The thermal part of power plants is discussed in sufficient detail in the course “General Energy”. However, here, in this course, it is advisable to return to the consideration of some issues of the thermal part. But this consideration must be made from the point of view of its influence on the electrical part of power plants.
2.1. Schemes of condensing power plants (CPS)
Feed water is also supplied to the boiler by the feed pump (PN), which is converted into steam under the influence of high temperature. Thus, at the boiler output, live steam is obtained with the following parameters: p=3...30 MPa, t=400...650°C. Live steam is supplied to the steam turbine (T). Here the steam energy is converted into mechanical energy of rotation of the turbine rotor. This energy is transferred to an electrical synchronous generator (G), where it is converted into electrical energy.
The exhaust steam from the turbine enters the condenser (K) (that’s why these stations are called condensing stations) and is cooled cold water and condenses. The condensate is supplied by a condensate pump (CP) to the water treatment system (WTP), and then, after replenishing with chemically purified water (now called feed water), it is supplied to the boiler by the feed pump.
Sources of cold water, which is supplied to the condenser by a circulation pump (CP), can be a river, lake, artificial reservoir, as well as cooling towers and spray ponds. Passing the main part of the steam through the condenser leads to the fact that 60...70% of the thermal energy generated by the boiler is carried away by the circulating water.
Gaseous products of fuel combustion from the boiler are removed by smoke exhausters (Ds) and released into the atmosphere through chimney 100...250 m high (the tallest chimney with a height of 420 m is listed in the Guinness Book of Records), and particulate matter by the hydraulic ash removal system (GZU) are sent to the ash dump.
All these devices and units (dust feeders, blower fans, smoke exhausters, feed pumps, etc.) designed to ensure the technological process and normal operation of the main equipment (boilers, turbines, generators) are called auxiliary mechanisms (S.N.). At block stations the mechanisms of S.N. They are divided into block ones, designed to ensure the operation of only one unit, and general station ones - for the operation of the station as a whole.
The main mechanisms of S.N. are:
– blower fan (DV) for supplying air to the boiler;
– a smoke exhauster (Ds) for the emission of gaseous (and largely solid suspended particles) fuel combustion products from the boiler into a chimney 100...250 m high (420 m in the Guinness Book);
– circulation pump (CP) for supplying cold circulating water to the condenser;
– condensate pump (KN) for pumping condensate from the condenser;
– feed pump (PN) to supply feed water to the boiler and to create the required pressure in the process circuit.
The power plant also uses other auxiliary mechanisms for fuel supply and fuel preparation, in the chemical water treatment and slag and ash removal systems, in control systems for various gate valves, taps and valves, etc. etc. It is not advisable to list all of them in this course, but nevertheless we will consider most of them in the process of studying the material.
Mechanisms S.N. divided into responsible and irresponsible.
Responsible are those mechanisms whose short-term stop leads to an emergency shutdown or unloading of the main units of the station. A short-term interruption in the operation of non-critical auxiliary mechanisms does not lead to an immediate emergency stop of the main equipment. However, in order not to disrupt the technological cycle of electricity production, after a short period of time they must be put into operation again.
In the boiler room, the responsible mechanisms are smoke exhausters, blower fans, and dust feeders. Stopping the operation of smoke exhausters, blower fans and dust feeders leads to the extinguishing of the torch and stopping the steam boiler. The non-responsible ones include flushing and trap pumps of the hydraulic ash removal system (GZU), as well as electric precipitators.
Critical engine room machinery includes feed, circulation and condensate pumps, turbine and generator oil pumps, generator gas cooler lift pumps and generator shaft seal oil pumps. Irrelevant mechanisms include drain pumps of regenerative heaters, drainage pumps, ejectors.
An important place in the station's technological cycle is occupied by feed pumps that supply feed water to steam boilers. The power of electric drives of high-pressure feed pumps reaches 40% (for gas-oil CPPs) of the total power of consumers of their own needs, i.e. several megawatts. Stopping feed pumps leads to emergency shutdown of steam boilers by technological protections. It is especially difficult for once-through boilers at block power plants to endure such a shutdown.
Shutting off condensate and circulation pumps leads to a breakdown of the vacuum of the turbines and to their emergency shutdown.
Particularly critical auxiliary mechanisms, the shutdown of which can lead to damage to the main units, include oil pumps of the turbogenerator lubrication system and generator shaft seals. Refusal to turn on backup oil pumps during emergency stop station with loss of power supply for its own needs can lead to disruption of the oil supply to turbine and generator bearings and melting of their bearings. Therefore, power supply for turbine oil pumps and generator shaft seals is backed up by batteries.
A special place at thermal power plants is occupied by fuel preparation and fuel supply mechanisms: crushers, coal grinding mills, mill fans, conveyors and conveyors for fuel supply and dust plant bunkers, loader cranes in a coal warehouse, car dumpers. A short-term stop of these mechanisms usually does not lead to disruption of the technological cycle for the production of electrical and thermal energy, and therefore these mechanisms can be classified as irresponsible. Indeed, there is always a supply of raw coal in the bunkers, and therefore stopping conveyors or coal crushing devices does not lead to a cessation of fuel supply to the combustion chambers. It is also possible to stop drum ball mills, since when they are used at power plants there are usually intermediate bunkers with a supply of coal dust designed for approximately two hours of boiler operation at rated output. When hammer mills are used, intermediate bunkers are usually not provided, but at least three mills are installed on each boiler. When one of them stops, the remaining ones provide at least 90% of productivity.
General station mechanisms include pumps for chemical water treatment and domestic water supply. Most of them can be classified as irresponsible consumers, since a short-term stop of chemical water treatment pumps should not lead to an emergency in the water supply to boiler units. An exception is the pumps for supplying chemically purified water to the turbine compartment, since if the balance between their performance and feedwater consumption is disturbed, an emergency situation at the station is possible.
