(698d) Experimental Investigation of a High-Temperature Thermochemical Storage Reactor

The integration of thermal energy storage to concentrated solar power plants enables round-the-clock dispatchability of industrial process heat and/or electricity. State-of-the-art molten salt storage systems are typically limited to applications at below about 600°C. Higher operating temperatures are desirable for efficient power generation and for driving energy-intensive thermochemical processes such as minerals and metals extraction, cement manufacturing, and fuels production. In particular, thermochemical heat storage (TCS) employs reversible endothermic/exothermic reactions to store heat with relatively high energy density and for long-term and seasonal storage. In the case of gas-solid reactions, the reaction equilibrium can be adjusted by controlling pressure and/or temperature, therefore controlling the uptake/release of heat.

We report on the design, fabrication, and experimental investigation of a TCS reactor applied for gas-solid reactions. It consists of an Inconel tubular shell filled with a packed bed of the solid reactive material. A concentric inner porous ceramic tube allows the gaseous reactant and product to flow radially through the packed bed with minimal pressure drop. A numerical heat and mass transfer model of the reactor has been developed for elucidating rate-controlling mechanisms and determining optimal design parameters. Model validation is accomplished with experimental data obtained with a lab-scale reactor prototype using the reversible calcination/carbonation of limestone as a model reaction: CaCO₃ ↔ CaO + CO₂. Powder and granules of CaCO₃ containing MgO (42 wt%) as an inert stabilizer exhibited stable cyclic performance for up to 100 consecutive carbonation-calcination cycles performed in a thermogravimeter analyzer (TG). During the TG measurements, a pure CO₂ environment at atmospheric pressure was maintained while swinging the temperature between 825°C (carbonation onset) to 925°C (calcination onset). The reaction extent was 96.48% for powder but only 31.99% for granules because of sintering at 1260°C during manufacturing. Similarly, the TCS reactor setup is operated in a temperature-swing mode at ambient pressure, which is ensured by storing the evolved gas (CO₂) in an inflatable bag during calcination and measuring the inflow/outflow of CO₂. The gas atmosphere is controlled by repeated evacuation and re-filling of the setup, and further verified by gas chromatography. The reactor tube contains 0.37L of the reactive material and multiple thermocouples for measurement of the radial temperature distribution, which in turn validates the transient conduction-convection-radiation heat transfer model within the packed bed. The engineering details of the TCS reactor as well as model and experimental results will be presented.