The chemical mechanism of exhaust gas recirculation on polycyclic

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The chemical mechanism of exhaust gas recirculation on polycyclic aromatic hydrocarbons formation based on LIF measurement Peng Liu, Yiran Zhang, Lijun Wang, Bo Tian, Bin Guan, Dong Han, Zhen Huang, and He Lin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00422 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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The chemical mechanism of exhaust gas recirculation on polycyclic aromatic hydrocarbons formation based on LIF measurement Peng Liua, Yiran Zhanga, Lijun Wanga, Bo Tianb, Bin Guana, Dong Hana, Zhen Huanga, He Lin*a a

Key Laboratory for Power Machinery and Engineering of Ministry of Education, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China b

Department of Engineering, University of Cambridge, CB21PZ, Cambridge, UK

Abstract: Exhaust gas recirculation (EGR) has been widely used in engine to meet current emission regulations. Investigating the chemical mechanism of EGR on PAHs (polycyclic aromatic hydrocarbons, the precursor of soot) formation in premixed flames contributes to understanding the EGR-dependence on soot formation in engine. In this study, the influences of flame temperature, equivalence ratio and CO2 addition on the formation of PAHs was systematically investigated in premixed C2H4/O2/Ar/CO2 flames using laser induced fluorescence (LIF) technology. The temperature dependence of PAHs formation was studied at fixed equivalence ratio and dilution ratio. It was found that the LIF signal of PAH reaches the maximum value around 1730 K, and decreases at lower or higher temperature in this study. The LIF signal of PAHs almost increases linearly with equivalence ratio, as the maximum flame temperature and dilution ratio are kept constant. The experimental results show that the CO2 addition in the inlet gas suppresses PAHs formation due to the chemical inhibition effect. The thermal effect of CO2 addition on PAHs formation is highly sensitive to flame temperature. The PAHs reaction mechanisms proposed by Appel et al. and Wang et al. are used to clarify the experimental results. The first-order temperature sensitivity analysis showed that the hydrogen-abstraction-carbon-addition pathway with high reaction reversibility should account for temperature effects on PAHs formation. The pathway sensitivity analysis showed that CO2 inhibition chemical effect is realized thought the route CO2 (+H) → OH → C3H3 (C2H2) → A1 → PAHs with the assistance of the entrance reaction CO2+H=CO+OH.

Keyword: LIF; PAHs Formation; CO2; Sensitivity Analysis; EGR.

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1.Introduction The formation of polycyclic aromatic hydrocarbons (PAHs) and soot particles in combustion has been extensively investigated during the last two decades due to their great influence on human health and natural environment.1-5 Vehicle engines contribute more than 20% PAHs and particle matter (PM) emissions in metropolitans6. To reduce the emissions of these pollutants from vehicle engines, strict emission regulations have been implemented in many countries. Exhaust gas recirculation (EGR) has been widely used in diesel engine to meet current emission regulations.7,8 It is well known that EGR can reduce the NOx emissions by decreasing the combustion temperature. However, the influences of EGR on the soot emissions are complex.9 For example, soot emissions increase rapidly with increased EGR rate in conventional diesel combustion mode.10-13 However, it was reported that the ultra-low soot and NOx emissions can be achieved in the low temperature combustion (LTC) strategy when EGR is heavily used.8,14 As a major constituent of EGR, the influences of CO2 addition on soot formation are receiving increased attention. Most studies have shown that the addition of CO2 in flame suppresses the formation of soot.15-17 But no consensus is obtained in terms of the reduction mechanism by the CO2 addition. McLintock15 and Oh et al16 claimed that the soot reduction results from the chemical effect of CO2. While, Schug et al17 indicated that the soot reduction is completely caused by the thermal effect of CO2. Some experimental findings18,19 showed that the thermal and chemical effect of CO2 should both account for the soot reduction. On the other hand, it was found that the soot formation can also be promoted by CO2 addition in some cases. Recently, Abián found that the soot formation in the thermal decomposition of C2H4O2 mixtures is promoted with 25 % CO2 addition.20 Further, Teini 21 observed an increase in soot generation when a small amount CO2 is added to the rich C2H2/N2/O2/Ar mixtures in a rapid compression facility. Based on the above analysis, the influences of CO2 on soot formation in flame may depend on the combustion condition, and the mechanism of EGR on soot formation is still under determined. Accurate soot model is the key to understand the mechanism of EGR on soot formation. To date, one of the challenges to obtain reliable soot model is to develop accurate gas-phase PAHs chemistry, which determines the soot particle dynamics.22,23 In recent 20 years, significant progress has been obtained in the PAHs formation mechanisms.3,24-31 Appel et al.22 proposed a PAHs mechanism (called ABF mechanism) for C2 fuels based on the hydrogen-abstraction-carbon-addition (HACA) route. Wang et al.32 proposed a PAHs mechanism for C1-C4 fuels (called KM2 mechanism) with the USC mech II33 as the base mechanism, and emphasized the synergistic effect of fuels in the PAHs formation.32,34,35 Nowadays, the PAHs mechanism is widely merged into the liquid transportation fuel mechanisms for PAHs and soot prediction in engine.3,23,25,32,36-38 Wang et al37 embedded a detailed PAHs mechanism in a n-heptane-n-butanol mechanism to investigate the influence of fuel structure on PAHs and soot formation. Liu et al38 developed a reduced toluene reference fuel-2,5-dimethylfuran-PAHs mechanism, which is applied to study the effect of 2 ACS Paragon Plus Environment

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combustion mode on PAHs and soot formation in spark ignition and compression ignition engines. However, the validation of PAHs mechanisms was mainly focused on the prediction accuracy of PAHs concentration in several target flames.3,24-29 As is known, the PAHs formation is very sensitive to the combustion temperature, equivalence ratio and initial mixture composition.39,40 However, the dependence of the PAHs formation on these parameters is rarely considered in the PAHs mechanism validation due to the lack of experimental data. This may limit the application of PAHs mechanism to the EGR-condition flames. For example, Liu et al.41 attempted to clarify the influence of CO2 addition on soot formation in C2H4/air diffusion flames by numerical simulation with the PAHs mechanism proposed by Slavinskaya and Frank.42 However, the simulation results failed to reproduce the main features of the experimental measurement. Therefore, there is an urgent demand for the temperature-dependent, equivalence-ratio-dependent and CO2content-dependent PAHs profiles. This work attempts to investigate the influences of temperature, equivalence ratio and CO2 addition on the PAHs formation in premixed C2H4/O2/Ar (CO2) flames using the laser induced fluorescence (LIF) technology and kinetic analysis. Further, the toughness of two PAHs mechanisms at different temperatures, equivalence ratios and CO2 addition condition are examined based on the PAHs detection. Finally, the sensitivity and reaction pathway analysis are carried out to clarify the experimental results.

