Experimental and Kinetic Modeling Study of - American Chemical

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Experimental and Kinetic Modeling Study of n‑Butanol Pyrolysis and Combustion Jianghuai Cai, Lidong Zhang, Feng Zhang, Zhandong Wang, Zhanjun Cheng, Wenhao Yuan, and Fei Qi* National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China S Supporting Information *

ABSTRACT: n-Butanol pyrolysis in a flow reactor was investigated at the pressures of 5, 30, 80, 200, and 760 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used for the identification of pyrolysis species and the measurement of their mole fractions. A detailed kinetic model consisting of 121 species and 658 reactions was developed to simulate the nbutanol pyrolysis. To enhance the accuracy of the model, the rate constants of unimolecular reactions of n-butanol and β-scission reactions of four n-butanol radicals (C4H8OH) were calculated with the variable reaction coordinate−transition-state theory (VRC−TST) and the Rice−Ramsperger−Kassel−Marcus (RRKM) theory coupled with the master equation method. These rates are very sensitive to the mole fractions of pyrolysis species and have been well-validated by the pyrolysis experiment. The model was further validated by the low-pressure premixed flames at different equivalence ratios, oxidation data from the jetstirred reactor, and ignition delay times. The comparison between predicted and measured results exhibited a good performance of this model. The reaction product analysis and sensitivity analysis were performed to elucidate the chemistry under different conditions. al.,18 were used to validate the kinetic model. Besides, oxidation data, such as jet-stirred reactor data at 1 atm16 and 10 atm15 and ignition delay times,2,3,7,9 were also used to validate the present kinetic model.

1. INTRODUCTION Bio-alcohol, as a typical biofuel, has received more and more attention because it is renewable and environmentally friendly. Biomethanol and bioethanol have already been used widely as additives to fossil fuel in many countries. Recently, n-butanol is considered as a potential fuel because it offers several advantages over lighter alcohols, e.g., higher energy density, better miscibility with practical fuels, lower water absorption, and higher suitability for conventional engines.1 Many experimental studies have been performed to investigate the high-temperature chemistry of n-butanol, including the ignition delay times, 2−9 laminar flame speed,10−13 species profiles measured in the pyrolysis,14 jetstirred reactor,15,16 and premixed flames.17−19 The detailed conditions of these experiments are summarized in Table 1. Several kinetic models2,3,15,16,18,20 have been proposed to describe the combustion chemistry of n-butanol and validated against the experimental data. The goal of this work is to develop a n-butanol mechanism over a wide range of temperatures, pressures, and equivalence ratios. To enhance the accuracy of the model, the rate constants of dominant unimolecular decomposition channels of n-butanol and β-scission of n-butanol radicals (C4H8OH) were calculated with the variable reaction coordinate−transition-state theory (VRC−TST) and the Rice−Ramsperger−Kassel−Marcus (RRKM) theory coupled with the master equation (RRKM/ ME) method. The pyrolysis of n-butanol in a flow reactor at pressures of 5, 30, 80, 200, and 760 Torr were studied to understand the pressure effect on the high-temperature chemistry of n-butanol. The pyrolysis experimental results were used to validate the n-butanol mechanism. Eight premixed n-butanol flames, including the experiments studied in this work, the work of Osswald et al.,19 and the work of Hensen et © 2012 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Pyrolysis Experiment. The experiments were carried out at the National Synchrotron Radiation Laboratory in Hefei, China. A detailed description of the two beamlines and pyrolysis apparatus used in this work has been published elsewhere.21−25 As shown in Figure 1a, the pyrolysis apparatus is composed of a pyrolysis chamber, a differentially pumped chamber, and a photoionization chamber with a homemade reflectron time-of-flight mass spectrometer (RTOF−MS). The mixture of Ar [970 standard cubic centimeter per minute (sccm)] and n-butanol (30 sccm) in the gas phase was fed into a 6.8 mm inner diameter alumina flow tube with 150 mm heated by a furnace in the pyrolysis chamber. Then, the pyrolysis species were sampled at 10 mm downstream from the outlet by a quartz cone-like nozzle with a 40° cone angle. We used several nozzles with different orifices (70−500 μm) for experiments under different pressures. The sampled pyrolysis species formed a molecular beam in the differentially pumped chamber and passed into the photoionization chamber through a nickel skimmer. The molecular beam was crossed by the tunable synchrotron vacuum ultraviolet (VUV) light in the photoionization chamber, and the ionized species were detected by a RTOF−MS. The furnace temperature was monitored by a thermocouple in the middle region of the heating wire outside the flow tube. The temperature measured by this thermocouple is referred to as Toutside. After the experiment, we chose different Toutside and measured the temperature profiles along the centerline of the flow tube by another thermocouple. Hence, the relationship between the Toutside and the temperature profiles (Tin) inside the flow tube was obtained. Figure 1b Received: July 17, 2012 Revised: August 24, 2012 Published: August 27, 2012 5550

dx.doi.org/10.1021/ef3011965 | Energy Fuels 2012, 26, 5550−5568

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Table 1. Previous Studies on n-Butanol Combustion Experiment pyrolysis year

temperature (K)

2011

923−1078

year

temperature (K)

2009 2009

750−1100 750−1250

pressure (atm) 1.70 oxidation pressure (atm)

reactor

reference

flow reactor

Harper et al.14

equivalence ratio

10 0.5, 1.0, 2.0 1 0.25, 0.5, 1.0, 2.0 premixed laminar flame

year

equivalence ratio

2011 2011

1.70 1.4, 1.2, 1.0

pressure (Torr)

reference Dagaut et al.15 Sarathy et al.16 reference

Osswald et al.19 Hansen et al.18

30 15, 25 ignition delay times

year

temperature (K)

pressure

equivalence ratio

2008 2010 2010

1200−1800 1100−1800 1070−1760

1−4 bar 1, 2.6, and 8 atm 2, 10, and 12 atm

0.25, 0.5, 1.0 0.5, 1.0, 2.0 0.5, 1.0, 2.0

2011 2011 2011

770−1250 902−1040 795−1200

10−42 bar 2.86−3.35 atm 61−92 bar

1.0 1.0 1.0

2011 2012

675−925 1050−1600

15 and 30 bar 0.5, 1.0, 2.0 1.5−43 atm 0.5, 1.0 laminar burning velocity

year

pressure

2009 2010 2010 2011

0.1 and 0.25 MPa 1 atm 0.1, 0.25, 0.5, and 0.75 MPa 0.1 MPa with different dilution ratios of N2

reference Moss et al.2 Black et al.3 Noorani et al.8 Heufer et al.4 Karwat et al.9 Vranckx et al.5 Weber et al.6 Stranic et al.7

