Methane conversion to C2 hydrocarbons and hydrogen has been investigated in a needle-to-plate reactor by pulsed streamer and pulsed spark discharges and in a wire-to-cylinder dielectric barrier discharge (DBD) reactor by pulsed DC DBD and AC DBD at atmospheric pressure and ambient temperature. In the former two electric discharge processes, acetylene is the dominating C2 products. Pulsed spark discharges gives the highest acetylene yield (54%) and H2 yield (51%) with 69% of methane conversion in a pure methane system and at 10 SCCM of flow rate and 12 Wof discharge power. In the two DBD processes, ethane is the major C2 products and pulsed DC DBD provides the highest ethane yield. Of the four electric discharge techniques, ethylene yield is less than 2%. Energy costs for methane conversion, acetylene or ethane (for DBD processes) formation, and H2 formation increase with methane conversion percentage, and were found to be: in pulsed spark discharges (methane conversion 18–69%), 14–25, 35–65 and 10–17 eV/molecule; in pulsed streamer discharges (methane conversion 19–41%), 17–21, 38–59, and 12–19 eV/molecule; in pulsed DBD (methane conversion 6–13%), 38–57, 137–227 and 47–75 eV/molecule; in AC DBD (methane conversion 5–8%), 116–175, 446–637, and 151–205 eV/molecule, respectively. The immersion of the γ-Al2O3 pellets in the pulsed streamer discharges, or in the pulsed DC DBD, or in the AC DBD has a positive effect on increasing methane conversion and C2 yield. © 2004 Elsevier B.V. All rights reserved.

The oxidative dehydrogenation of ethane to ethylene and acetylene with carbon dioxide at ambient temperature and atmospheric pressure by pulse corona plasma over various catalysts has been investigated. The products included C2H4, C2H2 and syngas (H2 and CO). The conversion of ethane and distribution of products depend on the catalyst used, the C2H6/CO2 feed ratio, the energy density of plasma, etc. The rare earth metal oxides catalyst such as La2O3/γ-Al2O3 and CeO2/γ-Al2O3 enhance the conversion of ethane and the yield of ethylene and acetylene. The sequence of ethane conversion and yield of ethylene and acetylene is from CeO2/γ-Al2O3 to La2O3/γ-Al2O3. The metal catalyst Pd/γ-Al2O3 exhibits high ethylene selectivity. The optimum C2H6/CO2 ratio in the feed for oxidative dehydrogenation of ethane under plasma catalytic conditions is 1/1. The conversion of ethane and the yield of ethylene and acetylene increase with increasing of the energy density of plasma. © 2003 Elsevier B.V. All rights reserved.

At ambient temperature and pressure. C2H2 and H2 are the dominating products from pure methane conversion under corona discharge (PCD). When the energy density of 194-1788 kJ/mol was applied, 7%-30% of C2H2 yield and 6%-35% of H2 yield per pass have been obtained. These resuits are higher than the maximum thermodynamic yield of H2H2 (5.1%) and H2 (3.8%) at 100 kPa and 1100 K, respectively. Thereby, pulsed corona discharge is a very effective tool for “beyond-thermal-equilibrium” conversion of methane of C2H2 and H2 at ambient temperature and pressure. In the PCD energy density range of 339-822 kJ/mol, the carbon distribution of the methane conversion products is found to be: C2H2 86%-89%, C2H6 4%-6%, C2H4 4%-6%, C3～2%, C4～1%. Through comparison of the product from pure methane, ethane and ethylene conversion at the same discharge conditions, it can be concluded that three pathways may be responsible for the C2H2 formation via CH, radicals produced from the collisions of CH4 molecules with energized electrons in the PCD plasma: (i) C2H2 is formed directly from free radical reactions, (ii) C2H2 is formed through the dehydrogenation of C2H2, which is formed via free radical reactions primarily, and (iii) C2H6 is the primary product and then dehydrogenates to C2H4 (secondary product) and followed by C2H4 dehydrogenation to C2H2.

At normal temperature and pressure, pulse corona plasma was used as a new method for the dehydrogenative coupling of methane in the absence of oxygen. The effects of voltage polarity and input energy on the dehydrogenative coupling of methane were investigated. The parameter “energy efficiency” was introduced to examine the coupling of the input energy and the dehydrogenative coupling of methane. The experimental results show that positive corona gives higher energy efficiency than negative corona. When the positive corona was chosen, C2 yield per pass was 31.6% and acetylene yield per pass was 30.1% with 44.6% methane conversion at an input energy density of 1788kJ/mol and a pulse repetition frequency of 66Hz. The function of input energy density towards methane conversion may be expressed as a formula of –ln (1-X) = k (P/F). In the range of input energy employed, C2 yield is proportional to input energy density, but energy efficiency drops off with increasing input energy density.