Experimental and Modeling Study of Low Temperature Oxidation of

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Combustion

An experimental and modeling study of low temperature oxidation of iso-propylbenzene with JSR Bing-Yin Wang, Yue-Xi Liu, Jun-Jie Weng, and Zhen-Yu Tian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01492 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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Energy & Fuels

An experimental and modeling study of low temperature oxidation of iso-propylbenzene with JSR Bing Yin Wanga,b, Yue Xi Liua,b, Jun Jie Wenga, Zhen Yu Tiana,b,* a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd., Beijing 100190, China b

University of Chinese Academy of Sciences, 19A Yuquan Rd., Beijing 100049, China

Abstract: Oxidation of iso-propylbenzene (IPB) has been studied over temperature ranging from 700 to 1100 K in a jet-stirred reactor (JSR) at Low-temperature, which is operated at atmospheric pressure from fuel-lean to fuel-rich condition with residence time from 1.06 to 1.67 sec. Reactants and 25 species were identified and quantified by online GC-MS and GC analysis. A new model involving 306 species and 1985 reactions for low-temperature oxidation of IPB was developed, whose predictions were in good agreement with the measured profiles of mole fraction. Sensitivity analysis indicates that the primary H-atom abstraction from the side iso-propyl chain has significant promoting effect and H-abstraction from the tertiary site of side iso-propyl tends to play an inhibiting effect under fuel-lean and fuel-rich cases. The predominant consumption pathway of IPB proceeds through primary benzylic H-atom abstraction to form iso-phenylpropyl radicals for both fuel-lean and fuel-rich cases. However, compared to fuel-rich condition, 1-iso-phenylpropyl is favorable kinetically under fuel-lean condition. Both simulated and experimental results show that styrene and phenol are most abundant and stable monocyclic aromatic intermediates for IPB oxidation at low-temperature. These experimental and modeling works will expand the research area of low-temperature oxidation of IPB, and provide insights on understanding the combustion mechanism of IPB.

Keywords: Iso-propylbenzene; Low-temperature oxidation; Jet-stirred reactor; Speciation profiles;

*

Corresponding author. Tel/Fax: +86 10 8254 3184, E-mail: [email protected]. 1

ACS Paragon Plus Environment

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Modeling.

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Energy & Fuels

1. Introduction Heightened awareness of improving the burning efficiency and of dwindling pollutant emission has brought the demands of understanding the combustion characteristic of contemporary transportation fuels. Today’s transport fuels are comprised of a large proportion of alkanes, alkenes and aromatic compounds. Unfortunately, it is difficult to develop a detailed kinetic mechanism for real fuel because of the complex nature of these fuels. It is imperative to investigate the individual fuel constituent to understand the combustion characteristic of different chemical classes. These fuels are designed to be combined into surrogate fuels to emulate the combustion of real fuels and reduce the size of practical fuel model. Branched alkylbenzene compounds are major representative components of many surrogates for transportation fuels, which could simulate simultaneously the combustion characteristics of branched chain alkanes and alkylbenzenes. As representatives and common components of branched alkylbenzene in surrogate fuels, n-propylbenzene (NPB) and iso-propylbenzene (IPB) are complex enough to represent the branched alkylbenzene contents of practical transport fuels1. Previous investigations were mainly focused on benzene, mono-substituted n-alkylbenzene like toluene and ethylbenzene. However, scarce works of NPB and IPB oxidation have been published in the existing literature. Former investigations of combustion characteristics of NPB and IPB were mainly operated in laminar premixed flames, counter-flow diffusion flames and shock tube. IPB experiment has been studied scarcely at relatively high temperature2. In 2005, the laminar burning speeds of NPB were investigated in a stainless steel vessel at temperature of 450 K and pressure of 304 kPa under equivalence ratios (Ф) from 0.8 to 1.4 by Johnston et al.3. Hui et al.4 reported the experimental and modelling investigation of laminar flame velocity of NPB at temperature of 400 and 450 K with equivalence ratios from 0.7 to 1.4 d, and then the effects of temperature ranging from 350 to 470 K 3