2. Experimental and Simulation Methodologies 2.1. Burners and flames In this study, the premixed C2H4/O2/Ar (CO2) flames were stabilized on a movable burner consisting of a water-cooled bronze porous plug of 5 cm in diameter at atmospheric pressure. The shielding argon flowed through a concentric porous ring to isolate the flame from the ambient air. The aluminium sheet was positioned at the distance of 30 mm above the burner surface to stabilize the flame. In the experiment, it is very difficult to keep a single variable (temperature, equivalence ratio, dilution ratio and total flow) constant. Given that the PAHs formation is highly sensitive to temperature and equivalence ratio,24 the total flow was changed in each experiment matrix to keep other parameters constant as the influences of a given variable on PAHs formation are investigated in this study. Therefore, all experimental data were compared at a constant residence time to exclude the effects of total flow change. The residence time was calculated by considering the axial convective velocity and thermophoresis in the limit of hard sphere and free molecular regime.43 The operating flow conditions for the premixed C2H4/O2/Ar (CO2) flames are shown in Table 1. Specifically, the premixed C2H4 flames with different total flow including 5, 6, 7, 8, 9 L/min at fixed equivalence ratio (Φ=2.3) and fixed dilution ratio (60 %) were performed to investigate the influences of temperature. The equivalence ratios including 2.1, 2.2, 2.3 and 2.4 at fixed maximum flame temperature and 3 ACS Paragon Plus Environment

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fixed dilution ratio (60 %) were carried out to study the influences of equivalence ratio. The chemical effect of CO2 on PAHs formation was paid great attention in this study. To this end, the maximum flame temperature and dilution ratio were fixed, only the ratios of CO2 in total dilution gas (mole fraction of CO2/(mole fraction of CO2+ mole fraction of Ar)) change with the value of 0%, 20%, 40% and 60% respectively, as shown in Table 1.

Table 1. Flow conditions for the laminar premixed C2H4/O2/Ar flames.

Equivalent ratio (Φ)

C2H4 flow (L/min)

O2 flow

Shield gas Ar flow

CO2 flow

(L/min)

(L/min)

(L/min)

Total flow (Q, L/min)

Maximum flame temperature (Tmax, K)

2.3

0.868

1.132

3.000

0

5.000

1681.5 (±70.5)

2.3

1.042

1.358

3.600

0

6.000

1732.5 (±79.5)

2.3

1.215

1.585

4.200

0

7.000

1769.0 (±87.0)

2.3

1.389

1.811

4.800

0

8.000

1801.5 (±78.5)

2.3

1.562

2.038

5.400

0

9.000

1821.0 (±79.0)

2.1

0.873

1.247

3.180

0

5.300

1731.5 (±76.0)

2.2

0.932

1.272

3.306

0

5.510

1727.0 (±75.0)

2.3

1.042

1.358

3.600

0

6.000

1732.5 (±79.5)

2.4

1.142

1.428

3.855

0

6.425

1731.5 (±72.5)

2.3

1.105

1.440

3.817

0

6.362

1778.5 (±91.5)

2.3

1.207

1.573

3.336

0.834

6.951

1779.0 (±89.5)

2.3

1.389

1.811

2.880

1.920

8.000

1775.0 (±84.0)

2.3

1.534

2.000

2.121

3.181

8.836

1777.0 (±87.0)

2.2 Temperature measurement In this study, the premixed C2H4/O2/Ar (CO2) flames can be treated as a 1-D flame due to its high symmetry. Therefore, only the axial flame temperature profiles were measured via a S-type fine wire thermocouple. To minimize the radiation heat losses and prevent catalysis on thermocouple surface, YCl3/BeO mixtures were used to coat the thermocouple.44 The diameter of thermocouple wire is approximately 0.13 mm, and increases to approximately 0.38 mm after coating. Radiation correction was 4 ACS Paragon Plus Environment

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carried out through Shaddix procedure,45 where the flow properties and local transport are calculated using an updated OPPDIF code.46 We were unable to measure the emissivity of the coating in this study, thus, the upper and lower limits (0.3 and 0.6) values found in literature were used in radiation correction.47 The average of two corrected temperatures using the upper and lower limits of emissivity is assumed to be the radiation-corrected flame temperature. To obtain a more reliable corrected temperature profile, the viscosity and conductivity of the gas were calculated via detailed chemistry kinetic modelling using the USC Mech II consisting of 111 species and 784 reactions.33 2.3. Optical diagnostic facility The schematic of the LIF measurement facility is presented in Fig.1. The excitation wavelength is 266 nm generated by 4th harmonics of an Nd:YAG laser (Quantel Smart 850). The ICCD camera (Princeton Instruments PI-MAX3) is placed at the exit of spectrometer (Acton 2500i) equipped with a high resolution 1200 g/mm 500 nm blazed grating. The spectrometer is calibrated using spectral lines from a mercury lamp. Here the laser beam firstly passes through the aperture slot with diameter equalling to 4 mm, then passes through the center of flame. The aperture slot with a smaller diameter is tried in this study for higher position resolution, but the signal-noise ratio in some flames is too weak to detect. This study emphasizes on the PAHs formation comparisons between different flames. On the other hand, the experimental data used in comparisons are obtained from the post-flame region, where the PAHs concentration converges to a constant along heights above the burner surface (HAB). In this way, the PAHs information was measured with a higher signal-noise ratio instead of a higher position resolution with smaller aperture slot. The fibre-optics probe is employed to collect the fluorescence signal, and positioned at the right angle with respect to the laser beam in the same plane throughout the experiment. The burner is moved vertically to obtain the evolution of fluorescence signal along the HABs. The gatewidth for the camera is set at 50 ns. To avoid the signal disturbance from the laser induced incandescence (LII), the very low laser energy (45 mJ/cm2) is used as suggested by Bejaoui.48 Both the prompt detection and delayed detection were carried out in this study. In the prompt detection (starting when the laser pulse arrives the flame center), both LIF and LII processes may contribute to the measured signal. In the delayed detection (starting 50 ns after the laser pulse passed through the flame center), the signals were likely emitted in sole LII way due to the short lifetime of PAHs fluorescence signals (shorter than 50 nm). For each test, 50 single shots were recorded and averaged to decrease the influences of the fluctuation of laser energy, as well as the fluctuations of flame. The maximum standard deviation of signal statistics is 61.82 counts in experiment, where the intensity of fluorescence signal reaches about 1831 counts after subtracting the background signal. The fluctuation of experimental signal is within 8 %.


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Fig.1 Experimental setup for LIF measurement of PAHs 2.4. Time resolved LII intensity In order to evaluate the LII intensity in prompt detection, the time resolved LII intensity were simulated by a well-known energy balance model, which was originally formulated by Melton49 and subsequently refined by a number of researchers over the years.50-52 The model includes heat addition by the laser pulse and various cooling terms in the general form, as shown in equation (1).

