A comprehensive experimental and kinetic modeling study of n

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A comprehensive experimental and kinetic modeling study of n-tetradecane combustion Meirong Zeng, Wenhao Yuan, Wei Li, Yan Zhang, Chuangchuang Cao, Tianyu Li, and Jiabiao Zou Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01114 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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A comprehensive experimental and kinetic modeling study of n-tetradecane combustion Meirong Zeng1, Wenhao Yuan2,3*, Wei Li1, Yan Zhang1, Chuangchuang Cao1, Tianyu Li1, Jiabiao Zou3 1. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, P. R. China 2. Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiao Tong University, Shanghai 200240, P. R. China 3. Key Laboratory for Power Machinery and Engineering of MOE, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Abstract: N-tetradecane is a typical heavy n-alkane component in transportation fuels. Nowadays experimental data about n-tetradecane is rare, especially for the gas-phase kinetic experiment, which limits the development and updating of the combustion kinetic model of n-tetradecane. This paper reports the first effort on investigating the gas phase pyrolysis of n-tetradecane. The experimental conditions cover temperatures of 832-1281 K and pressures of 30 and 760 Torr, while photoionization mass spectrometry was applied for speciation. A comprehensive model of n-tetradecane combustion was also constructed. The validation data include the new pyrolysis data, literature oxidation data and global combustion parameters. Modeling analysis tools were also applied to reveal major reactions for the pyrolysis and

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oxidation of n-tetradecane. Primary dissociation and H-abstraction reactions have the largest contributions and sensitivity coefficients for n-tetradecane decomposition under present pyrolysis conditions. HO2 plays a significant role on ignition delay times, while C0-C3 reactions dominate laminar burning velocities of n-tetradecane. Keywords: n-tetradecane; pyrolysis; comprehensive model; validation; modeling analysis

1. Introduction Long chain n-alkanes with seven and more carbon atoms are abundant in gasoline1, kerosene2 and diesel fuels3,4. Among them, C10-C16 n-alkanes with even carbon numbers usually exist with high concentrations in kerosene and diesel fuels. Their physical and chemical properties are very close to the total paraffin components in these transportation fuels. Therefore they have been widely used as representatives of paraffin components in the surrogate fuels of kerosene and diesel4-11. In particular, n-tetradecane has been widely used to represent the heavy n-alkane components in diesel5-8. Great attentions have been paid on the combustion experiments of C10 and C12 n-alkanes, as summarized in references12-14. Comprehensive kinetic models of C10 and C12 n-alkanes have also been developed and validated under wide combustion conditions12-15. In contrast, very limited experimental studies on n-tetradecane combustion have been performed, probably due to their extremely high boiling points (~523 K for n-tetradecane, compared to ~447 K for n-decane and ~489 K for n-dodecane). Most of previous experimental investigations on n-tetradecane combustion were performed under condensed phase of fuel. Vranos and coworkers16 performed the pyrolysis experiment of n-tetradecane in free droplet vaporization in a laminar flow furnace. Barbella and coworkers17 investigated n-tetradecane combustion using direct injection diesel


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engine. Cavaliere and coworkers18 investigated n-tetradecane oxidation by injecting liquid fuel into engine-like conditions. Song and coworkers19 studied the liquid phase n-tetradecane pyrolysis using gas chromatography for gaseous products and gas chromatography-mass spectrometry for liquid products. The multiphase circumstances in these condensed phase experiments dramatically increase the complexities of physical models and the difficulties to perform kinetic simulations, making them hardly used to validate n-tetradecane models. Recently, Shen and coworkers20 investigated n-tetradecane ignition under elevated pressures and low temperatures. Li and coworkers21 investigated laminar burning velocities for n-tetradecane at 1 atm and two heating temperatures. Mzé-Ahmed and coworkers22 studied n-tetradecane oxidation with jet stirred reactor (JSR) . Some mechanistic descriptions of n-tetradecane combustion can be found in Westbrook’s13 and Ranzi’s23 n-alkane models, and in previous n-tetradecane models21,24. However, the lack of experimental data of gas phase n-tetradecane combustion, especially the species profile measurements, strongly limits the development and validation of n-tetradecane models and kinetic models of surrogate fuels containing n-tetradecane. In this work, gas phase n-tetradecane pyrolysis at low and atmospheric pressures was studied under 832-1281 K. Photoionization mass spectrometry (PIMS) was applied to detect both radicals and stable pyrolysis products. A kinetic model of n-tetradecane combustion focusing on the low-to-high temperature chemistry was constructed from our reported n-decane model12 for high temperature reactions and from previous n-dodecane model25 for low temperature reactions. It was then validated using the new n-tetradecane pyrolysis experiment and literature experiments, such as JSR oxidation reported by Mzé-Ahmed and coworkers22, ignition delay times reported by Shen and coworkers20 and laminar burning velocities reported by Li and coworkers21.


