Computational modeling of CO₂ conversion by a solar-enhanced microwave plasma reactor

Abstract

The use of renewable energy to convert carbon dioxide (CO2) into higher-value products can help meet the demand for fuels and chemicals while reducing CO2 emissions. Solar-Enhanced Microwave Plasma (SEMP) CO2 conversion aims to combine the scalability and sustainability of solar thermochemical methods with the high efficiency and continuous operation of plasmachemical approaches. A computational study of a built SEMP reactor operating with up to 1250 W of microwave power together with up to 525 W of incident solar power at atmospheric pressure is presented. The study is based on a fully-coupled 2D computational model comprising the description of fluid flow, heat transfer, Ar-CO2 chemical kinetics, energy conservation for electrons and heavy-species, electrostatics, and radiative transport in participating media through the discharge tube, together with the description of the microwave electromagnetic field through the waveguide and the discharge tube. Numerical simulations reveal that the plasma is concentrated near the location of incident microwave energy, which is aligned with the radiation focal point, and that CO2 decomposition is highest in that region. The incident solar radiation flux leads to more uniform distributions of heavy-species temperature with moderately greater values throughout most of the discharge tube. Modelling results show that, at 700 W of electric power, conversion efficiency increases from 6.8% to 10.0% with increasing solar power from 0 to 525 W, in good agreement with the experimental findings of 6.4% to 9.2%. The enhanced process performance is a consequence of the greater power density of the microwave plasma due to the absorption of solar radiation.

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

“…In particular, the tip of the coaxial resonator at the bottom is neglected. The origin of the coordinate system (z, r) corresponds to the centre of the heat source where it assumes its peak value, equation (11) below. This point is located on the discharge tube axis close to the middle of the cylindrical resonator.…”

“…However, there are indications that for the experiments of D'Isa et al [4,22] a shape less peaked in the middle could be more appropriate. Therefore, while in the reference calculations (11) is always applied, several cases were repeated with flat z-profile to demonstrate that the final solution is not strongly affected by this assumption, see section 4.1. Parameters α r , α z in (11) are reconstructed from the 'plasma diameter' d pl and 'plasma length' l pl reported in [4], figures 9 and 11 there respectively.…”

“…Therefore, while in the reference calculations (11) is always applied, several cases were repeated with flat z-profile to demonstrate that the final solution is not strongly affected by this assumption, see section 4.1. Parameters α r , α z in (11) are reconstructed from the 'plasma diameter' d pl and 'plasma length' l pl reported in [4], figures 9 and 11 there respectively. The plasma radius d pl /2 is defined in [4,22] as the radius where the radiation intensity is 15% of its maximum.…”

“…In this table, the column 'reference' is the solution obtained with the reference model of section 3.1. The next two columns show the results of using for Q (z, r) other types of spatial shape than function (11). The column 'flat Q(z)' stands for the heat source described by the function:…”

“…2D modelling studies of plasmas sustained by radio-frequency waves including chemical reactions have been done for N 2 [9,10]. Recently, a fully selfconsistent 2D modelling of the CO 2 plasma chemical conversion in MW reactors has been published [11], however, only for CO 2 diluted in Ar in the proportion 1:7. Most relevant for practical applications is the conversion at atmospheric pressure which does not require vacuum equipment.…”