Experimental and kinetic modeling study of n-pentanol pyrolysis and combustion

Combustion and Flame xxx (2015) xxx–xxx

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Combustion and Flame j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e

Experimental and kinetic modeling study of n-pentanol pyrolysis and combustion Gao Wang a, Wenhao Yuan a, Yuyang Li b,⇑, Long Zhao a, Fei Qi a,b,⇑ a b

National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, PR China Key Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong University, Shanghai 200240, PR China

a r t i c l e

i n f o

Article history: Received 10 May 2015 Received in revised form 17 May 2015 Accepted 20 May 2015 Available online xxxx Keywords: n-Pentanol Flow reactor pyrolysis Laminar premixed flame SVUV-PIMS Kinetic model

a b s t r a c t The flow reactor pyrolysis of n-pentanol at 30, 150, and 760 Torr and laminar premixed flames of n-pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr are investigated using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis products and flame species are identified and the mole fraction profiles are measured. A detailed kinetic model of n-pentanol is developed and validated on the new experimental results. The C–C bond dissociation reactions play a significant role in the pyrolysis of n-pentanol. The contribution of the water elimination reaction becomes less important in the pyrolysis of n-pentanol than in the pyrolysis of n-butanol. Olefins and CnH2nO species are found to be the two major product families in the pyrolysis and combustion of n-pentanol, and specific products are also observed for most of the H-atom abstraction reactions of n-pentanol. The model is further validated on the experimental data of n-pentanol combustion in literature, including the species profiles in jet-stirred reactor oxidation, the laminar flame speeds and ignition delay times. Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction Biofuels play a significant role in meeting the challenge of energy sustainability caused by rapid consumption of fossil fuels, and its percentage in global transport fuel is expected to increase from around 2% in 2011 to 27% by 2050 [1]. As prospective biofuels, long chain alcohols like n-butanol and n-pentanol have a lot of advantages over ethanol which is the first generation biofuel, such as higher energy density, better miscibility with fossil-derived transport fuels, lower water absorption, and higher suitability for conventional engines [2]. In particular, growing attentions have been paid to the novel production method of n-pentanol from biomass in recent years [3,4], since it has more similar physical and chemical properties to gasoline than n-butanol. Compared with the experimental studies of n-butanol combustion, only a limited number of experimental studies have been performed on n-pentanol combustion [5], most of which focused on the measurement of global combustion parameters such as ignition delay times and laminar flame speeds. Yacoub et al. [6] and Gautam et al. [7,8] investigated the performance and emission characteristics of pentanol/gasoline blends in engines. Heufer ⇑ Corresponding authors at: Key Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong University, Shanghai 200240, PR China (Y.Y. Li and F. Qi). E-mail addresses: [email protected] (Y. Li), [email protected] (F. Qi).

et al. [9] investigated the ignition delay times of n-pentanol at the pressures of 9–30 bar, temperatures of 640–1200 K, and equivalence ratio (/) of 1.0 using a shock tube and a rapid compression machine (RCM). Tang et al. [10] measured the high temperature ignition delay times of C5 primary alcohols behind reflected shock waves at the pressures of 1.0 and 2.6 atm, temperatures of 1100– 1500 K, equivalence ratios of 0.25, 0.5, and 1.0, and initial fuel mole fractions of 0.25% and 0.50%. Togbé et al. [11] measured the laminar flame speeds of n-pentanol at the pressure of 1 atm and initial temperature of 423 K. Li et al. [12] measured the laminar flame speeds and flame instability of n-pentanol at the pressures of 0.10–0.75 MPa and initial temperatures of 393–473 K. Up to now, only Togbé et al. [11] performed a speciation measurement on n-pentanol combustion. They measured the concentration profiles of stable oxidation products using gas chromatography in the jet-stirred reactor (JSR) oxidation of n-pentanol at the pressure of 10 atm, temperatures of 770–1220 K, and equivalence ratios of 0.35, 0.5, 1, 2, and 4. Besides the experimental work, Zhao et al. [13] performed a theoretical investigation on the unimolecular decomposition reactions of pentanol isomers using CBS-QB3 method and calculated the pressure-dependent rate constants by solving the RRKM/master equation. Compared with the great number of kinetic models of n-butanol, only two kinetic models have been developed for n-pentanol [5]. Togbé et al. [11] developed the first kinetic model of n-pentanol and validated it on their JSR oxidation and laminar flame speed

http://dx.doi.org/10.1016/j.combustflame.2015.05.017 0010-2180/Ó 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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G. Wang et al. / Combustion and Flame xxx (2015) xxx–xxx