Figure 1. (a) Schematic diagram of the pyrolysis apparatus with a molecular-beam sampling mass spectrometer. The red circles inside the furnace denote an electronically heating wire with the length of 150 mm in this work. A tungsten−rhenium (W−Re) thermocouple that connected to a temperature controller is put close to the middle region of the heating wire. (b) Temperature profiles along the centerline of the flow tube are measured by moving a B-type thermocouple (not shown in the figure) from the tube inlet to the sampling point of the quartz nozzle. Three temperature profiles are chosen to illustrate variation along the tube.

references Gu et Veloo Gu et Gu et

al.10 et al.53 al.13 al.12

Two experimental modes were used in this work, including measurement of photoionization efficiency (PIE) spectra to identify pyrolysis products and temperature scan to plot mole fraction profiles of pyrolysis species versus Tmax. In this work, the mass spectra were measured from Tmax = 700 K to the temperatures where more than 90% of n-butanol was consumed. Photon energies of 16.64, 11.70, 11.00, 10.50, 10.00, and 9.50 eV were selected to obtain near-threshold photoionization. The evaluation method of the mole fraction has been reported in detail previously.26 The photoionization cross-sections (PICSs) used in this work are from our online database.28 The experimental uncertainties of mole fractions are evaluated to be ±25% for the products with known PICSs and a factor of 2 for those with estimated PICSs. The experimental uncertainties of C, H, and O balances are all within ±10% compared to inlet fluxes of C, H, and O elements. 2.2. Premixed n-Butanol/O2/Ar Flame Experiment. The premixed n-butanol/O2/Ar flames were studied at four equivalence ratios, including 0.70, 1.00, 1.30, and 1.80, using synchrotron VUV photoionization mass spectrometry (SVUV−PIMS) to further validate the kinetic model, especially the hydrogen-abstraction reactions by radical (e.g., H, O, or OH radical) attacking. For a better comparison of the flame chemistry, pressure, inlet percentage of Ar, and cold gas velocity of the inlet mixture were kept identical in all flames. Detailed descriptions of the experimental instruments and the methodologies of intermediate identification and mole fraction evaluation have been reported previously.21,22,29 The flame conditions are listed in Table 2. The measurements were performed along the axial direction of the burner at photon energies of 16.64, 11.70, 11.00, 10.50, 10.00, and 9.50 eV to obtain near-threshold photoionization of flame species. The uncertainties of evaluated mole fractions are within 10% for major species, 25% for intermediates with known PICSs, and a factor of 2 for those with estimated PICSs. The flame temperatures were measured

shows three representative temperature profiles, and the complete temperature profiles are provided in the Supporting Information. Each temperature profile is named by its maximum temperature (Tmax), which was used as the experimental temperature. The uncertainty of Tmax is estimated to be within ±30 K. The temperature profiles are not sensitive to pressure. Slight pressure differences were observed between 30 and 760 Torr. However, carbon deposition on the heating wire surface and its aging probably leads to the change of temperature distribution. Thus, the temperature measurement was performed immediately after each pyrolysis experiment. In this work, the pressures of 5, 30, 80, 200, and 760 Torr were chosen to study the effect of the pressure on the n-butanol pyrolysis. The calculated Reynolds number (Re) of the flow was always less than 2000 when Tmax varied from 600 to 1900 K and Poutlet changed from 5 to 760 Torr. Thus, it is a laminar flow under the experimental conditions. The pressure distributions along the flow tube are evaluated using E1 and assuming that the gas inside the flow tube is pure Ar because the Ar concentration is higher than 90% in most cases. The detailed calculations can be found in previous work24

v dv + dp / ρ + dF = 0

(E1)

where v, p, ρ, and F are velocity, pressure, density, and frictional resistance, respectively. The detailed calculations can be found in previous work.26,27 Pressure profiles should be used in the simulation with pressures below 30 Torr. When the pressures are above 30 Torr, the difference between the inlet and outlet pressures are less than 10%; that is to say, the pressure profiles can be treated as a constant. 5551


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Table 2. Flame Conditionsa ϕ

C/O

P (Torr)

Xfuel

XO2

XAr

V (cm s−1)

0.70 1.00 1.30 1.80

0.22 0.31 0.39 0.52

30 30 30 30

5.2 7.1 8.9 11.5

44.8 42.9 41.1 38.5

50.0 50.0 50.0 50.0

50.0 50.0 50.0 50.0

constants of dominant unimolecular decomposition channels of n-butanol were calculated with VRC−TST and RRKM/ME methods using the Variflex program.35 Reactions with tight transition states were treated with the canonical transition-state theory,36 while the numbers of states were evaluated on the basis of the rigid-rotor harmonic oscillator (RRHO) assumption. The transition-state partition functions were evaluated at the E/J (energy E and total angular momentum J) resolved calculations,37 in which the energy E covers the range from −29 900 to 50 000 cm−1 with an energy grain size of 100 cm−1 and the angular momentum J ranges from 1 to 241 with a step size of 10. This E and J choice is consistent with our previous work.38 The pressure-dependent rate constants were calculated with a one-dimensional (1D) master equation, while the master equation was solved by an eigenvalue-solver-based approach for the dissociation processes.39,40 In this work, Ar is the bath gas. Lennard−Jones potential was chosen to model the interaction between the reactant and bath gas. The Lennard− Jones pairwise parameters for Ar applied to our rate constant calculations are σ = 3.465 Å and ε/k = 78.89 cm−1.41 For the Lennard−Jones parameters of n-butanol and its radicals, these values are estimated by the following equations:42

Xi is the inlet percentage of species i, and V is the flow velocity of the inlet mixture at 300 K. a

using a 0.1 mm diameter Pt−6% Rh/Pt−30% Rh thermocouple coated with Y2O3−BeO anticatalytic ceramic30 and then were corrected for the radiative heat loss and cooling effects of the sampling nozzle.31 The uncertainty of the maximum flame temperature was estimated to be ±100 K.

3. THEORETICAL CALCULATIONS AND KINETIC MODEL 3.1. Theoretical Methods. The TGt conformer of nbutanol (see Figure 2) has been proven to be the lowest energy conformer by Black and co-workers.3 Hence, the TGt conformer was chosen as the commencement for the reaction pathways and rate constant calculations for unimolecular decomposition of n-butanol. The decomposition pathways of n-butanol and its four radical isomers (C4H8OH) were computed at the CBS−APNO32 level, which starts from the geometry optimization and frequency calculations at the HF/6311G(d,p) level and then followed by a second geometry optimization at the QCISD/6-311G(d,p) level, and then this second-optimized geometry is used for further single-point energy corrections by QCISD(T), MP2, and CBS extrapolation calculations to provide the complete basis set extrapolation energy. Studying the direct unimolecular bond dissociation channels without apparent transition states requires multireference methods. In this work, the potential energy curves for the C−C and C−O bond dissociation of n-butanol were calculated by the CASPT2(2e,2o)//CASSCF(2e,2o) method with the 6-311G(d,p) basis set, 33 scanning along the dissociation bond length by 0.2 Å intervals. The active space was chosen as the bonding orbital σ and an antibonding orbital σ* of the breaking bond. All of the ab initio calculations were performed with Gaussian 09 program.34 The pressure-dependent (at 5, 30, 70, 200, 760, 7600, and 76 000 Torr) and temperature-dependent (800−2000 K) rate

