1. Air Pollution
Chapter 5

2. Control of Air Pollutants
Air Quality Standards
Air quality standards are provided by many groups and organisations such as:
National Environment Protection Council Standards
WHO
US EPA
NSW EPA/DECC standards
Standards Australia

3. Control of Air Pollutants
Particulates
Particulate matter the most obvious form of air pollution – therefore receives the most effort in pollution reduction measures
Process Modifications
fuel substitution
encapsulation and wet operation can also greatly reduce the amount of fugitive particles emanating from a potential pollutant source

4. Control of Air Pollutants
Particulates – Cyclones
separation by centrifugation - most common form of particulate removal
gas is spun rapidly - heavier particulate matter to collect on outside of separator by centrifugal force, where it is collected and removed
cyclone separator

5. Control of Air Pollutants

6. Control of Air Pollutants
Particulates – Filtration
Fibre bags commonly used for control of particulate emissions with very high dust loadings and smaller particles
As the gas changes direction, large particles are removed by inertial separation and collected in the hopper
dust is collected on the inside of the bag surface and the filtered gas is discharged to the atmosphere - 99% efficient

7. Control of Air Pollutants

8. Control of Air Pollutants
Particulates – Wet Scrubbers
spray systems where fine water droplets are sprayed at high velocity at right angles to the emerging gas
Most of the particles in the gas stream are scavenged by the water droplets, which fall and are collected along with the particles
relatively low efficiencies (80-90%) and is usually employed as a pre-cleaner to remove particles larger than 5m

9. Control of Air Pollutants

10. Control of Air Pollutants
Particulates – Electrostatic Precipitators
pass dirty gas through a series of fine wires (coronas) charged with DC current – causes particles to coalesce & precipitate
Alternatively corona produces negative ions that cause particles in the gas stream to become negatively charged, and attracted to positive terminal – where they coalesce and fall into a collection hopper
Large precipitators and low gas flow rates give better results

11. Control of Air Pollutants

12. Control of Air Pollutants
Gaseous pollutants - Process Modifications
simplest and least expensive methods for the control of gaseous pollutants
fuel substitution e.g. low sulfur coal, or fuel oils in place of cheaper coal can greatly reduce the amount of SO2 emissions at the source
This type of source control is always the best approach wherever possible

13. Control of Air Pollutants
Gaseous pollutants - Combustion
involves a series of complex chemical reactions in which oxygen is combined with organic molecules, to form CO2 and H2O
commonly referred to as incineration or afterburning
afterburning is applicable when the treatment process is located downstream of a primary combustion process

14. Control of Air Pollutants
Gaseous pollutants - Combustion
Incineration applied to effluent streams containing combustible gases
Incineration can be used to eliminate;
malodourants such as mercaptans and H2S
organic aerosols and visible plumes such as those produced by coffee roaster and enamel bake ovens
combustible gases produced by refineries, and
solvent vapours produced by a variety of industrial processes

15. Control of Air Pollutants
Gaseous pollutants - Combustion
3 types of combustion systems commonly utilised for pollution control
direct flame,
thermal, and
catalytic incineration systems

16. Control of Air Pollutants
Gaseous pollutants - Adsorption
physical adsorption to solid surfaces
Reversible - adsorbate removed from the adsorbent by increasing temp. or lowering pressure
widely used for solvent recovery in dry cleaning, metal degreasing operations, surface coating, and rayon, plastic, and rubber processing

17. Control of Air Pollutants
Gaseous pollutants - Adsorption
limited use in solving ambient air pollution problems – with its main use involved in the reduction of odour
Adsorbents with large surface area to volume ratio (activated carbon) preferred agents for gaseous pollutant control
Efficiencies to 99%

18. Control of Air Pollutants
Gaseous pollutants - Absorption
Scrubbers remove gases by chemical absorption in a medium that may be a liquid or a liquid-solid slurry
water is the most commonly used scrubbing medium
Additives commonly employed to increase chemical reactivity and absorption capacity

19. Control of Air Pollutants

20. Control of Air Pollutants
Gaseous pollutants – Dry Scrubbing
used to remove large amounts of SOx from flue gases using a dry alkaline absorbent (usually lime or sodium carbonate)
several advantages over wet scrubbers.
do not suffer from scaling or residue build up
do not require elaborate sludge handling systems for waste materials
less maintenance as there is less corrosion
they use up to 50% less power and water

21. Control of Air Pollutants
Gaseous pollutants – Odour
The main approaches include
wet scrubbing,
charcoal filtration and
incineration

22. Control of Air Pollutants
Gas pollutants – Vehicle emissions
generally involve simple procedures such as maintaining the correct tuning for the engine, or the use of catalytic converters
catalytic converters use Pt and Pd attached to some form of ceramic material
extremely high surface area (in hundreds of m2) allows catalytic materials to contact exhaust gases, oxidising them to CO2 and water vapour

23. Control of Air Pollutants
Gas pollutants – Vehicle emissions
all the measures which decrease CO and hydrocarbon emissions, increase NOx emissions
measures such as changing engine spark plug timing and reduction of compression ratios allow NOx emissions to be lowered without greatly increasing other pollutant emissions

24. Control of Air Pollutants