Mechanisms for general station purposes also include backup exciters, acid washing pumps, fire-fighting pumps (these mechanisms do not operate under normal operating conditions of the units), ventilation devices, air main compressors, crane facilities, workshops, chargers batteries, open switchgear and integrated auxiliary housing mechanisms. Most of these mechanisms can be classified as non-responsible. Some of the auxiliary mechanisms of the electrical part of the station are responsible: motor-generators of dust feeders and cooling fans of powerful transformers, which blow through oil coolers and forcefully circulate oil. When the generator operates on a backup exciter, the latter also belongs to the responsible mechanisms for its own needs.
As a rule, electric motors are used as drives for auxiliary mechanisms and only at stations with higher power units to reduce currents short circuit steam turbines can be used in the power supply system for auxiliary needs (this will be discussed below). To power electrical consumers S.N. At the stations, a S.N. power supply system is provided. with a special power source, which is usually a TSN transformer connected to the generator voltage.
The features of IES are as follows:
1) are built as close as possible to fuel deposits or electrical energy consumption;
2) the overwhelming majority of the generated electrical energy is given to electrical networks high voltages (110...750 kV);
The first two points determine the purpose of condensing-type stations - power supply to regional networks (if the station is built in an area where electrical energy is consumed) and power supply to the system (when constructing a station in places where fuel is produced).
3) operate according to a free (independent of heat consumers) electricity generation schedule - power can vary from the calculated maximum to the technological minimum (determined mainly by the stability of the flame combustion in the boiler);
4) low maneuverability - turning the turbines and loading the load from a cold state requires approximately 3...10 hours;
Points 3 and 4 determine the operating mode of such stations - they operate mainly in the base part of the system load schedule.
5) require more cooling water to supply it to turbine condensers;
This feature determines the construction site of the station - near a reservoir with a sufficient amount of water.
6) have a relatively low efficiency - 30...40%.
1.2. CHP schemes
Combined heat and power plants are intended for the centralized supply of heat and electricity to industrial enterprises and cities. Therefore, unlike CES, CHP plants, in addition to electrical energy, produce heat in the form of steam or hot water for the needs of production, heating, ventilation and hot water supply. For these purposes, the thermal power plant has significant extractions of steam, partially exhausted in the turbine. With such a combined generation of electrical and thermal energy, significant fuel savings are achieved compared to separate power supply, i.e. generating electricity at CPPs and receiving heat from local boiler houses.
Most Applications The thermal power plant received turbines with one and two adjustable steam extractions and condensers. Adjustable extractions make it possible to independently regulate heat supply and electricity generation within certain limits.
At partial thermal load, they can, if necessary, develop rated power by passing steam to the condensers. When there is a large and constant consumption of steam in technological processes, turbines with back pressure without condensers are also used. The operating power of such units is completely determined by the thermal load. The most widespread are units with a capacity of 50 MW and higher (up to 250 MW).
The mechanisms for auxiliary needs at CHP plants are similar to those at CPPs, but are supplemented with mechanisms that ensure the delivery of thermal energy to the consumer. These include: network pumps (SN), boiler condensate pumps, heating network feed pumps, return condensate pumps (RCP), and other mechanisms.
Combined generation of thermal and electrical energy significantly complicates the technological scheme of a thermal power plant and determines the dependence of electrical energy production on heat consumer. The CHP mode - daily and seasonal - is determined mainly by heat consumption. The station operates most economically if its electrical power matches the heat output. At the same time, it enters the capacitors minimum quantity pair. During periods when heat consumption is relatively low, for example in summer, as well as in winter when the air temperature is higher than the design temperature and at night, the electric power of the thermal power plant corresponding to heat consumption decreases. If the power system needs electrical power, the thermal power plant must switch to mixed mode, in which the supply of steam to parts increases low pressure turbines and condensers. In addition, in order to avoid overheating of the tail section of the turbine, a certain amount of steam must be passed through it in all modes. At the same time, the efficiency of the power plant decreases. When the electrical load at the thermal power plant is reduced below the power of thermal consumption, the thermal energy necessary for consumers can be obtained using a reduction-cooling unit ROU, powered by live steam from the boiler.
Radius of action of powerful thermal power plants - supply hot water for heating - does not exceed 10 km. Suburban CHP plants transmit hot water at a higher initial temperature over a distance of up to 45 km. Steam for production processes at a pressure of 0.8...1.6 MPa can be transmitted no further than 2...3 km.
With an average heat load density, the power of a thermal power plant usually does not exceed 300...500 MW. Only in the largest cities (Moscow, St. Petersburg) with a high load density, thermal power plants with a capacity of up to 1000...1500 MW are appropriate.
The features of the thermal power plant are as follows:
1) are built near thermal energy consumers;
2) usually operate on imported fuel (most thermal power plants use gas transported through gas pipelines);
3) most of the generated electricity is distributed to consumers in the nearby area (at generator or increased voltage);
4) operate according to a partially forced electricity generation schedule (i.e. the schedule depends on the heat consumer);
5) low maneuverability (like IES);
6) have a relatively high total efficiency (60...75% with significant steam extraction for production and domestic needs).
1.3. NPP diagrams
Nuclear power plants are thermal stations that use the energy of nuclear reactions. The thermal energy released in the reactor during the fission reaction of uranium nuclei is removed from the core using a coolant that is pumped under pressure through the core. The most common coolant is water, which is thoroughly purified in inorganic filters.
Nuclear power plants are designed and constructed with reactors of various types using thermal or fast neutrons using a single-circuit, double-circuit or triple-circuit design. The equipment of the last circuit, which includes a turbine and a condenser, is similar to the equipment of thermal power plants. The first, radioactive circuit contains a reactor, a steam generator and a feed pump.
