Factors affecting the electrical resistivity of kraft recovery boiler precipitator ash

The electrical resistivity of ash particles is an important parameter that determines the efficiency of electrostatic precipitators. This systematic study examines the resistivity of recovery boiler precipitator ash as a function of electrical field strength, time of exposure, particle composition, and gas composition and temperature. Synthetic ash and actual ash samples from several pulp mills are used. The results show that most ash samples tested had a resistivity between 109 and 1010 Ω·cm, but one of the samples had an unusually high resistivity, 1012 Ω·cm. The resistivity increases with temperature up to about 140°C, then decreases. At a given temperature, the resistivity decreases with increasing moisture and sulfur dioxide concentration in the gas. Resistivity also increases with an increase in chloride content in the ash, but is not affected by the carbonate, sulfate, and potassium contents. The results imply that recovery boilers burning liquors with high solids and high chloride contents produce ash with higher resistivity, making it more difficult for electrostatic precipitators to capture.