Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Mechanical Engineering

First Advisor

Robert C. Brown


Fluidized bed combustion has grown with the prospect that it can burn coal and low grade fuels in an environmentally acceptable manner. Among the technical problems that exist, however, is the inherent inability to produce large variations in heat transfer rate from the fluidized bed. Generally, changes in heat transfer rate (load turndown control) by conventional means are modest and are accompanied by degradation in combustion;The objective of this research was to investigate a new concept in fluidized bed design that improves load turndown capability. Load turndown ratio is defined as the ratio of maximum to minimum fuel firing rates. Load turndown improvement was accomplished by independently controlling heat transfer and combustion in the combustor. The design consisted of two fluidized beds: one central and one annular bed. The central bed served as the combustion bed. The annular bed was fluidized separately from the combustion bed and its level of fluidization determined the overall heat transfer rate from the combustion bed to a surrounding water jacket. Simple theoretical considerations suggested a load turndown ratio exceeding ten was possible for this design;A computational model consisting of energy and mass balances and a simple combustion kinetics model for the two-bed combustor were developed to predict the detailed performance of the combustor and to assist in the design of the unit. Combustion tests were performed in an optimally-designed combustor. Three coal-based fuel forms were used: crushed coal, coal-limestone briquettes, and coal-water-limestone mixture (CWLM). A load turndown ratio of 12.3 was obtained while burning crushed coal. Slightly lower turndowns were obtained for briquettes and CWLM. Sulfur dioxide emission reductions ranging from 42% to 93% were observed for a fuel calcium-to-sulfur molar ratio of 2.0. For most tests, emissions of NO[subscript] x and SO[subscript]2 met EPA New Source Performance Standards. Computer model predictions were in reasonable agreement with experimental observations.



Digital Repository @ Iowa State University,

Copyright Owner

James Everett Foley



Proquest ID


File Format


File Size

134 pages