| Chemical looping combustion(CLC), which has the characteristic of CO2 inherent separation, is a novel fossil fuel utilization technology. CLC has been viewed as one of the most promising techniques to address the global warming issue. Traditional one-stage combustion process is split into two separated reduction and oxidation reactions in CLC, thus realizing the transformation from gas composition separation process in traditional CO2 separation techniques to gas-solid separation process in CLC, and such changing contributes to decreasing the exergy loss in overall electric energy production process. Currently, interconnected fluidized bed is regarded as the most feasible type of the CLC reactor by most of CLC researchers. However, due to the complex multiphase reactive flows present in CLC reactors, it is difficult to study it in details by the costly experimental method. Numerical simulation provides a promising way to understand the hydrodynamics, chemical reaction, heat and mass transfer in the CLC reactors, which are very important for the rational design and optimal operation of the CLC reactors.Therefore, a 50 kWth CLC dual circulation fluidized bed reactors, which was developed by our research group, was firstly simulated by CPFD software Barracuda. The simulation results point out that this software was fairly suitable for investigating the operation characteristic variables, like solid circulation rate, of CLC reactors; the flow in the lower section of the reactor within a bubbling fluidization regime, while in the upper section of the reactor within a pneumatic transport regime. The solid circulation rate increased as the superficial gas velocity and the overall solid inventory increased. The solid inventory in FR increased as the overall solid inventory increased, while decreased along with the increasing of the superficial gas velocity.For improving the accuracy of the simulation, the multiphase flow drag model and heterogeneous reaction model was studied.By taking the solid volume fraction gradient as an independent variable for modifying the drag force calculated from the traditional drag correlation, a new inter-phase drag model in which heterogeneous characteristics in fast fluidized beds be considered in CFD simulation was proposed in the present work. A CFD simulation for a two dimensional riser was conducted for validating the proposed drag model. The simulation results were in good agreement with the experimental measurements. Many experimental conclusions(e.g. the S-shaped distribution for axial voidage, the distribution change properties and the constant solid mass flux along with the solids inventory changing) were reproduced in the CFD simulation, which reveals that the new model was superior to the conventional model in the simulation of fast fluidized bed.Based on the thermogravimetric analyzer, the reaction kinetics model of hematite reduced by CO was investigated. The affection of the type of reactor(TGA and batch fluidized bed reactor) on the test of the reaction kinetics of OC was also studied. In the TGA experiment, the first reduction stage was controlled by the gas diffusion in the boundary layer on the particle surface, while the second reduction stage was the chemical reaction control. The reaction order for the reduction in the second stage was determined as 1.5, the apparent activation energy is 110.75 kJ/mol and the pre-exponential factor is 88.55 m4.5mol-1.5s-1. The experimental results in the fluidized bed reactor reveal that the first reduction stage was controlled by aeration rate of reduction gas, and the second reduction stage was the chemical reaction control. Therefore, the kinetics model obtained from the TGA experiment is applicable for CFD simulation to describe the process in the batch fluidized bed reactor. |
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