Combined-cycle power plants are a combination of steam and gas turbines. Such a combination makes it possible to reduce the waste heat losses of gas turbines or the heat of flue gases of steam boilers, which ensures an increase in the efficiency of combined-cycle plants (CCGT) compared to separately taken steam turbine and gas turbine plants.
Currently, there are two types of combined-cycle plants:
a) with high-pressure boilers and with the discharge of exhaust gases from the turbine into the combustion chamber of a conventional boiler;
b) using the heat of the exhaust gases of the turbine in the boiler.
Schematic diagrams of CCGT of these two types are shown in fig. 2.7 and 2.8.
On fig. 2.7 shows a schematic diagram of a CCGT with a high-pressure steam boiler (HSG) 1 , which is supplied with water and fuel, as in a conventional thermal station for the production of steam. High pressure steam enters the condensing turbine 5 , on the same shaft with which the generator is located 8 . The exhaust steam from the turbine first enters the condenser. 6 and then with a pump 7 goes back to the cauldron 1 .
Fig 2.7. Schematic diagram of a CCGT with VPG
At the same time, the gases formed during the combustion of fuel in the boiler, having a high temperature and pressure, are sent to the gas turbine 2 . On the same shaft with it are a compressor 3 , as in a conventional gas turbine, and another electric generator 4 . The compressor is designed to pump air into the combustion chamber of the boiler. Turbine exhaust 2 also heat the boiler feed water.
Such a CCGT scheme has the advantage that it does not require a smoke exhauster to remove the boiler flue gases. It should be noted that the function of the blower fan is performed by the compressor 3 . The efficiency of such a CCGT can reach 43%.
On fig. 2.8 shows a schematic diagram of another type of CCGT. In contrast to the PGU shown in Fig. 2.7, gas to turbine 2 comes from the combustion chamber 9 and not from the boiler 1 . Further spent in the turbine 2 gases saturated with up to 16–18% oxygen due to the presence of a compressor enter the boiler 1 .
Such a scheme (Fig. 2.8) has an advantage over the CCGT discussed above (Fig. 2.7), since it uses a boiler of conventional design with the ability to use any type of fuel, including solid fuel. In the combustion chamber 3 at the same time, much less expensive gas or liquid fuel is burned than in the CCGT scheme with a high-pressure steam boiler.
Fig 2.8. Schematic diagram of the CCGT (discharge circuit)
Such a combination of two units (steam and gas) into a common combined cycle unit creates the possibility of obtaining also higher maneuverability compared to a conventional thermal power plant.
Schematic diagram of nuclear power plants
In terms of purpose and technological principle of operation, nuclear power plants practically do not differ from traditional thermal power plants. Their significant difference lies, firstly, in the fact that at a nuclear power plant, unlike a thermal power plant, steam is generated not in a boiler, but in the reactor core, and secondly, in the fact that nuclear fuel is used at a nuclear power plant, which includes isotopes of uranium-235 (U-235) and uranium-238 (U-238).
A feature of the technological process at nuclear power plants is also the formation of significant amounts of radioactive fission products, in connection with which nuclear power plants are technically more complex than thermal power plants.
The NPP scheme can be single-circuit, double-circuit and three-circuit (Fig. 2.9).
Rice.2.9. NPP schematic diagrams
The single-circuit scheme (Fig. 2.9, a) is the simplest. Released in a nuclear reactor 1 due to the chain reaction of nuclear fission of heavy elements, heat is transferred by the coolant. Often, steam is used as a heat carrier, which is then used as in conventional steam turbine power plants. However, the steam generated in the reactor is radioactive. Therefore, in order to protect nuclear power plant personnel and the environment, most of the equipment must be shielded from radiation.
According to two- and three-loop schemes (Fig. 2.9, b and 2.9, c), heat is removed from the reactor by a coolant, which then transfers this heat to the working medium directly (for example, as in a two-loop scheme through a steam generator 3 ) or through the intermediate circuit coolant (e.g. as in a three-circuit circuit between an intermediate heat exchanger 2 and steam generator 3 ). On fig. 2.9 digits 5 , 6 And 7 the condenser and pumps are indicated, performing the same functions as in a conventional thermal power plant.
