(264f) Development and Testing of a Novel Combined Moving-Fluidized Bed Oxidation Reactor for Efficient High-Temperature Discharge of Thermochemical Energy Storage Particles

Thermochemical energy storage (TCES) using metal oxide particles has emerged as a promising technology to enable renewable energy dispatchability. The basic principle of TCES is to store energy as chemical bonds in metal oxide particles for short or long-term periods, which can then be released as heat when needed. In addition to providing energy storage, TCES offers several other advantages, including high energy density, decoupled charging and discharging steps for long-term storage, and delivery of high temperature heat (~1000 °C). However, there are inherent mass and heat transfer limitations associated with the oxidation reaction that takes place during the discharge process due to the interaction of solids and gases. While the design space for oxidation reactors offers numerous possibilities, including solid-gas contacting patterns like moving beds, fixed beds, and fluidized beds, there is currently no effective decoupled discharge reactor design that can accommodate a heat transfer fluid different than the process gas or fluidizing gas.

To enable effective extraction of high-temperature heat from TCES, a reactor concept has been proposed in which a counter-flow fluidized bed with heat exchanger is located between two moving beds. The operation of the reactor involves introducing reduced magnesium manganese oxide particles into the reactor, which undergo an exothermic oxidation reaction under air. The moving beds enable sensible heat exchange between gas and solids to completely preheat and cool down gas and particles, whereas the fluidized bed facilitates the oxidation reaction and heat extraction. The heat transfer fluid (HTF) destined for the high-temperature application passes through the heat exchanger, removing the reactive heat from the fluidized bed. The proposed design offers several advantages, including feeding and removal of particles at ambient temperature, enabling the integration of different HTFs such as air or sCO2, preventing the carriage of fines downstream with the HTF, and enhancing heat transfer between reactive particles and HTF.

Low order mathematical models were developed to aid in the design and to estimate the effect of operating parameters such as solids and gas flowrates, particle size, final extent of oxidation, and HTF type on the performance of the moving beds, fluidized bed, and heat exchanger plates. The design calculations indicate that a heat output between 0.8W to 1.2W from the particles undergoing oxidation can be produced with a particle flowrate between 1 to 2 g/s, achieving conversions between 70% to 90% using air as process gas. Furthermore, due to the counterflow nature of the operation, the top and bottom moving beds have the potential to completely preheat from 25 °C to 1000 °C the particles and the incoming process gas respectively in a short distance. A finned, narrow channel heat exchanger is employed to accommodate different heat transfer fluids and to maximize heat transfer from the particle bed to the HTF to achieve high temperatures at the outlet.

In addition, a 1kW reactor has been constructed using high-temperature resistant materials such as superalloy Hastelloy X for the heat exchangers and fluidized bed, and non-porous alumina for the top and bottom moving beds. The ceramic-metal joints are achieved by using a combination of ceramic based insulating putty and a high-temperature high-thermal expansion adhesive. The reactor is modular, enabling easy access for maintenance and potential scalability to test larger reactive beds. The system includes an "L-valve" pneumatic conveyance mechanism for particle flowrate control. Initial testing using N₂ as a heat transfer fluid indicates that HTF outlet temperatures between 800 °C to 1000 °C can be obtained in the heat exchangers from ~1kW of heat produced on the particle side. The reactor concept has the potential to be used in several applications such as concentrated solar power plants storing chemical energy during the day, and another potential application is in industrial processes, where excess heat from a process can be stored and reused at a later time, leading to energy savings and reduced carbon emissions.

The proposed reactor design has several advantages over existing TCES technologies. One major advantage is the decoupled charging and discharging steps for long-term storage. The proposed reactor design separates these two steps, allowing for greater flexibility and efficiency in energy storage and retrieval. The use of a counter-flow fluidized bed with a heat exchanger also allows for the integration of different heat transfer fluids such as air or supercritical CO₂. This makes the system adaptable to different types of applications and allows for the optimization of heat transfer and energy efficiency. Additionally, the use of a narrow-channel heat exchanger with fins maximizes heat transfer between the particle bed and the HTF, leading to higher temperatures at the outlet.

In conclusion, the presented reactor design for TCES using metal oxide particles represents a potential advancement in the field of energy storage. The implementation of the system in various thermal applications is enhanced by the decoupled charging and discharging steps, integration of different HTFs, and the ability to feed and remove solids and gas at ambient temperature. With further development and refinement, this reactor concept has the potential to advance the way we store and utilize energy from renewable sources.