CFD Simulation of Chemical Looping Combustion System
AuthorUddin, Md Helal
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Emerging technologies for greenhouse-gas mitigation have assumed growing importance due to the imminent threat of climate change. The American Clean Energy Security Act and the American Power Act project that about 30% of fossil-fuel-based electricity generation to come from power plants with carbon capture and sequestration (CCS) by 2040, rising to approximately 59% by 2050. Chemical looping combustion (CLC) is one of the most promising cost-effective technologies that can be retrofitted onto existing power plants for CCS. The main drawback attributed to CLC is a very low confidence level as a consequence of the lack of maturity of the technology. Use of computational fluid dynamics (CFD) has the potential to boost the development and implementation of commercial-scale CLC units. This dissertation focuses on designing a novel semi-batch CLC unit using fluidized-bed reactors and modeling the hydrodynamics of fluidized bed reactors with use of CFD. The National Energy Technology Laboratory’s (NETL, USA) open-source code MFIX is used in this study as flow solver for CFD models.In this dissertation, a conceptual design is developed that leads to fabrication of a 100-kWth semi-batch CLC prototype unit by ZERE Energy and Biofuels, Inc. San Jose, California. The hydrodynamics of the prototype unit are extensively studied using mathematical modeling and CFD. A multi-stage numerical model has been developed to investigate the behavior of a fuel reactor used in CLC unit. To predict the behavior of mass transfer in the CLC reactor, a combination of perturbation theory and semi-empirical correlation is suggested. Much of the work presented in this dissertation is focused on improving the ability to use CFD for process development. The grid size used in numerical simulations should be sufficiently small so that the meso-scale structures prevailing in the gas-fluidized beds can be captured explicitly. This restricts CFD in studying industrial-scale fluidized bed reactors. Thus, a generalized grid size that is sufficient to obtain a grid-independent solution of two-fluid CFD model is suggested in this study. In order to fully understand the complex interaction between fluid phases of CFD models, a 3-D face-masking algorithm is developed and applied to assist post-processing CFD results for identification and tracking of gas bubbles in a fluidized bed. Finally, the hydrodynamics of multiphase flow reactor at high-temperature is investigated through the particle-particle restitution coefficient in numerical simulations. In conclusion, findings of this dissertation will be useful for scale-up, design, or process optimization for reliable commercial CLC plants reducing economic risk, and potentially allowing for rapid scale-up.