data. Then Heufer et al. [14] developed the other model and validated it on their own ignition delay time data [9] and the JSR oxidation and laminar flame speed data reported by Togbé et al. [11]. It is recognized that the validation of n-pentanol models is limited due to the very few speciation information on n-pentanol combustion. Consequently, new experimental efforts on the speciation of n-pentanol pyrolysis and combustion, as well as a detailed kinetic model with comprehensive validation, are needed. In this work, the flow reactor pyrolysis of n-pentanol at various pressures (30, 150, and 760 Torr) and the low pressure (30 Torr) laminar premixed flames of n-pentanol at lean and rich conditions (/ = 0.7 and 1.8) are investigated using the synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis and flame species are detected and their mole fractions are evaluated. A detailed kinetic model of n-pentanol is also developed and validated on the new data measured in this work and the experimental data of n-pentanol combustion in literature, such as species profiles in the JSR oxidation at 10 atm and global combustion parameters including ignition delay times and laminar flame speeds at a variety of conditions. 2. Experimental method 2.1. Flow reactor pyrolysis Both the flow reactor pyrolysis and laminar premixed flame experiments are carried out at National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. Descriptions of the beamlines and flow reactor pyrolysis apparatus used in this work have been reported in detail previously [15–17]. In brief, the pyrolysis apparatus is composed of three chambers, i.e. a pyrolysis chamber to hold the flow reactor, a differentially pumped chamber to sample the pyrolysis species and transfer the formed molecular beam to the photoionization region, and a photoionization chamber to ionize the pyrolysis species with the synchrotron VUV light. The formed ions are detected by a home-made reflectron time-of-flight mass spectrometer (RTOF-MS). Table 1 lists the experimental conditions of the flow reactor pyrolysis of n-pentanol in this work. The gas flow rate of Ar is 970 standard cubic centimeters per minute (SCCM), and the liquid flow rate of n-pentanol is 0.145 ml/min (equal to 30 SCCM in gas phase). Therefore the inlet mole fractions of fuel and Ar are 3% and 97%, respectively. The fuel is vaporized in a vaporizer heated at 453 K and mixed with Ar before entering the flow tube. The flow tube with an inner diameter of 7.0 mm and a heating length of 150 mm is made of a-Al2O3 to reduce wall catalytic effects [18– 20]. Three pressures in the pyrolysis chamber are investigated, i.e. 30, 150, and 760 Torr. Detailed introduction of the method of temperature measurement using an S-type thermocouple along the flow tube centerline can be found in our previous work [16,21,22]. Each measured temperature profile is named as its maximum value (Tmax). The uncertainty of Tmax is estimated to be within ±30 K. Descriptions of the methods of species identification and mole fraction evaluation have been described in detail elsewhere [16]. The uncertainties of measured mole fractions are estimated to be within ±25% for pyrolysis products with known

Table 1 Experimental conditions of the flow reactor pyrolysis of n-pentanol in this work.

Table 2 Experimental conditions of the laminar premixed flames of n-pentanol in this work. /

C/O

P (Torr)

Xn-pentanol

XO2

XAr

V (cm/s)

0.7 1.8

0.22 0.54

30 30

0.043 0.097

0.457 0.403

0.500 0.500

60 60

Note: Xi is the inlet mole fraction of species i; V is the flow velocity of the inlet mixture at 300 K.

photoionization cross sections (PICSs), and a factor of 2 for those with estimated PICSs. The PICSs of pyrolysis species are available from the online database [23]. 2.2. Laminar premixed flame The laminar premixed flame apparatus at NSRL has been introduced in detail in our previous work [15,17,24,25]. In brief, the apparatus consists of a low-pressure flame chamber, a differentially pumped chamber with a molecular-beam sampling system, and a photoionization chamber with a home-made RTOF-MS. The flame is stabilized on a 60-mm-diameter McKenna burner in the flame chamber. A quartz nozzle with a 500 lm orifice on the tip and a 40° cone angle is used to sample the flame species. A molecular beam formed by the sampled flame species passes through a nickel skimmer with a 1.5 mm aperture and enters the photoionization chamber where it is crossed and ionized by the synchrotron VUV light. The ions are detected by the RTOF-MS. Movement of the burner controlled by a step motor allows the flame species to be sampled at any desired position along the flame centerline. Table 2 lists the experimental conditions of the laminar premixed flames of n-pentanol in this work. Two flames with equivalence ratios of 0.7 and 1.8 are investigated at 30 Torr (4.0 kPa). The vaporization and inlet processes are similar to those in the pyrolysis experiment, and will not be introduced in detail. The flame temperature is measured using a 0.10-mm-diameter Pt–6%Rh/Pt– 30%Rh thermocouple coated with Y2O3–BeO anti-catalytic ceramic to inhibit the catalytic effects [26], and is corrected for radiation heat loss [27] and cooling effects of the sampling nozzle [28]. The uncertainty of the maximum flame temperature is estimated to be within ±100 K. Methodologies of intermediate identification and mole fraction evaluation have been reported in detail previously [25,29]. The uncertainties of measured mole fractions are estimated to be around ±10% for major flame species, ±25% for intermediates with known photoionization cross sections (PICSs), and a factor of 2 for those with estimated PICSs. The PICSs of flame species are also available from the online database [23]. 3. Kinetic modeling A detailed model of n-pentanol (C5H11OH) consisting of 314 species and 1602 reactions is developed in this work. The C0–C4 sub-mechanism used in the present model is mainly taken from our recently reported models of butanol isomers [21,22], and a new sub-mechanism of n-pentanol is constructed. Table S1 in the Supplementary Material lists some important reactions in the sub-mechanism of n-pentanol. The pressure-dependent rate constants of the water (H2O) elimination reaction (R1) and C–C bond dissociation reactions (R2–R5) of n-pentanol are taken from the recent theoretical work reported by Zhao et al. [13].

P (Torr)

T (K)

Xn-pentanol

XAr

Total flow Rate (SCCM)

C5 H11 OH ¼ C5 H10 þ H2 O

ðR1Þ

30 150 760

910–1390 910–1255 810–1155

0.03 0.03 0.03

0.97 0.97 0.97

1000 1000 1000

C5 H11 OH ¼ pC4 H9 þ CH2 OH

ðR2Þ

C5 H11 OH ¼ nC3 H7 þ C2 H4 OH

ðR3Þ

Note: Xi is the inlet mole fraction of species i.

