How an Electrostatic Precipitator Works

 

Introduction

Currently, throughout the country, many places and people are pushing for a “greener” way of

life.  This doesn’t mean that they want us to turn into Kermit the Frog.  It means that they want

us to take a look at the way in which we do things.  They want us to look at more fuel efficient

vehicles, what things are made of, and how much pollution is entering our environment.  There

are many other factors to the whole “greener” phenomenon, but these seem to be the leading

problems. 

Air pollution standards change.  With that said, it is important for major refineries to update their

pollution equipment according to the new standards.  This costs millions of dollars, but they have

no choice.  Electrostatic precipitators are designed to deal mostly with coal fired refineries.  In

this country, we have more coal then we know what to do with, however, it does not burn

cleanly.  Lately though, coal has been making a comeback in big industries. Electrostatic

precipitators separate the clean from the unclean, and in the end what you have is a refinery that

is releasing air that is almost 99% cleaner.  Figure 1 shows an overhead view of how an

electrostatic precipitator works.

 

 

Figure 1. Top View of ESP.

 

  ESP’s are similar to the way an ionic breeze works.  They take the incoming dirty air and pass it

through a filtration device that purifies the air.  Any solid particles left over will fall into a large

storage container called a hopper and the clean air is brought out.  Figure 2 illustrates the

operation of an electrostatic precipitator in its simplest form.  This procedure will inform you on

what makes up an electrostatic precipitator and how they work once the pieces are put together.

 

                                                                                  

 

 

 

 

 

The main components of an ESP are:

-         Collecting Plates

-         Discharge Electrodes

-         Rappers

-         Hoppers

 

  

 

A. Collecting Plates

 

Figure 3. ESP Plate.

 

 

Figure 3 shows a typical collecting plate used in electrostatic precipitators. 

1. Collecting plates are made from rolled steel, and are welded together in the factory to

reduce the installation time at the jobsite.  Each plate contains electrodes which are positively

charged.  When the particulate gas enters the electrostatic precipitator and is struck with a

negative charge electrode, the positively charged plates act as a magnet and pull the

particulate gas to them.

2. The plates have both top and bottom stiffeners and plates which allow better mounting and

the ability to deal with more abuse from the rappers.  The plates are arranged to form a series

of gas passages.

3. There can be anywhere from 30-35 passages inside the electrostatic precipitator shell.  The

plates are placed parallel to the incoming particulate gas.

 

B. Discharge Electrodes

 

1. Discharging electrodes are a high voltage unit that negatively charges the particulate gas as

it enters.  These electrodes were once wires that were suspended from the ceiling and

weighted at the bottom, but are now a rigid mast. 

2. There are many types of configurations of discharge electrodes which can be tailored to

your needs.  The most common types are the two spiked and multiple spiked electrodes.

 Figure 4 shows the two spiked discharge electrode.

 

                                                   Figure 4. Discharge Electrode

                                               

 

 

3. The discharge electrode is mounted to a frame in between the collecting plates.  The

negatively charged particles that pass by the electrodes and then the counter charge of the

collecting plates; make the particulate like the magnet on a refrigerator.

 

 

 

 

 

C. Rappers

 

1. There are many types of rappers.  Rappers are used to dislodge the particulate from the

collecting plates.  Some types of rappers are mechanical, pneumatic, and the MIGI rapper. 

2. Mechanical rappers work like a hammer and chisel.  The hammers are attached to the rods

which are attached to a rolling cam above.  The cam is turned by an external motor and gear.

3.  The pneumatic rapper uses compressed air to operate.  Pneumatic powered rappers work

really well as long as the conditions permit and the factory has compressed air on tap.

4.  MIGI stands for magnetic impulse gravity impact rapper. The MIGI uses magnetic coils

to drive the hammer up and down. The magnetic coil is wrapped around the hammer.  When

the magnet is electrically energized, the hammer will be pulled up, similar to the way a

positive and negative magnet are attracted.  Once the electricity is turned off from the

magnet, the hammer falls, similar to the way two negative or positive magnets repel.

