The panorama of plasma-assisted non-oxidative methane reforming

Abstract

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.

Kinetic roles of vibrational excitation in RF plasma assisted methane pyrolysis

“…Natural gas resource thus became one of the most promising alternatives to oil as energy and chemical raw materials in the future and have received great attention in the fields of chemical and energy industry. Simultaneously, the sustainable process of converting natural gas into high value-added chemicals and fuels has been considered a challenge for the 21st century [2] .For traditional methane utilization, high temperature [3,4] and catalysts [5,6] are needed to activate methane molecules because of its high C -H bond energy (434 kJ/mol). This process thus has big challenges, high energy cost and easy deactivation of catalysts.…”

Non‐oxidative methane conversion in diffuse, filamentary, and spark regimes of nanosecond repetitively pulsed discharge with negative polarity

“…Other mechanisms, such as direct excitation from the ground state CH, were not well known. The Equations (11,12) are intermediate processes of the step-by-step electron dissociation as shown in Equation (6). It means the electron dissociation reactions are important in both diffuse discharge and filamentary discharge, but they are negligible in the spark discharge.…”

“…[37] Craggs et al [38] investigated atmospheric hydrogen spark discharge, and he also suggested the electron excitation appears to be the dominant mechanism rather than pure recombination or thermal excitation mechanism. Both electron dissociation and thermal pyrolysis are able to produce the H radicals as Equations (6,10). When the gas temperature is high enough, the H radicals become so reactive that they can extract H atoms from the hydrocarbons (Equation (17)) and thus trigger a radical chain reaction, which explains the high-conversion rate and hydrogen selectivity in the spark discharge.…”

“…Gao et al investigated the effects of voltage, frequency, pulse width, flow rate, and gap distance on the conversion performance, and achieved methane conversion rate up to 91.2% with an energy conversion efficiency of 44.3% in a microsecond pulsed spark discharge. Scapinello et al reviewed the conversion performances of all typical NTP technologies and found that the pulsed discharge had the closest performance of methane conversion and acetylene yield at same energy input than other NTP technologies. Lotfalipour et al found that the vibrational excitation is the main methane dissociation channel in NRP, which requires less energy than thermal dissociation.…”

“…In the past few decades, various atmospheric nonthermal plasma (NTP) technologies had been developed for direct methane conversion, including corona discharge, plasma jet, dielectric barrier discharge (DBD), pulsed discharge, spark discharge, gliding arc discharge, radiofrequency discharge, and microwave discharge. [4][5][6][7] The NTP technologies can generate high reactive species at relatively low temperature in favor of reducing the manufactory requirements and possibly enhancing the catalysis performance. [8,9] Among these NTP technologies, the pulsed discharge, especially the nanosecond repetitively plasma (NRP), has already shown promising results for methane conversion.…”