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Electricity, in particular thermal power generated using fossil fuels, produces the largest amounts of industrial greenhouse gas emissions. This is because fossil fuels contain carbon which when burned in air produces CO2.
In this series on power stations we look at three different types of fuel supplied to the thermal power stations, this first article being on coal fuel, then natural gas and ending with biomass fuel. We will examine the different methods of producing electricity and, fume treatment, both the present and future methods.
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The UK coal industry was dismantled by Maggie Thatcher’s Government in one of the bitterest disputes between a government and the trade unions ever witnessed. Since then mines have been continuously shut down until now we have only four surface mines and four deep mines operating in Britain today.
However we still use coal in our homes and for power generation fuel, so now most of the steaming coal used in the power stations is imported from as far away as Russia, South Africa, and Australia.
The coal arrives in Britain by ship and is transported by rail to the power station stockpile.
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Power Station Stockpile
It is distributed into heaps in the stockpile by a bucket wheel machine. This is a device with large buckets arranged in a circle on the end of a rotating head. From the heaps the coal is loaded onto an enclosed conveyor.
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From the stockpile a conveyor system conveys the coal to the pulverisers.
The coal pulverisers are really large ball mills having numerous hollow steel balls weighing over a ton each and about 750mm diameter. The pulveriser drum rotates causing the balls to move in a circular motion inside as the coal is fed into the drum. The coal is not large lumps like we use at home, but in small pieces of between 25 and 50mm known as steaming coal.
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Water Tube Boilers
The pulverised coal, now really coal dust is blown by the primary air fan to the boiler coal burners and into the furnace. Forced draught fans propel the combustion air from the top of the boiler room through an air heater into the furnace combining with the coal dust. The mixture ignites and boils the water in the boiler tubes, turning it first to wet steam in the steam drum and then to superheated steam, in the superheater, which is now used to power a steam turbine.
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The steam turbine can be one of several mechanical arrangements; the one shown is an in-line, reheat, single casing arrangement which has three stages. Superheated steam enters the High Pressure (HP) stage; the expanded steam is returned to the boiler and reheated before it enters the Intermediate Pressure (IP) stage. From here the exiting steam passes onto the dual Low Pressure (IP) stage where it expands before exiting to a vacuum condenser and hotwell from which the condensate is pumped into the boiler feed system.
This steam turbine drives a generator producing electricity which after passing through transformers is fed to the National Grid.
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Ash and Fume Extraction
The furnace ashes are gathered and sold to the construction industry, with any surplus usually going to a disposal site.
The furnace combustion gasses pass through the superheater, economizer, and air heater before being discharged out the flue after being purified by the fume extraction system.
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Dust and Particle Extraction
The fumes are extracted by the induced draught fan drawing the gases through the particle extractor/filtration system of bag filters or electrostatic precipitators. These reduce the fly ash and other dust particles, with the residue being sold to the construction industry as aggregate.
The fumes are now passed through a gas cooler reducing the gas temperature, and then compressed before passing on into the Flue Gas Desulphurisation Plant (FGD plant) which removes the sulphur dioxide. The fumes enter the lower part of the unit where they are washed again to reduce the temperature and subjected to a limestone slurry treatment. The slurry is injected into the gas stream through an array of jets situated at different levels as they pass up through the FGD tower. The fumes pass from here through the extraction fan and up the chimney to atmosphere. The chimneys are built at a height that the extracted fumes are distributed high into the air and prevailing wind.
Meanwhile the slurry is recirculated and the chemical combination of calcium, lime and sulphur dioxide produce a product known as gypsum which is gradually siphoned off. This is separated from the water, dried, and sold off to be used in the cement, plasterboard, and fertiliser industries.
Waste water from the gypsum process is passed through a water treatment plant where it is treated with chemicals and clarified to remove solids and heavy metals such as mercury, before being passed through gravity sand filters and discharged.
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CO2 Emissions to Atmosphere
At present there is government legislation that will prevent any future power plants in Britain to be built without a method of CO2 abatement such as Carbon Capture and Sequestration (CCS). This involves isolating the CO2 by scrubbing with an ammonia solution and then impounding this gas in an underground or undersea cavern such as a disused deep coal mine pit or exhausted oil/gas reservoir. The amine solution can be recycled and reused.
The other method of reduction of CO2 emissions is to make the plant as efficient as possible; much like the one I have described using pulverised coal and energy efficient turbines. In addition to these measures, mixing biomass such as wood sawdust or pellets with the coal dust supply to the furnace will also reduce CO2 emission levels.