(365e) Fluidized and Inductively Heated Bed for Enhanced CO2 Capture Using Solid Sorbents

Solid CO2 sorbents have potential as an energy efficient, economic, and scalable platform for point source and direct air capture (DAC). Many such sorbents use Temperature Swing Adsorption (TSA) processes, and they are conventionally performed in fixed-bed gas-solid contactors. In an ideal absorption step, mass transfer would be enhanced, particularly with dilute CO2 steams. Furthermore, the temperature would be ideally stabilized to a set value with minimal temperature gradients within the system — an important factor considering the scale needed for DAC. In an ideal regeneration step, the reactor bed would be heated quickly, volumetrically, efficiently, and uniformly to minimize the fraction of time and energy spent for regeneration. However, in many implementations, scaled contactor systems operate in less-than-ideal conditions and feature sub-optimal mass and heat transfer properties.

Proposals to overcome heat transfer limitations of fixed-bed gas-solid contactors include alternative bed configurations, structured sorbents, and various heating approaches. We propose a novel reactor configuration comprising solid sorbent particles in a porous three-dimensional electrically conductive baffle that can be inductively heated to enable internal volumetric heating control. Absorption is performed with sorbents suspended in a fluidized bed (i.e., gas flow is above the minimum fluidization velocity) and regeneration is performed with the sorbents in fixed bed mode (i.e., gas flow is below the minimum fluidization velocity). Internal heating of the baffle via magnetic induction is used to control the bed temperature during absorption and enable rapid heating during regeneration. These concepts build on prior concepts developed in our group, where we recently proposed a metamaterial reactor in which a helical coil is used to inductively heat a volumetric, electrically conducting open cell baffle. When a high frequency alternating current is driven through the coil, an alternating magnetic field within the coil is produced that excites eddy currents throughout the cross section of the baffle that dissipate into heat. When the electromagnetic properties of the baffle are co-designed with power electronics, this heat is generated with high efficiency.

In terms of sorbent selection, we utilize a sorbent of potassium carbonate impregnated onto aluminosilicate support for both high surface area and attrition resistance. We choose potassium carbonate as it is a common low temperature sorbent that has the highest CO2 capture capacity out of alkali metal carbonate-based sorbents.

We validate the benefits of our proposed CO2 capture system with an experimental demonstration and modeling, beginning with a focus on fluidization. Hydrodynamic studies of fluidized beds with fixed internals have been mostly limited to vertical and horizontal inserts and no work has been found to focus on open cell foams. Some compound internals, which combine features of both horizontal and vertical baffles, have been studied and are used industrially in fluid catalytic cracking applications. We perform hydrodynamic studies using Geldart B particles with open cell foam and other compound internals by pressure drop measurements with and without compound internals at varying feed flow rates. These studies not only extend understanding of compound internals in fluidized beds, but also guide our experimental selection of feed flow rates such that bubbling fluidization occurs to promote mass and heat transfer without a significant reduction of residence time.

We experimentally run cycles of CO2 adsorption and regeneration in our metamaterial reactor at different CO2 concentrations. Adsorption occurs in fluidized bed mode while regeneration occurs in fixed bed mode, and both include inductive heating of the compound internal susceptor. Compared to other studies of inductively heated CO2 capture, we combine knowledge of power electronics and the impedance of our susceptor to set the alternating current frequency for optimal plug to susceptor power transfer, enabling high energy efficiency operation. We also experimentally measure the temperature axially and radially to verify enhanced heat transfer and build a model to understand scalability. This 1D multiphysics model integrates electromagnetics and heat-transfer and shows both the fast heat transfer rates of our system and enhanced temperature uniformity at various scales throughout adsorption and regeneration. We also model the kinetics of our fluidized bed reactor as a CSTR with sorbent activity decay using the deactivation model.

Research Interests:

Electrification, process intensification, fluidization, inductive heating, CO2 capture