Thermal electricity production began in the 1880s, following the coupling of industrial steam engines to current generators. Today, these thermal power plants represent nearly 75% of global electricity production.

In this article, SirEnergies introduces you to the different types of thermal power plants, whether they use coal, fuel, gas, nuclear or renewable energies.

What is thermal energy?

Thermal energy was discovered in 1847 by the English physicist and mathematician James Prescott Joule, after whom the unit of energy and Joule's law was named.

Thermal energy comes from displacement of atoms and molecules constituting a solid, liquid or gaseous body. This displacement is accompanied by a rise in its temperature and therefore by warmth.

This thermal energy can be produced from various processes. Physics, Chemicals, Mechanics or Nuclear. It represents the quantity of energy contained in a body (petroleum, coal, gas, biomass, uranium, etc.) depending on its mass and temperature.

How does a thermal power plant work?

In a thermal power plant, the principle is always the same, We use the heat released by combustion coal, fuel, gas, etc. or by the fission of uranium atoms to produce thermal energy.

The production of electricity is then carried out by converting this thermal energy using the expansion of a heated and compressed gas. This gas may come either from combustion fumes or from water vapor produced with the heat released by the combustion or fission of atoms.

It then sets in motion either a combustion turbine or a steam turbine and thus transmits its energy to an alternator.

In concrete terms, here are the different stages of operation:

• In the boiler, energy is released for Heat water And the Turn into steam at high pressure;
• This vapor is transformed into mechanical energy In a turbine Who results in a alternator ;
• When the alternator is rotated, it generates between its rotor and its stator a magnetic field Who spawns a Electric current of high intensity;
• A transformer increases the voltage of this electrical energy to facilitate its transport;
• At the exit of the turbine, the steam is again transformed into water thanks to a condenser connected to a cold source, sea or river;
• Once cooled, the water is recovered and circulated again in the boiler, a new cycle then begins;
• The water from the cold spring is returned to its natural environment, the sea or the river, with a few degrees of temperature in addition;
• The hot gases, or fumes, resulting from combustion, are dedusted using filters, then evacuated through chimneys to be released into the open air.
  

The combustion process

The combustion process consists in burning fuel. sturdy, liquid or gaseous. This fuel then reacts with oxygen, the oxidizer, to produce heat. This exothermic reaction also produces carbon dioxide (CO₂).

To recover the maximum amount of heat and therefore obtain the best combustion efficiency, the fuel must be burned in its entirety. The aim of this combustion is to generate heat ; it is also accompanied by light in the form of a flame.

Steam production

Water vapor is used in power plants as driving force to transform the heat emitted by the combustion process into mechanical energy.

For this, we Take advantage of the change in phase of water between liquid and gaseous states according to the Rankine cycle in a circuit where the water will be vaporized and heated in the boiler (boiler) by heat exchange with the hot source, then the steam will be expanded in a turbine to provide work and therefore mechanical energy. Finally, the steam will be condensed in a condenser in contact with cold water to return to liquid state.

Various optimizations of the Rankine cycle make it possible to improve the energy efficiency of thermal power plants, such as:

• By superheating the steam (in a superheater),
• By preheating compressed water to a liquid state with combustion gases (economizer).

This cycle is illustrated by the following diagram:

Electricity production

In a thermal power plant, steam generates work that makes it possible to do Turning a turbine causing an alternator to rotate. Thanks to the mechanical energy supplied by the turbine, the alternator generates alternating electrical current.

Recycling

In the Rankine cycle described above, at the outlet of the turbine, the steam is transformed into water using a condenser using a cold source, generally water coming from the sea or from a river. The steam is thus condensed into water and can go back to the boiler to start a new cycle.

Flame-fired power plants

The flame-fired power plant, called classical power plant, is the most widespread and oldest type of power plant. She uses fossil fuels and works with a steam boiler.

There are different types of flame-fired power plants depending on the type of fuel used:

Coal-fired power plant

The coal is ground into fine particles then mixed with heated air and finally injected under pressure into the boiler burner where combustion will take place.

Gas-fired power plant

The gas is injected as it is into the burner to be burned there. It may be natural gas or gas recovered from industrial processes but cannot be stored or transported, so-called fatal gas, resulting from blast furnaces or industrial chemical or extraction processes.

Oil fired power plant

The Fuel oil, too viscous, is liquefied before being burned. To do this, it is heated before being injected into the burner.