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and pressure of 1-3 atm were considered under equivalence ratio from 0.7 to 1.3. The flame mole fraction profiles of NPB with methane were reported by Anderson et al.5. The mole fraction profiles of reactants, intermediates and products of NPB flame under fuel-rich condition (Ф = 1.79) were investigated by Wang et al.6. Beside high temperature oxidation and macroscopic investigations, experimental data of low-temperature oxidation of propylbenzenes are scarce in the existing literature. Dagaut et al.7 investigated the low-temperature oxidation of NPB in a JSR over temperature range of 900-1250 K and atmospheric pressure with Ф varying from 0.5 to 1.5. They utilized GC to obtain the mole fraction profiles for 23 species. Litzinger et al.2 presented the oxidation of IPB in a flow reactor at temperature ranging from 900 to 1300 K and pressure of 1 atm. Based upon their regression analysis to the rate equation of IPB oxidation, benzylic H-atom abstraction by combination of H, O and OH radicals dominated the consumption of IPB with up to 75%. However, investigations of IPB about low-temperature oxidation experiment and detailed chemical kinetic mechanism describing combustion of IPB still remain to be lacked. It is thus desirable to investigate the oxidation of IPB and build a kinetic model as basics of further work developing surrogate mechanism. This work focuses on the low-temperature oxidation of IPB in a JSR at temperature ranging from 700 to 1100 K with Ф (0.4 and 2.0) by using GC-MS and GC techniques. Based on experimental observations and recent related NPB works, a new kinetic model was proposed. The sensitivity and flux analysis were carried out to understand the oxidation route of IPB and formation pathway of major intermediates at low-temperature. The proposed mechanism could benefit the understanding of low-temperature oxidation processes of gasoline and jet fuel.

2. Experimental 4


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Energy & Fuels

The experiments were performed over temperature range of 700-1100 K and equivalence ratios of 0.4 and 2.0 at atmospheric pressure in the low temperature oxidation installation (LTOI). The experimental conditions are listed in Table 1. Detailed verification information about LTOI has been given in earlier publications8 and a brief description about experimental operation is shown in the Supplemental Material (SM). Fuel, intermediates and products were analyzed online by a gas chromatograph equipped with FID and TCD. The calibration of gas chromatograph was carried out through injecting a certain amount of known pure gaseous substances. The reliability of quantification was guaranteed by calculating carbon balance between the inlet mole fractions and outlet mole fractions of the JSR, as shown in Fig. S2 in SM. Each experimental data point was measured at least three times under same condition, which can guarantee a good experimental repeatability and reproducibility. The detection limitations of FID and TCD were about 0.1 ppm and 10 ppm, respectively. The approximated uncertainty of major species and intermediates were about ±5% and ±10%. Table 1 Experimental conditions Φ

C/O

XIPB

XO2

XAr

TFRa (sccm)

0.4 2.0

0.15 0.75

1.00 1.00

30.00 6.00

69.00 93.00

1200.00 1200.00

Note: Xi is the percentage of inlet species i; TFR a means the total flow rate.

3. Modeling IPB characterize two types of H atom-primary H atom and tertiary H atom-in the side iso-propyl chain, which result in different behaviour of their relevant rate coefficients and consumption pathways compared to linear alkylbenzene. Interesting points for the following description of established kinetic mechanism are provided by its molecular structure. The kinetic

