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 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

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

Control of Air Pollutants

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

Control of Air Pollutants

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

Control of Air Pollutants

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

Control of Air Pollutants

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

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

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

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

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

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%

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

Control of Air Pollutants

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

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

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

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

Control of Air Pollutants