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G. Wang et al. / Combustion and Flame xxx (2015) xxx–xxx Table 3 Summary of the experimental data of n-pentanol used to validate the present model. Experimental type

Temperature

Pressure

Equivalence ratio

Reference

Flow reactor pyrolysis Laminar premixed flame Jet-stirred reactor oxidation

810–1390 K 400–2100 K 770–1220 K

30, 150, 760 Torr 30 Torr 10 atm

1 0.7, 1.8 0.35, 0.50, 1.00, 2.00, 4.00

This work This work [11]

Laminar flame speed

Tu = 423 K Tu = 393 K Tu = 433 K Tu = 473 K

1.0 atm 0.10 MPa 0.10, 0.25, 0.50, 0.75 MPa 0.10 MPa

0.7–1.4 0.7–1.8 0.6–1.8 0.6–1.8

[11] [12] [12] [12]

Ignition delay time

1100–1500 K 640–1200 K

1.0, 2.6 atm 9–30 bar

0.25, 0.50, 1.00 1.0

[10] [9]

C5 H11 OH ¼ C2 H5 þ cC3 H6 OH

ðR4Þ

C5 H11 OH ¼ CH3 þ dC4 H8 OH

ðR5Þ

The rate constants of the H-atom abstraction reactions of n-pentanol by H atom (R6–R10) are estimated from the calculated results of the analogue reactions of n-butanol [30]. The H-atom abstraction reactions of n-pentanol by OH radical (R11–R15), HO2 radical, and O2 are taken from the oxidation model of n-pentanol reported by Heufer et al. [14], while the H-atom abstraction reactions of n-pentanol by O atom and other small radicals (e.g. methyl, formyl, and ethyl radicals) are referred to the analogous reactions of n-butanol [31]. For five C5H10OH radicals, the rate constants of their b-scission reactions are estimated from the b-scission reactions of four C4H8OH radicals [21]. The sub-mechanism of 1-pentene (C5H10) is mainly taken from the JetSurF 2.0 model [32] and partly from the experimental work of 1-hexene decomposition investigated by Kiefer et al. [33]. The low temperature oxidation sub-mechanism used to simulate the ignition delay times at low temperatures is adopted from the oxidation model of n-pentanol reported by Heufer et al. [14]. The thermodynamic data of C0– C4 species used in this work are mainly taken from the previous models of butanol isomers [21,22]. The thermodynamic data of the species involved in the sub-mechanism of n-pentanol are primarily taken from the previous model of n-pentanol [14] and the database reported by Burcat and Ruscic [34]. The transport data are also mainly taken from the previous models of butanol isomers [21,22] and n-pentanol [14]. The simulation work is performed with the Chemkin-PRO software [35] and the modules used to simulate various validation experiments are introduced in the following section.

C5 H11 OH þ H=OH ¼ aC5 H10 OH þ H2 =H2 O

ðR6=R11Þ

C5 H11 OH þ H=OH ¼ bC5 H10 OH þ H2 =H2 O

ðR7=R12Þ

C5 H11 OH þ H=OH ¼ cC5 H10 OH þ H2 =H2 O

ðR8=R13Þ

C5 H11 OH þ H=OH ¼ dC5 H10 OH þ H2 =H2 O

ðR9=R14Þ

C5 H11 OH þ H=OH ¼ eC5 H10 OH þ H2 =H2 O

ðR10=R15Þ

4. Results and discussion The new experimental data of flow reactor pyrolysis and laminar premixed flame of n-pentanol are reported in Sections 4.1 and 4.2, respectively. As shown in Table 3, a vast amount of experimental data of n-pentanol combustion have been used to validate the present model, including both species profiles and global combustion parameters: (1) Flow reactor pyrolysis in this work. (2) Laminar premixed flames in this work.

(3) Jet-stirred reactor oxidation. (4) Ignition delay times. (5) Laminar flame speeds. 4.1. Flow reactor pyrolysis More than 20 pyrolysis products, especially some radicals such as methyl (CH3), propargyl (C3H3), and allyl (aC3H5) radicals and unstable molecules like ethenol (C2H3OH), are detected in the pyrolysis of n-pentanol. The simulation work is performed with the Plug Flow Reactor module in the Chemkin-PRO software [35]. The measured temperature distributions along the flow tube centerline are used in the simulation, while the pressure distributions are not considered since the pressure drops along the centerline were found to be very small at pressures greater than 30 Torr [21,36]. The experimental and simulated mole fraction profiles of n-pentanol, H2, hydrocarbon products, and oxygenated products are shown in Figs. 1–3. The rate of production (ROP) analysis and sensitivity analysis are performed at 1330 K for the 30 Torr pyrolysis and 1130 K for the 760 Torr pyrolysis to explore the key reactions in the decomposition of n-pentanol at low and atmospheric pressures. At the two conditions, the conversion ratios of n-pentanol are both around 75%, while all detected products have been produced and most of them reach high concentration levels. The discussion in this section will also be focused on the two pressures. Figures 4 and S1 illustrate the reaction network at 30 and 760 Torr summarized from the ROP analysis, respectively. 4.1.1. Primary decomposition of n-pentanol Figure 1a shows the experimental and simulated mole fraction profiles of n-pentanol at the three pressures investigated, demonstrating the good performance of the present model in simulating the decomposition of n-pentanol. For the primary decomposition of n-pentanol, the ROP analysis demonstrates that the unimolecular decomposition reactions and the H-atom abstraction reactions are the two major types of reactions. 43% and 15% of n-pentanol decomposes through the unimolecular decomposition reactions at 30 and 760 Torr, respectively. The recent theoretical investigation on the unimolecular decomposition of n-pentanol [13] demonstrates that two types of unimolecular decomposition reactions are the most favored. The water elimination reaction producing 1-pentene (R1) is the unimolecular decomposition reaction of n-pentanol with the lowest energy barrier (67.5 kcal/mol [13]), and plays an important role in the consumption of n-pentanol at pyrolysis conditions (8% and 4% at 30 and 760 Torr, respectively). The experimental and simulated mole fraction profiles of 1-pentene and water are shown in Figs. 1b and 3f, respectively. By comparing the concentration levels of water elimination products of n-pentanol and n-butanol, it is noticed that the maximum mole fractions of 1-pentene in the pyrolysis of n-pentanol are much lower than those of 1-butene (1-C4H8) in the pyrolysis of n-butanol [21]. For example, the maximum mole fraction of

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Fig. 1. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) n-pentanol, (b) 1-pentene, (c) 1-butene, (d) 2-butene, (e) 1,3-butadiene, and (f) vinylacetylene in the pyrolysis of n-pentanol.

