Electrostatic smoke precipitators

Last updated: December 9, 2008.

No-one likes smoke. Whether it's rooms full of cigarette fumes or cities choking with pollution, we'd all much prefer to be breathing cool, clean air. But industry is the lifeblood of our economic system and many people assume we have to put up with factory and power-plant smokestacks that choke up the air. In fact, technology has had a pretty good solution to the problem for over a century, in the shape of electrostatic smoke precipitators. Also known as scrubbers, these cunning, static-electric filters snatch the soot from dirty air as it flows along a pipe, greatly reducing pollution and helping to improve the environment. Let's take a closer look at how they work!

Photo: An electrostatic smoke precipitator scrubs the waste gases at the McNeil biomass power plant in Burlington, VT. The precipitator is the smokestack on the left closest to the camera. Photo by Warren Gretz courtesy of US DOE National Renewable Energy Laboratory (NREL).

The old balloon trick

Have you tried that trick where you rub a balloon repeatedly on your sweater until it sticks by the sheer force of static electricity? As you rub the balloon, electrons break free from the molecules of rubber that are inside it and gather on your sweater. The balloon (losing electrons) becomes positively charged while your sweater (gaining electrons) become negatively charged. Because opposite charges attract, the balloon sticks to your sweater.

Photo (right): Smoke is an aerosol of soot particles dispersed in hot, rising air. Photo by courtesy of US Fish and Wildlife Service.

What does all this have to do with smoke? Smoke might look like a gas but it's actually an aerosol. Most aerosols (such as air fresheners and polishes that come out of aerosol cans) are made of liquid droplets dispersed through gases, but smoke is a bit different: it's a solid dispersed in a gas. Smoke consists of microscopically tiny particles of soot (unburned carbon) dispersed through hot, rising air. Imagine if you could "rub" all the tiny smoke particles as they entered the bottom of a smokestack to give them a tiny electrical charge... and you wrapped something like a sweater round the inside of the smokestack at the top. In theory, the electrically charged smoke particles would cling to the sweater, removing the dirt from the smoke and leaving clean hot air to emerge by itself!

How electrostatic smoke precipitators work

Photo (left): Electrostatic smoke precipitator equipment in close-up. Photo by Dave Parsons courtesy of US DOE National Renewable Energy Laboratory (NREL).

That's the basic idea behind electrostatic smoke precipitators. They pass a sooty gas through a pipe past a set of wires charged to a very high negative voltage. As the soot particles move past the wires, they pick up a negative charge. Higher up the pipe (or further along, if it's a horizontal pipe), there are metal plates charged to a high positive voltage. Since like charges attract, the negatively charged soot particles are attracted to the positively charged plates and stick there. From time to time, the plates have to be shaken to empty away the soot; that can be done either manually (by someone brushing the plates clean) or automatically (by some kind of automated shaking mechanism).

Here's how a precipitator works in a nutshell:

Illustration: Electrostatic smoke precipitators use static electricity to remove the soot from smoke. Dirty air moves past negatively charged wires (shown as −ve) so the soot particles pick up a negative charge. The negatively charged particles of soot then stick to positively charged plates (shown as +ve) further along the pipe. Much cleaner air emerges from the pipe as a result. Most precipitators also have simple filters at each end of the pipe (shown by the orange bars) to remove larger soot particles.