Combined steam and CO2 reforming of CH4 for syngas production in a gliding arc discharge plasma

Journal of CO₂ Utilization 37 (2020) 248–259

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Combined steam and CO2 reforming of CH4 for syngas production in a gliding arc discharge plasma

T

Yun Xiaa, Na Lua,b,*, Jie Lia,b, Nan Jianga,b, Kefeng Shanga,b, Yan Wua,b a b

Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), Dalian University of Technology, Dalian, 116024, PR China School of Electrical Engineering, Dalian University of Technology, Dalian, 116024, PR China

ARTICLE INFO

ABSTRACT

Keywords: Gliding arc discharge (GAD) CO2 reforming of methane Steam reforming of methane Syngas

To reduce soot formation during the CO2 reforming of CH4 and to optimize the H2/CO ratio of produced syngas, the combination of steam and CO2 reforming of CH4 (SDMR) in a gliding arc discharge (GAD) plasma reactor is investigated. The effect of the addition of steam on gliding arc discharge characteristics, methane reforming performance, and SDMR at varying CH4/CO2 molar ratios are discussed. When the steam-to-carbon (S/C) molar ratio increases from 0 to 2.3, the CH4 conversion and energy conversion efficiency of syngas production (ECEsyngas) increase initially and then decrease, reaching their maximum values (55.6 % and 36 %, respectively) at an S/C ratio of 0.58. The increased number of OH radicals and H atoms generated from H2O in the plasma promotes CH4 dissociation, H2 selectivity and H2 yield. The H2/CO molar ratio increases from 0.76 to 1.1. The addition of steam significantly suppresses the soot formation, allowing the CH4/CO2 molar ratio to range between 1/3 and 3/1 with a carbon balance of > 82 % and a broadly adjustable H2/CO ratio (ranging between 0.4 and 3.0). The ECEsyngas decreases while the energy conversion efficiency for fuel production (ECEfuel) increases alongside increasing CH4/CO2 molar ratios. The ECEfuel peaks at 49.3 % at a CH4/CO2 molar ratio of 3/1. Acetic acid is the primary constituent of the resulting liquid product. A possible reaction mechanism of SDMR is also proposed.

1. Introduction In recent years, the development of renewable energy sources and the reduction of greenhouse gas emissions have become increasingly urgent global issues [1–5]. The capturing, storage, and utilization of CO2 (as a major greenhouse gas) have garnered significant attention [6]. Reforming CO2 into high value-added chemicals or fuels is an effective means of CO2 sequestration or CO2-neutral fuel recycling [7]. CH4, another greenhouse gas, is also a renewable energy source, abundant in natural gas obtained from petroleum reserves and landfill gas. CH4 can react with CO2 as a reductant – a technique known as dry methane reforming (DMR, Eq. 1) [8]. DMR simultaneously converts two greenhouse gases and produces a gas mixture of H2 and CO (also known as syngas) that is utilizable as a fuel or feedstock in Fischer-Tropsch (FT) synthesis to produce higher-value products such as methanol, acetic acid, and ammonia [9,10]. Considering that DMR is a highly endothermic process, providing it with energy from a non-hydrocarbon source (e.g., solar or nuclear energy) enables the storage and transportation of energy to locations where energy sources are scarce

[11,12].

CH 4 +CO2

2CO+2H2

H298K = 247.3kJ/mol

(1)

Due to the chemical stability of both CO2 and CH4, DMR reaction is usually conducted at temperatures ranging between 800−1000 °C in the presence of a noble metal catalyst (i.e., Rh, Ru, Pd, or Pt) or nonnoble metal catalyst (i.e., Fe, Co, or Ni) [13,14]. The elevated temperatures required for high equilibrium conversion in this reaction increase the costs associated with equipment and energy. The problematic deactivation of catalysts caused by sintering and carbon deposition also makes this method economically unfeasible [15,16]. Non-thermal plasma (NTP) technology is an appealing alternative to DMR compared with thermal-catalytic method. Plasma is created by the breakdown of gas under applied electric power. Plasma consists of numerous reactive species and electrons with an energy of 1−25 eV [17,18], capable of breaking most chemical bonds (the ionization energy of CH4 and CO2 equals 13.58 eV and 13.76 eV, respectively [19,20]) and inducing chemical reactions while the gas maintained at room temperature [21]. The high reaction rate and rapid steady-state

⁎ Corresponding author at: Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), Dalian University of Technology, Dalian, 116024, PR China. E-mail address: [email protected] (N. Lu).

https://doi.org/10.1016/j.jcou.2019.12.016 Received 6 June 2019; Received in revised form 22 November 2019; Accepted 22 December 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic diagram of the GAD plasma reforming system.

attainment of the plasma process allow for rapid startup and shutdown compared to other thermal treatment technologies, offering significant advantages in terms of energy-efficiency and flexibility [22,23]. Different kinds of NTP have been adopted for DMR, including dielectric barrier discharge (DBD), corona discharge, microwave discharge, and gliding arc discharge (GAD) [24–28]. Among these, GAD plasma is a transitional plasma that can be used effectively to create DMR reaction due its relatively high electron density (1017–1018 cm−3), similar to that of thermal plasma, and significantly higher than that of other non-thermal plasma (e.g., DBD and corona discharge), thus offering high reactant conversion rates and large processing capacities [26,27]. However, the formation of soot inside discharge plasma reactors during DMR is reported to negatively affect the plasma process. When the CH4/CO2 molar ratio in the feed exceeds 1/1, soot forms and deposits on the reactor walls and electrodes, incurring pipe blockages, abnormal discharge, and preventing long-term processing [21,24]. At lower CH4/CO2 molar ratios, the produced syngas (with an H2/CO molar ratio below 1.0) is unsuitable for further application in FT synthesis [29,30]. The addition of steam in DMR is an effective way of limiting soot formation and increasing the H2/CO ratio during the NTP process. H2O is an inexpensive and abundant resource that reacts with CH4 – a process known as steam methane reforming (SMR, Eq. (2)) – and is combinable with water-gas shift reactions (Eq. (3)) to produce H2.

CH 4 + H2 O CO+H2 O

CO+3H2 CO2 + H2

H298K = 206.0kJ/mol H298K =

41.2kJ/mol

catalyst with the addition of steam, and an H2/CO ratio peaking at 1.7 with a water content of 20 %. Le et al. [32] developed a point-to-point plasma reactor to measure the fraction of CH4-CO2-H2O in SDMR, finding that a higher mole fraction of water in the feed resulted in higher CH4 and CO2 conversion. Superior CH4 conversion (45 %) and CO2 conversion (20 %) occurred at 37.5 % CH4, 25 % CO2, and 37.5 % H2O. Supat et al. [33] performed SDMR inside a corona discharge reactor with a CH4/CO2 molar ratio of 3/1. With the addition of 50 % H2O, CH4 and CO2 conversion increased significantly, and energy consumption decreased from 33 eV to 15 eV per molecule of C converted. These studies demonstrate the positive effects of the addition of steam in DMR for improving reactant conversion, H2/CO ratios, and reducing energy consumption and carbon deposition. However, reactant conversion and energy consumption in SDMR could still be improved. Furthermore, the effects of steam on the discharge characteristics, reaction mechanisms, and detailed product analysis are still not fully understood. For this paper, an AC-powered GAD plasma reactor was used to convert CH4 and CO2 with the addition of steam. The effects of steam on the discharge characteristics, reforming performance, and product distribution were analyzed. The electrical waveform signals and optical emission spectroscopy (OES) were recorded and analyzed to gain a better understanding of the effects of steam addition on the discharge characteristics and radical reactions. The reaction performances at varying CH4/CO2/H2O molar ratios were investigated in terms of reactant conversion, product selectivity, yield, and energy conversion efficiency. Liquid chemicals were also produced during GAD-induced SDMR, and the effects of varying reactant ratios on the production of these liquid chemicals were investigated. To gain a better understanding of the effects of steam on the plasma reforming process and the production of gas and liquid chemicals, the possible reaction mechanisms and pathways are discussed.

