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.
The main components of an ESP are:
- Collecting Plates
- Discharge Electrodes
- Rappers
- Hoppers
A. Collecting Plates
Figure above 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 bellow shows the two spiked 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.
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 bellow gives a general view of what a hopper looks like.
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.
Thanks for the very detailed information on Electrostatic Precipiators.
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