Pyrolysis and Oxidation of Methane in a RF Plasma Reactor

Plasma Chem Plasma Process (2017) 37:1551–1571 DOI 10.1007/s11090-017-9844-4

ORIGINAL PAPER

1,3 2 3 3 Qi Chen • Xiaofang Yang • Jintao Sun • Xiaojun Zhang • 3 1 2 Xingian Mao • Yiguang Ju • Bruce E. Koel

Received: 26 October 2016 / Accepted: 29 August 2017 / Published online: 10 October 2017 Ó Springer Science+Business Media, LLC 2017

Abstract The chemical kinetic effects of RF plasma on the pyrolysis and oxidation of methane were studied experimentally and computationally in a laminar flow reactor at 100 Torr and 373 K with and without oxygen addition into He/CH4 mixtures. The formation of excited species as well as intermediate species and products in the RF plasma reactor was measured with optical emission spectrometer and Gas Chromatography and the data were used to validate the kinetic model. The kinetic analysis was performed to understand the key reaction pathways. The experimental results showed that H2,C2 and C3 hydrocarbon formation was the major pathways for plasma assisted pyrolysis of methane. In contrast, with oxygen addition, C2 and C3 formation dramatically decreased, and syngas (H2 and CO) became the major products. The above results revealed oxygen addition significantly modified the chemistry of plasma assisted fuel pyrolysis in a RF discharge. Moreover, an increase of E/n was found to be more beneficial for the formation of higher hydrocarbons while a small amount of oxygen was presented in a He/CH4 mixture. A reaction path flux analysis showed that in a RF plasma, the formation of active species such 1 as CH3,CH2, CH, H, O and O ( D) via the electron impact dissociation reactions played a critical role in the subsequent processes of radical chain propagating and products formation. The results showed that the electronically excitation, ionization, and dissociation processes as well as the products formation were selective and strongly dependent on the reduced electric field.

& Yiguang Ju [email protected]

1 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA 2 Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544, USA 3 School of Mechanical and Electronic Control Engineering, Beijing Jiaotong University, Beijing 100044, China 123 1552 Plasma Chem Plasma Process (2017) 37:1551–1571

Keywords RF plasma Á Plasma assisted combustion Á Methane pyrolysis and oxidation Á Reaction kinetics Á Methane reforming

Introduction

Directly converting CH4 into transportation fuels and petrochemicals is a potentially important process to supply alternative fuels to the current petroleum derived fuels, and to alleviate the environmental problem caused by greenhouse gas emissions (CO2,CH4, etc.) [1–4]. However, since CH4 has a high C-H bond strength, negligible electron affinity, large ionization energy, and low polarizability [5], its direct conversion to useful hydrocarbon products remains an enormous challenge. Currently, industrial CH4 conversion relies on two-step catalytic processes including steam reforming and F-T/methanol syntheses, which are normally carried out under high reaction temperatures or high pressure conditions [5–9]. A number of non-equilibrium plasma technologies have been recently developed to assist low temperature CH4 reforming to take the advantage of higher electron energy with low neutral gas temperature [10]. Thus, the chemical reactions for fuel reforming can proceed at much lower temperatures than that in the conventional catalytic reforming processes. Recent experimental study of pyrolysis and oxidation of methane in non-equilibrium plasmas has drawn great attention for the production of various chemicals: higher hydrocarbons, syngas, aromatics, oxygenates-methanol, formaldehyde, as well as carbon nanotubes, diamond powders and so on. Numerous efforts have been made to search for optimal conditions for high selectivity of target chemicals and further to make clear the kinetic processes of low temperature methane reforming in various non-equilibrium plasma discharges including radio frequency discharge (RF) [11–15], gliding arc discharge [16–18], microwave discharge [19–21], and nano-second pulsed discharge (NSD) [23–25]. The reported experimental results [11–25] showed diverse values depending upon the conditions of the plasmas, and mainly aimed at interpreting the dependence of methane conversion and chemicals selectivity on gas pressure, gas composition, electrode geometry and material, electrical settings such as operating voltages and frequencies, unfortunately, poorly referred to reaction mechanisms and kinetic process of chemicals formation in different discharge plasmas. In these ways, the direct methane reforming by the RF plasma-driven technology [11–15] which can generate homogeneous discharges and suf- ficient high electron energy at low-pressure, has potential with a single-stage, non-catalytic methane conversion process. In order to study plasma chemistry in gas phase process and to have more insight into the chemical mechanism, kinetic modelling has been proven a useful technique. Jemmer [26] built a physiochemically-motivated reaction–advection–diffusion model using the symbolic algebra package mathematics to describe the chemistry of plasma enhanced chemical vapor deposition of thin films. The results originally presented a schematic pathway for direct methane decomposition in an audio-frequency plasma. Ropcke et al. [27] developed a chemical model with 57 reactions to generally predict the species concentrations and major reaction scheme in a H2–O2–CH4 microwave plasma. The later modelling works by Nair, Tomohiro Nozaki [28, 29], Yiguang Ju and Wenting Sun [3, 23], Young-Hoon Song [30] and others [31–37], presented more detailed reaction schemes for direct methane or methane oxidative conversion in non-thermal as well as thermal plasmas, and obtained good agreement with the experimental values for conversion and some key species concentration. Despite these initial simulation understanding of reactive organic 123 Plasma Chem Plasma Process (2017) 37:1551–1571 1553 plasma dynamics, the kinetic roles of plasma generated species on product formation in the gas phase and the detailed reaction pathway flux are, however, still lack of interpreting. It is therefore necessary to study reaction kinetics under well-defined conditions and to obtain detailed species information to develop and validate kinetic models for plasma assisted methane reforming. Building an accurate model for a specific plasma reactor with a complex He/CH4/O2 chemistry is a big task. In this calculation, we focused on the chemical kinetics of He/CH4 and He/CH4/O2 in non-equilibrium discharges with special attention to the kinetic effect of vibrational excitation on electron-impact ionization and dissociation. Accordingly, the goals of the present research are to investigate the kinetic contributions of O2 additive and plasma generated species on the methane pyrolysis pathways as well as key reactions influencing desired product formation by combining experiments and kinetic modeling. To characterize the performance of RF discharge in He/CH4 64/36 and He/CH4/ O2 55/36/9 mixtures, the concentrations of intermediate species, reactants and products are measured with the optical emission spectroscopy (OES) and the on-line Gas Chromatog- raphy (GC-TCD), respectively. The experimental data are used to validate the kinetic models. Path flux analysis for species production and consumption is conducted. The effect of reduced electric field and the oxygen addition on the reaction kinetics and production formation is investigated. Key reaction pathways for RF plasma activated methane reforming are identified.