(2) (3)

Combining SMR with DMR (SDMR) offers two key advantages compared to single DMR reactions, namely that (i) H2O provides additional O, thus increasing the O/C ratio in the DMR feedstock which helps to oxidize soot; (ii) H2O provides extra H atoms for the formation of H2, thereby adjusting the H2/CO molar ratio in the product stream. Wang et al. [31] investigated the effect of the addition of steam on DMR in a DBD reactor compacted with Ni/Al2O3 catalyst at 773 K. The results indicate a significant reduction in carbon deposition on the 249

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2. Experimental

CCH4 (%) =

2.1. Experimental setup

S/C =

H2 O input(mols-1) CH 4 + CO2 input(mols-1)

CO2 converted(mols CO2 input(mols 1)

CCO2 (%) =

The basic experimental arrangement is shown in Fig. 1. Pure CH4 (99.999 %) and pure CO2 (99.99 %) controlled by two mass flow controllers (MFCs) are used as feed and flowed into the reactor after a gas mixer. The total gas flow rate of CH4 and CO2 is 360 SCCM (standard cubic centimeter per minute) with the CH4/CO2 ratio varies between 1/3 to 3/1. Deionized water (H2O) is controlled by a peristaltic pump (LongerPump YZ-1515X) and vaporized by a pipe furnace (∼180 °C) before injected into the plasma reactor. Steam-to-carbon (S/ C) molar ratio is adopted to illustrate the amount of steam in the feeding gas, which determines the ratio of H2O molecules and total carbon atoms in the CH4 and CO2 in the feeding gas (Eq. (4)).

CH 4 converted(mols 1) × 100% CH 4 input(mols 1) 1)

(5)

× 100%

(6)

Selectivity of CxHy (including C2H6, C2H4, C2H2, C3H8, and C3H6) or CxHyOz (including formic acid (CH2O2) and acetic acid (C2H4O2) is defined as:

x×C x Hy (or Cx Hy Oz )produced(mols 1)

SCx Hy orSCx Hy Oz (%) =

CH 4 converted(mols 1) +CO2 converted(mols 1) (7)

× 100% Selectivity of H2:

SH2 (%) =

H2 produced(mols 1) × 100% 2 × CH 4 converted(mols 1)

(8)

Yields of H2 and CO：

(4)

A high-frequency AC power (Tesman CTP-2000 K, 5000 Hz) provides high voltage for the plasma reactor. Input power is measured by a power meter integrated inside the power supplier and it is kept constant at 80 W by adjusting applied voltage between 6−10 kV. Electrical signals are monitored by a high voltage probe (Tek P6015), a current probe (Tek P6021) and a digital oscilloscope (Tek TDS2014). Discharge images are taken by a camera (Casio Ex-zr1500). The emission spectra of GAD are recorded by a spectrograph (Princeton Instruments, Acton SP2750) equipped with an ICCD camera (Princeton Instruments, PIMAX3) in the range of 250−800 nm via an optical fiber pointing to the electrode gap. The outlet gas of the reactor firstly passes through a cold trap (placed in an ice-water bath) to separate gaseous products and liquid products. Then the gaseous products are sent to a gas chromatograph (GC, Tianmei GC7900) equipped with a thermal conductivity detector and a flame ionized detector for analysis. The total gas flow rate before and after reaction is measured by a soap-film flowmeter. The run time of each set of experiments is about 60 min. It takes 5–10 min for the reactor to heat up and for the outlet gas to become stable in component and concentration. After that, the sampling and analysis processes are carried out. Gas samples are taken every 10 min and repeated 3 times for the calculation of average value and standard error. Liquid products are collected after replacing the cold trap with a dry clean one, simultaneously starting accurate timing of 30 min. The collections are analyzed by a gas chromatography-mass spectrometer (GC–MS, Agilent GC 6890 N and Agilent MSD 5975) equipped with an HP-FFAP column (30 m × 0.53 mm × 1.0 μm) and a headspace autosampling unit. A thermocouple is used to estimate the approximate temperature inside the reactor which is placed near the gas outlet at a distance 2 cm below the electrodes. The gas temperature in the afterglow of the discharge is varying from 310 to 280 °C with the S/C molar ratio increasing from 0 to 2.3 at an applied power of 80 W and a total gas flow rate of 360 SCCM. The GAD reactor is a Teflon box with an inner size of 85 × 52 × 10 mm. A piece of quartz plate is employed as a transparent cover for the convenience of observing and taking photos. A pair of knife-shaped electrodes (made of stainless steel) are fastened symmetrically to the middle of the reactor. One electrode is connected to the high voltage and the other is grounded. The minimum electrode gap is fixed to 2 mm. Feeding gases flow into the reactor from a stainless-steel nozzle heading towards discharge zone and flow out from a tube opposite the nozzle.

YH2 (%) =

H2 produced(mols 1) × 100% 2 × CH 4 input(mols 1)

YCO (%) =

CO produced(mols 1) × 100% CH 4 +CO2 input(mols 1)

(9) (10)

Carbon balance is calculated by the following equation. Here carbon in gaseous products includes the carbon in CO, C2 hydrocarbon and C3 hydrocarbon. Soot and liquid product are excluded.

carbon in gaseous product(mols 1) Cbalance (%) =

+ (CH 4 + CO2)unconverted(mols 1) × 100% CH 4 + CO2 input(mols 1)

(11)

In the plasma reforming process, the specific energy input (SEI) is defined as the ratio of input power to the mole of gases inputted, and the equation is as follows:

SEI(kJ/mol) =

discharge power(kJs-1) CH 4 +CO2 + H2 O input(mols-1)

(12)

Energy conversion efficiency (ECE) is used to evaluate the percentage of energy recovered in the total energy consumed during the plasma reforming process. ECE of syngas and fuels are defined as the following equations. Here the fuel refers to H2, CO, and C2H2. LHV denotes the lowest heat value. (LHVCH4 =803.7 kJ/mol; LHVH2 =241.6 kJ/mol; LHVCO =283.0 kJ/mol; LHVC2 H2 = 1265.4 kJ/mol.) ECEsyngas (%) =

H2 produced(mols 1) × LHVH2 (kJ mol 1) +CO produced(mols 1) × LHVCO (kJ mol 1) CH 4 converted(mols 1) × LHVCH4 (kJ mol 1) + discharge power(kJs 1) × 100%

ECEfuel (%) =

fuel(mols-1)

(13)

mol-1)

× LHV (kJ CH4 converted(mols-1) × LHVCH4 (kJ mol-1) +discharge power(kJs-1) × 100%