σ = 2.44(Tc/Pc)1/3

(E2)

ε /k b = 0.77Tc

(E3)

where kb is the Boltzmann constant. The critical temperature (Tc = 563.06 K) and pressure (Pc = 44.24 bar) for n-butanol were from the work of Poling et al.43 According to E2 and E3, the Lennard−Jones parameters are σ = 5.697 Å and ε/k = 301.32 cm−1. The collision energy transfer was treated using an exponential-down model with ⟨ΔEdown⟩ = 150 × (T/300)0.85 cm−1.44−46 The same collisional parameters were used for C4H8OH radicals. Low-frequency vibrational modes corresponding to internal rotation were treated as hindered rotors by the computed hindrance potentials at the B3LYP/6-31G(d) level.47 The barrierless bond cleavage channels were treated using variational transition-state theory.48 The calculated CASPT2 potential curves were fitted into Morse potential V(R) = De{1 − exp[−β(R − Re)]2}, where De is the electronic binding energy, excluding the zero-point energy (ZPE), and Re is the equilibrium bond length. Here, De was corrected by the

Figure 2. Calculated major decomposition pathways of n-butanol at the CBS−APNO level, including ZPE correction (energy unit: kcal/mol). 5552


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Table 3. Calculation Results and Selected Reactions in the n-Butanol Modela selected reactions 1

nC4H9OH = C4H8 + H2O

2

nC4H9OH = nC3H7 + CH2OH

3

nC4H9OH = C2H5 + CH2CH2OH

4

nC4H9OH = CH3 + CH2CH2CH2OH

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH nC4H9OH

20

aC4H8OH = C2H3OH + C2H5

21

aC4H8OH = C4H7OH1−1

22

aC4H8OH = C3H7CHO + H

+ + + + + + + + + + + + + + +

H = aC4H8OH + H2 H = bC4H8OH + H2 H = cC4H8OH + H2 H = dC4H8OH + H2 H = C4H9O + H2 OH = aC4H8OH + H2O OH = bC4H8OH + H2O OH = cC4H8OH + H2O OH = dC4H8OH + H2O OH = C4H9O + H2O CH3 = aC4H8OH + CH4 CH3 = bC4H8OH + CH4 CH3 = cC4H8OH + CH4 CH3 = dC4H8OH + CH4 CH3 = C4H9O + CH4

A

n

Reactions of C4H9OH 4.11 × 1083 −20.67 3.25 × 1077 −18.76 1.39 × 1073 −17.43 2.215 × 1068 −15.98 2.142 × 1060 −13.60 4.829 × 1044 −9.00 1.248 × 1029 −4.465 8.43 × 10101 −25.53 1.19 × 10100 −24.71 3.91 × 1097 −23.86 2.11 × 1094 −22.79 4.26 × 1087 −20.71 4.45 × 1071 −15.91 4.28 × 1052 −10.32 2.44 × 10103 −26.02 2.61 × 10102 −25.42 2.84 × 10100 −24.69 5.22 × 1097 −23.76 4.61 × 1091 −21.84 1.32 × 1076 −17.12 2.10 × 1056 −11.28 1.00 × 10103 −26.05 1.46 × 10102 −25.48 2.30 × 10100 −24.80 4.48 × 1097 −23.87 4.54 × 1091 −21.96 1.19 × 1076 −17.23 1.67 × 1056 −11.36 9.25 × 105 2.28 2.26 × 106 2.15 1.14 × 106 2.25 3.47 × 106 2.27 1.54 × 105 2.48 3.61 × 103 2.890 1.54 × 100 3.70 1.14 × 103 2.87 5.28 × 109 0.97 5.88 × 102 2.82 1.99 × 101 3.37 8.02 × 100 3.23 1.51 × 100 3.46 4.53 × 10−1 3.65 1.02 × 100 3.57 Reactions of C4H9O Isomers 1.76 × 1024 −4.94 5.61 × 1027 −5.69 5.34 × 1029 −6.14 3.34 × 1032 −6.78 3.47 × 1036 −7.73 2.17 × 1050 −11.27 9.30 × 1054 −12.23 1.61 × 1021 −5.27 5.49 × 1025 −6.16 1.36 × 1028 −6.66 3.02 × 1031 −7.39 1.90 × 1036 −8.47 1.64 × 1053 −12.76 5.84 × 1060 −14.41 9.12 × 1022 −5.05 6.37 × 1026 −5.86 5553

E

pressure (Torr)

reference

102698 100769 98970 96774 92789 84357 75432 122927 124635 124610 123938 121583 114141 103881 125685 128121 128556 128368 126653 119918 109480 125924 128466 129028 128871 127237 120522 110115 3760 5660 5440 7880 8920 −2291 −3736 −2926 1586 −5850 7634 6461 5481 7154 8221

5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000

this this this this this this this this this this this this this this this this this this this this this this this this this this this this 14 14 14 14 14 20 20 20 20 20 3 3 3 3 3

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20126 22942 24593 26991 30560 42437 48452 22642 25611 27397 30059 34118 48389 56993 20778 23807

5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30

this this this this this this this this this this this this this this this this

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Table 3. continued selected reactions

23

bC4H8OH = C4H8 + OH

24

bC4H8OH = C3H5OH + CH3

25

bC4H8OH = C4H7OH1−1

26

bC4H8OH = C4H7OH2−1

27

cC4H8OH = C3H6 + CH2OH

28

cC4H8OH = C4H7OH2−1 + H

29

cC4H8OH = C4H7OH1−4 + H

30

dC4H8OH = C2H4 + C2H4OH

A

n

Reactions of C4H9O 7.52 × 1028 7.58 × 1031 2.38 × 1036 3.05 × 1051 2.02 × 1057 3.85 × 1018 2.64 × 1021 1.04 × 1023 2.11 × 1025 7.99 × 1028 2.82 × 1040 1.44 × 1043 6.56 × 1030 2.95 × 1036 2.63 × 1039 2.11 × 1043 1.43 × 1048 1.17 × 1056 1.65 × 1053 5.22 × 1015 9.68 × 1019 1.43 × 1022 2.13 × 1025 8.39 × 1029 4.69 × 1046 1.18 × 1053 1.25 × 1014 1.10 × 1019 4.18 × 1021 1.34 × 1025 1.41 × 1030 1.74 × 1048 1.08 × 1056 7.86 × 1021 1.73 × 1025 1.53 × 1027 8.85 × 1029 1.00 × 1034 5.12 × 1046 2.86 × 1049 1.11 × 1018 1.68 × 1023 6.17 × 1025 2.33 × 1029 3.97 × 1034 1.01 × 1052 6.39 × 1058 9.82 × 1017 1.18 × 1023 5.47 × 1025 2.30 × 1029 4.01 × 1034 9.88 × 1051 6.74 × 1058 2.41 × 1026 4.46 × 1030 9.94 × 1032 2.03 × 1036 1.04 × 1041 1.82 × 1053 5554

