AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/7/7-6)
Unit 7 · Topic 7.6
7.6 Reduction of Air Pollutants
You can reduce air pollution in two ways: prevent it from forming (use less fuel or cleaner fuel) or capture it before it escapes. This topic covers the control devices you need to know, such as catalytic converters, scrubbers, electrostatic precipitators and vapor recovery nozzles, and what each removes.
Key terms
- catalytic converter
- scrubber
- electrostatic precipitator
- vapor recovery nozzle
- fluidized bed combustion
Prevention first
The cheapest pollution to deal with is pollution that never forms. Prevention strategies include conserving energy, switching from coal to natural gas or renewables, burning low-sulfur coal or removing sulfur from fuel before burning it, driving less, using public transit, and raising fuel-economy standards. Regulations and incentives such as emissions limits and cap-and-trade programs push industries toward these choices.
Controls on vehicles and gas stations
- Catalytic converter: a device in a car's exhaust system coated with metals such as platinum, palladium and rhodium. Required on new US cars since the mid-1970s, it converts carbon monoxide into CO₂, unburned hydrocarbons into CO₂ and water, and nitrogen oxides into nitrogen gas and oxygen. It does not remove CO₂.
- Vapor recovery nozzle: a gas pump nozzle with a rubber boot and return line that captures gasoline vapors (VOCs) while you fill up and sends them back to the underground tank, instead of letting them escape and form smog.
Controls at power plants and factories
| Device | How it works | What it removes |
|---|---|---|
| Wet scrubber | Sprays exhaust gas with a water slurry of lime or limestone, which reacts with SO₂ to form a solid (calcium sulfite or sulfate, which can become gypsum) | Mainly SO₂, plus some particulates |
| Dry scrubber | Sprays a fine powder or mist of lime into the hot exhaust, where it reacts with SO₂ and dries into a solid that is collected with the dust | SO₂ and other acid gases, plus some particulates |
| Electrostatic precipitator | Gives particles in the exhaust an electric charge, then oppositely charged plates attract and collect them; the plates are shaken to drop the dust into hoppers | Particulates (over 99% of them), not gases |
| Baghouse filter | Pushes exhaust through large fabric bags that trap particles, like a vacuum cleaner bag | Particulates |
| Fluidized bed combustion | Burns crushed coal with limestone in a bed of hot particles kept churning by air blown from below; the limestone captures sulfur, and the lower burning temperature makes less NOₓ | SO₂ and NOₓ (prevents them forming) |
Matching the device to the pollutant
Exam questions often name a pollutant and ask for a device that reduces it. Use this quick guide: SO₂ → scrubber, fluidized bed combustion or low-sulfur coal; particulates → electrostatic precipitator or baghouse; NOₓ and CO from cars → catalytic converter; VOCs at gas stations → vapor recovery nozzle.
Control devices have trade-offs. Scrubbers make large amounts of sludge or gypsum to dispose of; precipitators use electricity; and all of them add cost. None of them reduce CO₂, which requires burning less fossil fuel or capturing carbon separately.
Worked examples
Try each one yourself first, then open the solution.
- Example 1Calculator allowed
How much SO₂ does a scrubber remove?
Before installing a scrubber, a coal plant released 12,000 metric tons of SO₂ per year. The scrubber removes 95% of the SO₂. (a) How much SO₂ does the plant release per year after installation? (b) How much is removed?
Show the solutionHide the solution
- Step 1: If 95% is removed, 100% − 95% = 5% still escapes.
- Step 2: (a) Released: 12,000 × 0.05 = 600 metric tons per year.
- Step 3: (b) Removed: 12,000 × 0.95 = 11,400 metric tons per year (check: 600 + 11,400 = 12,000).
Answer: (a) 600 metric tons per year. (b) 11,400 metric tons per year removed.
- Example 2
Trap: the wrong device for the pollutant
A city wants to reduce acid rain caused by a coal plant. An engineer proposes adding an electrostatic precipitator. Will this work? Explain.
Show the solutionHide the solution
- Step 1: Acid rain from coal plants comes mainly from SO₂ (plus NOₓ), which are gases.
- Step 2: An electrostatic precipitator removes particles by charging them and collecting them on plates. It does not capture gases like SO₂.
- Step 3: To cut SO₂, the plant needs a scrubber, fluidized bed combustion or lower-sulfur coal.
Answer: No. A precipitator removes particulates, not SO₂. A scrubber (or fluidized bed combustion, or low-sulfur coal) would reduce acid rain.
Common mistakes
- Saying catalytic converters remove CO₂. They convert CO, hydrocarbons and NOₓ; CO₂ still comes out.
- Using an electrostatic precipitator for SO₂. Precipitators catch particles, not gases.
- Describing a scrubber as a filter. It removes SO₂ by a chemical reaction with lime or limestone.
On the exam
- The most common question: name a device or method that reduces a specific pollutant and explain how it works. Match the device to the pollutant carefully.
- Percent-removal calculations show up in free response. Subtract the percent removed from 100% to find what still escapes.
Connected topics
Videos
Check yourself
4 questions on 7.6 Reduction of Air Pollutants. Pick an answer to see if you got it, and why.
A catalytic converter reduces air pollution from cars by converting carbon monoxide, nitrogen oxides and unburned hydrocarbons mainly into which products?
A coal-fired power plant must cut both its sulfur dioxide and its particulate emissions. Which combination of technologies best fits these goals?
In fluidized bed combustion, crushed coal is burned while mixed with limestone particles. What is the main benefit of adding the limestone?
An electrostatic precipitator removes particulates from a power plant's exhaust by
0 of 4 answered