Steam power plant

Steam boiler and associated supporting structure in a coal-fired power plant

A steam power plant is a power plant for generating electricity , in which the thermal energy of water vapor in a steam turbine (previously also in a piston steam engine ) is used. A steam power plant includes a heat source, a steam generator , a turbine with a generator, and a device for cooling the hot steam.

Scheme of a steam power plant

Principle diagram of a steam power plant with reheating

Thermal circuit diagram of Staudinger power plant (Großkrotzenburg am Main), Block 5

The types of steam power plants differ mainly according to their heat source. Coal power plants , oil power plants, and combined cycle power plants burn fossil fuels . In nuclear power plants , the heat is generated by fission . And in the rarer solar thermal power plants , the heat comes from the sun.

Many steam power plants are used exclusively to generate electricity : lignite power plants and nuclear power plants mainly for base load ; Hard coal power stations, especially for medium loads ; Gas power plants especially for peak loads. Gas-fired power plants can be started up and controlled more quickly than coal-fired power plants or nuclear power plants and are therefore better suited for load following operation than these.

process description

Basic process



Steam generator steam



Steam turbine steam



Condenser water



Feed pump water

The power required to operate the steam turbine, steam is used in a steam boiler made from previously cleaned and treated water. As the steam is further heated in the superheater , the temperature and the specific volume of the steam increase. From the steam boiler, the steam flows via pipes into the steam turbine where it some of its previously recorded energy as kinetic energy outputs it to the turbine. A generator is coupled to the turbine , which converts the mechanical power into electrical power. The relaxed and cooled steam then flows into the condenser , where it condenses through heat transfer to the environment and collects as liquid water at the lowest point of the condenser. The water is temporarily stored in a feed water tank via the condensate pump and the preheater and then fed back to the steam boiler via the feed pump .

Water-steam cycles in modern power plants have more complicated circuits in order to convert the fuel enthalpy into electrical power with the highest degree of efficiency. Above all, evaporation energy is used to preheat the feed water by extracting steam at the right pressure from the turbine and condensing it in heat exchangers. See adjacent thermal circuit diagram.

Steam boilers are mostly fired with conventional fuels such as oil , natural gas , hard coal or lignite . There are power plants whose main task is waste incineration . In addition, the steam boilers of large power plants are also used for the thermal disposal of liquid, flammable or non-flammable waste such as oil-water mixtures. Thanks to subsidies from the Renewable Energy Sources Act (EEG), a large number of biomass steam boiler systems have been built in recent years in which fresh and waste wood are used as fuel.

Steam boilers can generate up to 830 kg of water vapor per second (just under 1 kg / s per MW of electricity generated from it). The condenser, with its shell-and- tube heat exchanger design, is usually connected to a cooling tower , via which the no longer usable heat of the steam is released into the environment with the aid of cooling water . This application of the steam power plant circuit for generating electricity is subject to the laws of thermodynamics , with the help of which a statement can also be made about the efficiency and possible optimization steps of a steam power plant. These relationships can be shown very clearly in the Ts diagram.

Steam power plant process in the TS diagram and in the HS diagram

Cycle of the steam power plant for various design parameters; yellow: vapor pressure: 50 bar / live steam temperature: 400 ° C / condensation temperature: 35 ° C / no reheating; pink: vapor pressure: 150 bar / live steam temperature: 550 ° C / condensation temperature: 20 ° C / simple reheating

The heat is the product of the entropy difference and the absolute temperature. If the changes in state of a steam power plant process ( Clausius-Rankine process ) are entered in the temperature-entropy diagram (TS diagram), then the area below the curve represents the heat supplied (change in state from left to right) or removed (change in state from right) to the left). ${\ displaystyle \ delta Q}$

{\ displaystyle \ delta Q = {T} \ cdot dS}

The diagram shows the power plant process (see block diagram) without reheating ( yellow ) and with reheating ( pink ). The turbine is assumed to be ideal ( reversible change of state).

The cornerstones of the cycle denote the following changes in state:

1–2: pressure increase of the feed water

2–3: isobaric heating of the feed water to the boiling point
3-4: isobaric evaporation
4–5: overheating
5–6: Relaxation on the turbine
6–1: condensation

only process with reheating ( pink ):

5–5a: Relaxation on the high pressure turbine
5a-5b: reheating
5b – 6: Relaxation on the LP turbine (LP = low pressure)

In the diagram, the specific heat supplied and removed (based on 1 kg of water) for the respective process parameters can be read off as the area below the curve. Neglecting the technical work carried out on the feedwater pump, neglecting heat losses and assuming an ideal turbine (reversible expansion), the following enthalpy exchange occurs between the system boundaries of the power plant and the environment:

{\ displaystyle Q _ {\ text {fuel}} = Q _ {\ text {condensation}} + Q _ {\ text {exhaust gas}} + W _ {\ text {technical}}}

