Date of Award
Doctor of Philosophy
Robert C. Brown
This study examined the fundamental reaction dynamics associated with calcination and sulfation of limestone particles in fluidized bed combustors. Although this subject has been extensively treated in the literature, investigations to date have been primarily conducted in small laboratory reactors that do not well simulate combustor conditions. A review of the literature shows disagreement as to the controlling mechanisms for the calcination and sulfation reactions. In this investigation, experiments were conducted both on laboratory-scale and full-scale fluidized bed combustors (FBC's);Transient gas analysis of combustor emissions was used to investigate calcination and sulfation reactions in FBC's. Measurements of carbon dioxide and sulfur dioxide were made continuously after batch additions of limestone for many different operating conditions. Experimental variables included bed temperature, air and fuel flow rates, oxygen and carbon dioxide concentrations, batch size and particle size. Time constants for calcination and sulfation which characterized the reactions were defined. Integration of CO2 and SO2 transients gave the extents of calcination and sulfation;From this study it was revealed that the calcination reaction is controlled by the chemical reaction rate. This result was deduced from the particle size dependence and observed activation energy of the calcination reaction rate. Intra-particle diffusion also influenced the calcination reaction rate at high temperatures in large particles. The sulfation reaction appeared to be controlled by film and bulk mass transfer at early times in the laboratory reactor. At later times pore diffusion and pore-plugging controlled the reaction for the relatively non-porous limestones studied. The type of limestone and the conditions of calcination strongly determined the rate and extent of sulfation;Particle analysis was done for comparison to transient gas analysis results. Calcination appeared to take place by a shrinking particle core mechanism. Scanning electron microscopy (SEM) revealed a calcined outer layer for particles extracted during the middle of the CO2 transient. Evidence of structural changes in the calcine as the reaction proceeded inward was observed. Sulfated particles demonstrated an outer layer that increased in thickness and concentration over the course of the reaction.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Theodore Donald Thiede
Thiede, Theodore Donald, "Analysis of limestone calcination and sulfation using transient gas analysis " (1996). Retrospective Theses and Dissertations. 11495.