At nuclear power plants in the CIS they use nuclear reactors the following main types:
RBMK (high power reactor, channel) - thermal neutron reactor, water-graphite;
VVER (water-cooled power reactor) – thermal neutron reactor, vessel type;
BN (fast neutrons) is a fast neutron reactor with liquid metal sodium coolant.
The unit capacity of nuclear power units reached 1,500 MW. Currently, it is believed that the unit power of a nuclear power plant is limited not so much by technical considerations as by safety conditions in case of reactor accidents.
Water-cooled reactors can operate in water or steam mode. In the second case, steam is produced directly in the reactor core.
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| Rice. 2.6. Single-circuit diagram of a nuclear power plant |
A single-circuit scheme with a boiling water reactor and a graphite moderator of the RBMK-1000 type was used at the Leningrad NPP. The reactor operates in a block with two condensing turbines of the K-500-65/3000 type and two generators with a capacity of 500 MW. The boiling reactor is a steam generator and thus predetermines the possibility of using a single-circuit circuit. Initial parameters of saturated steam in front of the turbine: temperature 284°C, steam pressure 7.0 MPa. The single-circuit circuit is relatively simple, but radioactivity spreads to all elements of the unit, which complicates biological protection.
The three-circuit scheme is used at nuclear power plants with fast neutron reactors with sodium coolant of the BN-600 type. To prevent contact of radioactive sodium with water, a second circuit with non-radioactive sodium is constructed. Thus, the circuit turns out to be three-circuit. The BN-600 reactor operates in a unit with three K-200-130 condensing turbines with an initial steam pressure of 13 MPa and a temperature of 500°C.
The world's first industrial Obninsk nuclear power plant with a capacity of 5 MW was put into operation in the USSR on June 27, 1954. In 1956...1957. Nuclear power plant units were launched in England (Calder Hall with a capacity of 92 MW) and in the USA (Shippingport Nuclear Power Plant with a capacity of 60 MW). Subsequently, nuclear power plant construction programs began to be accelerated in England, the USA, Japan, France, Canada, Germany, Sweden and a number of other countries. It was assumed that by 2000, electricity generation from nuclear power plants in the world could reach 50% of total electricity generation. However, currently the pace of development nuclear energy in the world, due to a number of reasons, have decreased significantly.
The features of the nuclear power plant are as follows:
1) can be built in any geographical location, including hard-to-reach places;
2) in their mode they are autonomous from the series external factors;
3) require a small amount of fuel;
4) can work according to a free workload schedule;
5) sensitive to alternating conditions, especially nuclear power plants with fast neutron reactors; for this reason, and also taking into account the requirements for economical operation, the basic part of the power system load schedule is allocated for nuclear power plants (duration of use of the installed capacity 6500...7000 h/year);
6) lightly pollute the atmosphere; emissions of radioactive gases and aerosols are insignificant and do not exceed permissible values sanitary standards. In this regard, nuclear power plants are cleaner than thermal power plants.
1.4. Hydroelectric power station schemes
When constructing a hydroelectric power station, the following goals are usually pursued:
Electricity generation;
Improving conditions for navigation on the river;
Improving irrigation conditions for adjacent lands.
The power of a hydroelectric power station depends on the water flow through the turbine and the pressure (the difference in the levels of the upper and lower pools).
Units for each hydroelectric power station, as a rule, are designed individually, in relation to the characteristics of this hydroelectric power station.
For low pressures, run-of-river (Uglich and Rybinsk hydroelectric power stations) or combined (Volzhsky hydroelectric power stations named after V.I. Lenin and named after the XXII Congress of the CPSU) hydroelectric power stations are built, and for significant pressures (more than 30...35 m) - dam hydroelectric power stations (DneproGES, Bratsk hydroelectric power station). IN mountainous areas they are constructing diversion hydroelectric power stations (Gyumush hydroelectric power station, Farhad hydroelectric power station) with high pressures at low flow rates.
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| Rice. 6 |
Hydroelectric power plants usually have reservoirs that allow them to accumulate water and regulate its flow and, consequently, the operating power of the station so as to provide the most favorable mode for the energy system as a whole.
The regulatory process is as follows. For some time, when the load on the power system is low (or the natural inflow of water in the river is large), the hydroelectric power plant consumes water in an amount less than the natural inflow. In this case, water accumulates in the reservoir, and the operating capacity of the station is relatively small. At other times, when the system load is high (or the water inflow is small), the hydroelectric power station consumes water in an amount exceeding the natural inflow. In this case, the water accumulated in the reservoir is consumed, and the operating power of the station increases to maximum. Depending on the volume of the reservoir, the regulation period, or the time required to fill and operate the reservoir, can be a day, a week, several months or more. During this time, the hydroelectric power station can consume a strictly defined amount of water, determined by natural inflow.
At working together Hydroelectric power plants with thermal power plants and nuclear power plants distribute the load of the energy system between them so that, at a given water consumption during the period under review, the demand for electricity is met with minimal fuel consumption (or minimal costs for fuel) in the system. Experience in operating energy systems shows that during most of the year it is advisable to use hydroelectric power plants in peak mode. This means that during the day the operating power of a hydroelectric power station should vary within wide limits - from minimum during hours when the load on the power system is low to maximum during hours heaviest load systems. With this use of hydroelectric power stations, the load of thermal stations is leveled and their operation becomes more economical.
During periods of flood, it is advisable to use hydroelectric power stations around the clock with an operating capacity close to maximum, and thus reduce idle reset water through the dam.
The operation of hydroelectric power plants is characterized by frequent starts and stops of units, a rapid change in operating power from zero to nominal. Hydraulic turbines by their nature are adapted to this regime. For hydrogenerators, this mode is also acceptable, since, unlike steam turbine generators, the axial length of the hydrogenerator is relatively small and temperature deformations of the winding rods are less pronounced. The process of starting the hydraulic unit and gaining power is fully automated and requires only a few minutes.