5


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model of IPB was proposed mainly by adding primary mechanism of IPB and elementary steps of relevant radicals to the oxidation model of NPB proposed by Liu et al.9 at low temperature. The hierarchical nature of combustion mechanism is the basics of detailed chemical model in this work, which contains a C0-C2 core submechanism, a model to emulate the oxidation of C3-C5 species and aromatic compounds, especially propylbenzene subset. The C0-C2 submechanism is based on previous work about the low temperature oxidation of acetylene10. The C3-C5 subset is taken from Yuan et al.11. The developed IPB kinetic mechanism with 306 species and 1985 reactions is available in Chemkin format in SM. In IPB sub-mechanism, apart from the formation heat of most species taken from Goos et al.12, thermochemical data was primarily calculated by THERGAS13, which is developed according to the group additivity methods proposed by Benson14. By analogy to the unimolecular decomposition, H-abstraction and metatheses reaction of iso-butane15, and consulting three deductive routes for IPB oxidation proposed by Litzinger et al.2 and initiatory attempt combustion model of multi-component including IPB by Ra et al.16, a comprehensive IPB primary mechanism has been proposed and is listed in Table S1 in SM, along with the references relevant to kinetic model. It should be noted that the rate coefficients involved in this kinetic model are scarcely available in the existing literature. The unimolecular decompositions involving cleavage of C-H bond and C-C bond, bimolecular initiations with O2, and H-atom abstractions with R (fuel radical), RO, H, CH3, O, OH, HO2 and O2 are considered in current mechanism. The bond dissociation energy (BDE) of C-H bond of side isopropyl group in the IPB molecular structure is 74 Kcal/mole17, which is close to that of iso-propyl group with 84 Kcal/mole in iso-butane molecular structure. New reaction classes for IPB were added to better predict low-temperature oxidation behaviour. The selection of different reaction classes and various rate coefficients are described to explain how to propose this kinetic model. In order to 6


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Energy & Fuels

clarify the establishment process, several major Low-temperature reactions are classified: (Q1 refers to a 2-phenylethyl radical, Q2 refers to a 1-iso-phenylpropyl radical and Q3 refers to a 2-iso-phenylpropyl radical) are described in here. 1.

Q1+CH3(+M) = Q1CH3(+M)

2.

Q2+H(+M) = Q2H(+M)

3.

Q3+H(+M) = Q3H(+M)

4.

Q2H+H(CH3, O, OH, HO2, O2) = Q2+H2(CH4, OH, HO2, H2O2, HO2)

5.

Q3H+H(CH3, O, OH, HO2, O2) = Q3+H2(CH4, OH, HO2, H2O2, HO2)

(i) Unimolecular fuel decomposition: These classes of reactions include unimolecular decomposition of C-H bond and C-C bond from side iso-propyl chain. The unimolecular decomposition steps are reverse reactions, and the reaction coefficients are calculated according to the rule of microscopic reversibility. These reactions produce two alkyl radicals or alkyl radical and H atom. Because benzene ring is a resonance-stabilized structure, therefore only cleavages of C-H and C-C bonds from side iso-propyl chain are considered in current mechanism. These reaction types are very important for low-temperature oxidation, because they serve as sinks of radicals. The unimolecular decomposition rates were updated based on analogies with iso-butane homolysis. The difference in aromatic structure and bond strengths are the basics of analogy of iso-butane elementary steps chosen. Unimolecular decomposition depends on the local environment of C-C and C-H bond. Therefore, we consider unimolecular decomposition of IPB to be analogous to 1.2 times the rate of decomposition of iso-butane. (ii) Primary H-atom abstraction: these elementary step classes include primary H-atom abstractions attacked by R (fuel radical), RO, H, CH3, O, OH, HO2 and O2. The H-atom abstractions are reverse elementary steps, and these reaction coefficients are also calculated according to the rule 7


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of microscopic reversibility during the modeling. Type of H-atom being abstracted and relevant radical species influence largely the rate coefficients of elementary steps in this mechanism. Abstractions from alpha sites in IPB are expected to be faster due to the weaker steric-hindrance effect. therefore, the rate coefficient was analogous to H-atom abstraction from the primary position of iso-butane, which owns the similar branched iso-propyl structure as IPB. Primary H atoms have the weakest steric-hindrance effect, which leads the H atom easy to abstract. Several reactions relevant to 1-iso-phenylpropyl radical were included in the secondary mechanism of Pousse et al.18, who studied the premixed laminar flame of methane mixed with diesel fuel components. The rates of primary H abstractions attacked by H, CH3, O, OH, HO2 and O2 were proposed by analogy to iso-butane in the work by Pousse et al.18. (iii) Tertiary H-atom abstraction: these classes of elementary steps include H-atom abstraction of tertiary site from the side iso-propyl chain, which is attacked by R (fuel radical), RO, H, CH3, O, OH, HO2 and O2. Tertiary H atom has the strongest steric-hindrance effect, which leads the H atom difficult to abstract, although the inductive effect of presence of benzene ring group results in the weakness of beta C-H bond. Abstractions from beta sites in IPB are expected to be slower due to the strongest steric-hindrance effect. Therefore, the reaction rates of H-atom from tertiary position of side iso-propyl chain were referenced when estimating the reaction coefficients of tertiary H-atom abstractions. The rates of tertiary H-atom abstraction attacked by H, CH3, O, OH, HO2 and O2 were developed by analogy to iso-butane, which has a similar isopropyl group with IPB. The detailed experimental data, names, nomenclatures and structures for key aromatic species mentioned in this work but not presented in previous work are available in SM. All kinetic modelling were carried out by utilizing PSR code in Chemkin-II software19, and the predicted mole fraction profiles were compared to experimental results obtained in this work. For purpose of understanding 8