Fig. 2. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) propene, (b) allyl radical, (c) allene, (d) propyne, (e) propargyl radical, (f) ethylene, (g) acetylene, (h) methane, and (i) hydrogen in the pyrolysis of n-pentanol.

1-pentene in the 30 Torr pyrolysis of n-pentanol is only 5.6  104, while that of 1-butene in the 30 Torr pyrolysis of n-butanol is 2.0  103 at the same inlet mole fraction of fuels (3.0  102) [21]. This phenomenon indicates the decreasing importance of the water elimination reaction in the pyrolysis of n-alcohols as the length of fuel carbon skeleton increases, which is closely related to its decreasing branching ratio in the unimolecular decomposition reactions of n-alcohols [13,21]. The relatively low reaction fluxes of the water elimination reaction of n-pentanol also influence its contributions to the formation of 1-pentene. The water elimination reaction of n-pentanol contributes about 30– 60% to the formation of 1-pentene, while the water elimination of n-butanol contributes about 40–80% to the formation of 1-butene at similar fuel conversion ratios [21].

Among the four types of bonds in n-pentanol, the C–C bonds have the lowest bond dissociation energies (BDEs) [37]. Therefore the C–C bond dissociation reactions (R2), (R3), (R4), (R5) play a much more important role in the primary decomposition of n-pentanol than the C–H, C–O, and O–H bond dissociation reactions. Here the different carbon groups in n-pentanol are labeled as Ce– Cd–Cc–Cb–Ca–OH. The ROP analysis shows that the dissociation reactions of Ca–Cb, Cb–Cc and Cc–Cd bonds (R2), (R3), (R4) consume comparable portions of n-pentanol to the water elimination reaction (R1) at both 30 and 760 Torr, while the contribution of Cd–Ce bond dissociation reaction (R5) is much lower than those of R2– R4. To simply the figures, R5 is not shown in Figs. 4 and S1. Besides the unimolecular decomposition reactions, the H-atom abstraction reactions of n-pentanol by H atom, OH radical, and CH3

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Fig. 3. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) 2-propen-1-ol, (b) ethenol, (c) acetaldehyde, (d) formaldehyde, (e) carbon monoxide, and (f) water in the pyrolysis of n-pentanol.

Fig. 4. Reaction network of n-pentanol decomposition at 1330 K and 30 Torr in the pyrolysis of n-pentanol. The thickness of each arrow denotes the reaction flux of corresponding pathway. The number on each arrow represents the percentage in the total reaction flux of n-pentanol decomposition. The blue arrows denote the unimolecular decomposition pathways of n-pentanol. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

radical consume almost the rest of n-pentanol. The dominant products are five C5H10OH radicals, while only a very limited portion of n-pentanol can suffer H-atom abstraction from the O–H bond forming C5H11O radical. Among the H-atom abstraction reactions, those by H atom (R6–R10) contribute the most to the consumption of n-pentanol, and they are also the main source of H2. Different from the situations in the pyrolysis of hydrocarbon fuels [16,36,38,39], the H-atom abstraction reactions by OH radical (R11–R15) also play an important role in the decomposition of n-pentanol because of the high production rate of OH radical.

Fig. 5. Sensitivity analysis of n-pentanol at 1330 K, 30 Torr (gray) and 1130 K, 760 Torr (black) in the pyrolysis of n-pentanol.

Figure 5 shows the sensitivity analysis of n-pentanol at 30 and 760 Torr, respectively. The unimolecular decomposition reactions show very high sensitivities to the consumption of n-pentanol. In particular, the dissociation reactions of Ca–Cb, Cb–Cc and Cc–Cd bonds (R2), (R3), (R4) are the top three sensitive reactions at both 30 and 760 Torr since they are the most important chain initiation steps at the pyrolysis conditions investigated. The sensitivity analysis also reveals that the H-atom abstraction reactions become much more influential on the decomposition of n-pentanol at 760 Torr than at 30 Torr, which is consistent with the results of ROP analysis. 4.1.2. Decomposition of primary decomposition products Due to the great reaction fluxes from the H-atom abstraction reactions, C5H10OH radicals play significant roles in the further decomposition of n-pentanol. The five carbon groups in n-pentanol

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G. Wang et al. / Combustion and Flame xxx (2015) xxx–xxx

correspond to five C5H10OH radicals, which are named as aC5H10OH (C–C–C–C–C–OH), bC5H10OH (C–C–C–C–C–OH), cC5H10OH (C–C–C–C–C–OH), dC5H10OH (C–C–C–C–C–OH), and eC5H10OH (C–C–C–C–C–OH) in this model. According to the ROP analysis, these C5H10OH radicals mainly decompose through b-scission reactions, especially b-C–C and b-C–O scission reactions. The decomposition of aC5H10OH radical almost totally proceeds through the b-C-C scission reaction (R16) producing n-propyl radical (nC3H7) and ethenol (C2H3OH). As seen from Figs. 4 and S1, n-propyl radical can also be produced from the Cb–Cc bond dissociation reaction of n-pentanol (R3), and is dominantly consumed via the b-C–C scission reaction (R17) producing ethylene (C2H4) and methyl radical. Ethenol is exclusively produced through R16, and it can be consumed by the isomerization reactions (R18), (R19) producing acetaldehyde (CH3CHO).

