Air Pollution

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