Isomers −6.31 −6.99 −8.04 −11.89 −13.13 −3.36 −3.94 −4.28 −4.80 −5.64 −8.58 −9.04 −6.92 −8.24 −8.93 −9.87 −11.02 −12.89 −11.71 −3.60 −4.42 −4.85 −5.52 −6.54 −10.72 −12.06 −3.54 −4.50 −5.01 −5.74 −6.84 −11.32 −12.99 −4.28 −4.99 −5.43 −6.07 −7.03 −10.27 −10.70 −4.66 −5.68 −6.21 −6.97 −8.14 −12.50 −13.89 −4.66 −5.66 −6.21 −6.98 −8.15 −12.51 −13.91 −5.52 −6.49 −7.04 −7.82 −8.96 −12.05

E

pressure (Torr)

25520 28071 32008 44961 52127 14184 16419 17747 19737 22908 33125 37585 26202 30835 33368 36824 41391 49469 51223 17406 19819 21320 23741 27565 41947 49932 19014 21127 22684 25150 29141 44481 53658 17661 20372 22025 24423 28077 39310 44158 21297 24358 26192 28988 33417 48344 57007 21367 24311 26210 29034 33461 48375 57060 22329 25917 27965 30924 35278 46604

70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600 76000 5 30 70 200 760 7600

reference this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this this

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Table 3. continued selected reactions

31

dC4H8OH = C4H7OH1−4 + H

aC4H8OH + O2 = C3H7CHO + HO2

a

32 33

C2H3OH + H = CH3CHO + H C2H3OH = CH3CHO

34 35 36 37 38 39

C2H3OH + H = CH2CHO + H2 C2H3OH + OH = CH2CHO + H2O C2H3OH + O = CH2CHO + OH C2H3OH + HO2 = CH3CHO + H2O C2H3OH + HO2 = CH3CHOH + O2 CH2CO + H (+M) = CH2CHO (+M) LOW/ TROE/0.337

A

n

Reactions of C4H9O Isomers 3.05 × 1053 −11.76 3.87 × 1024 −5.89 3.02 × 1029 −6.94 1.48 × 1032 −7.54 8.36 × 1035 −8.40 2.52 × 1041 −9.70 3.73 × 1056 −13.52 4.18 × 1058 −13.65 4.68 × 1011 0.332 Reactions of C2H3OH 7.42 × 10 4.42 × 1042 2.90 × 1027 1.50 × 107 7.46 × 1011 2.81 × 1013 1.49 × 105 4.11 × 106 3.30 × 1014 3.80 × 1041 1707

−10.56 −9.09 −4.35 1.60 0.30 0.00 1.67 1.62 −0.06 −7.64 3200

46

E

pressure (Torr)

50035 24244 28112 30371 33640 38601 52357 57700 −1063.5

76000 5 30 70 200 760 7600 76000

67200 67069 61612 3038 1634 5200 6810 15440 8500 11900 4131

76 760 76000

reference this this this this this this this this 19

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19b 20

53 53 53 59 59 52

Units are s−1, cm3, and cal/mol. bThe rate constants are CHEB format.

Figure 3. Reaction rate constants of R1−R4 at 760 Torr in the present model, Black et al. model,3 Sarathy et al. model,16 and Moss et al. model.2

and thermodynamic and transport data are available in the Supporting Information. 3.3. Submechanism of n-Butanol. Figure 2 shows the calculated reaction pathways of n-butanol. Among them, the dominant channels are

computed CBS−APNO dissociation energy. The quantum tunneling correction was calculated by the Eckart model.49 3.2. Brief Description of the Kinetic Model. The present kinetic model consisting of 121 species and 658 reactions was mainly developed from our recently reported model of three butene isomers25 and the USC Mech II model.50 The submechanism of n-butanol was developed on the basis of theoretical calculations in this work, which will be described briefly in the next section. The simulations were performed using Chemkin-Pro software.51 Most thermodynamic and transport data were taken from the USC Mech II model,50 the database reported by Goos, Burcat, and Ruscic,52 and previous n-butanol models.3,18,20,53 The reaction mechanism 5555

nC4 H 9OH = C4 H8 + H 2O

(R1)

nC4 H 9OH = nC3H 7 + CH 2OH

(R2)

nC4 H 9OH = C2H5 + CH 2CH 2OH

(R3)

nC4 H 9OH = CH3 + CH 2CH 2CH 2OH

(R4)


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Figure 4. Experimental mole fraction profiles (symbols) and modeling results (lines) of nC4H9OH, H2, H2O, and C4H8 from n-butanol pyrolysis at various pressures.

predicted using the rate constants calculated by Zhou et al.54 Therefore, they estimated the rate constants of nC4H9OH + OH using a combination of available theoretical results and high-55 and low-temperature experimental measurements.56 These rate constant data are adopted by the present model. The data for hydrogen-abstraction by other small radicals, such as CH3, HCO, and C2H5, are from the work of Black et al.3 3.4. Submechanism of C4H8OH Radicals. The rate constants of β-scission reactions of four C4H8OH radicals (aC4H8OH, CH3CH2CH2CH*OH; bC4H8OH, CH3CH2CH*CH2OH; cC4H8OH, CH3CH*CH2CH2OH; and dC4H8OH, CH2*CH2CH2CH2OH) were also computed by the same method as the unimolecular decomposition of nbutanol, as listed in Table 3. The isomerization reactions of C4H8OH radicals used in this work were from the work of Harper et al.,14 which were solved by the reaction mechanism generator (RMG) package using modified strong collision approximation. The reaction between aC4H8OH and O2 producing n-butanal (R5) plays an important role in the nC4H9OH flame and oxidation.

