Optimization of the Loop Seal in the Counter-Current Reactor of the Dual Circulating Fluidized Bed System for Chemical Looping Processes

The dual circulating fluidized bed (DCFB) system for chemical looping processes includes an internal solids loop on the fuel reactor (FR) side. The particles elutriated from the fuel reactor are separated from the gas stream in a cyclone and directed back into the fuel reactor through a fluidized J-type loop seal, the so-called internal loop seal (ILS). The present work seeks to improve operation stability of this internal solids loop by modification of the design. The study is carried out in a cold flow model specially built for the fluid-dynamic characterization of the 120 kW chemical looping pilot plant unit at the Vienna University of Technology. The cold flow rig is provided with a system for determination of solids residence time distribution using ferromagnetic tracer particles. Pressure profiles and residence time distribution curves are used for the evaluation of the proposed geometry changes. The increase in the loop seal size has avoided the seal failure at low internal solids circulation rates. The back-flow of particles from the bottom of the reactor to the loop seal via the return leg was corrected. The slugging in the downcomer of the ILS has been eliminated by using a larger downcomer diameter. By shifting the ILS upward high pressures at the discharge side are avoided and a smaller amount of particles need to be accumulated in the downcomer and are available for circulation. Overloading of bed material at the bottom of the FR seems to be avoided and particles concentration grows in the upper part of the reactor.