200 250 300 350 400 450

Temperature, F (b)

FIG. 5.17.6 Variation of fly ash resistivity with coal sulfur content and flue-gas temperature.

Increasing the temperature above 350°F results in excessive heat loss from the stack.

Resistivity decreases with increased sulfur content in coal because of the increased adsorption of conductive gases on the fly ash. Resistivity changes were responsible for the increased fly ash emissions when power plants switched from high-sulfur coal to low-sulfur coal to reduce SO2 emissions in the United States.

In some cases of high resistivity in dust, adding a conditioning agent to the effluent gases substantially reduces resistivity and enhances particle collection. Examples are adding SO3 to gas from power generator boilers and NH3 to gas from catalytic cracking units used in petroleum refining. The overall efficiency can increase from 80% before injection to 99% after injection.

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