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2. Experimental method Present experiments were carried out at National Synchrotron Radiation Laboratory. Experimental equipment was introduced in detail elsewhere26-29. In short, this equipment is composed of three parts. The first part is the reactor chamber, which mainly contains a flow reactor with electrical heating. The second and third parts are two chambers for differential pumping and photoionization. Before entering the reactor, n-tetradecane was vaporized by a vaporizer heated at 570 K and mixed with Ar. The total flow rate is 1 SLM and the inlet proportion of n-tetradecane is 2%. Then the mixture was fed into the flow tube which is made of α-Al2O3 to reduce surface catalytic effects30. The heating length is 15 cm and the inner diameter is 0.68 cm. Pyrolysis chamber was kept at pressures of 30 and 760 Torr in this work. The sampled pyrolysis products were detected by synchrotron VUV PIMS with two modes. One is the measurements of photoionization efficiency spectra for species identification and the other one is the measurements of mass spectra under various temperatures for concentration evaluation. Detailed descriptions of data evaluation methods can be found elsewhere31,32. Experimental uncertainties were evaluated as ±10% for major products, ±25% for minor products with known photoionization cross sections (PICSs), and a factor of 2 for those with estimated PICSs. The PICSs of pyrolysis species can be found in our database33. In present work, centerline temperature profiles of the reactor were recorded by an S-type thermocouple. Detailed descriptions of the method can be found in our recent work26,31,34. The measured temperature profiles were named by their maximum values (Tmaxs) and the temperatures discussed in the following sections all refer to Tmaxs. The uncertainty of Tmaxs is around ±30 K. These temperature profiles (provided in the Supporting Information 1) will be used in simulation. On the other hand, the pressure inside the reactor was kept constant during


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simulation since the pressure drops along the axis of flow tube were found to be very small at both 30 and 760 Torr26,35. Residence times under present flow reactor conditions are around 0.005 second and 0.15 second, for 30 Torr and 760 Torr, respectively.

3. Kinetic modeling Present n-tetradecane model was constructed based on our previous n-decane model12. As for the submechanism of n-alkane, it is quite well known that the most important reactions are H-abstraction reactions to form alkyls and corresponding small species, and bond fission to produce two radicals12. In present n-tetradecane model, H-abstraction reactions of fuel via H attack have been estimated from similar reactions of n-butane, which is a recent experimental and theoretical work by Peukert and coworkers36. Those via CH3, C2H3 and C2H5 attack were estimated from similar reactions from Healy and coworkers37. Those via OH attack were taken from evaluations by Sivaramakrishnan and Michael38. Those via O2, O and HO2 attack were taken from the Westbrook model13. Bond dissociation reactions of the fuel were estimated from similar reactions of n-butane39. Alkyls are important intermediates for product distributions in n-tetradecane combustion. Tsang and coworkers40,41 reported rate constants for isomerization and β-C-C scission of octyl and hexyl. Their rate constants were adopted in present model and also used to estimate rate constants for similar reactions of C7 and C9-C14 alkyls. Low temperature oxidation reactions of n-tetradecane were referred to the similar reactions of n-dodecane from the recently optimized n-dodecane kinetic model of Cai and coworkers25, which incorporates typical low temperature reaction classes. Low temperature reactions for small species were directly taken from the Cai model25, which originates from the model of Zhang and coworkers42. Recently, third O2 addition process was proposed as a new low temperature reaction pathway and it was found to have potential effects on the low temperature oxidation of 2-methylhexane43 and 5 ACS Paragon Plus Environment

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2,5-dimethylhexane44. In present work, these reaction pathways are not considered due to lack of reliable rate constants for the third O2 addition pathways as well as suitable experimental data to validate these reactions. Thermodynamic and transport data were mainly from our n-decane model12, the Cai model25 and Westbrook model13. Thermodynamic data for C13 and C14 species were from the Westbrook model13. Those for C10-C12 species and other low temperature species (C≤12) were from the Cai model25. The high temperature mechanism (254 species and 2217 reactions), low-to-high temperature mechanism (1975 species and 6444 reactions) are provided in Supporting Information 2 and 3. Thermodynamic data is provided in Supporting Information 4. Transport data is provided in Supporting Information 5. Simulations were carried out with Chemkin-PRO software45. Flow reactor pyrolysis and laminar burning velocity simulations were performed using the high temperature mechanism. JSR oxidation and ignition delay time simulations were carried out using low-to-high temperature mechanism. Details for the simulations will be introduced below.