(14)

3. Results and discussions 3.1. Effect of steam on the discharge characteristics of GAD 3.1.1. Images and electric signals of the GAD plasma with different S/C molar ratio To understand the effect of steam addition on discharge characteristics, the images of GAD plasma with different S/C molar ratios are shown in Fig. 2 (a1)–(a6). The corresponding waveforms of discharge voltage and current are shown in Fig. 2(b1)–(b6). The total flow rates of CH4 and CO2 are kept constant at 360 SCCM and the CH4/CO2 molar ratio is 1/1.5. Adjusting the flow rate of H2O from 0 to 0.67 ml/min to have the S/C molar ratio increasing from 0 to 2.3. The corresponding

2.2. Calculation of parameters In the plasma reforming of CH4, the conversions of CH4 and CO2 are defined as: 250

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Fig. 2. Discharge images and discharge waveforms of voltage and current in GAD plasma at different S/C molar ratio; (a1) to (a6) represent the discharge images of GAD with S/C molar ratio increased from 0.00–2.30 (exposure time 0.05 s); (b1) to (b6) represent the discharge voltage and current of GAD with S/C molar ratio increased from 0 to 2.3.

residence time decreases from 0.74 to 0.22 s. Fig. 2(a1) and (b1) shows the GAD without steam. When the arc occurs at the minimum gap of the electrodes, the discharge voltage sharply drops to several hundred volts with the appearance of current peaks. Then the arc moves along the electrode under the force of airflow, the discharge voltage rises and the current peak decreases in both amplitude and numbers. The discharge mode gradually transits from arc discharge to glow discharge [27]. At the end of a gliding period, the arc reaches its maximum length with large amounts of energy released. Intense collisions between electrons, molecules, and atoms lead to emission of strong yellow light. Then the arc extinguishes and a new arc starts from the minimum gap of the electrodes. After the addition of steam, the yellow light at the end of the arc gradually weakened. The maximum length and the max moving distance of arc are both shortened. Correspondingly, the max voltage decreased and the time for one gliding period decreased. From the distribution of current peaks, it can be inferred that the time for glow discharge mode is shortened by the addition of steam, and the discharge is weakened. This is due to the fact that with the increase of the S/C molar ratio, the electronegativity of the H2O additive becomes more important in the discharge progress. More energetic electrons are adsorbed by H2O and less energy could be used to sustain the discharge arc [34–36]. These results indicate that excessive steam addition inhibits the development of the discharge arc. And this would further impact the plasma-induced reactions.

CH4/CO2/H2O in a GAD plasma with different of S/C molar ratio are shown in Fig. 3. The spectrum of S/C = 0 represents the GAD plasma of CH4/CO2 without steam. The high peak of hydrogen Balmer Hα (3d2 D 2p2P 0 ) centered at 656 nm comes from the step-wise disX2 ) sociation of CH4 (Eq. (15–17)) [37–41]. The band of CH (A2 3 3 X u ) at 516 nm and 558 nm at 461 nm and C2 swan bands (A g might be linked with the production of hydrocarbons (Eq. (18–20) [37–41]). Besides, CH has been considered the direct source of soot (Eq. 17), and the C2 species have been reported to have an important role in A1 ) have parsoot formation [42]. The CO angstrom bands (B1 tially overlapped C2 swan bands in the range 430–560 nm which mainly derive from the dissociation of CO2 (Eq. (21–23)). Emission from CO2+ X 2 ) is also observed at 358 nm (Eq. (22)). The formation of (A2 CO2+ ions requires at least 17.3 eV, which suggests a high energy tail of the electron energy distribution function of GAD plasma [43]. O 3p5P ) at 777 nm can also be found as an intermediate during (3s5D0 X 2 ) around 309 nm CO2 dissociation. The emission of OH (A2 + indicates the coupling of O and H atoms during plasma process (Eq. (24)) [37–39]. With the increase of S/C molar ratio, the reactive species produced in the GAD plasma have no obvious change. However, the spectral intensity of different species shows different changes.

CH 4 + e CH 4 + M

251

3)

CH3 + H + M(M presents any molecules)

(15) (16)

C+H+e

(17)

CH3+ CH3

C2 H 6

(18)

CH2 + CH2

C2 H 4

(19)

CH + e

3.1.2. Optical emission spectra of GAD with different S/C molar ratio OES is an important diagnostic method for providing in-situ information of the complex chemical reactions in plasma. The OES of

CHx +(4-x )H + e(x = 1

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Fig. 3. Comparison of spectra emitted from GAD SDRM with different S/C molar ratios. (a) OES ranging from 280 to 800 nm at different S/C molar ratio; (b) the peak intensity of main species as a function of S/C molar ratio; (c) electron density and OH rotational temperature at different S/C.

CH + CH

C2 H2

(20)

CO2 + M

CO + O + M (M present any molecules)

(21)

e + CO2

2e + CO+2

(22)

e + CO+2 CO + O

(23)

O+H

(24)

OH

steam. This can be interpreted as the H2O molecules in plasma system are prone to adsorb high-energy electrons, leading to the decrease of electron density and the decreasing opportunities for CH, C2 radicals and CO2 molecules to be excited or ionized by electrons.

e +H2 O

e + OH + H

(25)

e+ OH

e+O+H

(26)

Fig. 3(c) shows the electron densities and OH rotational temperatures of the GAD plasma with different S/C molar ratios. The electron densities are calculated by Stark-boarding of Hα peak method, and the rotational temperatures of OH are calculated from the OH bands (306−312 nm) by best fitting method using Lifbase software. The results show that electron density for GAD without steam is 8.85 × 1016 cm−3 and gradually decreases with increasing S/C molar ratio further proving that H2O tends to adsorb electrons. The rotational temperature of OH increases slightly from 3800 to 4100 K when the S/C molar ratio increases from 0 to 2.3. This might also be the result of

The peak values of the main species in each spectrum have been plotted in Fig. 3(b) as a function of S/C molar ratio. The peak values of OH and Hα both increase obviously, suggesting that H2O molecules dissociated during GAD process and more amount of OH radicals and H atoms are formed (Eq. 25). The spectral line of O atom at 777 nm increases slightly due to the dissociation of OH radicals (Eq. 26). However, the peaks of all C-containing radicals decrease more or less. The strong peak of C2 decreases rapidly with increasing S/C molar ratio. The peaks of CO2+ and CH band almost disappear after the addition of 252

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Fig. 4. The effect of different steam content on reforming reaction. (a) conversion of reactants (b) selectivity of syngas and H2/CO ratio (c) selectivity of hydrocarbons (d) yields and carbon balance (e) ECE and SEI.

adsorption of electrons by H2O molecules. The kinetic energy of the electrons that transferred to H2O molecules by adsorption increases the temperature of heavy particles, resulting in an increase of OH rotational temperature [44].

increasing S/C molar ratio from 0 to 2.3. A maximum value of 55.6 % is achieved at the S/C molar ratio of 0.58, which indicates that the moderate addition of steam benefits the CH4 conversion. In DRM without steam, inside the discharge arc, CH4 molecules can be decomposed by reacting with energetic electrons, various radicals and any molecules M [40,41]. Among these reactions, CH4 reacting with OH and C2H3 radicals (Eq. 27–29) has the greatest contribution to the total CH4 loss [41]. When adding a small amount of steam (S/C < 0.58), the density of OH radicals and H atoms increases rapidly. The increment density of OH enhanced the CH4 dehydrogenation reaction through Eq. 27, and the increment of H atoms benefits the formation of C2H3 (Eq. 28). Even though the increase of the S/C molar ratio decreased the residence time, which would have inhibited the conversion of CH4, the

