Types of smoke precipitators

Electrostatic smoke precipitators all work in essentially this way, with dirt particles gaining an electric charge from one wire or plate before being attracted to a second wire or plate with the opposite electric charge for collection and disposal. Even so, there are many variations, with ESP equipment designed to work in different ways for different-sized dirt particles made from different
chemicals and different amounts of pollution (different flue-gas flow rates). The coal burned in power plants around the world varies substantially in its chemical composition and the designers of ESP equipment have to take this into account. Some power plants try to minimize sulfur dioxide pollution (which causes acid rain) by burning low-sulfur coal that typically produces more ash. Since the coal is less calorific (energy-containing), you need to burn more of it to make the same amount of energy, which means more ash again. Low-sulfur coal produces a different kind of ash that has a higher electrical resistivity. This makes ESP equipment less effective so, to achieve the same reduction in pollution, it generally has to be scaled-up in size to compensate. The effectiveness of ESP is also affected by the temperature and moisture content of the flue gas (low-sulfur coal burns at a lower-temperature and often contains more moisture). ESP isn't 100 percent effective and some pollution will always remain behind. In some states and regions, the air pollution laws are stricter than in others, and that might mean that a power plant needs larger plates (or multiple sets of them) to ensure the gas emerging from a smokestack is cleaner.

All sorts of other factors affect the design of ESP equipment. For example, if you want to scrub out very large amounts of dust or dirt, you need to focus not just on removing pollution from the air but on how you'll continually remove large amounts of dirt from the scrubber with an automated rapping mechanism (motorized steel brushes, vibrating hammers, and some kind of hopper underneath). If the air you're scrubbing contains more than one type of pollutant, as it does if it's the flue-gas from a power plant, ESP might be only one stage of a multi-stage scrubbing process including other steps such as catalytic cleaning (used to remove nitrogen oxides) and wet flue-gas desulfurization (WFGD, used to remove sulfur dioxide). If you're running a mineral processing plant, maybe the pollutant you're worried about is quite valuable and it might pay you to capture it and recycle it, so you'll need to design some automated way of doing this.

The point is that while electrostatic smoke precipitation is a general technology, every place where it's used has unique features and problems, and equipment needs to be designed with these in mind if it's going to be effective at keeping air pollution to a minimum.

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