Electric charge limits on settled powders

Abstract

In flows of dry particulate systems, electric charge is generated on particle surfaces by their collision with walls and with other particles. Charge build-up on single particles can yield local charge values high enough to surpass the limiting electric field for corona discharge into the surrounding gas. Then, local charge is decreased to a lower value that becomes stabilized when flows stop and particles deposit in a container. In this paper, we have used a Faraday pail system to measure the residual particle charge after using two different devices—tribochargers—for particle charging. One of the tribochargers allowed us to directly measure the total charge that was transferred from the walls to the particles, and this was compared to the final values in the bulk powder once it was collected in the Faraday pail. The results show that the electric charge of particles dispersed in gas is limited by corona discharge and depends mainly on the particle size. In addition, we present a simple model of the discharge of the collected powder based on electrostatic considerations. If the powder effective conductivity and the electric charge of the settling particles are known, the model predicts the temporal evolution of the total charge of the collected powder and the spatial distribution of the electric charge and electric field.