The pressure- and temperature-dependent rate constants of dominant channels in n-butanol unimolecular decomposition are adopted from our calculations. Detailed theoretical methods have been discussed above. The calculated rate constants of the channels R1−R4 at 5, 30, 70, 200, 760, 7600, and 76 000 Torr in the temperature range of 800−2000 K are listed in Table 3. Figure 3 shows the comparison between the calculated rate constants at 760 Torr used in our model and those in the previous models.2,3,16,18 The rate constants of R1 in different models are close to each other, except the one in the Hansen et al. model,18 which is about 10 times smaller than the others in the whole temperature range. The rate constants of C−C scission reactions (R2−R4) used in various models are quite different from one another. The pressure dependence of rate constants in the Black et al. model3 is estimated by the highpressure limit expressions together with a chemical activation formulation based on QRRK theory. These data were also adopted by the model developed by Sarathy et al. in 2012.20 However, the rate constants of R2 and R3 in the Black et al. model3 are about 10 times smaller than those calculated in the present work at all studied pressures. These differences will affect the simulation results of n-butanol pyrolysis, especially at low pressure, which will be discussed in detail in the following section. Hydrogen-abstraction reactions of n-butanol by small radicals are important in the n-butanol chemistry. In previous models, most rate constants for the nC4H9OH + H reactions were referred to analogous abstraction reactions, such as ethanol + H or alkane + H. In the Hansen et al. model,18 the rate constants of the nC4H9OH + H reaction were calculated at the CBS− QB3 level, which was adopted by the present model. The rate constants of nC4H9OH + OH have been calculated by Zhou et al.54 based on high-level quantum chemical calculations. Their computed total rate constants are in good agreement with the shock tube measurement by Vasu et al.55 However, Sarathy et al.20 indicated that the ignition delay times cannot be accurately

aC4 H8OH + O2 = C3H 7CHO + HO2

(R5)

The rate constant of R5 is referred to the analogous reaction CH3CHOH + O2 in the work of Zádor et al.57 The submechanism of n-butanal, including its C−C scission and hydrogen-abstraction reactions, has been proposed by Harper et al.14 and validated by n-butanal ignition delay times. This submechanism is included in the present model. 3.5. Submechanism of Ethenol. Ethenol is an important intermediate in n-butanol high-temperature chemistry, which can be easily produced via β-scission of the aC4H8OH radical (R6).

5556

aC4 H8OH = C2H3OH + C2H5

(R6)

C2H3OH = CH3CHO

(R7)


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Figure 5. Experimental mole fraction profiles (symbols) and modeling results (lines) of C4H6, C3H6, aC3H5, and CH3 from n-butanol pyrolysis at various pressures.

Figure 6. Experimental mole fraction profiles (symbols) and modeling results (lines) of C3H7CHO, C3H5OH-1, C2H3OH, and CH3CHO from nbutanol pyrolysis at various pressures.

C2H3OH + H = CH3CHO + H

(R8)

C2H3OH + HO2 = CH3CHO + HO2

(R9)

the present model. The hydrogen-atom-assisted tautomerization reaction (R8) is believed to be important in ethenol chemistry.14,20 The rate constant of R8 in this model is taken from the work of Hansen et al.18 Recently, da Silva and Bozzelli59 found that ethenol can react with HO2, producing αhydroxyethyl + O2, which is quite rapid at combustion temperatures. These reactions can contribute to the con-

Some theoretical studies have been performed to investigate ethanol reactions.58−61 da Silva et al.58 calculated the conversion of ethenol to acetaldehyde (R7) at the CBS− APNO level. The pressure-dependent rate constants of R7 were estimated using RMG by Hansen et al.18 and are adopted by 5557


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Figure 7. Experimental mole fraction profiles (symbols) and modeling results (lines) of C2H4, C2H2, CH2O, and CO from n-butanol pyrolysis at various pressures.

sumption of ethenol given a suitably high level of HO2. Thus, these reactions are also considered in the present model.

4. RESULTS AND DISCUSSION 4.1. n-Butanol Pyrolysis. Figures 4−7 show the measured and predicted mole fraction profiles of n-butanol pyrolysis at various pressures. Generally, the model can predict most of the mole fractions of pyrolysis species quite well. The sensitivity analysis and rate of production (ROP) analysis at 5 and 760 Torr were performed to study the effect of pressure. For the simulation at 5 Torr, the sensitivity analysis and ROP analysis were performed at Tmax = 1450 K; the same analysis were performed at Tmax = 1100 K for the simulation at 760 Torr. At those temperatures, most of the intermediate species reach their maximum mole fractions. Figure 8 shows the sensitivity analysis of nC4H9OH at pressures of 5 and 760 Torr. On the basis of the ROP analysis, the reaction networks of nC4H9OH pyrolysis at the pressures of 5 and 760 Torr are listed in Figures 9 and 10, respectively. The mole fraction profiles of nC4H9OH decomposition are shown in Figure 4. The sensitivity analysis in Figure 8 indicates that nC4H9OH consumption is sensitive to the unimolecular reactions R1−R4 at both 5 and 760 Torr. The ROP analysis shows that 61% of nC4H9OH decomposes through the unimolecular reactions at 5 Torr; however, the decomposition ratio decreases to 27% at 760 Torr. The simulated mole fractions of nC4H9OH using different nC4H9OH unimolecular decomposition rate coefficients are illustrated in Figure 11a. It indicates that changing the rate coefficients of R1−R4 by a factor of 2 leads to about a 30 K shift of the nC4H9OH simulation results at 5 Torr but only a 10 K shift at 760 Torr. Therefore, the nC4H9OH decomposition is more sensitive to the rate coefficients of R1−R4 at low pressure than at high pressure. This conclusion is also consistent with the ROP analysis. Figure 12a shows the simulated results of nC4H9OH decomposition at 760 Torr by the present model and six other

Figure 8. Sensitivity analysis of nC4H9OH in pyrolysis at Tmax = 1450 K under 5 Torr (gray) and Tmax = 1100 K under 760 Torr (black). Only those reactions with sensitivities greater than 0.02 are listed.

published models.2,3,16,18,20,53 All models can predict the mole fractions of nC4H9OH within the experimental uncertainty. As shown in Figure 13, the reaction R1 is extremely important for C4H8 (1-butene) formation, especially at low pressure. At high pressure, the reaction sequence of nC4H9OH + H → bC4H8OH → C4H8 plays a more significant role. The ROP analysis indicates that, at 5 Torr, about 25% of nC4H9OH decomposes via R1, which contributes to about 80% of C4H8 and 61% of H2O formation. At 760 Torr, only 10% of nC4H9OH decomposes through R1, contributing to 52% of C4H8 and 36% of H2O formation. To obtain insight for C4H8 formation, the sensitivity of the C4H8 concentration to R1 was 5558


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Figure 9. Reaction network of nC4H9OH decomposition in pyrolysis at Tmax = 1450 K and 5 Torr. The thickness of each arrow represents the carbon flux of corresponding reaction(s). The percentage in the figure means the carbon flux of the pathway divided by the total carbon flux of nbutanol consumption. The blue arrows are the unimolecular pathways.

Figure 10. Reaction network of nC4H9OH decomposition in pyrolysis at Tmax = 1100 K and 760 Torr. The thickness of each arrow represents the carbon flux of corresponding reaction(s). The percentage in the figure means the carbon flux of the pathway divided by the total carbon flux of nbutanol consumption. The blue arrows are the unimolecular pathways.

