(719b) Reactivity and Energy Efficiency of an Atmospheric Non Thermal Plasma Used for the Non Oxidative Conversion of Methane

The non oxidative conversion of natural gas by non thermal plasmas offers a promising route to produce higher value products, such as hydrogen and unsatured C2 hydrocarbons that can then be used as process gases. In its most widespread application, the pulsed corona, a pulsating high voltage is applied for very short times (tens of nanoseconds) between two electrodes and a transient plasma phase is generated. Its properties are the considerably high reaction rate, due to the elevated concentration of active species that can promote a fast radical mechanism, and the low average temperature of the gas phase, often not far from room temperature. Also, the product distribution is not limited by thermodynamics. However in order to be competitive on the industrial scale, the specific energy cost of the plasma process should be smaller than that of the competitive thermal processes.

In this work are presented the results of a theoretical research aimed to study the reactivity of methane exposed to a plasma discharge and to determine the operating conditions that maximize the energy efficiency. For this purpose several models were used in order to investigate different aspects of the process. First it was developed a 2D axial symmetric finite element computational code aimed to describe the plasma discharge. Then microkinetic simulations were performed in order to understand which are the main reaction paths activated by the plasma discharge. Finally the fluid dynamics of the plasma was studied with the purpose of estimating the importance of transport phenomena.

The simulations results are compared and validated with literature experimental results, which have demonstrated the possibility to achieve elevated acetylene yields with energy efficiencies higher than those of thermal processes. One of the main findings of this study is that the local temperature evolution in the plasma volume plays a key role in determining the system reactivity and its energy consumption. In fact, though such discharges are often considered non-thermal, our calculations show that the local temperature rapidly increases with the heat produced by radical recombination reactions, which is then dissipated by transport. This indicates that methane plasma conversion through pulsed corona plasmas can be interpreted as a pyrolytic process with extremely rapid heating and quenching times. Differently from pyrolysis however, the conversion of methane through plasma does not lead to the formation of soot, which is known to decrease significantly the energy efficiency of methane thermal conversion processes, since the rapid temperature decrease effectively blocks PAH formation pathways. The main results of this study are formulated in the form of a set of rational guidelines that can be used to optimize the energy efficiency of plasma methane conversion processes operating on large scales.