Experimental and Kinetic Modeling Study of 2-Methylfuran Pyrolysis at

Nov 25, 2016 - Experimental and Kinetic Modeling Study of 2-Methylfuran Pyrolysis at Low and Atmospheric Pressures ... *Telephone: +86-22-85356251...
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Experimental and Kinetic Modeling Study of 2‑Methylfuran Pyrolysis at Low and Atmospheric Pressures Zhanjun Cheng,†,‡,§ Sirong He,‡ Lili Xing,∥ Lixia Wei,‡ Wei Li,∥ Tianyu Li,∥ Beibei Yan,*,†,§ Wenchao Ma,†,§ and Guanyi Chen†,‡,§ †

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, People’s Republic of China National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People’s Republic of China ‡ State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China § Tianjin Engineering Research Center of Biomass-Derived Gas/Oil Technology, Tianjin 300072, People’s Republic of China ∥

S Supporting Information *

ABSTRACT: The pyrolysis of 2-methylfuran (MF) was studied from 900 to 1530 K in a flow reactor at 30 and 760 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography were used for identification and mole fraction measurements of pyrolysis products, especially free radicals and aromatics. Specific products were observed for the two main unimolecular decomposition pathways of MF, such as propargyl radical, methyl radical, 1-butyne, and carbon monoxide. A previous kinetic model of MF was updated on the basis of recent theoretical and modeling progresses, especially for the aromatic formation sub-mechanism. The updated model was validated by the new pyrolysis data. Kinetic analyses including rate of production analysis, and sensitivity analysis were used to provide insight into the pyrolysis chemistry of MF, especially the decomposition of MF and aromatic formation. The pyrolysis of MF was found with the kinetic characteristics of both furan pyrolysis and 2,5-dimethylfuran (DMF) pyrolysis. The decomposition of MF in pyrolysis is initiated by the unimolecular dissociation pathway, producing propargyl and acetyl radicals. Aromatics were observed with higher concentration levels in MF pyrolysis than those in DMF pyrolysis. The high concentration levels of precursors of aromatics, especially propargyl radical, are responsible for the enhanced aromatic formation in MF pyrolysis.

1. INTRODUCTION Furan and its derivatives, such as 2-methylfuran (MF) and 2,5dimethylfuran (DMF), are being considered as a new kind of biofuel as a result of the potentially high efficiency production from cellulosic biomass.1−3 Among them, MF is the simplest alkylated furanic compound and can be formed in the manufacture of DMF.1 It has a larger octane number than gasoline and DMF4 and has more significant advantages when it is applied in engine combustion.5−8 On the other hand, the unsaturated molecular structures of furanic fuels have the potential to yield a considerable amount of aromatic hydrocarbons, which are widely acknowledged as the key precursors of soot.9 Pyrolysis experiments are broadly used to understand the thermal decomposition mechanism and molecular growth mechanism in combustion models, because the isolation from the oxidizer provides a relatively simple and specific reaction system. It is recognized that most of the previous studies focused on the oxidation and flames of MF, including ignition delay times,10,11 laminar burning velocities,7,8,10 and species profiles in premixed flames.12−15 Only two studies have been performed on the pyrolysis of MF. Grela et al.16 studied the pyrolysis of MF in a flow reactor at 1 mTorr using online electron ionization molecular-beam mass spectrometry (EIMBMS). They proposed the decomposition pathways of MF but did not report the quantitative results of concentrations. Lifshitz et al.17 investigated the pyrolysis of MF behind © 2016 American Chemical Society