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Energy & Fuels

the process of low-temperature oxidation of IPB and perfecting the simulation performance of IPB mechanism, the discussion of IPB oxidation at low-temperature has been carried out based on modelling analysis such as sensitivity analysis and rate of production (ROP) analysis.

4. Results and discussion 4.1 Mole fraction profiles In general, conversions of fuel and formation of most products and stable intermediates are well reproduced by present mechanism at Ф=0.4 and 2. Formation of ethane, propyne, toluene, ethanol and acrolein is underestimated with deviation under fuel-rich condition. The current model tends to slightly overpredict the formation of indene and phenol. Formations of propene, bibenzyl and stilbene are well predicted under fuel-lean condition, but underestimated under fuel-rich condition. In general, relatively large deviations are encountered under fuel-rich condition. It may be attributed to that formations of some large polycyclic hydrocarbons and soot were hardly taken into account in current mechanism. Figure 1 shows the evolution of fuel and major products mole fractions with temperature. There is a reasonable agreement between measurement and prediction for IPB. The initiation temperature of IPB consumption under lean condition is 775 K, which is lower than that under rich condition by about 75 K. when Ф increasing, the initiation temperature of IPB oxidation shifts to high temperature region. It is suggested that the conversion of IPB is strongly dependent on Ф. The ROP analysis shows that oxygen and its derived radicals play significant roles in the chain initiation of low temperature oxidation of IPB. For low-temperature oxidation of IPB, the predominant consumption pathway is H-atom abstractions attacked by H/OH/CH/O radicals. Especially, concentration of OH radical will decrease greatly when Ф increasing. As shown in Fig. 1 b-d, the mole fraction profiles of CO, CO2 and H2 are reproduced fairly well 9


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by current mechanism. Initiation temperatures of CO, CO2 and H2 formation under rich condition in the oxidation of IPB are 900, 950 and 900 K, respectively. Mole fractions for CO formation increase with increasing temperature and similar tendency is observed for CO2 and H2. Under rich condition, the peak concentrations of CO2 and CO are 2×10-2 and 4×10-2, respectively. Compared to relatively low concentration of CO2, CO becomes the major oxidation products because of insufficient oxygen under fuel-rich condition. When reaction temperature rises to 1100 K, the peak mole fractions of both CO and CO2 under fuel-rich condition are generally higher than that under fuel-lean condition. For low-temperature oxidation of IPB under lean condition, the initiation formation temperatures of CO and CO2 are decreased to 825 and 925 K, respectively.

Fig. 1 Experimental (symbols) and modeling (lines) profiles of the major species. Because of having similar formation route (HCO+ O2→CO+HO2 and HCCO+O2→2CO+OH) and oxidation pathway (CO+OH→CO2+H) of CO at low-temperature with our previous work20, mole fraction profile of CO has a are similar trend with previous acetylene investigation20. According to the above flux analysis, the accumulation of CO under fuel-rich condition was promoted by decreased OH radical. The production of oxygenated specie will be inhibited due to insufficient oxygen concentration under fuel-rich condition. As depicted in Fig. 2, the major produced light hydrocarbons were methane, ethylene, ethane 10