The duration of use of the installed capacity of hydroelectric power plants is usually less than that of thermal power plants. It is 1500...3000 hours for peak stations and up to 5000...6000 hours for base stations. It is advisable to build hydroelectric power stations on mountain and semi-mountain rivers.
3-4. Mechanisms for auxiliary needs of hydroelectric power plants
Mechanisms for the auxiliary needs of hydroelectric power stations are divided into aggregate and general station ones according to their purpose.
The auxiliary aggregate mechanisms ensure the start, stop and normal operation of hydraulic generators and step-up power transformers associated with them in block diagrams. These include:
Oil pumps of the hydraulic turbine control system;
Cooling pumps and fans for power transformers;
Oil or water pumps of the unit lubrication system;
Direct water cooling pumps for generators;
Unit braking compressors;
Pumps for pumping water from the turbine cover;
Auxiliary devices for the generator excitation system;
Pathogens in self-excitation systems. Public ones include:
Pumps for pumping water out of spiral chambers and suction pipes;
Domestic water supply pumps;
Drainage pumps;
Devices for charging, heating and ventilation of batteries;
Cranes, lifting mechanisms for dam gates, shields, suction pipe stops, debris-holding grates;
Outdoor switchgear compressors;
Heating, lighting and ventilation of premises and structures;
Heating devices for shutters, grilles and grooves.
With a centralized system for supplying units with compressed air, the station-wide compressors also include compressors for oil pressure units and unit braking.
The composition and power of electrical receivers for the auxiliary needs of hydroelectric power plants are influenced by climatic conditions: in a harsh climate, a significant (several thousand kilowatts) heating load appears on switches, oil tanks, oil-filled cable terminations, grilles, gates, grooves; In hot climates, these loads are absent, but energy consumption for equipment cooling, ventilation, and air conditioning increases.
At hydroelectric power plants, a relatively small proportion of auxiliary mechanisms operate continuously in a long-term mode. These include: pumps and cooling fans for generators and transformers; auxiliary devices excitation systems; pumps for water or oil lubrication of bearings. These mechanisms are among the most critical and allow a power interruption for the duration of the automatic transfer of reserve (ATS). Pumps for technical water supply and electric heating devices also operate in continuous mode. The rest of the electrical receivers operate repeatedly, briefly, for a short time, or even only occasionally. Responsible mechanisms for own needs also include fire pumps, pumps for oil pressure installations, some drainage pumps, outdoor switchgear compressors, and closing mechanisms for pressure pipeline valves. These mechanisms allow a power interruption of up to several minutes without disrupting the normal and safe operation of the units. The remaining consumers of their own needs can be classified as irresponsible.
The oil pressure units of hydraulic units have a sufficient energy reserve to close the guide vane and brake the unit even in the event of an emergency loss of voltage in the auxiliary system. Therefore, to ensure the safety of equipment in the event of a loss of voltage at hydroelectric power stations, autonomous sources in the form of batteries and diesel generators are not required.
The unit power of auxiliary mechanisms ranges from units to hundreds of kilowatts. The most powerful mechanisms for own needs are technical water supply pumps, pumps for pumping water out of suction pipes, and some lifting mechanisms. At most hydroelectric power stations, with the exception of diversion-type hydroelectric power stations, consumers of their own needs are concentrated in a limited area, within the station building and dam.
Unlike thermal power plants, the auxiliary mechanisms of hydroelectric power plants do not require continuous regulation of productivity; Intermittent and short-term operating mode (oil pumps, compressors) is sufficient.
The features of the hydroelectric power station are as follows:
1) are built where there are water resources and conditions for construction, which usually does not coincide with the location of the electrical load;
2) most of the electrical energy is supplied to high-voltage electrical networks;
3) work on a flexible schedule (if there is a reservoir);
4) highly maneuverable (turning and gaining load takes approximately 3...5 minutes);
5) have high efficiency (up to 85%).
hydroelectric power station in relation to operating parameters have a number of advantages over thermal power plants. However, at present, thermal and nuclear power plants are mainly being built. The determining factors here are the size of capital investments and the time of construction of power plants. (There are data on specific capital investments, cost of electricity and construction time various types email stations).
The specific cost of hydroelectric power plants (RUB/MW) is higher than the specific cost of thermal power plants of the same capacity due to the larger volume construction work. The construction time of a hydroelectric power station is also longer. However, the cost of electricity is lower, since operating costs do not include the cost of fuel.
Pumped storage power plants.
The purpose of pumped storage power plants is to level out the daily load schedule of the electrical system and increase the efficiency of thermal power plants and nuclear power plants. During the hours of minimum system load, PSPP units operate in pumping mode, pumping water from the lower reservoir to the upper one and thereby increasing the load of thermal power plants and nuclear power plants. During the hours of maximum system load, they operate in turbine mode, drawing water from the upper reservoir and thereby unloading thermal power plants and nuclear power plants from short-term peak loads. PSPP units are also used as rotating backup units and as synchronous compensators.
Peak pumped storage power plants are designed, as a rule, to operate in turbine mode for 4...6 hours per day. The duration of operation of a pumped storage power plant in pumping mode is 7...8 hours with a ratio of pumping to turbine power of 1.05...1.10. The annual use of pumped storage power plant capacity is 1000...1500 hours.
PSPPs are built in systems where there are no hydroelectric power stations or their capacity is insufficient to cover the load during peak hours. They are made from a number of blocks that produce energy in a high-voltage network and receive it from the network when operating in pump mode. The units are highly maneuverable and can be quickly transferred from pump mode to generator mode or to synchronous compensator mode. The efficiency of pumped storage power plants is 70...75%. They require a small number of maintenance personnel. Pumped storage power plants can be built where there are sources of water supply and local geological conditions allow the creation of a pressure reservoir.
1.4. Gas turbine units
1.7. Solar power plants.
Among solar power plants (solar power plants), two types of power plants can be distinguished - with a steam boiler and with silicon photocells. Such power plants have found application in a number of countries with a significant number of sunny days a year. According to published data, their efficiency can be increased to 20%.
