5 to 10.5 atm. Santhanam et al. [6] observed dissociation and vibrational relaxation in shock waves in 2, 5, and 10% isobutene mixtures with krypton by using the laser-Schlieren technique. And very recently, Yasanuga et al. [7] studied pyrolysis * Corresponding author. Tel.: +32 10 47 22 40; fax: +32 10 45 26 92. E-mail addresses: [email protected] (V. Dias), Jacques.vandooren@ uclouvain.be (J. Vandooren). 0016-2361/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2010.05.001

and oxidation of iso-butene behind shock waves in the temperature range of 1000–1800 K and at pressure between 1.0 and 2.7 atm. A reaction mechanism was elaborated and validated against experimental results. None of previous studies allowed studying the combustion of iso-butene at low pressure and thus no reaction mechanism could be validated in these conditions of pressure. Moreover, during a previous study of the n-butane and isobutane flames [38], many differences have been noticed. To understand the kinetic of branched alkanes, the investigation of the iso-butene oxidation in flames will help to know which products are formed during the front. To the best of our knowledge, laminar premixed flat iso-butene flames have not yet been investigated in terms of their structure. So, the aim of this work is to measure mole fraction profiles in a flat lean iso-butene flame (/ = 0.225) at low pressure, and to build a reaction sub-mechanism which will be incorporated into an already validated model for rich ethylene/dimethoxymethane flames [8]. 2. Experimental A lean premixed iso-butene/hydrogen/oxygen/argon flat flame (2.7% iC4H8, 4.5% H2, 83.0% O2, 9.8% Ar), at equivalence ratio (/) of 0.225, has been stabilized on a Spalding-Botha type burner. The presence of hydrogen allows the stabilization of the flame and facilitates its combustion at low pressure. The iso-butene has a too weak flow rate at the exit of gas feeding to insure a stable flow rate at the burner. The presence of hydrogen allows the stability of this poor studied flame. Also, a mixture with H2 enhances reactions radicals with iso-butene.

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The setup consists of a combustion chamber where a flat flame is stabilized at low pressure (40 mbar) on a movable flat burner of 8 cm in diameter. Facing the burner surface, the top of a conical quartz probe with a 45° angle within 2 cm and with small hole of 0.1 mm allows sampling to be performed in the flame. Behind this probe, three differentially pumped chambers lead to the formation of a molecular beam that is directed to the electronic ionization source of a quadrupole mass spectrometer (Balzers QMG 511). The axis of the mass spectrometer is perpendicular to the molecular beam one. Such an arrangement allows the initial composition of the sampling to be ‘‘frozen’’ at the point where the mass spectrometer is reached and therefore is stable for reactive species to be detected and monitored. Moreover, the molecular beam is chopped at 30 Hz, allowing a phase detection of the signal coming from the electron multiplier which strongly enhances the signalto-noise ratio. The complete experimental setup was described elsewhere [9]. The initial flow velocity is 53.4 cm/s and the total mass flux is 6.44 l/min. For every species, interferences from fragmentation during the electron impact or from overlapping of species with similar mass have been kept low or taken into account. The conversion of signal intensities to mole fractions has been performed by using a calibrated mixture for stable compounds and by estimating ionization cross sections for carbon-containing radicals [9]. For stable species such as Ar, CO, CO2, O2, H2O, the ratios Si/SAr = Ii.XAr/IAr.Xi can be determined using the sensitivity ratios derived from the calibration gas mixture. In post flame zone, the system H2/O2 is in partial equilibrium. Three equilibrium equations can be written to determine the concentration of H, O, OH, if the concentration of H2, O2, and H2O and temperature are known. The final two equations needed to determine the compositions are derivated from the sum of mole fractions equal to 1.0 and the equation of conservation of the total number of atoms. For the stable species that are not in the burned gas, such as C2H2, C2H4, C3H6 and C4H6, the calibration must be done in the reaction zone. The concentration of species is calibrated by comparison with the fresh gas mixture. The calibration of species present in low concentration, such as CH3, CH2O, CH3CHO, C3H4, C3H6O, C4H6O and C4H8O, has been provided by using the ionization cross section Qi. To a first approximation Qi may be determined from additivity of the atomic ionization cross sections. The mole fractions may then be determined by the ratio of the ionization cross sections so determined for species with similar molecular weight as follows: Xi = Qj.Ii.Xj/Qi.Ij. This method takes into account ionization potential of the chemical species and this method is reliable where from the experimental errors of the orders of 10–20%. However, because of some measurements have to be performed close to the ionization potential to avoid interferences, error bars could be larger: for C4H6O and C4H8O species. The detected species throughout the flame thickness were: H2, CH3, O, OH, H2O, C2H2 (acetylene), CO, C2H4 (ethylene), CH2O (formaldehyde), O2, HO2, Ar, C3H6 (propene), CO2, CH3CHO (acetaldehyde), C4H6 (1,3-butadiene), iC4H8 (iso-butene), C3H6O (acetone), C4H6O (1 propen-1-one, 2-methyl) and C4H8O (prop1-en-1-ol, 2-methyl and propanal, 2-methyl). The final flame temperature, measured using Pt/PtRh10% coated thermocouples 0.1 mm in diameter and located close to the sampling cone tip, was 1720 K (Fig. 1). The temperature profile has been corrected for radiation losses by the electrical compensation method [10]. Standard deviation on experiment measurements is estimated to ±50 K.

