(345d) Design of a Combined Moving-Fluidized Bed Oxidation Reactor for High Temperature Solid-State Thermochemical Energy Storage

The increased interest in harvesting solar energy has led to the development of multiple Thermochemical Energy Storage (TCES) materials that enable the accumulation of solar energy in the form of chemical energy. In addition to the TCES materials, suitable reactors for the charging and discharging processes must be developed. The use of Concentrating Solar Power (CSP) enables the use of heat transfer fluids (HTF) at high temperatures, which then can be used to run thermal powerplants or can also be used to charge TCES materials by means of endothermic reduction reactions for storage of thermal energy into chemical energy. In the system studied, a high temperature redox material is charged via the endothermic reduction. Moreover, the discharge of TCES involves a high temperature exothermic oxidation reaction, returning the stored chemical energy in the form of thermal energy. Higher temperature discharge allows for higher power generation efficiency, at the expense of more difficult process conditions. Presented here is the latest in our development of a reactor for high temperature (900-1000 °C) oxidation of solid TCES redox particles. The design presented here consists of a top moving bed, a middle fluidized bed with heat exchanger and a bottom moving bed. The top and bottom moving beds will serve as heat recuperation devices, and the middle fluidized bed with heat exchanger will be used to enhance mass and heat transfer properties between particles, fluidizing gas, and heat transfer fluid. This concept is convenient in the sense that both gases and solids are cold at both the inlet and outlet, eliminating the need to handle hot materials. The fluidized bed is where the brunt oxidation reaction takes place and the heat of reaction is transferred to the HTF. A heat output of 1 kW is used a basis to size the oxidation reactor/heat exchanger and top and bottom moving beds. The design process consisted of flowrate estimations based on heat duty and sensible heat recuperation, reactor volume estimations based on the solid’s extent of reaction and mean residence time, and heat transfer estimations based on particle properties, reactor material, heat exchanger configuration, and heat transfer fluid properties. The calculations show that the fluidized bed reactor with a finned narrow flat plate heat exchanger could deliver a heat transfer fluid exit temperature of around 1000 °C. Additionally, prototype testing shows that a spouting fluidization regime in the reaction zone enhances particle movement by recirculation and enables the transition from moving to fluidized to moving bed regimes.