200 Torr, while the experimental uncertainty is ±25%. On the basis of the above sensitivity analysis, the underprediction of C4H8 most likely resulted from the underestimation of hydrogen-abstraction reactions, producing bC4H8OH. The βscission of the bC4H8OH radical will produce allyl alcohol (C3H5OH-1). As shown in Figure 6, C3H5OH-1 is also underpredicted. However, the measured signal of C3H5OH-1 is

examined at 5 and 760 Torr using various kR1; the simulated results are shown in Figure 11b. Obviously, C4H8 formation is highly sensitive to kR1 at 5 Torr, while it is less sensitive to kR1 at 760 Torr. As shown in Figure 4, the mole fraction of C4H8 is well-predicted at 5 and 30 Torr by the present model but underpredicted at 80−760 Torr. For instance, the mole fraction of C4H8 underpredicted by about 30% at 80 Torr and 40% at 5559


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Figure 11. (a) Experimental mole fraction profiles (symbols) and modeling results (lines) of nC4H9OH in pyrolysis at 5 and 760 Torr. The dash lines and dotted lines are the simulation results of nC4H9OH using different rate coefficients of n-butanol overall unimolecular decomposition (R1−R4). (b) Experimental mole fraction profiles (symbols) and modeling results (lines) of 1-butene (C4H8) in n-butanol pyrolysis at 5 and 760 Torr. The dash lines and dotted lines are the simulation results of C4H8 using different rate coefficients of nC4H9OH = C4H8 + H2O (R1).

Figure 12. Experimental mole fraction profiles (symbols) and simulated results (lines) of nC4H9OH, C3H7CHO, C4H8, and C2H3OH from nbutanol pyrolysis at 760 Torr by the present model, Black et al. model,3 Sarathy et al. 2009 model,16 Moss et al. model,2 Veloo et al. model,53 Sarathy et al. 2012 model,20 and Hansen et al. model.18

in the Veloo et al. model;53 consequently, the mole fraction of C4H8 was overpredicted. Oppositely, it is underestimated in the Hansen et al. model;18 as a result, the C4H8 concentration is underpredicted, as shown in Figure 12c. In the Moss et al. model,2 the rate constant of nC4H9OH + H = aC4H8OH + H2 is so large compared to those in other models, so that about 78% of nC4H9OH is consumed through this reaction. As a result, the mole fractions of CH3CHO and C3H7CHO are largely overpredicted in the Moss et al. model,2 while C4H8 is underpredicted. At low pressure, C4H8 is mainly consumed by R10 (41%), which contributes to C4H8 formation at 760 Torr. The below H attack reactions (R11−R14) are dominant pathways for C4H8 decomposition, producing C3H6, C2H4, saxC4H7 (CH2

quite weak, which raises the uncertainty of its measured mole fractions. Also, the C3H5OH-1 submechanism is far from complete; further calculations and experiments are needed. Figure 12c shows the experimental data of C4H8 at 760 Torr and the simulated results by the present and previous models.2,3,16,18,20,53 The simulation results of the present model, Black et al. model,3 Sarathy et al. 2009 model,16 and Sarathy et al. 2012 model20 are within the experimental uncertainty. We checked the rate constants of R1 used in various models, because it significantly impacts the simulated C4H8 concentration. As shown in Figure 3, the reaction rates of R1 at 760 Torr in the present model, Black et al. model,3 Sarathy et al. 2009 model,16 and Sarathy et al. 2012 model20 are similar to each other. The rate constant of R1 is overestimated 5560


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the present work at all pressures. Because the unimolecular decomposition reactions are more important at low pressure, the simulation results of C2H4 and CH2O at 5 Torr by the present model and Sarathy et al. 2012 model20 are illustrated in Figure 14. It shows that C2H4 and CH2O, which are the followup product of R2, are well-reproduced by the present model, while they are underpredicted by the Sarathy et al. 2012 model.20

Figure 13. Sensitivity analysis of C4H8 in n-butanol pyrolysis at Tmax = 1450 K under 5 Torr (gray) and Tmax = 1100 K under 760 Torr (black). Only those reactions with sensitivities greater than 0.02 are listed.

CHCH*CH3), and 4-C4H7 (CH2CHCH2CH2*). The saxC4H7 radical is an important precursor of 1,3-C4H6 (1,3butadiene) at both high and low pressures. C4 H8 = aC3H5 + CH3

(R10)

C4 H8 + H = C3H6 + CH3

(R11)

C4 H8 + H = C2H4 + C2H5

(R12)

C4 H8 + H = saxC4 H 7 + H 2

(R13)

C4 H8 + H = 4‐C4 H 7 + H 2

(R14)

saxC4H 7 = 1, 3‐C4H6 + H

(R15)

Figure 14. Experimental mole fraction profiles (symbols) and simulated results (lines) of C2H4 and CH2O from n-butanol pyrolysis by the present model and Sarathy et al. 2012 model.20

Besides the unimolecular decomposition pathways, the hydrogen-abstraction reactions by the hydrogen atom producing four C4H8OH isomers also contribute to the destruction of nC4H9OH. Among the four C4H8OH isomers, aC4H8OH is the most preferred hydrogen-abstraction product, because the C− H bond is about 3−6 kcal/mol weaker than the others. aC4H8OH mainly decomposes to C2H3OH (ethenol) and C2H5 (R21) via β-C−C scission, while a few aC4H8OH produce n-butanal (C3H7CHO) through β-C−H scission (R22).

The C−C scission of nC4H9OH, producing nC3H7 (n-propyl radical) and CH2OH (R2), is the most important one among all possible C−C bond cleavage reactions because its bond dissociation energy is about 3 kcal/mol lower than the others. Figure 8 also demonstrates that the decomposition of nC4H9OH is sensitive to R2. nC3H7 produced by R2 mainly decomposes to C3H6 (R16) and C2H4 (R17) via β-scission. nC3H 7 = C3H6 + H

(R16)

nC3H 7 = C2H4 + CH3

(R17)

(R18)

CH 2OH + H = CH 2O + H 2

(R19)

CH 2OH + CH3 = CH 2O + CH4

(R20)

(R21)

aC4 H8OH = C3H 7CHO + H

(R22)

Figure 6 shows the experimental and simulated mole fractions of C3H7CHO and C2H3OH at various pressures. The maximum mole fractions of C3H7CHO detected in the pyrolysis experiment are about 20 times lower than detected ethenol. The present model predicts the mole fractions of nbutanal and ethenol within the experimental uncertainty at all pressures. Figure 12b shows the simulation results of C3H7CHO at 760 Torr by the present model and previous models.2,3,16,18,20,53 Most of the other models overpredicted the mole fraction of n-butanal, probably because of the overestimation of the R22/R21 branch ratio. The reactions involving C2H3OH are listed in Table 3. The simulation results show that the hydrogen-atom-assisted tautomerization reaction (R8) is the dominant pathway of ethenol consumption and CH3CHO formation. CH3CHO was detected and quantified in this experiment and illustrated in Figure 6. The predicted mole fraction profiles of C2H3OH by previous models at 760 Torr are presented in Figure 12d. The present

Further consumption of CH2OH leads to the formation of CH 2 O, which is mainly produced by R18−R20 and decomposes following the reaction sequence CH2O → HCO → CO. The mole fractions of CH2O and CO are well-predicted by the model (see Figure 7). CH 2OH = CH 2O + H

aC4 H8OH = C2H3OH + C2H5

As shown in Figure 3, the reaction rates of R2 in the Black et al. model,3 which were also adopted by the Sarathy et al. 2012 model,20 are about 10 times smaller than the rates calculated by 5561