reflected shock waves at 2−3 atm. Gas chromatography (GC) combined with mass spectrometry (MS) was used to measure only some stable species. A simple mechanism was developed by estimation to analyze the experimental results. However, no information on free radicals was reported in the pyrolysis of MF. On the basis of the previous work of MF, several kinetic models of MF combustion have been developed in recent years. Somers et al. developed a MF model10 and updated it with recently calculated reaction parameters.18 This model was validated against most of the previous experimental results of MF, including ignition delay times,10,11 laminar burning velocities,7,10 and species profiles in shock-tube pyrolysis17 and premixed flames.13 Tran et al.13,14 also developed a kinetic model of MF combustion and demonstrated the chemical structures of low-pressure premixed laminar flames of MF as well as the influences of the substituents on the formation of polycyclic aromatic hydrocarbons (PAHs). In this work, the pyrolysis of MF was investigated in a flow reactor at 30 and 760 Torr. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) and GC were used to achieve comprehensive measurements of the pyrolysis products, especially free radicals and aromatic products. For deep insight into the kinetics in the Received: September 18, 2016 Revised: November 6, 2016 Published: November 25, 2016 896

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Energy & Fuels

chemical structures of the important species discussed in this work are also listed in Table S2 of the Supporting Information.

decomposition of MF and the formation of aromatics, a modified model based on Somers model18 was used to simulate the new pyrolysis experiments.

4. RESULTS AND DISCUSSION A series of pyrolysis products were detected and identified in this work, especially radicals, such as methyl (CH3), propargyl (C3H3), and benzyl (C6H5CH2), which play important roles in the processes of MF decomposition and aromatic formation/ growth. The experimental and simulated mole fraction profiles of fuel and decomposition products in MF pyrolysis at 30 and 760 Torr are shown in Figures 1−3. The mole fraction profiles

2. EXPERIMENTAL SECTION The experiments were performed at the National Synchrotron Radiation Laboratory in Hefei, China. For the SVUV-PIMS experiments, detailed descriptions of the beamlines and pyrolysis apparatus have been reported elsewhere.19−28 Only a brief description of the pyrolysis apparatus will be provided here. The pyrolysis experiments were carried out in an α-alumina flow tube with 7.0 mm inner diameter and 150 mm length heated by a high-temperature furnace. The pyrolysis species were sampled at 10 mm downstream from the outlet of the flow tube by a quartz nozzle with different orifices at the tip (500 μm for 30 Torr and 50 μm for 760 Torr). The experimental conditions are listed in Table 1. The methods of mole fraction

Table 1. Experimental Conditions

a

p (Torr)

Xfuel (%)

XAr (%)

Qa (SLM)

MF (mL/min)

30 760

2 2

98 98

1.000 1.000

0.081 0.081

Q is the total flow rate of the inlet mixture at 300 K.

evaluation, temperature distribution measurement, and pressure calculation along the reactor centerline have been introduced in detail elsewhere.23,29,30 The temperature profiles can be found in the Supporting Information. Each temperature profile is named after its maximum value (Tmax). The uncertainties of Tmax are estimated to be within ±30 K. The photoionization cross sections (PICSs) of most pyrolysis products are available in our online database.31 The uncertainties of mole fractions are evaluated to be within ±25% for products with known PICSs and a factor of 2 for those with estimated PICSs. In addition, GC/MS was also used to measure and identify the MF pyrolysis products at 30 and 760 Torr. The pyrolysis apparatus used in the GC experiments is the same as that used in the SVUV-PIMS experiments, and the experimental conditions are identical in both experiments. A detailed description of the apparatus can be found elsewhere.32 The methods of directly injecting standards when available and effective carbon number were used for calibration. Relative uncertainty in mole fractions was estimated to within ±10% for products with direct calibration and within ±20% for the rest.

Figure 1. Experimental and simulated mole fraction profiles of 2methylfuran (MF), 1-butyne (C4H6-1), ethane (C2H6), methyl (CH3), propargyl (C3H3), and CO. Open symbols, solid symbols, solid lines, and dotted lines represent the SVUV-PIMS data, GC data, the simulated results by the updated model, and the simulated results by the Somers model,18 respectively.

of the species detected by SVUV-PIMS and GC are in good agreement with each other, which are shown in Figure S1 of the

