Methane conversion to synthesis gas in corona discharge

Abstract

Synthesis gas is a versatile feedstock for many synthesis processes. There are several conventional reactions to produce synthesis gas i.e. steam reforming, partial oxidation, and carbon dioxide reforming but these catalytic processes have to be operated at high temperatures. Because of non-equilibrium property of low temperature plasma, it is thought to be an alternative way to drive the methane reforming reaction to synthesis gas instead of high temperature catalytic processes. In this study, synthesis gas production from methane using an ac corona discharge was conducted with and without catalysts. To study partial oxidation of methane, air was used as feed gas for reducing investment and operating cost as compared to pure oxygen. The methane conversion dropped dramatically but oxygen conversion increased with addition of ethane to the feed gas. The nitrogen in air not only acts as a dilute gas but also affects the reactions. The results show that oxygen is the most effective active species to reduce carbon formation and increases methane conversion as well as lower the specific energy consumption. For this reason, steam reforming could not be operated alone under corona discharge to convert methane into synthesis gas because of the carbon formation. For carbon dioxide reforming with methane in low temperature plasmas, methane and carbon dioxide conversions both increased with increasing voltage, gap width, and carbon dioxide to methane feed mole ratio but decreased with increasing frequency and flow rate. Under the studied conditions, methane conversion was always higher than carbon dioxide conversion. Sinusoidal and square waveforms gave negligibly different results of the reactant conversions and the product distribution of partial oxidation of methane with air and carbon dioxide reforming with methane. To find the way to increase the efficiency of producing synthesis gas, the partial oxidation of methane with carbon dioxide was carried out in the presence and absence of Pt loaded KL zeolite (Pt/KL) and Pt/ZrO₂. The results showed that the combination of catalyst and electric discharge gave a higher oxygen conversion but a little bit lower methane conversion. The presence of catalyst did not show that synergetic effect on both partial oxidation and carbon dioxide reforming. The challenging method to improve synthesis gas production efficiency by introducing water in feed steam was investigated. Combined carbon dioxide and steam reforming with methane produced higher methane conversion and CO/C₂ ratio than either carbon dioxide or steam reforming. In case of the combined partial oxidation and steam reforming, the energy consumed to convert a methane molecule decreased dramatically from 68 to 13 eV/m[subscript c] with increasing the percentage of watervapor from 0 to 50% at a CH₄/O₂ ratio of 2:1