For the conversion of fossil energy into electrical energy, the technology of a thermal power plant is often used. At the moment in which a thermal power station is fossil fuels fossilized from a source of non-renewable energy generation.
A thermal power plant (or thermoelectric plant) is a plant that generates electricity by transforming heat. Historically heat energy is converted into electricity by transferring heat to a working fluid and then transforming the energy of this fluid into mechanical energy. Finally, mechanical energy is transformed into electricity. The typical thermal power plant is divided into several segments: the area of the boiler in which the heat is transferred to the working fluid, a turbine, an alternator, and a condenser.
The main thermodynamic cycles exploited in these plants are the Rankine cycle, possibly overheated, and the Brayton-Joule cycle, and their possible combinations, even if there are no central stations equipped with diesel cycle engines, or with other types of cycles.
From the point of view of the energy source, virtually any substance can be used to produce electricity. Among the most common fuels are fossil fuels (coal, oil and natural gas), uranium and plutonium in nuclear power plants, but less conventional fuels can also be used, for example, sludge. In this case we talk about non- renewable energy sources, but the thermal source can also be solar radiation. In this case it is about renewable energy plants, solar thermal installations.
Steam thermal power plants
Thermal steam power plants are characterized by the use of water or other liquid, which is found in two different phases during the work cycle, often in the form of vapor and liquid. In recent years, supercritical technologies have also spread, which has led to the absence of a phase transition, properly so called, which used to be the characteristic of these facilities.
These thermal plants can be divided into several sections: the power line, the steam generator, the steam turbine and the condenser. Although the definition of thermal power plant is quite restrictive, different types of thermodynamic cycles can be observed that satisfy these requirements, in particular the most widespread are the Rankine cycles and the Hirn cycles.
Line of feeding of a steam thermal power station
Before entering the boiler, the feed water goes through a preheating and compression phase. In fact, when entering the boiler there are several regenerators, that is to say, heat exchangers in which steam, partially or completely expanded, preheats the working fluid. This allows to enter the steam generator at higher temperatures, which results in greater efficiency of the plant.
A degasser is often provided within the heat supply line. To reduce the presence of incondensable in the working fluid. Compression of the working fluid can take place in a single pump at the discharge of the condenser, a preferred solution in small plants, or in more pumps or turbo pumps placed appropriately along the entire supply line, a more optimal solution in large steam thermal power plants.
In the steam generator of a thermal power station, the water at constant pressure is brought to the boiling point. Water undergoes a phase transition and is often overheated in the form of steam. This is achieved by means of a properly designed heat exchanger divided into different parts: the economizer, the evaporator and the superheater. These can be exchanged with a liquid, usually diathermic oil or water under pressure, or with hot gases produced by combustion, this is the most common configuration for larger plants.
For particularly large systems, the exchangers are placed in the combustion chamber itself and also obtain a radiative exchange with the flames. Special attention is paid to avoid overheating of the heat exchangers, as this could result in a reduction in its useful life or, worse, in its structural failure that would cause considerable damage to the system.
In supercritical systems, the working liquid no longer undergoes a real phase transition, the pressure being above the critical point, however, the structure is similar, even if the distinctions between the three types of banks are much lower. However, there are still three zones: one at relatively low temperatures where the working fluid is liquid, another at temperatures close to the critical point and areas where the fluid is in the gaseous state. This solution, which therefore involves the passage of the fluid to a supercritical state, is used particularly for large steam power plants or for smaller power plants with organic fluids, in this case in order to better approximate the cooling curve. Of the gases with which the heat is exchanged.
Steam expansion in turbine
The steam that leaves the steam generator is sent to a machine, usually a steam turbine or, more rarely, an alternative machine (capor motor). The first part of the expansion is often carried out through an initial stage of action, often in the form of some stages of Curtis, to guarantee the possibility of partializing and adjusting the turbine to the different loads.
Subsequently, only the reaction stages follow due to their greater efficiency. For large thermal power plants at a certain expansion point, the steam is sent back to the steam generator for overheating, to increase the work extracted from the turbine and at the same time reduce the presence of condensate in the discharge of the same, in if fluids are used. small complex.