(1)

• • • • dU in = Qa − Qs − Qc − Qr dt •

•

•

•

where Uin, Qa , Qs , Qc , and Qr represent the internal energy, the laser energy absorption rate, the sublimation rate of carbonaceous materials, the rate of heat loss via thermal conduction, and the rate of heat loss via thermal radiation respectively. The calculation details of each item in equation (1) are provided in the supplementary materials. Combining the time resolved profile of LII intensity and the LII intensity in delayed detection, the potential LII intensity in prompt detection can be evaluated. The LIF intensity can be abstracted by subtracting the potential LII intensity from the total intensity in prompt detection. In this way, the disturbance of LII is minimized. 2.5. Kinetic Simulation In order to further explain the experimental phenomenon and validate the PAHs mechanisms, the concentrations of some important intermediates including C2H2, C3H3 and PAHs up to pyrene (A4) were 6 ACS Paragon Plus Environment

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calculated using the premixed stagnation burner model in the Chemkin-Pro software. In simulation, the gas temperatures are fixed as the measured temperature profile. The mixture-averaged transport formulation is used to determine the species ordinary diffusion coefficients and fluxes. The maximum number of grid points allowed to solve the computational problem is set as 1000, which makes that the calculation results are independent of the number of gird. The maximum gradient and curvature allowed between grid points is set as 0.2 and 0.5, respectively. The parameters of flame condition are in accord to experimental parameters. Other parameters in the equation solver are the default values. Two PAHs mechanisms are chosen in this study. One proposed by Appel et al.22 with 101 species and 543 reactions is named as ABF mechanism, and the PAHs can grow up to A4 via the HACA reaction route. The other named KM2 mechanism is suggested by Wang et al. with 202 species and 1351 reactions.32 The KM2 mechanism extends the formation of PAHs up to coronene (A7) based on the USC Mech II,33 and features the synergistic effect on PAHs formation prediction. The reason for choosing these mechanisms is that the ABF mechanism is the footstone for many PAHs mechanisms, and the widely used KM2 mechanisms were published recently and showed good prediction accuracy in terms of PAHs concentrations. 23,32

3. Results and discussion 3.1 Temperature profiles The PAHs formation is a chemistry-controlled process, and extremely sensitive to the temperature. Thereby, the starting point of this study is to discuss the flame temperature. As shown in Fig.2 (a)-(d), the temperature rapidly increases to the maximum value in a short time (HAB < 0.3 cm) due to the heat release. The maximum flame temperature can be maintained in a narrower region, resulting from the thermal balance between the heat release and heat loss. For example, the temperature fluctuation in the range of HAB= 0.190.34 cm is within 3 K for the Q = 9 L/min case as shown in Fig.2 (d). As the HAB increases, the flame temperature gently descends until HAB = 2 cm. Subsequently, the flame temperatures rapidly drop to the temperature of the stabilized plate. As a reference, the calculated temperature profiles via a one-dimensional OPPDIF code using USC Mech II53,54 were presented for C2H4/O2/Ar flames. As showed in Figs.2-3, the measured maximum temperature is slightly higher than the calculated values. The largest deviation between the experimental and calculated data mainly exists in the post-flame region. For example, the calculated temperatures in the ranges of HAB = 1.2-2 cm do not reach the error bar of the measured temperatures in Q = 7 L/min case as shown in Fig.2 (b). The discrepancy can be explained for two reasons. One is the uncertainty in the emissivity estimate of coating in radiation correction. The other is the positional uncertainty of the 7 ACS Paragon Plus Environment

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thermocouple, because the slight bending of thermocouple may occur in flame. In most cases, the calculated temperature profiles are located in the error bar of the measured temperature profiles for C2H4/O2/Ar flames.

(a)

(b)

(c) (d) Fig 2. The centreline temperature profiles measured (symbols) and computed (lines) for C2H4/O2/Ar flame with different total flow rates, (a) Q = 5 L/min, (b) Q = 7 L/min, (c) Q = 8 L/min, (d) Q = 9 L/min.

(a)

(b)


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(c)

(d)

Fig 3. Centreline temperature profiles measured (symbols) and computed (lines) for C2H4/O2/Ar flame with different equivalence ratios: (a) Ф = 2.1, (b) Ф = 2.2, (c) Ф = 2.3, (d) Ф = 2.4. To investigate the influences of the flame temperature on PAHs formation, the temperature window should be designed as broad as possible. In this study, the designed total flow is restricted to the flame stability (the lower limit) and the signal-noise ratio (the upper limit). It was found that the flames with the total flow rate varying from 5 L/min to 9 L/min have high stability and relatively high signal-noise ratio in this study. The maximum flame temperatures increase with the total flow due to the increased fuel chemical enthalpy as shown in Fig.2 (a)-(d) and Fig.3 (c). Specifically, the maximum flame temperatures increase from 1681.5 (±70.5), 1732.5 (±79.5), 1769.0 (±87.0), 1801.5 (±78.5), to 1821.0 (±79.0) K for Q = 5, 6, 7, 8, to 9 L/min cases. In fuel-rich flames, the flame temperature increases at lower equivalence ratio due to the higher combustion efficiency. To isolate the temperature disturbance when investigating the correlation between equivalence ratio and PAHs formation, the total flow increases with the equivalence ratio. As shown in Fig. 3 (a)-(d), the maximum flame temperatures are almost kept constant, and the values are 1731.5 (±76.0), 1727.0 (±75.0), 1732.5 (±79.5) and 1731.5 (±72.5) K for the flames with Ф = 2.1, 2.2, 2.3 and 2.4.

Fig 4. The measured centerline temperature profiles for C2H4/O2/Ar/CO2 flames. 9 ACS Paragon Plus Environment

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The measured centerline temperature profiles for C2H4/O2/Ar/CO2 flames with different CO2 concentration were presented in Fig.4. The maximum flame region highly coincides with each other, and the fluctuation is within 5 K. The specific values are 1778.5 (±91.5), 1779.0 (±89.5), 1775.0 (±84.0), and 1777.0 (±87.0) K for 0 %, 20 %, 40 %, and 60 % cases respectively. Here, the calculated temperature profiles are not presented because the maximum temperature discrepancy is as high as 23 K. We chose to respect the measured results, and deduced that the imperfect reactions involving CO2 in the USC Mech II should be responsible for the temperature discrepancy. 3.2 Fluorescence signals of PAHs The normalized fluorescence spectra at different HABs was measured in premixed C2H4/O2/Ar flame (Φ=2.3, Q=6 L/min), as shown in Fig.5. It is notable that the shape of fluorescence spectra remains unchanged in different HABs ranging from 2 to 20 mm and centers round 500 nm, which is accord with the results of Bejaoui et al.48

Fig 5. The fluorescence spectra of PAHs in C2H4/O2/Ar flame (Φ=2.3, Q=6 L/min). To further distinguish specific PAHs information from the detected fluorescence signals, we first discuss the definition of fluorescence signals, as shown in equation (2).

(2) χ P E ηopt dvc [ PAH ]σ (λ , T)φ (λ ,T, P) hc / λ kT where E is the laser energy [J/cm2], h is the Planck constant, c is the velocity of light, λ is the wavelength of

Sf =

excitation, ηopt is the overall efficiency of the collection optics, dvc is the collection volume [cm3], χPAH is the mole fraction of PAH, P is the pressure, T is the temperature, k is the Boltzmann constant. The final two variables are the molecular absorption cross-section of PAH [σ, cm2] and the fluorescence quantum yield φ . These two variables are sensitive to the wavelength of excitation and temperature. In addition, the fluorescence quantum yield φ can be influenced by the pressure of flame to a lesser extent.