3.2. Effect of steam on CH4-CO2 reforming performance The effect of steam addition on GAD reforming performance was studied across S/C molar ratios ranging from 0 to 2.3. The operational parameters were identical to those used in Section 3.1. The respective reactant conversion, product selectivity, yield, and ECEsyngas are presented as functions of the S/C molar ratio in Fig. 4(a)–(e). The CH4 conversion initially increases then decreases with the 253

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Fig. 5. The effect of CH4/CO2 molar ratio on the steam combined DRM. (a) conversion of reactants; (b) selectivity of syngas and H2/CO ratio; (c) selectivity of hydrocarbons; (d) yields and carbon balance; (e) ECE.

change in residence time didn’t reverse the increasing trend of CH4 conversion due to that the steam addition was still small. When S/ C > 0.58, the increment rates of OH and H slow while the electron density still decreases rapidly with the increase of H2O as shown in the results of OES, indicating that the promotion effect of OH and H is reduced. The effect of decreased residence time on the CH4 conversion becomes more important with the S/C molar ratio keep increasing. Therefore, the CH4 conversion decreased when the S/C molar ratio exceeds 0.58.

CH 4 + OH

CH 4 +C2 H3

CH3+H2 O

CH3 +C2 H 4

C2 H 4 + H

CO2 + H CO2 + OH CO + O

C2 H3+H2

CO + OH CO +H2 O CO2

(29) (30) (31) (32)

The declined CO2 conversion from 44 % to 38 % with increasing S/C molar ratio from 0 to 2.3 should be blamed on the decreased electron density with increasing steam addition. In a GAD plasma process, CO2 decomposition occurs through reaction with energetic electrons, H atoms, OH radicals, or any molecules M [40,41]. The reaction of CO2 (mainly from the vibrational levels) with H atoms is the dominant loss

(27) (28) 254

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process [41]. Even though the density of H atoms increases with increasing S/C molar ratio, the decreased electron density reduces the vibrational excitation of CO2, resulting in decreased CO2 conversion. However, it should be noted that the CO2 conversion at an S/C molar ratio of 2.3 is still higher than that of CO2 alone treated by discharge plasma (of which the CO2 conversion is 12 % under the same power and gas flow rate). That is because the CH radicals and H atoms react with O atoms preventing the recombination of CO and O from forming CO2 again [45–47]. The selectivity of H2, as expected, increases significantly from 63.3%–78.2% when the S/C molar ratio increases from 0 to 2.3. The selectivity of CO increases slightly from 70.5 %–78.5 %. The H2/CO ratio rises from 0.76 to 1.1. It can be explained that with the increasing steam in the reaction system, more H2O molecules take part in the reforming reaction and are converted into H2 (Eq. 25 and 26). As to CO, the increasing O atoms in the system enhance the oxidation of C-containing species resulting in the increase of CO selectivity. Fig. 4(c) presents the hydrocarbon selectivity. C2H2 selectivity decreases as the S/C molar ratio increases, whereas C2H6 and C3H8 selectivity increases. It is widely accepted that the radicals formed during the stepwise decomposition of CH4 to CH3, CH2, and CH strongly affects the formation of hydrocarbons. The combination of these radicals generates hydrocarbon (Eq. 17–20). Further decomposition of CH leads to the formation of soot (Eq. 21). With the addition of steam, the proclivity of deep decomposition of CH4 to CH or C is limited, as proven by the decreased emission spectrum intensity of the CH band. More CH3 and CH2 radicals combine to form C2H6 and C2H4, rather than further decomposing into CH radicals or soot. The addition of steam thereby reduces soot formation and C2H2. The increased O atoms that result from higher S/C molar ratios is another factor that suppresses soot formation by increasing the opportunity of C atoms to react with O atoms during the plasma process. With the increase of the S/C molar ratio, the carbon balance increases from 83.5%–88.5 %, as shown in Fig. 4(d), and carbon deposition inside the reactor is hardly detectable when the S/C molar ratio reaches 2.3. Although soot formation is not completely eliminated, carbon deposition inside the reactor is dramatically suppressed, and only a negligible amount of black carbon particles is found in the liquid collected from the cold trap. The decreasing of soot is beneficial to the long-term stable operation of GAD reactors. The yield of H2 increases with the S/C molar ratio while the yield of CO shows an opposite trend. The ECEsyngas rises initially then falls which is similar to CH4 conversion. The best ECEsyngas is 36 % at a S/C molar ratio of 0.58, as shown in Fig. 4(e). Thus, excessive H2O in the reaction system should be avoided and the S/C molar ratio of 0.58 is chosen for the following study.

that H2 selectivity decreases slightly from 68.6 %–65.2%, whereas CO selectivity decreases significantly from 90 % to 35.6 % as CO2 concentrations decrease. Higher CH4 concentration may increases CHx radicals generation, further promoting the production of C2H6, C2H4, and C2H2. The results presented in Fig. 5(c) confirm this process in which hydrocarbon selectivity increases ubiquitously alongside increasing CH4/CO2 molar ratios. C2H2 selectivity surpasses CO selectivity, reaching 39.5 % at a CH4/CO2 molar ratio of 3/1, and becoming the predominant product. Consequently, both H2 and CO yields decrease. However, the H2/CO ratio in the product increases from 0.4 to nearly 3.0, exhibiting a widely adjustable range. Thus, the CH4/CO2 molar ratio should be carefully controlled to have a balance between high H2/ CO ratio and high syngas yield. The ECE of syngas production shows a tendency to decline due to decreased H2 and CO yields alongside increasing CH4/CO2 molar ratios, as indicated in Fig. 5(e). However, when taking C2H2 (another vital chemical industrial fuel) into account, ECEfuel first decreases slightly to 42.3 % then increases alongside increasing CH4/CO2 molar ratios, reaching 49.3 % at CH4/CO2 = 3/1. This result suggests that higher CH4/CO2 molar ratios would benefit energy recovery. Considering the requirement of H2/CO ratio to be 2 in F-T synthesis, the CH4/CO2 molar ratio should be controlled around 2/1. It should be noticed that, in the absence of steam, the GAD plasma with CH4/CO2 molar ratio beyond 1/1 could not process in long term due to the formation of soot. When the CH4/CO2 molar ratio exceeds 1.5/1, carbon filaments form between the electrodes and accumulate quickly in the plasma reactor that results in conduction of the electrodes and stops the discharge plasma [21,27]. On the contrary, under the condition of steam addition, even at the CH4/CO2 molar ratio of 3/ 1, the carbon balance is still as high as 82 %. The discharge plasma could process smoothly without carbon deposition between the electrodes. These results proved that the combination of DRM with steam in GAD can effectively inhibit the formation of soot, which is of great significance for maintaining stable discharge, improving energy efficiency, and promoting the CH4 conversion. The reactant conversion, production rate and energy efficiency are three critical parameters to evaluate reaction performance. Therefore, the experimental conditions, reactant conversions, production rates, C converted per kJ for CO2 reduction with CH4 (with and without steam addition) in various non-thermal plasmas are represented in Table 1. It is found that the reactant conversion, energy efficiency and syngas production rate in the DBD reactor and the corona discharge reactor are all lower than those of a GAD plasma reactor. DBD plasma powered by pulsed power shows high energy efficiency (1.98 mmol/kJ) but the reactant conversion and syngas production rate are much lower. DRM in an AC spark discharge reactor and a microwave plasma reactor show higher reactant conversion than the GAD plasma reactor. And a high processing capacity and H2 production rate in microwave plasma reactor was reported by [58,59]. However, the energy efficiency of the AC spark discharge reactor and the microwave plasma reactor (1.10 and 1.56 mmol/kJ, respectively) are lower than the that of the GAD reactor. Compared this work with DRM in a GAD reactor reported by [21], it can be seen that even though the addition of steam slightly decreased the energy efficiency, it is still a highly efficient reforming technology for syngas production with a high reactant conversion and a high H2/CO molar ratio.