3. Modeling Previously, we have developed a reaction mechanism validated against premixed rich C2H4/O2/Ar and C2H4/Dimethoxymethane/

Fig. 1. Temperature profile in the iso-butene flame.

O2/Ar flames (/ = 2.50), which describes in detail the formation of soot precursors and more precisely the main pathways involving benzene [8]. The kinetics for H2/O2 related species is based on the mechanism of Westbrook et al. [11] and the evaluation of Baulch et al. [12]. The C1 and C2 related processes are mainly taken from Miller and Melius’ [13] mechanism, from the GRI-Mech 1.2 [14] and from Warnatz’s [15] mechanisms. The C3 and C4 sub-mechanism was taken from Davis et al. [16] which describe propene pyrolysis based on their studies of propyne (p-C3H4) and allene (a-C3H4) oxidation. Their kinetic data were obtained through an ab initio quantum mechanical and RRKM analysis. Kinetics of C5 to C10 hydrocarbons has been elaborated from several models in the literature [16–20] and by optimization of the simulated mole fraction profiles against experimental data of the C2H4 flame. Recently, the sub-mechanism for dimethoxymethane (DMM, C3H8O) has been added to the mechanism, with the oxygenated species involved in C3H8O combustion [21]. In this work, we have extended the original model by building a sub-mechanism taking into account the formation and the consumption of species involved in iso-butene combustion: iC4H8 (iso-butene), iC4H7 (2-methylallyl radical), C4H6O (1-propen-1one, 2-methyl), C4H8O (prop-1-en-1-ol, 2-methyl), mC4H8O (propanal, 2-methyl), iC4H9 (iso-butyl radical), tC4H9 (ter-butyl radical), C3H6O (acetone) and C3H5O (CH2CO(CH3) radical). The sub-mechanism for iso-butene decomposition is presented in Table 1, it contains 40 elementary reactions and 9 additional chemical species. Using kinetic data from the literature about iso-butene, we were able to build a new mechanism containing 520 elementary reactions and involving 99 chemical species. (Note: The complete mechanism is available as supplementary material). Transport properties and thermochemical quantities from the Burcat’s database [22] were used. The numerical simulation of the mechanism of the investigated one-dimensional flames has been performed by using the CosilabÓ software from SoftPredict [23]. Molecular and thermal diffusion are considered in this code. The modeling has been performed using as input parameters the initial composition of the flame, the initial total mass flux and the experimental temperature profile measured in similar conditions as the mole fraction. The cold gas velocity is calculated for 298 K.

4. Results and Discussion Figs. 2–5, 7 and 8 show experimental and simulated mole fraction profiles of chemical species detected in the lean iso-butene flame (H2, CH3, O, OH, H2O, C2H2, CO, C2H4, CH2O, O2, HO2, Ar, C3H6, CO2, CH3CHO, C4H6, iC4H8, C3H6O, C4H6O and C4H8O). (The flames of iso-butene were never studied and the comparison between the contents of the species of this flame with another

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V. Dias, J. Vandooren / Fuel 89 (2010) 2633–2639 Table 1 Elementary reactions and rate constant expressions. No

Reaction

A

n

Ea

Reference

R.481 R.482 R.483 R.484 R.485 R.486 R.487 R.488 R.489 R.490 R.491 R.492 R.493 R.494 R.495 R.496 R.497 R.498 R.499 R.500 R.501 R.502 R.503 R.504 R.505 R.506 R.507 R.508 R.509 R.510 R.511 R.512 R.513 R.514 R.515 R.516 R.517 R.518 R.519 R.520

