(345c) Superlocal Equilibrium in Low Temperature Plasma Reactors

Prominent methods of harvesting renewable energy involve the production of electricity for societal utilization. Compared to petroleum, which is a source of both heat and chemicals, renewable energy sources only provide work. In a sustainable society, electricity must be used to act upon material flows to increase the free energy of the compounds to make platform chemicals from which commodity products can be synthesized. Ideally, the synthesis of platform chemicals would involve removing carbon dioxide from the atmosphere. Low temperature plasma (LTP), which is a partially ionized gas in which extremely hot electrons with a temperature in the range from 10,000 to 100,000 K are immersed in a molecular background at 300 to 1000 K, comprises an open driven system capable of causing chemical transformations that proceed away from the local equilibrium state at the background temperature. Thus LTP is capable of increasing the specific free energy of mass flows. If one cannot use the idea of local equilibrium at the background temperature, then how can one predict the direction of a chemical reaction occurring in LTP? In this presentation, a proposed idea of superlocal equilibrium will be presented that can be used to make such predictions. The basic idea is that thermal equilibrium is local in both space and chemical species. Experimental evidence will be presented that in LTP, chemical reactions proceed towards a steady state that is entirely different from the local equilibrium state at the background temperature. The superlocal state depends on the background temperature, total pressure, electron temperature, plasma density, and relative amounts of different elements in the system; but is independent of species mol fractions at the inlet. Specifically, at low background temperature < 1000 K, it will be shown that the effluent from a plasma containing Ar + CO₂ approaches a steady state comprised of mostly CO + O₂, independent of whether CO₂ or CO+1/2O₂ were fed into the reactor. The experimental results were used to calculate the chemical potential of hot electrons present LTP at the conditions used in this work.