3.3. Combined reforming at different CH4/CO2 molar ratios in feed The effects of reagent composition on SDMR were determined by varying the CH4/CO2 molar ratios between 3/1 to 1/3 at a constant gas flow rate of 360 SCCM. The water flow rate was fixed at 0.17 mL/min to maintain a constant S/C molar ratio of 0.58. The discharge power was 80 W, and the SEI was constant at 3.16 kJ/mol. With the CH4/CO2 molar ratio increases, the CH4 conversion declines and the CO2 conversion increases, as shown in Fig. 5(a). Similar results have been reported in the literature [48–51]. In a plasma process, increasing the concentration of a reactant generally lowers its conversion due to the enhancement of backward reactions. The increased number of CH3 and H species derived from higher CH4 concentration leads to higher collision and recombination probability of the two intermediates (Eq. 15–17), which in turn lower the conversion of CH4. The decreased CO2 concentration suppressed the recombination probability of CO and O (Eq. 32), so the CO2 conversion increases. It should be noted that CH4 conversion is consistently higher than CO2 conversion, attributable to the higher energy required for CO2 electron excitation compared to CH4 electron excitation [52,53]. Fig. 5(b) shows

3.4. Analysis of liquid products In the process of methane reforming with CO2 and H2O, the feeding gas contains C/H/O three elements. Through the complex reactions between atoms, ions and free radicals in discharge plasma, small molecular organic matters containing C, H, and O could be synthesized as products. For the aim of investigating the product distribution of DRM in GAD, the liquid collected in the cold trap was analyzed using a GC–MS analyzer to identify the constitution. Fig. 6 shows the GC–MS 255

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Table 1 Comparison of the conversion and energy efficiency of methane reforming in different plasmas. Conversion

Syngas production rate

Plasma

Gas composition

Gas flow rate

CH4

CO2

C converted per kJ

H2

CO

H2/CO ratio

Reference

Corona DBD DBD + Ni/Al2O3 (at 773 K) Pulsed DBD AC spark Microwave plasma Microwave plasma GAD GAD

CH4/CO2/H2O = 3/2/3 CH4/CO2 = 1/1 CH4/CO2/H2O = 1/1/0.5 CH4/CO2 = 1/1 CH4/CO2 = 3/2 CH4/CO2 = 3/2 CH4/CO2/H2O = 2.5/1/2.06 CH4/CO2 = 1/1 CH4/CO2/H2O = 1/1/0.58

(SCCM) 200 40 65.5 200 150 200 111300 7500 360

(%) 38 47 62 19 69 70 22 13 55

16 25 41 13 61 68 – 8 43

(mmol/kJ) 1.04 0.08 0.22 1.98 1.10 1.56 – 2.14 1.89

(g/h) 0.24 0.05 – 0.08 0.44 0.84 192 0.65 0.68

(g/h) – 0.56 – 0.91 4.97 7.78 – 24.93 8.7

– 1.3 1.3 1.1 1.5 1.5 – 0.4 1.1

[32] [54] [31] [55] [56] [57] [58] [21] This work

Fig. 6. GC–MS analysis of liquid compounds after the SDRM reforming process. (Power input is 80 W, CH4/CO2 molar ratio is 1/1 with a total gas flow rate of 360 mL/min.).

spectrum of the liquid collection. The result shows that a wide range of oxygenates including methanol, acetaldehyde, acetone, formic acid, acetic acid, and propanoic acid was identified in the liquid by-products. Similar results of direct coupling of CH4 and CO2 to form oxygenates have been reported by density functional theory modeling on Zn-Ce catalysts [60] and in lowtemperature DBD plasmas [61]. In order to have better insights on the effect of steam on the formation of liquid by-products, the selectivities and production rates are calculated under different experimental parameters. It is found that the production rates of methanol, acetaldehyde, acetone and propanoic acid are always smaller than 0.005 mmol/min, far below that of acetic acid and formic acid. So only acetic acid and formic acid are taken as the major liquid products and their production rates and selectivities as functions of S/C molar ratios and CH4/CO2 molar ratios are plotted in Fig. 7. Fig. 7(a) exhibits the production rates and selectivities of acetic acid and formic acid with the S/C molar ratio ranging from 0 to 2.3. Both of the two products increase with the increasing S/C molar ratio in production rate and selectivity. That is due to the increasing H atoms and OH radicals promote the formation of carboxyl radicals (COOH) (Eq. 33 and 34) [61], which results in the increase of corresponding end-products. Fig. 7(b) shows the acid production at different CH4/CO2 molar ratio. The production rate of formic acid decreases with the increasing CH4/CO2 molar ratio. Formic acid is obtained by radical coupling between H and COOH (Eq. 35). In the situation of with steam, where H atoms and OH radicals are excessive, CO radicals become the determining factor for the production of formic acid. The increasing CH4/ CO2 molar ratio reduces the CO2 concentration leading to fewer CO

Fig. 7. Production rate and selectivity of acetic acid and formic acid as a function of (a) S/C molar ratio; (b)CH4/CO2 molar ratio.

radicals in the system. Consequently, the production rate of formic acid shows a decreasing trend. With respect to acetic acid, it initially increases with the increasing CH4/CO2 molar ratio and reaches its max value at the CH4/CO2 molar ratio of 1/1. When the CH4/CO2 molar ratio further increased to 3/1, the production rate decreased. For the formation of acetic acid, two possible reaction pathways have been proposed. One is the direct coupling of COOH and CH3 radicals through Eq. 36 and Eq. 37 [62]. The other is the reaction of CH3 and CO radicals to form CH3CO which further reacts with OH to produce acetic acid (Eq. 38–40) [63]. Both of the two pathways demand the participation of CO and CH3 radicals. Therefore, the balance between these two radicals is the key point to maximize acetic acid production. It can be inferred that in this work, the best balance between CH3 and CO radicals was achieved at the CH4/CO2 molar ratio of 1/1. In addition, the high 256

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reasonable range to have a balance between high H2/CO and high syngas selectivity. Soot formation is suppressed through the addition of steam by preventing deep dissociation of CH4 and enhancing the oxidation of C by O atoms. The formation of small organic acids (e.g., formic acid and acetic acid) increases with increasing S/C molar ratios. CH4/CO2 molar ratios exhibit a different effect on formic acid and acetic acid production rates. The former decreases with increasing CH4/CO2 molar ratio while the latter reaches its peak production rate at CH4/CO2 = 1/1. In general, the selectivity of the small organic acids is relatively low compared with gaseous products. The possible reaction pathways in SDRM to major productions have also been proposed. These results indicate that the SDMR reaction performed in a GAD plasma reactor is a highly efficient method for the simultaneous production of syngas. The addition of steam reduces problematic carbon deposition during the single DMR reaction and optimizes the H2/CO ratio of syngas, enabling its extensive utilization in F-T synthesis and other applications.

