The recent demolition of the Athlone power station cooling towers sparked some discussion around coal power. I was very surprised to learn that very few people actually understand the inner workings of a coal power plant, and even fewer really understand the relationship to carbon emissions. Perhaps those of us in the renewable energy industry take these things for granted? So this month I want to try to provide an uncomplicated description of what is actually going on inside one of these filthy giants.
I think the best place to start is with the generator. You are likely to be imagining that dirty diesel generator sitting next to the dam on your uncle’s farm, but this is only one form of an electric generator. A generator is a device which converts mechanical energy to electrical energy. In other words it converts movement into electricity. The process is relatively simple, but let’s leave the technicalities out for now. Think of it as the opposite of an electric motor, similar to the one in your lawnmower. You put electricity in your lawnmower and the resulting output is movement, or rotation, of the blades. A power generator works due to the very same principle, however the other way around – you put rotation in and electricity comes out.
So, all you need to produce electricity is the rotation of a shaft in a generator. This can be done in a variety of ways. The diesel generator at your uncle’s farm uses the combustion of fuel (diesel) inside an engine to provide rotation, much like an engine turns the wheels of your car. The flow of water in a river can do the same, i.e. hydro power. Or wind can turn the blades of a wind turbine, and so forth. There is another way of doing it though – with steam.
If you create steam and try to contain it, pressure will build up inside the container. This pressure can be harnessed to provide mechanical energy. Think of a steam train – coal is burned to create steam which provides the necessary energy to move the wheels. This is, in principle, exactly how coal power stations produce electricity. Coal is burned in a furnace to heat water which creates steam. This steam is then forced through turbine blades to rotate a shaft in the generator, which generates electricity. Electricity is produced in the very same way at a nuclear power station, however there they are using heat from nuclear reactions to produce steam. So you can see that the mechanism of electricity generation in many technologies is essentially the same. The main difference lies in the source of the required mechanical energy, or the fuel.
Coal is primarily made up of carbon, the element central to all life on earth. All living things, including the plants in your garden, are forms of carbon-based life. Plants absorb carbon dioxide from the air and use it to grow. The carbon then forms part of the plant until it dies and decomposes, at which point the carbon is released back into the environment ( keep a look out for an upcoming article on the “Carbon Cycle”). To make coal all we need is lots of dead carbon-based things and we need to compress them under very high pressures for a very long time. The carbon then becomes concentrated into a very effective fuel. The problem is that when it is burnt, all the carbon is released again in the form of carbon dioxide.
Now I can imagine you’re thinking that since the carbon was part of the system in the first place, it shouldn’t be a problem when it is released back into the system? The problem here is down to time. One lump of coal comprises carbon from hundreds of thousands of plants. Our coal reserves are a result of millions of years of accumulated organic material. If we burnt the coal as slowly as it was formed, there wouldn’t be any problem. The problem is that we releasing large amounts of carbon in a very short period of time by burning millions of plants in an instant. This in turn leads to the increase of carbon dioxide in our atmosphere, which of course, leads to global warming.
These are both very straightforward descriptions of electricity production, as well as global warming, hopefully providing a basic understanding of the fundamental principles.
The bottom line here is that not only is the combustion of coal bad for the environment, it is going to eventually run out. So, the more ways we can find to turn shafts in generators without burning coal, the better. May I ask that my fellow engineers/scientists humour my overly colloquial descriptions and analogies in this article. Anyone requiring more detailed information is welcome to contact me.
Gary Fahy (gary@90×2030.org.za)