The chemical enthalpy contained in the fuel is converted into the technical work on the turbine shaft and the waste heat in the flue gas, as well as the waste heat that has to be dissipated via the condenser. The hatched areas in the diagram describe the condensation heat to be dissipated. The usable technical work is represented by the monochrome areas. The efficiency of the steam power process can be derived from:

{\ displaystyle \ eta = {\ frac {Q _ {\ text {fuel}} - Q _ {\ text {condensation}} - Q _ {\ text {exhaust gas}}} {Q _ {\ text {fuel}}}}.}

Turbine process of the Staudinger power plant (block 5) in the hs diagram. The picture shows the real course of the adiabatic expansion before and after reheating (red lines), which is not isentropic due to frictional shock and throttling losses.

Cycle process of the Staudinger power plant, (block 5) in the Ts diagram. The red circles on the left mark the preheating stages of the feed water, on the right the withdrawals, some of which are also identified by the associated isobars. This diagram clearly shows that this is a supercritical system (the process does not go through the wet steam area). The pressure loss in the boiler is emphasized by the two isobars for 300 bar and 255 bar.

Due to the second law of thermodynamics, the efficiency cannot exceed the Carnot efficiency . The Carnot efficiency is formed from the mean temperatures of the heat input and the heat dissipation of a process. In the steam power plant cycle, these are the mean water-steam temperature in the boiler and the condensation temperature (if - as happened above - only the water cycle is considered) or the mean flue gas temperature and the ambient temperature (if the heat transfer is included in the balance). Using the diagram, the efficiency of the process can be calculated and measures for an efficiency optimization can be derived graphically:

Increase in vapor pressure,
Increase in the live steam temperature (FD temperature)
low condensation temperature.

The intermediate overheating (ZÜ) increases the efficiency via the higher mean temperature of the heat supply. It is even indispensable at higher steam pressures because it avoids erosion on the blades of the “cold end” (last blades in the low-pressure part) due to excessive steam moisture. The permissible proportion of liquid water in the exhaust steam is around 10% (steam proportion x = 0.9).

Efficiency

The theoretical description of the steam power process is based on the Rankine cycle .

The efficiency of a steam power plant depends on the corner temperatures through which the steam passes. Further optimization is possible if an attempt is made to approximate the real process as closely as possible to the Carnot process through reheating (possibly several times) and regenerative feed water preheating (extraction from the turbine) .

With the average temperature of the heat supply, which results from the corner temperatures of the process (feed water inlet, FD, ZÜ, and condensation), the upper limit of the exergetic or Carnot efficiency can be derived for a steam power plant process with the Carnot factor .

Development of the mechanical or electrical efficiency of steam power plants or mechanical drives

{\ displaystyle \ eta _ {c} = 1 - {\ frac {T _ {\ mathrm {k}}} {T _ {\ mathrm {m, zu}}}}}

With:

{\ displaystyle T _ {\ mathrm {m, zu}}}

: absolute mean temperature of the heat input in K

{\ displaystyle T _ {\ mathrm {k}}}

: absolute condensation temperature in K

The following Carnot efficiencies can be derived for steam power processes from the history of development: Newcomen (saturated steam process without regenerative feed water preheating 100 ° C / 30 ° C) :; Steam power plant to 1900 (10 bar, 350 ° C / 30 ° C, with an ideal regenerative preheating) ; Modern steam power plant with intermediate superheating heat according to diagram (256 bar, 543 ° C / 562 ° C / 18 ° C, preheating to 276 ° C): . The actually achievable degrees of efficiency are significantly lower. ${\ displaystyle \ eta _ {c} = 18 {,} 7 \, \%}$ ${\ displaystyle \ eta _ {c} = 34 {,} 6 \, \%}$ ${\ displaystyle \ eta _ {c} = 53 \, \%}$

The live steam temperature can be influenced by the design of the steam generator. A further increase in the temperature on the superheater as the heating surface with the highest temperature can only be implemented in small steps. A live steam temperature of 600 ° C currently represents the technical and economic limit, since if the superheater was increased further, it would no longer be possible to manufacture from (expensive) austenitic steels , but from materials based on nickel-based alloys, which are extremely expensive. Such large-scale tests are currently underway; the resulting temperatures of over 700 ° C make the system parts involved, such as pipes and fittings, already visibly glow .

The steam temperature at the outlet of the LP condensation turbine is determined by the condenser pressure , which should be as low as possible. The lowest condensation pressures are achieved by water cooling in a tube bundle heat exchanger. In this case, the power plant must be built on a river from which water can be taken for cooling purposes. The inlet temperature when the cooling water is returned is limited, however. On hot summer days with a low water level in the body of water it can happen that the power plant output has to be reduced. The tube bundles of the condenser are polluted by algae growth and salt deposits and impair the heat transfer on the cooling water side. The pipes must therefore be cleaned, using the Taprogge process , for example .