Fig. 8. Possible reaction pathways for SDRM and the formation of major products. (M presents any molecules).

production rate of acetic acid compared with formic acid and other liquid-products could be explained by the different energy barriers of different radical reactions. The reaction of radical coupling between CH3 and COOH occurs without any barrier while the reaction between H and COOH needs to conquer an energy barrier of 23.4 kJ/mol [61]. Therefore, the formation of acetic acid is much easier than other liquid products in plasma process which provides a potential method for acetic acid production.

CO + OH

CO2 + H

COOH

COOH

COOH + H

CH3+ CO

COOH

CH3 CO

CH3 CO + OH

CH3+ CO

CH3 COOH

CH3 CO

CRediT authorship contribution statement Yun Xia: Conceptualization, Methodology, Software, Data curation, Writing - original draft, Writing - review & editing, Visualization, Investigation, Formal analysis, Validation. Na Lu: Supervision, Writing - review & editing, Funding acquisition, Resources. Jie Li: Project administration. Nan Jiang: Project administration. Kefeng Shang: Project administration. Yan Wu: Project administration.

(33) (34) (35) (36)

Declaration of Competing Interest

(37)

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

(38)

CH3 CO + OH

CH3 COOH

(39)

CH3+ COOH

CH3 COOH

(40)

Acknowledgments

The possible reaction mechanism of SDRM is schematically shown in Fig. 8. As have been discussed above, with the addition of steam, the increasing concentrations of H, OH, and O species influence the chemical equilibrium in the plasma system and thereby change the selectivities of different products. With certainty, the increased H atoms in the system enhance the production of H2, which follows the Le Chatelier's principle [64]. A higher concentration of OH radicals accelerates the decomposition of CH4 and promotes radical reactions between CH3, CO and OH radicals resulting in the increased production rates of acetic acid. The increased O atoms in the plasma system promote the oxidation of carbon thus to reduce the soot.

This work was supported in part by the Joint Funds of the National Natural Science Foundation of China under Grant No. U1462105. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jcou.2019.12.016. References [1] D. Pakhare, J. Spivey, A review of dry (CO2) reforming of methane over noble metal catalysts, Chem. Soc. Rev. 43 (2014) 7813–7837, https://doi.org/10.1039/ c3cs60395d. [2] H.J. Alves, C. Bley Junior, R.R. Niklevicz, E.P. Frigo, M.S. Frigo, C.H. CoimbraAraújo, Overview of hydrogen production technologies from biogas and the applications in fuel cells, Int. J. Hydrogen Energy 38 (2013) 5215–5225, https://doi. org/10.1016/j.ijhydene.2013.02.057. [3] P. Nikolaidis, A. Poullikkas, A comparative overview of hydrogen production processes, Renew. Sustain. Energy Rev. 67 (2017) 597–611, https://doi.org/10.1016/j. rser.2016.09.044. [4] S.E. Hosseini, M.A. Wahid, Hydrogen production from renewable and sustainable energy resources: promising green energy carrier for clean development, Renew. Sustain. Energy Rev. 57 (2016) 850–866, https://doi.org/10.1016/j.rser.2015.12. 112. [5] E. Alper, O.Y. Orhan, CO2 utilization: developments in conversion processes, Petroleum 1 (2016) 109–126, https://doi.org/10.1016/j.petlm.2016.11.003. [6] L. Li, N. Zhao, W. Wei, Y. Sun, A review of research progress on CO2 capture, storage, and utilization in Chinese Academy of Sciences, Fuel 108 (2013) 112–130 http://10.1016/j.fuel.2011.08.022. [7] M. Mikkelsen, M. Jørgensen, F.C. Krebs, The teraton challenge. A review of fixation and transformation of carbon dioxide, Energy Environ. Sci. 3 (2010) 43–81 http:// 10.1039/B912904A. [8] M. Usman, W.M.A. Wan Daud, H.F. Abbas, Dry reforming of methane: influence of process parameters—a review, Renew. Sustain. Energy Rev 45 (2015) 710–744 http://10.1016/j.rser.2015.02.026.

4. Conclusion In this work, the SDMR was conducted inside a GAD plasma reactor to investigate the effect of steam addition on the gliding arc discharge characteristics and methane reforming performance. The addition of steam increases the number of reactive species (e.g., OH, H), but decreases the electron density in plasma and reduces the residence time. With the addition of steam with S/C molar ratio lower than 0.58, the CH4 conversion increases due to the enhanced reaction with increasing OH radicals and H atoms. H2 selectivity, as well as ECEsyngas were also improved. Excessive amounts of H2O in the discharge plasma decrease the CH4 conversion and ECEsyngas due to the decreased electron density and shortened residence time in the reactor. The CH4/CO2 molar ratio dramatically affects product distribution. The H2/CO molar ratio increases from 0.4–3 when the CH4/CO2 molar ratio increases from 1/3 to 3/1, but the main carbon-containing product shifts from CO to C2H2. When the CH4/CO2 molar ratio equals 3/1, a superior ECEfuel of 49.3 % is achieved but the ECEsyngas decreased to 28.5 %. Thus, the CH4/CO2 molar ratio should be controlled at a 257