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Energy & Fuels

and n-butene with maximum mole fractions of 5×10-3, 2×10-3, 1×10-3 and 1.3×10-5 under fuel-rich condition. In fuel-lean condition, the peak mole concentration of methane, ethylene, ethane and n-butene decrease to 2×10-3, 5×10-4, 2×10-4 and 6×10-6, respectively. From comparisons of peak concentration between lean and rich condition, formation of C2 and C4 compounds was favored kinetically under rich condition. Moreover, the profiles of propyne and propene under fuel-lean condition show a peak-shape distribution. When equivalence ratio increasing, mole fractions of both products decrease obviously. These phenomena may be attributed to unimolecular decomposition of IPB, which is favorable under lean condition and facilitates the formation of C3 intermediates by cleavage of C-C bond.

Fig. 2 Experimental (symbols) and modeling (lines) profiles of the light hydrocarbons. As illustrated in Fig. 3, peak mole concentrations of major monocyclic aromatic compounds like styrene, benzene and toluene were 4×10-3, 1.2×10-3 and 3×10-4, respectively. While production of cyclopentadiene, benzene and ethylbenzene significantly increases with Φ increasing, formations of α-methylstyrene and styrene are kinetically favored under lean condition but less rely on the variation of Φ. α-Methylstyrene is mainly formed through H-atom abstraction followed by C–H bond dissociation. All monocyclic aromatic intermediates were predicted well by present model. Both peaks and tendencies of simulated aromatic intermediates are in good agreements with experimental 11


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results. These peaks generally appear over temperature range of 900 to 1000 K at Ф = 2.0, while mole fraction profile of benzene shows a gradually growing tendency under rich condition. Benzene was formed by two main pathways: ipso-addition of phenol and methylcyclopentadiene (A1OH+H=A1+OH and CH3+C5H5CH3=>CH4+A1+H). Benzyl radical was an important precursor of ethylbenzene and bibenzyl. Most of styrene came from H-abstraction of 1-iso-phenylpropyl radicals (A1CH(CH3)CH2). Reaction flux analysis indicated that more than 60% of IPB converts to styrene under both conditions, which means styrene is one of abundant and important monocyclic aromatic intermediates during low-temperature oxidation IPB.

Fig. 3 Experimental (symbols) and modeling (lines) profiles of aromatic species.

Figure 4 shows that the polycyclic aromatic hydrocarbons (indene, naphthalene, bibenzyl and stilbene) are formed in lower amounts within peak value ranges of (5.9×10-6-1.2×10-5), (2.1×10-5-4×10-5), (9×10-6-1.2×10-5) and (6×10-6-9×10-6), respectively. Peak locations of bibenzyl and stilbene appear at above 900 K with concentration of 1.0×10-5 and 6×10-6 under rich condition, respectively. By comparison, corresponding values of bibenzyl and stilbene under lean condition increase by 20% and 50%. Peak temperature of two species shifts to low temperature zone with Φ decreasing, that is consistent with the shifting tendency of IPB oxidation. Dominant formation 12


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Energy & Fuels

pathways for these two species are the combination of benzyl radical and its dehydrogenation via series of peroxide intermediates. It should be noted that concentration of naphthalene increase sharply within temperature range between 900 and 950 K under fuel-rich condition. Naphthalene performed a steadily increasing profile above 950 K. In previous study 21, several formation channels, such as combination of two cyclopentadienyl radicals, phenyl with vinylacetylene, benzyl plus propargyl radical and ethylphenyl radical with acetylene, were reported. However, the major production route for naphthalene in low-temperature oxidation of IPB is reaction of styrene with acetylenyl radical: A1C2H3+C2H=A2+H. Under the investigated conditions, α-methylstyrene plays a key

role

in

indene

formation.

Reaction

sequence

A1CH(CH3)CH2→A1CH2CHCH3→A1CH2CHCH2→A1CHCHCH2→indene is the major formation channel of indene in low-temperature oxidation of IPB.