The nuclear reactor is often referred to as the "heart" of a nuclear power plant. Currently, there are quite a few types of reactors.
Depending on the energy level of neutrons, under the influence of which fission of nuclear fuel occurs, nuclear power plants can be divided into two groups:
NPP with thermal neutron reactors;
NPP with fast neutron reactors.
Under the influence of thermal neutrons, only uranium-235 isotopes are capable of fission, the content of which in natural uranium is only 0.7%, the remaining 99.3% are uranium-238 isotopes. Under the influence of a neutron flux of a higher energy level (fast neutrons), artificial nuclear fuel plutonium-239 is formed from uranium-238, which is used in fast neutron reactors. The vast majority of power reactors currently in operation are of the first type.
A schematic diagram of a nuclear power reactor used in a double-circuit NPP is shown in fig. 2.10.
A nuclear reactor consists of an active zone, a reflector, a cooling system, a control, regulation and control system, a housing and biological protection.
The reactor core is the area where the fission chain reaction is maintained. It consists of a fissile material, a coolant neutron moderator and reflector, control rods, and structural materials. The main elements of the reactor core, which provide energy release and self-sustaining the reaction, are the fissile material and the moderator. The active zone is remote from external devices and personnel work by a protection zone.
For the production of electricity, combined steam and gas plants (CCGT), combined in a single thermal circuit, are used. At the same time, a reduction in specific fuel consumption and capital costs is achieved. CCGT units with a high-pressure steam generating unit (VNPPU) and with a low-pressure steam generating unit (NNPPU) are most widely used. Sometimes VNPPU are called high-pressure boilers.
Unlike boilers operating under vacuum from the gas side, in the combustion chamber and gas ducts of high-pressure and pressurized boilers, relatively low pressure is created at NNPPU (0.005-0.01 MPa) and increased at VNPPU (0.5-0.7 MPa) .
The work of the boiler under pressure is characterized by a number of positive features. Thus, air suction into the furnace and gas ducts is completely excluded, which leads to a decrease in heat loss with outgoing gases, as well as to a decrease in
reducing the consumption of electricity for their pumping. An increase in pressure in the combustion chamber opens up the possibility of overcoming all air and gas resistances due to the blower fan (smoke draft may be absent), which also leads to a decrease in electricity consumption due to the operation of the blower device in cold air.
The creation of excess pressure in the combustion chamber leads to a corresponding intensification of the fuel combustion process and allows you to significantly increase the speed of gases in the convective elements of the boiler up to 200-300 m/s. At the same time, the coefficient of heat transfer from gases to the heating surface increases, which leads to a decrease in the dimensions of the boiler. At the same time, its operation under pressure requires dense lining and various devices to prevent combustion products from being knocked out into the room.
Rice. 15.1. Schematic diagram of a combined cycle plant with VNPPU:
/ - air intake; 2 - compressor; 3 - fuel; 4 - combustion chamber; 5 - gas turbine; 6 - exhaust gases; 7 - electric generator; 8 - boiler; 9 - steam turbine; 10 - capacitor; // - pump; 12 - high pressure heater; 13 - regenerative exhaust gas heater (economizer)
On fig. 15.1 shows a diagram of a combined cycle plant (CCGT) with a high-pressure boiler. Combustion of fuel in the furnace of such a boiler occurs under pressure up to 0.6-0.7 MPa, which leads to a significant reduction in the cost of metal on heat-receiving surfaces. After the boiler, the combustion products enter the gas turbine, on the shaft of which are air compressor and electric generator
torus The steam from the boiler enters the turbine with another electric generator.
The thermodynamic efficiency of a combined steam-gas cycle with a high-pressure boiler, gas and steam-water turbines is shown in fig. 15.2. On the T, n-diagram: areas 1-2-3-4-1 - the work of the gas stage bt, the area sye\abc - the work of the steam stage bn; 1-5-6-7-1 - heat loss with outgoing gases; cbdc - loss of heat in the condenser. The gas stage is partially built over the steam stage, which leads to a significant increase in the thermal efficiency of the installation.