3. KINETIC MODELING The simulations were performed with the plug flow reactor module in the CHEMKIN-PRO software.33 Basically, in the plug flow reactor code, “fix gas temperature” is considered as the problem type to start. The temperature profile along the reactor axis, inner diameter of the reactor, pressures, and inlet concentration of reactants were used as input parameters during the simulation. The predicted concentration at the reactor outlet is the simulation result that is used to compare to the experimental result at corresponding Tmax. Somers model18 was updated to better simulate the pyrolysis of MF, especially the formation of aromatics by modifying and adding the aromatic sub-mechanism. For example, aromatics observed in this work, including phenylacetylene (C6H5C2H), indene (C9H8), indenyl radical (C9H7), and naphthalene (C10H8) were not included in the Somers model. The updated reactions and thermodynamic data were mainly taken from the recently reported comprehensive toluene model.34,37 The updated model had been validated against our premixed flame study.38 The modifications of some key reactions updated in our recent work are listed in Table S1 of the Supporting Information. The

Figure 2. Experimental (symbols) and simulated (lines) mole fraction profiles of hydrogen (H2), methane (CH4), propyne (C3H4-p), allene (C3H4-a), vinylacetylene (C4H4), and furan. Open symbols represent SVUV-PIMS data, and solid symbols represent GC data. 897

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Figure 3. Experimental (symbols) and simulated (lines) mole fraction profiles of 1-oxo-1,3-butadiene (CH2CHCHCO) and furan. Open symbols represent SVUV-PIMS data, and solid symbols represent GC data. Figure 5. Sensitivity analysis of MF at 1442 K and 30 Torr and at 1237 K and 760 Torr.

Supporting Information. Comparisons of simulated results by the updated model and Somers model for this work and the shock tube pyrolysis experiments of Lifshitz et al.17 are shown in Figures S2 and S3 of the Supporting Information. On the basis of the rate of production (ROP) analysis, the reaction networks of MF decomposition at 1442 K and 30 Torr and at 1237 K and 760 Torr are drawn in Figure 4, where about 75% of MF is consumed and most products reach high concentration levels. The sensitivity analyses of MF at 1442 K and 30 Torr and at 1237 K and 760 Torr are displayed in Figure 5 to reveal the most sensitive reactions to MF decomposition at both low and atmosphere pressures.

4.1. Decomposition of MF. Figure 1a shows the experimental and simulated mole fraction profiles of MF. The updated model can reasonably reproduce the decomposition trends of MF. According to the ROP analysis, the consumption of MF is mainly controlled by the unimolecular isomerization reactions (reactions R1 and R2) and radical attack reactions (reactions R3−R8). MF = CH3CHCCHCHO‐t

(R1)

Figure 4. Reaction network in the decomposition of MF at (a) 1442 K and 30 Torr and (b) 1237 K and 760 Torr. The numbers reflect the percentages of corresponding pathways in total reaction flux. 898

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Energy & Fuels MF = CH3COCHCCH 2‐t

(R2)

MF + H = MF22J + H 2

(R3)

MF + CH3 = MF22J + CH4

(R4)

MF + C3H3 = MF22J + C3H4‐p/C3H4‐a

(R5)

MF + H = CH 2CHCHCO + CH3

(R6)

MF + H = furan + CH3

(R7)

MF + H = C4 H 71‐1 + CO

(R8)

