Analysis of reactivity and energy efficiency of methane conversion through non thermal plasmas

The non oxidative conversion of natural gas by non thermal plasmas offers a promising route to produce higher value products, such as hydrogen and C-2 hydrocarbons, at a low energetic cost. In this work we present the results of a theoretical research aimed to study the reactivity of methane treated with atmospheric plasmas. The effects of various process parameters on energy efficiency, conversion, and selectivity to acetylene, ethylene, and ethane have been investigated for different reactor configurations. For this purpose two different models were used. The first consists of a detailed kinetic model and a simplified reactor description, based on the assumption that the plasma volume is homogeneous and adiabatic. It is apt to describe stationary discharges such as microwave and radio frequency plasmas. The second is a time dependent micro-discharge model based on similar assumptions that is suitable to study pulsed discharges. The results evidence a strong dependence between the energy efficiency of the discharge and the specific energy supplied. A kinetic analysis was performed in order to understand the main reaction paths activated by the plasma discharge. One of the main findings of this study is that the temperature evolution in the plasma volume plays a key role in determining the system reactivity, in particular in the case of atmospheric pulsed discharges. In fact, though such plasmas are often considered non-thermal, our calculations show that in the few milliseconds that follow a pulsed discharge the local temperature rapidly rises up to the point where endothermic reactions, such as homolytic scissions, get activated. Our simulations show that the inception of these reactions is accompanied by an increase of the process energy efficiency. Furthermore, since in micro-discharges transport phenomena can be important, fluid dynamic simulations of pulsed corona plasmas have been performed using a simplified kinetic scheme. Such computations have been useful to understand the main physical features of micro-discharges. Among the conclusions of this study are reported a set of guidelines that may be useful to improve the energy efficiency of plasma methane conversion processes. (C) 2012 Elsevier Ltd. All rights reserved.