Non-Conventional Heating Methods for CO2 Regeneration

Abstract

Carbon dioxide makes a significant contribution to global warming and climate change. Microwave heating offers a promising approach for efficient desorption in carbon capture. Microwave-assisted desorption reduces the desorption time and selectively heats the adsorbent material, resulting in better regeneration and reducing the system's total energy usage.

This thesis considers alternative heating methods to the conventional heating for the temperature swing adsorption process to desorb CO2. The two non-convective heating sources considered here are the microwave heating and the infrared radiation heating. Both of these methods are electromagnetic heating methods, eliminating the need for heating mediums. The microwave heating method heats the sorbent material to desorb the CO2. While the IR methods target the CO2 molecules to excite them, they gain enough kinetic energy to overcome the activation energy.

The COMSOL Multiphysics simulation study described here optimized the microwave cavity design for lab-pilot scale, revealing that optimal dimensions can absorb over 98% of input power. Positioning the waveguide at the cavity's middle height proved most effective, not only absorbing 1.6% more power but, possibly more importantly, providing uniform heating distribution. This configuration is ideal for fluidized bed reactors, potentially enhancing energy efficiency in carbon capture processes.

In addition, this work presents novel industrial-scale reactor designs for continuous microwave desorption of CO2, filling a critical gap in current microwave heating systems. Traditional microwave technologies have limited efficiency and efficacy in gas desorption operations, including CO2 removal. This study describes innovative reactor designs that use microwave radiation's unique heating capabilities to improve desorption efficiency and selectivity. The study assures optimum microwave power use by optimizing reactor size and configurations using numerical modeling, reducing energy consumption while attaining the intended outcomes. Three unique reactor designs are offered to outperform and save energy compared to current desorption procedures by allowing for continuous operation. The reactors combine sorbent pellets from numerous adsorption reactors into a single desorption unit, eliminating the constraints of classic paired adsorption-desorption systems and increasing production efficiency. The research looks at both horizontal and vertical continuous microwave reactor designs. The horizontal design includes a modified conveyor belt system with cleated belts and Teflon sidewalls, which are ideal for gas desorption. In contrast, the vertical design employs a cascade gate opening mechanism, allowing for precise control over microwave power and exposure time in each tray, maximizing desorption kinetics and efficacy. The study's findings offer valuable insights into designing and optimizing microwave reactors for CO2 desorption, demonstrating microwave technology's potential to revolutionize desorption processes and progress the sector.

Moreover, this work employs numerical analysis to investigate temperature distribution during microwave-assisted CO2 desorption with zeolite 13X. The model includes the electromagnetic frequency domain, heat, and mass transfer, and investigates the effects of microwave forward power, purge flow rate, and adding MW-CNT nanoparticles to the sorbent material. The results reveal that increased microwave power accelerates heating and desorption rates, whereas lower power causes steady temperature rises. Adding 2% MW-CNT nanoparticles improves the energy absorbed by the sorbent bed by 14% due to the improvement of the dielectric characteristics. Lower flow rates minimize convective heat loss, resulting in a more uniform temperature distribution. These findings offer important insights into enhancing microwave-assisted CO2 desorption, emphasizing the significance of power levels, flow rates, and nanoparticle additions in increasing CO2 desorption efficiency.

Lastly, this work examines the microwave-assisted regeneration process using a packed-bed reactor under direct air capture (DAC) application. For the regeneration process, a commercial Lewatit VP OC 1065 (Lewatit) was selected as the sorbent, and microwave ovens were used as the heating source. This study examines the influence of microwave initial power on CO2 regeneration kinetics, regeneration efficiency, and energy consumption since no study has been performed on this sorbent for this analysis. The regeneration temperatures were varied from 40 °C to 70 °C, and the microwave power was changed from 10 W to 30 W to investigate their effect on the CO2 desorption characteristic. This study also investigates the effect of multiple microwave on/off cycles on the regeneration time and efficiency. The results show that the heating time can be reduced to 1490 seconds under microwave heating (30 W and 70 °C) compared to 5154 seconds under a conventional heating system. The results also show that the energy consumption for a 70% regeneration is lower compared to a 100% regeneration under various microwave conditions. The results illustrate that the energy required for CO2 regeneration can be reduced to around 9.6 MJ/kg CO2 if the regeneration temperatures are 40 °C and 45 °C. It was found that CO2 regeneration at a near-ambient temperature is possible using a microwave-assisted regeneration system. For the case of 70 °C, the regeneration time can be reduced by 16% with the case of two on/off microwave cycles compared to only one continuous microwave on cycle. For a 45 °C desorption temperature with the same microwave on time, the multiple on/off cycles increase the regression efficiency by about 10% compared to one continuous microwave on.