Mechanism of the effect of alkali metal on the electrostatic precipitability of fly ash

Abstract

The alkali element content is second only to the sulfur content as the most important factor affecting dust-specific resistivity. Size sieving, soaking in distilled water, and ion migration at a high voltage are effective methods of changing the sodium and potassium contents in fly ash. Ash samples with different alkali contents were prepared by the aforementioned methods. The effects of sodium and potassium contents and morphosis on the specific resistance of fly ash were analyzed. Based on the morphosis of alkali compounds in coal and on the analysis of the combustion process, the mechanisms underlying the effects of sodium and potassium on volume resistivity and surface resistivity were determined. Sodium and potassium were related to the specific resistance in fly ash, i.e., a higher alkali content resulted in a lower specific resistance of fly ash. However, the instability of sodium causes it to react easily with water vapor, HCl, and SO3 in a boiler to produce Na2SO4 and NaCl. In an electrostatic precipitator, the electrically conductive component is distributed outside the particle and makes contact with water vapor to form a liquid film, which in turn becomes the charge carrier for fly ash conduction. These reactions significantly improve the conductivity and decrease the surface resistance of fly ash.