« Physics - 11th grade"
Power generation
Electricity is produced at power plants mainly using electromechanical induction generators.
There are two main types of power plants: thermal and hydroelectric.
These power plants differ in the engines that rotate the generator rotors.
At thermal power plants, the source of energy is fuel: coal, gas, oil, fuel oil, oil shale.
The rotors of electric generators are driven by steam and gas turbines or internal combustion engines.
Thermal steam turbine power plants - TPP most economical.
In a steam boiler, over 90% of the energy released by the fuel is transferred to steam.
In the turbine, the kinetic energy of the steam jets is transferred to the rotor.
The turbine shaft is rigidly connected to the generator shaft.
Steam turbogenerators are very fast: the rotor speed is several thousand per minute.
The efficiency of heat engines increases with increasing initial temperature of the working fluid (steam, gas).
Therefore, the steam entering the turbine is brought to high parameters: temperature - almost 550 ° C and pressure - up to 25 MPa.
The efficiency of thermal power plants reaches 40%. Most of the energy is lost along with the hot exhaust steam.
Thermal power plants - CHP allow a significant part of the waste steam energy to be used at industrial enterprises and for domestic needs.
As a result, the efficiency of the thermal power plant reaches 60-70%.
In Russia, thermal power plants provide about 40% of all electricity and supply hundreds of cities with electricity.
On hydroelectric power plants - hydroelectric power station The potential energy of water is used to rotate the generator rotors.
The rotors of electric generators are driven by hydraulic turbines.
The power of such a station depends on the pressure created by the dam and the mass of water passing through the turbine every second.
Hydroelectric power plants provide about 20% of all electricity generated in our country.
Nuclear power plants - nuclear power plants in Russia they provide about 10% of electricity.
Electricity usage
The main consumer of electricity is industry - 70% of the electricity produced.
Transport is also a major consumer.
Most of the electricity used is now converted into mechanical energy because... Almost all machinery in industry is driven by electric motors.
Electricity transmission
Electricity cannot be conserved on a large scale.It must be consumed immediately upon receipt.
Therefore, there is a need to transmit electricity over long distances.
The transmission of electricity is associated with noticeable losses, as the electric current heats the wires of the power lines. In accordance with the Joule-Lenz law, the energy spent on heating the line wires is determined by the formula
Where
R- line resistance,
U- transmitted voltage,
R- power of the current source.
If the line length is very long, energy transmission may become economically unprofitable.
It is practically very difficult to significantly reduce the line resistance R, so it is necessary to reduce the current I.
Since the power of the current source P is equal to the product of the current I and the voltage U, then to reduce the transmitted power it is necessary to increase the transmitted voltage in the transmission line.
For this purpose, step-up transformers are installed at large power plants.
The transformer increases the voltage in the line by the same number of times as it reduces the current.
The longer the transmission line, the more beneficial it is to use a higher voltage. Alternating current generators are set to voltages not exceeding 16-20 kV. Higher voltages would require complex special measures to insulate the windings and other parts of the generators.
This is achieved using step-down transformers.
The voltage decrease (and, accordingly, the current increase) is carried out in stages.
If the voltage is very high, a discharge may begin between the wires, leading to energy loss.
The permissible amplitude of the alternating voltage must be such that, for a given cross-sectional area of the wire, energy losses due to the discharge are insignificant.
Electric stations are connected by high-voltage power lines, forming a common electrical network to which consumers are connected.
This connection, called a power grid, makes it possible to distribute energy consumption loads.
The power system ensures uninterrupted supply of energy to consumers.
Now our country has a Unified Energy System for the European part of the country.
Electricity usage
The demand for electricity is constantly increasing both in industry, transport, scientific institutions, and in everyday life. There are two main ways to satisfy this need.
The first is the construction of new powerful power plants: thermal, hydraulic and nuclear.
However, building a large power plant requires several years and high costs.
In addition, thermal power plants consume non-renewable natural resources: coal, oil and gas.
At the same time, they cause great damage to the balance on our planet.
Advanced technologies make it possible to meet energy needs in a different way.
The second is the efficient use of electricity: modern fluorescent lamps, lighting savings.
Great hopes are placed on obtaining energy using controlled thermonuclear reactions.
Priority should be given to increasing energy efficiency rather than increasing power plant capacity.
Electricity makes people's lives better, brighter and cleaner. But before it can travel along high-voltage power lines and then be distributed to homes and businesses, electrical energy must be generated by a power plant.
How is electricity generated?
In 1831, M. Faraday discovered that when a magnet rotates around a coil of wire, an electric current flows in the conductor. An electricity generator is a device that converts another form of energy into electrical energy. These units operate based on the interaction of electric and magnetic fields. Almost all of the power consumed is produced by generators that convert mechanical energy into electrical energy.
The production of electricity in the usual way is carried out by a generator with an electromagnet. It has a series of insulated coils of wire forming a stationary cylinder (stator). Inside the cylinder there is a rotating electromagnetic shaft (rotor). When the electromagnetic shaft rotates, an electric current arises in the stator coils, which is then transmitted through power lines to consumers.
In power plants, turbines are used as generators to produce electrical energy, which come in various types:
- steam;
- gas combustion turbines;
- water;
- wind.