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Y. Xia, et al. [9] Y. Lu, T. Lee, Influence of the feed gas composition on the Fischer-Tropsch synthesis in commercial operations, J. Nat. Gas Chem. 16 (2007) 329–341, https://doi.org/ 10.1016/s1003-9953(08)60001-8. [10] X. Peng, K. Cheng, J. Kang, B. Gu, X. Yu, Q. Zhang, Y. Wang, Impact of hydrogenolysis on the selectivity of the Fischer-Tropsch synthesis: diesel fuel production over mesoporous zeolite-y-supported cobalt nanoparticles, Angew. Chem. Int. Ed. Engl. 54 (2015) 4553–4556, https://doi.org/10.1002/anie.201411708. [11] M. Levy, R. Levitan, E. Meirovitch, A. Segal, H. Rosin, R. Rubin, Chemical reactions in a solar furnace 2: direct heating of a vertical reactor in an insulated receiver. Experiments and computer simulations, Sol. Energy 48 (1992) 395–402, https:// doi.org/10.1016/0038-092X(92)90048-F. [12] D. Fraenkel, R. Levitan, M.A. Levy, Solar thermochemical pipe based on the CO2/ CH4 (1: 1) system, Int. J. Hydrog. Energy 11 (1986) 267–277, https://doi.org/10. 1016/0360-3199(86)90187-4. [13] Y.J.O. Asencios, E.M. Assaf, Combination of dry reforming and partial oxidation of methane on NiO–MgO–ZrO2 catalyst: effect of nickel content, Fuel Process Technol 106 (2013) 247–252 http://10.1016/j.fuproc.2012.08.004. [14] D. Liu, X.Y. Quek, W.N.E. Cheo, R. Lau, A. Borgna, Y. Yang, MCM-41 supported nickel-based bimetallic catalysts with superior stability during carbon dioxide reforming of methane: Effect of strong metal–support interaction, J. Catal. 266 (2009) 380–390 http://10.1016/j.jcat.2009.07.004. [15] Y.T. Yuen, P.N. Sharratt, B. Jie, Carbon dioxide mineralization process design and evaluation: concepts, case studies, and considerations, Environ. Sci. Pollut. R. 23 (2016) 22309–22330, https://doi.org/10.1007/s11356-016-6512-9. [16] T. Nozaki, K. Okazaki, Non-thermal plasma catalysis of methane: principles, energy efficiency, and applications, Catal. Today 211 (2013) 29–38, https://doi.org/10. 1016/j.cattod.2013.04.002. [17] B. Eliasson, U. Kogelschatz, Modeling and applications of silent discharge plasmas, IEEE Trans. Plasma Sci. 19 (2) (1991) 309–323 http://10.1021/acs.est.8b00655. [18] A.M. Harling, D.J. Glover, J.C. Whitehead, K. Zhang, Novel method for enhancing the destruction of environmental pollutants by the combination of multiple plasma discharges, Environ. Sci. Technol. 42 (2008) 4546–4550 http://10.1021/ es703213p. [19] D.K. Jain, S.P. Khare, Ionizing collisions of electrons with CO2, CO, H2O, CH4 and NH3, J. Phys. B-At. Mol. Opt 9 (1976) 1429–1438 http://10.1088/0022-3700/9/8/ 023. [20] M. Stano, S. Matejcik, J.D. Skalny, T.D. Märk, Electron impact ionization of CH4: ionization energies and temperature effects, J. Phys. B-At. Mol. Opt 36 (2003) 261–271 http://10.1088/0953-4075/36/2/307. [21] X. Tu, J.C. Whitehead, Plasma dry reforming of methane in an atmospheric pressure AC gliding arc discharge: co-generation of syngas and carbon nanomaterials, Int. J. Hydrogen Energy 39 (2014) 9658–9669, https://doi.org/10.1016/j.ijhydene.2014. 04.073. [22] N. Lu, D. Sun, Y. Xia, K. Shang, B. Wang, N. Jiang, J. Li, Y. Wu, Dry reforming of CH4-CO2 in AC rotating gliding arc discharge: effect of electrode structure and gas parameters, Int. J. Hydrogen Energy 43 (2018) 13098–13109, https://doi.org/10. 1016/j.ijhydene.2018.05.053. [23] N. Jiang, L. Guo, C. Qiu, Y. Zhang, K. Shang, N. Lu, J. Li, Y. Wu, Reactive species distribution characteristics and toluene destruction in the three-electrode DBD reactor energized by different pulsed modes, Chem. Eng. J. 350 (2018) 12–19, https://doi.org/10.1016/j.cej.2018.05.154. [24] X. Tu, J.C. Whitehead, Plasma-catalytic dry reforming of methane in an atmospheric dielectric barrier discharge: understanding the synergistic effect at low temperature, Appl. Catal. B-Environ. 125 (2012) 439–448, https://doi.org/10. 1016/j.apcatb.2012.06.006. [25] M. Li, G. Xu, Y. Tian, L. Chen, H. Fu, Carbon dioxide reforming of methane using DC corona discharge plasma reaction, J. Phys. Chem. A 108 (2004) 1687–1693, https://doi.org/10.1021/jp037008q. [26] M. Jasiński, M. Dors, J. Mizeraczyk, Production of hydrogen via methane reforming using atmospheric pressure microwave plasma, J. Power Sources 181 (2008) 41–45, https://doi.org/10.1016/j.jpowsour.2007.10.058. [27] Y. Xia, N. Lu, B. Wang, J. Li, K. Shang, N. Jiang, Y. Wu, Dry reforming of CO2-CH4 assisted by high-frequency AC gliding arc discharge: electrical characteristics and the effects of different parameters, Int. J. Hydrogen Energy 42 (2017) 22776–22785, https://doi.org/10.1016/j.ijhydene.2017.07.104. [28] X. Wang, Y. Gao, S. Zhang, H. Sun, J. Li, T. Shao, Nanosecond pulsed plasma assisted dry reforming of CH4: the effect of plasma operating parameters, Appl. Energy 243 (2019) 132–144, https://doi.org/10.1016/j.apenergy.2019.03.193. [29] A. Indarto, J. Choi, H. Lee, H. Song, Effect of additive gases on methane conversion using gliding arc discharge, Energy 31 (2006) 2986–2995, https://doi.org/10. 1016/j.energy.2005.10.034. [30] X. Tao, M. Bai, X. Li, H. Long, S. Shang, Y. Yin, X. Dai, CH4–CO2 reforming by plasma–challenges and opportunities, Prog. Energy Combust. 37 (2011) 113–124, https://doi.org/10.1016/j.pecs.2010.05.001. [31] Q. Wang, H. Shi, B. Yan, Y. Jin, Y. Cheng, Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor, Int. J. Hydrogen Energy 36 (2011) 8301–8306, https://doi.org/10.1016/j.ijhydene.2011.04.084. [32] H. Le, T. Hoang, R. Mallinson, L. Lobban, Combined steam reforming and dry reforming of methane using AC discharge, Stud. Surf. Sci. Catal. 147 (2004) 175–180, https://doi.org/10.1016/S0167-2991(04)80047-8. [33] K. Supat, S. Chavadej, L. Lobban, R. Mallinson, Combined Carbon Dioxide and Steam Reforming With Methane in Low-temperature Plasmas, Utilization of Greenhouse Gases, ACS Symposium Series, 2003, pp. 83–99, https://doi.org/10. 1021/bk-2003-0852 ch005. [34] C. Zhang, T. Shao, Y.X. Zhou, F. Zhi, P. Yan, W.J. Yang, Effect of O2 additive on spatial uniformity of atmospheric-pressure helium plasma jet array driven by

[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] [61]

[62]