The steam, reheated or not, continues its expansion in the turbine, expanding and cooling, this can cause an excessive volumetric flow that involves special precautions both in the structure of the coating and, possibly, in the use of multiple turbine bodies.
In the lower pressure zone, working with simple liquids, there is a partial condensation of the working fluid, this can be extremely damaging to the steam turbine since the drops of liquid water do not follow the same trajectories of the vapor, which results in a hammering and damage to the pallets. Once the expansion is completed, the steam leaves the turbine and is sent to the condenser, for simple fluids, or to a desuperheater followed by the condenser, for fluids with a retrograde saturation bell.
During the expansion, in the large groups of water and steam, a steam sample is taken in different sections of the turbine: this steam is used in the heat exchangers to heat the water of the cycle before it enters the boiler. In addition, the enormous losses of steam due to leaks in the different discontinuous sections of the turbine (given the high pressures and temperatures that the sealing system is not performing) are generally transferred to a heat exchanger and then re-inserted into the circuit; The high costs of water demineralization and its overheating justify the use of this energy and the recovery of materials.
The condenser of a thermal power station is the component in which the condensation of the working fluid takes place. This instrument is at very low pressures in water cycles, while it can be at higher pressures, even higher than atmospheric pressures, for cycles fed with other working fluids. In water cycles, or in any case with fluids with low pressure at the condensation temperature, it is essential to have a condenser capable of preventing air leaks inside the condenser, since the oxygen that finally enters the working fluid is particularly aggressive. the time the working fluid will be brought to high temperatures.
Conversion of mechanical energy into electrical energy and the main electrical system of the plant
The expansion of steam in the turbine allows the transfer of mechanical energy to the rotor blades. The resistive torque necessary to stabilize the rotation of the rotor is absorbed by the alternator, a three-phase synchronous generator connected directly to the main electrical system of the power plant and indirectly, by means of the voltage increasing station and the switches placed in the busbars, to the electric transmission network.
In fact, this pair of resistance is converted into electrical energy through phenomena of electromagnetomechanical conversion of the energy present within the alternator. In addition the excitation system of the DC synchronous generator,
Demineralization of water
The water used in the cycles of the thermoelectric plants can be sea water or fresh groundwater or river water. On the basis of its origin, it will undergo a different previous treatment, which in the case of salt water is called desalination.
The previous treatment of the water is carried out in tanks for flocculation and precipitation of solid substances grouped in flocs obtained through chemical products. Water is purified from solid waste and impure substances.
Plants with complex working fluids and mixtures
There are applications that exploit fluids with high molecular complexity, therefore, with molecules with high degrees of freedom. The saturation bell of the complex fluids is decidedly deformed compared to that of the simplest fluids, this allows to have an exhaust fluid of the turbine still in a state of steam, which is generally cooled initially by means of an exchanger up to its temperature of saturation. Possibly in an order of cogeneration or preheating of the condensed fluid.
Other applications exploit fluid mixtures, appropriately selected, to exploit particular characteristics of the resulting mixtures. The mixtures, if synthesized and operated correctly, can present condensation curves of bubbles and / or non-isothermal, which allows the exploitation of particular non-isothermal thermal sources, such as the exhaust gases of a small-sized plant, or a easier. Sizing of a cogeneration heat recovery of the cooling and condensation phase.
Gas power plants
This type of thermal power plant is characterized by the use of a fluid in the form of a gas that does not undergo phase transitions. Plants of this type generally consist of four sections: gas compression, gas heating, gas expansion, exhaust or gas cooling. Typically these sections are joined in a turbogas.
Gas compression generally takes place through an axial turbocharger, or for smaller radial systems, it is typical to have the first stator moving stages to allow the machine to be controlled more easily. During compression, for large machines it is common practice for the air to be blown and then to cool the combustion chamber and the turbine.
The gas heating can be done through an exchanger, when it is necessary to keep the combustion of the working fluid separate, or more commonly in a combustion chamber where a fuel is burned in the working fluid, necessarily air or oxygen. The expansion takes place in a turbine that is usually completely reactive, since it is no longer necessary to operate the machine to control it. In the case of plants that operate with air, there is also an important section to filter and purify the intake air.
