AP Syllabus focus:
‘Wet and dry scrubbers remove particulate matter and/or gases from industrial exhaust streams.’
Industrial scrubbers are end-of-pipe air-pollution control devices that treat hot exhaust before it leaves a smokestack. They reduce human and ecosystem exposure by physically removing particles and chemically capturing reactive gases.
What scrubbers control (and why)
Scrubbers are designed to reduce:
Particulate matter (PM): soot, ash, metal-bearing dusts that can penetrate lungs and reduce visibility.
Acid gases: especially sulfur dioxide (SO₂) and sometimes hydrogen chloride (HCl) and hydrogen fluoride (HF), which contribute to acid deposition and respiratory irritation.
In some systems, certain toxic metals and mists attached to particles are also captured.
Core idea: contacting exhaust with a capture medium
Scrubbers work by forcing polluted exhaust (flue gas) to contact a medium that can trap pollutants:
For PM, removal is mostly physical (collisions with droplets or surfaces).
For gases, removal is mainly chemical/physical absorption followed by neutralisation with an alkaline material.
Wet scrubbers
How wet scrubbers operate
A wet scrubber sprays or circulates a liquid (usually water with additives) through the exhaust stream so pollutants transfer from gas to liquid.
Exhaust enters the scrubber body.
Liquid is introduced as sprays, films, or turbulent droplets.
PM impacts and sticks to droplets; droplets then collect and drain.
Soluble gases dissolve into the liquid; alkaline chemicals may be added to react with the dissolved gas.
A mist eliminator reduces liquid carryover before the cleaned gas exits the stack.
Chevron (vane) mist eliminator schematic (vertical flow): The diagram shows how the gas stream is forced through a zig-zag channel, causing entrained liquid droplets to impact the vanes, coalesce, and drain back downward. This illustrates why mist eliminators are placed near the outlet of wet scrubbers: they reduce droplet “carryover” so the stack exhaust is a cleaned gas rather than a wet aerosol plume. Source
Wet scrubber: An air-pollution control device that uses liquid droplets to capture particulate matter and/or absorb gases from an exhaust stream, producing a liquid waste or slurry.
Typical targets and chemistry
Wet scrubbers are widely used for SO₂ control in forms of flue gas desulfurization (FGD), often using limestone or lime.
Key design/operation factors:
Liquid-to-gas ratio (more liquid contact generally increases capture)
Droplet size (smaller droplets increase surface area)
pH control (higher alkalinity improves acid-gas neutralisation)
Temperature (affects solubility and reaction rates)
Trade-offs (wet)
Advantages:
High removal for acid gases and many fine particles
Can handle hot, humid, variable exhaust streams
Limitations:
Produces wastewater/sludge (must be treated/disposed)
Potential corrosion and higher maintenance
Uses water and can cause visible plumes if not managed
Dry scrubbers
How dry scrubbers operate
A dry scrubber captures pollutants using a dry or semi-dry alkaline material, limiting liquid waste.
An alkaline sorbent (commonly lime or sodium-based compounds) is injected into the exhaust.
Acid gases react with the sorbent to form solid salts.
The resulting particles are removed downstream by particle control equipment (often fabric filters), leaving cleaner gas to exit the stack.
Dry/semi-dry scrubber (spray dryer absorber) cutaway: The figure traces flue-gas entry, turbulent mixing, and slurry/sorbent atomization inside the absorber, where acid gases react to form solid products. The outlet labeled “To Particulate Collection” emphasizes that these systems shift pollution control to downstream capture of the newly formed particles (e.g., in a baghouse/fabric filter). Source
Dry scrubber: An air-pollution control device that injects a dry or semi-dry alkaline sorbent into exhaust so acid gases form solid particles that can be captured as dry waste.
Dry vs wet (practical differences)
Dry scrubbers are often chosen when:
Water availability is limited
Minimising liquid effluent is a priority
Retrofitting space or operational constraints favour simpler plumbing
Trade-offs:
Typically lower SO₂ removal than the best wet FGD (system-dependent)
Produces a dry solid waste stream that still requires disposal or reuse
Performance depends strongly on mixing, contact time, and sorbent distribution
Industrial context and environmental significance
Scrubbers are commonly applied to major stationary sources, especially:
Coal- and oil-fired power plants
Industrial boilers
Waste incinerators
Metal smelters and cement kilns
By removing particulate matter and/or gases from industrial exhaust streams, wet and dry scrubbers reduce downwind impacts such as respiratory disease risk, haze, and acidifying deposition, while shifting pollution management toward controlled waste handling.
FAQ
Choice depends on reagent cost, local availability, target gases, by-product value, and handling safety.
Sodium sorbents can be highly reactive; limestone is often cheaper but may require more contact time.
Wet systems may produce sludge that is dewatered; some FGD by-products can be processed into gypsum.
Dry systems generate collected solids that are landfilled or, less commonly, beneficially reused if uncontaminated.
Acid gases must collide with sorbent particles.
If sorbent is unevenly distributed, “untreated” channels of flue gas pass through, reducing overall capture efficiency.
They use mist eliminators (baffles or mesh) to capture entrained droplets.
Good drainage and maintenance prevent re-entrainment during high flow events.
No. Efficiency depends on design; very fine particles can be harder to capture.
Devices that increase turbulence and droplet surface area generally improve fine-particle removal, but may increase energy use and pressure drop.
Practice Questions
State two types of pollutants that wet and dry scrubbers can remove from industrial exhaust streams. (2 marks)
Any two from: particulate matter/particles; sulfur dioxide; other acid gases (e.g. HCl/HF). (1 mark each)
Describe how a wet scrubber removes pollutants and explain two disadvantages of using wet scrubbers compared with dry scrubbers. (5 marks)
Describes liquid–gas contact (sprays/films/droplets) enabling pollutant capture. (1)
Particulate matter removed by impaction/collection into droplets and draining. (1)
Acid gases absorbed/dissolved and neutralised with an alkaline additive. (1)
Disadvantage: produces wastewater/sludge requiring treatment/disposal. (1)
Disadvantage: higher corrosion/maintenance or higher water use (credit either). (1)