Among them, H2, CH4, and C3H4-p/C3H4-a are mainly formed from the H-abstraction reactions of MF by the H atom (reaction R3), CH3 (reaction R4), and C3H3 (reaction R5), while 1-oxo-1,3-butadiene (CH2CHCHCO) and furan are mainly produced from the H addition reactions (reactions R6 and R7). Besides, H-addition reactions of MF (reaction R8) also lead to the formation of 1-buten-1-yl (C4H71-1) with minor contribution. By H-abstraction reactions, MF is transformed to 2-furanylmethyl (MF22J), which mainly leads to the formation of C4H4 by isomerization and subsequent decomposition reaction. Furthermore, the sensitivity analysis of MF in Figure 5 can reveal the influences of the unimolecular decomposition and Habstraction reactions on the consumption of MF at both 30 and 760 Torr. It is observed that the unimolecular decomposition pathway of reactions R2 and R10 producing C3H3 and CH3CO are sensitive at both low and atmospheric pressures. This is reasonable because this pathway produces two radicals, while the other unimolecular decomposition pathway (reactions R1 and R9) produces two stable species (C4H6-1 and CO). Thus, reactions R2 and R10 play key roles as the chain initiation steps in MF pyrolysis. Sensitivity analysis also shows that these two unimolecular reaction pathways are more important at 30 Torr than at 760 Torr, and the H-abstraction reactions become more and more important with the pressure increasing. On the basis of the above discussion, the products of unimolecular decomposition, H-addition, and H-abstraction reactions should play key roles in the pyrolysis of MF, including acetylene (C2H2), ethene (C2H4), ethane (C2H6), propene (C3H6), 1,3-butadiene (C4H6), 1,2-butadiene (C4H6-12), furan, and CH2CHCHCO. The simulated and experimental mole fraction profiles of these species are shown in Figure 3 and Figure S4 of the Supporting Information. Only the results of CH3, C2H6, furan, and CH2CHCHCO are discussed here. The formation of C2H6 is dominated by the self-combination reaction of CH3, CH3 + CH3 = C2H6 (reaction R11). It was found that Somers model overpredicted C2H6 by a factor of 2 (Figure 1c) and underestimated CH3 (Figure 1d). In fact, the rate constant of reaction R11 is taken from Wang et al.35 in Somers model. Thus, the rate constant in GRI36 for reaction R11 was adopted in this work. It can be seen that this replacement improves the prediction of C2H6 and CH3. With regard to CH2CHCHCO and furan, both of them were produced from the H-addition reactions on MF via reactions R6 and R7, respectively. Because reactions R6 and R7 are competing reactions and the products are a pair of isomers, clarifying the formation of these two species may improve the understanding of MF pyrolysis. The experimental and simulated results of these two species are shown in Figure 3. It should be pointed out that the PICS of CH2CHCHCO is adopt from that of 1-oxo-1,3,4-pentatriene (P134TE1O) by analogy, as described in the recent work.38,40 As a result of the close ionization energy, it is difficult to distinguish these two species totally. Therefore, we assume that the products of m/z 68 all corresponded to CH2CHCHCO in PIMS experiments. It can be seen that this species is overpredicted. This can be attributed to the high error bar of the experimental results and the estimated parameters of the consumption reactions of CH2CHCHCO. In the future, more experimental and theoretical work should be focused on the reactions of CH2CHCHCO. However, the updated model can still well reproduce the formation of furan at both low and atmospheric pressures in MF pyrolysis.

Among them, the unimolecular isomerization pathways of reactions R1 and R2 via the corresponding H migration to produce the respective cyclic carbenes have higher contributions to the consumption of MF, as shown in Figure 4. For reaction R1, the H atom on C(3) migrated to C(2) to form a carbene, which can easily transform to 1-oxo-2,3-pentadiene (CH3CHCCHCHO-t). The subsequent decomposition reaction of CH3CHCCHCHO-t leads to the formation of 1-butyne (C4H6-1) and carbon monoxide (CO). CH3CHCCHCHO‐t = C4H6‐1 + CO

(R9)

ROP analysis indicates that the formation of C4H6-1 is totally controlled by reaction R9 at both pressures. The mole fraction profiles of C4H6-1 and CO are shown in panels b and f of Figure 1. It can be seen that the peak mole fraction of C4H6-1 drops substantially with the increase of the pressure. This reveals the decreasing importance of the unimolecular decomposition reactions at a higher pressure in MF pyrolysis. The other important unimolecular decomposition pathway is through β-carbene, yielding the intermediate of 2-oxo-3,4pentadiene (CH3COCHCCH2-t) by the migration of the H atom on C(4) to C(5). The decomposition reaction of CH3COCHCCH2-t leads to two important radicals, C3H3 and acetyl (CH3CO). CH3COCHCCH 2‐t = C3H3 + CH3CO

(R10)

