Electrostatic Precipitation (ESP) Fundamentals
PARTICULATE & OPACITY CONTROL
Fundamentals of Precipitators
How Does ESP Work?
In the simplest terms, a precipitator is a device that utilizes electrostatic forces for the collection of particulate. The particulate laden gases enter one area of the vessel and exit another area clean. Inside, high voltage electrodes impart a charge to the particles. Most modern precipitators utilize negative voltage electrodes though positive voltage electrodes do exist. These charged particles are then attracted to a collecting surface (grounded, neutral or oppositely charged). The particles build up on the collection surfaces over time and are then periodically removed, typically at set intervals.
What are the Different Types of ESP’s?
The vast majority of Industrial Electrostatic Precipitators are of the dry type, though wet precipitators are gaining popularity. Dry Electrostatic Precipitators are often referred to as Dry ESP's or DESP's while the Wet Electrostatic Precipitators are referred to as Wet ESP's or WESP's. Regarding particulate removal, the surfaces in a Dry ESP are rapped or vibrated, and the tubes in a Wet ESP are flushed , causing the particles to separate from the collection surface. In a Dry ESP the particles are usually evacuated from the hopper by rotary screws or drag chain conveyors . In a Wet ESP the hoppers are designed to gravity drain or pump the flush water for designated treatment or disposal.
Wide Range of Collectible Particles
Wide Range of Collectible Particles
Precipitators can be used to collect many different kinds of particles with a wide range of physical properties. Some of the compounds that can be collected include:
- Ammonium Sulfates
- Bagasse Ash
- Coal Ash
- Diesel Smoke
- Fly Ash
- Hydrochloric Acid
- Hydrofluoric Acid
- Incinerator Ash
- Iron Oxide
- Salt Cake
- Sulfuric Acid Mist
- Wood Ash
- Wood Acids
- Wood Tar
Processes Employing Precipitators
Processes Employing Precipitators
Precipitators have been employed on many separate processes and pieces of process equipment such as:
- Biofuel Production
- Biomass to Energy
- Ethanol Production
- Food Processing
- Glass Making
- Oil Refineries
- Paint Processing
- Pellet Mills
- Plastic Processing
- Pulp and Paper
- Sugar Mills
- Waste Disposal
- Waste to Energy
- Wood Products
The basic size of a precipitator can be determined through the Deutsch-Anderson Equation.
- A = -( Q / w ) x [ ln ( 1 - Eff ) ]
- A = collection area required in square feet.
- w = Drift or Migration velocity of the particles to be collected.
- Q = Gas Flow Rate in cubic feet per second.
- Eff = Fractional collection efficiency desired.
- Drift velocity or migration velocity can be specifically defined as:
- w = dp x Eo x Ep / (4πµ)
- dp =diameter of particle, microns
- Eo =strength of field in which particles are charged, V/m (V/ft)
- Ep = strength of field in which particles are collected, V/m (V/ft)
- µ = Gas viscosity, Pa *s (cp)
- π = 3.1415
Each type of particulate or process gas stream has a different migration or drift velocity associated with it. Once the process or particulate source is known a range of drift velocities can be selected. It is important to note that the drift velocities shown below are highly dependent upon not only the upstream process but the configuration and design of the precipitator as well. For example two identical processes each with their own precipitator made by a different manufacturer will often have different apparent drift velocities. This is because each manufacturer has a different physical design in terms of gas flow, electrode design and power supplies.
The calculator below can be used to determine the rough size of a precipitator.
There are many factors that can affect the correct sizing of a precipitator.
The gas stream moisture content affects the flow of electrons and the subsequent collection of particulates in that gas stream. The graph to the right shows the effects of moisture upon a gas stream containing cement dust. Normal dust resistivity ranges between 107 ohm-cm and 1010 ohm-cm. Precipitators whose gas streams exhibit high dust resistivity can improve their collection efficiency by adjusting temperature or introducing additional water through evaporation or steam addition or by adding other gas stream conditioning agents.
Temperature alone can affect the resistivity of the dust particles. The graph below shows how resistivity changes as temperature changes for a variety of dusts. Precipitators whose gas streams exhibit high resistivity can improve their collection efficiency by adjusting temperature or introducing additional water through evaporation or steam addition or by adding other gas stream conditioning agents.
