M.S. Thesis: Numerical Simulation of Flow and Mixing Behavior of Solids on a Moving Grate Combustion System

By Sara Barone

Advisor: Prof. Nickolas J. Themelis

Department of Earth and Environmental Engineering
Fu Foundation School of Engineering & Applied Science
Columbia University

April 2015

The residence time of the waste particles on a moving grate is a very important parameter, and to a large extent influences the combustion process. Municipal solid waste (MSW) is not a uniform fuel, it varies widely in chemical composition and physical properties. Determining the particle residence time is a difficult task requiring pilot or industrial scale experimental work. Trying to minimize time and capital investment, mathematical models have been developed for the mixing process of a moving bed, estimating residence time of its particles, burnout rate, and evaporation parameters. In the past, the adjustment of the waste-to-energy power plants was accomplished empirically by studying the response of the system on changing operation parameters. However, numerical simulations have been established for the modeling of material and gas flows in the presence of chemical reactions. It is a very effective tool, since it is much less costly to perform simulations than run furnace experiments.

At the present time, it is impossible to model a complete waste-to-energy combustor with the currently available commercial programs due to the complex interaction of the solids in the moving bed and the gas phase passing though the bed or flowing above it (Nakamura M. 2008). Hence, developing appropriate tools is necessary for the description of the combustion reactions in waste combustion plants. In this study, the waste combustion on the grate and the furnace is modeled. The motion of the grate and the force of gravity transport the waste material in the cross flow to a gaseous fluid. The average residence time of the material can be influenced by adjustment of the bed height and the local transport velocity. Furthermore, an adequate mixing is necessary to provide continuous contact between gases and solids. In order to achieve these conditions, several different grate designs are in use. Based on ESyS-Particle HPC (Discrete Element Method code), granular flow and mixing on different types of waste-to-energy grates are studied in two dimensions. Discrete element simulation provides detailed information on particle positions and velocities over time. This information is used to derive quantities characterizing the dynamic process of mixing. The mixing parameters are used to compare the mixing process on different grate designs.

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