Design and Fluid Dynamic Analysis of a Three-Fluidized-Bed Reactor System for Chemical-Looping Hydrogen Generation

Chemical-looping hydrogen generation (CLHG) can produce hydrogen from fossils fuels with inherent separation of CO2. Iron oxide is a suitable oxygen carrier for this process. The CLHG process basically involves three reactors, a fuel reactor (FR), a steam reactor (SR), and an air reactor (AR). In the FR, the carbon-containing fuel gases react with hematite (Fe2O3). The product solids are wustite (FeO), and the product stream is a mixture of carbon dioxide and water vapor. After water condensation, pure carbon dioxide can be obtained. FeO then enters the SR and react with steam, giving the gas product hydrogen and the solid product magnetite (Fe3O4). In the AR, Fe3O4 is reoxidized to Fe3O3. Through this cycle, hydrogen is generated with inherent separation of CO2. In this article, a novel compact fluidized-bed fuel reactor is proposed. It integrates a bubbling fluidized bed and a riser to obtain full conversion of unreacted fuel gases through the thermodynamic equilibrium limit. Based on this fuel reactor, a cold-flow model of the three-fluidized-bed reactor system with a 50-kW CLHG design scheme was built to test the feasibility of this CLHG process. A series of tests with respect to solids circulation rate, gas leakage, and stability of long-term operation were performed by varying the inlet gas flow and total solids inventory. The results showed that the three-fluidized-bed reactor system can run steadily. The solids circulation rate could be changed in a wide range by adjusting the inlet gas flows. The gas leakage was associated with both the solids circulation rate and the pressure difference balanced by the downcomer. The system showed a stable pressure difference and solids circulation rate during a test of long-term operation.