Experimental and Simulation Methods

CH4 Reforming in a RF Plasma

Dielectric barrier discharge (DBD) has been widely studied in methane reforming because of its low tendency to form arcs and sparks [23]. In this study, a wire-to-tube double dielectric discharge was adopted to produce a uniform plasma discharge. The plasma was generated by a CESAR radio frequency power supply with a matching network, which supplies 13.56 MHz continuous sinusoidal waves. Automatic matching function was used for RF power supply tuning. Zero reflecting of RF power energy and 100% matching were displayed in each experiment. During the plasma discharge, the pressure and temperature of the reactor were controlled at 100 Torr and 373 K respectively. Experiments were conducted in a nearly isothermal flow reactor to minimize the temperature changes from the reaction zone in the DBD reactor. A Type-K thermocouple probe (± 1 K) of 1-mm- diameter was located at the end of the reaction zone after plasma discharge to measure the mixture temperature. CH4 and O2 were the principle reactants for the experiments. A methane fuel-containing mixture was highly diluted in a helium gas, in order to achieve lower breakdown voltages and further to ensure the accuracy of GC sampling. As shown in Fig. 1a, the inner and outer diameters of the cylindrical plasma reactor were 6.35 and 12.7 mm, respectively. A high-voltage copper electrode was placed in the center of the inner quartz tube. The outer concentric copper electrode was attached to the outside wall of the quartz tube and was connected to the ground. The gas flow rate (methane, oxygen and helium) was controlled by mass flow controllers (Brooks Instrument) and the gases were premixed at room temperature before entering the reactor. The mean flow velocity of the gas mixture across the cylindrical plasma reactor remained unchanged in each experiment. The current during the RF discharge was measured by a Pearson Coil Current probe 123 1554 Plasma Chem Plasma Process (2017) 37:1551–1571

Fig. 1 a Schematic diagram of experimental setup; b voltage and current curves for He/CH4 plasma

(Model 6585). The voltage applied to the high-voltage electrode was measured by a Lecroy high voltage probe (PPE20 KV). All the voltage signals were obtained with a Tektronix DPO 7104C oscilloscope. The reduced electric ﬁeld (E/n) in the continuous RF discharge at experimental conditions was calculated by the voltages applied to the high-voltage electrode, providing boundary conditions for simulation prediction. Typical voltage and current waveforms measured by a LeCroy high voltage probe (PPE20KV) and a Pearson Coil Current probe (Model 6585), are shown in Fig. 1b. In order to provide the species information for modeling validation, GC-TCD (Inﬁcon3000) and optical emission spectrometer (OES, Ocean Instrument) were used to quantify species concentration generated in the RF plasma. Product composition was reported in terms of the moles of product formed per mole of CH4 converted. Product selectivity equals to moles of product formed divided by moles of C detected at the reactor output.

123 Plasma Chem Plasma Process (2017) 37:1551–1571 1555

Optical emission spectroscopy (OES) method was used for the diagnostics of species in He/CH4 and He/CH4/O2 plasmas. The main advantage of this method is non-intrusive. The low pressure reactor has an optical quartz window for spectroscopic diagnostics, as shown in Fig. 1a. The resolution of the spectrometer is 0.1 nm. To improve the signal-to-noise ratio, the signal is integrated over a time period of 1 s. Spectra of the plasma discharge were measured by emission spectroscopy in the wavelength range of 179–874 nm. The optical emission spectrum of the discharge is shown in Fig. 2. The strong light ? emissions from O, OH, CH, C , HCO, CH2O, CO and He indicate the processes of electron excitation and active radical formation in the discharge [23, 38]. The most sig- niﬁcant radical lines detected in our experimental conditions are the 615.8/502 nm O atomic oxygen lines, the 388 nm CH radical and the 722.3 nm C? carbon line. These lines correspond to the electron dissociative excitation or direct impact excitation of CH4 and 1 O2, which are formed by the following pathways respectively, e ? O2 ? O ? O( D), ? e ? CH4 ? e ? CH ? H2 ? H, and CH4 ? e ? 2e ? C ? 2H2. It is also observed that the intensity of CH line increases (from 1785 to 3778 counts) and the intensity of C? line decreases (from 5659 to 4059 counts) with a small amount of O2 addition. Thus, the atom oxygen produced in He/CH4/O2 mixture shows higher oxidative reactivity and further decreases the energy barrier associated with CH4 dissociation reactions such as 1 1 O( D) ? CH4 = CH3 ? OH, and O( D) ? CH4 = CH2OH ? H[3]. It is interesting to note that a considerable amount of CH2O is formed in the plasma discharge in the He/CH4/ O2 mixture. This suggests that CH2O formation via the reactions of CH3 ? O = CH2- 1 O ? O and O( D) ? CH4 = CH2O ? H2 may also play a role in methane oxidation in a

10000

7500

5000 He/CH4 He501.6nm He 492.24nm 2500 CH 388nm

10000 7500 O 347.8nm 3 O 369.8nm 318.3

5000 2 2

Intensity(counts) He/CH4/O2 CO 451.1nm O 502nm OH 311.2nm, HCO311.5nm O CH HCO 336nm 2500 CH 0 150 200 250 300 350 400 450 500 550 600 Wavelength (nm)

30000 He/CH 20000 4

10000 722.3nm + He 587.6nm He 706.5nm C 0 He 667.8nm 30000 He/CH /O 4 2 20000 Intensity(counts)

10000 He 656.3nm and H656nm CO 561nm O 615.8nm 0 550 600 650 700 750 800 850 Wavelength (nm)

Fig. 2 Emission spectrum (179–874 nm) to illustrate the formation of intermediate species in He/CH4 and He/CH4/O2 plasmas 123 1556 Plasma Chem Plasma Process (2017) 37:1551–1571 plasma [3]. It is also noted from the intensity of C? line decreasing that collisional dissociation of methane by electrons has great effects on carbon deposition. Furthermore, helium spectrum intensity in Fig. 2 presents the strongest among all the emissions. However, only small percentage of the excited helium contributes to the energy transfer in a discharge due to its high ﬁrst excitation energy (19.8 eV). Detailed kinetic process will be discussed in the next sections.

Numerical Modeling

Bolsig ? and Chemkin PRO were combined in this study to explain the plasma chemistry in He/CH4 and He/CH4/O2 mixture discharges. The rate coefficients of the electron impact elementary reactions were calculated by solving the steady state, two-term expansion Boltzmann equation. The cross sections of electron elastic impact, electron excitation and dissociation used in this study were taken from the online LXCAT (Plasma Data Exchange Project) database and literatures [39–41] and then were integrated over the electron energy distribution function (EEDF) to get the rate coefficients of electron involving reactions. During the calculation process, all the electrons with global mean electron energy were used to substitute for the partial energetic electrons. HP mechanism [42] with some update of elementary reaction rates was integrated to decide rate coefficients of radical/ion reactions at low temperature. Temporal evolution of the electrons, free radicals, charged and excited species, and the final products was calculated by a set of conversion equations including charge balance equation, species conservation equation, energy conservation equation and state equation. Species included in this kinetic model are shown in Table 1. Transport processes were not

Table 1 Free radicals, charged and excited species, and ﬁnal products included in numerical modeling Reactants Radicals Charged species Excited Molecular species species

? ? 3 He/CH4 CH3,CH2,CH2(S), CH, C, H, e,He ,CH5 He(2 S) He,CH4,C2H2,C2H4,C2H6, ? ? C2H, H2CC, C2H3,C2H5, CH4 ,CH3 CH4m1, aC3H4,pC3H4, c-C3H4, ? ? C3H3,aC3H5,tC3H5,sC3H5, CH2 ,H2 CH4m2 C3H6, c-C3H6,C3H8,C4H4, iC3H7,nC3H7,C4H2,iC4H3, C4H6-12, C4H6-13, C4H6-2, nC4H3,iC4H5, nC4H5, H2,C4H8-1, C4H8-2, iC4H8, C4H7,iC4H7,iC4H9,tC4H9, C4H10,iC4H10 sC4H9,pC4H9 ? ? ? 3 He/CH4/ O, O3, H, OH, HO2,H2O2, e, O ,O2 ,He , He(2 S), He, CH4,O2, CO, H2,CO2, ? ? 1 O2 CH3,CH2, CH2(S), CH, C, H ,CH5 , O( D), H2O, CH2O, CH3OH, C2H2, ? ?, 1 HCO, CH2OH, HCHO, CH4 ,CH3 O( S) C2H4,C2H6,aC3H4,pC3H4, ? ? - HOCO, CH3O2, CH3O, CH2 ,H2 ,O2, O2m1, c-C3H4, C3H6,C3H8 - HOCH2O, HCOH, C2H, O O2m2, HOCHO, CH3O2H, H2CC, C2H3,C2H5, O2m3, CH3CHO,CH2CO, CH3CO, O2m4, HCCOH,C2H5OH, CH2CHO,HCCO,C2O, CH4m1, CH3OCHO, C2H4O1-2, C2H5O,CH2CH2OH, CH4m2 CH2CHOH, C2H5O2H, CH3CHOH, C2H5O2, CH3OCO, C2H4O2H, C2H3O1-2, CH2OCHO, C3H3,aC3H5,tC3H5,sC3H5, iC3H7,nC3H7,