CH3CO can easily decompose to CH3 and CO. The mole fraction profiles of CH3 and C3H3 are shown in panels d and e of Figure 1. According to the ROP analysis, the two unimolecular decomposition pathways, reactions R1 and R2, contribute about half of MF consumption together at 30 Torr. The contribution of unimolecular decomposition in MF pyrolysis is much higher than that in DMF pyrolysis (only 16%22) at 30 Torr. It can be seen that the reaction pathway of reactions R1 and R9 producing C4H6-1 and CO is similar to the dominant unimolecular decomposition pathway producing propyne (C3H4-p) and CO in furan pyrolysis.39 The other reaction pathway of reactions R2 and R10 producing C3H3 and CH3CO is similar to the main unimolecular decomposition pathway producing 1,2-butadien-1-yl (C4H5-1s) and CH3CO in DMF pyrolysis.22 Therefore, the unimolecular decomposition kinetics of MF pyrolysis show the characteristics of those of both furan and DMF pyrolysis. According to the ROP analysis, H-addition and Habstraction reactions contribute substantially to the consumption of MF. The contributions of these reactions become more important at a higher pressure. H-addition and H-abstraction reactions of MF lead to the formations of hydrogen (H2), methane (CH4), C3H4-p, allene (C3H4-a), vinylacetylene (C4H4), and furan (C4H4O). The experimental and simulated mole fraction profiles of these species are shown in Figure 2. 899

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Energy & Fuels 4.2. Formation of Aromatics. A series of monocyclic aromatic hydrocarbons (MAHs) and PAHs, including benzene (C6H6), C6H5CH2, toluene (C6H5CH3), C6H5C2H, styrene (C6H5C2H3), ethylbenzene (C6H5C2H5), C9H8, C10H8, etc., were detected and measured in this work. The experimental and simulated mole fraction profiles of these aromatics are shown in Figure 6. The main reaction network of aromatic formation in MF pyrolysis is shown in Figure 7.

C6H6 is the most important aromatic product in MF pyrolysis, with its highest yields and its crucial role in the formation of other aromatic products, as shown in Figure 6a and Figure 7, respectively. Its formation is mainly controlled by the self-combination reaction of C3H3 (reaction R12, C3H3 + C3H3 = C6H6) at a low pressure. As seen from Figure 6a, Somers model overpredicts C6H6 at 30 Torr and slightly underestimates C6H6 at 760 Torr. Therefore, in this work, the rate constants of reactions R12 and R13 (C3H3 + C3H3 = C6H5 + H) adapted from the work41 in Somers model were updated from the kinetic model of allene and propyne flames.42 As shown in Figures 1e and 6a, the simulated results of C3H3 and C6H6 by the updated model agree well with the experimental results. At 760 Torr, the reaction of C4H4 + vinyl radical (C2H3) = C6H6 + H is more important for C6H6 formation as a result of the abundant yields of C4H4 and C2H4 in MF pyrolysis. Sensitivity analysis of C3H3 and C6H6 at 30 Torr are displayed in Figure S5 of the Supporting Information. It indicates that the consumption of C3H3 and the formation of C6H6 are both controlled by reaction R12. Furthermore, Somers et al.18 proposed that C6H6 can also be produced from the combination of C3H3 + allyl radical (C3H5-a) in their model. However, theoretical studies43,44 indicated that the combination of C3H3 and C3H5-a tends to produce fulvene rather than C6H6. Thus, reaction 14 (C3H3 + C3H5-a = fulvene + H + H) was adopted in the updated model, with the calculated rate coefficient reported by Georgievskii et al.44 Two C7 aromatics, C6H5CH2 and C6H5CH3, were detected in this work, as shown in panels b and c of Figure 6. It can be seen that Somers model well reproduced C6H5CH2. ROP analysis indicates that C6H5CH2 was mainly produced from the combination reaction of C3H3 and C4H4 (reaction 15, C3H3 + C4H4 = C6H5CH2). Recently, Trogolo et al.45 performed a comprehensive theoretical study on the reactions between C3H3 and C4H4, and their calculated rate constants were adopted in the updated model. The ROP analysis shows that reaction 15 contributes minor to C6H5CH2 formation at both 30 and 760 Torr. Thus, there should be another reaction accounting for the formation of C6H5CH2. Klippenstein et al.46 proposed that the chemically activated process C6H5CH2 + H = phenyl (C6H5) + CH3 (reaction R16) also contributes to the formation of C6H5CH2 and theoretically calculated its rate constant, which was adopted in the updated model. On the basis of the ROP analysis, reaction R16 was found to dominate the formation of C6H5CH2 at 30 Torr as a result of the abundant production of C6H5 from reaction R13. It can be seen that the simulated results of C6H5CH2 was overpredicted by a factor of 2. This can be attributed to the higher uncertainty on PICSs of C6H5CH2. For C6H5CH3, it can be formed from the combination reactions of C3H3 + C4H6 (R17) and C6H5 + CH3 at 30 Torr, and R17 also controls its formation at 760 Torr. It is mainly consumed by H atom abstraction reactions producing C6H5CH2 at both 30 and 760 Torr.