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In this study, the excitation wavelength, the overall efficiency of the collection optics, collection volume and pressure are constants. The fluorescence signals linearly change with the mole fraction of PAH when investigating the influences of equivalence ratio and CO2 addition on PAHs formation, where temperatures are almost kept constant. The maximum flame temperature varies from 1681.5 (±70.5) to 1821.0 (±79.0) K when investigating the influences of temperature on PAHs. The variables σ and φ are assumed to be independent of the investigated flame temperature. This is resulted from the lack of high temperature gas-phase PAHs fluorescence data. Rather, many flames detection using the LIF methods have the same problems and have to be treated, in principle, in a similar manner.34, 55-58 Generally, the variables σ and φ is likely to slightly decrease at higher temperature, which may results in a relatively weaker PAHs fluorescence signals at higher flame temperature. However, the observed PAHs signal profiles along maximum flame temperature is parabola opening down instead of monotonically decreasing. This means the accuracy of analysis in temperature-dependent experiment is reliable and almost not sensitive to the assumption for variables σ and φ . Our previous study59 showed that the PAHs fluorescence wavelength is greatly sensitive to its structure. The emission wavelengths of PAHs without five-membered ring are shorter than 450 nm, and the LIF signals of PAHs detected at shorter (longer) wavelength are likely emitted by relatively smaller (larger) PAHs. The emission wavelengths of PAHs with five-membered ring are within the visible region, and insensitive to the structure size. Generally, the fluorescence signal detected at 320-360 nm were reported to represent two- and three-ring aromatic species, and four-ring aromatic species should be responsible for fluorescence signal detected at 370-410 nm as shown in Table 259-61 Five-membered ring species play an important role in PAHs growth and oxidation. Therefore, we investigated the influences of combustion parameters on the evolution of five-membered ring species by averaging the fluorescence signal located in 500-550 nm.59 It should be noted that the overlapping of PAHs spectra is inevitable.59 But the fluorescence signals contributed from some species can be ignored by considering its relative low concentration in flame. For example, the mole fraction of naphthalene (A2) and phenanthrene (A3) is higher than that of other twoor three-ring species by at least 15 times as shown in Fig.6. The rank of PAHs mole fraction results indicates that the combination of A2 and A3 is qualified to represent the evolution of species with two- and three-ring in LIF measurement, and the interference from other species can be ignored. In the ABF mechanism, the A4 is the sole four-ring species and acenaphthylene (A2R5) is the sole PAH with five-membered ring. Therefore, A2-A3, A4 and A2R5 are selected to represent two- and three-ring, four-ring, and five-membered ring species respectively for comparison in this study. This treatment is also supported by the concentration rank of aromatic ring species calculated by the KM2 mechanism. Table 2. The spectra of some PAHs. Species

Selected spectra (nm)59-61 11 ACS Paragon Plus Environment

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Two- and three-ring aromatic species Four-ring aromatic species Five-membered ring species

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320-360 370-410 500-550

(a) two-rings aromatic species

(b) three-rings aromatic species

Fig 6. The mole fraction of PAHs with two- and three-ring species along HAB in C2H4/O2/Ar flame (Φ=2.3, Q=6 L/min) using ABF mechanism. Species nomenclature please refer to 3. In this study, the trend lines of temperature-dependent, equivalence-ratio-dependent, and CO2-contentdependent PAHs concentration were investigated. In order to compare with the predicted results, the fluorescence signals are normalized in each experiment matrix. The residence time decreases with the increased flow rate at the same HAB. To vanish the influences of the total flow fluctuation, the temperaturedependent, equivalence-ratio-dependent and CO2-content-dependent experimental data are all compared at t = 42.7 ms, which is determined by the maximum flow rate used in this study (Q = 9 L/min, HAB = 20 mm, t = 42.7 ms). In each experimental matrix, the strongest signal value used as the denominator to normalize comes from different flame condition. Specifically, the denominator to normalize comes from the C2H4/O2/Ar flame with Q = 6 L/min, Q = 6.425 L/min and Q = 6.362 L/min cases in temperature-dependent, CO2-dependent and equivalence-ratio-dependent experimental matrix respectively. The denominator to normalize for all predicted data is the value obtained at the same place with experiment. 3.3 Influence of temperature The correlation between the relative concentration of PAHs and maximum flame temperature was shown in Fig.7. It is obvious that the measured trend lines for two- and three-ring aromatic, four-ring aromatic and five-membered ring species are similar to each other. The signals firstly increase with temperature and reach the maximum value around 1732 K, then decrease with temperature. However their temperature sensitivity is different, for example, the signal value detected at 1821 K is 0.50 times lower than that at 1732 K for two- and three-ring aromatic species, 0.37 times for four-ring aromatic species and 0.33 12 ACS Paragon Plus Environment

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times for five-membered ring species. The predicted trend lines of A2+A3 concentration using the KM2 and ABF mechanisms were presented in Fig.7 (a). The predicted results using the KM2 mechanism match the experimental results well. However, the performance of the ABF mechanisms is less satisfactory. The ABF mechanism seems insensitive to temperature. For example, the predicted concentrations of A2+A3 in the temperature range of 1732-1801 K are almost the same. The point of inflection was found in the investigated temperature range for ABF and KM2 mechanism. The point of inflection captured by the KM2 mechanism locates at 1732 K, which is consistent with the experiment results as shown in Fig.7 (b). It should be noted that the decline magnitude of A2R5 concentration is only 1.2 times from 1732 K to 1821 K in the KM2 mechanism, however it reaches 3.0 times in experiment. For ABF mechanism, the predicted point of inflection moves to 1801.5 K, which is higher than the experimental values by 69 K. The KM2 mechanism fails to capture the point of inflection for the trend line of A4 concentration, as shown in Fig.7 (c). The A4 can grow up to A7 in the KM2 mechanism in theoretical calculation, which has not been tested against experimental data. We deduced that it may be responsible for the lack of the point of inflection in the trend line of A4 concentration. Completely contrary to the KM2 mechanisms, the predicted concentration of A4 with the ABF mechanism increases with temperature, which is adverse to corresponding experimental data in temperature ranges of 1732-1821 K.

(a) Two- and three-ring aromatic species

(b) Five-membered ring species

(c) Four-ring aromatic species Fig 7. The influence of temperature on PAHs formation.


