Some temperature effects on stability and carbon formation in low temperature ac plasma conversion of methane

Methane conversion using a low temperature plasma generated by an ac corona discharge has recently been extensively studied. Different products can be produced: C2, C3, H2, CO, CO2 and coke. The understanding of the role of temperature has been limited. This paper discusses the use of an IR thermal imaging and measurement system to study the effects of temperature on the stability and carbon formation in low temperature ac plasma conversion of methane. Three types of coke are observed during the methane conversion process using plasma generated by an ac corona discharge under various conditions. The first type of coke is dark brown and powder coke. The second type of coke is dark greenish and forms a soft coating. The third type is coke filaments, which strongly affects the discharge stability by quickly connecting the two electrodes. In the partial oxidation or steam reforming of methane, the temperature is generally less than 300 °C under normal experimental conditions. Increasing the feed temperature in the steam reforming of methane increases the reactor temperature, causing the formation of coke filaments when the water concentration is not high enough. Therefore, feeding liquid water at room temperature into the reactor helps improve the stability.