In a turbogenerator, moving liquid or gas (steam) hits blades mounted on a shaft and rotates the shaft connected to the generator. Thus, the mechanical energy of water or gas is converted into electrical energy.
Interesting. Currently, 93% of the world's electricity comes from steam, gas and water turbines using biomass, coal, geothermal, nuclear energy, and natural gas.
Other types of devices that generate electricity:
- electrochemical batteries;
- fuel devices;
- solar photovoltaic cells;
- thermoelectric generators.
History of the electric power industry
Before the advent of electricity, people burned vegetable oil, wax candles, fat, kerosene, and gasified coal to illuminate houses, streets, and workshops. Electricity made it possible to have clean, safe, bright lighting, for which the first power plant was built. Thomas Edison launched it in lower Manhattan (New York) in 1882 and pushed aside the darkness forever, opening up a new world. The coal-fired Pearl Street Station became the prototype for the entire emerging energy industry. It consisted of six dynamo generators, each weighing 27 tons and producing 100 kW.
In Russia, the first power plants began to appear in the late 80s-90s of the 19th century in Moscow, St. Petersburg and Odessa. As electricity transmission developed, power plants were enlarged and moved closer to sources of raw materials. A powerful impetus to the production and use of electrical energy was given by the GOELRO plan adopted in 1920.
Fossil fuel stations
Fossil fuels are the remains of plant and animal life that have been subjected to high temperatures, high pressures over millions of years and come out in the form of carbons: peat, coal, oil and natural gas. Unlike electricity itself, fossil fuels can be stored in large quantities. Fossil fuel power plants are generally reliable and last for decades.
Disadvantages of thermal power plants:
- Fuel combustion results in sulfur dioxide and nitrogen oxide pollution, requiring expensive treatment systems;
- Wastewater from used steam can carry pollutants into water bodies;
- Current difficulties are large amounts of carbon dioxide and coal ash.
Important! The extraction and transportation of fossil resources creates environmental problems that can lead to catastrophic consequences for ecosystems.
The efficiency of thermal power plants is below 50%. To increase it, thermal power plants are used, in which the thermal energy of the used steam is used for heating and supplying hot water. At the same time, efficiency increases to 70%.
Gas turbines and biomass plants
Some natural gas units can produce electricity without steam. They use turbines very similar to jet airplane turbines. However, instead of jet fuel, they burn natural gas to power a generator. Such installations are convenient because they can be brought online quickly in response to temporary surges in electricity demand.
There are units whose operation is based on the combustion of biomass. This term applies to wood waste or other renewable plant materials. For example, the Okeelanta plant in Florida burns grass waste from sugar cane processing for part of the year and wood waste for the remainder of the year.
Hydroelectric power stations
There are two types of hydroelectric power plants operating in the world. The first type takes energy from a fast-moving stream to turn a turbine. Water flow in most rivers can vary widely depending on rainfall, and there are several suitable locations along the riverbed for the construction of power plants.
Most hydroelectric power plants use a reservoir to compensate for periods of drought and increase water pressure in the turbines. These artificial reservoirs cover large areas, creating picturesque features. The massive dams required are also useful for flood control. In the past, few doubted that the benefits of their construction exceeded the costs.
However, now the point of view has changed:
- Huge areas of land for reservoirs are being lost;
- The dams have displaced people and destroyed wildlife habitat and archaeological sites.
Some costs can be offset, for example, by building fish passages in the dam. However, others remain, and the construction of hydroelectric dams is causing widespread protests from local residents.
The second type of hydroelectric power station is pumped storage power plant, or pumped storage power plant. The units operate in two modes: pumping and generator. Pumped storage power plants use periods of low demand (night) to pump water into a reservoir. When demand increases, some of this water is sent to hydro turbines to generate electricity. These stations are economically profitable because they use cheap electricity for pumping and generate expensive electricity.
NPP
Despite some important technical differences, nuclear power plants are thermal and produce electricity in much the same way as fossil fuel plants. The difference is that they generate steam using the heat of atomic fission rather than from burning coal, oil or gas. Then the steam works in the same way as in thermal units.
Features of the nuclear power plant:
- Nuclear plants do not use much fuel and are rarely refueled, unlike coal plants, which are loaded with fuel by railcar;
- Greenhouse gases and harmful emissions are minimal when properly operated, which makes nuclear power attractive to people concerned about air quality;
- The wastewater is hotter, large cooling towers are designed to solve this problem.
The emerging desire for nuclear energy faltered in the face of social problems related to environmental and economic safety issues. Creating better safety mechanisms increases construction and operating costs. The problem of disposal of spent nuclear fuel and contaminated accessories, which can remain dangerous for thousands of years, has not yet been resolved.
Important! The Three Mile Island accident in 1979 and Chernobyl in 1986 were serious disasters. Ongoing economic problems have made nuclear power plants less attractive. Despite producing 16% of the world's electricity, the future of nuclear power is uncertain and hotly debated.
Wind energy
Wind farms do not require water storage and do not pollute the air, which carries much less energy than water. Therefore, it is necessary to build either very large units or many small ones. Construction costs can be high.