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The prediction accuracy of A2 concentration is the guarantee for precisely predicting other large size PAHs. To understand the experimental phenomenon, the pathways and first-order temperature sensitivity analysis of A2 formation was carried out using the KM2 mechanisms due to its good performance in predicting the concentration trend line of A2+A3. As shown in Fig.8 (a), the A2 is mainly formed via the pathways of A1 (+C2H2) → A1C2H (+C2H2) → A2 (HACA), A1/A1- (+CH2/CH3) → ACH2 (+C3H3) → A2, C5H5 + C5H5 → A2 and A2CH3 + H → A2 + CH3. The first-order temperature sensitivity analysis of A2 formation showed that the HACA pathway with high reaction reversibility should account for temperature effects on A2 formation. As shown in Fig.8 (b), the main promoting reactions of A2 formation in KM2 mechanism are R1: H + O2 = OH + O, R1224: A1C2H + H => A1C2H- + H2, R812: A1C2H- + C2H2 => A1C2HC2H2 and R1197: A2-1 + H2 => A2 + H reactions. Once formed, the A1C2HC2H2 molecule rapidly converts to A2- due to the low energy barrier (only 4.48 kcal/mol) .32

(a)

(b)

Fig 8. The pathways (a), and the normalized first-order temperature sensitivity analysis of A2 formation (b) (Φ=2.3, Q=6 L/min, HAB=1 cm) using KM2 mechanism. In view of the importance of A1 in HACA pathway, the formation of A1 at different flame temperatures is further investigated. The C2H2+C4Hx and C3H3+C3H3 reactions are the main pathways leading to the formation of A1 and A1- in C2H4 flame.22, 23, 32, 62, 63 Therefore, the mole fractions of C2H2 and C3H3 along different flame temperatures are also calculated. As shown in Fig. 9, the concentration trend lines of A1 and C3H3 are a parabola opening down in the investigated temperature range, and that of C2H2 is a parabola opening up. In this study, it was found that the contribution of C3H3+C3H3 reaction to A1 formation is more than 90 %. This explains the similarity of the concentration trends of A1 and C3H3. The mole fractions of reactants A1 and C2H2 determine the yield of A2 along A1 (+C2H2) → A1C2H (+C2H2) → A2 pathway. In the temperature region of 1681.5 K to 1732.5 K, the mole fraction of A1 increases to a factor of 1.14 and that of C2H2 only decreases to a factor of 0.96, leading to the enhancement of A2 formation. In the temperature 14 ACS Paragon Plus Environment

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region of 1732.5 K to 1821 K, the mole fraction of A1 drops 28 %. However, the fluctuation of C2H2 concentration is within 6 %. Therefore, the trend line of A2+A3 concentration follows that of A1, decreasing at higher flame temperature.

Fig 9. The calculated normalized mole fraction of A1, C3H3, C2H2 using KM2 mechanism.

3.4 Influence of equivalence ratio As EGR rate increases, the global equivalence ratio in internal combustion engine increases due to the reduction of O2 and increase of CO2. To better understand the mechanism of EGR on PAHs formation, the premixed C2H4/O2/Ar flame with equivalence ratios changing from 2.1 to 2.4 at fixed maximum flame temperature and dilution ratio were investigated. As shown in Fig.10 (a)-(c), the detected PAHs fluorescence signals almost linearly increase with equivalence ratio due to the increased concentration of C2H2 molecules. 64

The two mechanisms successfully reproduce the main features of experimental measurement, as shown in

Fig.10 (a)-(c). Generally, the deviation between the predicted slope of the trend line using KM2 and ABF mechanisms and the measured slope is less than 1.23 times.




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(c) Four-ring aromatic species Fig 10. The influence of equivalence ratio on PAHs formation.

3.5 Influence of CO2 addition CO2 is the main component in exhausted gas. However, few research paid attention to the influences of CO2 addition on PAHs formation. In this study, the premixed C2H4/O2/Ar/CO2 flames with different CO2 blending ratio were investigated. Specifically, only the chemical effect of CO2 addition was discussed, it’s thermal and dilution effects were excluded via fixing the maximum flame temperature and dilution rate. As shown in Fig.11 (a)-(c), the fluorescence signals detected for two- and three-ring aromatic species, fivemembered ring, and four-ring aromatic species decrease with CO2 content, and the reduction of PAHs signals is more obvious at the initial stage (blending ratio < 40 %). In addition, the signals detected for twoand three-ring aromatic species have a more intensive drop. For example, the signal detected for two- and three-ring aromatic species drops by 65 % when the CO2 content changes from 0 % to 60 %, and the value is only 47 % and 52 % for five-membered ring and four-ring aromatic species respectively. The predicted results also presented in Fig.11 (a)-(c), three predicted trend lines provided by the ABF mechanism are similar to each other. However, only the curve of A2+A3 concentration matches the experimental data well, and other two curves located below the corresponding experimental curves and the deviation increases with higher CO2 content. In ABF mechanism, PAH grows only along HACA route and the concentrations of A2R5 and A4 are greatly sensitive to that of A2 and A3. This may account for the similarity of three curves predicted by the ABF mechanism. The trends line of A2+A3 given by KM2 mechanism is almost insensitive to CO2 addition, and even adverse to the detected trend line as shown in Fig. 11 (a). The predicted trends line of A2R5 and A4 with KM2 mechanism are closer to the experimental trends lines than that with ABF mechanism. In KM2 mechanism, other pathways beyond HACA pathway contribute a lot to the formation of A2R5 and A4, and the concentrations of A2R5 and A4 are little sensitive to that of A2 and A3. For example, the A4 is mainly generated by the recombination reaction of two indene radicals and the addition reaction of indene radical and toluene radical, instead of A3- + C2H2 → A4 + H reaction in KM2 mechanism.


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(c) Four-ring aromatic species Fig 11. The influence of CO2 addition on PAHs formation. Both PAHs and CO2 are saturated species, and tend to react with other unsaturated species, which is evidenced by ABF and KM2 mechanisms. Therefore, it is unlikely that the CO2 addition directly affect the formation of PAHs. The CO2 addition is more likely to change the concentration of gas-phase small molecule or radical species. As mentioned above, the formation of A1 is the footstone of PAHs formation and is greatly dependent on the concentration of some gas-phase small molecules and radical species. Understanding the chemical mechanism of CO2 addition on A1 formation contributes to knowing the chemical mechanism of CO2 addition on PAHs formation. Here, ABF mechanism is selected to carry out the sensitivity analysis of A1 formation towards CO2 entrance reactions, as ABF mechanism shows better performance in terms of predicting the concentration trend lines of A2+A3 along CO2 addition. The CO2 entrance reactions consist of CO2+H=CO+OH, CO2+O=CO+O2, CO2+OH=CO+HO2, CO2+CH=HCO+CO, and CO2+H=HCO+O reactions in ABF mechanism. In this study, the peak mole fraction of A1 was calculated on the condition that the reaction rate of CO2 entrance reaction decreases by a factor of 10 individually. The sensitivity of CO2 entrance reaction towards A1 formation can be evaluated by comparing the A1 concentrations calculated with the unchanged and changed rate constant. As shown in Fig. 12, the A1 formation is mainly sensitive to the reaction of CO2+H=CO+OH, and can be affected by reaction of CO2+OH=CO+HO2 to a minor extent. Specifically, the A1 formation is promoted when the reaction rate of CO2+H=CO+OH decreases, and is suppressed when the reaction rate of CO2+OH=CO+HO2 decreases.