aC5 H10 OH ¼ nC3 H7 þ C2 H3 OH

ðR16Þ

nC3 H7 ¼ C2 H4 þ CH3

ðR17Þ

C2 H3 OH ¼ CH3 CHO

ðR18Þ

C2 H3 OH þ H ¼ CH3 CHO þ H

ðR19Þ

Acetaldehyde is known as an important oxygenated pollutant [40,41], and its formation is a crucial topic in the combustion studies of alcohols [2]. In the previous modeling study of n-pentanol oxidation by Togbé et al. [11], a direct decomposition reaction of aC5H10OH radical was proposed to form acetaldehyde. However, by adopting this reaction in the present model, the predicted formation temperatures of acetaldehyde become much lower than the experimental results in this work. This observation indicates that acetaldehyde is probably not formed from the direct decomposition of aC5H10OH radical, especially considering the strong competition of R16 in the consumption of aC5H10OH radical. As seen from Fig. 3b and c, ethenol is produced more than 50 K earlier than acetaldehyde and has a comparable concentration level to acetaldehyde at each pressure. By including the two isomerization reactions of ethenol (R18), (R19), the present model can reproduce the experimental results of both ethenol and acetaldehyde. The ROP analysis indicates that R18 and R19 dominate the consumption of ethenol and produce more than 96% of acetaldehyde at both 30 and 760 Torr. Therefore ethenol is concluded as the most important precursor of acetaldehyde in the pyrolysis of n-pentanol. As shown in Figs. 4 and S1, bC5H10OH radical can mainly decompose through two pathways, i.e. the b-C–O scission reaction (R20) producing 1-pentene + OH radical and the b-C–C scission reaction (R21) producing ethyl radial (C2H5) + 2-propen-1-ol (C3H5OH). As mentioned above, 1-pentene can also be directly formed from the water elimination of n-pentanol (R1). R1 is found to be the dominant formation pathway of 1-pentene at 30 Torr, while its importance is replaced by R20 at 760 Torr. Due to the existence of an allylic C–C bond with an extremely low BDE (74.3 kcal/mol [37]), 1-pentene is very easy to decompose via the allylic C–C bond dissociation reaction (R22) producing allyl and ethyl radicals. This reaction contributes over 50% to the consumption of 1-pentene at both 30 and 760 Torr. Besides, 1-pentene can also be consumed by a series of reactions with H atom, e.g. the ethyl-substitution by H atom (R23) producing propene (C3H6) and ethyl radical, respectively. For 2-propen-1-ol, its consumption mainly proceeds through reactions with H atom, producing propene + OH radical and ethylene + hydroxymethyl radical (CH2OH).

bC5 H10 OH ¼ C5 H10 þ OH

ðR20Þ

bC5 H10 OH ¼ C2 H5 þ C3 H5 OH

ðR21Þ

C5 H10 ¼ aC3 H5 þ C2 H5

ðR22Þ

C5 H10 þ H ¼ C3 H6 þ C2 H5

ðR23Þ

The decomposition of cC5H10OH radical is dominated by two b-C–C scission reactions producing 1-butene + hydroxymethyl radical (R24) and 3-buten-1-ol (C4H7OH) + methyl radical (R25). R24 is the controlling step and contributes more than 94% to the consumption of cC5H10OH radical at the conditions investigated. The results of 1-butene are shown in Fig. 1c. It is mainly consumed through the methyl-substitution by H atom producing propene and the H-atom abstraction reactions producing C4H7 radicals which are major precursors of smaller C4 products shown in Fig. 1(e and f).

cC5 H10 OH ¼ 1  C4 H8 þ CH2 OH

ðR24Þ

cC5 H10 OH ¼ CH3 þ C4 H7 OH

ðR25Þ

The ROP analysis demonstrates that dC5H10OH radical attracts more reaction fluxes than other C5H10OH radicals except aC5H10OH radical, which reveals the major difference between the reactivity of n-butanol and n-pentanol. As concluded by Heufer et al. [14], the longer carbon skeleton of n-pentanol enables an H-atom abstraction reaction on a secondary ‘‘alkane-like’’ carbon at the d-position, while in butanol, the four carbon atoms can only be divided as two types, i.e. the hydroxyl-influenced carbons at the a- to c-positions and a primary ‘‘alkane-like’’ carbon at the d-position. The decomposition of dC5H10OH radical almost totally proceeds through the b-C–C scission reaction (R26) producing propene and 2-hydroxyethyl radical (C2H4OH). The latter product can easily decompose to ethylene and OH radical. The experimental and simulated mole fraction profiles of propene are shown in Fig. 2a.

dC5 H10 OH ¼ C3 H6 þ C2 H4 OH

ðR26Þ

The decomposition of eC5H10OH radical is dominated by the b-C–C scission reaction (R27) producing ethylene and 3-hydroxylpropyl radical (cC3H6OH). The latter product quickly converts to ethylene and hydroxymethyl radical via R28. In the pyrolysis of n-pentanol, ethylene is the most abundant hydrocarbon product and can be produced from a variety of pathways. The most important pathways are shown in Figs. 4 and S1. As discussed above, hydroxymethyl radical can also be produced from the Ca–Cb bond dissociation reaction of n-pentanol (R2) and the b-C–C scission reaction of cC5H10OH radical (R24). It is mainly consumed by the b-O–H scission reaction producing formaldehyde which is another important oxygenated pollutant [40].