iC4H8 + H = iC4H7 + H2 iC4H8 + H = CH3 + C2H6 iC4H8 + OH = iC4H7 + H2O iC4H8 + OH = C2H5 + CH3CHO iC4H8 + OH = iC3H7 + CH2O iC4H8 + O = C2H5 + CH3CO iC4H7 + O = C4H6O + H iC4H8 + O = iC3H7 + HCO iC4H8 + O = iC4H7 + OH iC4H8 + O = C4H8O iC4H8 + O2 = iC4H7 + HO2 iC4H8 + HO2 = C4H8O + OH iC4H8 = iC4H7 + H iC4H7 = CH3 + aC3H4 iC4H7 + H = CH4 + aC3H4 iC4H7 + pC3H4 = C6H6 + H2 + CH3 C4H8O = mC4H8O iC4H8O = iC3H7 + HCO mC4H8O = iC4H7 + HCO iC4H9 = iC4H8 + H tC4H9 = iC4H8 + H iC4H9 = tC4H9 iC4H9 = C3H6 + CH3 tC4H9 = C3H6 + CH3 tC4H9 + O2 = iC4H8 + HO2 tC4H9 + HO2 = C3H6O + CH3 + OH tC4H9 + H = iC4H8 + H2 tC4H9 + O = iC4H8 + OH tC4H9 + O = C3H6O+CH3 tC4H9 + OH = iC4H8 + H2O iC3H7 + O = C3H6O + H C3H6O + H = C3H5O + H2 C3H6O + OH = C3H5O + H2O C3H6O + O = C3H5O + H2O C3H6 + OH = C3H6O + H C4H6O + O = C3H6O + CO C4H6O + OH = C3H6O + HCO C4H6O + H = C3H5 + CH2O C3H6O = CH3 + CH3CO C3H5O = CH3 + CH2CO

1.72E + 014 1.72E + 013 2.70E + 013 1.00E + 011 1.40E + 012 4.50E + 012 1.74E + 011 3.50E + 013 4.50E + 013 5.00E + 007 2.40E + 013 1.02E + 012 1.50E + 015 1.00E + 013 6.31E + 013 1.00E + 012 4.00E + 013 6.00E + 013 2.44E + 016 1.91E + 029 8.30E + 013 3.57E + 010 7.00E + 012 3.00E + 014 1.50E + 011 1.80E + 013 5.40E + 012 4.16E + 014 1.04E + 014 1.81E + 013 4.82E + 013 2.30E + 007 2.00E + 013 1.00E + 013 9.89E + 010 1.00E + 013 1.00E + 012 1.86E + 013 1.13E + 016 2.00E + 013

0 0 0 0 0 0 0.70 0 0 1.28 0 0 0 0 0 0 0 0 0 -5.246 0 0.88 0 0 0 0 0 0 0 0 0 2.00 0 0 0 0 0 0 0 0

8000 3597 3000 0 2000 0 2961 6000 4500 -1079 37,800 14,964 83,000 214,000 0 0 57,200 57,200 84,128 39,758 38,200 34,600 26,200 46,300 2000 0 0 0 0 0 0 5000 3000 6000 0 6000 1200 6400 81,700 32,000

[7] [29] [7] see text [7] [7] [30] [7] [7] [4] [7] [4] [7] [3] [7] [7] [4] [4] [4] [4] [26] [7] [28] [4] [3] [4] [4] [4] [4] [26] [34] [35] [35] [33] [37] see text see text see text [35] [36]

Rate constants in the form, ATn exp( Ea/RT) in cm, mol, cal and K units.

2.8E-01

3.0E-02

x 10

2.5E-02

2.0E-01

Mole fraction

Mole fraction

2.4E-01

1.6E-01 1.2E-01 8.0E-02

/ 10

4.0E-02

x 10

0.0E+00

2.0E-02 1.5E-02 1.0E-02 5.0E-03 0.0E+00

0

10 20 Distance from burner (mm)

30

0

5 10 15 20 25 Distance from burner (mm)

30

Fig. 2. Experimental (symbols) and simulated (lines) mole fraction profiles of main species in the iso-butene flame: iC4H8 (grey triangles), H2 (black circles), O2 (white circles), H2O (grey circles), CO2 (white triangles) and CO (grey diamonds).

Fig. 3. Experimental (symbols) and simulated (lines) mole fraction profiles of O (grey diamonds), OH (white triangles) and HO2 (black circles) radicals in the isobutene flame.

iso-butene flame cannot be made. However, we detected the same chemical species as in studies if iso-butene in shock tube [4,7]). Fig. 2 presents experimental and calculated mole fraction profiles for main species: H2, iC4H8, O2, H2O, CO and CO2. We observe an excellent agreement between experimental results and the simulation.

From the model, the main consumption of iso-butene results from the reaction with the hydroxyl radical (OH): iC4H8 + OH = iC4H7 + H2O (R.483) using the rate coefficient suggested by Yasunaga et al. [7], k483 = 2.7  1013 exp( 1510/T) [cm3 mol 1 s 1]. The iC4H7 radical reacts with a hydrogen atom to produce methane and allene (iC4H7 + H = CH4 + aC3H4 (R.495)), with

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V. Dias, J. Vandooren / Fuel 89 (2010) 2633–2639

8.E-04

1.4E-03

7.E-04

1.2E-03 1.0E-03

5.E-04

Mole fraction

Mole fraction

6.E-04

4.E-04 3.E-04

6.0E-04

2.E-04

4.0E-04

1.E-04

2.0E-04

0.E+00

0.0E+00

0

5 10 Distance from burner (mm)

15

Fig. 4. Experimental (symbols) and simulated (lines) mole fraction profiles of CH3 (black diamonds), C2H2 (white circles) and C2H4 (grey triangles) radicals in the isobutene flame.

