Manuscript

Methane Conversion Using Dielectric Barrier Discharge: Comparison with Thermal Process and Catalyst Effects

Antonius Indarto†, Jae-Wook Choi, Hwaung Lee, Hyung Keun Song Korea Institute of Science & Technology, Clean Technology Research Center, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

†

Corresponding author: Antonius Indarto; e-mail: [email protected] 1

Abstract The direct conversion of methane using a dielectric barrier discharge has been experimentally studied. Different values of flow rates and discharge voltages have been used to investigate the effects on the conversion reaction products both qualitatively and quantitatively. Experimental results indicate that the maximum conversion of methane was 80% at an input flow rate of 5 mL/min and discharge voltage of 3.5 kV. Experimental results show that the optimum condition was occurred at high discharge voltage and higher input flow rate. In term of products distribution, higher flow rate or shorter residence time can increase the selectivity of higher hydrocarbons. No hydrocarbon product was detected using thermal method, except hydrogen and C. Increasing selectivity of ethane was found when Pt and Ru catalyst were existed in the plasma reaction. Hydrogenation of acetylene in the catalyst surface can be the reason for this phenomenon as the selectivity of acetylene in the products was decreasing.

Keywords: plasma, dielectric barrier discharge, methane conversion, catalyst

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1. Introduction

Methane, which is primary constituent of most natural gas reserves, is currently being used for home and industrial heating as well as for the generation of electrical power. In many respects, methane is an ideal fuel for these purposes because of its availability in most populated centers, its ease of purification to remove sulfur compounds and the fact that among the hydrocarbons, it has the largest heat of combustion relative to the amount of CO2 formed. On the other hand, methane is a greatly underutilized resource for chemicals and liquid fuels. Moreover, the reserves are increasing more rapidly than those of liquid petroleum. Much of the methane is found in regions that are far removed from industrial complexes and often it is produced off shore [1]. Pipelines may not be available for transporting this remote gas to potential markets and liquefaction for shipping by ocean-going vessels is expensive. Approximately 11% of this gas is re-injected, and unfortunately, another 4% is flared or vented [2], which is a waste of a hydrocarbon resource. Both methane itself and carbon dioxide derived from methane combustion are greenhouse gases. Natural gas can be converted to fuels and chemicals in two ways, either via synthesis gas or directly into higher hydrocarbons or methanol. Today, most commercial processes for natural gas conversion involve synthesis gas as an intermediate. However, extensive research is presently being carried out on different possible routes for the direct conversion of natural gas to improve the selectivity and yield of higher hydrocarbons to meet the industrial and economical demand.

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The investigations of methane conversion to higher hydrocarbons in non-thermal plasmas have been made worldwide by corona discharges, spark discharges, gliding arc, or dielectric barrier discharges (DBD) at atmospheric pressure and ambient temperature. Some researchers have tried to add auxiliary gases in the process, such as hydrogen [3], air [3-5], oxygen [4-7], noble gas [5, 8]. Among them, mixing gas between methane and CO2 was found to be the most promising technique to produce more valuable products, such as synthesis gas [9-10]. Some others used the different types of plasma discharges that possibly produced different products distribution [10-11]. In the present research, comprehensive study on the performance of plasma and thermal treatment for methane conversion has been undertaken. Plasma process was done on a dielectric barrier discharge (DBD). A series of metal catalyst (Ni, Ru, Pt), supported by Al2O3, has been prepared to investigate the effect of the products distributions.

2. Experimental Setup Figure 1

Figure 1 shows a schematic diagram of the experimental setup in this study. Methane, as source gas, was introduced into a cylindrical reactor at atmospheric pressure. Gases were analyzed by gas chromatography. Details of each part of the system are described in the next section.

A. Reactor The reactor is a concentric cylinder with an inner metal electrode and an outer electrode

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of silver film coated around the tube. The plasma reactor consists of a pyrex tube (7.5 mm ID) with 2 parallel-straight wire (0.2-mm diameter, stainless steel) high-voltage electrode. The effective gas volume and length of the reactor were 8.8 mL and 200 mm, respectively. High voltage and frequency AC power supply were used in this experiment. In order to maintain the similar configuration, e.g. gap distance, the reactor capacitance was measured before experiment. The reactor capacitance was in the range of 8.2-8.8 pF in air-gap environment. The same reactor size and configuration was used for the thermal (non-plasma) process.