123 Plasma Chem Plasma Process (2017) 37:1551–1571 1557 taken into account during the modeling. It is noted that the charge separation and sheath formation near the electrodes was not considered either. Therefore, the Poisson equation for the electric field was not solved during the kinetic modeling. The initial value of the electron number density was treated as an adjustable parameter to fit the methane conversion measurements, due to the sensitivity of CH4 dissociation excitation to the electron number density and electron energy distribution. The measured E/n values were deter- mined by the measured average applied voltage, the discharge channel distance, the quartz layer thickness, and the dielectric constant [23]. The kinetic model has 56 species, 385 elementary reactions and 83 species, 890 elementary reactions, respectively, for He/CH4 and He/CH4/O2 reforming in RF plasmas. The aim of this calculation is mainly to identify the reactions which can influence the final product selectivity and hence analyze the detailed reaction pathways of methane pyrolysis and oxidation by low-pressure RF discharge at low temperature. Formation of C3 oxygenates were not considered in the present case as was observed in the experiments. Key reactions related to the discharge chemistry are listed in Table 2.

Results and Discussion

Species Selectivity and Reactants Conversion for Methane Reforming

The main purpose of this experimental study was to understand the kinetic role of oxygen addition in affecting the kinetic process of methane reforming activated by RF plasma, and to obtain experimental data to validate the assembled kinetic model. Therefore, in this set of tests, the flow rate of methane in both He/CH4 and He/CH4/O2 discharge mixtures was fixed at 36 ml/min to consider the kinetic effect of O2 in CH4 reforming. It is seen that electronically excited dissociation, ionization and radical recombination in the methane mixture discharge led to the production of C2 and C3. Here, C2 denotes for hydrocarbons including C2H6,C2H4 and C2H2,C3 for C3H8,C3H6 and C3H4. With 9% oxygen addition, a large amount of syngas (CO and H2) as well as oxygenated species (CH2O and CH3OH) were produced. These species are all beneficial by-products for synthetic fuels and industrial chemicals. At the same time, H2O and CO2 were also formed by slow oxidation process. Furthermore, tar-like products or sometimes solid carbon deposits were observed near the electrode. GC-TCD was used to measure the final products after the RF discharge. At all experimental conditions, more than 90% carbon was recovered by the GC measurements by avoiding arc-discharge and eliminating carbon deposit in the DBD channel. Figure 3 shows the selectivity of C2,C3 and syngas with respect to RF power. It can be seen that in the range of 50–140 W RF power (E/n values of 80–102.7 Td), all the products change linearly with the RF power. As shown in Fig. 3,H2 production was very prominent in both non-oxidative and oxidative processes. Moreover, C2 selectivity was greater than C3 at least a factor of two, indicating that H2 and C2 hydrocarbon formation was the major pathway for methane pyrolysis in plasma. However, with 9% oxygen addition, both C2 and C3 formation dramatically decreased, and syngas becomes the major products. This observation suggests the different electron energy transfer mechanisms in non-oxidation and oxidation methane reforming mixtures with respect to RF power. It can be found from the changing trends of species selectivity in Fig. 3 that the discharge power localized within excitation and ionization channels in CH4 molecules is effectively competed by 123 1558 Plasma Chem Plasma Process (2017) 37:1551–1571

Table 2 List of key reactions related to He/CH4/O2 discharge Number Reaction Rate coefﬁcient, ATnexp(-E/RT) References

A (mol cm3 s) n (T in K) E (cal/mol)

Electron collision reactions mechanism

1e? CH4 ? e ? CH3 ? H r [40]

2e? CH4 ? e ? CH2 ? H2 r [40]

3e? CH4 ? e ? CH2 ? H?H r [40]

4e? CH4 ? e ? CH ? H2 ? H r [40]

5e? O2 ? e ? O2* r [41]

6e? O2 ? e ? O ? O(1D) r [41] ? 7e? O2 ? O2 ? 2e r [41] 8e? He ? e ? He* r [41]

C2,C3 formation

9 He* ? CH4 ? He ? CH ? H2 ? H 3.3723E ? 11 0 0 [42]

10 C2H3 ? H = C2H2 ? H2 9.00E ? 13 0 0 [42]

11 CH2 ? CH2 = C2H2 ? H?H 7.094E ? 13 0.002 8.9 [42]

12 CH2 ? CH3 = C2H4 ? H 1.20E ? 15 -0.343 153.1 [42]

13 C2H5 ? OH = C2H4 ? H2O 2.409E ? 13 0 0 [42]

14 CH ? C2H6 = C2H4 ? CH3 6.185E ? 13 0 -262.3 [42]

15 CH3 ? CH3(?M) = C2H6(?M) 3.61E ? 13 0 0 [42] LOW 1.990E ? 50 -7.080 6685.0 TROE 0.8422 125.0 2219.0 6882.0

H2 = 2.0 H2O = 6.0 CH4 = 2.0 CO = 1.5 CO2 = 2.0 C2H6 = 3.0 CH3OH = 3.0

16 C2H5 ? CH3O = C2H6 ? CH2O 1.00E ? 13 0 0 [42]

17 C2H5 ? H(?M) = C2H6(?M) 5.210E ? 17 -0.99 1580 [42] LOW 1.990E ? 41 -7.080 6685.0 TROE 0.8422 125.0 2219.0 6882.0

H2 = 2.0 H2O = 6.0 CH4 = 2.0 CO = 1.5 CO2 = 2.0 C2H6 = 3.0 CH3OH = 3.0

18 H ? sC3H5 = aC3H4 ? H2 3.00E ? 13 0 0 [42]

19 H ? sC3H5 = pC3H4 ? H2 3.00E ? 13 0 0 [42]

20 CH ? C2H4 = aC3H4 ? H 9.23E ? 13 0 -344.0 [43]

21 CH ? C2H4 = pC3H4 ? H2 3.96E ? 13 0 -344.0 [43] 22 aC3H5 ? H(?M) = C3H6(?M) 2.00E ? 14 0 0 [42] LOW 1.33E ? 60 -12.0 5967.8 TROE 0.02 1097.0 10967.0 6860.0