4. Results and discussion 4.1 n-Tetradecane pyrolysis in flow reactor This work detected lots of pyrolysis products for n-tetradecane (n-C14H30), including C2-C12 1-alkenes, 2-alkenes, dialkenes, alkynes, radicals and so on. Plug Flow Reactor code45 was adopted in simulating the experiments. Details for simulation methods are available elsewhere26,31,46. Results of important species from experiments and simulations are presented in Fig. 1 and Figs. 4-6. Modeling analysis was conducted to help understanding the kinetics in n-C14H30 decomposition. Table S1 in the Supporting Information 6 presents the molecular structures of C3 and larger species involved in discussion below. 6 ACS Paragon Plus Environment

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a) Decomposition of n-tetradecane Figure 1(a) shows the measured and modeling results of n-C14H30. Rate of production (ROP) analysis demonstrates n-C14H30 decomposition is initiated by unimolecular reactions. C-C bond dissociation reactions producing C1-C13 alkyls (R1-R7) dominate the unimolecular decomposition reactions of n-C14H30. Among reactions R1-R7, R7 has the lowest contribution to the decomposition of n-C14H30 because only one C(7)-C(8) bond exists in the molecule of n-C14H30. Meanwhile, R1 has the second lowest contribution since the primary carbon-secondary carbon bond in n-alkane molecules is stronger than those of secondary carbon-secondary carbon bonds47. n-C14H30 (+M) = C13H27X1 + CH3 (+M)

(R1)

n-C14H30 (+M) = C12H25X1 + C2H5 (+M)

(R2)

n-C14H30 (+M) = C11H23X1 + NXC3H7 (+M)

(R3)

n-C14H30 (+M) = C10H21X1 + PXC4H9 (+M)

(R4)

n-C14H30 (+M) = C9H19X1 + C5H11X1 (+M)

(R5)

n-C14H30 (+M) = C8H17X1 + C6H13X1 (+M)

(R6)

n-C14H30 (+M) = C7H15X1 + C7H15X1 (+M)

(R7)


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Figure 1 C14-C8 species in n-C14H30 pyrolysis at 30 and 760 Torr. Symbols represent present experiments. Lines represent simulations. H-abstraction reactions (R8-R21) forming primary tetradecyl (C14H29X1) and six secondary tetradecyls (C14H29X2-C14H29X7) are other important consumption pathways of fuel at both low and atmospheric pressure conditions. They also control the formation of hydrogen (H2) and methane (CH4). Figures 6(f) and 6(e) present results of H2 and CH4, respectively. H-abstraction reactions via attack of large radicals, such as vinyl (C2H3) and ethyl (C2H5), were also considered in present model. But these reactions are only minor consumption reactions for n-C14H30 at investigated conditions. Among H-abstraction on different C sites, those on the primary sites (R8, R15) contribute less than those on the secondary sites (R9-R14, R16-R21). n-C14H30 + H/CH3 = C14H29X1+ H2/CH4

(R8/R15)

n-C14H30 + H/CH3 = C14H29X2 + H2/CH4

(R9/R16)

n-C14H30 + H/CH3 = C14H29X3 + H2/CH4

(R10/R17)

n-C14H30 + H/CH3 = C14H29X4 + H2/CH4

(R11/R18)


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n-C14H30 + H/CH3 = C14H29X5 + H2/CH4

(R12/R19)

n-C14H30 + H/CH3 = C14H29X6 + H2/CH4

(R13/R20)

n-C14H30 + H/CH3 = C14H29X7 + H2/CH4

(R14/R21)

Figure 2 compares the total contributions of the three major reaction classes to n-C14H30 decomposition, i.e. bond fission (R1-R7), H-abstraction via H attack (R8-R14) and via CH3 attack (R15-R21), at different temperatures and pressures according to the ROP analysis. The temperatures used in Fig. 2 cover 0%-100% conversion of n-C14H30 at both low and atmospheric pressure conditions. As shown in Fig. 2, most of n-C14H30 decomposes via R8-R14 at both pressures. C-C bond dissociation has a greater contribution than H-abstraction via CH3 attack at 30 Torr, while the opposite results can be observed at most temperatures at 760 Torr. The dominance of R8-R14 in H-abstraction at both pressures can be verified by the high concentrations of H2 compared with those of CH4, as shown in Figs. 6(f) and 6(e).

