air pollution chapter 5. control of air pollutants air quality standards air quality standards are...

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


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


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


dirty gas inlet

cleaned gas outlet

particulate outlet


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


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


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


charged electrodes soot laden smoke inlet

soot free gas escape

removal of soot particles earth point


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


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


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


Gaseous pollutants - Combustion

• 3 types of combustion systems commonly utilised for pollution control– direct flame, – thermal, and – catalytic incineration systems


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


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%


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


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


Gaseous pollutants – OdourThe main approaches include

• wet scrubbing,

• charcoal filtration and

• incineration


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


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

air pollution chapter 5. control of air pollutants air quality standards air quality standards are...

Documents

possible slide

efficient slide

air pollution chapter

cyclone separator slide

type of source control

obvious form of air

large particles

gas stream