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Fig 12. The calculated peak mole fraction of A1 in C2H4 premixed flame with the specific reaction rate decreasing 10 times. (1 atm, C2H4/O2/Ar/CO2 = 17.4/22.6/24/36). To further explore the details, the concentrations of OH, C2H2, C3H3 and A1 in premixed C2H4 flames with different CO2 content were calculated. As shown in Fig.13, the calculated concentrations of C2H2 and C3H3 are lower at higher CO2 content. The A1 forms mainly via the addition reaction of C2H2 and C4Hx and the recombination reaction of two C3H3 radicals in premixed C2H4 flames.24 Therefore, it is understanding that the A1 formation is suppressed at higher CO2 content. The concentration of OH radical goes up with CO2 addition, resulting from the reaction of CO2+H=CO+OH. In conclusion, the reduction of PAHs concentration can be realized by CO2 chemical effect thought the route CO2 (+H) → OH → C3H3 (C2H2) → A1 → PAHs.

(a)

(b)


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(c)

(d)

Fig. 13 The calculated mole fraction of C2H2, C3H3, A1 and OH species in premixed C2H4/O2/Ar(CO2) flames. CO2 addition is likely to lower the flame temperature due to its high heat capacity. Considering the trend line of temperature-dependent PAHs concentration, it is possible that the thermal effect of CO2 addition on PAHs or soot reduction is not observed if the maximum flame temperature is around or below 1730 K. In addition, the possibility that the formation of PAHs or soot is enhanced with CO2 addition also exists if the maximum flame temperature is higher than 1730 K in experiment. In regular diesel combustion mode, the soot emissions are higher with EGR application. Researches generally explained it by two factors, 1) the formation of soot is restrained due to the lower temperature, 2) the oxidation of soot is also weakened as the oxygen content and temperature simultaneously decreases, and the weakened intensity in oxidation process is more significant than that in formation process.65-68 The maximum combustion temperature in regular combustion mode of diesel engine is easily higher than 2000 K without EGR application, and generally not lower than 1730 K with EGR application.69 In this study, the experimental results showed that the formation of PAHs (the precursor of soot) is enhanced at lower temperature (like temperature drops from T0 to T1, but T1 >1730 K) and higher equivalence ratio. The increased concentration of PAHs cannot be compensated by the chemical effect of CO2. Therefore, the soot emission is more serious with EGR application in regular combustion mode. When the combustion temperature is lower than 1730 K like LTC mode, the total PAHs concentration may be restrained due to the lower temperature effect (T0 < 1730 K) and the chemical effect of CO2, resulting in the reduction of soot emission. Briefly speaking, the chemical mechanism of EGR addition on soot formation in internal combustion engine was further clarified from the angle of PAHs formation in premixed C2H4/O2/Ar flame. 4. Conclusion The influence of EGR on PAHs formation was investigated based on LIF measurement in this study. Specifically, the temperature, equivalence ratio and CO2 content dependences were studied. The detected 19 ACS Paragon Plus Environment

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results were further clarified by of two mechanisms named ABF and KM2. Several conclusions can be drawn. It was found that the PAHs formation is extremely sensitive to the flame temperature. The concentration of PAHs reaches the maximum value (the point of inflection) around 1730 K, and decreases at lower or higher temperature. This trend is partially predicted by the KM2 mechanism. The mechanism of flame temperature on PAHs formation can be explained by the route Flame temperature → C3H3 → A1 → PAHs. The PAHs concentration almost increases linearly with equivalence ratio due to higher C content, which is predicted by the ABF and KM2 mechanisms. The experimental results showed that the PAHs formation is hindered by CO2 addition due to its chemical effect. This feature is predicted by the ABF mechanisms, and partially predicted by the KM2 mechanism. The pathway sensitivity analysis showed that CO2 chemical effect is realized thought the route CO2 (+H) → OH → C3H3 (C2H2) → A1 → PAHs. In addition, the thermal effect of CO2 addition on PAHs formation is dependent on the flame temperature.

Supporting Information The details of time resolved LII intensity calculation. Notes The authors declare no competing financial interest. Acknowledgments This work was supported by National Natural Science Foundation of China (91441129, 51210010) and the National Basic Research Program of China (973 Program) (2013CB228502). Reference (1) Minutolo, P.; D'Anna, A.; D'Alessio, A., On detection of nanoparticles below the sooting threshold. Combust. Flame 2008, 152, 287-292. (2) Denissenko, M. F.; Pao, A.; Tang, M.-s.; Pfeifer, G. P., Preferential Formation of Benzo[a]pyrene Adducts at Lung Cancer Mutational Hotspots in P53. Science 1996, 274, 430-432. (3) Liu, P.; Lin, H.; Yang, Y.; Shao, C.; Guan, B.; Huang, Z., Investigating the Role of CH2 Radicals in the HACA Mechanism. J. Phys. Chem. A 2015, 119, 3261-3268. 20 ACS Paragon Plus Environment

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(4) Perera, F. P., Environment and Cancer: Who Are Susceptible? Science 1997, 278, 1068-1073. (5) An, Y. Z.; Pei, Y. Q.; Qin, J.; Zhao, H.; Li, X.; Kohse-Höinghaus, K., Kinetic modeling of polycyclic aromatic hydrocarbons formation process for gasoline surrogate fuels. Energ. Convers. Manage. 2015, 100, 249-261. (6) Dai, J.; Xu, M.; Chen, J.; Yang, X.; Ke, Z., PCDD/F, PAH and heavy metals in the sewage sludge from six wastewater treatment plants in Beijing, China. Chemosphere 2007, 66, 353-361. (7) Choi, S.; Park, W.; Lee, S.; Min, K.; Choi, H., Methods for in-cylinder EGR stratification and its effects on combustion and emission characteristics in a diesel engine. Energy 2011, 36, 6948-6959. (8) An, Y. Z.; Jaasim, M.; Raman, V.; Im, H. G.; Johansson, B., In-Cylinder Combustion and Soot Evolution in the Transition from Conventional Compression Ignition (CI) Mode to Partially Premixed Combustion (PPC) Mode. Energ. Fuel 2018, 32, 2306-2320. (9) Bhaskar, K.; Nagarajan, G.; Sampath, S., Optimization of FOME (fish oil methyl esters) blend and EGR (exhaust gas recirculation) for simultaneous control of NOx and particulate matter emissions in diesel engines. Energy 2013, 62, 224-234. (10) Feng, H.; Zheng, Z.; Yao, M.; Cheng, G.; Wang, M.; Wang, X., Effects of exhaust gas recirculation on low temperature combustion using wide distillation range diesel. Energy 2013, 51, 291-296. (11) An, Y. Z.; Jaasim, M.; Raman, V.; Vedharaj, S.; Im, H. G.; Johansson, B., Numerical simulation of combustion and soot under partially premixed combustion of low-octane gasoline. Fuel 2018, 211, 420-431. (12) Agarwal, D.; Singh, S. K.; Agarwal, A. K., Effect of Exhaust Gas Recirculation (EGR) on performance, emissions, deposits and durability of a constant speed compression ignition engine. Appl. Energy 2011, 88, 2900-2907. (13) Koegl, M.; Hofbeck, B.; Will, S.; Zigan, L., Investigation of soot formation and oxidation of ethanol and butanol fuel blends in a DISI engine at different exhaust gas recirculation rates. Appl. Energy 2018, 209, 426-434. (14) Li, T.; Yin, T.; Wang, B., Anatomy of the cooled EGR effects on soot emission reduction in boosted spark-ignited direct-injection engines. Appl. Energy 2017, 190, 43-56. (15) McLintock, I. S., The effect of various diluents on soot production in laminar ethylene diffusion flames. Combust. Flame 1968, 12, 217-225. (16) Oh, K. C.; Shin, H. D., The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames. Fuel 2006, 85, 615-624. (17) Schug, K., Manheimer-Timnat. Y., Yaccarino, P., and Glassman, I. Combust. Sci. Technol. 22235 1980, 250. (18) Du, D. X.; Axelbaum, R. L.; Law, C. K., The influence of carbon dioxide and oxygen as additives on soot formation in diffusion flames. Symp. (Int) Combust. 1991, 23, 1501-1507. (19) Zhang, C.; Atreya, A.; Lee, K., Sooting structure of methane counterflow diffusion flames with preheated reactants and dilution by products of combustion. Symp. (Int.) Combust. 1992, 24, 1049-1057. 21 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

