Electricity has traditionally been generated from coal – a fossil fuel. Though with growing environmental concerns over the effect of greenhouse gases, we are starting to produce energy using natural or renewable sources such as wind, the sun and water.
Coal is mined at open cut or underground mines, then crushed, washed and transported to power stations to be stockpiled and used as fuel.
For efficient combustion, the coal is ground to the consistency of talcum powder in pulverising mills before being blown into the boiler furnace chamber in a stream of pre-heated air. The pulverised coal is burned at very high temperature, converting water circulating in the boiler tubes into high pressure steam.
The steam produced by the boiler is injected at very high pressure into the turbine, spinning the fan-like blades mounted along the main drive shaft. This shaft continues like an axle from one end of the turbine to the other.
The turbine is divided into three stages with different levels of pressure and different sized blades. Stage 1
is a high-pressure cylinder where the fan blades are smallest in diameter. Once the steam has passed through Stage 1, for maximum efficiency, it is reheated in the boiler as some of its energy has been spent. The steam is then passed to Stage 2
, which is an intermediate pressure cylinder with larger fan blades. As the heat and pressure lessen, the steam passes through Stage 3
of the turbine, which has two low-pressure cylinders and the largest blades. Once the spent steam leaves the turbine, it is cooled back to water as it passes over a series of condenser tubes through which cold water from the cooling system is circulated. The reclaimed water is then recirculated to the boiler to produce more steam.
The generator consists of two main sections - the revolving section called the rotor
, which is directly coupled to the steam turbine's drive shaft, and the stator
, a series of wire coils which form a cylinder around the rotor. The rotor, which is really an electro-magnet, revolves at high speed to generate electricity (alternating current) in the stator. A separate static excite
energises the wire coils of the rotor.
Electricity is produced in modern generators at 23,000 volts. It then passes through a transformer that increases the voltage to as high as 500,000 volts. Then it passes into the adjacent power station switchyard where transmission lines carry it to where it is needed, via a vast network of interconnected high voltage transmission lines called a grid.
Impact on the environment
An extensive filter system between the boiler and the emission stack extracts almost all of the ash particles from the exhaust gases generated by the boiler. The filter system consists of up to 48,000 fabric bags with a total surface area of 120 hectares. This system is 99.9 per cent efficient
and reduces emissions to the atmosphere to a barely visible level. The 'smoke' you see coming out of power stations is usually just steam, not pollution. You can tell this because it evaporates from the extremity rather than forming a long plume across the sky, as smoke would.