258

microsecond-duration pulses, Appl. Phys. Lett. 105 (2014) 044102, , https://doi. org/10.1063/1.4887992. N. Srivastava, C. Wang, Effects of water addition on OH radical generation and plasma properties in an atmospheric argon microwave plasma jet, J. Appl. Phys. 110 (2011) 53304, https://doi.org/10.1063/1.3632970. D.X. Liu, P. Bruggeman, F. Iza, M.Z. Rong, M.G. Kong, Global model of low-temperature atmospheric-pressure He+H2O plasmas, Plasma Sources Sci. Technol. 19 (2010) 25018 http://10.1088/0963-0252/19/2/025018. W. Chung, K. Pan, H. Lee, M. Chang, Dry reforming of methane with dielectric barrier discharge and ferroelectric packed-bed reactors, Energ. Fuel. 28 (2014) 7621–7631, https://doi.org/10.1021/ef5020555. K. Pornmai, N. Arthiwet, N. Rueangjitt, H. Sekiguchi, S. Chavadej, Synthesis gas production by combined reforming of co2-containing natural gas with steam and partial oxidation in a multistage gliding arc discharge system, Ind. Eng. Chem. Res. 53 (2014) 11891–11900, https://doi.org/10.1021/ie500516m. H. Zhang, W. Wang, X. Li, L. Han, M. Yan, Y. Zhong, X. Tu, Plasma activation of methane for hydrogen production in a N2 rotating gliding arc warm plasma: a chemical kinetics study, Chem. Eng. J. 345 (2018) 67–78, https://doi.org/10.1016/ j.cej.2018.03.123. R. Snoeckx, R. Aerts, X. Tu, A. Bogaerts, Plasma-based dry reforming: a computational study ranging from the nanoseconds to seconds time scale, J. Phys. Chem. C 117 (2013) 4957–4970, https://doi.org/10.1021/jp311912b. E. Cleiren, S. Heijkers, M. Ramakers, A. Bogaerts, Dry reforming of methane in a gliding arc plasmatron: towards a better understanding of the plasma chemistry, Chem. Sus. Chem. 10 (2017) 4025–4036, https://doi.org/10.1002/cssc. 201701274. T. Sakka, K. Saito, Y.H. Ogata, Confinement effect of laser ablation plume in liquids probed by self-absorption of C2 Swan band emission, J. Appl. Phys. 97 (2005) 14902, https://doi.org/10.1063/1.1828214. S. Xu, J.C. Whitehead, P.A. Martin, CO2 conversion in a non-thermal, barium titanate packed bed plasma reactor: the effect of dilution by Ar and N2, Chem. Eng. J. 327 (2017) 764–773, https://doi.org/10.1016/j.cej.2017.06.090. P. Bruggeman, D.C. Schram, M.G. Kong, C. Leys, Is the rotational temperature of OH (A-X) for discharges in and in contact with liquids a good diagnostic for determining the gas temperature? Plasma Process. Polym. 6 (2009) 751–762, https://doi.org/ 10.1002/ppap.200950014. W. Wang, D. Mei, X. Tu, A. Bogaerts, Gliding arc plasma for CO2conversion: Better insights by a combined experimental and modelling approach, Chem. Eng. J. 330 (2017) 11–25, https://doi.org/10.1016/j.cej.2017.07.133. M. Ramakers, G. Trenchev, S. Heijkers, W. Wang, A. Bogaerts, Gliding arc plasmatron: providing an alternative method for carbon dioxide conversion, Chem. Sus. Chem 10 (2017) 2642–2652, https://doi.org/10.1002/cssc.201700589. J. Liu, H. Park, W. Chung, D. Park, High-efficient conversion of CO2 in AC-pulsed tornado gliding arc plasma, Plasma Chem. Plasma P. 36 (2016) 437–449, https:// doi.org/10.1007/s11090-015-9649-2. A.H. Khoja, M. Tahir, N.A. Saidina Amin, Process optimization of DBD plasma dry reforming of methane over Ni/La2O3MgAl2O4 using multiple response surface methodology, Int. J. Hydrogen Energ 44 (2019) 11774–11787 http://10.1016/j. ijhydene.2019.03.059. D.K. Dinh, S. Choi, D.H. Lee, S. Jo, K. Kim, Y. Song, Energy efficient dry reforming process using low temperature arcs, Plasma Process. Polym. 15 (2018) 1700203 http://10.1002/ppap.201700203. X. Zhu, K. Li, J. Liu, X. Li, A. Zhu, Effect of CO2/CH4 ratio on biogas reforming with added O2 through an unique spark-shade plasma, Int. J. Hydrogen Energ 39 (2014) 13902–13908 http://10.1016/j.ijhydene.2014.01.040. X. Li, B. Zhu, C. Shi, Y. Xu, A. Zhu, Carbon dioxide reforming of methane in kilohertz spark-discharge plasma at atmospheric pressure, AIChE J. 57 (2011) 2854–2860 http://10.1002/aic.12472. Hayashi Database, (2019) retrieved on January 23 www.lxcat.net. Morgan Database, (2019) retrieved on March 1 www.lxcat.net. H.K. Song, J. Choi, S.H. Yue, H. Lee, B. Na, Synthesis gas production via dielectric barrier discharge over Ni/γ-Al2O3 catalyst, Catal. Today 89 (2004) 27–33, https:// doi.org/10.1016/j.cattod.2003.11.009. V. Goujard, J. Tatibouët, C. Batiot-Dupeyrat, Use of a non-thermal plasma for the production of synthesis gas from biogas, Appl. Catal. A Gen. 353 (2009) 228–235, https://doi.org/10.1016/j.apcata.2008.10.050. B. Zhu, X. Li, C. Shi, J. Liu, T. Zhao, A. Zhu, Pressurization effect on dry reforming of biogas in kilohertz spark-discharge plasma, Int. J. Hydrogen Energy 37 (2012) 4945–4954, https://doi.org/10.1016/j.ijhydene.2011.12.062. J. Zhang, J. Zhang, Y. Yang, Q. Liu, Oxidative coupling and reforming of methane with carbon dioxide using a pulsed microwave plasma under atmospheric pressure, Energy Fuel. 17 (2003) 54–59, https://doi.org/10.1021/ef020069n. D. Czylkowski, B. Hrycak, M. Jasiński, M. Dors, J. Mizeraczyk, Microwave plasmabased method of hydrogen production via combined steam reforming of methane, Energy 113 (2016) 653–661, https://doi.org/10.1016/j.energy.2016.07.088. J. Mizeraczyk, M. Jasiński, Plasma processing methods for hydrogen production, Eur. Phys. J-Appl. Phys. 75 (2016) 24702, https://doi.org/10.1051/epjap/ 2016150561. J. Wang, C. Liu, Y. Zhang, B. Eliasson, A DFT study of synthesis of acetic acid from methane and carbon dioxide, Chem. Phys. Lett. 368 (2003) 313–318, https://doi. org/10.1016/S0009-2614(02)01866-3. L. Wang, Y. Yi, C. Wu, H. Guo, X. Tu, One-step reforming of CO2 and CH4 into highvalue liquid chemicals and fuels at room temperature by plasma-driven catalysis, Angew. Chemie Int. Ed. English 56 (2017) 13679–13683, https://doi.org/10.1002/ anie.201707131. L.M. Martini, G. Dilecce, G. Guella, A. Maranzana, G. Tonachini, P. Tosi, Oxidation

Journal of CO₂ Utilization 37 (2020) 248–259

Y. Xia, et al. of CH4 by CO2 in a dielectric barrier discharge, Chem. Phys. Lett. 593 (2014) 55–60, https://doi.org/10.1016/j.cplett.2013.12.069. [63] D.L. Baulch, C.J. Cobos, R.A. Cox, P. Frank, G. Hayman, T. Just, J.A. Kerr, T. Murrells, M.J. Pilling, J. Troe, R.W. Walker, J. Warnatz, Evaluated kinetic data for combustion modeling, Supplement I, J. Phys. Chem. Ref. Data 23 (1994)

847–848, https://doi.org/10.1063/1.555953. [64] S. Liu, D. Mei, L. Wang, X. Tu, Steam reforming of toluene as biomass tar model compound in a gliding arc discharge reactor, Chem. Eng. J. 307 (2017) 793–802, https://doi.org/10.1016/j.cej.2016.08.005.

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