Revised Manuscript

Effec t of Additi ve Gases on Me th ane C on versi on usi ng Glidi ng Arc Dis ch ar ge

Ant on ius Indar to † , Jae- Woo k Ch oi , Hwaun g Lee and Hyun g Keun Son g

Korea Institute of Science & Technology, Clean Technology Research Center,

P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Received 16 November 2004

Abstra ct - Methane conversion using gliding arc plasma has been studied. The process was conducted at atmospheric pressure. Four kinds of additives gaseshelium, argon, nitrogen, and CO2were used to investigate their effects on methane conversion, as well as products selectivity, and discharged power. Methane conversion was increased with the increasing concentration of helium, argon, and nitrogen in the feed gas but decreased when CO2 concentration increased. Qualitatively, hydrogen and acetylene were the major gas products. No liquid product was produced.

Keywords: Methane conversion, plasma, gliding arc discharge, additive gas

Intr oduct io n

The conversion of methane into more valuable compounds, such as hydrogen, synthesis †

Corresponding author: E-mail:[email protected], Tel:+82-19-352-1981

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gas, acetylene, and other higher hydrocarbon or black carbon is still becoming a challenge [1]. Many studies have been done intensively for several decades especially for direct methane conversion. The major problem on this route came from the strong C-H bond of methane. Many research groups used the catalytic method to overcome this problem. Although they reported some good results, some problems were found also. Carbon solid deposition on catalyst surface that was produced by chemical reaction became the greatest barrier to transfer this technology from the laboratory to the industrial scale. The catalyst was needed a specific temperature which was usually 100-200oC higher than room temperature to activate the catalytic site. It means heat supply was significantly required. Another reported problem was the small flow of injected raw gas.

Currently, more and more investigations have been deeply performed using nonconventional technology, like plasma technology. Plasmas, both thermal and nonthermal plasmas, have been extensively studied for methane conversion. Different kinds of plasmas and operation conditions produced different product distribution. This characteristic made it suitable for chemical synthesis selection. Methane utilization using glow discharge [2-4], Dielectric Barrier Discharge (DBD) [5-11], Corona [3,12], Spark [3], arc plasma-jet [13], Radio Frequency (RF) plasma [14,15], thermal plasma [16,17] have been investigated as well as the influence of additive gases effect. Other plasma variables effect on CH4 plasma reactions such as a plasma power generator [18,19], catalyst process-assisted [20,21], water vapor injection [22] were also experimentally investigated.

Cold plasmas such as corona, glow discharge, and DBD were very cheap and easy to

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handle, making them a promising possibility to be applied in industry. The main problem was the plasma density which is very low. It made it rather difficult to achieve a higher conversion at a higher flow rate. However, hot plasmas which typically high temperature arc plasmas produced very high density of plasma and capable to maintain high injection gas flow rate. But the instrument cost was very expensive and it used more power. To overcome these problems, plasma devices which are located in the transition region between the glow and arc state ware introduced. Gliding arc plasma at low current intensity, which is also called glowing arc, became a favor due to its characteristics under transition region, such as higher electron density, higher flame overheating, and high injection flow rate. Its applications have been increasing. Decomposition of H2S [23], N2O [24], CHCl3 and CCl4 [25,26], which were employing Gliding Arc as the destruction tool, have been investigated and studied. High percentage of destruction efficiency has been claimed using this method. Many papers were also discussing on the discharge behavior of gliding arc plasma. Theoretical and numerical study of gliding arc to describe it has been published with showing many mathematical equations [27-31]. In this study, Gliding Arc plasma was used to convert methane into higher hydrocarbon like acetylene and other valuable products such as solid carbon black, hydrogen, and synthesis gas. The investigation was deeply concerned on the effect of additive gases such as argon, helium, CO2, and nitrogen to the methane conversion, product distribution and power consumption.