The gas stream velocity through the precipitator can affect performance in three fundamental ways.
- The gas velocity can be so fast that the electrostatic attraction forces cannot overcome the velocity head or turbulence created by the flow of the gas stream
- High gas velocity can strip collected particles off of the collecting surfaces, reducing overall collection efficiency.
- Increased gas velocity means decreased residence time resulting in the particles spending less time in the active collection areas.
Large amounts of particulate can create excessive sparking inside the precipitator. Sparking is a normal phenomena where the current of the high voltage areas rushes to a grounded surface. Normal spark rates are less than 100 sparks per minute. Examining the equation for drift velocity presented above one can see that the charging voltage has a direct impact upon the calculation of w. The graph from "An Electrostatic Precipitator Performance Model (EPA-650/2-74-132)” shows that the higher the grain loading, the higher the reduction in charge intensity.
The larger the particle the more readily it can be collected in a precipitator. The graphs below provided in the EPA publication EPA-600/8-77-020b dated January 1978 shows the relationship between the particle diameter and collection efficiency as well as particle size and migration velocity.
The equation presented above (w=dpEoEp/(4πµ) gives an indication how the collecting and discharge voltages affect drift velocity; the higher the collecting and discharge voltage the higher the value of migration (drift) velocity will be. Examining the Deutsch-Anderson Equation, also presented above, ( A= -[ Q / w ] x [ ln ( 1 - Eff ) ] reveals that as "w" (the denominator) increases in size the required collection plate area decreases. Simply put, for a given efficiency, as voltage increases the collection plate area required decreases. Therefore, for a given fixed amount of collection plate area, as voltage increases the efficiency increases.
The potential voltage that can be generated is based upon two primary limitations 1) the physical capabilities of the transformer and 2) the dielectric value of the gap between the high voltage electrode and collection electrode. All things remaining the same, the larger the gap the larger the dielectric value and the greater the potential voltage; larger gap = higher potential voltage.
Years ago, Japan had changed the particulate emission requirements for power plants with the result that many of their existing precipitators would have to be replaced or if room permitted added on to. Research and experimentation proved that the internal components could be removed and replaced with those with a larger gap between the high voltage electrodes and the collection plates while at the same time they installed transformers that could generate much higher voltages. The increasing of the gap resulted in less actual collection plate area because the external shell of the precipitators was a fixed sized. The result was an increase in efficiency with less collection plate area because of the increase migration velocity.
The Environmental Protection Agency (EPA) has recognized this feature, that voltage directly impacts collection efficiency, and has established criteria for the monitoring the power of consumed by precipitators as an indicator of performance. See EPA excerpt table.
The physical design of the precipitator can impact collection efficiencies in three primary ways: Airflow, Voltage Potential, and Corona Generation.
1) Airflow, as already discussed, can be too fast through the unit causing the re-entrainment of particles or the prevention of collection due to velocity head. Improper inlet duct design and improper air distribution can create situations where portions of the precipitator are receiving more flow per square foot of cross sectional area than others leading to reduces residence time, increased velocity head and re-entrainment in those areas with the net overall effect of the precipitator being less efficient.
2) Voltage potential, efficiencies as shown in equations presented above, has a direct impact upon collection. The physical design of the precipitator, specifically the clear space distance between the high voltage components and the grounded (or collection) surfaces dictate the maximum achievable voltage. When voltage reaches a point where its potential is greater than that of the resistance created by the distance between high voltage and collection surface the system will discharge this potential rapidly creating a spark or arc. Take for instance a high voltage electrode that is separated from ground by one quarter of an inch, the voltage required to spark over (jump the gap) is much less than if that gap was five inches. The closer the gap the higher the current and the greater the gap the higher the voltage. Two extremes of this concept can be visualized by connecting the high voltage electrode directly to ground, a short circuit and separating the high voltage from ground by ten feet, an open circuit. In a short circuit there is no voltage between the points and very high current, in the open circuit there is full voltage but no current. Precipitators operate at point between these two extremes, at a point where there is high voltage, enough to generate corona, and relatively low current.
3) Corona generation, also called ion generation, is directly dependent upon the high voltage electrode (discharge electrode) design. Sharp points and fine wires generate corona at much lower voltages than rounded or smooth corners. The collection of efficiency and current consumption rapidly increases once corona starts to occur. Corona can be seen as a visible blue or violet glow around the generation points.
