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How they handle Smoke, Gasses and Chemical emissions
at a Coal Fired Power Station
Various gases and ash are produced when coal is burned in a power station. Some of these byproducts are harmful to people and animals, as well as to the environment.
Today we’re going into Eskom’s Grootvlei coal fired power station to have a look at the way in which these harmful gases and ash are handled at a South African coal fired power station.
Producing electricity from coal, starts when the coal is pulverized in huge mills into a fine powder. This fine powder is blown into huge kettles, called boilers.
The coal burns to generate heat to turn water into steam.
The steam is used to turn the blades of a a turbine.
The turbine shaft turns a generator and the generator produces an electric current, which is stepped up in voltage by transformers and sent to the homes and factories of consumers via power lines.
Coal contains various elements which are harmful to the environment when the coal is burned.
Some of these include oxides of sulphur, oxides of nitrogen, particulates, and various trace elements such as mercury.
Another byproduct of burning coal is the tiny particles of fly ash that are so small that they can be suspended in the air. Clean coal technologies have been developed to remove or reduce pollutant emissions to the atmosphere.
Low NOx burners
Nitrogen Oxides are controlled using low NOx burners. A burner regulates the coal to air ratio that are blown into the furnace.
Low NOx burners are designed to create larger and more branched flames. Peak flame temperature is thereby reduced, and results in less Nitrogen Oxide formation.
After the coal has been burned in the furnace it’s exhausted through a filtration unit. South African plants use two methods of particulate & ash removal, a Bag Filter system and an Electrostatic Precipitator.
Most baghouses use long, cylindrical bags made of non-woven or felted fabric as a filter medium.
Dust-laden air enters the baghouse and is drawn through the bags, either on the inside or the outside depending on cleaning method. The particles accumulate on the filter media surface until the air can no longer move through it. At this point the bags are cleaned using a very special cleaning technique.
A short burst of compressed air injected through a manifold over a row of bags. This compressed-air burst acts as a fast moving air bubble, traveling through the entire length of the bag and causing the bag surface to flex. This flexing of the bags breaks the dust cake, and the dust falls into a storage hopper below.
Bag filters are able to capture more than 99% of the fly ash and dust, even when particle size is very small.
The second method of removing fly ash from gases exhausted from the furnace is known as an electrostatic precipitator.
An electrostatic precipitator is a particulate collector that removes fine particles, like fly ash, from a flowing gas stream using the force of an induced electrostatic charge
The most basic precipitator contains a row of thin vertical wires, and followed by sheets of large flat metal plates on each side of the row of wires, oriented vertically. The air stream flows horizontally through the spaces between the wires, and the sheet of plates.
A negative voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough, an electric corona discharge ionizes the air around the electrodes, which then ionizes the particles in the air stream.
The ionized particles are diverted towards the grounded plates. Particles build up on the collection plates and are removed from the air stream.
A system of hammers knocks the particles off the collection plates and the dust falls into a hopper at the base of the electrostatic precipitator.
The ash is conditioned with water and is carried on conveyors to the ashing facilitywhich is located nearby.
Modern ashing facilities are lined with plastic to prevent the leaching of toxins and heavy metals into the surrounding soil and ground water.
With world-wide awareness and actions taken to prevent toxins from damaging our environment, new systems are being developed and tested on a continuous basis.
Coal-fired electricity generation is far cleaner today than ever before.
Kusile & Medupe
THE NEW GENERATION OF SOUTH AFRICAN COAL-FIRED POWER STATIONS, KUSILE AND MEDUPI, WHICH ARE UNDER CONSTRUCTION IN MPUMALANGA and Limpopo AT PRESENT, WILL BE FITTED WITH FABRIC FILTER PLANTS TO REDUCE PARTICULATE EMISSIONS, LOW NOX BURNERS TO REDUCE NITROGEN OXIDE EMISSIONS, AND FLUE GAS DESULPHURISATION TO REDUCE SULPHUR DIOXIDE EMISSIONS.
Modern coal plants and older plants retrofitted with modern technologies to reduce pollution are a success story.
Undoubtedly, pollution emissions from South African coal-fired power plants will continue to be reduced as technologies improve.