eC5 H10 OH ¼ C2 H4 þ cC3 H6 OH

ðR27Þ

cC3H6 OH ¼ C2 H4 þ CH2 OH

ðR28Þ

In general, the main decomposition pathways of n-pentanol are quite similar at 30 and 760 Torr while the contributions of individual pathways change with pressures, as shown in Figs. 4 and S1. Olefins (e.g. ethylene, propene, 1-pentene, and 1-butene) and CnH2nO species (e.g. formaldehyde, ethenol, acetaldehyde, and 2-propen-1-ol) are observed as the two major product families. Specific decomposition products are observed for most of C5H10OH radicals in this work, such as ethenol for aC5H10OH radical, 2-propen-1-ol for bC5H10OH radical, 1-butene for cC5H10OH radical, and propene for dC5H10OH radical. Consequently, the measurements of these decomposition products in this work provide useful experimental data for validating the H-atom abstraction reactions of n-pentanol. Figure. S2 shows the simulated results of

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the 30 Torr pyrolysis by the present model and previous n-pentanol models reported by Heufer et al. [14] (referred as the Heufer model) and Togbé et al. [11] (referred as the Togbé model). 4.2. Laminar premixed flames More than 30 flame species are detected in the laminar premixed flames of n-pentanol at lean (/ = 0.7) and rich (/ = 1.8) conditions. Figure 6 shows the experimental and simulated mole fraction profiles of major flame species in the two flames, including reactants (n-pentanol and O2), inert gas (Ar), and major products (H2, H2O, CO, and CO2). The simulation is conducted with the Premixed Burner Stabilized Flame module in the Chemkin-PRO software [35] using the measured temperature profiles as input parameters. The experimental and simulated mole fraction profiles of intermediates are shown in Figs. 7–9. In general, the present model reasonably predicts the experimental results. The ROP analysis is also performed for both the lean and rich flames. Due to the abundant production of radicals in flames, the H-atom abstraction reactions become more dominant to the consumption of n-pentanol in both flames than in the pyrolysis. The ROP analysis indicates that around 76% of n-pentanol is consumed by the H-atom abstraction reactions in the rich flame, while in the lean flame, almost all of n-pentanol is consumed by the H-atom abstraction reactions. Among different types of H-atom abstraction reactions of n-pentanol, the H-atom abstraction by OH radical plays the most important role in both flames, consuming about 74% and 38% of n-pentanol in the lean and rich flames, respectively. The second important one is the H-atom abstraction by H atom. It contributes over 16% to the decomposition of n-pentanol in the lean flame, while in the rich flame its contribution (32%) becomes comparable to the H-atom abstraction by OH radical due to the reduced oxidative circumstance. Therefore the premixed flame experiments provide useful validation to the H-atom abstraction reactions by H atom and OH radical. Other H-atom abstraction

Fig. 6. Experimental (symbols) and simulated (solid lines) mole fraction profiles of major species and measured temperature profiles (dash lines) in the laminar premixed flames of n-pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr.

Fig. 7. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) 1-pentene, (b) 1,3-pentadiene, (c) 1-butene, (d) 2-butene, (e) 1,3-butadiene, and (f) vinylacetylene in the laminar premixed flames of n-pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr.

reactions consume less than 10% of n-pentanol together in both flames. Furthermore, the unimolecular decomposition reactions of n-pentanol behave quite differently in the lean and rich flames. These reactions have only negligible contributions to the decomposition of n-pentanol in the lean flame, while they become important in the rich flame and consume almost the rest of n-pentanol. The major difference in the consumption of C5H10OH radicals is that they can also react with O2 to form enols and aldehydes in flames. For example, aC5H10OH radical can be consumed via reactions with O2 to form n-pentanol (C4H9CHO) (R29) and aC5H10OH–OO radical (R30). The oxidation reactions are much more significant in the lean flame. For aC5H10OH radical, R29 contributes around 26% to its consumption in the rich flame, while the contribution increases to 60% in the lean flame. The rest of aC5H10OH radical is mainly consumed by the b-C–C scission reaction (R16) producing n-propyl radical and ethenol. Similar to the situation in the pyrolysis, n-propyl radical is dominantly consumed via the b-C–C scission reaction (R17) to produce ethylene and methyl radical. For ethenol, its main consumption pathway in the rich flame is still the isomerization to acetaldehyde, not only via the enol-keto tautomerization (R18) and the isomerization reaction catalyzed by H atom (R19), but also the isomerization reaction catalyzed by HO2 radical (R31). However in the lean flame the H-atom abstraction reactions of ethenol by OH radical, O atom, and H atom become slightly more favored.

aC5 H10 OH þ O2 ¼ C4 H9 CHO þ HO2

ðR29Þ

aC5 H10 OH þ O2 ¼ aC5 H10 OH  OO

ðR30Þ

C2 H3 OH þ HO2 ðþMÞ ¼ CH3 CHO þ HO2 ðþMÞ

ðR31Þ

For other C5H10OH radicals, the most favored pathways are still the b-scission reactions, while the oxidation reactions only have minor contributions. For example, the reactions of bC5H10OH radical with O2 only contribute 10% and 1% to its consumption in the lean and rich flames, respectively, while more than 70% of bC5H10OH radical decomposes to 1-pentene and OH radical via R20 and the rest decomposes to ethyl radical and 2-propen-1-ol

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Fig. 8. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) propene, (b) allyl radical, (c) allene, (d) propyne, (e) propargyl radical, (f) ethyl radical, (g) ethylene, (h) acetylene, and (i) methyl radical in the laminar premixed flames of n-pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr.

