Enhanced solar radiation absorption by carbon dioxide in thermodynamic nonequilibrium: A computational study

Abstract

The conversion of carbon dioxide (CO2) into higher value products using solar energy can help fulfill the increasing need for fuels and chemicals while mitigating environmental emissions. Solar thermochemical conversion approaches rely on the absorption of radiative energy by a solid catalytic medium. The direct absorption of solar energy by CO2 before reaching the catalyst could lead to more efficient conversion. Greater solar photon absorption can be achieved through the use of an electrical discharge to drive CO2 to a low-temperature plasma state in which free electrons and heavy-species are in a state of Non-Local Thermodynamic-Equilibrium (NLTE). The absorption of solar energy by a plasma-enhanced solar reactor for atmospheric pressure CO2 conversion is analyzed via zero- and one-dimensional models. The models are comprised by the spectral radiative transfer equation (RTE) and energy conservation equations for electrons and heavy-species. Radiative properties are computed by a line-by-line radiative code and transport properties using kinetic theory. Simulation results for a representative 10 cm long solar-plasma reactor show up to 48% and 90% net-absorption of solar radiation by CO2 in NLTE at electron temperatures of 1 and 2 eV, respectively, and heavy-species temperatures of ~2000 K, common in solar thermochemical processes. In contrast, less than 0.2% of solar radiation is absorbed by CO2 in LTE under similar conditions. The results suggest that solar-plasma direct-receiver reactors, due to increased direct solar energy absorption, have the potential for greater efficiency CO2 decomposition processes.