0

4.0E-04

1.8E-03

3.5E-04

1.6E-03

3.0E-04

Mole fraction

1.2E-03 1.0E-03 8.0E-04 6.0E-04

5 10 Distance from burner (mm)

15

Fig. 7. Experimental (symbols) and simulated (lines) mole fraction profiles of CH2O (black diamonds), and CH3CHO (grey triangles) in the iso-butene flame.

2.0E-03

1.4E-03

Mole fraction

8.0E-04

2.5E-04 2.0E-04 1.5E-04 1.0E-04

4.0E-04

5.0E-05

2.0E-04

0.0E+00

0.0E+00 0

5 10 Distance from burner (mm)

15

Fig. 5. Experimental (symbols) and simulated (lines) mole fraction profiles of C3H6 (black diamonds) and C4H6 (grey triangles) radicals in the iso-butene flame.

iC4H8 iC4H7

OH

O2

H

tC4H9

H

H

iC4H9

aC3H4 OH

C3H3

C3H6

OH

O

C3H2

CH2CO H

O2

HCCO O2

C2H3O CO OH

CO2 Fig. 6. Reaction pathways of formation for important intermediate species in the lean iso-butene flame.

k495 = 6.31  1013 [cm3 mol 1 s 1] from Santhanam et al. [6]. The propargyl radical is produced from allene by the reaction R.171:

0

5 10 Distance from burner (mm)

15

Fig. 8. Experimental (symbols) and simulated (lines) mole fraction profiles of C3H6O (grey diamonds), C4H6O (black triangles) and C4H8O (white circles) in the iso-butene flame.

aC3H4 + OH = C3H3 + H2O, with k171 = 2.0  107 T2.0 exp( 503/T) [cm3 mol 1 s 1] from Miller and Melius [13]. The propargyl radical then reacts with the hydroxyl one to form C3H2 (R.158: C3H3 + OH = C3H2 + H2O [13]). In this lean flame, C3H2 radical reacting with oxygen allows the formation of HCCO and CO, by the global reaction: C3H2 + O2 => HCCO + H + CO (R.149), k149 = 2.0  1012 [cm3 mol 1 s 1] from Pauwels et al. [24]. Carbon monoxide is also produced from HCCO by the reaction HCCO + O2 => CO + CO + OH (R.35), k35 = 1.63  1012 exp( 432/T) [cm3 mol 1 s 1] from Peeters et al. [25]. Carbon monoxide is consumed by the classical pathway with the reaction CO + OH = CO2 + H (R.17), k17 = 6.32  106 T1.5 exp(+250/T) [cm3 mol 1 s 1] from Baulch et al.’s evaluation [12]. A secondary path of iC4H8 consumption comes from the reaction with the hydrogen atom: iC4H8 + H = tC4H9 (R.501), k-501 = 8.30  1013 exp( 1922/T) [cm3 mol 1 s 1] from Tsang’s evaluation [26]. Tertiobutyl radical (tC4H9) isomerizes into iC4H9 (R.502), k-502 = 3.57  1010 T0.88 exp( 17,413/T) [s 1] from Matheu et al. [27] used by Yasunuaga et al. [7]. By decomposition of this last radical, propene and methyl radical are produced: iC4H9 = C3H6 + CH3 (R.503), k503 = 7.0  1012 exp( 13,190/T) [s 1] from Warnatz’s recommendation [28]. Reactions R.501 and R.503 are different from the ones suggested by Yasunaga et al. [7] to improve the simulated profiles of species. The reaction 501 was modified to reduce the production of the acetone (C3H6O). Indeed, the reaction R.-501 of

