An Electrostatic Precipitator is a device that removes particles from a flow of gas.
We want to do this in order to reduce pollutants emitted into the atmosphere. Electrostatic precipitators are generally able to remove more than 99% of the particles in exhaust, without having to force the air through a filter. Filters would have to be replaced quite often, and the pressure drop caused by having to push the air through the filter reduces the efficiency of the plant.
How do these wondrous devices work? Well, you pass the gas by a very
high voltage electrode, up to 80 kilovolts, which will then impart some negative
electrical charge to any particles in the air. Despite the high voltage needed, the current is very low, and thus the power used is low. Therefore, the electricity needed only costs a few dollars per hour of operation.
These negatively charged particles will then be drawn away from the negatively charged electrodes, towards positively charged plates, where the particles collect.
There are two types of electrostatic precipitators, dry, and wet. Dry is more common than wet.
Dry precipitators work on particles that, shockingly, are dry. Dust and ash and the like. The particles will be drawn towards plates hanging on the side of the device, where they will collect. Periodically, the plates will be struck by a hammer, or something like it, or vibrated, causing the particles, now discharged, to fall off. They are then collected, and disposed of accordingly.
Wet precipitators on the other hand, are used to strip gasses of "wet" particles, such as resin, oil, paint, tar, acid, or anything that can't be handled well dry. Sometimes, the gas will be saturated with steam before it enters the precipitator. In this case, the steam / particulate mix will be drawn to the sides, and simply flow down the sides. In other cases, the gas flow is unchanged before it enters the precipitator, which will cause whatever is in the gas flow to deposit on the sides. In some cases, this will simply drip down on its own, in other cases it must be periodically flushed off with water.
Wet precipitators are generally more expensive than their dry counterpart. They have to be made out of non corrosive materials such as stainless steel, and the flushing mechanisms are more complicated than the collection method of the dry precipitator. As well, they need to go to the extra step of purifying the water used to clean the gunk off the sides, which depending on what you're cleaning from the gas flow, can be quite an intensive project on its own.
Naturally, this is a simplified view of how an electrostatic precipitator works. There will not be a single electrode, or a single collecting plate, but quite a number.The actual setup, naturally, varies from installation to installation. Electrostatic precipitators are quite useful in manufacturing plants, and power plants, especially coal burning power plants. While the initial setup of an electrostatic precipitator will be more expensive than other systems such as filters, or scrubbers (soaking the gas with steam, and then condensing it, along with the particles out), over time they are more efficient. The only real cost is the electricity, as opposed to the drop of pressure, and associated loss of overall plant efficiency that occurs with other gas cleansing methods. Over time, electrostatic precipitators will be both more energy and cost efficient.
Sources:
Gerry Graham. "Controlling Stack Emissions in the Wood Products Industry," PPC Industries - Electrostatic Precipitator. <www.ppcesp.com/ppcart.html> (December 30, 2004).
Kenneth Friedman. "How an Electrostatic Precipitator Works," Kenneth Friedman's Home Page. 2000. <www.lehigh.edu/~kaf3/envt/HowanElectrostaticPrecipitatorWorks.htm> (December 30, 2004).
PPC Industries. "What is an Electrostatic Precipitator," PPC Biofilter, PPC Industries, Wet Dry Electrostatic Precipitator, Wet Dry ESP. <www.ppcbio.com/ppcdespwhatis.htm> (December 30, 2004).