Figure 2 Contributions of three reaction classes to n-C14H30 decomposition in n-C14H30 pyrolysis at (a) 30 Torr and (b) 760 Torr. Sensitivity analysis also indicates that these three reaction classes (R1-R7, R8-R14 and R15-R21) are the most important reaction classes for n-C14H30 decomposition. For these three reaction classes, sensitivity coefficients of same reaction class were summed together to reveal the total influence of corresponding reaction class to n-C14H30 consumption at different 9 ACS Paragon Plus Environment

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temperatures and pressures. Figure 3(a) shows the sensitivity analysis of n-C14H30 for the three reaction classes at 760 Torr and the temperatures of 991, 1044 and 1096 K, corresponding to about 25%, 65% and 95% conversion of n-C14H30, respectively. It shows that sensitivity coefficients of R1-R7 are always the largest at all three temperatures. Furthermore, H-abstraction reaction by CH3 has a much greater sensitivity coefficient than that by H at 991 K, while the opposite result can be found at 1096 K. Figure 3(b) shows the sensitivity analysis of n-C14H30 for the three reaction classes at 30 Torr, 1189 K and 760 Torr, 1044 K, both corresponding to about 65% conversion of n-C14H30. At the two conditions, R1-R7 also have the largest sensitivity coefficients among the three reaction classes. Consequently, the high sensitivity coefficients of the C-C bond dissociation of n-C14H30 (R1-R7) at different temperatures and pressures indicate that the pyrolysis experiment at various pressures is useful for the validation of their pressure-dependent rate constants.

Figure 3 (a) Sensitivity analysis of n-C14H30 at the pressure of 760 Torr and temperatures of 991, 1044 and 1096 K; (b) Sensitivity analysis of n-C14H30 at 30 Torr, 1189 K and 760 Torr, 1044 K. b) Formation and consumption of alkenes 10 ACS Paragon Plus Environment

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As mentioned above, the primary decomposition of n-C14H30 via R1-R21 produces abundant alkyls. Tsang and coworkers41 performed the shock tube pyrolysis of n-octyl iodide to investigate the reactions of octyls. They concluded that β-scission and isomerization are crucial pathways for octyls. Their results were used for the analogous reactions of larger alkyls in present model. β-C-C scission of alkyls will produce smaller 1-alkenes and primary alkyls. β-C-H scission of alkyls will produce H and alkenes. Isomerization reactions of a specific alkyl can produce its isomers with different radical positions. In particular, the isomerization reactions of C13 and smaller primary alkyls are dominant formation pathways of C13 and smaller secondary alkyls since these secondary alkyls can hardly be produced from other sources in n-alkane pyrolysis.

Figure 4 C7-C5 species in n-C14H30 pyrolysis at 30 and 760 Torr. Symbols represent present experiments. Lines represent simulations. In present work, C2-C12 1-alkenes were measured, as presented in Fig. 1 and Figs. 4-6. Generally, present model reproduces measured results satisfactorily. ROP analysis demonstrates


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C2-C12 1-alkenes are mostly yielded by β-scission of larger alkyls, especially from β-C-C scission reactions due to their lower energy barriers than those of β-C-H scission reactions. For C3-C12 1-alkenes observed in this work, their dominant formation pathways are concluded to be β-C-C scission of larger secondary alkyls. Taking 1-octene (1-C8H16) as an example, most of 1-C8H16 is produced from the β-C-C scission reactions of six C9-C14 secondary alkyls, including 7-tetradecyl radical (C14H29X7), 7-tridecyl radical (C13H27X7), 6-dodecyl radical (C12H25X6), 5-undecyl radical (C11H23X5), 4-decyl radical (C10H21X4) and 3-nonyl radical (C9H19X3) (R22-R27) at both 30 and 760 Torr, while only a little is formed from β-C-H scission of C8H17. Among formation pathways of 1-C8H16, β-C-C scission of C14H29X7 (R22) contributes the highest, while that of C11H23X5 (R25) is the second important one. The former can be explained by that C14H29X7 can be produced directly from the primary decomposition of n-C14H30, such as H-abstraction of n-C14H30 (R14, R21), and from isomerization reactions of its C14H29 isomers. The latter is caused by the great carbon flux from the primary undecyl (C11H23X1) to C11H23X5 since 1,5-H shift is quite favored in isomerization of large primary alkyls41.