Figure 6. Experimental and simulated mole fraction profiles of benzene (C6H6), benzyl radical (C6H5CH2), toluene (C6H5CH3), phenylacetylene (C6H5C2H), styrene (C6H5C2H3), ethylbenzene (C6H5C2H5), indene (C9H8), and nathalene (C10H8). Symbols, solid lines, and dotted lines represent the SVUV-PIMS data, the simulated results by the updated model, and the simulated results by the Somers model,18 respectively.

C6H5CH 2 + H = C6H5 + CH3

(R16)

C3H3 + C4 H6 = C6H5CH3 + H

(R17)

For C8 aromatics, C6H5C2H, C6H5C2H3, and C6H5C2H5 were detected and measured in this work, as shown in panels d−f of Figure 6. According to the ROP analysis, C6H5C2H is mainly formed from the combination reaction of C6H5 + C2H2/C4H4 at both 30 and 760 Torr. For C6H5C2H3, its formation is mainly controlled by the combination reactions of two C4H5

Figure 7. Main reaction network of aromatic formation at 30 Torr (red arrows) and 760 Torr (black arrows) in MF pyrolysis.

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Energy & Fuels radicals (C4H5-n and C4H5-2) + C4H4 and the β-C−H scission reaction of C6H5CHCH3, which is the H atom abstraction product of C6H5C2H5 at both pressures. Slavinskaya and Frank47 proposed a combination reaction of cyclopentadienyl (C5H5) and C3H3 radicals (reaction R18, C5H5 + C3H3 = C6H5C2H3) in their aromatic formation mechanism. The rate constant of reaction R18 is analogous to reaction R12, and this reaction was added to the updated model. The uncertainty of the rate constants of these reactions may account for the discrepancies between the experimental and simulated results of C6H5C2H3. As a result of the large formation of C6H5CH2 and CH3, the formation of C6H5C2H5 is controlled by the combination of C6H5CH2 and CH3 (reaction R19, C6H5CH2 + CH3 = C6H5C2H5). In Somers model, the rate constant of reaction R19 taken from ref 48 can lead to overconsumption of C6 H5CH2 and, hence, the overpredicted formation of C6H5C2H5. The rate constant of reaction R19 taken from the literature48 was calculated by the statistical adiabatic channel model (SACM) to agree with the measured dissociation rate constant of C6H5C2H5 from their laser photolysis experiment. The rate constant with pressure dependence of reaction R19 is also still lacking, and SACM is a single-fit-parameter version of the theory. These combined factors lead to the uncertainty in the simulations. Thus, the rate constant of reaction R19 is taken from the recent toluene combustion work.34 It can be seen that the simulated results by the updated model can well reproduce the experimental results. Figure 6 also displays the experimental and simulated mole fraction profiles of two PAHs, i.e., C9H8, and C10H8. On the basis of the ROP analysis, the formation of C9H8 is controlled by the reaction C6H5 + C3H3 = C9H8 (reaction R20) at 30 Torr. At 760 Torr, C9H8 is produced from the combination reactions of C6H5CH2 + C2H2, C9H7 + H, and fulvenallenyl radical (C7H5) + C2H4. The formation of C7H5 is controlled by the combination reaction of C4H2 and C3H3 at both 30 and 760 Torr. Besides, the formation of C10H8 is mainly from the combination reactions of C6H5CH2 + C3H3, C9H7 + CH3 and C6H5 + C4H4 at both 30 and 760 Torr, and it can also be produced by the combination reaction of C7H5 + C3H3 at 760 Torr. On the basis of the above discussions, it is concluded that C3H3 and C4 species are the most important precursors of MAHs in MF pyrolysis, while PAHs are mainly produced from the combination reactions of C5−C7 precursors (especially C6− C7 MAHs) and C2−C4 species. Most of these aromatic products were also detected in DMF pyrolysis.22 Figure 8 shows the measured mole fractions of C6H6, C6H5CH2, C6H5C2H, and C10H8 at the same conversion of fuel in MF pyrolysis and DMF pyrolysis. It is obvious that their mole fractions in MF pyrolysis are generally higher than those in DMF pyrolysis. In MF pyrolysis, direct production of C3H3 from the initial decomposition of fuel will greatly elevate its concentration levels, and the formation of a large amount of C4H5-n also results in high concentration levels of C4 species. In fact, the concentration levels of C3H3 in MF pyrolysis are more than an order of magnitude higher than those in DMF pyrolysis,22 which favors the formation of C6−C7 MAHs and, consequently, the formation of large MAHs and PAHs.