(20) Abián, M.; Millera, A.; Bilbao, R.; Alzueta, M. U., Experimental study on the effect of different CO2 concentrations on soot and gas products from ethylene thermal decomposition. Fuel 2012, 91, 307-312. (21) Teini, P. D.; Karwat, D. M. A.; Atreya, A., The effect of CO2/H2O on the formation of soot particles in the homogeneous environment of a rapid compression facility. Combust. Flame 2012, 159, 1090-1099. (22) Appel, J.; Bockhorn, H.; Frenklach, M., Kinetic modeling of soot formation with detailed chemistry and physics: laminar premixed flames of C2 hydrocarbons. Combust. Flame 2000, 121, 122-136. (23) Chernov, V.; Thomson, M. J.; Dworkin, S. B.; Slavinskaya, N. A.; Riedel, U., Soot formation with C1 and C2 fuels using an improved chemical mechanism for PAH growth. Combust. Flame 2014, 161, 592-601. (24) Frenklach, M., Reaction mechanism of soot formation in flames. Phys. Chem. Chem. Phys. 2002, 4, 2028-2037. (25) Lin, H.; Liu, P.; He, Z.; Zhang, Y.; Guan, B.; Huang, Z., Formation of the first aromatic ring through the self-recombination of but-1-ene-3-yne with H-assistance in combustion. Int. J. Hydrogen Energy 2016, 41, 13736-13746. (26) Wang, H.; Frenklach, M., Calculations of rate coefficients for the chemically activated reactions of acetylene with vinylic and aromatic radicals. J. Phys. Chem. 1994, 98, 11465-11489. (27) Frenklach, M.; Wang, H., Detailed modeling of soot particle nucleation and growth. Symp. (Int.) Combust. 1991, 23, 1559-1566. (28) An, Y. Z.; Li, X.; Teng, S. P.; Wang, K.; Pei, Y. Q.; Qin, J.; Zhao, H., Development of a soot particle model with PAHs as precursors through simulations and experiments. Fuel. 2016, 179, 246-257. (29) Simmie, J. M., Detailed chemical kinetic models for the combustion of hydrocarbon fuels. Prog. Energy Combust. Sci. 2003, 29, 599-634. (30) An, Y. Z.; Pei, Y. Q.; Qin, J.; Zhao, H.; Teng, S. P.; Li, B.; Li, X., Development of a PAH (polycyclic aromatic hydrocarbon) formation model for gasoline surrogates and its application for GDI (gasoline direct injection) engine CFD (computational fluid dynamics) simulation. Energy 2016, 94, 367-379. (31) An, Y. Z.; Teng, S. P.; Pei, Y. Q.; Qin, J.; Li, X.; Zhao, H.; An experimental study of polycyclic aromatic hydrocarbons and soot emissions from a GDI engine fueled with commercial gasoline. Fuel. 2016, 164, 160-171. (32) Wang, Y.; Raj, A.; Chung, S. H., A PAH growth mechanism and synergistic effect on PAH formation in counterflow diffusion flames. Combust. Flame 2013, 160, 1667-1676. (33) Wang, H.; You, X.; Joshi, A.; Davis, S.; Laskin, A.; Egolfopoulos F. N.; Law C. K.; USC Mech Version II. , 2007. (34) Lee, S.; Yoon, S.; Chung, S., Synergistic effect on soot formation in counterflow diffusion flames of ethylene–propane mixtures with benzene addition. Combust. flame 2004, 136, 493-500.


Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

(35) Yoon, S.; Anh, D.; Chung, S., Synergistic effect of mixing dimethyl ether with methane, ethane, propane, and ethylene fuels on polycyclic aromatic hydrocarbon and soot formation. Combust. Flame 2008, 154, 368-377. (36) Smooke, M. D.; McEnally, C. S.; Pfefferle, L. D.; Hall, R. J.; Colket, M. B., Computational and experimental study of soot formation in a coflow, laminar diffusion flame. Combust. Flame 1999, 117, 117139. (37) Wang, H.; Reitz, R. D.; Yao, M.; Yang, B.; Jiao, Q.; Qiu, L., Development of an n -heptane- n -butanolPAH mechanism and its application for combustion and soot prediction. Combust. Flame 2013, 160, 504519. (38) Liu, X.; Wang, H.; Wei, L.; Liu, J.; Reitz, R. D.; Yao, M., Development of a reduced toluene reference fuel (TRF)-2,5-dimethylfuran-polycyclic aromatic hydrocarbon (PAH) mechanism for engine applications. Combust. Flame 2016, 165, 453-465. (39) Ciajolo, A.; D'Anna, A.; Barbella, R.; Tregrossi, A.; Violi, A., The effect of temperature on soot inception in premixed ethylene flames. Symp. (int.) Combust. 1996, 26, 2327-2333. (40) Inal, F.; Senkan, S. M., Effects of equivalence ratio on species and soot concentrations in premixed nheptane flames. Combust. Flame 2002, 131, 16-28. (41) Liu, F.; Karataş, A. E.; Gülder, Ö. L.; Gu, M., Numerical and experimental study of the influence of CO2 and N2 dilution on soot formation in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20 atm. Combust. Flame 2015, 162, 2231-2247. (42) Slavinskaya, N.; Frank, P., A modelling study of aromatic soot precursors formation in laminar methane and ethene flames. Combust. Flame 2009, 156, 1705-1722. (43) Waldmann, L., Über die Kraft eines inhomogenen Gases auf kleine suspendierte Kugeln. In Zeitschrift für Naturforschung A, 1959; pp 589. (44) Kint, J. H., A noncatalytic coating for platinum-rhodium thermocouples. Combust. Flame 1970, 14, 279-281. (45) Shaddix, C. R., Correcting thermocouple measurements for radiation loss: A critical review. 1999. (46) Smooke, M. D.; Puri, I. K.; Seshadri, K. In A comparison between numerical calculations and experimental measurements of the structure of a counterflow diffusion flame burning diluted methane in diluted air, Symp. (Int.) on Combust. 1988, 1783-1792. (47) Peterson, R. C.; Laurendeau, N. M., The emittance of yttrium-beryllium oxide thermocouple coating. Combust. Flame 1985, 60, 279–284. (48) Bejaoui, S.; Mercier, X.; Desgroux, P.; Therssen, E., Laser induced fluorescence spectroscopy of aromatic species produced in atmospheric sooting flames using UV and visible excitation wavelengths. Combust. Flame 2014, 161, 2479-2491. (49) Melton, L. A., Soot diagnostics based on laser heating. Appl. Optics 1984, 23, 2201. 23 ACS Paragon Plus Environment