1.8. Geothermal power plants use cheap energy from underground thermal springs.
Geothermal power plants operate in Iceland, New Zealand, Papua, New Guinea, the USA, and in Italy they provide about 6% of all electricity generated. In Russia (on Komchatka), the Pauzhetskaya geothermal power plant was built.
1.9. Tidal power plants with so-called capsule hydroelectric units are built where there is a significant difference in water levels during high and low tides. The most powerful TPP Rance was built in 1966 in France: its capacity is 240 MW. PPPs are being designed in the USA with a capacity of 1000 MW, in the UK with a capacity of 7260 MW, etc. In Russia, on the Kola Peninsula, where tides reach 10...13 m, in 1968 the first stage of the experimental Kislogubskaya TPP (2·0.4 MW) came into operation.
1.10. Magnetohydrodynamic power plants use the principle of current generation when a moving conductor passes through a magnetic field. Low-temperature plasma (about 2700 C) is used as a working fluid, which is formed during the combustion of organic fuel and the supply of special ionizing additives to the combustion chamber. The working fluid passing through the superconducting magnetic system creates D.C., which is converted into alternating with the help of inverter converters. The working fluid, after passing through the magnetic system, enters the steam turbine part of the power plant, consisting of a steam generator and a conventional condensing steam turbine. Currently, at the Ryazan State District Power Plant, a 500 MW main MHD power unit has been built, including an MHD generator with a capacity of about 300 MW and a steam turbine unit with a capacity of 315 MW with a K-300-240 turbine. With an installed capacity of over 610 MW, the power output of the MHD power unit into the system is 500 MW due to significant energy consumption for its own needs in the MHD power unit. 
parts. The efficiency of MGD-500 exceeds 45%, specific fuel consumption is approximately 270 g/(kW*h). The main MHD power unit is designed to use natural gas, in the future it was planned to switch to solid fuel. However further development MHD installations were not developed due to the lack of materials capable of operating at such high temperatures.
Electricity is produced at power plants by using the energy hidden in various natural resources. As can be seen from table. 1.2 this happens mainly at thermal power plants (TPPs) and nuclear power plants (NPPs) operating according to the thermal cycle.
Types of thermal power plants
Based on the type of energy generated and released, thermal power plants are divided into two main types: condensing power plants (CHPs), intended only for the production of electricity, and heating plants, or combined heat and power plants (CHPs). Condensing power stations operating on fossil fuels are built near the places of its production, and combined heat and power plants are located near heat consumers - industrial enterprises and residential areas. CHP plants also operate on fossil fuels, but unlike CPPs, they generate both electrical and thermal energy in the form of hot water and steam for production and heating purposes. The main types of fuel of these power plants include: solid - coals, anthracite, semi-anthracite, brown coal, peat, shale; liquid - fuel oil and gaseous - natural, coke, blast furnace, etc. gas.
Table 1.2. Electricity generation in the world
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Indicator |
2010 (forecast) |
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Share of total output by power plants, % NPP Thermal power plant on gas TPP on fuel oil |
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Electricity generation by region, % Western Europe Eastern Europe Asia and Australia America Middle East and Africa |
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Installed capacity of power plants in the world (total), GW Including, % NPP Thermal power plant on gas TPP on fuel oil Thermal power plants using coal and other types of fuel Hydroelectric power stations and power plants using other renewable types of fuel |
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Electricity generation (total), billion kWh |
Nuclear power plants, predominantly of the condensing type, use the energy of nuclear fuel.
Depending on the type of thermal power plant for driving an electric generator, power plants are divided into steam turbine (STU), gas turbine (GTU), combined cycle (CCG) and power plants with internal combustion engines (ICE).
Depending on the duration of work TPP throughout the year Based on the coverage of energy load schedules, characterized by the number of hours of use of the installed capacity τ at the station, power plants are usually classified into: basic (τ at the station > 6000 h/year); half-peak (τ at station = 2000 – 5000 h/year); peak (τ at st< 2000 ч/год).
Basic power plants are those that carry the maximum possible constant load for most of the year. In the global energy industry, nuclear power plants, highly economical thermal power plants, and thermal power plants are used as base plants when operating according to a thermal schedule. Peak loads are covered by hydroelectric power plants, pumped storage power plants, gas turbine plants, which have maneuverability and mobility, i.e. quick start and stop. Peaking power plants are turned on at the hours when it is necessary to cover the peak part of the daily electrical load schedule. Half-peak power plants, when the total electrical load decreases, are either transferred to reduced power or put into reserve.
According to the technological structure, thermal power plants are divided into block and non-block. With a block diagram, the main and auxiliary equipment of a steam turbine plant does not have technological connections with the equipment of another installation of the power plant. For fossil fuel power plants, steam is supplied to each turbine from one or two boilers connected to it. With a non-block TPP scheme, steam from all boilers enters a common main and from there is distributed to individual turbines.
At condensing power plants included in large power systems, only block systems with intermediate steam superheating. Non-block circuits with cross-coupling of steam and water are used without intermediate overheating.
Operating principle and main energy characteristics of thermal power plants
Electricity at power plants is produced by using energy hidden in various natural resources (coal, gas, oil, fuel oil, uranium, etc.), according to sufficient simple principle, implementing energy conversion technology. The general diagram of a thermal power plant (see Fig. 1.1) reflects the sequence of such conversion of one type of energy into another and the use of the working fluid (water, steam) in the cycle of a thermal power plant. The fuel (in this case coal) burns in the boiler, heats the water and turns it into steam. The steam is supplied to turbines, which convert the thermal energy of the steam into mechanical energy and drive generators that produce electricity (see section 4.1).
A modern thermal power plant is a complex enterprise, including large number various equipment. The composition of the power plant equipment depends on the selected thermal circuit, the type of fuel used and the type of water supply system.
