Abstract
Hydrocarbon exhaust gases containing residual amounts of oxygen may pose challenges for their conversion into value added chemicals downstream, because oxygen may affect the process. This could be avoided by plasma treating the exhaust to convert hbox {O}_2 in presence of hydrocarbons into CO or hbox {CO}_2 on demand. The underlying reaction mechanisms of plasma conversion of hbox {O}_2 in the presence of hydrocarbons are analysed in a model experiment using a radio frequency atmospheric pressure helium plasma in a plug flow design with admixtures of hbox {O}_2 and of hbox {CH}_4. The plasma process is analysed with infrared absorption spectroscopy to monitor hbox {CH}_4 as well as the reaction products CO, hbox {CO}_2 and hbox {H}_2O. It is shown that the plasma reaction for oxygen (or methane removal) is triggered by the formation of oxygen atoms from hbox {O}_2 by electron. Oxygen atoms are efficiently converted into CO, hbox {CO}_2 and hbox {H}_2O with CO being an intermediate in that reaction sequence. However, at very high oxygen admixtures to the gas stream, the conversion efficiency saturates because electron induced hbox {O}_2 dissociation in the plasma seems to be counterbalanced by a reduction of the efficiency of electron heating at high admixtures of hbox {O}_2. The impact of a typical industrial manganese oxide catalyst is evaluated for methane conversion. It is shown that the conversion efficiency is enhanced by 15–20% already at temperatures of 430 K.
Highlights
Plasma oxidation of hydrocarbons is a well known process in the field of combustion or the removal of volatile organic compounds (VOC) [1, 2]
It is shown that the plasma reaction for oxygen is triggered by the formation of oxygen atoms from O2 by electron impact since the threshold energy for electron induced dissociation of oxygen molecules is smaller than that for methane
Oxygen atoms are efficiently converted into CO, CO2 and H2 O with CO being an intermediate in that reaction sequence
Summary
Plasma oxidation of hydrocarbons is a well known process in the field of combustion or the removal of volatile organic compounds (VOC) [1, 2]. Plasma-based removal of VOCs is a viable method to filter contaminated exhaust gas streams in industrial applications. Such VOC removal could be performed using filter technologies. VOC removal can be performed by thermal catalysis in packed bed reactors. The catalyst and the operation window (surface temperature, residence time) of such reactors has to be carefully chosen, because different hydrocarbons exhibit different reactivities in thermal catalysis and such systems are prone to catalyst poisoning by coke formation. Sensitive gas monitoring is required and to adjust and control the feed gas
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