B. Power supply and Heater The maximum voltage and frequency of the AC power supply (Auto electric, model A1831) were 10 kV and 20 kHz. For measurement of voltage and current waveforms, a digital oscilloscope (Agilent, model 54641A), a voltage divider (Tektronix, model P6015A), and a current probe (Fluke, model i400s) were used. The external input power was measured by a digital power meter (Metex, model M-3860M) inserted at the ac power input line. The amounts of actual supplied power were calculated by following equation: Actual power = ∫ (V (t ) × I (t ) ) dt × frequency

The waveform typical of voltage and current, used in this experiment, is shown in Figure 2. For thermal process, a tubular furnace with temperature controller (Daepoong Industry, Korea) was used. Figure 2

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C. Materials Preparation All experiments were carried out using pure methane (CH4) with purity higher than 99.99%. Flow rate of the source gas was controlled by a calibrated mass flow controller (Milipore, model FC-280SAV). Analysis of the gas sample was carried out using a gas chromatography (YoungLin, model M600D) with a thermal conductivity detector (TCD, Column: Hayesep D 80/100) for measuring H2 and CH4, and a flame ionized detector (FID) for measuring CH4 and higher hydrocarbons. The evaluation of system performance was done based on products selectivity and methane conversion which are formulated as: Selectivity of H 2 =

moles of H 2 produced × 100% 2 × moles of CH 4 converted

Selectivity of C x H y =

Conversion of CH 4 =

Energy efficiency =

x × moles of C x H y produced moles of CH 4 converted

× 100%

moles of CH 4 converted × 100% moles of initial CH 4

flow of converted CH 4 × 100% total power

(1)

(2)

(3)

(4)

A series of metal catalysts (Ni, Ru, Pt) was prepared by incipient wetness impregnation method with Al2O3 as the support. The aqueous solutions of Ni, Ru, and Pt were made by dissolving NiCl2.6H2O (Kanto Chemical Co., Japan), RuCl3.3H2O (Engerhard, England), and H2PtCl6.6H2O (Next Chimica, South Africa) precursors in aqua regia. All catalysts were dried at 388 K for 2 h, followed by calcination at 873 K for 3 h in O 2-rich gas condition. Amounts of metal loaded were all settled to be 3 wt% of the catalyst. In the plasma reaction experiment, 0.5 gram of catalyst was packed at the end of plasma

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zone to prevent the decomposition of products.

3. Result and Discussion

3.1. Plasma and Thermal Process Figure 3

Methane conversion under plasma and thermal process was conducted at an atmospheric pressure. In plasma experiments, the total flow rate was varied from 5 mL/min to 40 mL/min while the voltage was set at 3, 3.2, and 3.5 kV. Figure 3 shows the effects of input methane flow rate and voltage on the plasma process. Higher conversion is produced at a higher voltage and a lower flow rate. The maximum methane conversion reaches 80% at initial methane flow rate of 5 mL/min and voltage of 3.5 kV. It shows that discharge voltage has a straight correlation with methane conversion. The increase of the amplitude of voltage leads to the increase of plasma density [12]. It can be seen also from the curve trend that at the conversion rate decreases when the flow rate is increased. Lower flow rates will give longer chance for the molecules to collide with energetic species, such as electron. At a higher flow rate, the collisions are more effective although the percentage of conversion is lower, as seen in Figure 4. More numbers of methane molecules are converted at higher flow rates than that at lower ones. Instead of electron, energetic species from methane decomposition, e.g. methyl radical (CH3.) and H., can initiate another effective collision with CH4. At lower flow rates, the number of those species is less as the amount of methane flows to

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the reactor is also small. However, by increasing discharge voltage, the energy efficiency is slightly increasing. Following the previous statement, the plasma reaction is more effective to decompose methane at higher plasma density. This leads to the conclusion that the optimum process is occurred at higher flow rate with higher discharge voltage. Figure 4

Figure 5

Figure 5 shows the products distribution of methane conversion by plasma process. High selectivity of H2 is produced at a low input flow rate. Fragmentation of methane into smaller molecules, e.g. H2 and C, is more favorable that recombinant reaction to produce higher hydrocarbons. At lower flow rate or longer residence time, higher hydrocarbons that also produced during plasma reaction will be easily decomposed by electron and other energetic species. Electron that has a key factor to initiate the radical reaction in DBD process [13] can lead the reaction: CH4 + e  CH4. + e

(5)

CH4.  CH3. + H.