Fig. 9. Experimental (symbols) and simulated (solid lines) mole fraction profiles of (a) acrolein, (b) acetaldehyde, (c) ethenol, (d) ketene, (e) methanol, and (f) formaldehyde in the laminar premixed flames of n-pentanol with equivalence ratios of 0.7 and 1.8 at 30 Torr.

via R21. The most important consumption pathways of 1-pentene in the rich flame are the allylic C–C bond dissociation producing allyl and ethyl radicals (R22) and the ethyl-substitution by H atom (R23). In the lean flame the contribution of the bond dissociation reaction becomes less than 5%. 1-Pentene can also be consumed to produce C5H9 radicals which can subsequently convert to pentadiene isomers (C5H8). A comparison of Fig. 7a and c shows that the experimental and simulated maximum mole fractions of 1-butene are around 2 times as those of 1-pentene, which is similar to the situations in the

pyrolysis. The ROP analysis indicates that 1-pentene and 1-butene are the most favored decomposition products of bC5H10OH and cC5H10OH radicals, respectively. Therefore the observation of much higher concentrations of 1-butene than 1-pentene exhibits the greater importance of c-H-atom abstraction than b-H-atom abstraction in the consumption of n-pentanol, which is also supported by the lower BDE of the Cc–H bond than the Cb–H bond. The main decomposition pathways of 1-butene in the flames are almost the same as those in the pyrolysis, and will not be repeated herein. The alkane-like dC5H10OH and eC5H10OH radicals mainly decompose via b-C–C scission reactions forming propene + 2-hydroxyethyl radical and ethylene + 3-hydroxypropyl radical, respectively. Both 2-hydroxyethyl and 3-hydroxypropyl radicals ultimately converts to ethylene. Based on the discussion above, it can be concluded that C2–C5 olefins are abundantly produced from the decomposition of C5H10OH radicals in both the pyrolysis and flames. For example, 1-pentene, 1-butene, and propene are mainly formed from the decomposition of bC5H10OH, cC5H10OH and dC5H10OH radicals, while the dominant decomposition product of eC5H10OH radical is ethylene. Thus olefins become the most important hydrocarbon product family in the combustion of n-pentanol. In particular, ethylene is observed as the most important hydrocarbon intermediate in both the pyrolysis and flames, indicating that it affords extremely high reaction fluxes from n-pentanol. Three CnH2nO species, i.e. acetaldehyde, ethenol and formaldehyde, are the most abundant oxygenated intermediates as shown in Fig. 9. Besides, Fig. S3 shows the simulated results of the rich flame by the present model, the Heufer model [14], and the Togbé model [11]. 4.3. Model validation on literature data 4.3.1. Jet-stirred reactor oxidation The JSR experiments reported by Togbé et al. [11] were performed at the pressure of 10 atm, mean residence time of 0.7 s, and temperatures of 770–1220 K using gas chromatography. The

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Fig. 10. Validation on the species profiles in the JSR oxidation of n-pentanol at the pressure of 10 atm, residence time of 0.7 s, and equivalence ratio of 0.35 reported by Togbé et al. [11]. Symbols are the experimental results, and solid lines are the simulated results by the present model. In order to display the results of CO, CH2O, and C2H2 better, their experimental and simulated results are multiplied by factors of 5, 4, and 5, respectively.

Fig. 11. Validation on the species profiles in the JSR oxidation of n-pentanol at the pressure of 10 atm, residence time of 0.7 s, and equivalence ratio of 4 reported by Togbé et al. [11]. Symbols are the experimental results, and solid lines are the simulated results by the present model. In order to display the results of CH2O, CH3CHO, C4H8, and C5H10 better, their experimental and simulated results are multiplied by factors of 5, 8, 50, and 8, respectively.

equivalence ratio was varied from lean to rich conditions (0.35, 0.5, 1, 2, and 4). The initial concentrations of fuel were 1000 ppm (/ = 0.35-2) and 1500 ppm (/ = 4). Their data are used to validate the present model. The simulation work is performed with the Perfectly Stirred Reactor module in the Chemkin-PRO software [35]. The experimental and simulated mole fraction profiles of the reactants, stable intermediates, and final products at the leanest (/ = 0.35) and richest (/ = 4) conditions are shown in Figs. 10 and 11, while the results at other conditions are presented in Figs. S4–S6 in the Supplementary Material. It can be seen that the present model reproduces the experimental data well at all equivalence ratios. According to the ROP analysis, at all equivalence ratios, the leading consumption pathways of n-pentanol are the H-atom abstraction reactions producing five C5H10OH radicals, which is similar to the situation in the flames. At lean conditions and low temperatures, the H-atom abstraction reactions by OH radical are the dominant consumption pathways of n-pentanol, while at rich conditions and relatively high temperatures, the H-atom abstraction reactions by H atom become the main consumption pathways.