Fig. 4 Experimental (symbols) and modeling (lines) profiles of PAH species. Figure 5 exhibits the mole fraction profiles of oxygenated intermediates. Methanol, ethanol and acrolein are produced in significant amounts (peak values of 8×10-5, 5×10-5 and 9×10-5, respectively) with formation notably favored under fuel-rich condition. Other aromatic oxygenated compounds (phenol, benzaldehyde and benzofuran) were strongly effected by mole fraction of oxygen, and their maximum values decreased slightly when Φ becomes rich. Acrolein was underestimated by present 13


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model. Reaction flux analysis reveals that underprediction of propene (see Fig. 2e), which is an important precursor of acrolein via propene→propenyl→propenyloxy→acrolein, leads to low production of acrolein. Phenol is the major aromatic oxygenated species (mole fractions of 1.5×10-3 at least at all Φ). The maximum mole fraction of phenol was 5 times larger than that of benzaldehyde from lean to rich condition. More than 90% of phenol is from reaction sequence as following: benzaldehyde→ phenylcarbonyl→phenyl→phenylperoxy→phenoxy→phenol. Benzofuran, one of main product of benzaldehyde, was chiefly produced by combination of phenoxy and C2H2.

Fig. 5 Experimental (symbols) and modeling (lines) profiles of oxygenated species. Maximum concentration of benzaldehyde increases by a factor of 1.3 under Φ=0.4, in comparison with that at Φ=2.0. As a major precursor for other aromatic oxygenates like phenol and benzofuran during low temperature oxidation of IPB, benzaldehyde can be easily formed under lean condition with larger concentration. According to ROP analysis, the major formation source for benzaldehyde

is

the

reaction

of

2-phenylethyl

with

hydroperoxide

radical:

A1CHCH3+HO2=A1CHO+CH3+OH. Unimolecular decomposition of IPB would produce 2-phenylethyl radical, which could convert to series of oxygenated species and promote the production of OH radical. OH radical pool can significantly accelerate the reaction rate of following steps relevant to OH radicals. When Ф increasing, the concentration of hydroxyl radical will 14


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Energy & Fuels

accordingly decrease. Therefore, the decrease of radical concentration derived from H-atom abstraction attacked by OH radical lets the low-temperature oxidation profile of IPB shift to high temperature region. Similar laws were also observed for those species in the low-temperature oxidation of NPB by Liu et al.9. 4.2 Sensitivity and flux analysis Sensitivity and reaction flux analysis for consumption of IPB under lean (61% conversion at 850 K) and rich (60% conversion at 905 K) conditions are displayed in Figs. 6 and 7, respectively. As displayed in Fig. 6, primary H-atom abstraction from the side isopropyl group of IPB forming 1-iso-phenylpropyl radical is the most significant promoting reaction and H-abstraction from the tertiary position of side isopropyl of IPB forming 2-iso-phenylpropyl radical tends to play inhibiting effect for both fuel-lean and fuel-rich cases. The elementary steps involving formation of 2-iso-phenylpropyl radical from IPB have inhibiting effects, while that of 1-iso-phenylpropyl have promoting effects. Sensitivity analysis reveals that under lean condition OH radical plays more significant role than CH3 radical, while under rich condition elementary steps involving CH3 tend to have more important role for oxidation of IPB.

Fig. 6 Sensitivity analysis of IPB oxidation under lean and rich conditions, corresponding to 850 K, 61% and 905 K, 60% conversion, respectively. 15