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Figure 5 C4-C3 species in n-C14H30 pyrolysis at 30 and 760 Torr. Symbols represent present experiments. Lines represent simulations. C14H29X7 (+M) = 1-C8H16 + C6H13X1 (+M)

(R22)

C13H27X7 (+M) = 1-C8H16 + C5H11X1 (+M)

(R23)

C12H25X6 (+M) = 1-C8H16 + PXC4H9 (+M)

(R24)

C11H23X5 (+M) = 1-C8H16 + NXC3H7 (+M)

(R25)

C10H21X4 (+M) = 1-C8H16 + C2H5 (+M)

(R26)

C9H19X3 (+M) = 1-C8H16 + CH3 (+M)

(R27)

The main formation pathways of other C5-C12 1-alkenes are quite similar to those of 1-C8H16, thus will not be discussed herein. For 1-butene (1-C4H8), it can be yielded from larger secondary alkyls via β-C-C scission and from combination of allyl (C3H5XA) and CH3. Propene (C3H6) is mainly yielded from larger secondary alkyls via β-C-C scission. A small part of C3H6 is produced from larger 1-alkenes via retro-ene reactions. For ethylene (C2H4), its production is quite different from those of C3-C12 1-alkenes. β-C-C scission of C3-C14 primary alkyls and the β-C-H scission of C2H5 dominate its production together.


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Figure 6 C3-C0 species in n-C14H30 pyrolysis at 30 and 760 Torr. Symbols represent present experiments. Lines represent simulations. 1-Alkenes can be consumed by unimolecular dissociation and H-abstraction. For C6-C14 1-alkenes, the major consumption reactions are similar. Here 1-C8H16 is also used as an example. Its decomposition is dominated by C(3)-C(4) bond dissociation (R28) to produce C3H5XA and 1-pentyl radical (C5H11X1) at both pressures since this bond has a much lower bond dissociation energy (~73-75 kcal/mol47) than other C-C and C-H bonds in C4 and larger 1-alkenes. Another type of unimolecular decomposition reaction, i.e. the retro-ene reaction (R29), contributes much less to the consumption of 1-C8H16 than R28. This is in accordance with experimental observations in 1-hexene decomposition by Tsang and coworkers48. Moreover, 1-C8H16 can also be consumed via bimolecular reactions (e.g. R30). For C2-C5 1-alkenes, their consumptions are dominated by H-abstraction. 1-C8H16 = C3H5XA + C5H11X1

(R28)

1-C8H16 = 1-C5H10 + C3H6

(R29)

1-C8H16 + H = C5H11X1+ C3H6

(R30)

Sensitivity analysis results of 1-C8H16 at 30 Torr, 1189 K and 760 Torr, 1044 K are shown in Fig. 7. 1-C8H16 formation is sensitive to β-C-C scission of alkyls at both pressures, especially that of C14H29X7 (R22). Then it can be explained that the dominant formation pathway of C14H29X7 (R14) has positive sensitivity coefficient to 1-C8H16. Figure 7 also demonstrates that 1-C8H16 consumption is largely sensitive to R28. The other β-C-C scission pathway of C14H29X7 producing 1-nonene (1-C9H18) and n-pentyl radical (C5H11X1) (R31) shows the second largest negative sensitivity coefficient to the formation of 1-C8H16 due to its competition with R22 in the consumption of C14H29X7. C14H29X7 (+M) = 1-C9H18 + C5H11X1 (+M) 14 ACS Paragon Plus Environment

(R31)

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Figure 7 Sensitivity analysis of 1-C8H16 at 30 Torr, 1189 K and 760 Torr, 1044 K. For 1-alkenes shown in Fig. 1 and Figs. 4-6, maximum concentrations gradually increase with decreasing carbon chain length at both pressures. Similar phenomena can also be observed from previous pyrolysis, oxidation and flame experiments of n-alkanes12,49-52. This can be related to the increasing number of formation pathways with decreasing carbon chain length of 1-alkenes. For 1-dodecene (1-C12H24), it is yielded via β-C-C scission of 4-tetradecyl (C14H29X4) and 3-tridecyl (C13H27X3). For 1-C8H16, it is mainly yielded from alkyls via β-C-C scission. For 1-C4H8, C3H6 and C2H4, they all have ten and more similar formation pathways, as well as other types of important formation pathways as discussed above. Meanwhile, contribution of β-C-C scission reactions of C14H29 to 1-alkenes formation decreases with decreasing carbon chain length of 1-alkenes, e.g. from more than 98% in the formation of 1-C12H24 to around 30% in the formation of 1-pentene (1-C5H10). c) Formation and consumption of other products A series of 1,3-dialkenes were observed at both pressures, as presented in Figs. 4 and 5. In general, they are mostly yielded via H-loss reactions of 1-alkenyls which are H-abstraction products of 1-alkenes. Furthermore, 1,3-butadiene can also be formed from the β-C-C scission 15 ACS Paragon Plus Environment

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reactions of C5 and larger allylic 1-alkenyls. Besides C3H6, there are other four C3 products observed in this work, e.g. C3H5XA, C3H4 isomers and propargyl. C(3)-C(4) bond dissociation of 1-alkenes dominates C3H5XA production. Results of C3H5XA are presented in Fig. 5(e). The consumption of C3H5XA mainly follows the reaction sequence C3H5XA → aC3H4 → pC3H4 → C3H3. This sequence is also responsible for production of C3H4 isomers and C3H3 whose results are shown in Figs. 5(f) and 6(a-b).