5.  All three are effective in what they do.  Rappers knock the solid particulate off of the

collecting plates where it is collected and then trucked away.

An example of each can be found in Figure 5, 6, 7. 

 

 

Figure 5. MIGI Rapper.

 

Source: http://www.apcnetwork.com/ESP%20Photos/Posters%20Pictures/Rappers/Picture%2001.JPG

Figure 6. Pneumatic Rapper

 

 

 

 

 

 

 

Source:

 

 

 

 

 

 

 

  

D. Hoppers

 

1. Hoppers look like upside down triangular prisms.  They are generally made of steel, and their

only purpose is to store particulate. 

2. Once the rappers have done their job, it is then time to collect the falling particulate.  Once the

particulate has entered the hopper, it is stored there until it is emptied and the particulate is

carried away by a conveyor. 

3. Most hoppers are heated so that the presence of moisture will be minimized.  The worst thing

that can happen is that the solid particulate gets wet and hardens in the hopper.  This will cause

the hoppers to be unable to be emptied causing serious issues.

 

 

Figure 8 gives a general view of what a hopper looks like.

 

Figure 8. Hopper.

 

 

 

 

 

How They Work

 

1. Once all the previous items have been created, they are placed in a “shell.”  This shell is

basically the home of the precipitator.  Almost all of the previous components are located in the

shell.  The shell is typically comprised of carbon steel.  It has holes on either side for the inlet

and outlet ducts.  The shell is also insulated to reduce the risk of condensation build up. 

2. Condensation will form when the flue gas which can leave the refinery at 200oF hits the inside

of a cold precipitator.  Also condensation will interfere with the way in which the electrodes

work and render them useless because the condensation will collect on the walls and will begin

to collect the particulate before it can be properly taken care of. 

3. The hoppers are one of the major external elements.  Yes, there are wires, piping and

ductwork on the outside, but in the end the hoppers collect the particulate.  They, just like the

shell, are typically made of carbon steel.  Their main job is to collect what is rapped off the

collecting plates.  Once the particulate has been collected, and the hoppers are full, valves and

access doors are used to evacuate the hoppers.  The hoppers can have piping running to them for

a vacuum operated system which will pull the particulate from the hoppers and bring it to a

remote storage facility; or they can have trucks driven underneath them that will physically truck

the ash away.

4. Inside of the shell, the collecting electrodes are assembled parallel to the inlet duct.  They are

assembled this way because it is the most economical and efficient way to collect the

particulate. 

5. The discharge electrode is a different story.  The discharge electrode runs perpendicular to the

inlet and collecting electrodes.  As the flue gas comes out of the inlet and enters the precipitator

the rigid mast discharge electrodes greet the flue gas and negatively charge them; allowing the

positively charge plates that are running parallel to the inlet to collect the flue gas.

6. The other major external element is the rapper.  It sits on top of the roof of the precipitator,

and is programmed to deliver its powerful strike within a certain timed interval.  The hammer

end strikes the top of the collecting plates, making all the collected particulate fall into the

hoppers below.

7. Electricity is the major power source to operate just about everything on the precipitator.  It is

used to power the electrodes, both positive and negative, and powers the rappers.  It can also be

used to power the vacuum system on the evacuation of the hoppers, if the hoppers have this

option.  Most precipitators run auxiliary transformers to subsidize the amount of energy needed

to keep the precipitators running.

8. The final element of an electrostatic precipitator is the outlet duct.  Although not really

covered in this procedure very much, it does hold one vital role.  It is where the new clean gas

will leave the precipitator.  So in the end that final piece of ductwork gives the gas a final exit

strategy from inside the shell where all the work had been done previously.

 

Conclusion

 

Electrostatic precipitators have been a reliable technology since the early 1900's. They were

originally developed to stop serious smoke and air issues and have definitely helped the

environment.   Today electrostatic precipitators are found mainly on large power plants, cement

plants, incinerators, and various boiler applications.