Electric Characteristics And Flow Structures In Typical Configurations Of ESPs

With the increasingly attention of the environmental protection in China since 1970s, electrostatic precipitators (ESPs) are widely used in industry. Because of the differences of the configuration of wire-plate ESPs, electric field and flow field are different in the ESPs, which results in the discrepancy of collection efficiency between theory and application. In order to obtain the distribution of the electric field and flow field in three typical configurations of ESPs, theoretical studies and numerical computation are carried out in the present research.A computational program is obtained for the numerical simulation of the space charge density and V-I characteristic in ESPs in the absence of fly-ash when corona discharge occurs. An iterative numerical program based on the finite difference method, is implemented for solving Poisson's equation and current continuity equation. The V-I characteristics, which obtained by using the proposed program, is compared with the experimental results in the literature, and the agreement between computational and experimental values shows the numerical program is applicable for such theoretical estimations. In order to analyze the numerical results of three configuration ESPs, Seven indexes are introduced for evaluating the electric performances of the ESPs. The results show that under a given condition, the applied voltage for the three ESPs is much the same when the same mean current density on the collecting plate is obtained. There exist weak zones of electric field and current density on the collecting plate when flat plate is used, and back corona tends to occur when the resistivity of particles is high. The current density is distributed uniformly and the electric field is relatively high on the zigzag plate, however, because of the nonuniformity of the electric field on plate, spark voltage may be reduced, which incline to decrease the regime of applied voltage. The separation of collecting zone and charging zone restrict the occurrence of back corona in tri-electrode ESPs, moreover, the electric field and current density are distributed homogeneously on the plate, thus theESPs can operate in a wide regime of applied voltage. As a result, tri-electrode systems are obviously superior to the duct and zigzag type ESPs.Gas flow through ESPs is influenced by a secondary flow of electrical origin known as electric wind. This phenomenon arises when significant momentum is transferred from corona-generated ions to the gas molecules. The results indicate the secondary flow is not negligible, and strong flow interactions take place owing to the induced circulatory cells. The magnitude of the interactions between the primary and secondary flows is directly related to the applied voltage and the inlet velocity. The mean flow field is heavily influenced at high current densities, but the effect is not as dramatic as reducing inlet velocity. Numerical computation results of three configuration ESPs prove that, for charging and collecting particles, it is more suitable for flow gas distribution in tri-electrode systems.The results of analysis indicate that zigzag plate and tri-electrode have some advantages for application, and a tri-electrode system is the most optimum. However, wire-plate is an unsuitable way for waste gas cleaning.