INTRODUCTION TO ELECTROSTATIC PRECIPITATORS

Electrostatic precipitators (ESPs) are the most commonly used, effective, and reliable particulate control devices; they are employed mostly in power plants and other process industries. The particle-laden flue gas from the boiler flows through the ESP before it enters the environment. The ESP works as a cleaning device, using electrical forces to separate the dust particles from the flue gas. A typical ESP consists of an inlet diffuser known as an inlet evase, a rectangular collection chamber, and an outlet convergent duct known as an outlet evase. Perforated plates are placed inside the inlet and the outlet evase for the purpose of flow distribution. Inside the collection chamber there are a number of discharge electrodes (DEs) and collection electrodes (CEs). A set of discharge electrodes is suspended vertically between two collection electrodes in a typical wire-plate ESP channel. While the flue gas flows through the collection area, electrostatic precipitators accomplish particle separation through the use of an electric field in the following three steps. The electrical field does the following:

1. Imparts a positive or negative charge to the particles by means of discharge electrodes
2. Attracts the charged particles to oppositely charged or grounded collection electrodes
3. Removes the collected particles by vibrating or rapping the collection electrodes or spraying them with liquid

PERFORMANCE IMPROVEMENT TECHNIQUES

In recent years the particle emissions from process industries have been attracting more attention because of the anticipation of upcoming strict U.S.Environmental Protection Agency (EPA) regulations. Although electrostatic precipitators generally capture 99.5% of the particles from the flue gas in terms of mass volumes, the anticipated regulations on PM2.5 (particulate matter with particle sizes of 2.5 microns and less) have led power stations to explore improvement options to control the emissions of the fine particulate at a minimal cost.

Optimizing flow distribution inside the ESP by using perforated plates and screens could be the most important technique for improving performance with a view to addressing the problem of fine particulate emission. The flow distribution within the ESP has varying effects on its performance in capturing fly ash particles. It was extremely difficult to evaluate the flow impact on individual ESP performance until computational fluid dynamics (CFD) could be applied. CFD code FLUENT can be used to predict the flow field and electric field characteristics and particle trajectories inside the ESP and optimize flow distributions within the ESP by simulating proposed modifications. This ensures that the required flow profiles will be achieved, substantially reducing the outage time.

MODELING APPROACH

An ESP system consists of a flow field, an electrostatic field, and particle dynamics. Numerical simulations can be carried out by using CFD code FLUENT, and comparisons can be made with the experimental results. The ESP can be modeled in two steps. First, the three-dimensional (3D) fluid (air) flow can be modeled in accordance with the detailed geometrical configuration inside the ESP. The model has to be validated by the on-site measured data or by the data obtained from the laboratory experiments. In the second step, as the complete ESP system consists of an electric field and a particle phase in addition to the fluid flow field, a two-dimensional ESP model can be developed. The electrostatic force has to be applied to the flow equations through the use of user-defined functions (UDFs). A discrete phase model can be incorporated into this two-dimensional model to study the effect of particle size, electric field, and flue gas flow on the collection efficiency of particles inside the ESP. As a result of the symmetry in geometry, only one-half of the physical model can be considered for the simulation. Figure 1 shows a typical computational domain of an ESP with grids.

Figure 1: Computational grid of an ESP model

Two collection electrodes and three discharge electrodes were considered for creating the geometry of an ESP channel. Further simplification could be done by modeling half of the channel because of the geometrical symmetry. The configuration and dimensions of a typical ESP are shown in Figure 2.

Figure 2: Two-dimensional geometry configuration of an ESP channel

ESP PERFORMANCE IMPROVEMENT THROUGH OPTIMIZATION

Optimization of the Flow Field Using the Variable Porosity of Perforated Plate

A number of perforated plates or screens are situated within the inlet evase and outlet evase. The perforated plates can be modeled as a thin porous surface of finite thickness with directional permeability. The perforated plate that is closest to the ESP chamber can be modeled as a set of porous surfaces with variable porosity.  This modeling approach gives the ESP model versatility to optimize the flow by controlling the resistance of these surfaces. The nonuniformity of the velocity distribution inside the ESP can be removed after the introduction of a variable porosity of perforated plate inside the inlet evase.

Optimization of Flow Field: Insertion of Guide Vanes inside the Inlet Evase

It has been found that when the flow medium spreads over the front of the grid, the streamlines become distorted. The higher the resistance coefficient of the grid is, the sharper is the distortion of streamlines, and consequently the greater is the departure of the jets from the orifices to the periphery of the grid. With an increase in the resistance coefficient of the grid up to a certain value, the velocity profile is reversed. To avoid this situation, guide vanes can be inserted over the cross section of the inlet evase to distribute the flow all over the last perforated plate of the inlet evase. It was found from the simulated results that this new geometric feature guides the flow uniformly over the perforated plate. The perforated plate then gives the flow an additional uniformity that continues throughout the total collection area of the ESP.

Optimization of Flue Gas Velocity and Electric Potential

The particle collection inside the ESP depends on the flue gas velocity and the electrostatic force of the ESP system. The average gas velocity inside the collection chamber of an ESP varies from 0.5 m/s to 2 m/s. The flow stream with high velocity leaves the collection chamber with poor particle collection. The particles require sufficient treatment time to get charged and collected inside an ESP. The electrostatic force, which is generated after the application of electric potential to the discharge electrodes, is strongly related to the movement of the particles. Figure 3 shows how the particle collection efficiency of an ESP channel can be improved by increasing electric potential at the discharge electrodes and maintaining a reduced flow velocity at the inlet surface, which is 0.5 m/s in all four cases.

Figure 3: Particle collection efficiency of an ESP at different electric potentials
(inlet flow velocity 0.5 m/s)

CONCLUSION

A novel approach can be introduced for modeling the perforated plate as a set of porous surfaces with variable porosity.  This could give the ESP model flexibility to optimize the flow by adjusting the resistance of an individual surface. A novel guide vane can be placed inside the inlet evase of the ESP model to achieve improved flow distribution. The optimized velocity obtained from the three-dimensional model can be applied to the two-dimensional model. The CFD models thus developed reveal that particle collection efficiency can be improved by optimizing the flow field and increasing the electric force.