2. Expe riment al setup

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The schematic diagram of experimental setup is shown in figure 1. Methane with a purity of 99.97% and additive gases were used as a gas source. The details of each part of the system are described in the following section.

2.1. P la sma re act or a nd po wer sys tem ap pl ied

The reactor was made from a quartz-glass tube of inner diameter 450 mm and total volume 0.5 l. The upper part and bottom of the reactor supplied with a teflon seal comprising two electrodes made of stainless steel. The electrodes were of length 150 mm. The separation of electrodes in the narrowest section was 1 mm. The gas mixture was injected between the electrodes through a capillary of inner diameter 0.3 mm. A thermocouple, located 100 mm above the electrode, was provided to measure the outlet gas temperatures. A high frequency AC power supply (Auto electric, A1831) with a maximum voltage of 10 kV and a maximum current of 100 mA was connected to the electrode of gliding arc to generate plasma. The frequency could be adjusted from 10 to 20 kHz. Figure 2 shows typical waveforms of voltage and discharge current obtained in this experiment.

2.2. In put g as

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Injection of methane gas and additive gases were controlled by four calibrated Mass Flow Controllers (MFC): Tylan, FC-280S with maximum flow capacity of 20 L/min; 3 Bronkhorst Hi-Flow MFCs, with maximum flow capacity of 100, 500, and 1000 ml/min. Total gas flow rate was varied from 1 to 3 L/min. The mixture composition of the outlet reactor was analyzed before and after plasma operation.

2.3. Me asureme nt system

Two Gas Chromatographers (GCs) have been used to analyze the quantitative amount of products. The content of hydrogen, O2, CO, nitrogen, and CO2 in the gas mixture before and after the reaction was determined by a GC-TCD (YoungLin M600D, Column: SK Carbon) and the hydrocarbons by a GC-FID (Hewlett Packard 5890, Column: Haysep Q). The flow of gases to the GC was measured first by bubble flow meter. To check the expansion of gas, the end of main output line was connected to a wet test meter (Ritter TG5) to measure the fluctuation of main flow after and before experiment running.

The evaluation of system performance, selectivity and conversion, were 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 =

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

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(1)

(2)

(3)

Special case when CO2 was used as addictive gas, the formulation would be:

Selectivity of O2 =

moles of O2 produced × 100% moles of CO2 converted

Selectivity of C x H y =

Conversion of CO2 =

x × moles of C x H y produced moles of CH 4 converted + moles of CO2 converted

(4)

× 100% (5)

moles of CO2 converted × 100% moles of initial CO2

(6)

The wave form of voltage and current were captured by oscilloscope (Agilent 54641A) with a high voltage probe (Tektronix 6015A) and a current monitor (Pearson 4997). The amount of power supplied was calculated by following equation:

Power consumption = ∫ (V (t ) × I (t ) ) dt × frequency (Watt )

(7)

3. Resu lts a nd Dis cuss io n

3.1. P ure Met ha ne

Figure 1 shows the result of pure methane experiment as a function of frequency. In this experiment, total gas flow rate was maintained at 1.5 L/min. It is shown in figure 3a that the rising of power supply frequency makes conversion of methane become slightly higher. The conversion reaches 40% at 15 kHz and goes up to 45% at 20 kHz. Increasing frequency will increase total power that is supplied to plasma reactor. Generally, the power consumption has a linear function to the frequency. Formula 7 is

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used to calculate the voltage and the current into power as it is shown in figure 3b. The increment of input power will increase energy supply which probably produce higher dissociation of CH4 molecule and generate more energetic electrons. Song et al reports one advantage of methane conversion using the gliding arc plasma because of additional heat released from the plasma reaction [32]. The heat which is produced from the partial oxidation and combustion process of methane conversion in gliding system has to be counted as a significant factor for methane conversion.

