Methane is an important raw material for fuel and commodity chemicals production. Energyintensive steam methane catalytic reforming in gas-fired furnaces is the main industrial process for methane conversion to synthesis gas and further to other chemicals. Methane conversion by means of non-thermal plasma technologies has attracted attention in the last years, as no pre-heating of the feedstream at high temperatures is needed. Electric energy is consumed in producing energetic electrons for molecule bonds breaking, instead of gas heating, thereby overcoming the disadvantages of high operational temperatures. In this work, after introducing plasma classification and plasma chemistry, a comprehensive review of literature papers on non-thermal plasma-assisted methane coupling in the period 2010-2016 is presented and the best results that have been obtained with all different kinds of non-thermal plasma techniques are reported. Finally, as the energy cost is the main cost driver of the process after the raw material cost, comparison among all plasma techniques used for methane coupling is performed in terms of specific energy requirement to crack a mole of methane (SER, kJ/molCH4), efficiency (η %) and energy requirement to produce a mole of target product (ER, either kJ/molC2H2 or kJ/molC2H4). This is followed by a comparison between plasma-driven and thermal energy-driven methane coupling.

A possible way to store both renewable energy and CO 2 in chemical energy is to produce value-added chemicals and fuels starting from CO 2 and green electricity. This can be done by exploiting the non-equilibrium properties of gaseous electrical discharges. Discharges, in addition, can be switched on and off quickly, thus being suitable to be coupled with an intermittent energy source. In this study, we have used a nanosecond pulsed discharge to dissociate CO 2 and CH 4 in a 1:1 mixture at atmospheric pressure, and compared our results with literature data obtained by other discharges. The main products are CO, H 2 , C 2 H 2 , water and solid carbon. We estimate an energy efficiency of 40% for syngas (CO and H 2 ) production, higher if also other products are considered. Such values are among the highest compared to other discharges, and, although not very high on an absolute scale, are likely improvable along possible routes discussed in the paper and by coupling to the discharge a heterogeneous catalysis stage.

The catalytic effect of the electrode surface is investigated in a dielectric barrier discharge of CO 2 and CH 4 at atmospheric pressure. An increased production of carboxylic acids, especially formic acid, is observed for copper and nickel electrodes. This suggests the occurrence of CO 2 hydrogenation reactions on the metal surface.

We report that gas phase plasma-assisted non-oxidative methane coupling can lead to formation of ethylene as major product at ~20 % yield per pass. This is attained by using a nanosecond pulsed discharge (NPD) reactor, featuring rapid product quenching rates, (recyclable) hydrogen co-feeding (CH4:H2=1:1) and elevated pressures (5 bar) at which NPD is ignited.

In this work, we report on low energy cost methane conversion to ethylene in a hybrid plasmacatalytic reactor system. Methane is first converted to acetylene, reaching up to 23.5% yield per pass, by a nanosecond pulsed discharge (NPD), and subsequently, acetylene is hydrogenated to ethylene using a Pd-based catalyst, which is placed in the post-plasma zone. Overall, ethylene is formed as major product at 25.7% yield per pass, consuming 1642 kJ/molC2H4 , which is the lowest energy cost reported for plasma-assisted methane-to-ethylene conversion so far. The two-step process is carried out in a single reactor volume that aside from the discharge energy does not require any heat or hydrogen input since both are provided by methane cracking in the plasma zone itself.