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To explore the chemistry in different temperature regions, the ROP analysis is also performed at 850 K and 1070 K for the oxidation with the equivalence ratio of 1. The ROP analysis shows that 91% of n-pentanol is consumed via the H-atom abstraction reactions by OH radical at 850 K, while at 1070 K this percentage drops to 66% and the contribution of the H-atom abstraction reactions by H atom increases to 21%. Similar to the situations in the pyrolysis and flames, aC5H10OH radical is the most dominant H-atom abstraction product at both 850 and 1070 K. At 850 K, more than 80% of aC5H10OH radical is consumed via the reactions with O2 (R29) and (R30), while the b-C-C scission reaction (R16) to produce n-propyl radical and ethenol only contributes 18%. At 1070 K, about 89% of aC5H10OH radical is consumed by the b-C–C scission reaction, while R29 and R30 only contribute around 6%. n-Propyl radical mainly decomposes to ethylene and methyl radical, and this is also the main formation source of ethylene, especially at 1070 K. Other formation pathways of ethylene include the b-C–O scission of C2H4OH radical, the reaction of C2H5 radical with O2 and the b-C–C scission reactions of eC5H10OH and cC3H6OH radicals. The main consumption pathways of other C5H10OH radicals are the b-scission reactions, which are very similar to the situations in the flames. Furthermore, a comparison of the simulated results of four olefins by the present model, the Heufer model [14], and the Togbé model [11] is shown in Fig. S7 in the Supplementary Material. 4.3.2. Ignition delay times Heufer et al. [9] measured the ignition delay times of n-pentanol/air mixtures at the pressures of 9–30 bar, temperatures of 640–1200 K, and equivalence ratio of 1.0 using a shock tube and a RCM. Tang et al. [10] measured the ignition delay times of n-pentanol/O2/Ar mixtures behind reflected shock waves at the pressures of 1.0 and 2.6 atm, temperatures of 1100–1500 K, equivalence ratios of 0.25, 0.5, and 1.0, and initial fuel mole fractions of 0.25% and 0.50%. These experimental results are simulated to validate the present model. The simulation work is performed using the Closed Homogeneous Batch Reactor module in the Chemkin-PRO software [35]. For the shock tube simulation, the ignition delay time is determined as the point of maximum temperature rising, corresponding to the time when the concentration of OH reaches the half of its maximum value. For the RCM simulation, the ignition delay time is determined as the occurrence of an inflection point in the pressure-time history during the ignition-induced pressure rising. Moreover, the non-ideal facility effects are included during the simulation of the ignition delay times at the low temperature region reported by Heufer et al. [9]. Figure 12 presents the comparison of experimental and simulated results, showing that the present model can reasonably reproduce the ignition delay time data at these conditions. The sensitivity analysis of the high temperature ignition delay time experiment reported by Tang et al. [10] is performed at the pressure of 1 atm, temperature of 1350 K, equivalence ratio of 1.0, and initial fuel mole fraction of 0.50%. The key chain branching reaction H + O2 = OH + O has the greatest negative sensitivity on the ignition delay time of n-pentanol. The reaction of methyl radical with HO2 radical (CH3 + HO2 = CH3O + OH) has the second greatest negative sensitivity since it produces the reactive OH radical. Among the reactions of n-pentanol, the C–C bond dissociation reactions have negative sensitivities on the ignition delay time, while the H-atom abstraction reactions by OH radical have positive sensitivities. The sensitivity analysis of the low temperature ignition delay time experiment in a shock tube reported by Heufer et al. [9] is performed at the pressure of 30 bar and temperature of 750 K. The results indicate that the H-atom abstraction reactions of n-pentanol by OH radical have the greatest negative sensitivities on the ignition delay time. The O2 addition reaction to aC5H10OH

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Fig. 12. Validation on the ignition delay times of n-pentanol at a variety of initial fuel mole fractions (XFuel), pressures, temperatures, and equivalence ratios reported by (a and b) Tang et al. [10] and (c) Heufer et al. [9]. Symbols are the experimental results, and solid lines are the simulated results by the present model.

Fig. 13. Validation on the laminar flame speeds of n-pentanol/air mixtures at a variety of initial temperatures (Tu) and pressures (Pu) reported by (a and b) Li et al. [12] and (c) Togbé et al. [11]. Symbols are the experimental results, and solid lines are the simulated results by the present model.

radical producing aC5H10OH–OO radical also has a great negative sensitivity on the ignition delay time. In contrast, its competitive reaction R29 shows the greatest positive sensitivity. The chain branching reactions that control the high temperature ignition behaviors of n-pentanol only have small sensitivities on the ignition delay time at the low temperature region. 4.3.3. Laminar flame speeds Togbé et al. [11] measured the laminar flame speeds of n-pentanol/air mixtures at the pressure of 1 atm and initial temperature of 423 K. More recently, Li et al. [12] measured the laminar flame speeds of n-pentanol/air mixtures at the pressures of 0.10– 0.75 MPa, initial temperatures of 393–473 K and equivalence ratios from 0.6 to 1.8. These experimental data are also simulated in this work to validate the present model. The simulation work is performed with the Laminar Flame Speed Calculation module in the CHEMKIN-PRO software [35]. Figure 13 presents the comparison of experimental and simulated results. In general, the model can reasonably reproduce these laminar flame speed data at different conditions. For the experimental results reported by Togbé et al., the present model slightly over-predicts the flame speeds at the rich side.

SVUV-PIMS. A detailed kinetic model of n-pentanol is also developed and validated on the new experimental results. In both pyrolysis and premixed flames, olefins and CnH2nO species are observed to be the two major product families. The unimolecular C–C bond dissociation reactions are found to play a significant role in the decomposition of n-pentanol in the pyrolysis, while the water elimination reaction of n-pentanol is found to be less important in the pyrolysis of n-pentanol than in the pyrolysis of n-butanol. In the premixed flames, the H-atom abstraction reactions dominate the decomposition of n-pentanol, and the oxidation reactions also play important roles in the consumption of several key intermediates. The observation of specific decomposition products for most of the C5H10OH radicals in both the pyrolysis and flames also provides useful experimental data for validating the H-atom abstraction reactions of n-pentanol. The present model is further validated on different types of experimental data in literature, including species profiles in the JSR oxidation and global combustion parameters such as ignition delay times and laminar flame speeds. Good agreement between the simulated results by the present model and various experimental data indicates that the present model is suitable for simulating the combustion of n-pentanol at the validated pressure, temperature, and equivalence ratio conditions.

5. Conclusion

Acknowledgments

The flow reactor pyrolysis of n-pentanol at 30, 150, and 760 Torr and the low pressure laminar premixed n-pentanol flames at two equivalence ratios (/ = 0.7 and 1.8) are investigated using

Authors are grateful for the funding support from Natural Science Foundation of China (51476155, U1332208), National Basic Research Program of China (973 Program) (2013CB834602),

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