Under gliding arc plasma, Methane conversion produces hydrogen, acetylene (C2H2), and carbon (C) solid as the main gas products. The selectivity of hydrogen reaches 40% and acetylene reaches 20%. The rising of frequency or power did not give a significant effect on product distribution (figure 3c). The selectivity of hydrogen is slightly higher from 35% at 15 kHz to 40% at 20 kHz. However, the selectivity of acetylene is 20-22%.

3.2. Effec t of Arg on and Met ha ne Ga s

Figure 4 shows the effect of argon and helium gas to performance of methane conversion at a total gas flow rate of 1 L/min and a frequency of 20 kHz. Generally, argon and helium made a positive effect on methane conversion. The conversion is higher with increasing concentration of these gases (figure 4a). Comparison between argon and helium, these gases gives almost same characteristic on methane plasma reaction. No significant different between those gases on the product results. Song et al

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reports that argon and helium have a meta-stable energy level which could help the dissociation of carbon-hydrogen bond of methane molecule [33]. The existence of meta-stable argon or helium in the plasma system will increase the number of energetic unstable species and increase the possibility to react with methane. In term of products distribution, figure 4b shows the selectivity of hydrogen in the product stream gets increase when the concentration of argon or helium (β) is in a higher concentration. On the other hand, the selectivity of acetylene (C2H2) will be lower at the similar phenomena. It means that increasing concentration of argon or helium makes the plasma reaction tend to produce hydrogen rather than higher hydrocarbon, such as acetylene. Abundant amount of electrons and excited argon or helium atoms lead the reaction into fragmentation reaction rather than re-arrangement into higher species. Lower ionization potential of argon and helium affects in decreasing power consumption of methane reforming when those gases are existed in feed stream. Figure 4c shows the consumed power is significantly decrease from 170 Watt into 120 Watt when β has changed from 1 to 0.15.

3.3. Effec t of Nitr oge n Gas

The effect of nitrogen gas on methane conversion has been studied also. Figure 5 shows its effect on methane conversion, product selectivity, and power consumption at a total gas flow rate of 1 L/min and a frequency of 20 kHz. As shown in figure 5a, the

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methane conversion increases when the concentration of nitrogen in the mixed gas increases. The increasing rate of methane conversion is started from β=0.4 and it achieves 65% at β=0.8. It means that at lower concentration of nitrogen gas, conversion of methane is not significantly different and relatively stable. In our previous work, when the concentration of nitrogen was quite low, the nitrogen molecule was not involved in plasma-chemical reactions, but at higher concentration, it has a high possibility to contribute in the reaction mechanism by excitation of N2 molecules [34]. This excitation of N2 into higher vibrational level and meta-stable state (N2(A) and N2(a’)) will help to increase the conversion of methane.

The effect of nitrogen on product selectivity is presented in figure 5b. Increasing concentration of nitrogen in the mixed gas will increase the selectivity of hydrogen. In case of acetylene, the selectivity achieves 60% at β=0.5 and decreases to 40% at β=0.2. Compared with figure 4b, the selectivity ratio between H2 to C2H2 is lower than that when argon and helium is used as additive gases. Diamy et. al. has proposed the kinetic reaction of CH4/N2 on plasma reaction [2-4]. Compared to Argon and Helium, N2 has more important role to lead the mechanism reaction. The mechanism of reaction would be:

CH 4 + N 2 ( A) → CH 3 + H

(8)

CH 4 + N 2 (a' ) → C + 2 H 2

(9)

C + CH 3 → C 2 H 2 + H

(10)

C + CH 2 → C 2 H 2

(11)

and produces ratio of H2 to C2H2 close to 1:1.

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The methane conversion reaction goes into a complex reaction when nitrogen is used as an additive gas. Although the dissociation energy of nitrogen is relatively high (higher than dissociation of CH3-H by factor of 2), small amount of nitrogen has a possibility to react and produce products that contain N atom, such as HCN, NH3. However, similar to argon and helium, the addition of nitrogen produces lower energy consumption (figure 5c). The power consumption reduces up to 140 Watt at β=0.3.

