Abstract
Methane conversion by the dielectric-barrier discharge plasma method (DBD) was compared with our previous findings for the microwave plasma method (MW). The power (Pw), initial pressure (P0) and flow rate (F0) affect the collisions between electrons and molecules, so the dissociated radical species may change. Changing the Pw resulted in methane conversion (XCH4) of DBD as high as 9.6% at 44 W, but much lower than the XCH4 of MW (93.8%). Ethane was the main product (60%) of DBD. Propane, methylpropane and butane were also produced. Therefore, DBD promoted homologation. Acetylene was the main product (90%) of MW. Therefore, MW promoted dehydrogenation. Changing the P0 resulted in lower XCH4 of DBD than XCH4 of MW. Propane selectivity was increased with higher P0, and butane was produced at 101.4 kPa in DBD. Acetylene was the main product irrespective of P0 in MW. Changing the F0 resulted in lower XCH4 of DBD. XCH4 of MW increased until 0.8 mmol/min and remained constant after 1.26 mmol/min. However, changing the F0 showed no effect on product selectivity. Therefore, methane conversion may be caused by the pulse-plasma effect under pressure. The conversion of ethane, ethylene and ethane/hydrogen was investigated to clarify the reaction mechanism of the methane conversion by DBD. Methane, acetylene, propane and butane were produced irrespective of the starting gases. Ethylene was produced from starting ethane. Ethane was produced from starting ethylene. The process of methane decomposition is considered to occur as follows. Firstly, methane converts to ethane. Secondly, ethane converts to propane. Finally, butane is produced from coupling of CH3 and C3H7 obtained from propane, and butane is produced from coupling of C2H5. In contrast, dehydrogenation is slightly promoted.