Inlet Ducts and Nozzles
Inlet ducts and nozzles should be designed with air flow dynamics and particle fallout or sticking in mind. The duct work leading to the precipitator has to have sufficient velocity to keep the particles from falling out of suspension and accumulating on the bottom of the duct. When enough particles collect on surfaces it acts as insulation that can allow the surface of the metal to cool and condense water that can cause corrosion of the duct. When the velocity is too high the particles can wear away the duct at bend locations. Bends or turns in the ductwork can create unbalanced flow in that ductwork and upon entering the precipitator result in more flow in certain areas than others. Turning vanes should be used to reduce or eliminate non-uniform flow. The inlet nozzle itself should incorporate flow distribution devices or screens to equalize non-uniform flow and spread the gas stream across the cross sectional area of the precipitator. The increase in cross sectional area of the inlet nozzle compared to the inlet duct work will reduce the velocity of the gases dramatically allowing the larger suspended particles to fall out therefore, the inlet nozzle must be designed in a way that facilitates the removal of these particles.
Outlet Ducts and Nozzles
Many of the same design considerations for inlet ducts and nozzles apply to the outlet ducts and nozzles but in reverse order. Flow distribution devices in the outlet nozzle are often utilized to create uniformity of flow as the gases enter the outlet duct or stack. The outlet duct or stack is designed at velocities to eliminate corrosion or particle fallout. Regardless of whether the outlet nozzle is connected to a duct or stack if testing is to be performed then uniformity of flow is desired. Care should be taken, meaning turning vanes and flow uniformity devices should be utilized. In the case of stacks, velocity is vitally important because of rain and the fact that stacks are often not insulated. Two potential problems immediately come to mind if the velocity of the stack is insufficient. The first is that rain will be able to enter the stack and the velocity head of the flue gas will push that rain to the outer edges of the gas stream which would be the inside diameter of the stack. Most stacks on dry precipitators are constructed of carbon steel and the repetitive wetting of the stack will accelerate corrosion (rust). The second is that eddy currents can form along the side walls cooling the stack material that allows condensation to form with the same result.
The most important maintenance item in any precipitator are the insulators for if they fail then that field will cease to function. Many manufacturers utilize a purge air or heated air system to clean the insulators as clean as possible. Other areas of maintenance can include conveyors, ash removal systems and rotary seals.
Which Do I Need? Wet or Dry Precipitator?
Dry electrostatic precipitator ( ESP ) devices are employed on hot process exhausts(250 - 850 deg. F) that operate above the dew point of the gas stream. Dry electrostatic precipitator devices typically collect dry dust particles such as wood ash, incinerator ash, or coal ash from boiler or incinerator applications. Additional dry electrostatic precipitator applications include carbon anode ovens, cement kilns, and petroleum cat crackers. Dry electrostatic precipitator devices are attractive due to their ability to collect and transport the dust in a dry condition. This eliminates the use of water and the concerns of pollution, corrosion and dewatering efforts associated with scrubbers. If the dust particles can be collected and handled in a dry condition it is always more advantageous to employ a Dry ESP.
Wet electrostatic precipitator ( WESP ) devices are employed on exhausts that contain wet, sticky, tar like, tacky or oily particulates. Wet electrostatic precipitator ( WESP ) devices are an old technology originally designed in the 1920's to collect sulfuric acid mist using lead collection tubes. Today, WESP devices are employed on gas streams that include oily and sticky particulates or gas streams that must be cooled to saturation in order to condense aerosols that were formerly in the gas phase. Due to the different characteristics of the collected precipitate, the mechanical removal systems (rappers and vibrators) in Dry electrostatic precipitator devices are not effective. Consequently, the Wet electrostatic precipitator uses a water flushing system to remove the particles from the collecting surface. The gas stream is either saturated before entering the collection area or the collecting surface is continually wetted to prevent agglomerations from forming. Some mist aerosols simply gravity flow down the collecting surfaces. Wet electrostatic precipitator ( WESP ) devices are effective on acid mist, oil and tar based condensed aerosols or applications where dry dust particles combine with condensables to form paste like residues. Due to the wet environment of wet electrostatic precipitator devices, they are typically fabricated out of corrosion resistant materials such as stainless steel or special alloys.