3.4. Effec t of CO 2 G as

The addition of CO2 to the methane reaction has been studied to produce the synthesis gas (CO+H2) [6,10,17]. Before applying CO2 as additive gas on methane reaction, the experiment is done by pure CO2 decomposition. Figure 6 shows the decomposition of CO2 as a function of total gas flow rate from 1.3 to 2.2 L/min by gliding arc plasma. The conversion is decreasing with increasing total gas flow rate (figure 6a). Increasing total gas flow rate reduces the resident time of gas inside the reactor and reduce the chance of CO2 to collide with electron or other exited species in the plasma reaction.

In case of products selectivity, the main products (figure 6b) are CO and O2. Calculation of material balance of oxygen atom from these two compounds achieves more than 80% for all experiment condition. In term of power consumption (figure 6c), CO2 treatment requires higher consumed energy than CH4. It can be proposed because of the chemical bond of C-O in CO2 is much stronger and stable than bond of C-H in CH4.

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Figure 7 shows the experiment result of methane conversion which is influenced by CO2 at a total gas flow rate of 1 L/min and a frequency of 20 kHz. The conversion has a tendency to decrease with increasing concentration of CO2 in the mixed gas (figure 7a). The conversion drops from 45% at β=1 to 35% at β=0.2. In case of CO2, the conversion of CO2 increases with increasing CH4 concentration. The maximum conversion of CO2 reaches 33% at β=0.9. Different from the previous gases, argon and helium, CO2 is difficult to be transformed into meta-stable state and not giving a positive effect on methane conversion. On the other hand, lower meta-stable level of CH4 makes the existence of CH4 in the plasma reaction increase decomposition efficiency of CO2.

Although it produces negative effect on the methane conversion, existing CO2 in the mixed gas has an important role to govern the distribution of products (figure 7b). The selectivity of hydrogen was down to 40% at β=0.8 but continuing up to 60% at β=0.2. It means higher concentration of CO2 would increase the production of H2. Decreasing trend was shown by CO selectivity when the concentration of CO2 decreased. It is caused by reduction of oxygen in the feed gas. Dissociation of CO2 will produce CO and radical O as products. At higher concentration of CO2, more radical O will be produced and give higher possibility to react with CH4

CH 4 + O → CO + 2H 2

(12)

On the other hand, increasing fraction of CH4 will reduce the selectivity of CO (figure 7b) and the plasma reaction tents to produce higher hydrocarbon products, such as acetylene.

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Interesting point of products selectivity is shown at β=0.7 (figure 7b). Calculation of material balance at this point is close to reaction of:

CO 2 +3CH 4 → 2CO + C 2 H 2 + 5 H 2

(13)

At β≠0.7, this mechanism reaction is not applicable to the plasma products distribution.

In term of power consumption, increasing concentration of CO2 increases the power consumption (figure 7c). It because CO2 has a stronger bond than CH4 and require more supplied energy to break the bond.

4. Co nc lusi on

The effect of additive gases on methane conversion using gliding arc plasma was investigated. Using Gliding Arc, pure methane is converted mostly into C solid, hydrogen, and acetylene. Hydrogen selectivity reaches 50% and C2H2 around 20-22%. Increasing frequency produces a higher conversion from ~40% up to ~50% when it increases from 15 kHz to 20 kHz. Argon, helium, and nitrogen produce a positive effect on methane conversion and reduce the power consumption. The conversion reaches ~65% at 10% CH4 diluted with 90% of argon or helium. Increasing dilute gas ratio produces higher selectivity of hydrogen and reduces the selectivity of C2H2. CO2 is converted into CO and O2 in the gliding arc plasma. Selectivity of CO reaches 5060% and O2 reaches 30-40%. When methane is mixed with CO2, the conversion of methane gets lower. Hydrogen, CO, and acetylene are the main products of plasma reaction.

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Ack no wled gements

This study was supported by National Research Laboratory Program of Korea Minister of Science and Technology.

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