Mustafa Erguvan¹, Matt Atwood², Shahriar Amini¹

1. University of Alabama, Department of Mechanical Engineering, Tuscaloosa, AL, USA
2. AirCapture, LLC, CA, USA

Abstract

Over 190 countries have agreed to reduce greenhouse gas emissions as well as limit the global temperature increase to 2 ⁰C through the Kyoto Protocol and the Paris Agreement. To fulfill the requirements of this agreement, negative emission technologies will play an important role in the next decades. Direct Air capture (DAC) systems, which is defined simply as removal of CO₂ from the atmosphere, has been considered as one of the most promising ways to reduce and maintain the CO₂ concentration of the air. Although several experimental and numerical studies related to the DAC system have been performed, the main feasibility challenge for implementation of DAC systems is the high capital and energy costs of CO₂ capture.

There are two main DAC systems namely, high temperature aqueous solution and low temperature solid sorbent. While high temperature systems require over 800 ⁰C for the desorption process, 80-100 ⁰C could be sufficient using solid sorbent technology, which makes the low temperature system more advantageous since even waste heat from industrial plants can be used as a heat source.

As the reaction binding CO₂ to sorbents is exothermal in adsorption, the addition of thermal energy is required to regenerate the sorbent and capture the CO₂ during desorption. Conventionally used desorption systems include temperature swing adsorption (TSA), pressure swing adsorption (PSA) and vacuum swing adsorption (VSA) where desorption of CO₂ is normally done by changing the temperature and pressure (TVSA). This desorption step represents the majority of overall DAC system capital cost and energy demand, although an argument is often made that leveraging waste heat reduces desorption costs; however, this approach limits the market potential of DAC by requiring co-location with waste thermal heat sources and requires costly capital equipment associated with steam production, management and thermal recovery, while impacting overall system carbon lifecycle [1]. Therefore, approaches that can target desorption energy to the sorbent-CO₂ bond while limiting or avoiding the sensible heat loss associated with the contactor substrate and related DAC component parts can maximize second law efficiency leading to lower.

The use of microwave (MW) irradiation, replacing the conventional heating technique, can result in process intensification, faster scale up and energy costs reduction. The electromagnetic energy is directly converted into thermal energy inside the adsorbent, overcoming the heat transfer limitations. Several researchers have explored the use of MW for CO₂ desorption[2-7].

In this study, a microwave assisted regeneration technique has been used as a heat source to capture CO₂ with a solid contactor which was made of a titanium oxide monolith impregnated with polyethylenimine (PEI). A modified microwave oven was used, heating the contactor to temperatures below 130 °C.

Several parametric studies have been performed to investigate the adsorption and desorption processes by evaluating CO₂ concentration and temperature of the released gas. While microwave power varied between 336 W and 840 W, the volumetric flow rate of the inlet gas changed from 40 ml/min to 100 ml/min. Furthermore, repeated experiment cycles with the same conditions have also been performed to evaluate the adsorption and desorption stabilities of the contactor. The results showed that the microwave power has more effect than inlet gas flow rate on the desorption process.

The use of microwave has shown to be effective in CO₂ desorption of amino-polymer sorbents on the contactor. This addresses the heat transfer limitations occurring in conventional thermal desorption processes.

Acknowledgment

The funding from Alabama Transportation Institute (ATI) is acknowledged to carry out this work.

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