Vapour plants

In particular, thermoelectric plants exploit vapour energy, which is produced by a “boiler” that burns a liquid fuel, such as fuel oil and naphtha or methane (usually modern boilers can burn the three types of fuel without distinction). Usually large thermoelectric power plants are installed close to big consumption centres and need suitable water supplies for vapour production and fuel storage. The combustion occurs in a part of the boiler that is called “combustion chamber”, with the walls made up of a series of pipes where water heats and gradually converts into vapour. The combustion chamber receives the fuels by means of adequate openings through which air passes, pushed by special ventilators. According to a determined route, the gases resulting from the combustion release a part of their heat and, at the boiler exit, they pass through the pre-heaters that release the air, which will enter the boiler. Then, they pass through a series of treating filters and finally they get to the chimney, that disperses them into the air. The vapour turns the blades of a turbine, which is connected to an alternator for the production of electric power. Vapour turbines are approximately similar to hydraulic ones, but they differ a lot because they do not work with water, but with superheated vapour, with all the subsequent temperature and resistance problems deriving from it.
Pollution abatement
Flue gas of thermoelectric power plants contains pollutants produced from fuel oil combustion. These include:

  • sulphur dioxide (SO2): arises from the oxidation of the sulphur contained within fossil fuels;
  • nitrogen oxides (NOx): arise from the oxidation of the nitrogen contained within fossil fuels and present in air;
  • dust particles: produced during the complex physical and chemical processes which the fuel particles undergo inside the combustion chamber;
  • carbon dioxide (CO2): produced in all combustion reactions.

It goes without saying that the effects on the environment of the above-mentioned substances depend on their concentration. To reduce polluting emissions modern thermoelectric power plants are equipped with systems which are based on different technologies:

  • denitrification: nitrogen oxides are converted to water and molecular nitrogen (not polluting) through a reaction with ammonia and oxygen;
  • dust precipitators: thanks to the effect of electrical fields or filtration devices, solid particles are trapped and are not released into the atmosphere (currently precipitators collect 99.9% of the dust);
  • flue gas desulphurisation: an operation which allows the removal of up to 97% of the sulphur compounds present in fossil fuel power plant flues;
  • water treatment: there are different uses of water in power plants; in all cases, however, before being discharged, water must be treated in order to eliminate possible polluting substances, and it can be released among the flue gases or into the sea only when the concentration of hazardous substances and the temperatures comply with legal regulations.

Ultimately all substances are filtered and treated by the pollution abatement systems present in the power plants. In order to favour the dispersion at higher altitudes of the remaining flue gas components and hence avoid soil pollution, flue stacks are very tall, in some cases over 200 metres high.

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