Figure 8 Comparisons of modeling results for (a) n-C14H30, (b) H2, (c) 1-C8H16 and (d) C2H4. Symbols represent present experiments. Lines represent simulations by present model (solid), Westbrook model13 (dash), and Ranzi model23 (dot), respectively. Furthermore, the performance of present model is also compared with two previous long chain n-alkane models with n-C14H30 sub-mechanisms, i.e. the Westbrook model13 and Ranzi model23. Figure 8 shows comparisons for n-C14H30, H2, 1-C8H16 and C2H4, representing fuel, final products, large alkenes and small alkenes, respectively. Generally, present model has satisfactory performance in reproducing experiments, especially at low pressure. Results of n-tetradecane pyrolysis are further compared with those of n-decane pyrolysis under similar conditions12. Figure S1 in the Supporting Information 6 presents results of fuel (i.e. n-C10H22 and n-C14H30) and some representative pyrolysis products (H2, 1-C8H16 and C2H4). As 16 ACS Paragon Plus Environment

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shown in Fig. S1, n-tetradecane has lower decomposition temperatures than n-decane at both 30 and 760 Torr. Besides, in n-tetradecane pyrolysis, 1-C8H16 and C2H4 have higher concentrations than those in n-decane pyrolysis. 4.2 Model validations with literature data a) Jet stirred reactor oxidation Mzé-Ahmed and coworkers22 investigated n-tetradecane oxidation using JSR (P = 10 bar, φ = 0.5, 1.0 and 2.0, τ = 1 s, T = 560-1030 K). Figure 9 presents results of n-C14H30, O2 and products at φ = 2.0. Results for φ = 0.5 and 1.0 are illustrated in Figs. S2 and S3 in the Supporting Information 6. Simulations were carried out with Perfectly-Stirred Reactor code45. ROP analysis has been performed at T = 620 K, 720 K and 800 K for n-tetradecane oxidation at φ = 2.0. The analysis shows n-tetradecane decomposition is controlled by H-abstraction via OH attack, forming tetradecyls and H2O. Tetradecyls have different consumption pathways according to the temperatures in the reactor. At 620 K, over 80% of tetradecyls is consumed by O2-addition reaction to form tetradecyl peroxides. As the temperature increases to 800 K, isomerization reactions and β–scission reactions become the dominant pathways to consume tetradecyls. The following chain propagation and chain branching processes are also different at different temperatures. For instance, at 620 K, over 80% of tetradecyls is consumed via O2-addition reactions, forming tetradecyl peroxides. Tetradecyl peroxides mainly proceeds isomerization reactions forming QOOH radicals, which competes with concerted elimination reactions producing an HO2 radical and an olefin. QOOH has multiple consumption pathways such as O2-addition, cyclic ether production, etc. Among them, O2-addition reactions of QOOH producing O2QOOH are dominant. Further decompositions of O2QOOH produce ketohydroperoxides, which serves as chain branching reactions. At 800 K, 17 ACS Paragon Plus Environment

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O2-addition reactions producing tetradecyl peroxides are still the dominant consumption pathways of tetradecyls. However, at this temperature tetradecyl peroxides mainly proceeds concerted elimination reactions rather than isomerization reactions, thus low temperature chain branching is suppressed at this temperature.

Figure 9 Results of n-tetradecane JSR oxidation (P = 10 bar, τ = 1 s, φ = 2.0). Symbols represent literature experiments from Mzé-Ahmed and coworkers22. Solid lines represent simulations by present model. Furthermore, Fig. S4 in the Supporting Information 6 illustrates JSR oxidation experiments22 simulated using literature models, i.e. the Westbrook model13, Mzé-Ahmed model22 and Chang model24. The Westbrook model simulates earlier initial consumption for n-tetradecane, while the Chang model can hardly reproduce n-tetradecane consumption under three equivalence ratios. Besides, the Chang model also over-predicts the formation of C3H6. The Mzé-Ahmed model can well reproduce the consumptions of n-tetradecane and O2. b) Ignition delay times Shen and coworkers20 carried out ignition delay time measurements for n-tetradecane under various conditions. Their results were simulated with Closed Homogeneous Batch Reactor code45. Comparisons of their results and simulations by present model are shown in Fig. 10


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which shows good agreements. In addition, simulations by the Chang model24, Westbrook model13 and Mzé-Ahmed model22 are also presented in Fig. 10.