V. Dias, J. Vandooren / Fuel 89 (2010) 2633–2639

Yasunuga et al. [7] produced a concentration of tC4H9 too important. This radical is responsible for the formation of C3H6O by the reaction: tC4H9 + HO2 = C3H6O + CH3 + OH (R.506). With the constant kinetics of Tsang [26], the concentration of tC4H9 is weaker and thus also decreases that of the C3H6O, who was overestimated by the model. The reaction R.503, iC4H9 = C3H6 + CH3, has been modified to reduce the concentration of C3H6, with the rate constant of Warnatz [18] which is a little slower than Yasunaga et al. [7]. We should underline that the main propene production comes directly from the iso-butene through the reaction R.482, iC4H8 + H = C3H6 + CH3 with the rate constant k482 = 1.72  1013 exp( 1812/T) [cm3 mol 1 s 1] from Tsang and Walker [29]. Propene consumption permits forming ketene (CH2CO) and the methyl radical: C3H6 + O => CH2CO + CH3 + H (R.189), k189 = 1.2  108 T1.65 exp( 165/T) [cm3 mol 1 s 1] from the Tsang’s evaluation [30]. By the reverse reaction, ketene produces CH3CO: CH2CO + H = CH3CO (R.-138), k-138 = 3.0  1013 exp( 4040/T) [cm3 mol 1 s 1] [31]. By the decomposition of CH3CO, methyl radical and carbon monoxide are produced: CH3CO = CH3 + CO (R.139), with k139 = 3.16  1014 exp( 6060/T) [s 1] from Lifshitz and Ben–Hamou [31]. Fig. 3 presents mole fraction profiles for O, OH and HO2 radicals. Simulations carried out with our model predict very well experimental data for OH radical concentrations and underestimate slightly O and HO2 radical concentrations. But a simulated value with an error of more or less 30% from the experimental value stays a good modeling. The radical HO2 is mainly formed by the reaction O2 + HCO = CO + HO2 (R.25) of Baulch et al. [12] and not by the reaction iC4H8 + O2. The contribution of this reaction iC4H8 + O2 = iC4H7 + HO2 (R.491) is rather weak, in spite of the rate constant (k = 2.4x1013 exp ( 19,029 / T) (cm3 mol 1 s 1) of Yasunaga et al. [7]. We can underline the presence of O and OH in the post-combustion zone, contrary to HO2 radical which is produced and consumed in the flame front. Moreover, the experimental profile of the atomic oxygen presents a formation of the atom near the burner, contrary to the modeling which predicts its production from 5 mm of the burner. According to the mechanism, this formation takes place in the same place as the production of the radical OH. This observation can be justified by an experimental effect. Indeed, the radical O is formed experimentally close to the burner by fragmentation of another compound early in the flame, or by mass spectrometer interferences of CH4. But its kinetics indicates that it has to form at the same time as the radical OH. Methyl radical (CH3), acetylene (C2H2) and ethylene (C2H4) mole fraction profiles are presented in Fig. 4. In the iso-butene flame, these three species are intermediates: there are produced and consumed in the flame front. We can see a very good agreement between simulation and experimental results. Indeed, the initial mechanism has been established and validated in rich premixed C2H4 flames [8] where ethylene was the reactant. Fig. 5 shows experimental and simulated mole fraction profiles of propene (C3H6) and 1,3-butadiene (C4H6). The simulated profile of propene agrees very well with the experimental one. For C4H6, the model underpredicts significantly the maximum concentration value. We can justify this observation by the number of isomers for C4H6 species at the m/e = 54. Indeed, only 1,3-butadiene is present in the mechanism but it is likely that others C4H6 species like 1,2butadiene, 1-butyne or 2-butyne, etc., must be included. As it is written above, the main formation of propene comes from the reaction of iso-butene with hydrogen atom (iC4H8 + H = C3H6 + CH3, R.482 [29]). Propene is the main intermediate during the combustion of iso-butene. Along the mechanism, 1,3-butadiene is produced from the decomposition reaction of the n-C4H7 radical: n-C4H7 = C4H6 + H (R.302), k302 = 2.28  1052 T 12 exp( 25,770/T) [s 1] from Wang and Frenklach [17]. C4H6 reacts with the oxygen