Additionally, there are few places where the wind blows predictably. Turbines are designed using a special gear to spin the rotor at a constant speed.
Alternative Energy
- Geothermal. A clear example of the heat available underground is seen in geyser eruptions. The disadvantage of geothermal power plants is the need for construction in areas with seismic hazard;
- Solar. Solar panels themselves are a generator. They take advantage of the ability to convert solar radiation into electricity. Until recently, solar cells were expensive, increasing their efficiency is also a difficult task;
- Fuel cells. They are used, in particular, in spacecraft. There they chemically combine hydrogen and oxygen to form water and produce electricity. So far, such installations are expensive and have not found widespread use. Although a central fuel cell power plant has already been created in Japan.
Electricity usage
- Two thirds of the energy generated goes to industry;
- The second main direction is the use of electricity in transport. Electric transport: railways, trams, trolleybuses, metro operate on direct and alternating current. Recently, more and more electric vehicles are appearing, for which a network of gas stations is being built;
- The household sector consumes the least amount of electricity: residential buildings, shops, offices, educational institutions, hospitals, etc.
As power generation technologies improve and environmental safety improves, the very concept of building large centralized power plants is being called into question. In most cases, it is no longer economically viable to heat houses from the center. Further developments in fuel cells and solar panels could completely change the landscape of electricity generation and transmission. This opportunity is all the more attractive given the cost and objections associated with the construction of large power plants and transmission lines.
Video
Let us consider the movement of a conductor in a plane perpendicular to the direction of the field, when one end of the conductor is stationary and the other describes a circle. The electromotive force at the ends of the conductor is determined by the formula of the law of electromagnetic induction. A machine running...
Energy production should be understood as the transformation of energy from an “inconvenient” form for human use to a “convenient” one. For example, sunlight can be used by receiving it directly from the Sun, or it can be generated from it, which in turn will be converted into light indoors. You can burn gas in an internal combustion engine, converting it into - shaft rotation. Or you can burn gas in a fuel cell, converting the same chemical energy of bonds into electromagnetic energy, which will then be converted into mechanical energy of shaft rotation. The efficiency of different energy conversion algorithms varies. However, this is not a consequence of the “damage” of certain energy chains. The reason for the difference in efficiency is the different level of technology development. For example, the efficiency of large diesel engines installed on ocean-going oil tankers and container ships is significantly higher than the efficiency of automobile diesel engines. However, many times more horsepower is removed from a car engine, and in the end you have to pay in terms of reduced efficiency.In general, centralized energy looks attractive only at first glance
For example, hydroelectric power stations provide a lot of free electricity, but they are very expensive to build, have a destructive impact on the ecology of the region, and force settlements to be moved and cities to be built. And in arid countries, the consequences of the construction of hydroelectric power stations lead to the dehydration of entire regions, where residents do not even have enough water for drinking, let alone for agriculture. Nuclear power plants look attractive, but production creates the problem of disposal and disposal of highly radioactive waste. Thermal plants aren't so bad either, since they account for the vast majority of production and electricity. But they release carbon dioxide into the atmosphere and reduce mineral reserves. But why are we building all these stations, transmitting, converting and losing huge amounts of energy. The fact is that we need specific energy - electricity. But it is possible to build such production and life processes when there is no need to either produce energy at a significant distance from the consumer or transmit it over long distances. For example, the problem of obtaining hydrogen will be very difficult if we start producing it as fuel for cars on a global scale. The separation of hydrogen from water by electrolysis is a very energy-intensive process that will require doubling global electricity production if all cars are converted to hydrogen.But is it really necessary to “plant” hydrogen production at old capacities?
After all, it is possible to separate hydrogen from ocean water on floating platforms using solar energy. Then it turns out that solar energy is reliably “canned” in hydrogen fuel and transported wherever needed. After all, this is much more profitable than transmitting and storing electricity. Today, the following devices and structures are used for energy production: furnaces, internal combustion engines, electric generators, turbines, solar panels, wind turbines and power plants, dams and hydroelectric power stations, tidal stations, geothermal stations, nuclear power plants, thermonuclear reactors.Introduction
This publication provides general information about the processes of production, transmission and consumption of electrical and thermal energy, the mutual connection and objective laws of these processes, about various types of power plants, their characteristics, conditions for joint work and integrated use. A separate chapter discusses energy saving issues.
Production of electrical and thermal energy
General provisions
Energy is a set of natural, natural and artificial, man-made systems designed to obtain, transform, distribute and use energy resources of all types. Energy resources are all material objects in which energy is concentrated for possible use by humans.
Among the various types of energy used by people, electricity has a number of significant advantages. This is the relative simplicity of its production, the possibility of transmission over very long distances, the ease of conversion into mechanical, thermal, light and other energy, which makes electric power the most important sector of human life.
The processes occurring during the production, distribution, and consumption of electrical energy are inextricably interconnected. Installations for the generation, transmission, distribution and conversion of electricity are also interconnected and integrated. Such associations are called electric power systems (Fig. 1.1) and are an integral part of the energy system. In accordance with the energy system, they call a set of power stations, boiler houses, electrical and heating networks, interconnected and connected by a common mode in the continuous process of production, conversion and distribution of electricity and heat with the general control of these modes.