H2 = 2.0 CH4 = 2.0 C2H6 = 3.0 He = 0.7

23 C2H6 ? CH = C3H6 ? H 1.8066E ? 13 0 0 [44]

24 CH2 ? C2H4 = C3H6 3.952E ? 10 0 -4788 [45]

25 H ? iC3H7 = C3H6 ? H2 3.61E ? 13 0 0 [42]

26 H ? i-C3H7 (?M) = C3H8 (?M) 2.40E ? 13 0 0 [46] LOW 1.70E ? 58 -12.08 11263.7 TROE 0.649 1213.1 1213.1 13369.7

H2 = 2.0 H2O = 6.0 CH4 = 2.0 CO = 1.5 CO2 = 2.0 C2H6 = 3.0

27 CH2 ? C2H6 = C3H8 2.89E ? 12 0 0 [47]

123 Plasma Chem Plasma Process (2017) 37:1551–1571 1559

Table 2 continued

Number Reaction Rate coefﬁcient, ATnexp(-E/RT) References

A (mol cm3 s) n (T in K) E (cal/mol)

Syngas formation (H2 and CO) 1 28 O( D) ? CH4 ? CH2O ? H2 4.52E ? 12 0 0 [42] 29 HCO ? O2 = CO ? HO2 1.204E ? 10 0.807 -727.0 [42]

30 HCO ? H = CO ? H2 1.100E ? 14 0 0 [42]

31 CH3 ? O ? CO ? H2 ? H 1.686E ? 13 0 0 [42]

32 CH2 ? O = CO ? H2 8.190E ? 13 0 536.5 [42]

33 CH2 ? H = CH ? H2 1.205E ? 14 0 0 [42]

CH2O, CH3OH formation

34 O ? CH3 = H?CH2O 6.745E ? 13 0 0 [42] 35 CH2OH ? O2 = CH2O ? HO2 8.130E ? 09 0.89 -866.1 [42]

36 CH3O2 ? OH = CH3OH ? O2 6.022E ? 13 0 0 [42]

37 CH2OH ? O=CH2O ? OH 6.564E ? 13 0 -693.2 [42]

38 CH2O ? OH = HCO ? H2O 7.82E ? 07 1.63 -1055 [42] Carbon deposition

39 H ? CH = C ? H2 1.20E ? 14 0 0 [42]

40 CH4 ? e?e ? C ? 2H2 r

41 C ? O2 = CO ? O 6.624E ? 13 0 635.9 [42] 42 C ? OH = CO ? H 5.00E ? 13 0 0 [42]

100 H2, methane pyrolysis H2,partial oxidation C2,methane pyrolysis 80 C2,partial oxidation C2, non-oxidation C3,methane pyrolysis C3,partial oxidation CO,partial oxidation 60 H2, non-oxidation CO, oxidation

40 H2, oxidation

Selectivity (%) C3, non-oxidation C2,oxidation 20 C3, oxidation

0 40 60 80 100 120 140 RF power output (W)

Fig. 3 Experimental results of C2,C3 and syngas selectivity as a function of RF power in He/CH4 64/36 (non-oxidation) and He/CH4/O2 55/36/9 (oxidation) mixtures. Solid and dotted lines are used for experimental data ﬁtting in oxidation as well as non-oxidation reforming mixtures respectively, to clearly show the product change tendency, with the RF power increasing

123 1560 Plasma Chem Plasma Process (2017) 37:1551–1571

different freedom degrees of O2 molecules when O2 is added, leading to the dramatic decrease of higher hydrocarbon formation in the He/CH4/O2 mixture. The individual species selectivity measurements of C2 and C3 hydrocarbons with respect to RF power are plotted in Fig. 4a, b. It can be seen that C2H6 and C3H8 selectivities generally decrease with the increase of RF power at the non-oxidation experimental conditions. However, as seen in the right part of Fig. 4a, b, their selectivities increase oppositely with respect to RF power increasing in the methane oxidation process. Numerical analysis of the reaction pathways later in Fig. 11 reveals that this increase is due to the accelerated formation of CH3 and CH2 radicals (major sources of C2H6 and C3H8 yielding) at relatively higher E/n values.

(a) 45 C2H4,non-oxidation 40 C2H6,non-oxidation C2H6, non-oxidation C2H2,non-oxidation 35 C2H4,oxidation C2H6,oxidation 30 C2H2,oxidation 25 C2H2, non-oxidation 20 C2H6, oxidation

selectivity (%) 15 C2H4, non-oxidation 10 C2H2, oxidation 5 C2H4, oxidation

0 40 60 80 100 120 140 RF power output (W)

16 (b) C3H6, non-oxidation C3H6,oxidation 14 C3H8,oxidation C3H4,oxidation 12 C3H8, non-oxidation C3H6,non-oxidation 10 C3H8,non-oxidation C3H8, oxidation C3H4,non-oxidation 8 C3H6, oxidation 6

seletivity (%) 4

2 C3H4, non-oxidation C3H4, oxidation

40 60 80 100 120 140 RF power output (W)

Fig. 4 Product selectivity as a function of RF power in 64/36 He/CH4 (non-oxidation) and 55/36/9 He/CH4/ O2 (oxidation) mixtures; a C2 hydrocarbon species, b C3 hydrocarbon species. Solid and dotted lines are used for experimental data ﬁtting in oxidation as well as non-oxidation reforming mixtures respectively, to clearly show the product change tendency, with the RF power increasing 123 Plasma Chem Plasma Process (2017) 37:1551–1571 1561

100 CH4-exp, non-oxidation O2-exp, oxidation 80 CH4-sim, oxidation CH4-exp, oxidation O2-sim, oxidation

Conversion rate (%) 20 The same experimental conditions for non-oxidation and oxidation CH 4 reforming at 70-100W RF power 0 50 60 70 80 90 100 110 120 130 140 RF power output (W)

Fig. 5 Dependence of CH4 and O2 conversion rates on RF power

Figure 5 presents the measured and predicted conversion rates of CH4 and O2 as a function of RF power. It was experimentally observed that low-pressure RF plasma can decompose methane sufﬁciently and rapidly. The overall decomposition rate of methane as well as oxygen increased proportionally to the supplied RF power. It is further seen that in the range of 50–140 W RF power input (E/n values of 80–102.7 Td), the fuel and oxygen conversion rates are predicted to be within 5 and 20% discrepancies from the experimental results respectively, which is within the experimental uncertainty. As discussed above, due to the sensitivity of CH4 dissociation excitation to the electron number density and electron energy distribution, the initial value of the electron number density was treated as an adjustable parameter to ﬁt the methane conversion measurements. The resulting CH4 conversion is thus in good agreement with the experimental values. The relative agreement

50 70000 H2-exp C3H8-exp CO-exp CO-sim H2O-exp H2O-sim H2-sim C3H8-sim 60000 40 C2H6-exp C2H6-sim 50000 30 40000 20 30000 CO2-exp CO2-sim 10 CH2O-exp CH2O-sim CH3OH-exp Selectivity (%) 20000 CH3OH-sim

0 10000 C3H6-exp C3H6-sim C2H4-exp C2H4-sim

-10 species concentration (ppm) 0 60 70 80 90 100 110 120 130 140 150 60 70 80 90 100 110 120 130 140 150 RF power output (W) RF power out put (W) (a) (b)