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The flux analysis of IPB oxidation is shown in Fig. 7. The consumption pathway of IPB is mainly through H-abstraction to produce iso-phenylpropyl radicals for both lean (89%) and rich (79%) condition. At Φ =2.0, 68% of IPB is consumed by H-abstractions attacked by OH (35%), CH3 (25%), H (4%) and HO2 (2%) radical to form 1-iso-phenylpropyl radicals and 11% to produce 2-iso-phenylpropyl radicals. The difference in decomposition of IPB between lean and rich condition is obvious. It may be mainly attributed to the difference in concentration of OH radicals. Compared to fuel-lean condition, fuel-rich condition lacks of any vigorous initiation reaction, which produces the chain carrier OH radical. OH radical will benefit the formation of 1-iso-phenylpropyl radical in low-temperature oxidation of IPB. Another important consumption channel (19%) of IPB is decomposition reaction by scission of C–C bond to give 2-phenylethyl radicals at Φ=2.0. In comparison, corresponding fraction at Φ=0.4 is negligible (2%). 96% of 1-iso-phenylpropyl radicals convert to styrene via decomposition reaction by releasing CH3 radical and 3% to α-methylstyrene by H-abstraction. Styrene is a commonly stable and important intermediate for both fuel-lean and fuel-rich cases, independent of Φ. These results match with experimental data that styrene was the most abundant aromatic intermediate in IPB oxidation. ROP analysis shows that ipso-additions are major consumption pathways for styrene. At 905 K, 40% of styrene is consumed by addition to form phenylethyl radicals, 18% by ipso-addition of O atoms producing phenylcarbonyl and methyl radicals, 18% by ipso-addition of O-atoms yielding benzyl and formyl group, 10% to produce 2-phenylethenyl radicals and 8% to 2-phenylethenyl radicals.

16


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Energy & Fuels

7

Fig. 7 Flux analysis of IPB oxidation under lean (red and italic)and rich conditions, corresponding to 850 K, 61% and 905 K, 60% conversion, respectively. Flux analysis depicts the importance of H-abstraction of alkylbenzene by OH radicals, which has already been observed in previous study22. Noting that recombination of phenylethyl radicals with methyl radical to produce NPB is very minor in the consumption proportion of phenylethyl radical. Main reaction of phenyl is combination with oxygen, followed by O–O bond dissociation to yield phenoxy radical, that can further give phenol by disproportion with HO2 radical. Another difference between lean and rich condition is the fate of benzyl radical. Three major consumption pathways of benzyl radicals are by addition of methyl to give ethylbenzene, by ipso-addition of HO2 radical yielding benzyloxy and OH radical and by combination with another benzyl to produce bibenzyl molecule. This is consistent with previous work of Liu et al.9. that most of bibenzyl is formed through self-combination of benzyl radicals. At Φ=2.0, benzyl radicals mainly convert to ethylbenzene (49%), while such route accounts for less extent (12%) at Φ=0.4. Formations of benzyloxy radical (66%) and toluene (11%) were dominant consumption pathways for benzyl radicals at Φ=0.4. Phenyl radicals react mainly via phenylperoxy radicals to produce phenoxy 17


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radicals and oxygen. Phenoxy radicals react mainly with HO2 radicals yielding phenol, but minor consumption pathways also include formation of benzofuran by reaction with acetylene, combination with another phenoxy to yield two carbon monoxide and 1,2-dihydronaphthalene and decomposition to give cyclopentadienyl radicals. Benzaldehyde is formed mainly from the reaction of 2-phenylethyl with HO2: A1CHCH3+HO2=A1CHO+CH3+OH. The dominant low-temperature oxidation channels for IPB are H-atom abstractions attacked by OH and CH3 radicals, and the following steps involved in its derived species. The production channel of OH radical (CH3+O2=CH2O+OH) will be benefit for the low-temperature oxidation of IPB through yielding the chain carrier OH radicals. Styrene and phenol are major abundant and stable monocyclic aromatic intermediates for low-temperature oxidation of IPB. 4.3 Effect of C9H12 alkylbenzenes on representative intermediates in low-temperature oxidation Low-temperature oxidation of four C9H12 alkylbenzenes, namely 1,3,5-trimethylbenzene (T135MB)8,23, 1,2,4-trimethylbenzene (T124MB)24, NPB9 and IPB, have been investigated in the same JSR. To find the effect of fuel structure on representative intermediates, comparison is performed among these four C9H12 aromatic isomers. Comparisons of proportion value (peak values/ inlet mole fractions) of representative intermediates are shown in Fig. 8. For ethylbenzene and styrene, the dominant source were NPB and IPB, but not for benzene and toluene. Noting that proportion of styrene for IPB is much larger than other C9H12 isomers. This phenomenon can be attributed to high energy barrier of CH3 addition on TMBs (T135MB and T124MB) and relative low energy barrier of H-abstraction from side isopropyl of IPB. Proportion of toluene in low-temperature oxidation of TMBs is relatively higher than that of PBs (NPB and IPB). The differences in formation of toluene account for energy difference of C-C bond cleavage between TMBs and PBs. C-C bond energy between methyl and phenyl radical in TMBs is lower than that between ethyl and benzyl 18