The high-pressure boiler in operation, developed by NPO TsKTI, has a capacity of 62.5 kg/s. The boiler is water-tube, with forced circulation. Steam pressure 14 MPa, superheated steam temperature 545 °C. Fuel---gas (fuel oil), is burned with a heat release volumetric density of about 4 MW/m3. Combustion products leaving the boiler at temperatures up to 775 ° C and pressures up to 0.7 MPa expand in the gas turbine to a pressure close to atmospheric. The exhaust gases at a temperature of 460 °C enter the economizer, after which the exhaust gases have a temperature of about 120 °C.
The principal thermal diagram of a CCGT with a VNPPU with a power of 200 MW is shown in fig. 15.3. The installation includes a K-160-130 steam turbine and a GT-35/44-770 gas turbine. From the compressor, air enters the VNPPU furnace, where fuel is also supplied. High-pressure gases after the superheater at a temperature of 770 ° C enter the gas turbine, and then into the economizer. The scheme provides for an additional combustion chamber, which provides the nominal temperature of the gases in front of the GTU when the load changes. In combined CCGTs, the specific fuel consumption is 4-6% less than in conventional steam turbines, and capital investments are also reduced. 
Rice. 15.2. Т, ї-diagram for combined steam-gas cycle
What are the reasons for the introduction of CCGT in Russia, why is this decision difficult but necessary?
Why did they start building a CCGT
The decentralized market for the production of electricity and heat dictates the need for energy companies to increase the competitiveness of their products. The main importance for them is the minimization of investment risk and the real results that can be obtained using this technology.
The abolition of state regulation in the electricity and heat market, which will become a commercial product, will lead to increased competition between their producers. Therefore, in the future, only reliable and highly profitable power plants will be able to provide additional capital investments in the implementation of new projects.
CCGT selection criteria
The choice of one or another type of CCGT depends on many factors. One of the most important criteria in the implementation of the project are its economic viability and safety.
An analysis of the existing market for power plants shows a significant need for inexpensive, reliable in operation and highly efficient power plants. The modular, pre-configured design of this concept makes the plant highly adaptable to any local conditions and specific customer requirements.
Such products satisfy more than 70% of customers. These conditions are largely met by GT and SG-TPPs of the utilization (binary) type.
Energy dead end
An analysis of the Russian energy sector, carried out by a number of academic institutions, shows that even today the Russian electric power industry is practically losing 3-4 GW of its capacities annually. As a result, by 2005, according to RAO "UES of Russia", the volume of equipment that has worked out its physical resource will amount to 38% of the total capacity, and by 2010 this figure will already be 108 million kW (46%).
If events develop exactly according to this scenario, then most of the power units due to aging in the coming years will enter the zone of a serious risk of accidents. The problem of technical re-equipment of all types of existing power plants is exacerbated by the fact that even some of the relatively “young” 500-800 MW power units have exhausted the service life of the main units and require serious restoration work.
Read also: How do GTU and CCGT efficiency differ for domestic and foreign power plants
Reconstruction of power plants is easier and cheaper
Extending the life of plants with the replacement of large components of the main equipment (turbine rotors, heating surfaces of boilers, steam pipelines), of course, is much cheaper than building new power plants.
It is often convenient and profitable for power plants and manufacturing plants to replace equipment with a similar one that is being dismantled. However, this does not take advantage of the opportunities to significantly increase fuel economy, does not reduce pollution environment, modern means are not used automated systems new equipment, operating and maintenance costs increase.
Low efficiency of power plants
Russia is gradually entering the European energy market, joining the WTO, but at the same time, we have had an extremely difficult situation for many years. low level thermal efficiency of electric power industry. Average odds useful action power plants when operating in the condensing mode is 25%. This means that if the price of fuel rises to the world level, the price of electricity in our country will inevitably become one and a half to two times higher than the world price, which will affect other goods. Therefore, the reconstruction of power units and thermal stations should be carried out in such a way that the new equipment being introduced and individual components of power plants are at the modern world level.