The main equipment of the power plant includes: boiler and turbine units with electric generator and a capacitor. These units are standardized in terms of power, steam parameters, productivity, voltage and current, etc. The type and quantity of the main equipment of a thermal power plant correspond to the specified power and the intended operating mode. There is also auxiliary equipment used to supply heat to consumers and use turbine steam to heat boiler feedwater and meet the power plant’s own needs. This includes equipment for fuel supply systems, deaeration and feed installations, condensation installations, heating plants (for thermal power plants), technical water supply systems, oil supply systems, regenerative heating of feed water, chemical water treatment, distribution and transmission of electricity (see section 4).
All steam turbine plants use regenerative heating of feed water, which significantly increases the thermal and overall efficiency of the power plant, since in circuits with regenerative heating, the steam flows removed from the turbine to the regenerative heaters perform work without losses in the cold source (condenser). At the same time, for the same electric power of the turbogenerator, the steam flow in the condenser decreases and, as a result, efficiency installations are growing.
The type of steam boiler used (see section 2) depends on the type of fuel used in the power plant. For the most common fuels (fossil coal, gas, fuel oil, milling peat), boilers with a U-, T-shaped and tower layout and a combustion chamber designed in relation to a particular type of fuel are used. For fuels with low-melting ash, boilers with liquid ash removal are used. At the same time, high (up to 90%) ash collection in the firebox is achieved and abrasive wear of heating surfaces is reduced. For the same reasons, steam boilers with a four-pass arrangement are used for high-ash fuels, such as shale and coal preparation waste. Thermal power plants usually use drum or direct-flow boilers.
Turbines and electric generators are matched on a power scale. Each turbine has a specific type of generator. For block thermal condensing power plants, the power of the turbines corresponds to the power of the blocks, and the number of blocks is determined by the given power of the power plant. Modern units use 150, 200, 300, 500, 800 and 1200 MW condensing turbines with steam reheating.
Thermal power plants use turbines (see subsection 4.2) with back pressure (type P), with condensation and industrial steam extraction (type P), with condensation and one or two heating extractions (type T), as well as with condensation, industrial and heating extraction pair (PT type). PT turbines can also have one or two heating outlets. The choice of turbine type depends on the magnitude and ratio of thermal loads. If the heating load predominates, then in addition to the PT turbines, type T turbines with heating extraction can be installed, and if the industrial load predominates, type PR and R turbines with industrial extraction and back pressure can be installed.
Currently, the most widely used thermal power plants are installations with an electrical capacity of 100 and 50 MW, operating at initial parameters of 12.7 MPa, 540–560°C. For thermal power plants in large cities, installations with an electrical capacity of 175–185 MW and 250 MW (with a T-250-240 turbine) have been created. Installations with T-250-240 turbines are modular and operate at supercritical initial parameters (23.5 MPa, 540/540°C).
A feature of the operation of power stations in the network is that the total amount of electrical energy generated by them at each moment of time must fully correspond to the energy consumed. The main part of the power plants operates in parallel in the unified energy system, covering the total electrical load of the system, and the thermal power plant simultaneously covers the heat load of its area. There are local power plants designed to serve the area and not connected to the general power grid.
A graphical representation of the dependence of power consumption over time is called electrical load graph. Daily graphs of electrical load (Fig. 1.5) vary depending on the time of year, day of the week and are usually characterized by a minimum load at night and a maximum load during peak hours (the peak part of the graph). Along with daily graphs, annual graphs of electrical load (Fig. 1.6), which are constructed based on data from daily graphs, are of great importance.
Electrical load graphs are used when planning electrical loads of power plants and systems, distributing loads between individual power plants and units, in calculations for selecting the composition of working and backup equipment, determining the required installed power and the required reserve, the number and unit power of units, when developing equipment repair plans and determining the repair reserve, etc.
When operating at full load, the power plant equipment develops its rated or as long as possible power (performance), which is the main passport characteristic of the unit. At this maximum power (performance), the unit must operate for a long time at the nominal values of the main parameters. One of the main characteristics of a power plant is its installed capacity, which is defined as the sum of the rated capacities of all electric generators and heating equipment, taking into account the reserve.
The operation of the power plant is also characterized by the number of hours of use installed capacity, which depends on the mode in which the power plant operates. For base-load power plants, the number of hours of use of installed capacity is 6000–7500 hours/year, and for those operating in peak load coverage mode – less than 2000–3000 hours/year.
The load at which the unit operates with the greatest efficiency is called the economic load. The rated long-term load can be equal to the economic load. Sometimes it is possible to operate equipment for a short time with a load 10–20% higher than the rated load at lower efficiency. If the power plant equipment operates stably with the design load at the nominal values of the main parameters or when they change within acceptable limits, then this mode is called stationary.
Operating modes with steady loads, but different from the design ones, or with unsteady loads are called non-stationary or variable modes. In variable modes, some parameters remain unchanged and have nominal values, while others change within certain acceptable limits. Thus, at partial load of the unit, the pressure and temperature of the steam in front of the turbine can remain nominal, while the vacuum in the condenser and the steam parameters in the extractions will change in proportion to the load. Non-stationary modes are also possible, when all the main parameters change. Such modes occur, for example, when starting and stopping equipment, dumping and increasing the load on a turbogenerator, when operating on sliding parameters and are called non-stationary.
The thermal load of the power plant is used for technological processes and industrial installations, for heating and ventilation of industrial, residential and public buildings, air conditioning and domestic needs. For production purposes, steam pressure of 0.15 to 1.6 MPa is usually required. However, in order to reduce losses during transportation and avoid the need for continuous drainage of water from communications, steam is released from the power plant somewhat overheated. The thermal power plant usually supplies hot water with a temperature of 70 to 180°C for heating, ventilation and domestic needs.
