Plasma Chemistry Modelling for CO2 and CH4 Conversion in Various Plasma Types

Datum: 27 april 2020

Locatie: ONLINE verdediging - - - - -

Tijdstip: 10 uur

Organisatie / co-organisatie: Departement Chemie

Promovendus: Stijn Heijkers

Promotor: Annemie Bogaerts

Korte beschrijving: ONLINE Doctoraatsverdediging Stijn Heijkers - Faculteit Wetenschappen, Departement Chemie

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The ever increasing atmospheric CO2 concentrations lead to accelerated global warming. Therefore, we should reduce our greenhouse gas emission drastically by shifting towards renewable energy and by storing this (fluctuating) energy through simultaneously converting greenhouse gases into fuels or value-added chemicals. One emerging technology for this purpose is plasma technology. Plasma chemical kinetics modelling is very suitable to gain more knowledge in the underlying plasma processes, needed for further optimization. Therefore, in this PhD thesis we focus on chemical kinetic modelling of CO2 and CH4 in different plasma reactors.

We studied the most important processes in pure CO2, CO2/CH4 and CO2/N2 mixtures in a gliding arc plasmatron (GAP). The GAP shows the advantage of intense vibrational excitation at atmospheric pressure, beneficial for industrial implementation. However, the CO2 dissociation mainly occurs from the lowest vibrational levels, due to the high temperature in the arc (3000 K), so that the vibrational-translational non-equilibrium is negligible. Adding CH4 enhances the CO2 conversion, and the overall performance in terms of energy cost / energy efficiency reaches values above the required efficiency target, due to the reaction of CO2 with H atoms, formed upon dissociation of CH4. The addition of N2 causes the formation of NO and NO2. However, the NOx concentrations reached are somewhat too low to be valuable for N2 fixation.

Pure CO2 splitting was also studied in a nanosecond repetitively pulsed (NRP) discharge, which shows promising results by stimulating vibrational excitation. More than 20 % of all CO2 dissociation occurs from the highest asymmetric stretch mode levels. However, in between the pulses, fresh gas entering the plasma, VT relaxation and recombination reactions limit the overall conversion and energy efficiency.

Finally, we studied CH4 conversion in different plasma reactors, i.e., dielectric barrier discharge (DBD), microwave (MW) plasma and GAP. Higher temperatures, especially in the GAP but also in atmospheric pressure MW plasmas, result in more CH4 conversion, and in neutral dissociation and dehydrogenation of the hydrocarbons created, forming especially C2H2 and H2, and (some) C2H4. Low temperature plasmas, such as DBD and reduced pressure MW plasmas, result in more electron impact dissociation and three-body recombination, creating more saturated compounds, i.e., mainly C2H6, but also higher hydrocarbons..

Overall, the results of this thesis give valuable insight in the possibilities and limitations of plasma-based CO2 and CH4 conversion.