Energy chooses combined cycle technologies
Now, despite the difficult financial situation, the design bureaus of power engineering and aircraft engine research institutes have resumed the development of new equipment systems for thermal power plants. In particular, we are talking about the creation of condensing steam-gas power plants with an efficiency of up to 54-60%.
Economic assessments made by various domestic organizations indicate a real opportunity to reduce the costs of electricity production in Russia if such power plants are built.
Even simple gas turbines will be more efficient in terms of efficiency
At CHPPs, it is not necessary to universally use CCGTs of this type, such as CCGT-325 and CCGT-450. Circuit solutions may be different depending on specific conditions, in particular, on the ratio of thermal and electrical loads.
Read also: Choice of the cycle of the combined cycle plant and the circuit diagram of the CCGT
In the simplest case, when using the heat of the gases exhausted in the gas turbine for heat supply or production of process steam, the electrical efficiency of the CHPP with modern gas turbines will reach a level of 35%, which is also significantly higher than those existing today. About the differences in the efficiency of GTU and PTU - read in the article How the efficiency of GTU and CCGT efficiency differ for domestic and foreign power plants
The use of gas turbines in thermal power plants can be very wide. Currently, about 300 steam turbine units of CHPP with a capacity of 50-120 MW are fed with steam from boilers that burn 90 percent or more of natural gas. In principle, all of them are candidates for technical re-equipment using gas turbines with a unit capacity of 60-150 MW.
Difficulties with the introduction of GTU and CCGT
However, the process of industrial introduction of GTU and CCGT in our country is extremely slow. main reason- investment difficulties associated with the need for sufficiently large financial investments in the shortest possible time.
Another limiting circumstance is related to the actual absence in the range of domestic manufacturers of purely power gas turbines that have been proven in large-scale operation. GTUs of a new generation can be taken as prototypes of such gas turbines.
Binary CCGT without regeneration
Binary CCGTs have a certain advantage, as they are the cheapest and most reliable in operation. The steam part of binary CCGTs is very simple, since steam regeneration is unprofitable and is not used. The temperature of the superheated steam is 20-50 °C lower than the temperature of the exhaust gases in the gas turbine. At present, it has reached the standard level in the energy sector of 535-565 °C. The live steam pressure is chosen so as to provide acceptable humidity in the last stages, the operating conditions and blade sizes of which are approximately the same as in powerful steam turbines.
Influence of steam pressure on the efficiency of CCGT
Of course, economic and cost factors are taken into account, since the steam pressure has little effect on the thermal efficiency of the CCGT. To reduce the temperature difference between the gases and the steam-water medium and in the best way to use the heat of the gases exhausted in the gas turbine with less thermodynamic losses, the evaporation of the feed water is organized at two or three pressure levels. The steam generated at reduced pressures is mixed in at intermediate points of the flow path of the turbine. Steam reheating is also carried out.
Read also: Reliability of CCGT Combined-Cycle Plants
Influence of flue gas temperature on CCGT efficiency
With an increase in the gas temperature at the turbine inlet and outlet, the steam parameters and the efficiency of the steam part of the GTP cycle increase, contributing to the overall increase in the CCGT efficiency.
The choice of specific directions for the creation, improvement and large-scale production of power machines should be decided taking into account not only thermodynamic perfection, but also the investment attractiveness of projects. The investment attractiveness of Russian technical and industrial projects for potential investors is the most important and urgent problem, on the solution of which the revival of the Russian economy largely depends.
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Depending on what is chosen steam-gas cycles, what choice will be optimal, and what will the CCGT process flow diagram look like?
Once the capital parity and the roll configuration are known, the cycle pre-selection can begin.
The range extends from very simple “single pressure cycles” to extremely complex “reheat triple pressure cycles”. The efficiency of the cycle increases with increasing complexity, but capital costs also increase. The key to choosing the right cycle is to determine the pressure cycle that works best for a given efficiency and cost.
