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.

1. Introduction

The conversion of methane into more valuable compounds, such as hydrogen, synthesis 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 100–200 1C 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 non-conventional technology, like plasma technology. Plasmas, both thermal and non-thermal 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, dielectric barrier discharge (DBD) , Corona , Spark , arc plasma-jet ,
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 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.