Biomass thermal power plant

The fuel used is biomass vegetable or animal. The main advantage of this non-fossil fuel is that it is inexhaustible if produced in an ecological way.

The main sources of these organic materials, called biomass, are:

• Solid matter such as wood, straw, plants, agricultural waste such as sugar cane and even organic household waste;
• liquid material such as vegetable oils;
• Gaseous matter such as biogas.

Nuclear thermal power plants

In a nuclear power plant, heat is produced from the fission uranium atoms. A nuclear power plant is therefore a steam engine in which the heating of water takes place without combustion but by means of fissile materials.

The fission of uranium or plutonium nuclei creates a chain reaction that releases large amounts of heat and allows water to be heated.

Source: IRSN

In this type of power plant, a primary circuit, completely closed, makes it possible to extract the heat produced by the nuclear reaction. The water that circulates there reaches over 300°C and is kept under pressure to remain liquid.

Then, as in a conventional thermal power plant, a circuit, called secondary, makes it possible to produce steam. This secondary circuit uses the primary circuit as a hot source and water from a river or sea as a cold source.

This type of power plant makes it possible to produce nearly 70% of the electricity of French energy mix.

Renewable heat power plants

There are other very specific types of thermal power plants that do not use fuel.

Power plant geothermal

Les geothermal power plants use Earth's heat as a heat source by pumping water at more than 150°C into aquifers under very high pressure; water that will vaporize as it rises to the surface. Then, as in a conventional thermal power plant, the steam will be transformed into electrical energy.

Solar thermal power plant

In a solar thermal power plant, steam is produced from Sun radiation. These power plants rely on the use of mirrors to capture and concentrate the sun's rays at a specific point and generate very high temperatures in excess of 400°C.

Pressurized water circulates in this receiver, which will rise to a very high temperature and constitute the hot source of a secondary circuit exploiting the Rankine cycle, as in conventional thermal power plants.

What are the advantages and disadvantages of a thermal power plant?

As we have just seen, thermal power plants are of different types even if they are all based on The principle of the steam engine : a gas propels a turbine that drives an alternator generating an electric current.

Depending on the fuel they use and the technology of their boiler, they are more or less polluting and can generate environmental risks or, on the contrary, present a attractive alternative to fossil fuels.

The advantages of thermal power plants

• They are quick to build and require few investments (except for nuclear power plants);
• They are available almost in real time to meet demand;
• They are very variable in size, from a few MW to thousands of GW;
• They only depend on the availability of fuel (except for solar thermal power plants).

It should also be noted that:

• The gas-fired combined cycle power plant, by combining a combustion turbine and a steam turbine, makes it possible to increase energy efficiency ;
• The cogeneration power plant makes it possible to produce heat and electricity at the same time and thus increase the efficiency of the installation;
• Solar thermal and geothermal power plants are ecological.
  

The disadvantages of flame-fired power plants

• They are responsible for half of greenhouse gas emissions;
• They also emit air pollutants;
• They use limited and non-renewable fossil sources;
• They lead to dependencies on producing countries.
  

Energy transition: What future for thermal power plants in France?

In France, the policy in favor of energy transition has led to the decision to close the most polluting thermal power plants.

Thus, on March 31, 2018, the last French oil-fired electricity production unit was closed. This unit with a unit capacity of 700 MW was located at the Cordemais power plant, in Loire-Atlantique.

Likewise, the closure of the last 2 coal-fired power plants located in Cordemais and Saint-Avold was planned for 2022, but the current energy crisis has resulted in the resumption of these power plants in order to pass the peak periods of winter 2022—23.

It should be noted that some of these power plants are going to be converted to biomass. As for gas-fired power plants, they remain essential to the security of supply and no closure is planned.

Finally, despite the closure of the Fessenheim nuclear power plant on June 30, 2020, the renewal of nuclear power was announced by President Macron with the construction of 6 new EPRs.

Thermal power plants make up most of the electricity production capacity in France, especially thanks to Nuclear power park.

Although in France the last 2 coal-fired power plants are in the process of being closed, it should be understood that there are 2,400 in the world and that 600 construction projects exist, mainly in Asia.

Source: Carbonbrief

This makes coal the main source of electricity with more than a third of global electricity production. The steam engine therefore still has a bright future ahead of it despite the global climate change crisis.