Fig. 6 Comparisons between experimental and modeling results in an oxidation reforming mixture as a function of RF power; a selectivity for H2,C2H6,C2H4,C3H8,C3H6 species, b concentrations of oxygenated products including CO, CO2,H2O, CH2O and CH3OH 123 1562 Plasma Chem Plasma Process (2017) 37:1551–1571

of O2 conversion in the comparison between the modeling and experimental results indicates that the related electron collision reaction rates are well modeled in this study. Figure 6a shows the comparison of the model prediction and the GC measured selectivity for H2,C2H6,C2H4,C3H8 and C3H6 with respect to RF power. The kinetic model can well predict C2H4 and C3H6 formation for all experimental cases. However, this model considerably over-predicts H2 (up to 4.5–58.5%) except for the case of 70 W RF power output. From the reaction paths ﬂux predicted by this kinetic model for the oxidation methane reforming (as shown in later Fig. 11b), the over-prediction of H2 formation is probably due to the overall underestimation of higher hydrocarbons. Moreover, the C2H6 and C3H8 predicted and measured results display the opposite trends in different cases. The failure of the model to predict C2H6 measurements at 70 and 80 W RF power in Fig. 6ais largely due to the over-consumption of CH3 radical by the oxidation pathways CH3 ? O2 (?M) = CH3O2 (?M) and CH3 ? O = CH2O ? H. Additionally, C3H8 selectivity is under-predicted by a factor of 0.6–4.3 at relatively lower energy input while it is over- predicted at higher RF power. C3H8 is primarily formed from H and C3H7 recombination. H atom was mainly from the methane collisional dissociation by electrons and C3H7 radical from the recombination reactions of C2H6 with CH. All the above reaction pathways suggest that the over-prediction of C3H8 is mainly caused by the failure prediction of C2H6 formation. Figure 6b presents the measured and predicted species concentrations as a function of RF power. The H2O predicted concentrations are compared with the experimental measurements. The model captures the relative concentrations of H2O and the correct trend, but over-predicts the absolute concentrations. The experimental results of methane reforming with a small amount of oxygen addition are very different from that in the non- oxidation process, as shown in Fig. 3.C2 and C3 selectivity decreased sharply, and CO and H2O had the highest concentrations with oxygen addition. Considerable amount of CO2, CH2O and CH3OH were also detected. All these results suggest the prominent oxidation effect of low-ratio O2 addition on methane reforming in a discharge. Path ﬂux analysis later in Figs. 11 and 12 further demonstrates that oxygen atom formation in plasma initiated many chain reactions, which gave rise to OH, CH2O and HCO radicals. H2O, CH2O and CH3OH were formed during these chain reactions. CO2 was mainly produced from CO oxidation.

CH4 and O2 Activation by Electron Impact

Kinetic modeling was conducted to understand the electron impact excitation, dissociation and ionization, radial production and product formation in a continuous RF discharge. The rotational degree of freedom was neglected in this calculation because its excitation was important only at low (\1 Td) E/n values. Vibrational molecules (2 levels for CH4 and 4 levels for O2, as shown in Table 2) were included in this model, considering the effect of vibrational states on rate constants for electron impact dissociation and ionization reactions. The solved EEDF allows calculation of the energy branches for various elementary reactions. Energy branches at varied reduced electric ﬁeld E/n in He/CH4 and He/CH4/O2 discharges are presented in Fig. 7. Most of the discharge power is located within vibrational excitation of methane molecules at relatively lower E/n. As can be seen in Fig. 7a and b, the contribution of dissociation and ionization can effectively compete with vibrational excitation at higher E/n values. The electron energy distribution between different excitation channels strongly depends on E/n values as well as gas composition. 123 Plasma Chem Plasma Process (2017) 37:1551–1571 1563

100% 100% He/O /CH discharge CH vibration 4 2 4 CH4 dissociation CH4 dissociation

O dissociation 2 CH4 vibration CH ionization He/CH discharge 4 4 O electronic excitation 10% 2 CH , ionization 10% 4

He elastic He elastic Energy loss fraction Energy loss fraction He electronic excitation O ionization O vibration 2 CH4 elastic 1% 2 1% 0 20 40 60 80 100 0 20406080100 E/N (Td) E/N (Td) (a) (b)

Fig. 7 Fractional power dissipated by electrons into the different molecular degrees of freedom as a function of E/n; a He/CH4/O2 mixture discharge, b He/CH4 mixture discharge

Comparing Fig. 7a and b, addition of O2 changes the energy distribution dramatically. The efﬁciency of the dissociative and electronic excitation of oxygen molecular become sig- niﬁcant, leading to an abrupt increase of oxygen atoms (O and O (1D)) in their concentrations. It is therefore found that the higher hydrocarbon formation including C2 and C3 species decreased sharply when O2 is added, as shown in Figs. 4 and 5. Figure 7 shows that at relatively lower E/n, a major fraction of electron energy can selectively go into the vibrational excitation of methane molecules. In order to demonstrate the kinetic effects of the vibrational excitation on the plasma chemistry, rate constants for typical electron impact dissociation and ionization reactions were calculated by solving the Boltzmann equation, with and without consideration of vibrational molecules. It can be seen in Fig. 8 that in the experimental region in this study (80–102.7 Td), the rate constants for main electron impact dissociation and ionization reactions were within 15% variations in cases of consideration of vibrational excitation or not, demonstrating minor effects of vibrational excitation on electron impact dissociation and ionization process in a plasma. Accordingly, the kinetic effects of rotational as well as vibrational excitation on the

Fig. 8 Rate constants variations e+CH4=>e+CH3+H for main electron impact e+O2=>e+O+O(D) -9 e+O2=>e+O+O dissociation and ionization 10 reactions as a function of E/n, with and without consideration of e+CH4=>e+CH+H2+H

/sec) -12 vibrational molecules 3 10 e+CH4=>e+CH2+H2 cm

-15 e+CH4=>2e+CH4+ experimental 10 region

e+O2=>2e+O2+ 10-18

Rate Constant( without consideration of vibrational levels with consideration of vibrational levels He/CH /O 55/36/9 mixture 4 2 10-21 20 40 60 80 100 E/n(Td)

123 1564 Plasma Chem Plasma Process (2017) 37:1551–1571 process of plasma assisted methane conversion were neglected in this calculation, in order to make the reaction pathway more efficient. In addition, gas heating term from V–V (vibrational–vibrational) and V–T (vibrational–translational) energy relaxations was removed from the calculation due to the isothermal reaction process in the plasma chemistry system. All the active species simultaneously generated in the plasma discharge, including active radicals, excited species, electrons and other intermediate species are rapidly increasing with discharge time. The maximum electron density reaches 1014 cm-3 order of magnitude at the end of discharge. CH, CH2,CH3 and H are quickly decreasing after discharge termination to produce higher hydrocarbon, syngas and oxygenates, as shown in Fig. 9. At the discharge termination, the stable products including C2 and C3 hydrocarbon species as well as syngas reached 100–1000 times than the active species. As discussed above, the presence of low-ratio O2 additive dramatically changes the energy distribution mechanism. Specifically, the dissociation excitation of oxygen, O2 ? e ? O ? O(1D) ? e, becomes the most efficient energy branch and leads to the highest production of the atom oxygen O in its concentration during the discharge, which has been validated in Fig. 9d. Simultaneously, a great number of O (1D) are generated at this stage and play an important role in CH4 oxidation. The major reactions of methane oxidation activated by 1 1 1 O( D) include O( D) ? CH4 ? CH3 ? OH,O( D) ? CH4 ? CH2OH ? H, and 1 O( D) ? CH4 ? CH2O ? H2, etc. The above analysis provides a good explanation for active species production and quenching in the methane reforming discharge, which are dominantly governed by radical chain propagating and termination reactions.