An integral part of the electrical power system is the power supply system, which is a set of electrical installations designed to provide consumers with electrical energy.
A similar definition can be given to a heat supply system.
Thermal power plants
Obtaining energy from fuel and energy resources (FER) by burning them is currently the simplest and most affordable way to produce energy. Therefore, up to 75% of all electricity in the country is generated at thermal power plants (TPPs). In this case, both joint production of thermal and electrical energy is possible, for example, at thermal power plants (CHP), and their separate production (Fig. 1.2).
The block diagram of the thermal power plant is shown in Fig. 1.3. The work proceeds as follows. The fuel supply system 1 ensures the supply of solid, liquid or gaseous fuel to the burner 2 of the steam boiler 3. The fuel is pre-prepared accordingly, for example, coal is crushed to a powder state in the crusher 4, dried and saturated with air, which is blown by a blower fan 5 from the air intake 6 through heater 7 is also supplied to the burner. The heat generated in the boiler furnace is used to heat water in heat exchangers 8 and generate steam. Water is supplied by pump 9 after it goes through a special water treatment system 10. Steam from drum 11 at high pressure and temperature enters steam turbine 12, where steam energy is converted into mechanical energy of rotation of the turbine shaft and electric generator 13. The synchronous generator produces alternating three-phase current . The steam exhausted in the turbine is condensed in the condenser 14. To speed up this process, cold water from a natural or artificial reservoir 15 or special coolers - cooling towers - are used. The condensate is pumped back into the steam generator (boiler). This cycle is called a condensation cycle. Power plants using this cycle (PPS) produce only electrical energy. At a thermal power plant, part of the steam from the turbine is taken at a certain pressure to the condenser and is used for the needs of heat consumers.
Rice. 1.1.
G - electricity generators; T - transformers; P - electrical loads;
W - power lines (power lines); AT - autotransformers
Fig.1.2.
a - combined production; b - separate production
Fig.1.3.
Fuel and its preparation. Thermal power plants use solid, liquid or gaseous organic fuel. Its general classification is shown in table 1.1.
Table 1.1. General fuel classification
The fuel in the form in which it is burned is called “working fuel”. The composition of the working fuel (solid and liquid) includes: carbon C, hydrogen H, oxygen O, nitrogen N, ash A and moisture W. Expressing the fuel components as a percentage , referred to one kilogram of mass, an equation for the composition of the working mass of fuel is obtained.
Sulfur is called volatile and makes up part of the total amount of sulfur found in the fuel; the rest of the non-combustible part of the sulfur is part of mineral impurities.
Natural gaseous fuels contain: methane, ethane, propane, butane, hydrocarbons, nitrogen, carbon dioxide. The last two components are ballast. Artificial gaseous fuel contains methane, carbon monoxide, hydrogen, carbon dioxide, water vapor, nitrogen, and resins.
The main thermal technical characteristic of fuel is the heat of combustion, which shows how much heat in kilojoules is released when burning one kilogram of solid, liquid or one cubic meter of gaseous fuel. There are higher and lower calorific values.
The higher calorific value of fuel is the amount of heat released by the fuel during its complete combustion, taking into account the heat released during the condensation of water vapor that is formed during combustion.
The lower calorific value differs from the highest calorific value in that it does not take into account the heat expended on the formation of water vapor that is found in the combustion products. When calculating, the lower calorific value is used, because the heat of water vapor is uselessly lost with combustion products going into the chimney.
The relationship between the higher and lower calorific values for the working mass of fuel is determined by the equation
To compare different types of fuel in terms of calorific value, the concept of “conventional fuel” (c.f.) was introduced. Conventional fuel is considered to be the fuel whose lower calorific value at operating mass is 293 kJ/kg for solid and liquid fuels or 29,300 kJ/m3 for gaseous fuels. In accordance with this, each fuel has its own thermal equivalent Et = QНР / 29300.
Converting the consumption of working natural fuel into conditional fuel is carried out according to the equation
Woosl = Et? Tue.
A brief description of individual types of fuel is given in Table 1.2.
Table 1.2. Fuel characteristics
Particularly noteworthy is the lower calorific value in kJ/kg of fuel oil - 38000...39000, natural gas - 34000...36000, associated gas - 50000...60000. In addition, this fuel contains virtually no moisture or mineral impurities.
Before supplying fuel to the furnace, it is prepared. The system for preparing solid fuel is especially complex, which successively undergoes cleaning from mechanical impurities and foreign objects, crushing, drying, dust preparation, and mixing with air.
The system for preparing liquid and especially gaseous fuels is much simpler. In addition, this fuel is more environmentally friendly and has virtually no ash content.
Simplicity of transportation, ease of automation of combustion process control, and high calorific value make natural gas promising for use in the energy sector. However, supplies of this raw material are limited.
Water treatment. Water, being the coolant at thermal power plants, continuously circulates in a closed circuit. In this case, the purification of water supplied to the boiler is of particular importance. Condensate from the steam turbine (Fig. 1.3) enters system 10 for purification from chemical impurities (chemical water treatment - CWO) and free gases (deaeration). In the water-steam-condensate technological cycle, losses are inevitable. Therefore, the water path is recharged from an external source 15 (pond, river) through the water intake 16. The water entering the boiler is preheated in the economizer (heat exchanger) by 17 exhaust combustion products.
