(455c) Low-Temperature CO2 Capture from Flue Gas Using Alkali Metal Carbonate-Based Solid Sorbents in a Magnetically Stabilized Fluidized Bed Reactor

Although renewable resources are being increasingly used for power generation, fossil fuels such as natural gas will continue to play a significant role in the foreseeable future. However, burning fossil fuels emits carbon dioxide (CO₂) into the atmosphere, a significant contributor to greenhouse gas emissions. CO₂ levels have risen rapidly since the Industrial Revolution due to growing anthropogenic activities, resulting in unwarranted climate change. This is one of the biggest problems the world is currently facing, which calls for an urgent need to develop reliable, scalable, and cost-effective CO₂ capture technologies for Natural Gas Combined Cycle (NGCC) power plants.

There are three main approaches to capturing CO₂: solvent-based, sorbent-based, and membrane-based. Currently, solvent-based technologies are considered state-of-the-art for capturing carbon emissions from NGCC power plants. However, sorbent-based technologies have the potential to be better. This method involves using a solid material to adsorb and then release the CO₂, either through temperature or pressure changes. Compared to solvent-based methods, sorbent-based technologies are less energy-intensive, can operate at a broader range of temperatures, and are easier to handle and dispose of, resulting in less environmental impact. Alkali metal carbonate-based solid sorbents are commonly used for post-combustion CO₂ capture due to their low cost and high CO₂ sorption capabilities. However, there are challenges with using this technology, such as pressure drops across the bed, and inefficient contact between CO₂ and the sorbent material. With the development of more efficient and stable sorbents along with a technology that enables good mass and heat transfer characteristics through efficient contact of gas and solids, would allow the solid-sorbent technology to compete with solvent-based methods. A possible solution is to use a magnetically stabilized fluidized bed reactor with a magnetically susceptible bed, which improves CO₂ uptake by providing efficient heat and mass transfer through bubble-less fluidization.

This study explores using potassium carbonate (K₂CO₃)-based solid sorbent for capturing carbon dioxide from simulated flue gas streams. The gas contains 4% CO₂, 10% water, and is balanced with an inert gas. The sorbent picks up CO₂ from 50°C to 90°C and can be regenerated at mild temperatures of 100°C to 150°C. The active material, K₂CO₃, is impregnated into the pores of metal oxide-based supports such as ZrO₂, Al₂O₃, and MgO, as well as other supports such as activated carbon and zeolite 13X. This helps to disperse the active material and increase the surface area available for CO₂ sorption. The impregnated sorbents are characterized using techniques like x-ray diffraction (XRD), nitrogen physisorption, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Recyclability and stability tests are conducted using a Thermogravimetric Analyzer (TGA) with consecutive cycles of adsorption and regeneration enabled through temperature swings. The sorbents are also tested at larger scales using fixed bed reactor experiments, and the CO₂ breakthrough analyses are carried out to determine their performance. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to analyze how CO₂ and H₂O interact with the sorbent during sorption reactions, revealing insights into the reaction mechanism. After cyclic testing, the sample is further characterized to study the stability and performance of the material for long-term applications.

To enhance the heat and mass transfer characteristics of the sorption reaction, a magnetically stabilized fluidized bed (MSFB) is tested by generating a uniform magnetic field in the bed using a Helmholtz coil. The effect of magnetization on the pressure drop across the bed is studied. Different magnetically susceptible bed materials are utilized, and lower pressure drops are observed when magnetic fields are applied as compared to cases in the absence of a magnetic field. CFD simulation incorporating magnetism is studied to investigate the hydrodynamics and heat transfer inside the bed with and without a magnetic field. K₂CO₃ sorbent designed in the previous section is then impregnated onto different magnetic materials and tested for its CO₂ capture performance in a fixed bed reactor. The optimized magnetic sorbent is then tested in a hot MSFB setup under reaction conditions and the results are compared with the non-magnetic K₂CO₃ sorbent to validate the advantage of using an MSFB. The next phase in the project is to de-risk commercialization efforts by designing and fabricating a bench scale unit to study the different modes of operation: fixed bed, bubbling fluidized bed, and magnetically stabilized bed to quantify and validate the performance of MSFB.

Besides the efforts to improve gas-solid contact in an MSFB, future work on material development would focus on studying the kinetics of adsorption and regeneration steps to facilitate a better understanding of the mechanism. Promoters would be added to improve the performance of the sorbent, and the effect of mesoporous supports with high surface areas like mesoporous alumina and SBA-15 would be studied. The effect of impurities in flue gas like SO_x and NO_x on solid sorbent and hence CO₂ uptake will be investigated.

This research work would contribute scientific and technical knowledge to the carbon capture community through a deeper understanding of solid sorbent development and using novel methods like a magnetically stabilized fluidized bed for improved capture efficiency and overall CO₂ uptake. Addressing climate change by developing low-cost carbon capture technologies is the need of the hour, and this work is a promising step toward achieving this goal by capturing CO₂ from natural gas-based power plants using inexpensive, readily available materials.