Combined-cycle plant with single pressure cycle
This cycle is often used for more favorable degraded quality fuels such as crude oil and high sulfur heavy fuel oil.
Compared to complex cycles, investments in CCGT of simple cycles are insignificant.
The diagram shows a CCGT with an additional evaporator coil at the cold end of the waste heat boiler. This evaporator removes additional heat from the exhaust gases and gives steam to the deaerator in order to use it to heat the feed water.
This eliminates the need for steam extraction for the deaerator from the steam turbine. The result compared to the simplest circuit one pressure is to improve the efficiency, however, the capital investment increases accordingly.
PGU with a cycle of two pressures
Most combined plants in operation have dual pressure cycles. Water is supplied by two separate feed pumps to the dual pressure economizer.
Read also: How to choose a gas turbine plant for a CCGT plant
The low pressure water then enters the first evaporator coil and the high pressure water is heated in the economizer before it evaporates and superheats in the hot end of the HRSG. Extraction from the low pressure drum supplies steam to the deaerator and steam turbine.
The efficiency of a dual pressure cycle, as shown in the T-S diagram in the figure, is higher than the efficiency of a single pressure cycle due to more full use the energy of the exhaust gases of the gas turbine (additional area SS "D" D).
However, this increases the capital investment for additional equipment, such as feed pumps, dual pressure economizers, evaporators, low pressure pipelines and two LP steam lines to the steam turbine. Therefore, the considered cycle is applied only at high capital parity.
CCGT with triple pressure cycle
This is one of the most complex schemes that are currently in use. It is used in cases of very high capital parity, where high efficiency can only be obtained at high cost.
A third stage is added to the waste heat boiler, which additionally uses the heat of the exhaust gases. The high pressure pump supplies feed water to the three-stage high pressure economizer and then to the high pressure drum separator. The medium pressure feed pump supplies water to the medium pressure separator drum.
Part of the feed water from the medium pressure pump through the throttle device enters the drum - low pressure separator. The steam from the high pressure drum enters the superheater and then to the high pressure part of the steam turbine. The steam exhausted in the high pressure part (HPP) mixes with the steam coming from the medium pressure drum, overheats and enters the inlet of the low pressure part (LPP) of the steam turbine.
Read also: Why build Combined Cycle Thermal Power Plants? What are the advantages of combined cycle plants.
The efficiency can be further increased by heating the fuel with high pressure water before it enters the gas turbine.
Cycle Selection Diagram
Cycle types from single pressure cycle to triple pressure cycle with reheat are presented as functions of supply parity.
The cycle is selected by determining which of the cycles are appropriate for a given capital parity for a particular application. If, for example, the capital parity is $1,800. US/kW, the dual or triple pressure cycle is selected.
As a first approximation, the decision is made in favor of the triple pressure cycle, since at a constant capital parity, the efficiency and capacity are higher. However, on closer examination of the parameters, it may be more appropriate to choose a dual pressure cycle to meet other requirements.
There are cases for which the cycle selection diagram is not applicable. The most common example of such a case is when the customer wants to have at his disposal electrical power as soon as possible and optimization is less important to him than short delivery times.
Depending on the circumstances, it may be advantageous to prefer a single pressure cycle to a multi-pressure cycle, as the time required is less. For this purpose, a series of standardized cycles with given parameters can be developed, which are successfully used in such cases.
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CCGT Installation designed for simultaneous conversion of the energy of two working bodies of steam and gas into mechanical energy. [GOST 26691 85] combined-cycle plant A device that includes radiative and convective heating surfaces, ... ...
Combined-cycle plant- a device that includes radiative and convective heating surfaces that generate and superheat steam for the operation of a steam turbine by burning organic fuel and utilizing the heat of combustion products used in gas turbine in… … Official terminology
Combined-cycle plant- GTU 15. Combined-cycle plant A plant designed to simultaneously convert the energy of two working bodies of steam and gas into mechanical energy Source: GOST 26691 85: Thermal power engineering. Terms and definitions original document 3.13 paro... Dictionary-reference book of terms of normative and technical documentation
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