Steam boiler. The boiler is a steam generator at a thermal power plant. The main structures are presented in Fig. 1.4.
The drum-type boiler has a steel drum 1, in the upper part of which steam is collected. Feed water is heated in economizer 2, located in flue gas chamber 3, and enters the drum. Manifold 4 closes the steam-water cycle of the boiler. In combustion chamber 5, combustion of fuel at a temperature of 1500...20000C ensures boiling of water. Through steel lifting pipes 6, having a diameter of 30...90 mm and covering the surface of the combustion chamber, water and steam enter the drum. Steam from the drum is supplied to the turbine through a tubular superheater 7. The superheater can be made in two or three stages and is designed for additional heating and drying of steam. The system has 8 drop pipes through which water from the bottom of the drum falls into the collector.
In a drum-type boiler, natural circulation of water and steam-water mixture is ensured due to their different densities.
Such a system makes it possible to obtain subcritical parameters of steam (critical is the point of state at which the difference in the properties of liquid and steam disappears): pressure up to 22.5 MPa, and practically no more than 20 MPa; temperature up to 374°C (without superheater). At higher pressures, the natural circulation of water and steam is disrupted. Forced circulation has not yet found application in powerful drum boilers due to its complexity. Therefore, boilers of this type are used in power units with a capacity of up to 500 MW with a steam output of up to 1600 tons per hour.
In a direct-flow boiler, special pumps carry out forced circulation of water and steam. Feedwater is supplied by pump 9 through economizer 2 to evaporator pipes 10, where it is converted into steam. Through the superheater 7 steam enters the turbine. The absence of a drum and forced circulation of water and steam make it possible to obtain supercritical steam parameters: pressure up to 30 MPa and temperature up to 590°C. This corresponds to power units with a capacity of up to 1200 MW and a steam production capacity of up to 4000 t/h.
Boilers intended only for heat supply and installed in local or district boiler houses are made on the same principles as discussed above. However, the parameters of the coolant, determined by the requirements of heat consumers, differ significantly from those discussed earlier (some technical characteristics of such boilers are given in Table 1.3).
Table 1.3. Technical data of heating system boilers
For example, boiler houses attached to buildings allow the use of boilers with steam pressure up to 0.17 MPa and water temperature up to 1150C, and the maximum power of built-in boiler houses should not exceed 3.5 MW when operating on liquid and gaseous fuels or I.7 MW when operating on liquid and gaseous fuels. working on solid fuel. Heating system boilers differ in the type of coolant (water, steam), in productivity and thermal power, in design (cast iron and steel, small-sized and tent-type, etc.).
The efficiency of a steam generation or hot water preparation system is largely determined by the coefficient of performance (COP) of the boiler unit.
In general, the efficiency of a steam boiler and fuel consumption are determined by the expressions:
Kg/s, (1.1)
where hk is the efficiency of the steam boiler, %; q2, q3, q4, q5, q6 - heat loss, respectively, with exhaust gases, chemical underburning, mechanical underburning, for external cooling, with slag, %; B - total fuel consumption, kg/s; QPC is the heat absorbed by the working environment in the steam boiler, kJ/m; - available heat of fuel entering the furnace, kJ/kg.
Fig.1.4.
a - drum type; b - direct-flow type
1- drum; 2 - economizer; 3 - exhaust gas chamber; 4 - collector; 5 - combustion chamber; 6 - lifting pipes; 7 - steam superheater; 8 - lowering pipes; 9 - pump; 10 - evaporator pipes
If the heat of the flue gases is not used, then
and with an open system for drying fuel with exhaust gases
where Nux, Notb, are the enthalpy of exhaust gases, gases at the point of selection for drying and cold air, respectively, kJ/kg; r is the proportion of gases taken for drying; ?yx - excess air in the exhaust gases.
The enthalpy of a gas at temperature T is numerically equal to the amount of heat that is supplied to the gas during the process of heating it from zero degrees Kelvin to temperature T at constant pressure.
With an open drying system, all fuel data refers to dried fuel.
In this case, the consumption of raw fuel when the humidity changes from WP to Wdry is
where Dry is the consumption of dried fuel according to (1.1), kg/s; Wdry, WP - humidity of dried and undried fuel, %.
When humidity changes, the lower calorific value of fuel also changes from to:
KJ/kg (1.4)
The lowest calorific value corresponds to the amount of heat released by the fuel during its complete combustion, without taking into account the heat expended on the formation of water vapor that is found in the combustion products.
Total available heat of fuel entering the furnace
KJ/kg, (1.5)
where is the lower heating value of fuel, kJ/kg; - additional heat introduced into the boiler by air heated from outside, steam blast, etc., kJ/kg.
For approximate calculations.
Heat perceived by the working environment in a steam boiler
KJ/s, (1.6)
where Dp is the steam output of the boiler, kg/s; hpp, hpv - enthalpy of superheated steam and feed water, kJ/kg; ?Qpk - additionally perceived heat in the presence of a superheater in the boiler, blowing with water, etc., kJ/s.