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atom to produce the C3H5 radical: C4H6 + O = C3H5 + HCO (R.286), k286 = 6.0  108 T1.45 exp(+433/T) [cm3 mol 1 s 1] from Adusei and Fontijn [32]. In Fig. 6, the reaction schemes for the iso-butene consumption are schematized. The pathways are determined via a reaction-flow analysis. The fatter arrows represent reaction pathways having a relative contribution more important with regard to the other ways. There are two main pathways: from iC4H7 radical and from tC4H9 radical. In this kinetic scheme, only iso-butene, propene, carbon monoxide and carbon dioxide have been measured and allow testing the reliability of the model by comparison with experimental profiles. Experimental and simulated mole fraction profiles of light oxygenated species like CH2O and CH3CHO are presented in Fig. 7. The simulation of acetaldehyde (CH3CHO) concentration is excellent compared to experimental data. For the formaldehyde (CH2O), the model overestimates by 30% the experimental maximum mole fraction value. However, the shape and the position of the calculated profile agree well with that measured. Acetaldehyde is produced from the C2H5 radical via the reaction with oxygen atom R.144: C2H5 + O = CH3CHO + H, k144 = 6.62  1013 [cm3 mol 1 s 1] from Baulch et al. [12]. Indeed, iso-butene forms directly C2H5, by iC4H8 + O = C2H5 + CH3CO (R.486), k486 = 4.50  1012 [cm3 mol 1 s 1] from Yasunaga et al. [7]. Acetaldehyde reacts mainly with the hydroxyl radical to finally produce a methyl radical, carbon monoxide and water (CH3CHO + OH = CH3 + CO + H2O, R.57 [28]). Formaldehyde is formed by the well know important reaction R.81: CH3 + O = CH2O + H [12]. Fig. 8 presents mole fraction profiles for the heaviest oxygenated species detected, which is C3H6O for acetone, C4H6O for 1-propen-1-one, 2-methyl; and C4H8O for prop-1-en-1-ol, 2-methyl (pC4H8O) or propanal, 2-methyl (mC4H8O). By mass spectrometry, we obtain experimental profiles for m/e ratio and the identification of species is performed by comparison with previous iso-butene studies presented in the literature [3,4,7]. Considering the location of the mole fraction maxima and the shape of profiles, we can suggest that these three species are oxygenated species. According to the m/e, we can identify acetone (m/e = 58), 1 propen-1-one, 2-methyl (m/e = 70), (CH3)2CCHOH and (CH3)2CHCHO (m/e = 72). For the last species, we cannot determine precisely what is the dominant chemical structure, so both compounds are considered in the mechanism. Both C4H8O simulated mole fraction profiles are summed up to be compared to the experimental one. We should underline the predominance by a factor of 14 of the (CH3)2CCHOH (pC4H8O) mole fraction compared to the (CH3)2CHCHO (mC4H8O) one. In Fig. 8, we observe a good agreement between simulated and experimental mole fraction profiles for all species. For C4H6O and C4H8O mole fraction profiles, maximum calculated values are shifted of 1.2 mm and 0.8 mm, respectively, toward burnt gases compared to measured ones. The formation and consumption for these three species can be described in detail. From iso-butene, pC4H8O is produced by the reaction R.490: iC4H8 + O = pC4H8O from Dagaut et Cathonnet [4], with a rate constant divided by two (k490 = 5.0  107 T1.28 exp(+543/T) [cm3 mol 1 s 1]). By isomerisation, (CH3)2CCHOH (pC4H8O) forms (CH3)2CHCHO (mC4H8O): pC4H8O = mC4H8O (R.497), k497 = 4.0  1013 exp( 28,790/T) [s 1] [4]. By decomposition, both C4H8O species produce iC3H7 radical: pC4H8O = iC3H7 + HCO (R.498), k498 = 6.0  1013 exp( 28,790/T) [s 1] [4]; and mC4H8O = iC3H7 + HCO (R.499), k499 = 2.44  1016 exp( 42,325/T) [s 1] [4]. As we described previously, the iC4H7 radical formation comes directly from the reaction of OH with iso-butene, R.483: iC4H8 + OH = iC4H7 + H2O [7]. And the iC4H7 radical reacts with oxygen atom to form C4H6O ((CH3)2CCO): iC4H7 + O = C4H6O + H (R.487). The

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V. Dias, J. Vandooren / Fuel 89 (2010) 2633–2639

tC4H9

H

iC4H8

O

pC 4H8O

mC4H8O

OH

iC4H7 O

Acknowledgements

C4H6O O

C3H6O

an original model [8], the complete mechanism contains 520 elementary reactions and 99 chemical species. The comparison between simulated and experimental mole fraction profiles allows the validation of this new model for a lean iso-butene flame. The future work consists in testing the reliability of this mechanism in a rich iso-butene flame to extend its validity range.

O

iC3H7

OH

C3H5O

CH3 + CH2CO Fig. 9. Reaction pathways of formation for oxygenated species: C3H6O, C4H6O and C4H8O, in the lean iso-butene flame.

rate coefficient is estimated by analogy with the reaction CH3CHCH2 + O => CH3CHCO + H + H, k487 = 1.74  1011 T0.7 exp( 1495/T) [cm3 mol 1 s 1] from Tsang’s evaluation [30]. The main consumption of C4H6O produces C3H6O via the reaction: C4H6O + O = C3H6O + CO (R.516). To estimate the rate constant, we have considered the reaction C3H6O + O = C3H5O + OH from Herron [33], with k516 = 1.0  1013 exp( 3020/T) [cm3 mol 1 s 1]. Acetone is also produced through the reaction of the iC3H7 radical with the oxygen atom: iC3H7 + O = C3H6O + H (R.511), k511 = 4.82  1013 [cm3 mol 1 s 1] from Tsang’s evaluation [34]; and from the reaction tC4H9 + HO2 => C3H6O + CH3 + OH (R.506), k506 = 1.80  1013 [cm3 mol 1 s 1] from Dagaut and Cathonnet [4], but with a minor contribution. Acetone consumption allows the formation of C3H5O radical by the reaction: C3H6O + OH = C3H5O + H2O (R.513), k513 = 2.0  1013 exp( 1510/T) [cm3 mol 1 s 1] from Sato and Hidaka [35]. The decomposition of the C3H5O radical produces a methyl radical and ketene (CH2CO): C3H5O = CH3 + CH2CO (R.520), k520 = 2.0  1013 exp( 16,160/T) [s 1] from Lesoil [36]. The reaction pathways for these three oxygenated species (C3H6O, C4H6O and C4H8O) are presented in Fig. 9. This scheme is elaborated after a reaction-flow analysis and the thickness of arrows represents the relative contribution of the way. The kinetic model of Yasunaga et al. [7] has already been tested in the premixed lean iso-butene flame and the simulation do not provide good results compared to experimental data for intermediate species (CH3, C2H2, C2H4, CH2O, C4H6). For these last species, the simulated mole fractions are overestimated compared to experimental ones. Moreover, kinetics of C3H6O, C4H6O and C4H8O are not present in the Yasunaga’s model [7]. 5. Conclusions The lean premixed iC4H8/H2/O2/Ar flame has been studied by molecular beam mass spectrometry at the equivalence ratio of 0.225 at low pressure (40 mbar). We have measured experimental mole fraction profiles of these detected species: H2, CH3, O, H2O, C2H2 (acetylene), CO, C2H4 (ethylene), CH2O (formaldehyde), O2, HO2, OH, Ar, C3H6 (propene), CO2, CH3CHO (acetaldehyde), C4H6 (1,3-butadiene), iC4H8 (iso-butene), C3H6O (acetone), C4H6O (1 propen-1-one, 2-methyl-) and C4H8O (prop-1-en-1-ol, 2-methyl- and propanal, 2-methyl-). A sub-mechanism for iso-butene taking into account the formation and the consumption of species involving in its combustion is elaborated according to several models from literature. Added to