Figure 8. Comparison of measured mole fractions of C6H6, C6H5CH2, C6H5C2H, and C10H8 at 75% fuel conversion in MF pyrolysis (gray) and DMF pyrolysis (black).

especially free radicals and aromatic species, and their mole fraction profiles were determined. The experimental results of both diagnostic tools are consistent with each other. A recent MF model of Somers et al.18 was updated in this work, especially for the aromatic growth sub-mechanism, and was validated by the new pyrolysis data. Kinetics in the decomposition of MF and the formation of aromatics were discussed. It is concluded that the pyrolysis of MF has the kinetic characteristics of both furan pyrolysis and DMF pyrolysis. The decomposition of MF in pyrolysis is initiated by the unimolecular dissociation pathway, producing propargyl and acetyl radicals, resulting in high concentration levels of the propargyl radical. Aromatic products were observed with high concentration levels in MF pyrolysis compared to those in DMF pyrolysis. The high concentration levels of precursors of aromatics, especially the propargyl radical, are responsible for the enhanced aromatic formation in MF pyrolysis.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02399. Figures S1−S5 and Tables S1 and S2 (PDF) Temperature profiles (XLSX) Reaction mechanism (TXT) Thermodynamic data (TXT) Transport data (TXT)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-22-85356251. E-mail: [email protected]. ORCID

Beibei Yan: 0000-0003-0499-8712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the funding support from the National Natural Science Foundation of China (51506143 and 51476155), the National Basic Research Program of China (973 Program, 2013CB834602), the Anhui Science and Technology Department (1408085J09), the Open Fund of

5. CONCLUSION The flow reactor pyrolysis of MF was studied at pressures of 30 and 760 Torr and temperatures of 900−1530 K using SVUVPIMS and GC. The pyrolysis products were identified, 901

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Article

Energy & Fuels

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Key Laboratory of Biomass Energy and Material in Jiangsu Province (JSBEM201405), and the Chinese Academy of Sciences. The authors are grateful to Dr. Jiuzhong Yang, Dr. Zhandong Wang, Dr. Wenhao Yuan, and Dr. Long Zhao for their help.

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NOMENCLATURE 2MF = 2-methylfuran DMF = 2,5-dimethylfuran SVUV-PIMS = synchrotron vacuum ultraviolet photoionization mass spectrometry EI-MBMS = electron ionization molecular-beam mass spectrometry GC/MS = gas chromatography combination with mass spectrometry PICS = photoionization cross section ROP = rate of production

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DOI: 10.1021/acs.energyfuels.6b02399 Energy Fuels 2017, 31, 896−903

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DOI: 10.1021/acs.energyfuels.6b02399 Energy Fuels 2017, 31, 896−903