For approximate calculations? Qpc=0.2…0.3 Dp(hpp - hpv).
where?un is the share of ash carryover with combustion products; Nshl - slag enthalpy, kJ/kg; AR - working ash content of fuel, %.
The values of q3, q4, q5, Wр, Ar are given in specialized literature, as well as in textbooks.
For solid slag removal, you can take?ух=1.2…1.25; ?un=0.95; Nshl=560 kJ/kg.
In addition, at an air temperature in front of the boiler of 300C = 223 kJ/kg, and at a flue gas temperature of 1200C Nux = 1256 kJ/kg.
Calculation example. Determine the efficiency and fuel consumption for a steam boiler under the following conditions: Dп=186 kg/s; fuel - dried Berezovsky coal with Wdry=13%; open-loop drying system, r=0.34; the gas taken for drying has Nob = 4000 kJ/kg; enthalpy of superheated steam and feed water, respectively, hpp = 3449 kJ/kg, hpv = 1086.5 kJ/kg.
Solution. Preliminarily, according to (1.4), the lower calorific value of the dried fuel is determined.
Here Wр=33% and =16200 kJ/kg are taken according to .
Taking by (1.5)
we find by (1.2)
We find: q3=1%, q4=0.2%, q5=0.26% and taking into account (1.7)
To calculate fuel consumption using (1.6) we find
Consumption of dried fuel according to (1.1)
The raw fuel consumption at Wр = 33% according to (1.3) is
Steam turbine. This is a heat engine in which the energy of steam is converted into mechanical energy of rotation of the rotor (shaft) and the working blades attached to it. A simplified diagram of the steam turbine design is shown in Fig. 1.5. Disks 2 with working blades 3 are attached to the turbine shaft 1. These blades are supplied with steam from the boiler from the nozzle 4, supplied through the steam line 5. The energy of the steam causes the turbine impeller to rotate, and the rotation of the shaft is transmitted through the coupling 6 to the shaft 7 of the synchronous generator. The exhaust steam is sent through chamber 8 to the condenser.
Steam turbines are divided by design into active and reactive. In an active turbine (Fig. 1.5c), the volume of steam V2 at the entrance to the working blades is equal to the volume of steam V3 at the exit from the blades. The expansion of the steam volume from V1 to V2 occurs only in the nozzles. There, the pressure changes from p1 to p2 and the steam velocity from c1 to c2. In this case, the steam pressure at the inlet p2 and the outlet p3 from the blades remains unchanged, and the steam speed drops from c2 to c3 due to the transfer of kinetic energy of the steam to the turbine blades:
Gp?(s2-s3)2 / 2 Gt?st2 / 2,
where Gp, Gt - mass of steam and turbine impeller; c2, c3, st - steam velocity at the inlet and outlet of the blades and the speed of movement of the impeller.
The design of the jet turbine blades is such (Fig. 1.5d) that the steam expands not only in the nozzles from V1 to V2, but also between the impeller blades from V2 to V3. In this case, the steam pressure changes from p2 to p3 and the steam velocity from c2 to c3. Since V2
where F is the area of the blade, m2; (p2 - p3) - pressure difference at the inlet and outlet of the blades, Pa; dS - blade displacement, m.
In this case, the work used to rotate the turbine impeller. Thus, in jet turbines, in addition to the centrifugal forces that arise when the speed of steam moves, reactive forces caused by the expansion of steam act on the blades.
Modern turbines are made both active and reactive. In powerful units, the steam input parameters approach the values of 30 MPa and 6000C. In this case, the outflow of steam from the nozzle occurs at a speed exceeding the speed of sound. This leads to the need for a high rotor speed. Enormous centrifugal forces arise that act on the rotating parts of the turbine.
In practice, the rotor rotation frequency, due to the design features of both the turbine itself and the synchronous generator, is 3000 1/min. In this case, the linear speed on the circumference of a turbine wheel with a diameter of one meter is 157 m/s. Under these conditions, particles tend to come off the wheel surface with a force 2,500 times their weight. Inertial loads are reduced by using speed and pressure steps. Not all of the steam energy is given to each stage, but only part of it. This also ensures an optimal heat drop per stage, which is 40...80 kJ/kg at a peripheral speed of 140...210 m/s. The total heat drop generated in modern turbines is 1400...1600 kJ/kg.
For design reasons, 5...12 stages are grouped in one housing, which is called a cylinder. A modern powerful turbine can have a high pressure cylinder (HPC) with an inlet steam pressure of 15...30 MPa, a medium pressure cylinder (MPC) with a pressure of 8...10 MPa and a low pressure cylinder (LPC) with a pressure of 3... 4 MPa. Turbines up to 50 MW are usually built in a single cylinder.