-2 -2 CH4 10 H 10 CH2 -5 CH3 -5 10 CH 10 pC3H4 H2 C2H2 c-C3H4 tC3H5 -8 sC3H5 -8 C 10 C2H3 10 C3H6 C2H6 -11 C3H3 10 -11 C2H4 C2H5 10

-14 -14 10 nC3H7 10 Mole Fraction Mole Fraction aC3H5 C3H8 -17 10 10-17 aC3H4

iC3H7 10-20 10-20 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Time(s) Time(s) (a) (b)

H2O 10-2 O2 10-2 CH3O2H OH CO O -5 -5 10 10 HCO O(1D) CH2O -8 -8 10 10 CH3O CH3O2 CH2OH -11 -11 10 CO2 10 HO2 CH3OH 10-14 10-14 Mole Fraction C2H5OH Mole Fraction -17 -17 10 CH3CHO 10

10-20 10-20 -9 -8 -7 -6 -5 -4 -3 -2 10 10 10 10 10 10 10 10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 Time(s) Time(s) (c) (d)

Fig. 9 Time evolution of radicals and products at E/n = 90 Td in He/CH4/O2 plasma 123 Plasma Chem Plasma Process (2017) 37:1551–1571 1565

-8 -1 1.0x10 10 Production,50Td Carbon mechanism Rate Constant -- CH+H=C+H2 1.99E-10 production -4 Consumption,50Td C+OH=CO+H 8.3E-11 consumption 10 -9 8.0x10 C+O2=CO+O 4.66E-11 consumption ) -7 e+CH2+=>C+H2 6.25E-7 production /s 10 Production,100Td 3 e+CH2+=>C+2H 6.85E-8 production CH3+C=C2H2+H 8.3E-11 consumption /s)

3 Consumption,100Td -10 -9 10 (cm 6.0x10 e+O =>e+O+O(1D) Production,150Td 2 -13 Consumption,150Td 10 Reactions of Production Electric field

mol/cm R26:CH+H=C+H -9 for RF discharge -16 2 4.0x10 10 R202:e+CH =>e+C+2H 4 2 + -19 R239:e+CH =>e+C+H 10 2 2 Rate ( Rate + -9 R240:e+CH =>e+C+2H Rate Constant 2.0x10 e+O =>e+O+O 2 2 10-22 Reactions of Consumption R358:CH +C=C H +H e+CH =>e+C+2H 3 2 2 4 2 10-25 0.0 10-15 10-13 10-11 10-9 10-7 10-5 10-3 20 40 60 80 100 120 140 160 180 200 Time(s) E/n(Td) (a) (b)

Fig. 10 Mechanism analysis of carbon deposit as a function of time; a balance between the carbon production and consumption, b dependence of rate coefficient on E/n Time evolution of carbon deposition during the process of methane pyrolysis and oxidation was predicted at the E/n values ranging from 50 to 150 Td. As presented in Fig. 10a, the primary mechanism of carbon formation is mainly the electronically dissociation of methane molecular, including the reactions, e ? CH4 ? e ? C ? 2H2, e ? CH2 ? e ? C ? H2,e? CH2 ? e ? C ? 2H. A noted exception to this formation mechanism is the radical recombination reaction from H and CH to C, i.e., H ? CH = H2 ? C. This reaction plays a role in carbon production primarily due to its fast reaction rate, as shown in Fig. 10b. In contrast, the primary mechanism of carbon consumption is mainly from the recombination reaction, CH3 ? C = C2H2 ? H. According to the prediction results in Fig. 10a, carbon formation via the fuel dissociation is predominant at the initial stage of discharge, however, would be quickly balanced by carbon consumption at the discharge termination. This indicates shortening discharge time or running the experiments at lower E/n values will be largely beneficial for carbon reduction. Figure 10b shows the depressed effect of O2 addition on carbon formation. In the RF region, E/n \ 100 Td, the discharge power localized within excitation and ionization channels in CH4 molecules is effectively competed by different freedom degrees of O2 molecules. It makes the carbon production in a He/CH4/O2 discharge mixture ineffi- cient. It is worth mentioning that as a carrier gas, helium ration in a reforming mixture has a great effect on carbon formation. With the helium ratio reduced, the average electron energy in a discharge declines due to the penning discharge effect, leading to the decrease of the rate coefficients of the methane dissociation reactions by electron impact. As discussed above, the major trends of reactants (CH4 and O2) consumption and primary species products are well captured by the present kinetic model, indicating that the electron collision rates and dominant pathways are well modeled. However, the modeling results also showed some significant disagreements of minor species including C2H2 and C3H4, which indicates the related reaction pathways need to be detailed analyzed. Overall, the mechanism proposed in this study is valid for predicting/explaining the electron impact excitation and dissociation, radial production and product formation in a continuous RF discharge. Path flux analysis for both non-oxidation and oxidation methane reforming reactions were performed at the reduced electric field 90 Td, as shown in Fig. 11a, b. The total flux of an element was calculated by summing up the element fluxes obtained from each reaction step, using the calculation method of Revel et al [48]. It is seen in Fig. 11a, b that CH4 pyrolysis and oxidation in a RF plasma volume begins with the formation of the active 123 1566 Plasma Chem Plasma Process (2017) 37:1551–1571

Fig. 11 Reaction paths ﬂux analysis for a non-oxidation and, b oxidation CH4 reforming at E/n = 90 Td species due to the energetic electron impact, of which the most important are free radicals, 1 including CH3,CH2, CH, H, O, and O ( D) etc. Once 9% of O2 is present, fuel oxidation process becomes dominant due to the signiﬁcant generation of O and O (1D) in a RF

123 Plasma Chem Plasma Process (2017) 37:1551–1571 1567 discharge. Unfortunately, quantitative measurement of these radicals is still challenging due to their short lifetime as well as very low concentration in the discharge mixture. Methane consumption is the primary process for methane pyrolysis and oxidation. There are three different major pathways for methane consumption. The first pathway is direct dissociation of methane to CH3,CH2, CH, C, H and H2 by electron impact in a RF plasma, which is primarily led by the following reactions, e ? CH4 ? e ? CH3 ? H, ? e ? CH4 ? 2e ? CH3 ? H, e ? CH4 ? e ? CH2 ? H2. The second pathway is the recombination reaction between methane molecules and dissociative radicals, for example, ? ? CH4 ? CH4 = CH5 ? CH3 and CH2 ? CH4 = C2H6. The last pathway is fuel oxi- 1 dation by plasma generated oxygen atoms (O, O( D), and OH) to form CH2O, CH2OH and CH3 radicals, when O2 is added. In contrast, methane formation by CH3 recombination with H, H ? CH3(?M) = CH4(?M), also needs to be noted. This process builds blocks for CH4 conversion and higher hydrocarbon formation, indicating that these recombination reactions must be detailed included in the model to accurately predict species concentrations [21]. It can be seen from Fig. 11a, b that the process occurring via either of the two following basic chemical mechanisms is necessarily important. One is the propagating of radical chain, which will terminate by radical stabilization (losing active). The other is the recombination of radical-radical or radical-molecule, leading to the formation of new larger molecules, for example, C2 and C3 hydrocarbons [49]. C2 and C3 hydrocarbons are the most important target products for methane reforming. As described above, C2 selectivity is higher than C3 by at least a factor of two. This is because the free radicals formed in the electron impact dissociation process are much more capable of reacting with each other than stable C2 molecules to form C3 species. We especially focus on the C2H4 formation due to its important industrial interest. From the pathway flux analysis in Fig. 11a, b, the dominant formation path of C2H4 is primarily via CH recombination with CH4,CH2 with CH3, i.e., CH ? CH4 = H ? C2H4, CH2 ? CH3 = C2H4 ? H. Simultaneously, for the view of sensitivity analysis, the electron impact reaction of e ? CH4 ? e ? CH ? H2 ? H instead makes great contribution for C2H4 formation. Accordingly, CH concentration is crucial for determining the concentration of C2H4. If we especially focus on C2H4 production not only for non-oxidation but also for oxidation CH4 reforming, plasma-catalyst processing [50] is necessarily needed to dissociate CH3 radical to CH and CH2. The path flux analysis of RF plasma assisted CH4 pyrolysis and oxidation in Fig. 11b also indicates that syngas formation is a very important process. As can be seen in the lower left of Fig. 11a, the dominant formation of H2 (about 44.75%) without oxygen additive is mainly via electron impact reactions. Among other formation pathways, approximate 36.59% of H2 is formed from H abstraction with CH and CH2. However, as shown in Fig. 12a, once atom oxygen is generated in a discharge, it would be rapidly consumed by methane and its radicals to generate H2, about 1.9% of H2 from the reaction of CH4 and O(1D), 6.4% from CH3 and O. As such, in order to predict H2 formation accurately, O2 consumption and atom oxygen formation need to be accurately captured in this kinetic model. It can be seen in Fig. 12b that approximate 71.6% of CO is mainly from HCO decomposition reactions, i.e., HCO ? M = CO ? H ? M, M = OH, H, O2. Moreover, as another important species of syngas, molecule hydrogen has the largest production. H2O and CO2 are also major products for CH4 pyrolysis and oxidation. As described in literatures, O begins chain reactions giving rise to OH and H radicals [51]. H2O is formed during these chain reactions. The recombination of CH2O with OH, CH2O ? OH = H2O ? CHO, is the most important reaction in H2O formation chain. 123 1568 Plasma Chem Plasma Process (2017) 37:1551–1571

