Particle charging mechanism and numerical methodology for industrial applications

Abstract

Particles in industrial flows can be charged under an action of external electric field, while in the absence of external electric field, tribo-electrostatic charges are almost unavoidable in gas-solid two-phase flows due to the consecutive particle contacts. The particle charging may be beneficial, or detrimental. In the past decade considerable progress has been made in understanding the physics of particles charging. However, the particle charging mechanism, especially in the gas-solid phase flow, is still poorly understood. The purpose of this review is to present a clear understanding of the particle charging and movement of charged particle in two-phase flow, by summarizing the charging mechanisms, physical models of particle charging, and methods of charging/charged particle entrained fluid flow simulations. In this review, charged particles in industry, which would be beneficial (triboelectrostatic separation, electrostatic precipitator) or detrimental (electrification in gas-solid fluidized bed and manufacturing plant) are discussed separately. The particle charging through collisions could be attributed to electron transfer, ion transfer, material transfer, and/or aqueous ion shift on particle surfaces. For conductive particle contacts, the difference in work function is often used to explain the charge transfer. For insulation particle contacts, the charging tendency can be explained by the ion transfer and material transfer. In addition, aqueous ion shift transfer would be an important charge transfer mechanism considering water content in environmental conditions and the influences of temperature and humidity. The charges on particle through collision can be quantitatively predicted by using the particle charging model. According to the differently induced ways of charge transfer, the charging models are related to the external electric field, asymmetry contact, and/or aqueous ion shift on particle surfaces. In fact, the motions of particles in industry are influenced by fluid flow. The effect of fluid on particle dynamics makes the particle charging more complicated. Thus it is more reasonable to study the particle charging from the viewpoint of the gas-solid two-phase flow. The method combining particle charging model with computational fluid dynamics and discrete element method is applicable to the studying of the particle charging/charged processes in gas-solid two phase flow in which the charge behaviors are significantly influenced by the fluid mechanics behavior. By this method, the influence factors of particle charging, such as gas-particle interaction, contact force, contact area, and various velocities, are described systematically. This review presents a clear understanding of the particle charging and provides theoretical references on controlling and utilizing the charging/charged particles in industrial technology.