An experimental study on microwave-assisted direct air capture of CO2 under fluidized bed conditions

Abstract

In this study, a proof-of-concept of microwave-assisted regeneration process coupled with a fluidized bed reactor under DAC conditions is presented. A commercial zeolite 13X with an average particle size of 213.5 μm was selected as the sorbent. Desorption characteristics, desorption kinetics, energy consumption and productivity of the proposed system along with temperature distribution under fluidized bed conditions were investigated by varying the regeneration temperature between 33 °C and 100 °C and microwave (MW) initial powers from 4 W to 30 W. The thermal energy consumption to regenerate sorbents and to release a kilogram of CO₂ was reduced to as low as 4.8 MJ in the best-case scenario. The electrical energy consumption for this process associated with fluidization was found to be in the range of 1–17% of the total energy consumption. The productivity and activation energy ranges were found to be 0.14–0.18 kg CO₂ per kg_s per day and 13.46–25.11 kJ mol⁻¹, respectively. While the MW initial power was found to be significant for high-temperature conditions, its effect on the low-temperature regeneration conditions was relatively low. The results obtained using an infrared camera indicated that while most of the heat was accumulated in the middle of the reactor under packed bed conditions, the heat was expanded through the reactor on fluidization, which created a homogeneous temperature distribution and prevented overheating. The volumetric heating of the microwave provided excellent heat management for the CO₂ desorption process, which made the regeneration at near-room temperature possible. The only advantage of the high regeneration temperatures was the achievement of fast desorption with 20% increment in the productivity; however, energy consumption increased from tenfold to twentyfold when the regeneration temperature increased from 33 °C to 100 °C. A limited number of humid experiments also showed an increase in the energy consumption under humid conditions. This work is the first study reported in the literature that investigates microwave-coupled fluidized regeneration process for DAC applications. It was proved that microwave-assisted fluidization accelerates regeneration with a uniform temperature distribution under near-room-temperature conditions, even at 33 °C with 4 W microwave power, for direct air capture units.