Reduction of Air Pollutants

Each pollution control device matches its method to the physical form of the pollutant. A scrubber targets sulfur dioxide gas rising in smokestack flue gas. It sprays a lime slurry, calcium hydroxide mixed with water, up through the gas stream. The sulfur dioxide reacts with the calcium hydroxide to form calcium sulfite, a solid that settles out and gets removed as sludge. The gas that escapes the stack is now mostly free of sulfur.

An electrostatic precipitator targets solid particles, soot and ash, rather than gas. Wires inside the precipitator give incoming particles a negative electric charge. Charged metal plates nearby, carrying the opposite charge, attract those particles out of the moving gas stream the way a charged balloon pulls at hair. The particles stick to the plates and get shaken loose into a collection hopper, while clean gas moves on.

A catalytic converter sits in a car's exhaust line and handles three pollutants at once because it runs two reactions in one chamber. Its precious metal coating, often platinum, palladium, or rhodium, speeds up oxidation of carbon monoxide and unburned hydrocarbons into carbon dioxide and water, and at the same time drives reduction of nitrogen oxides back into harmless nitrogen and oxygen gas. Selective catalytic reduction in large plants works on the same reduction principle, injecting ammonia or urea to convert nitrogen oxides into nitrogen gas and water, cutting that one pollutant sharply without touching sulfur or particulates, which is why plants often pair it with a scrubber and a precipitator.