Figure 10 Ignition delay times of n-tetradecane. Symbols represent literature experiments from Shen and coworkers20. Lines represent simulations by present model (solid), Westbrook model13 (dash), Chang model24 (dot), and Mzé-Ahmed model22 (dot dash), respectively. Brute force sensitivity analysis was carried out at φ =1.0, P =40 atm and T = 900 K to determine the most important reaction for the ignition process, as displayed in Fig. 11. Sensitivity coefficient (σ) can be calculated as: 2 2 ln 0.5 0.5 2 2

ln ln 0.5 0.5

ln

where τ(2k) stands for computed ignition delay time by doubled rate constant k, τ(0.5k) stands for those by halved k. Figure 11 demonstrates the consumption and formation reactions of HO2 play significant roles in ignition of n-C14H30 at 900 K. For instance, H2O2=OH+OH and HO2+HO2=H2O2+O2 have the largest negative and positive σ, respectively. This is because the former reaction produces two OH and the latter reaction terminates two HO2. HO2 can further


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react with allyl radical, producing OH and promotes the reactivity. The chain termination reaction C3H5XA+HO2=C3H6+O2, however, inhibites the reactivity. Moreover, reactions of tetradecyl peroxides that produce alkenes and HO2 are also inhibiting reactions under the temperature investigated.

Figure 11 Sensitivity analysis on the n-tetradecane ignition delay time (φ =1.0, 40 atm and 900 K). Ignition delay times of n-tetradecane are further compared with those of n-decane and n-dodecane under similar conditions20 in Fig. S5 (Supporting Information 6). Present model well predicts results of n-decane, n-dodecane and n-tetradecane. Meanwhile, both experimental and simulated results demonstrate that n-decane, n-dodecane and n-tetradecane have quite similar ignition delay times under the investigated conditions. c) Laminar burning velocities Li and coworkers21 carried out laminar burning velocity measurements of n-C14H30 (P = 1 atm, Tu = 423 and 443 K) with a counterflow twin flame apparatus. Their measurements21 were simulated using Premixed Laminar Flame Speed Calculation code45. Figure 12 presents measured and simulated results by present model. As seen from Fig. 12, present model can


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reproduce the measured data at most conditions, especially φ from 0.7 to 1.3 at both initial temperatures. At the very rich side (φ ≥ 1.3), the experimental uncertainties were reported to be much larger than those at leaner conditions21, while simulations of present model only slightly exceed the upper edge of the uncertainty ranges.

Figure 12 Laminar burning velocities of n-tetradecane (P = 1 atm, Tu = 423 and 443 K). Symbols represent experiments from Li and coworkers21. Solid lines represent simulations by present model, Chang model24, Li model21 and Ranzi model23, respectively. Besides present model, simulations of the Li model21, Chang model24 and Ranzi model23 were also presented in Fig. 12. Ranzi model23 well predicts laminar burning velocities of n-tetradecane at both temperatures. Li model21 can generally reproduce results at Tu = 423 K, but under-predicts results at Tu = 443 K. Chang model24 over-predicts results under most conditions, and their simulations reach maximums at richer conditions than experiments.


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Figure 13 Sensitivity analysis for laminar burning velocities of n-C14H30 (P = 1 atm, Tu = 423 K) at different equivalence ratios (φ). Sensitivity analysis was carried out at Tu = 423 K and φ = 0.7, 1.0 and 1.4 to reveal significant reactions influencing laminar burning velocities of n-tetradecane, as illustrated in Fig. 13. Here, positive (or negative) coefficient denotes the reaction can promote (or inhibit) laminar burning velocities. Figure 13 demonstrates laminar burning velocities of n-tetradecane are controlled by C0-C3 reactions, while the primary decomposition reactions of n-C14H30, e.g. R1-R21, only have negligible roles. Reactions with large positive coefficients are mostly chain branching, chain propagation and CO oxidation reactions at the three equivalence ratios, because these reactions are the major sources of reactive atoms and radicals (e.g. H, O and OH) and heat release. Some chain inhibition and chain termination reactions, such as the combination reaction of H and CH3 to produce CH4, show negative sensitivity coefficients.

