air pollution
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Air Pollution. Chapter 5. 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. Control of Air Pollutants. - PowerPoint PPT PresentationTRANSCRIPT
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Air Pollution
Chapter 5
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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
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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
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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
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Control of Air Pollutants
dirty gas inlet
cleaned gas outlet
particulate outlet
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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
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Control of Air Pollutants
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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 5m
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Control of Air Pollutants
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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
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Control of Air Pollutants
charged electrodes soot laden smoke inlet
soot free gas escape
removal of soot particles earth point
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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
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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
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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
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Control of Air Pollutants
Gaseous pollutants - Combustion
• 3 types of combustion systems commonly utilised for pollution control– direct flame, – thermal, and – catalytic incineration systems
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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
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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%
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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
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Control of Air Pollutants
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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
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Control of Air Pollutants
Gaseous pollutants – OdourThe main approaches include
• wet scrubbing,
• charcoal filtration and
• incineration
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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
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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
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Control of Air Pollutants