The heat load, determined by the heat consumption for production processes and domestic needs (hot water supply), depends on the outside air temperature. In the conditions of Ukraine in summer, this load (as well as electrical) is less than in winter. Industrial and domestic heat loads change during the day, in addition, the average daily heat load of the power plant, spent on domestic needs, changes on weekdays and weekends. Typical graphs of changes in the daily heat load of industrial enterprises and hot water supply to a residential area are shown in Figures 1.7 and 1.8.
The operating efficiency of thermal power plants is characterized by various technical and economic indicators, some of which assess the perfection of thermal processes (efficiency, heat and fuel consumption), while others characterize the conditions in which the thermal power plant operates. For example, in Fig. 1.9 (a,b) shows approximate heat balances of thermal power plants and CPPs.
As can be seen from the figures, the combined generation of electrical and thermal energy provides a significant increase in the thermal efficiency of power plants due to a reduction in heat losses in turbine condensers.
The most important and complete indicators of the operation of thermal power plants are the cost of electricity and heat.
Thermal power plants have both advantages and disadvantages compared to other types of power plants. The following advantages of TPP can be indicated:
- relatively free territorial distribution associated with the wide distribution of fuel resources;
- the ability (unlike hydroelectric power plants) to generate energy without seasonal power fluctuations;
- the area of alienation and withdrawal from economic circulation of land for the construction and operation of thermal power plants is, as a rule, much smaller than that required for nuclear power plants and hydroelectric power plants;
- Thermal power plants are built much faster than hydroelectric power plants or nuclear power plants, and their specific cost per unit of installed capacity is lower compared to nuclear power plants.
- At the same time, thermal power plants have major disadvantages:
- the operation of thermal power plants usually requires much more personnel than hydroelectric power plants, which is associated with the maintenance of a very large-scale fuel cycle;
- the operation of thermal power plants depends on the supply of fuel resources (coal, fuel oil, gas, peat, oil shale);
- variable operating modes of thermal power plants reduce efficiency, increase fuel consumption and lead to increased wear and tear of equipment;
- existing thermal power plants are characterized by relatively low efficiency. (mostly up to 40%);
- TPPs have a direct and adverse impact on environment and are not environmentally friendly sources of electricity.
- The greatest damage to the environment of the surrounding regions is caused by power plants burning coal, especially high-ash coal. Among thermal power plants, the “cleanest” ones are those that use natural gas in their technological process.
According to experts, thermal power plants around the world annually emit about 200–250 million tons of ash, more than 60 million tons of sulfur dioxide, large amounts of nitrogen oxides and carbon dioxide (causing the so-called greenhouse effect and leading to long-term global climate change), into the atmosphere. absorbing large amounts of oxygen. In addition, it has now been established that excess background radiation around coal-fired thermal power plants, on average in the world is 100 times higher than near nuclear power plants of the same power (coal almost always contains uranium, thorium and a radioactive carbon isotope as trace impurities). However, well-developed technologies for the construction, equipment and operation of thermal power plants, as well as the lower cost of their construction, lead to the fact that thermal power plants account for the bulk of global electricity production. For this reason, much attention is paid to improving TPP technologies and reducing their negative impact on the environment around the world (see section 6).
Purpose of the thermal power plant consists of converting the chemical energy of fuel into electrical energy. Since it turns out to be practically impossible to carry out such a transformation directly, it is necessary to first convert the chemical energy of the fuel into heat, which is produced by burning the fuel, then convert the heat into mechanical energy and, finally, convert this latter into electrical energy.
The figure below shows simplest scheme the thermal part of an electric power plant, often called a steam power plant. Fuel is burned in a furnace. At the same time. The resulting heat is transferred to the water in the steam boiler. As a result, the water heats up and then evaporates, forming so-called saturated steam, that is, steam at the same temperature as boiling water. Next, heat is supplied to the saturated steam, resulting in the formation of superheated steam, i.e. steam that has a higher temperature than water evaporating at the same pressure. Superheated steam is obtained from saturated steam in a superheater, which in most cases is a coil made of steel pipes. Steam moves inside the pipes, while on the outside the coil is washed by hot gases.
If the pressure in the boiler were equal to atmospheric pressure, then the water would need to be heated to a temperature of 100 ° C; with further heat it would begin to evaporate quickly. The resulting saturated steam would also have a temperature of 100 ° C. At atmospheric pressure, the steam will be superheated if its temperature is above 100 ° C. If the pressure in the boiler is higher than atmospheric, then the saturated steam has a temperature above 100 ° C. The temperature of the saturated The higher the pressure, the higher the vapor. Currently, steam boilers with pressure close to atmospheric are not used in the energy sector at all. It is much more profitable to use steam boilers designed for much higher pressure, about 100 atmospheres or more. The temperature of saturated steam is 310° C or more.
From the superheater, superheated water vapor is supplied through a steel pipeline to a heat engine, most often -. In existing steam power plants of power plants, other engines are almost never used. Superheated water vapor entering a heat engine contains a large supply of thermal energy released as a result of fuel combustion. The job of a heat engine is to convert the thermal energy of steam into mechanical energy.
The pressure and temperature of the steam at the inlet to the steam turbine, usually referred to as , are significantly higher than the pressure and temperature of the steam at the outlet of the turbine. The pressure and temperature of the steam at the outlet of the steam turbine, equal to the pressure and temperature in the condenser, are usually called. Currently, as already mentioned, the energy industry uses steam with very high initial parameters, with a pressure of up to 300 atmospheres and a temperature of up to 600 ° C. The final parameters, on the contrary, are chosen low: a pressure of about 0.04 atmospheres, i.e. 25 times less than atmospheric, and the temperature is about 30 ° C, i.e. close to ambient temperature. When steam expands in a turbine, due to a decrease in the pressure and temperature of the steam, the amount of thermal energy contained in it decreases significantly. Since the expansion process of steam occurs very quickly, during this very short time any significant transfer of heat from steam to the environment does not have time to take place. Where does the excess thermal energy go? It is known that, according to the basic law of nature - the law of conservation and transformation of energy - it is impossible to destroy or obtain “out of nothing” any, even the smallest, amount of energy. Energy can only move from one type to another. Obviously, it is precisely this kind of energy transformation that we are dealing with in this case. The excess thermal energy previously contained in the steam has turned into mechanical energy and can be used at our discretion.
