Computational Fluid Dynamics Modeling and Simulations of Fluidized Bed Reactors for Chemical Looping Combustion
Date of Award
Doctor of Philosophy (PhD)
Chemical looping combustion (CLC) is a next generation combustion technology that shows great promise as a solution for the need of high-efficiency low-cost carbon capture from fossil fueled power plants. To realize this technology on an industrial scale, the development of high-fidelity simulations is a necessary step to develop a thorough understanding of the CLC process. Although there have been a number of experimental studies on CLC in recent years, CFD simulations have been limited in the literature.
In this dissertation, reacting flow simulations of a CLC reactor are developed using the Eulerian approach based on a laboratory-scale experiment of a dual fluidized bed CLC system. The salient features of the fluidization behavior in the air reactor and fuel reactor beds representing a riser and a bubbling bed respectively are accurately captured in the simulation. This work is one of the first 3-D simulations of a complete circulating dual fluidized bed system; it highlights the importance of conducting 3-D simulations of CLC systems and the need for more accurate empirical reaction rate data for future CLC simulations.
Simulations of the multiphase flow with chemical reactions in a spouted bed fuel reactor for coal-direct CLC are performed based on the Lagrangian particle tracking approach. The Discrete Element Method (DEM) provides the means for tracking the motion of individual metal oxide particles in the CLC system as they react with the fuel and is coupled with CFD for capturing the solid-gas multiphase hydrodynamics. The overall results of the coupled CFD-DEM simulations using Fe-based oxygen carriers reacting with gaseous CH4 demonstrate that chemical reactions have been successfully incorporated into the CFD-DEM approach. The simulations show a strong dependence of the fluidization performance of the fuel reactor on the density of bed material and provide important insight into selecting the right oxygen carrier for the enhanced performance.
Given the high computing cost of CFD-DEM, it is necessary to develop a scaling methodology based on the principles of dynamic similarity that can be applied to expand the scope of this approach to larger CLC systems up to the industrial scale. A new scaling methodology based on the terminal velocity is proposed for spouted fluidized beds. Simulations of a laboratory-scale spouted fluidized bed are used to characterize the performance of the new scaling law in comparison with existing scaling laws in the literature. It is shown that the new model improves the accuracy of the simulation results compared to the other scaling methodologies while also providing the largest reduction in the number of particles and in turn in the computing cost.
CFD-DEM simulations are conducted of the binary particle bed associated with a coal-direct CLC system consisting of coal (represented by plastic beads) and oxygen carrier particles and validated against an experimental riser-based carbon stripper. The simulation results of the particle behavior and the separation ratio of the particles are in excellent agreement with the experiment. A credible simulation of a binary particle bed is of particular importance for understanding the details of the fluidization behavior; the baseline simulation established in this work can be used as a tool for designing and optimizing the performance of such systems.
The simulations conducted in this dissertation provide a strong foundation for future simulations of CD-CLC systems using solid coal as fuel, considering the additional complexities associated with the changing density and diameter of the coal particles as devolatilization and gasification process occur. A complete reacting flow simulation in the CFD-DEM framework will be crucial for the successful deployment of CD-CLC technology from the laboratory scale to pilot and industrial scale projects.
Ramesh K. Agarwal
Richard L. Axelbaum, Kenneth L. Jerina, Mark J. Meacham, David A. Peters
Permanent URL: https://doi.org/10.7936/K7959FZ3