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 25

(50) Schulz, C.; Kock, B. F.; Hofmann, M.; Michelsen, H.; Will, S.; Bougie, B.; Suntz, R.; Smallwood, G., Laser-induced incandescence: recent trends and current questions. Appl. Phys. B 2006, 83, 333-354. (51) Michelsen, H. A.; Liu, F.; Kock, B. F.; Bladh, H.; Boiarciuc, A.; Charwath, M.; Dreier, T.; Hadef, R.; Hofmann, M.; Reimann, J., Modeling laser-induced incandescence of soot: a summary and comparison of LII models. Appl. Phys. B 2007, 87, 503. (52) Michelsen, H. A.; Schulz, C.; Smallwood, G. J.; Will, S., Laser-induced incandescence: Particulate diagnostics for combustion, atmospheric, and industrial applications. Prog. Energy Combust. Sci. 2015, 51, 2-48. (53) Abid, A. D.; Camacho, J.; Sheen, D. A.; Wang, H., Quantitative measurement of soot particle size distribution in premixed flames – The burner-stabilized stagnation flame approach. Combust. Flame 2009, 156, 1862-1870. (54) Gu, C.; Lin, H.; Camacho, J.; Lin, B.; Shao, C.; Li, R.; Gu, H.; Guan, B.; Huang, Z.; Wang, H., Particle size distribution of nascent soot in lightly and heavily sooting premixed ethylene flames. Combust. Flame 2016, 165, 177-187. (55) De Andrade Oliveira, M. H.; Olofsson, N. E.; Johnsson, J.; Bladh, H.; Lantz, A.; Li, B.; Li, Z. S.; Aldén, M.; Bengtsson, P. E.; Luijten, C. C. M.; de Goey, L. P. H., Soot, PAH and OH measurements in vaporized liquid fuel flames. Fuel 2013, 112, 145-152. (56) Desgroux, P.; Mercier, X.; Thomson, K. A., Study of the formation of soot and its precursors in flames using optical diagnostics. Proc. Combust.Inst. 2013, 34, 1713-1738. (57) Wu, J.; Song, K. H.; Litzinger, T.; Lee, S. Y.; Santoro, R.; Linevsky, M.; Colket, M.; Liscinsky, D., Reduction of PAH and soot in premixed ethylene–air flames by addition of ethanol. Combust. Flame 2006, 144, 675-687. (58) Yoon, S. S.; Lee, S. M.; Chung, S. H., Effect of mixing methane, ethane, propane, and propene on the synergistic effect of PAH and soot formation in ethylene-base counterflow diffusion flames. Proc. Combust. Inst. 2005, 30, 1417-1424. (59) Liu, P.; He, Z.; Hou, G.-L.; Guan, B.; Lin, H.; Huang, Z., The Diagnostics of Laser-Induced Fluorescence (LIF) Spectra of PAHs in Flame with TD-DFT: Special Focus on Five-Membered Ring. J. Phys. Chem. A 2015, 119, 13009-13017. (60) Ciajolo, A.; Ragucci, R.; Apicella, B.; Barbella, R.; de Joannon, M.; Tregrossi, A., Fluorescence spectroscopy of aromatic species produced in rich premixed ethylene flames. Chemosphere 2001, 42, 835841. (61) Ciajolo, A.; Barbella, R.; Tregrossi, A.; Bonfanti, L., Spectroscopic and compositional signatures of pah-loaded mixtures in the soot inception region of a premixed ethylene flame. Symp. (Int.) Combust. 1998, 27, 1481-1487. (62) Castaldi, M. J.; Senkaw, S. M., PAH Formation in the Premixed Flame of Ethane. Combust. Sci. Technol. 1996, 116-117, 167-181. 24 ACS Paragon Plus Environment

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Energy & Fuels

(63) Liu, P.; Lin, H.; He, Z.; Zhang, Y.; Guan, B.; Huang, Z., The mechanism and kinetic analysis of C4H4 + C4H4 (but-1-ene-3-yne) reaction with features of H-transfer in combustion. Int. J. Hydrogen Energy 2016, 41, 3249-3258. (64) Melton, T. R.; Inal, F.; Senkan, S. M., The effects of equivalence ratio on the formation of polycyclic aromatic hydrocarbons and soot in premixed ethane flames. Combust. Flame 2000, 121, 671-678. (65) Asad, U.; Zheng, M., Efficacy of EGR and Boost in Single-Injection Enabled Low Temperature Combustion. SAE Int. Engines 2009, 2, 1085-1097. (66) Lewander, M.; Johansson, B.; Tunestål, P.; Keeler, N.; Tullis, S.; Milovanovic, N., Evaluation of the operating range of partially premixed combustion in a multi cylinder heavy duty engine with extensive EGR. Diesel 2009. (67) Adi, G.; Hall, C.; Snyder, D.; Bunce, M.; Satkoski, C.; Kumar, S.; Garimella, P.; Stanton, D.; Shaver, G., Soy-Biodiesel Impact on NOx Emissions and Fuel Economy for Diffusion-Dominated Combustion in a Turbo−Diesel Engine Incorporating Exhaust Gas Recirculation and Common Rail Fuel Injection. Energy Fuels 2009, 23, 5821-5829. (68) Pedrozo, V. B.; May, I.; Lanzanova, T. D. M.; Zhao, H., Potential of internal EGR and throttled operation for low load extension of ethanol–diesel dual-fuel reactivity controlled compression ignition combustion on a heavy-duty engine. Fuel 2016, 179, 391-405. (69) Eismark, J.; Balthasar, M.; Karlsson, A.; Benham, T.; Christensen, M.; Denbratt, I., Role of Late Soot Oxidation for Low Emission Combustion in a Diffusion-controlled, High-EGR, Heavy Duty Diesel Engine. SAE Int. Engines 2009.


The chemical mechanism of exhaust gas recirculation on polycyclic

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