How a steam turbine works is described in the article about.
Here we will only say that the stream of steam entering the turbine blades has a very high speed, often exceeding the speed of sound. The steam jet rotates the steam turbine disk and the shaft on which the disk is mounted. The turbine shaft can be connected, for example, to an electrical machine - a generator. The task of the generator is to convert the mechanical energy of shaft rotation into electrical energy. Thus, the chemical energy of the fuel in the steam power plant is converted into mechanical energy and then into electrical energy, which can be stored in an AC UPS.
The steam that has done work in the engine enters the condenser. Cooling water is continuously pumped through the condenser tubes, usually taken from some natural body of water: river, lake, sea. Cooling water takes heat from the steam entering the condenser, as a result of which the steam condenses, i.e. turns into water. The water formed as a result of condensation is pumped into a steam boiler, in which it evaporates again, and the whole process is repeated again.
This is, in principle, the operation of the steam power plant of a thermoelectric station. As you can see, steam serves as an intermediary, the so-called working fluid, with the help of which the chemical energy of the fuel, converted into thermal energy, is converted into mechanical energy.
You should not think, of course, that the design of a modern, powerful steam boiler or heat engine is as simple as shown in the figure above. On the contrary, the boiler and turbine, which are the most important elements of a steam power plant, have a very complex structure.
We now begin to explain the work.
Purpose of combined heat and power plants. Schematic diagram of a thermal power plant
CHP (combined heat and power plants)- designed for centralized supply of heat and electricity to consumers. Their difference from IES is that they use the heat of steam exhausted in turbines for the needs of production, heating, ventilation and hot water supply. Due to this combination of electricity and heat generation, significant fuel savings are achieved in comparison with separate energy supply (electricity generation at CPPs and thermal energy at local boiler houses). Thanks to this method of combined production, CHP plants achieve a fairly high efficiency, reaching up to 70%. Therefore, CHP plants have become widespread in areas and cities with high heat consumption. The maximum power of a CHP plant is less than that of a CPP.
CHP plants are tied to consumers, because The radius of heat transfer (steam, hot water) is approximately 15 km. Suburban thermal power plants transmit hot water at a higher initial temperature over a distance of up to 30 km. Steam for production needs with a pressure of 0.8-1.6 MPa can be transmitted over a distance of no more than 2-3 km. With an average heat load density, the power of a thermal power plant usually does not exceed 300-500 MW. Only in major cities, such as Moscow or St. Petersburg with a high heat load density, it makes sense to build stations with a capacity of up to 1000-1500 MW.
The power of the thermal power plant and the type of turbogenerator are selected in accordance with the heat requirements and parameters of the steam used in production processes and for heating. The most widely used are turbines with one and two adjustable steam extractions and condensers (see figure). Adjustable selections allow you to regulate the production of heat and electricity.
The CHP mode - daily and seasonal - is determined mainly by heat consumption. The station operates most economically if its electrical power matches the heat output. In this case, a minimum amount of steam enters the condensers. In winter, when the demand for heat is maximum, at the design air temperature during operating hours of industrial enterprises, the load of CHP generators is close to the nominal one. During periods when heat consumption is low, for example in summer, as well as in winter when the air temperature is higher than the design temperature and at night, the electric power of the thermal power plant corresponding to heat consumption decreases. If the power system needs electrical power, the thermal power plant must switch to mixed mode, which increases the flow of steam into the low pressure part of the turbines and into the condensers. At the same time, the efficiency of the power plant decreases.
Maximum electricity production by heating stations “on heat consumption” is possible only when working together with powerful CPPs and hydroelectric power plants, which take on a significant part of the load during hours of reduced heat consumption.
The impeller blades of this steam turbine are clearly visible.
A thermal power plant (CHP) uses the energy released by burning fossil fuels - coal, oil and natural gas - to convert water into high-pressure steam. This steam, having a pressure of about 240 kilograms per square centimeter and a temperature of 524°C (1000°F), drives the turbine. The turbine spins a giant magnet inside a generator, which produces electricity.
Modern thermal power plants convert about 40 percent of the heat released during fuel combustion into electricity, the rest is discharged into the environment. In Europe, many thermal power plants use waste heat to heat nearby homes and businesses. Combined heat and power generation increases the energy output of the power plant by up to 80 percent.
Steam turbine plant with electric generator
A typical steam turbine contains two groups of blades. High-pressure steam coming directly from the boiler enters the flow path of the turbine and rotates the impellers with the first group of blades. The steam is then heated in the superheater and again enters the turbine flow path to rotate impellers with a second group of blades, which operate at a lower steam pressure.
Sectional view
A typical thermal power plant (CHP) generator is driven directly by a steam turbine, which rotates at 3,000 revolutions per minute. In generators of this type, the magnet, also called the rotor, rotates, but the windings (stator) are stationary. The cooling system prevents the generator from overheating.
Power generation using steam
At a thermal power plant, fuel burns in a boiler, producing a high-temperature flame. The water passes through the tubes through the flame, is heated and turns into high-pressure steam. The steam spins a turbine, producing mechanical energy, which a generator converts into electricity. After leaving the turbine, the steam enters the condenser, where it washes the tubes with cold running water, and as a result turns back into liquid.
Oil, coal or gas boiler
Inside the boiler
The boiler is filled with intricately curved tubes through which heated water passes. The complex configuration of the tubes allows you to significantly increase the amount of heat transferred to the water and, as a result, produce much more steam.