Doctoral Dissertations

Date of Award

12-1988

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Gregory D. Reed

Committee Members

Wayne T. Davis, R. Bruce Robinson, Robert M. Counce

Abstract

The present study discusses the importance of slaking operation and its effect on the removal efficiency in a spray dryer operated FGD system using slaked lime slurry. Laboratory scale slaking experiments were run to look into various characteristics of resulting slurry. Pilot plant tests were made to quantify the effects of different parameters on spray removal efficiency. A vertical tower mill was used to grind calcium hydroxide slurry to attain smaller particle size. X-ray diffraction and SEM techniques were used to visualize the crystallinity and surface aspects of calcium hydroxide slurry. Sorbent particle size and surface area measurements were made for different slurries and an attempt was made to relate these parameters with the sorbent's performance in removing S02 across the spray dryer. A previous version of a mechanistic model for predicting S02 removal efficiency during constant rate period was modified to incorporate measured surface area of the sorbent into the model. The findings of the present work indicate that removal efficiencies increase with decrease in sorbent particle size. Removal efficiency is observed to increase with specific surface area of the sorbent, but there seems to be a limitation on the maximum possible efficiency that can be achieved. By increasing the surface area beyond certain values, the spray dryer system can be operated under gas-phase controlled conditions, which should give maximum possible efficiency. The new version of the model SPRAYMOD-N was used to mimic these conditions to arrive at the gas phase controlled efficiency and corresponding particle size to achieve those conditions. It is found that slaked lime slurry particles around 2.3 microns size can give an efficiency of around 75 percent in the spray dryer for an inlet S02 concentration of 1000 ppm, operated at a stoichiometric ratio (SR) of one and saturation approach temperature of 20° F. Pilot tests conducted in this study with 2.4 micron Ca(OH)2 slurry achieved this efficiency. The new version of the model for the constant rate period, was observed to predict reasonably well for both the present pilot test data and previous sets of data collected over a period of time. Predicted efficiency values (with constant rate period only) fell off at higher SR values, showing the significance of the falling rate period at those conditions. The surface area incorporation into the model resulted in decreasing the gap between actual values and under predicted values from a previous version of the model.

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