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Figure 15. Measured (symbols) and predicted (solid lines) mole fraction profiles of major species in four premixed nC4H9OH flames: (a) ϕ = 1.80, (b) ϕ = 1.30, (c) ϕ = 1.00, and (d) ϕ = 0.70.

model and Sarathy et al. 2012 model20 reproduce the mole fractions of C2H3OH within the uncertainty of the experiment. The overprediction of the Black et al. model3 and Veloo et al. model53 is due to the absence of the C2H3OH + H = CH3CHO + H reaction (R8). It is noteworthy that the reaction pathways of C2H3OH are much simpler in the pyrolysis than those in the flames because the pyrolysis excludes the complex oxidation reactions with the O2 molecule, O atom, and HO2 radicals. The maximum mole fractions of the most stable intermediates, such as 1-C4H8 (see Figure 4), C3H6 (see Figure 5), and C2H3OH (see Figure 6), do not change very much with varying pressure. However, the mole fractions of the radicals decrease dramatically as the pressure increases. The mole fractions of aC3H5 and CH3 at 5 Torr are 1 magnitude higher than these detected at 30 Torr, as shown in Figure 5. The ROP analysis shows that only 50% of produced CH3 is consumed at 5 Torr but more than 99% of produced CH3 is consumed at 760 Torr. The low-pressure experiment favors radical detection because there are few collisions at low pressures. 4.2. n-Butanol Flames. Figure 15 shows the mole fraction profiles of major species in the four premixed nC4H9OH flames, including inert gas (Ar), reactants (nC4H9OH and O2), and major products (H2, H2O, CO, and CO2). The equilibrium mole fractions of major flame species at the postflame zone as well as the features of the measured profiles, such as the exhaust positions of reactants and bending points of Ar and major products, are well-predicted by the present model. Figures 16 and 17 present the measured and predicted mole fraction profiles of several key intermediates in the nC4H9OH flames. We also simulate the n-butanol premixed flames performed by Osswald et al.19 and Hensen et al.;18 the results are provided in Figures S1−S3 of the Supporting Information. Generally, the present model reproduces the mole fraction profiles of the intermediates within the experimental uncertainties.

Figure 16. Measured (symbols) and predicted (solid lines) mole fraction profiles of selected C3−C4 intermediate species in four premixed nC4H9OH flames: (blue square) ϕ = 1.80, (red circle) ϕ = 1.30, (green triangle) ϕ = 1.00, and (black star) ϕ = 0.70.

The reaction flux analysis shows that most nC4H9OH is consumed through hydrogen-abstraction reactions to produce four C4H8OH radical isomers. The contributions of unimolecular reactions are 18% in the rich flame (ϕ = 1.80) and less than 1% in the lean flame (ϕ = 0.70). The analysis on the unimolecular reactions will be omitted in this section, because they have been discussed in detail in the Pyrolysis Experiment 5562


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and lean conditions. The β-C−C scission reaction of aC4H8OH produces C2H3OH and C2H5 radical (R21). The involved C2H3OH chemistry in the present model is referred to previous studies.3,14,20,58−61 The simulated results show that most C2H3OH consumed isomerizes into CH3CHO (R7 and R8), which contributes to 75% of the CH3CHO formation under the rich condition and 30% under the lean condition. Figure 17 shows that the present model slightly overpredicts the mole fraction of C2H3OH and largely underpredicts the CH3CHO formation, especially under the lean condition. Besides C2H3OH isomerization, CH3CHO can also be produced by oxidation of the C2H5 radical and the decomposition of the CH3CH2O radical. It is possible that additional pathways involving the formation of CH3CHO have not been considered in the n-butanol flames. C2H3OH can also generate CH2CHO, CHCHOH, and CH2COH radicals through hydrogen-abstraction reactions, among which CH2CHO is the preferred product because the O−H bond is the weakest bond in C2H3OH.58 Further consumption of CH2CHO leads to the production of CH2CO (R23 and R24). The formation of CH2CO is also attributed to the oxidation of C2H2 and C2H3.

Figure 17. Measured (symbols) and predicted (solid lines) mole fraction profiles of selected C1−C2 intermediate species in four premixed nC4H9OH flames: (blue square) ϕ = 1.80, (red circle) ϕ = 1.30, (green triangle) ϕ = 1.00, and (black star) ϕ = 0.70.

CH 2CHO = CH 2CO + H

(R23)

CH 2CHO + O2 = CH 2CO + HO2

(R24)

The β-scission of the bC4H8OH radical (R25) is dominant for C4H8 formation. Further destruction of C4H8 is dominated by the H attack reaction to produce C3H6, C2H4, and two C4H7 isomers, which is similar to that in the pyrolysis process.

section. In the rich flame (ϕ = 1.80), 39% of nC4H9OH decomposes through OH attack, which consumes 78% of nC4H9OH in the lean flame (ϕ = 0.70). As mentioned above, aC4H8OH is the most preferred product of the hydrogenabstraction reactions. About 35% of nC4H9OH is consumed via H or OH attack to form the aC4H8OH radical under both rich

bC4 H8OH = C4 H8 + OH

(R25)

The β-C−C scission of cC4H8OH leads to the formation of C3H6 and CH2OH, while the latter is an important precursor of CH2O. Figures 16 and 17 present the experimental and

Figure 18. Measured (symbols) and predicted (solid lines) mole fraction profiles of reactants, intermediates, and products from n-butanol oxidation in the jet-stirred reactor16 at 1 atm and ϕ = 0.5. 5563


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Figure 19. Measured (symbols) and predicted (solid lines) mole fraction profiles of reactants, intermediates, and products from n-butanol oxidation in the jet-stirred reactor16 at 1 atm and ϕ = 1.0.

Figure 20. Measured (symbols) and predicted (solid lines) mole fraction profiles of reactants, intermediates, and products for n-butanol oxidation in the jet-stirred reactor16 at 1 atm and ϕ = 2.0.