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Energy & Fuels

radical in PBs. Thus, decomposition of TMBs forming toluene is much easier than PBs, leading to larger formation of toluene under same condition. As common aromatic intermediates in low-temperature oxidation of alkylbenzenes, formation proportion of benzene is similar for TMBs and PBs. Thus, PBs prefers to produce styrene and ethylbenzene rather than toluene and xylenes. This reaction behavior also results in less production proportion of oxygenated intermediates for PBs, which is produced via the decomposition to form phenyl and propyl radicals. Relative concentration of acetaldehyde and acrolein were quite different among the four C9H12 alkylbenzenes. T135MB has largest proportion of acetaldehyde but smallest for acrolein. Acetaldehyde and acrolein are mainly formed from products of decomposition of C9H11O, which are largely formed in low-temperature oxidation of TMBs, but not in PBs. It is concluded that oxygenated intermediates is favorable kinetically produced in TMBs than PBs.

Fig. 8 Relative comparison of representative intermediates in low-temperature oxidation of C9H12 isomers: T135MB, T124MB, NPB and IPB.

5. Conclusion Oxidation of IPB at Low-temperature was studied in JSR over temperature ranging from 700 to 1100 K at atmospheric pressure under fuel-lean and fuel-rich condition. Based on 25 intermediates 19


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identified and quantified by GC-MS and GC analysis, IPB kinetic mechanism consisting of 306 species and 1985 reactions was established and reasonable agreement between experimental data and simulating result was obtained. Sensitivity analysis indicates that elementary steps relevant to hydroxyl radicals play significant role for the IPB oxidation at low-temperature. Under fuel-lean and fuel-rich cases, H-abstraction of primary site in the side isopropyl group of IPB has significant promoting effect; H-abstraction from the tertiary position of side isopropyl of IPB tends to play an inhibiting effect. ROP analysis demonstrates that dominant consumption channel of IPB proceeds via H-abstraction to produce iso-phenylpropyl radicals for both fuel-lean and fuel-rich cases. 1-iso-phenylpropyl radical, through H-abstraction of primary benzylic H atom from side isopropyl, kinetically prefers to form under fuel-lean condition than under fuel-rich condition. Styrene and phenol were important abundant and stable monocyclic aromatic intermediates in low-temperature oxidation of IPB. These experimental and modeling works will enrich the experimental database of IPB at low temperature, and the proposed kinetic model will benefit for further work to understand comprehensively the combustion mechanism of IPB.

Acknowledgement TZY thanks for the financial support from NSFC (No 91541102/51476168), MOST (2017YFA0402800) and Recruitment Program of Global Youth Experts.

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Table and Figure Captions Color figure in electronic versions only

Table 1 Experimental conditions Fig. 1 Experimental (symbols) and modeling (lines) profiles of the major species. Fig. 2 Experimental (symbols) and modeling (lines) profiles of the light hydrocarbons. Fig. 3 Experimental (symbols) and modeling (lines) profiles of aromatic species. Fig. 4 Experimental (symbols) and modeling (lines) profiles of PAH species. Fig. 5 Experimental (symbols) and modeling (lines) profiles of oxygenated species. Fig. 6 Sensitivity analysis of IPB oxidation under lean and rich conditions, corresponding to 850 K, 61% and 905 K, 60% conversion, respectively. Fig. 7 Flux analysis of IPB oxidation under lean (red and italic)and rich conditions, corresponding to 850 K, 61% and 905 K, 60% conversion, respectively. Fig. 8 Relative comparison of representative intermediates in low-temperature oxidation of C9H12 isomers: T135MB, T124MB, NPB and IPB.

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Experimental and Modeling Study of Low Temperature Oxidation of

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