A low condensation temperature is also achieved with evaporative cooling in cooling towers. The spraying of water and the evaporation that occurs saturate the air, so that the air is additionally cooled due to the release of the heat of evaporation . In this way, lower condensation temperatures can be achieved. When using air condensers (LuKo), the condensation temperatures are higher because the heat transfer to the air is poorer without the support of evaporation. The condensation temperatures are between 25 ° C and 40 ° C, depending on the process and time of year, and the corresponding condensation pressures are 0.026 to 0.068 bar, so that the condenser is always operated under vacuum.

Modern hard coal steam power plants have an efficiency of up to 46% (brown coal 43%). This means that most of the energy used in the form of heat cannot currently be used technically and is lost - mainly via the cooling tower. Assuming a technically feasible overheating of 700 ° C that heat is only supplied at this temperature (which is unrealistic), the comparative Carnot process achieved an efficiency of 70%. The waste heat loss of 30% would then be due to physical reasons and could not be undercut technically. The aim of many companies is the 50% mark, which is to be achieved primarily by increasing the temperature.

In addition to the highest possible inlet temperatures of the live steam, the lowest possible outlet temperatures of the exhaust steam and the double reheating of the turbine steam, regenerative feedwater preheating also plays a role in improving the efficiency . With this process, the feed water is preheated with bleed steam from the steam turbine before it is returned to the steam generator. In practice, up to six turbine taps are provided; This avoids mixing the cold feed water with the water content of the steam generator, which is at the evaporation temperature, which results in significant fuel savings .

According to the COORETEC study commissioned by the Federal Ministry of Economics, these current efficiencies of steam power plant processes can be increased to approx. 51 percent by 2010 through consistent further development, and even higher efficiencies are expected from 2020.

Combined heat and power (CHP)

The use of the primary energy used can be improved through so-called combined heat and power . The turbine is operated with counter pressure or a turbine tap is set up to extract steam at a temperature suitable for heating purposes for generating local or district heating (e.g. 100 ° C / pressure = 1 bar (abs)). Due to the higher exhaust pressure , the efficiency of power generation decreases (lower Carnot factor of the combustion heat supplied). In total, however, considerable savings in primary energy are achieved (less waste heat in the power plant and reduced use of primary energy for heating). Steam power plants without combined heat and power are called condensation power plants .

The reason for the reduced exergy losses through CHP is the low exergetic share of the heat that is usually required for space heating (the temperature level in relation to the ambient temperature is low). If the heat for heating a room (at 20 ° C) is provided by combustion, approx. 93% of the exergy of the fuel is lost at an ambient temperature of 0 ° C. If the heat is extracted from the power plant at 100 ° C, the exergy loss is only approx. 27%.

The degree of efficiency that is achieved when generating electrical power from the input fuel must not be confused with the thermal efficiency of the heat from the input fuel.

Compared to the combustion of fuel to evaporate the water, a much higher efficiency of power generation can be achieved by using hot exhaust gas from a gas turbine. Such power plants consisting of gas and steam turbines are called combined power plants or combined cycle power plants (gas and steam power plants). These systems are also operated as combined heat and power plants (CHP).

Further developments

In the past, considerations were repeatedly made to supplement the working medium water with other evaporating substances in the steam power plant. First and foremost, the metal mercury should be mentioned, which circulates in its own steam circuit, expands in its own steam turbine and then releases its heat of condensation in its own condenser to a steam circuit . The highest values for the mercury vapor cycle were 10 bar and 500 ° C in the systems built in the USA from 1914 onwards .

Furthermore, studies were commissioned around 1980 , which in an analogous manner included three-circuit systems of vaporous potassium , diphenyl and water. Each of these tools acts on its own steam turbine. In spite of the high efficiency of such processes, it has hitherto been dispensed with because of the high costs that such a steam power plant is implemented.

Alternative means of work gained new importance through geothermal power plants, because temperature levels above 150 ° C are rarely available there.

literature

Adolf J. Schwab: Electrical energy systems - generation, transport, transmission and distribution of electrical energy . Springer Verlag, 2006, ISBN 3-540-29664-6 .

Web links

Individual evidence

↑ Pictures of the Future Spring 2008, Energy for Billions: Highly Efficient Coal Power Plants. Siemens AG, accessed on October 29, 2014, 10:05 p.m.
↑ Research and development concept for low-emission fossil-fired power plants. Page V, accessed February 12, 2011

[1] Pictures of the Future Spring 2008, Energy for Billions: Highly Efficient Coal Power Plants. Siemens AG, accessed on October 29, 2014, 10:05 p.m.

[2] Research and development concept for low-emission fossil-fired power plants. Page V, accessed February 12, 2011