5. Conclusions 22 ACS Paragon Plus Environment

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Flow reactor pyrolysis of n-tetradecane (832-1281 K, 30 and 760 Torr) was investigated using SVUV-PIMS. A comprehensive model was constructed and its validation was based on the new pyrolysis data and a series of literature data, such as n-tetradecane JSR oxidation, laminar burning velocities and ignition delay times. For flow reactor pyrolysis, C-C bond dissociation and H-abstraction of n-C14H30 consume most n-tetradecane at both pressures. Sensitivity analysis reveals C-C bond dissociation reactions play a crucial role in n-tetradecane decomposition. For most 1-alkenes, β-C-C scission of alkyls and C(3)-C(4) bond dissociations of 1-alkenes are concluded as their dominant formation and consumption reactions at the pyrolysis conditions, respectively. For low temperature JSR oxidation, n-tetradecane decomposition is dominated by H-abstraction reactions of n-tetradecane to generate tetradecyls, whose further reactions belong to low temperature chain propagation and chain branching reactions. At high pressure and intermediate temperatures, the consumption and formation reactions of HO2 play significant roles on the ignition delay times of n-tetradecane. Lastly, sensitivity analysis indicates laminar burning velocities of n-tetradecane are dominated by C0-C3 reactions. For further development of n-tetradecane model and understanding of n-tetradecane chemistry, more experimental efforts on n-tetradecane combustion over wide conditions will be desired.

Supporting Information. 1. Experimental measured temperature profiles in the flow reactor. 2. High temperature mechanism. 3. Low and high temperature mechanism. 4. Thermodynamic data. 5. Transport data. 6. Table S1, Figures S1-S5. 23 ACS Paragon Plus Environment

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Corresponding author* E-mail: [email protected]. Telephone: +86-21-34204115.

Acknowledgments Authors appreciate funding supports from National Natural Science Foundation of China (91541201, 51622605, 91641205), National Postdoctoral Program for Innovative Talents (BX201600100), China Postdoctoral Science Foundation (2016M600312).

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Figure 1 C14-C8 species in n-C14H30 pyrolysis at 30 and 760 Torr. Symbols represent present experiments. Lines represent simulations. 88x97mm (600 x 600 DPI)

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Figure 2 Contributions of three reaction classes to n-C14H30 decomposition in n-C14H30 pyrolysis at (a) 30 Torr and (b) 760 Torr. 54x25mm (600 x 600 DPI)


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Figure 3 (a) Sensitivity analysis of n-C14H30 at the pressure of 760 Torr and temperatures of 991, 1044 and 1096 K; (b) Sensitivity analysis of n-C14H30 at 30 Torr, 1189 K and 760 Torr, 1044 K. 128x118mm (300 x 300 DPI)


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Figure 7 Sensitivity analysis of 1-C8H16 at 30 Torr, 1189 K and 760 Torr, 1044 K. 67x56mm (600 x 600 DPI)


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Figure 8 Comparisons of modeling results for (a) n-C14H30, (b) H2, (c) 1-C8H16 and (d) C2H4. Symbols represent present experiments. Lines represent simulations by present model (solid), Westbrook model13 (dash), and Ranzi model23 (dot), respectively. 62x48mm (600 x 600 DPI)


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Figure 9 Results of n-tetradecane JSR oxidation (P = 10 bar, τ = 1 s, Φ = 2.0). Symbols represent literature experiments from Mzé-Ahmed and coworkers22. Solid lines represent simulations by present model. 57x41mm (600 x 600 DPI)


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Figure 10 Ignition delay times of n-tetradecane. Symbols represent literature experiments from Shen and coworkers20. Lines represent simulations by present model (solid), Westbrook model13 (dash), Chang model24 (dot), and Mzé-Ahmed model22 (dot dash), respectively. 67x57mm (600 x 600 DPI)


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Figure 11 Sensitivity analysis on the n-tetradecane ignition delay time (Φ =1.0, 40 atm and 900 K). 116x96mm (300 x 300 DPI)


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Figure 12 Laminar burning velocities of n-tetradecane (P = 1 atm, Tu = 423 and 443 K). Symbols represent experiments from Li and coworkers21. Solid lines represent simulations by present model, Chang model24, Li model21 and Ranzi model23, respectively. 43x19mm (600 x 600 DPI)


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Figure 13 Sensitivity analysis for laminar burning velocities of n-C14H30 (P = 1 atm, Tu = 423 K) at different equivalence ratios (Φ). 156x175mm (300 x 300 DPI)


A comprehensive experimental and kinetic modeling study of n

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