Simulation study of packing of fine particles under electric fields

Abstract

Electrostatic precipitation processes have been widely applied to remove particulate matter from flue gases in coal-fired power stations. A high negative voltage is usually applied to a discharge electrode so that the gases are ionised in such processes. When the suspended particles in flue gases enter the ionised space, they are electrically charged and deposited on collection walls to form a layer of particle packing. Essentially, the underlying working principle of electrostatic precipitation processes is the packing of fine particles under electric fields. Despite the possibility of achieving high collection efficiencies of the precipitation processes through careful tuning of the electrical and aerodynamic conditions, improvements on the removal of fine particulate matter, such as PM2.5 have not been achieved. Understanding the formation process of particle packing under electric fields is the key to improve the collection efficiency of fine particulate matter. Yet, it is mostly impossible to carry out a study on packings of fine particles through practical experiments. In this thesis, we have developed a numerical model based on the discrete element method to simulate packings of fine particles under various electric fields. Both the formed packings under uniform and non-uniform electric fields are comprehensively examined. For the packings under uniform electric fields, the packing structures are characterised in terms of the packing fraction, coordination number, radial distribution function, and Voronoi tessellations. Our results indicate that the particle diameter and electric field are the two important parameters that determine the structure of the formed packings under electric fields. Such observations can be explained by the competition between the electric-field-induced electrostatic interactions and the interparticle van der Waals interactions during the formation of stable packings. For the packings under non-uniform electric fields, special emphasis is given to the elliptical-shaped packing structural profiles commonly observed in electrostatic precipitation processes. The results have demonstrated that non-uniform packing structures are formed as a result of the imposed non-uniform electric fields. Despite the non-uniformity of the overall structural profile, the local packing structures are correlated to the local electric fields in terms of packing fraction and coordination number. These findings may lead to better controls of the formed packings under various electric fields. In addition, a novel numerical method to evaluate electrical transport in the formed packings under electric fields is presented. Both the electrical potential and electric current on each and every particle in a packing are numerically solved to obtain the effective electrical conductivity of the packing. Here, the focus is given to the contacts between particles. Our results have shown that the three variables, the material properties of particles, the contact area between particles and packing structures all have determining effects on the effective conductivity. Furthermore, analyses on the electric current network and contact force network have revealed that the electric current flow is significantly influenced by the contact force distribution in a packing. Lastly, two mathematical models, that are applicable to industrial electrostatic precipitation processes, have been developed to predict the effective conductivity of the formed packings under electric fields. Conventional models predicting the effective conductivity require the use of the structural parameters of packings, such as the packing fraction, coordination number and contact diameter. However, such information is difficult to obtain in industrial applications. In contrast, based on the previously established relations, we have modelled the effective conductivity of packings using only the particle diameter, electric field and packing depth, which are all controllable parameters. Hence, the desirable electrical transport properties of the formed packings under electric fields can be achieved through changing the controllable parameters. Our findings may lead to better design and controls of electrostatic precipitation processes.