The presence of solid pollutants in the air is a very sensitive problem in gas plants and makes the installation of specific purification filters prevent their entry into the machine. In fact, these contaminants could melt due to the high temperatures reached in the turbine and solidify in the blades of the turbine, which would cause, over time, excessive wear of the machine.
In addition, even for turbines with relatively low temperatures, the particles can enter the cooling ducts of the turbine and clog them, which causes a local overheating of the machine that can cause the structural failure of the same.
Gas-steam combined cycle technology
In order to increase the energy efficiency of thermal power plants, the use of combined gas and steam cycles has been extended in recent years. The combined cycle of gas and steam is based on a turbogas consisting of a compressor, connected to the turbine and the alternator, which injects combustion air from the atmosphere into the combustion chamber. The mixture of air and gas injected is burned in the combustion chamber and the exhaust gases are used to obtain mechanical work in the turbine.
A subsequent recovery boiler uses the same hot fumes that come out of the turbine to generate steam that then expands in a steam turbine to generate more work. In general, combined cycle plants have the advantage of a lower environmental impact in terms of emissions, since they use light fuels such as methane gas or diesel fuel, as well as less use of water for condensation.
They also have a much higher efficiency than traditional thermoelectric plants, since the exhaust fumes are used to generate steam and generate electricity again. This performance (electrical) reaches up to almost 60%. In the case of cogeneration (electricity and heat), compared to a first class yield of around 87%, a slight decrease in electricity performance is observed.
All thermoelectric plants are obliged to control their emissions, this is particularly relevant for large power plants where there is an important section to reduce pollutants.
Reduction of sulfur oxides
Sulfur oxides, which are one of the causes of acid rain, are usually the result of coal combustion and are strictly regulated. Then they are cut, depending on when they are removed, there are three types of extraction: pre-combustion, boiler, post-combustion.
The reduction of pre-combustion can only take place if the coal can be pre-treated, as in the IGCC plants, so it is a rather rare process.
The destruction in the boiler is done by injecting calcium compounds that bind to the sulfur to give inert gypsum.
The reduction of afterburning is done by washing the fumes with a solution of calcium compounds that form the gypsum, this configuration is preferred for large plants, since the gypsum is produced pure, therefore, it can be sold, avoiding large disposal costs.
Oxidation of nitrogen oxides
The reduction of nitrogen oxides is a common problem in all combustion plants. In general, its production is effectively limited already at the origin through an adequate design of the burners and an equally studied distribution of the air currents of the food to the boiler or burner, avoiding portions of combustion gases at excessively high temperatures. high If this pollutant is still relevant, special purifiers with ammonia or urea are used.
Removal of ashes
Ash reduction is a typical problem for coal and fuel oil fossil fuel plants, since gas plants use both clean fuel and filtered air. The problem is also related to the light ashes, which are dragged by the air flow to the chimney. Then, the ashes are cut through a series of electrostatic filters, cyclones and filter covers of increasing efficiency to bring the emissions within legal limits. The heavier ash, on the other hand, is easily removed from the boiler and sent to an appropriate treatment and then deposited in landfills. Particular configurations of plants of advanced plants, such as the aforementioned IGCC, can also go to repair these heavy ashes,
Capture of carbon dioxide
In recent years, when traditional pollutants have been reduced, great attention has been paid to reducing carbon dioxide emissions due to their contribution to the greenhouse effect. This need has driven more and more efficient plants and the development and experimentation of plants with carbon capture and sequestration. The separation techniques are divided into three main groups:
The pre-combustion capture provides the removal of carbon and fuel that is fed to the plant, which, therefore, is to work by practically burning only hydrogen.
Oxycombustion involves combustion of the fuel in a pure oxygen atmosphere, so that it can then easily separate the carbon dioxide from the other components without the great dilution typical of combustion in the air.
The post-combustion capture provides, with techniques similar to those post-combustion for the elimination of sulfur oxides, to remove carbon dioxide from the flow to the discharge of the plant.
The carbon dioxide separated at this point is stored in depleted or deep aquifers or, more economically, it is pumped into active deposits, according to the technique of forced recovery of hydrocarbons, this latter technique, if combined with a strong imposition on the emissions, is the most promising economically.