The authors are very grateful to the Ministère de la Région Wallonne for the financial support (Visa n°07/47,509). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.fuel.2010.05.001. References [1] Bradley JN, West KO. Single-pulse Shock Tube Studies of Hydrocarbon Pyrolysis Part 6-The Pyrolysis of Isobutene. J. Chem. Soc., Faraday Trans. 1. 1976;72:558–67. [2] Brezinsky K, Dryer FL. A Flow Reactor of the Oxidation of Iso-butylene and an Iso-butylene/n-octane Mixture. Combust. Sci. Technol. 1986;45:225–32. [3] Curran HJ, Dunphy MP, Simmie JM, Westbrook CK, Pitz WJ. Shock Tube Ignition of Ethanol, Isobutene and MTBE: Experiments and Modeling. Proc. Combust. Inst. 1992;24:769–76. [4] Dagaut P, Cathonnet M. Isobutene Oxidation and Ignition: Experimental and Detailed Kinetic Modeling Study. Combust. Sci. Technol. 1998;137:237–75. [5] Bauge JC, Battin-Leclerc F, Baronnet F. Experimental and Modeling Study of the Oxidation of Isobutene. Int. J. Chem. Kinet. 2000;30:629–40. [6] Santhanam S, Kiefer JH, Tranter RS, Srinivasan NK. A Shock Tube LaserSchlieren Study of the Pyrolysis of Isobutene: Relaxation, Incubation, and Dissociation Rates. Int. J. Chem. Kinet. 2003;35:381–90. [7] Yasunaga K, Kuraguchi Y, Ikeuchi R, Masaoka H, Takahashi O, Koike H, et al. Shock tube and modeling study of isobutene pyrolysis and oxidation. Proc. Combust. Inst. 2009;32:453–60. [8] Dias V, Renard C, Vandooren J. Modeling of Rich Premixed C2H4/O2/Ar and C2H4/Dimethoxymethane/O2/Ar Flames. Z. Phys. Chem. 2009;223:565–77. [9] Vandooren J, Branch MC, Van Tiggelen PJ. Comparisons of the structure of stoichiometric CH4–N2O-Ar and CH4–O2-Ar flames by molecular beam sampling and mass spectrometric analysis. Combust. Flame 1992;90:247–58. [10] Wagner HGg, Bonne V, Grewer T, Z. Phys. Chem. 26 (1960) 93. [11] Westbrook CK, Dryer FL, Schug KP. Numerical Modeling of Ethylene Oxidation in Laminar Flames. Combust. Flame 1983;52:299–313. [12] Baulch DL, Cobos CJ, Cox RA, Esser C, Frank P, Just Th, Kerr JA, Pilling MJ, Troe J, Walker RW, Warnatz J, Evaluated Kinetic Data for Combustion Modeling, J. Phys. Chem. Ref. Data 21 (1992). [13] Miller JA, Melius CF. Kinetic and Thermodynamic Issues in the Formation of Aromatic Compounds in Flames of Aliphatic Fuels. Combust. Flame 1992;91:21–39. [14] GRI-Mech 1.2 (H. Frenklach, H. Wang, M. Goldenberg, G.P. Smith, D.M. Golden, C.T. Bowman, R.K. Hanson, W.C. Gardiner, V. Lissianski), (1995), GRI-Mech, An Optimized Detailed Chemical Reaction Mechanism for Methane Combustion, Report No. GRI-95/0058, November 1. [15] Warnatz J. Chemistry of High Temperature Combustion of Alkanes up to Octane. Proc. Combust. Inst. 1984;20:845–56. [16] Davis SG, Law CK, Wang H. Propene Pyrolysis and Oxidation Kinetics in a Flow Reactor and Laminar Flames. Combust. Flame 1999;119:375–99. [17] Wang H, Frenklach M. A Detailed Kinetic Modeling Study of Aromatics Formation in Laminar Premixed Acetylene, Ethylene Flames. Combust. Flame 1997;110:173–221. [18] Marinov NM, Pitz WJ, Westbrook CK, Vincitore AM, Castaldi MJ, Senkan SM, et al. Aromatic and Polycyclic Aromatic Hydrocarbon Formation in a Laminar Premixed n-Butane Flame. Combust. Flame 1998;114:192–213. [19] Richter H, Grieco WJ, Howard JB. Formation Mechanism of Polycyclic Aromatic Hydrocarbons and Fullerenes in Premixed Benzene Flames. Combust. Flame 1999;119:1–22. [20] Rasmussen CL, Skjøth-Rasmussen MS, Jensen AD, Glarborg P. Propargyl recombination: estimation of the high temperature, low pressure rate constant from flame measurements. Proc. Combust. Inst. 2005;30:1023–31. [21] Dias V, Lories X, Vandooren J. Lean And Rich Premixed Dimethoxymethane/ Oxygen/Argon Flames: Experimental and Modeling. Combust. Sci. Technol. 2010;182:350–64. [22] Burcat AIdeal Gas Thermodynamic Data in Polynomial Form for Combustion and Air Pollution Use. http://garfield.chem.elte.hu/Burcat/burcat.html, 2007. [23] COSILABÒ, The Combustion Simulation Laboratory, Version 2.1, www.SoftPredict.com, Rotexo GmbH & Co. KG, Haan (Germany), 2007. [24] Pauwels J-F, Volponi JV, Miller JA. The Oxidation of Allene in a Low Pressure H2/O2/Ar-C3H4 Flame. Combust. Sci. Technol. 1995;110–111:249–76.

