Particle Flow, Agglomeration, Mixing, Chemical and Physical Absorption in Circulating Fluidized Bed Absorbers

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Title: Particle Flow, Agglomeration, Mixing, Chemical and Physical Absorption in Circulating Fluidized Bed Absorbers
Author: Mao, Deming
Advisors: Dr. H. Henry Lamb, Committee Member
Dr. Hassan A. Hassan, Committee Member
Dr. Andrey V. Kuznetsov, Committee Co-Chair
Dr. Jack R. Edwards, Committee Chair
Abstract: Coal-utilization for energy production poses considerable environmental concerns as it results in emission of sulfur dioxide (SO2), nitrogen oxide (NOx), fine particulate matter (PM), and trace heavy metals such as mercury vapor (Hg) during coal-combustion. Circulating Fluidized Bed Adsorbers (CFBAs) are regarded as a potentially effective technology to capture some of the above pollutants. In particular, one could use limestone to remove sulfur dioxide by chemical adsorption, and activated carbon to remove elemental mercury by physical adsorption. Also, sorbent particles could be used to capture fine PM or promote formation of clusters of larger PM. In order to analyze CFBA systems in detail, a new approach has been developed for solving the Navier-Stokes equations for a gas-mixture/solids-mixture system. Sub-models are also developed to be combined with the gas/solids hydrodynamics model to simulate capture of multiple pollutants. Specific tasks accomplished include the following. 1. A model for fine particle agglomeration in CFBAs has been developed. It can model the influence of different factors on agglomeration, such as the geometry of a CFBAs, the superficial gas velocity, initial particle size distribution (PSD), and type of agglomeration mechanism. It is found that the Brownian agglomeration mechanism can be neglected compared to agglomeration by mean shear and turbulence. Sorbent particles are shown to capture fine particles effectively for certain conditions. A simplified version of this model has been developed for coupling with the hydrodynamics model. 2. A mixing model based on a core-annulus model of a CFBA has been developed to simulate the particle residence time distribution (RTD). Thus, macrochemical reaction can be simulated by combining microchemical reaction dynamics with the particle RTD. This has been applied to simulate SO_2 removal by chemical adsorption onto dry lime. 3. A 'gas mixture' and ``solids mixture' model has been developed to simulate fine particle agglomeration onto sorbent particles, sulfur dioxide removal through chemical adsorption with lime, and mercury vapor removal through physical adsorption with activated carbon. The 'gas mixture' is composed of fine PM, sulfur dioxide, mercury vapor, oxygen and inert gas; while 'solids mixture' is composed of solids-1 and solids-2. Solids-1 is composed of lime (CaO) and CaSO_4, and solids-2 is activated carbon. These equations are similar to continuity equations appearing in the gas-solids hydrodynamics system and are integrated fully coupled with that system. 4. A new approach for solving the Navier-Stokes equations governing gas/solids two-phase flow with chemical reaction has been developed. The approach combines a time-derivative preconditioning strategy for a gas/solids two-phase flow model with extensions of low-diffusion flux-splitting upwinding techniques. The combined framework is used to simulate jet-induced bubble formation within a minimally fluidized bed, flow within a circulating fluidized bed without chemical reaction, a downward fludized bed, and gas mixture/solids mixture flow within a prototype CFBA device. For bubble bed simulation, the model gives good results for bubble formation, growth and burst. For three-dimensional CFBAs, it can give good results compared with experimental data and can capture details of solids clustering phenomena. For downward fluidized bed, the model gives plausible results regarding the influence of superficial gas velocity on particle flow and mixing. Finally, simulations of a ``bench-scale' CFBA reactor with combined SO_2, Hg and fine PM capture give reasonable results for gas species and solids species, but further validation is needed.
Date: 2004-01-06
Degree: PhD
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/5087


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