The steam exhausted in the turbine enters the condenser for cooling and condensation. Cooling water at a temperature of 10...15°C is supplied to the tubular heat exchanger of the condenser, which promotes intense condensation of steam. For the same purpose, the pressure in the condenser is maintained within 3...4 kPa. The cooled condensate is again supplied to the boiler (Fig. 1.5), and the cooling water, heated to 20...25 ° C, is removed from the condenser. If cooling water is taken from a reservoir and then irretrievably discharged, the system is called an open-flow system. In closed cooling systems, water heated in the condenser is pumped to cooling towers - cone-shaped towers. From the top of the cooling towers, water flows down from a height of 40...80 m, cooling to the required temperature. The water then flows back into the condenser.
Both cooling systems have their advantages and disadvantages and are used in power plants.
Fig.1.5. Steam turbine design:
a - turbine impeller; b - diagram of a three-stage active turbine; c - steam work in the active stage of the turbine; d - work of steam in the reactive stage of the turbine.
1 - turbine shaft; 2 - disks; 3 - working blades; 4 - nozzles; 5 - steam line; 6 - coupling; 7 - synchronous generator shaft; 8 - exhaust steam chamber.
Turbines, in which all the steam supplied to them, after completing the work, enters the condenser, are called condensing and are used to produce only mechanical energy with its subsequent conversion into electrical energy. This cycle is called condensation and is used at state district power plants and thermal power plants. An example of a condensing turbine is K300-240 with a power of 300 MW with initial steam parameters of 23.5 MPa and 600°C.
In heating turbines, part of the steam is taken before the condenser and is used to heat water, which is then sent to the heat supply system of residential, administrative, and industrial buildings. The cycle is called heating and is used at thermal power plants and state district power plants. For example, the T100-130/565 turbine with a power of 100 MW for initial steam parameters of 13 MPa and 5650C has several adjustable steam extractions.
Industrial heating turbines have a condenser and several adjustable steam extractions for heating and industrial needs. They are used at thermal power plants and state district power plants. For example, a P150-130/7 turbine with a power of 50 MW for initial steam parameters of 13 MPa and 5650C provides industrial steam extraction at a pressure of 0.7 MPa.
Backpressure turbines operate without a condenser, and all exhaust steam goes to district heating and industrial consumers. The cycle is called back-pressure, and turbines are used at thermal power plants and state district power plants. For example, a turbine R50-130/5 with a power of 50 MW for an initial steam pressure of 13 MPa and a final pressure (back pressure) of 0.5 MPa with several steam extractions.
The use of a heating cycle makes it possible to achieve an efficiency of up to 70% at thermal power plants, taking into account the supply of heat to consumers. In the condensation cycle, the efficiency is 25...40% depending on the initial steam parameters and the power of the units. Therefore, CPPs are located in places where fuel is produced, which reduces transportation costs, and CHP plants are closer to heat consumers.
Synchronous generators. The design and characteristics of this machine, which converts mechanical energy into electrical energy, are discussed in detail in special disciplines. Therefore, we will limit ourselves to general information.
The main structural elements of a synchronous generator (Fig. 1.6): rotor 1, rotor winding 2, stator 3, stator winding 4, housing 5, exciter 6 - direct current source.
The non-salient pole rotor of high-speed machines - turbogenerators (n = 3000 1/min) is made of sheet electrical steel in the form of a cylinder located on shaft 7. Low-speed machines - hydrogenerators (n ? 1500 1/min) have a salient-pole rotor (shown in dotted lines). In the grooves on the surface of the rotor there is an insulated copper winding connected to the exciter using sliding contacts 8 (brushes). The stator is a complete cylinder made of electrical steel, on the inner surface of which three phase windings are located in grooves - A, B, C. The windings are made of copper insulated wire, are identical to each other and have axial symmetry, occupying 120° sectors. The beginnings of the phase windings A, B, C are led out through insulators, and the ends of the windings X, Y, Z are connected to a common point N - neutral.
The generator operates as follows. The excitation current iB in the rotor winding creates a magnetic flux Ф that crosses the stator windings. The generator shaft is driven by a turbine. This ensures uniform rotation of the rotor magnetic field with an angular frequency?=2?f, where f is the frequency of alternating current, 1/s is Hz. To obtain an alternating current frequency of 50 Hz with a number of pairs of magnetic poles p, the rotor rotation frequency n=60?f/p is required.
At p = 1, which corresponds to a salient-pole rotor, n = 3000 1/min. A rotating magnetic field crossing the stator windings induces an electromotive force (EMF) in them. In accordance with the law of electromagnetic induction, the instantaneous value of the emf
where w is the number of turns.
The EMF in the stator windings is induced synchronously with the change in the magnetic field as the rotor rotates.
Fig.1.6.
a - generator design; b - winding connection diagram;
c - EMF at the terminals of the generator windings
1 - rotor; 2 - rotor winding; 3 - stator; 4 - stator winding; 5 - body; 6 - pathogen; 7 - rotor shaft (axis); 8 - slip rings
With uniform rotation of the rotor and axial symmetry of the stator windings, the instantaneous values of the phase EMF are equal to:
where EM is the amplitude value of the EMF.
If an electrical load Z is connected to the terminals of the generator stator windings, an electrical current flows in the external circuit
where is the voltage at the terminals of the windings when current i flows through them and the stator winding resistance Zin.
In practice, it is more convenient to use not instantaneous, but effective values of electrical quantities. The necessary relationships are known from the course of physics and theoretical foundations of electrical engineering.