(6)

Methyl radical (CH3.) will initiate the recombinant reaction of higher hydrocarbons and hydrogen radical (H.) has tendency to be a reducer or decomposer of higher hydrocarbons. CH3. + CH3.  C2H6

(7)

CH3. + CH3.  C2H4 + H2

(8)

CxHy + H.  CxHy-1 + H2

(9)

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CH3. + C2H3  C3H6

(10)

Instate of H., decomposition of higher hydrocarbon was also caused by electron. Because electron collision with molecules is faster than that with ion or radical [14], the longer resident time of molecule in the plasma reaction will give a negative effect on higher hydrocarbon formation or produce more stable H2 and C than that in shorter resident time. In case of coke (C solid), it was produced mostly at the wall of the reactor and the surface of inner electrode. The research on coke formation has been experimentally done by Le et al. [15] and simulated by Rhallabi and Chaterine [16]. Coke will be formed at the plasma area where the temperature is relatively high, e.g. surface of the electrode. Close at the surface of the electrode, methane will be converted into C and 4 H or H2. CH4  C + 4 H

(11)

CH4  C + 2 H2

(12)

This phenomenon was also occurred on our previous research of methane conversion reaction by arc plasma [17] where the temperature is relatively higher than room temperature. Table 1

However, numerous amounts of H2 and C production were also found in the thermal process. Table 1 shows the products comparison between plasma and thermal process. Most of products of thermal process were dominated by H2 and C. Solid carbon was not measured but it was found many in the output line of reactor. At flow rate of 20 mL/min, the conversion of methane was started at temperature of 700oC. To achieve

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similar conversion of plasma experiment, it requires temperature more than 900oC. In thermal process, supplied heat is the most driving factor in dissociating methane molecules into H2 and C. CH4  C + 2 H2

(13)

The production of higher hydrocarbon was relatively small and un-detectable with our analysis instrument. Methane can be converted directly to higher hydrocarbons, such as C2 hydrocarbons and hydrogen via thermal coupling reactions: 2 CH4  C2H2 + 3 H2,

∆H298 = 376,5 kJ/mol

(14)

2 CH4  C2H4 + 2 H2,

∆H298 = 202,3 kJ/mol

(15)

2 CH4  C2H6 + H2,

∆H298 = 65,1 kJ/mol

(16)

All these reactions are highly endothermic, and high temperature operation is required to obtain favorable thermodynamics. In order to avoid the decomposition of products, very short time reaction, less than 0.01 s, should be done [18] and it makes a difficulty to achieve high conversion of methane. By comparing the experiment results, it shows that plasma process has an advantage on higher hydrocarbon production. The existing of electrons and methyl radical, produced during plasma process, could be the primary role on initiating the decomposition of methane and recombination reactions of higher hydrocarbons.

3.2. Catalyst Effect Table 2

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In order to improve the selectivity of products, 0.5 gram of three different catalysts has been put in the end of plasma zone of the reactor. The purpose of this is to prevent the produced higher hydrocarbon from the decomposition reactions. Table 2 shows the CH4 conversion and products distribution with different material packs. Experiment condition was done at methane flow rate of 20 mL/min. It shows that conversion of CH4 to other hydrocarbons was enhanced with the use of catalyst. Although the different relatively small, the existence of catalyst was also help the cracking process of methane molecules. Interesting phenomena was found when Pt/γ-Al2O3 was used as the catalyst. Compared to non-catalyst treatment, the selectivity of ethane (C2H6) is higher by factor of 1.35. On the other hand, no acetylene (C2H2) was detected by our analysis instrument. According to this result, the existence Pt catalyst in the plasma reaction will convert the produced acetylene into ethane by hydrogenation reaction in the surface of catalyst. Low activation energy (~0 kJ/mol) of acetylene adsorption on Pt will make acetylene easily attached to the surface of the catalyst [19]. C2H2 (g)  C2H2 (surf)

(17)

The existing of numerous H, also attached on the surface, can initiate the series of hydrogenation reactions: C2H2 (surf) + H (surf)  C2H3 (surf)

(18)

C2H3 (surf) + H (surf)  C2H4 (surf)

(19)

C2H4 (surf) + H (surf)  C2H5 (surf)

(20)

C2H5 (surf) + H (surf)  C2H6 (surf)

(21)

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In our previous experiment, under non-catalytic reaction, the above reactions were rarely occurred showed by small yields of C2H4 and C2H6 from C2H2 conversion [20]. The idea of this reaction mechanism, e.g. via intermediate C2H5, was supported by Dent et al’s investigation [21]. However, the similar phenomenon was also occurred when we put Ru/γ-Al2O3 catalyst. Although the condition is less clear than Pt, by adding Ru catalyst, the selectivity of ethane is increasing and followed by decreasing selectivity of acetylene. In term of Ni catalyst, our observations did not find significant effect on products distribution of methane conversion, in agreement with other experiment evidence that Ni is less active catalyst for hydrogenation than Pt [19].

