Semester

Fall

Date of Graduation

2010

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Mechanical and Aerospace Engineering

Committee Chair

John Kuhlman.

Abstract

Responsible carbon management will be required for the future utilization of fossil fuels for power generation. One technology that is showing tremendous potential for carbon capture is the chemical looping combustion (CLC). CLC involves combustion of fuels by heterogeneous chemical reactions with an oxygen carrier, usually a granular metal oxide, exchanged between two fluidized beds. In any regular combustion process the oxidizer for the fuel is air which essentially is a mixture of oxygen and nitrogen. This results in a flue gas stream which consists of primarily of relatively dilute CO 2 (15 -- 20%), mixed with steam and nitrogen. In CLC, CO2 separation is easily achieved because the oxygen for the reaction is supplied by the carrier (e.g. a metal oxide), resulting in a flue gas stream consisting almost entirely of carbon dioxide and steam. The steam can be easily condensed to give pure CO2 which can then be easily sequestrated. Thus the CLC process for power generation provides a sequestration ready CO2 stream without the need for using costly gas separation techniques. The only operational penalty for CLC is then the slight pressure losses required to circulate the carrier between reactors and the carrier make-up costs. CLC requires many unit operations involving gas-solid or granular flow. Herein a computational fluid dynamic study is presented to analyze the performance of CLC systems utilizing both gaseous and solid fuels. There have been extensive experimental studies in CLC, however CFD simulations of this concept to date are quite limited. The present simulations were performed using the interpenetrating fluid representation of dense multiphase flow. The ANSYS-FLUENT(TM) CFD solver was used in the present study. The granular phases are represented as continua whose dynamics are governed by Navier-Stokes like equations, coupled to the N-S equations describing the fluid flow. Detailed sub-models to account for fluid-particle and particle-particle interaction forces have been included. Heat transfer is fully accounted for. Heterogeneous reactions are used to describe the coal conversion and the reactions of the gaseous fuel with the carrier. Global chemical reaction models of fuel and carrier were utilized. Capability of the model to simulate the segregation processes, depending on particle density and size differences between the carrier and the fuel, allows the design of a reactor with the desired behavior. The results obtained from CFD have been compared with available experimental information. The transient CFD simulations provided a reasonable agreement with the reported experimental data for batch reactors using gaseous as well as solid fuels, and for a full circulating fluid bed CLC using gaseous fuels.;The CFD models that were developed and validated using available experimental results have been applied for design evaluations of fuel reactor of CLC system utilizing char as fuel. It would be very desirable to utilize coal directly in the fuel reactor, which requires in situ gasification in either a moving or bubbling fluidized bed reactor. In such a design, H 2O (and CO2) must be recycled from the product stream as the fluidizing medium, which allows in-bed heat transfer and mediates chemical reactions between the two solid feeds (carrier and fuel), and gasifies the coal char. The solid coal fuel must be heated by the recycled metal oxide, driving off moisture and volatile material, and the remaining char must then be gasified to provide complete fuel utilization. The gaseous products of these reactions must then contact the hot, oxidized carrier before leaving the bed to obtain complete conversion of the fuel to H2O and CO 2. Further, the reduced carrier particles must be removed from the bed and returned as a pure stream to the air reactor for regeneration. It is critical that no unburned fuel, i.e., char, be returned with the spent carrier as this material will rapidly burn in the air reactor and the resulting CO2 will escape capture. Three designs have been developed and analyzed with CFD. Special attention is paid to Fe-based carriers (due to their low cost relative to other carriers), which is somewhat complicated due to the multiple oxidation states displayed by Fe. The non-linear interaction of factors such as multiphase hydrodynamics, heat transfer and chemical reaction is fully coupled in the numerical simulations allowing evaluation of design options for a full circulating CLC system using solid fuels.

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