The operation of the generator largely depends on the excitation and cooling mode of the machine. Various excitation systems (independent and self-excitation, electric machine and thyristor, etc.) allow you to change the value of iB and, consequently, the magnetic flux Ф and EMF in the stator windings. This makes it possible to regulate the voltage at the generator terminals within certain limits (usually ±5%).
The amount of active power supplied by the turbogenerator to the electrical network is determined by the power on the turbine shaft and is regulated by the supply of steam to the turbine.
During operation of the generator, it heats up, primarily due to the release of heat in the windings flowing around the current. Therefore, the efficiency of the cooling system is essential.
Low power generators (1...30 MW) have air cooling of internal surfaces using a flow (open) or regenerative (closed) circuit. On medium-power generators (25...100 MW), surface hydrogen cooling is used in a closed circuit, which is more efficient, but requires the use of special safety measures. Powerful generators (more than 100 MW) have forced hydrogen, water or oil cooling, in which the coolant is pumped under pressure inside the stator, rotor, and windings through special cavities (channels).
Main technical characteristics of generators: rated voltage at the generator stator winding terminals, Unom: 6.3-10.5-21 kV (higher values correspond to more powerful generators); rated active power, Rnom, MW; rated power factor; nominal efficiency of 90...99%.
These parameters are related to each other:
Own needs of power plants. Not all electrical and thermal energy produced at thermal power plants is distributed to consumers. Some remains at the station and is used to ensure its operation. The main consumers of this energy are: the fuel transportation and preparation system; water and air supply pumps; purification system for water, air, exhaust gases, etc.; heating, lighting, ventilation of domestic and industrial premises, as well as a number of other consumers.
Many elements of own needs belong to the first category in terms of reliability of power supply. Therefore, they are connected to at least two independent energy sources, for example, to sources at their station and to the power grid.
Switchgear. Electricity generated by generators is collected at a switchgear (DS) and then distributed among consumers. For this purpose, the terminals of the generator stator windings are connected to the switchgear busbars through special switching devices (switches, disconnectors, etc.) with rigid or flexible conductors (busbars). Each connection to the switchgear is made through a special cell containing the necessary set of equipment. Since the transmission, distribution and generation of electricity, as well as its consumption, occur at different voltages, there are several switchgears at the station. For the rated voltage of generators, for example, 10.5 kV, generator voltage control is performed. Usually it is located in the station building and is closed by design (closed switchgear). Closely located consumers are connected to this switchgear. To transmit electricity through power transmission lines (PTL) over long distances and communicate with other stations and the system, it is necessary to use a voltage of 35...330 kV. Such communication is carried out using separate switchgears, usually open-type (OPU), where step-up transformers are installed. To connect consumers of their own needs, use RUSN. From the RUSN buses, electricity is transmitted directly and through step-down transformers to consumers at the power plant.
Similar principles are used in the distribution of thermal energy generated at thermal power plants. Special collectors, steam pipelines, and pumps provide heat supply to industrial and municipal consumers, as well as to the system’s own needs.
Interactive application “How CHP works”
The picture on the left is the Mosenergo power plant, where electricity and heat are generated for Moscow and the region. The most environmentally friendly fuel used is natural gas. At a thermal power plant, gas is supplied through a gas pipeline to a steam boiler. The gas burns in the boiler and heats the water.
To make the gas burn better, the boilers are equipped with draft mechanisms. Air is supplied to the boiler, which serves as an oxidizer during gas combustion. To reduce noise levels, the mechanisms are equipped with noise suppressors. The flue gases generated during fuel combustion are discharged into the chimney and dispersed into the atmosphere.
The hot gas rushes through the flue and heats the water passing through special boiler tubes. When heated, water turns into superheated steam, which enters the steam turbine. The steam enters the turbine and begins to rotate the turbine blades, which are connected to the generator rotor. Steam energy is converted into mechanical energy. In the generator, mechanical energy is converted into electrical energy, the rotor continues to rotate, creating an alternating electric current in the stator windings.
Through a step-up transformer and a step-down transformer substation, electricity is supplied to consumers via power lines. The steam exhausted in the turbine is sent to the condenser, where it turns into water and returns to the boiler. At a thermal power plant, water moves in a circle. Cooling towers are designed to cool water. CHP plants use fan and tower cooling towers. The water in cooling towers is cooled by atmospheric air. As a result, steam is released, which we see above the cooling tower in the form of clouds. The water in the cooling towers rises under pressure and falls like a waterfall into the front chamber, from where it flows back to the thermal power plant. To reduce droplet entrainment, cooling towers are equipped with water traps.
Water supply is provided from the Moscow River. In the chemical water treatment building, water is purified from mechanical impurities and supplied to groups of filters. In some, it is prepared to the level of purified water to feed the heating network, in others - to the level of demineralized water and is used to feed power units.
The cycle used for hot water supply and district heating is also closed. Part of the steam from the steam turbine is sent to water heaters. Next, the hot water is sent to heating points, where heat exchanges with water coming from the houses.
Highly qualified Mosenergo specialists support the production process around the clock, providing the huge metropolis with electricity and heat.