Fig. 12 Reaction paths ﬂux analysis for O2, CO, H2,H2O and CO2 in a He/CH4/O2 discharge plasma at E/n = 90 Td

Approximate 46.8% of H2O comes from this reaction. Additional rapid reaction mechanisms for H2O formation are OH, HO2,CH3 and CH4 reacted with OH, as shown in Fig. 12c. It can be further showed in Fig. 12d that CO2 is formed at the end of these chain reactions via slow oxidation process, mainly coming from CO.

Conclusion

The present study was conducted to investigate experimentally and computationally the kinetic effect of RF plasma generated species on methane pyrolysis and oxidation in He/ CH4 and He/CH4/O2 mixtures. The experimental results showed that oxygen addition significantly affected the reaction chemistry, plasma energy transfer, and the products of plasma affected methane reforming. In methane pyrolysis, H2,C2 and C3 are the major products. However, with oxygen addition, a large amount of syngas (CO and H2) as well as oxygenated species (CH2O and CH3OH) were produced. Moreover, both C2 and C3 formation dramatically decreased. It was also shown that higher E/n values were more beneficial for higher hydrocarbon formation when oxygen presented in a He/CH4 mixture. These results were in reasonable agreement with the modeling results. A kinetic mechanism for plasma assisted methane pyrolysis and oxidation was assembled and assessed by comparisons with experimental results. The modeling results showed that not only the efficient generations of active species (CH3,CH2, CH, H, O, O(1D), etc.) in RF plasma but also the radical chain-propagation and radical recombination were important for methane pyrolysis and oxidation. Time-evolution of radicals and species concentrations showed that electronically excited, ionization and dissociation processes were strongly dependent on the E/n value, the gas density and the electron 123 Plasma Chem Plasma Process (2017) 37:1551–1571 1569 concentrations. While the electrodes structure, the gas density and the reduced electric field were fixed, both species selectivity and reactant conversion at the discharge termination were modeled reasonably well in comparison with experiments. C2 selectivity was higher than C3 in a factor of two. This was because the related radical formation and chain propagation in the electron impact dissociation process had strong dependence on the reduced electric field. Further comparison of the measured and numerical predicted species concentrations in RF plasma assisted methane pyrolysis and oxidation showed that the kinetic model captured the major trends of reactants (CH4 and O2) consumption and primary species products, however, failed in accurately predicting the production of C2H2 and C3H4. This discrepancy indicated that more quantitative measurements of the radical species including CH3,CH2, CH, OH and H generated in the discharge are needed to provide validation targets for the kinetic modeling.

Acknowledgements This work was supported by the ACEE grand challenge grants, the Exxon Mobile research grant, the U.S. Department of Energy, Ofﬁce of Science (DE-SC0015735),and the National Natural Science Foundation of China (51373021).

References

1. Petitpasa G, Rollier J-D, Darmon A, Gonzalez-Aguilar J, Metkemeijer R, Fulcheri L (2007) A com- parative study of non-thermal plasma assisted reforming technologies-a review. Int J Hydrogen Energy 32:2848–2867 2. Jamal Y, Wyszynski ML (1994) On-board generation of hydrogen-rich gaseous fuels-a review. Int J Hydrogen Energy 19:557–572 3. Yiguang J, Sun W (2015) Plasma assisted combustion: dynamics and chemistry. Prog Energy Combust 48:21–83 4. Nozaki T, Okazaki K (2013) Non-thermal plasma catalysis of methane: principles, energy efﬁciency, and applications. Catal Today 211:29–38 5. Guo X, Fang G, Li G (2014) Direct, nonoxidative conversion of methane to ethylene, aromatics, and hydrogen. Science 344:616–619 6. Liu H, Wierzbicki D, Debek R, Motak M, Grzybek T, Da Costa P, Ga´lvez ME (2016) La-promoted Ni- hydrotalcite-derived catalysts for dry reforming of methane at low temperatures. Fuel 182:8–16 7. Zhu T, Yang Z, Han M (2015) Performance evaluation of solid oxide fuel cell with in-situ methane reforming. Fuel 161:168–173 8. Arp DJ, Sayavedra-Soto LA, Hommes NG (2002) Molecular biology and biochemistry of ammonia oxidation by Nitrosomonas europaea. Arch Microbiol 178:250–255 9. Balasubramanian R, Rosenzweig AC (2007) Structural and mechanistic insights into methane oxidation by particulate methane monooxygenase. Acc Chem Res 40:573–580 10. Yang E-C, Hao J-K, Tang T-H, Wang B-W, Cao Y-Y (2003) The theoretical study of reaction paths and transition states on coupling reaction of methane through plasma. J Mol Struct 626:121–126 11. Ren ZF, Huang ZP, Xu JW, Wang JH et al (1998) Synthesis of large arrays of well-aligned carbon nanotubes on glass. Science 282:1105–1107 12. Mitura S (1987) Nucleation of diamond powder particles in an RF methane plasma. J. Cryst Growth 80:417–424 13. Katayama K, Fukada S, Nishikawa M (2010) Direct decomposition of methane using helium RF plasma. Fusion Eng Des 85:1381–1385 14. Patino P, Perez Y, Caetano M (2005) Coupling and reforming of methane by means of low pressure radio-frequency plasmas. Fuel 84:2008–2014 15. Tsai C-H, Hsieh T-H (2004) New approach for methane conversion using an rf discharge reactor. 1. Inﬂuences of operating conditions on syngas production. Ind Eng Chem Res 43:4043–4047 16. Iskenderova K, Porshnev P, Gutsol A, Saveliev A, Fridman A, Kennedy L (2001) Methane conversion into syn-gas in gliding arc discharge. In: 15th international symposium on plasma chemistry, Orleans 17. Kalra CS, Gutsol AF, Fridman AA (2005) Gliding arc discharges as a source of intermediate plasma for methane partial oxidation. IEEE Trans Plasma Sci 33(1):32–41 123 1570 Plasma Chem Plasma Process (2017) 37:1551–1571

