Validation of the thermo-chemical approach to modelling of the CO2 conversion in sub-atmospheric pressure microwave gas discharges

The CO2→CO+½O2 conversion experiment [F~A~D’Isa et al. 2020 Plasma Sources Sci. Technol. 29 105009] has been compared with thermo-chemical calculations. The experiment is a 2.45 GHz plasma torch with straight channel in the effluent. The 1.5D model of the CO2/CO/O2/O/C mixture without turbulent transport has been applied with plasma acting only as prescribed heat source. The parameter range covered is specific energy input (SEI) 0.3-5 eV/molecule at pressure p=0.9 bar, and SEI=0.6-2 eV/molecule at p=0.5, 0.2 bar. The calculated conversion χ is always close to experimental values. At the same time, the calculated temperatures T deviate significantly from the experiment, especially for p=0.2 bar. The calculated T were also found to be sensitive with respect to the uncertain model parameters, but χ is not sensitive. According to the model the net conversion is driven mainly by the radial diffusion of CO and O from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The net conversion in the model is driven to large extent by the radial diffusion of CO and O from the hot core toward the wall and steep radial temperature gradients. The main factor which reduces the energy efficiency is re-oxidation of CO at the edge of the hot plasma region and downstream. The 1.5D approximation applied has the principle limitation that the impact of the realistic bulk flow field on the chemical process could not be studied. Hence the results must be considered as preliminary and have to be confirmed with a more elaborate and accurate model of the vortex stabilized flows inside the reactor.