On the basis of the above discussion, we conclude that the consumption pathways of nC4H9OH are similar in both the pyrolysis and premixed flames. The unimolecular decomposition pathways are more important in the pyrolysis, while the hydrogen-abstraction reactions are dominant in the flames. 4.3. n-Butanol Oxidation in the Jet-Stirred Reactor. The jet-stirred reactor is an important tool for studying the oxidation reactions of fuels. The oxidation of n-butanol in the

simulated results of C3H6 and CH2O. Most C3H6 decompose via H/OH attack to produce aC3H5 and C2H4. The main decomposition pathway of aC3H5 is through the reaction sequence aC3H5 → aC3H4 → pC3H4. The β-scission of dC4H8OH produces C2H4 and CH2CH2OH. Further decomposition of CH2CH2OH generates C2H4 (about 80%) and C2H3OH (about 20%) via β-scission at both the rich and lean conditions. 5564


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Figure 21. Ignition delay times of n-butanol measured by Black et al.,3 Moss et al.,2 and Stranic et al.7 (symbols) and predicted results (solid lines) by the present model.

jet-stirred reactor has been performed by Dagaut et al.,15,16 whose data have been tested against many previous n-butanol models.3,14,20 The present model was also validated against the n-butanol oxidation in the jet-stirred reactor at 1 atm16 and 10 atm.15 The simulation was performed using a perfectly stirred reactor code in the Chemkin-PRO program with an end time of 100 s to obtain steady-state mole fractions of oxidation species. Figures 18−20 present the simulation results at ϕ = 0.5, 1.0, and 2.0, respectively. It can be seen that the present model successfully predicts the experimental data, especially the sharp decrease of some species. The maximum mole fractions of most species are well-predicted, while the uncertainties are within a factor of 2 under all conditions. The simulated total mole fractions of C3H7CHO and butane-1-ol are compared to measured C3H7CHO, because these two isomers were not distinguished experimentally.16 As shown in Figures 18−20, the maximum mole fractions of C3H7CHO are well-predicted within the uncertainty by a factor of 2 at ϕ = 0.5 but are underpredicted at ϕ = 1.0 and 2.0. However, the simulation results of C3H7CHO in the jet-stirred reactor experiment at 10 atm15 are quite in agreement with the experimental results, which are provided in Figures S4−S6 of the Supporting Information. The ROP analysis shows that C3H7CHO is mainly produced by the oxidation of the 1-hydroxybutyl radical to form HO2 and C3H7CHO (R5). Some oxygenated species are underpredicted at low temperatures, such as C3H7CHO at 1 atm (900−1000 K) and CH2O and CH3CHO at 10 atm (800− 900 K). A possible reason is that the model does not account for the low-temperature oxidation chemistry, which is innegligible in this temperature region.

The main decomposition pathway of n-butanol in the oxidation experiment is hydrogen-abstraction reactions by H and OH producing four C4H8OH isomers. The further reactions of C4H8OH radicals are β-scission reactions, which are similar to those described in section 4.2. 4.4. Ignition Delay Times. The ignition delay times of nbutanol behind reflected shock waves have been measured over a wide range of pressures, temperatures, and equivalence ratios. The ignition delay times of n-butanol measured by Black et al.,3 Moss et al.,2 Stranic et al.,7 and Karwat et al.9 are used for validating the present model. The simulation was performed by running a constant-volume homogeneous batch reactor in Chemkin-Pro.51 The ignition delay times were determined as the point of maximum temperature rising, which corresponds to the time when reaching half the maximum of the OH concentration. The experimental and simulated results are illustrated in Figure 21. Black et al.3 measured the ignition delay times of n-butanol at pressures of 1, 2.6, and 8 atm and the temperature range from 1100 to 1800 K. Three equivalence ratios of 0.5, 1, and 2 were chosen for the measurements. At 1 atm, the present model can predict the ignition delay times quite well at ϕ = 0.5 and 1.0. However, the calculated ignition delay times at 1 atm and ϕ = 2.0 conditions are underpredicted. At 2.6 atm, two sets of measurements are both stoichiometric conditions but with different n-butanol mole fractions (0.6 and 3.5%). The ignition delay times predicted by the present model are consistent with experimental data at these two conditions. Moss et al.2 measured the ignition delay times for n-butanol/ O2/Ar mixtures from 1200 to 1800 K with pressure varying from 1 to 4 bar. They kept the equivalence ratio at 1.0 and 5565


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changed the mole fractions of n-butanol from 0.5 to 1.0%. Furthermore, they also kept the mole fractions of n-butanol at 1% and changed the equivalence ratios from 0.25 to 1.0. Figure 21d shows that the present model can accurately predicted these sets of data, except for those under the condition 1.3−1.4 bar and ϕ = 0.5, which are underpredicted in comparison to the experimental data. Recently, Stranic et al.7 measured the n-butanol ignition delay times at pressures from 1 to 45 atm. They also repeated n-butanol ignition delay time experiments at conditions used by Moss et al.2 and Black et al.3 However, the ignition delay times measured by Stranic et al.7 are shorter than other experiments.2,3 They stated that faulty post-shock temperature measurements would explain the disagreement with other experiments. They have checked their shock velocity measurement system and the thermodynamic properties of the driven gases and found that errors in temperature measurements in their work were unlikely. Panels e and f of Figure 21 show that the present model reproduces those data measured by Stranic et al. accurately under all studied conditions.

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-551-3602125. Fax: +86-551-5141078. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Natural Science Foundation of China (50925623), the National Basic Research Program of China (973 Program) (2012CB719701), and the Chinese Academy of Sciences.

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REFERENCES

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5. CONCLUSION The pyrolysis of n-butanol in the flow reactor at the pressures of 5, 30, 80, 200, and 760 Torr have been studied using SVUV− PIMS in this work. The intermediates were identified, and their mole fraction profiles were determined. A detailed kinetic model consisting of 121 species and 658 reactions was developed. The rate constants of unimolecular decomposition reactions of n-butanol and β-scission reactions of C4H8OH isomers at different pressures were calculated by the VRC− TST and RRKM/ME methods. Generally, the present model can predict the mole fractions of most pyrolysis species within experimental uncertainty at all investigated pressures. ROP analysis shows that unimolecular reactions are the key reactions of n-butanol pyrolysis, especially at low pressure. Sensitivity analysis indicates that the mole fractions of the fuel and the primary products show large sensitivity to the submechanism of fuel and primary products. Enol−keto isomerization assisted by a hydrogen atom is of great importance in successful prediction of C2H3OH and CH3CHO mole fractions. This reaction is the dominant pathway for C2H3OH consumption and CH3CHO formation. The mole fractions of radicals decrease dramatically as the pressure increases. Thus, the experiment is better to be performed at low pressures to study radical chemistry. The premixed n-butanol flames with four equivalence ratios (0.70, 1.00, 1.30, and 1.80) were performed for further validation of our model, especially for the hydrogen-abstraction reactions of n-butanol and β-scission of C4H8OH radicals. Furthermore, the present model was also validated by oxidation in a jet-stirred reactor and ignition delay times in a shock tube at pressures ranging from 1 to 45 atm and equivalence ratios of 0.25−2.0. Good agreement between the simulated results and various experimental data indicates that the present model is suitable for a wide range of conditions with the temperatures of 800−2200 K, the pressures of 5−7600 Torr, and the equivalence ratios from 0.25 to the pyrolysis condition.

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ASSOCIATED CONTENT

* Supporting Information S

Reaction mechanism, thermodynamic data, transport data, and measured and predicted mole fraction profiles (Figures S1− S6). This material is available free of charge via the Internet at http://pubs.acs.org. 5566


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