18. Czernikowski A (2001) Glidarc assisted preparation of the synthesis gas from natural gas and waste hydrocarbons gases. Oil Gas Sci Technol Rev IFP 2(56):181–198 19. Jasinski M, Dors M, Mizeraczyk J (2009) Destruction of Freon HFC-134a using a nozzleless microwave plasma source. Plasma Chem Plasma Process 29:363–372 20. Shen C, Sun D, Yang H (2011) Methane coupling in microwave plasma under atmospheric pressure. J Nat Gas Chem 20:449–456 21. Rusanov VD, Babaritskii AI, Geramisov EN, Deminskii MA, Demkin SA, Zhivotov VK (2003) Stimulation of the partial oxidation of methane in a microwave discharge. Dokl Phys 48(3):119–122 22. Babararistkii I, Baranov IR, Bibikov MB, Demkin SA, Zhivotov VK, Konovalov GM (2004) Partial hydrocarbon oxidation processes induced by atmospheric-pressure microwave-discharge plasma. High Energy Chem 38(6):407–410 23. Sun W, Uddi M, Won SH, Ombrello T, Carter C, Yiguang J (2012) Kinetic effects of non-equilibrium plasma-assisted methane oxidation on diffusion flame extinction limits. Combust Flame 159:221–229 24. Lefkowitz J, Uddi M, Windom BC, Lou G, Yiguang J (2014) In situ species diagnostics and kinetic study of plasma activated ethylene dissociation and oxidation in a low temperature flow reactor. Proc Combust Inst 35:3505–3512 25. Khalifeh O, Mosallanejad A, Taghvaei H, Rahimpour MR (2016) Decomposition of methane to hydrogen using nanosecond pulsed plasma reactor with different active volumes, voltages and frequencies. Appl Energy 169:585–596 26. Jemmer P (1999) Mathematical modeling and interpretation of reactive plasma chemistry. Math Comput Model 30:61–76 27. Ropcke J, Mechold L, Kaning M, Fan WY, Davies PB (1999) Tunable diode laser diagnostic studies of H(2)–Ar–O(2) microwave plasmas containing methane or methanol. Plasma Chem Plasma Process 19:395–419 28. Nair SA, Pemen AJM, Yan K, van Heesch EJM, Ptasinski KJ, Drinkenburg AAH (2004) Tar removal from biomass derived fuel gas by pulsed corona discharges—a chemical kinetic study. Ind Eng Chem Res 43:1649–1658 29. Nair SA, Nozaki T, Okazaki K (2007) Methane oxidative conversion pathways in a dielectric barrier discharge reactor—investigation of gas phase mechanism. Chem Eng J 132:85–95 30. Lee DH, Kim K-T, Cha MS, Song Y-H (2010) Plasma-controlled chemistry in plasma reforming of methane. Int J Hydrogen 35:10967–10976 31. Kraus M, Egli W, Haffner K, Eliasson B, Kogelschatz U, Wokaun A (2002) Investigation of mechanistic aspects of the catalytic CO2 reforming of methane in a dielectric-barrier discharge using optical emission spectroscopy and kinetic modeling. Phys Chem Chem Phys 4:668–675 32. Benilov MS, Naidis GV (2006) Modelling of hydrogen-rich gas production by plasma reforming of hydrocarbon fuels. Int J Hydrogen Energy 31:769–774 33. Hughes KJ, Turanyi T, Clague AR, Pilling MJ (2001) Development and testing of a comprehensive chemical mechanism for the oxidation of methane. Int J Chem Kinet 33:513–538 34. Indarto A, Choi JW, Lee H, Song HK (2005) Kinetic modelling of plasma methane conversion using gliding arc. J Nat Gas Chem 14:13–21 35. Luche Jocelyn, Aubry Olivier, Khacef Ahmed, Cormier Jean-Marie (2009) Syngas production from methane oxidation using a non-thermal plasma: experiments and kinetic modeling. Chem Eng J 149:35–41 36. Lee DH, Kim K-T, Song Y-H, Kang WS, Jo S (2013) Mapping plasma chemistry in hydrocarbon fuel processing processes. Plasma Chem Plasma Process 33:249–269 37. Fincke JR, Anderson RP, Hyde T, Detering BA (2002) Plasma thermal conversion of methane to acetylene. Plasma Chem Plasma Process 22(1):395–408 38. Bourig A, Lago V, Martin JP, Pliavaka K, Pliavaka F, Gorbatov S, Chernukho A, Naumov V (2007) Generation of singlet oxygen in HV pulsed ? DC crossed discharge at atmospheric pressure for oxygen-enhanced combustion. Int J Plasma Environ Sci Technol (IJEST) V1(1):57–63 39. Straub HC, Lin D, Lindsay BG, Smith KA, Stebbings RF (1997) Absolute partial cross sections for electron-impact ionization of CH4 from threshold to 1000 eV. J Chem Phys 106(11):4430–4435 40. Janev RK, Reiter D (2002) Collision processes of CHy and CHy hydrocarbons with plasma electrons and protons. Phys Plasmas 9(9):4071–4081 41. LAPLACE database (2016). http://www.lxcat.net. Accessed 29 Sept 2016 42. Shen X, Yang X, Santner J, Sun J, Ju Y (2015) Experimental and kinetic studies of acetylene flames at elevated pressures. Proc Combust Inst 35:721–728. http://engine.princeton.edu/mechanism.html 43. Goulay F, Trevitt AJ, Meloni G, Selby TM et al (2009) Cyclic versus linear lsomers produced by reaction of the methylidyne radical (CH) with small unsaturated hydrocarbons. J Am Chem Soc 131(3):993–1005

123 Plasma Chem Plasma Process (2017) 37:1551–1571 1571

44. Galland N, Caralp F, Hannachi Y et al (2003) Experimental and theoretical studies of the methylidyne CH (X2P) radical reaction with ethane (C2H6): overall rate constant and product channels. J Phys Chem A 107(28):5419–5426 45. Holroyd RA, Noyes WA Jr (1956) Photochemical studies. L. The ketene-oxygen system at higher temperatures. J Am Chem Soc 78(19):4831–4836 46. Wang H, You X, Joshi AV, Davis SG, Laskin A, Egolfopoulos F, Law CK (2007) USC mech version II. High-temperature combustion reaction model of H2/CO/C1–C4 compounds. http://ignis.usc.edu/USC_ MechII.htm 47. Halberstadt ML, Crump J (1973) Insertion of methylene into the carbon–hydrogen bonds of the C1 to C4 alkanes. J Photochem 1(4):295–305 48. Revel J, Boettner JC, Cathonnet M, Bachman JS (1994) Derivation of a global chemical kinetic mechanism for methane ignition and combustion. J Chim Phys 91(4):365–382 49. Zyn VI (1998) Kinetic identiﬁcation of a mechanism of complex plasma chemical reactions. Plasma Chem Plasma Process 18(3):395–408 50. Neyts EC, Ostrikov K, Sunkara MK, Bogaerts A (2015) Plasma catalysis: synergistic effects at the nanoscale. Chem Rev 115(24):13408–13446 51. Starikovskaia SM (2006) Plasma assisted ignition and combustion. J Phys D Appl Phys 39:R265–R299

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