4. Conclusions

The conversion of methane into hydrogen and higher hydrocarbons was experimentally investigated in a dielectric barrier discharge and a thermal process. In plasma process, the residence time of molecule has influenced the reaction mechanism of the products. Lower flow rate or longer residence time will make the reaction favors to be dissociation of methane into smaller molecules than recombinant reaction of higher hydrocarbons. By thermal process, the reaction will produce H2 and C. The existence of electrons in plasma process gives an advantage on the higher hydrocarbon production. In order to increase the selectivity of products, plasma-catalyst process has been performed. Pt and also Ru catalyst show a significant effect on increasing production of ethane by transforming the produced acetylene by hydrogenation reactions.

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Acknowledgment This research was supported by the National Research Laboratory program of the Korea Minister of Science and Technology.

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10. Song H K, Lee H, Choi J W, Na B K, Plasma Chem Plasma Process, 2004, 24(1). 11. Lia X S, Zhua A M, Wanga K J, et al. Catal Today, 2004, 98: 617. 12. Choi Y H, Kim J H, Hwang Y S. Thin Solid Films, 2005, in press 13. Eliasson B, Kogelschatz U. IEEE Trans Plasma Sci, 1991, 19(6) : 1063. 14. Lieberman M A, Lichtenberg A J, Principles of Plasma Discharges and Material Processing, 1st ed, New York: John Wiley & Sons, 1994, 224 15. Le H, Lobban L L, Mallinson R G. Catal. Today, 2004, 89: 15. 16. Rhallabi A, Chaterine Y. IEEE Trans Plasma Sci, 1991, 19(2): 270. 17. Indarto A, Choi J W, Lee H, Song H K, J Nat Gas Chem, 2005, 14:13. 18. Holmer A, Olsvik O, Rokstad O A. Fuel Process Tech, 1995, 42: 249. 19. Shustorovich E, Surface Sci, 1988, 205: 492. 20. Kim S S, Lee H, Na B K, Song H K, Korean J Chem Eng, 2003, 20(5): 869. 21. Dent A L, Kokes R J, J Physic Chem, 1970, 20: 3653.

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Plasma Reactor

AC power supply

catalyst

CH4

Bubble flow meter

MFC

heater

Thermal Process

Figure 1. Experimental Setup

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GC

6000

200 150

4000

2000

50

0

0 - 50

- 2000

- 100 - 4000

- 6000 - 25

voltage current

- 150 - 200

- 15

-5

5

15

25

Time cycle (µs)

Figure 2. Typical of waveform of voltage and current

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Current (mA)

Voltage (kV)

100

90

3 kV 3.2 kV 4 kV

80

Conversion (%)

70 60 50 40 30 20 10 0 0

10

20

30

40

50

Methane flow rate (mL/ min)

Figure 3. Effect of methane flow rates and discharge voltages on methane conversion

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0.16

Energy efficiency (mL/W)

0.14 0.12 0.1 0.08 0.06 0.04

3 kV 3.2 kV 4 kV

0.02 0 0

10

20

30

40

50

Methane flow rate (mL/ min)

Figure 4. Effect of methane flow rates and discharge voltages on process efficiency

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80

H2 C2 C3 C4

70

Selectivity (%)

60 50 40 30 20 10 0 0

10

20

30

40

50

Methane flow rate (mL/ min)

Figure 5. Effect of methane flow rates on products distribution at discharge voltage of 3.5 kV

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Table 1. Methane conversion and products distribution comparison between plasma and thermal process

plasma

thermal

CH4 conversion (%) 54.97 51.11 25.01 22.37 54.95 50.42 25.60

H2 35.29 29.22 34.77 38.46 65.73 68.58 77.57

13.35 7.05

84.04 91.60

C 2H2 4.85 7.36 5.06 9.30

C 2H6 20.89 23.19 25.48 42.75

selectivity (%) C 3H6 2.01 3.36 1.73 2.98

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C 3H8 12.42 13.33 13.89 19.79

n- C 4H10 5.90 6.11 4.42 5.85

i- C 4H10 4.64 7.37 4.95 7.67

Table 2. Effects of packed loading on methane conversion and products distribution

empty γ- Al2O3 Ni/ γ- Al2O3 Ru/ γ- Al2O3 Pt/ γ- Al2O3

CH4 conversion (%) 43.42 42.50 48.06

H2 34.52 39.69 38.89

C 2H2 5.13 7.62 5.23

48.13 47.00

33.55 32.92

2.65 0.00

selectivity (%) C 2H6 C 3H6 25.73 0.00 26.31 2.69 27.97 2.31 30.22 34.87

0.00 0.00

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C 3H8 11.73 15.85 14.02

n- C 4H10 5.25 5.74 4.92

i- C 4H10 4.81 5.56 5.31

13.47 16.36

5.08 5.51

5.36 6.00

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