V. Dias, J. Vandooren / Fuel 89 (2010) 2633–2639 [25] Peeters J, Van Look H, Ceursters B. Absolute Rate Coefficients of the Reactions of C2H with NO and H2 between 295 and 440 K. J. Phys. Chem. 1996;100: 15124–9. [26] Tsang W. Chemical kinetic data base for combustion chemistry. Part 4. Isobutane. J. Phys. Chem. Ref. Data 1990;19:1–68. [27] Matheu DM, Green Jr WH, Grenda JM. Capturing pressure-dependence in automated mechanism generation: Reactions through cycloalkyl intermediates. Int. J. Chem. Kinet. 2003;35:95–119. [28] Warnatz J. Combustion Chemistry, ed. W.C. Gardiner Jr., pub. NY: SpringerVerlag; 1984. [29] Tsang W, Walker JA. Mechanism and rate constants for the reactions of hydrogen atoms with isobutene at high temperatures. Proc. Combust. Inst. 1989;22:1015–22. [30] Tsang W. Chemical Kinetic Data Base for Combustion Chemistry. Part V. Propene. J. Phys. Chem. Ref. Data 1991;20:221–73. [31] Lifshitz A, Ben-Hamou H. Thermal Reactions of Cyclic Ethers at High Temperatures. 1Pyrolysis of Ethylene Oxide Behind Reflected Shocks. J. Phys. Chem. 1983;87:1782–7.

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[32] Adusei GY, Fontijn A. Kinetics of the Reaction between O(3P) Atoms and 1, 3Butadiene between 280 and 1015 K. J. Phys. Chem. 1993;97:1406–8. [33] Herron JT. Evaluated Chemical Kinetic Data for the Reactions of Atomic Oxygen O(3P) with Saturated Organic Compounds in the Gas Phase. J. Phys. Chem. Ref. Data 1988;17:967. [34] Tsang W, Chemical kinetic data base for combustion chemistry. Part 3. Propane, J. Phys. Chem. Ref. Data 17 (1988). [35] Sato K, Hidaka Y. Shock-Tube and Modeling Study of Acetone Pyrolysis and Oxidation. Combust. Flame 2000;112:291–311. [36] Lesoil H, Comparaison de la structure de flammes d’acétone et de propionaldéhyde en milieu riche, Dissertation, Université catholique de Louvain. 1995. [37] Westbrook CK, Pitz WJ. A Comprehensive Chemical Kinetic Reaction Mechanism for Oxidation and Pyrolysis of Propane and Propene. Combust. Sci. Technol. 1984;37:117–52. [38] Walravens B, Vandooren J, Van Tiggelen PJ. Peculiar Features in Lean Butane Flames. Combust. Sci. Technol. 1997;130:399–409.