Steam-electric power station

A steam power plant is the predominant construction of a power plant to generate electricity, at which the thermal energy of steam in a steam turbine ( in the early days: piston steam engine ) is exploited.

Basic structure: Heat source - Steam generator - turbine with generator - Cooling

There are different types of combined cycle power plant:

• Coal power plant
• Nuclear power station
• Oil power plant
• Solar thermal power plant
• Gas - and -steam combined cycle power plant ( CCPP )

The various primary energy sources together result in an energy or a mix.

Many steam power plants are used exclusively for power generation: lignite power plants and nuclear power plants, especially for base load; Coal-fired power plants, especially for medium load; Gas power plants mainly for peak load. Gas power plants can be faster approach and regulate than coal power plants or nuclear power plants and are thus more suitable for the load-following operation than this.

Process Description

Basic process

The power required to operate the steam turbine, steam is generated in a steam boiler from previously purified and treated water. By further heating of the steam in the superheater increases the temperature and specific volume of the steam. The boiler of the steam through the steam piping flows into the turbine, where it gives up some of its previously received energy as kinetic energy of the turbine. To the turbine, a generator is coupled, which converts the mechanical power to electrical power. Thereafter, the expanded and cooled steam enters the condenser where it is condensed by heat transfer to the surroundings, and collects at the lowest point of the condenser as the liquid water. Via the condensate pump and the feedwater heaters through which the water is temporarily stored in a feed water tank, and then re- fed through the feed pump to the boiler.

Water - steam circuits in modern power plants have complicated circuits to implement the fuel enthalpy with highest efficiency in electrical power. Mainly used by coupled out of the steam turbine with a suitable pressure, and condensed in heat exchangers for preheating the feed water vaporization. See accompanying heat diagram.

Steam boilers are usually fired with conventional fuels such as oil, natural gas, coal or lignite. There are also power plants, whose main task is to incineration. In addition, the steam boiler large power plants are also used for thermal disposal of liquid, flammable or non-flammable waste such as oil - water mixtures.

Aided by grants from the Renewable Energy Sources Act ( EEG), a large number have been built from biomass steam boiler plants where fresh water and waste wood used as fuel in recent years.

Today's steam boilers up to 830 kg of steam per second create ( almost 1 kg / s per MW generated from this electric). The capacitor is connected to its design as a tube bundle heat exchanger usually in conjunction with a cooling tower, over which no longer usable heat of the steam by means of cooling water is discharged to the environment.

This application of steam power plant cycle for power generation is subject to the laws of thermodynamics, with the help of which a statement about the efficiency and possible optimization steps of a steam power plant can be made. These relationships can be illustrated very clearly in the Ts diagram.

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

The heat is the product of the entropy, and absolute temperature. Plotting the changes of state of a steam power plant process ( Clausius- Rankine cycle ) in the temperature - entropy diagram (TS- diagram ), then provides the area under the curve, the supplied ( state change from left to right) or dissipated heat ( state change from the right dar. to the left)

In the diagram of power plant process (see block diagram) shown once without reheat (yellow) and reheat ( pink ). The turbine is assumed to be ideal (reversible change of state).

The vertices of the cycle denote the following state changes:

• 2-3: isobaric heating of the feed water to the boiling point
• 3-4: isobaric evaporation
• 4-5: Overheating
• 5-6: Relaxation at the turbine
• 6-1: Condensation

Only process with reheat ( pink ):

• 5 - 5a: Relaxing on the high pressure turbine
• 5a - 5b: reheat
• 5b - 6: Relaxation at the LP turbine (LP = low pressure)

In the diagram, the specific supplied and dissipated heat (relative to 1 kg of water ) for the relevant process parameters can be read as the area under the curve. Neglecting the supplied technical work on the feedwater pump, the neglect of heat losses and the assumption of an ideal turbine (reversible relaxation) following enthalpy exchange between the system boundaries of the power plant and the environment occurs:

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, and the waste heat, which is to be discharged via the condenser. The shaded areas in the diagram describe the dissipated heat of condensation. The useful technical work is represented by the solid areas. The efficiency of the steam process can be derived from:

The efficiency can be based on the second law of thermodynamics does not exceed the Carnot efficiency. The Carnot efficiency is formed from the average temperatures of the heat supply and the heat dissipation of a process. When steam power plant cycle, these are the average water -vapor temperature in the boiler and the condensing temperature ( if - as happened above - considered in isolation of the water cycle ) or the average flue gas temperature and the ambient temperature ( if heat transfer is included in the balance sheet ). From the diagram, the efficiency of the process can be calculated, and it can be derived graphically measures for optimizing efficiency:

• Increasing the vapor pressure,
• Increasing the main steam temperature (FD temperature)
• Low condensation temperature.

The reheat ( ZU ) increases the efficiency of the higher mean temperature of the heat. It is even indispensable at higher vapor pressures, because this erosion on the blades of the " cold end " (final blades in the low pressure part ) due to excessive steam wetness is avoided. The allowable content of liquid water in the exhaust is about 10 % ( vapor fraction x = 0.9).

Efficiency

The theoretical description of the steam power process is done with the Rankine cycle.

The efficiency of a steam power plant depends on the cutoff temperatures which passes through the steam. A further optimization is possible if an attempt is made (at least once ) and regenerative feedwater heating ( withdrawals from the turbine) as far as possible to approximate the real process by reheating the Carnot cycle.

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

With:

: Absolute mean temperature of the heat supply in K

: Absolute condensation temperature in K

For steam power processes from the development history following Carnot efficiencies can be derived: Newcomen ( saturated steam process without regenerative feedwater heating to 100 ° C / 30 ° C); Steam power plant in 1900 (10 bar, 350 ° C / 30 ° C, with ideal regenerative preheating ) :; Modern steam power plant with reheat heat according to diagram (256 bar, 543 ° C / 562 ° C / 18 ° C, preheating to 276 ° C). The actually achievable efficiencies are much lower.

The main steam temperature can be influenced by the design of the steam generator. A further increase in temperature at the superheater heating surface than the highest temperature can be implemented only in small steps. A live steam temperature of 600 ° C currently represents the technical and economic limit because upon further increase of the superheater should not be made ​​from expensive austenitic steels, but from materials based on nickel alloys which are extremely expensive. Such large-scale trials are currently in progress, the resulting temperatures of over 700 ° C allow the involved system components such as pipes and fittings glow already visible.

The steam temperature at the outlet of the LP condensing turbine is determined by the condenser pressure, which should be as low as possible. The lowest condensation pressures are reached by cooling water in a tube bundle heat exchanger. In this case, the power plant must be on a river, can be removed from the water for cooling purposes, be built. The Einleittemperatur in the repatriation of the cooling water is limited. So it can happen on hot summer days with low water level in the water that the plant capacity must be withdrawn. The tube bundle of the condenser dirtied with algae and salt deposits and deteriorate the heat transfer to the cooling water side. The tubes must therefore be purified using, for example, the Taprogge method is employed.

A low condensation temperature is achieved even with the evaporative cooling in cooling towers. By the spraying of water and evaporation is performed of the incoming saturation of the air, so that the air is further cooled due to the discharge of heat of evaporation. In this way, lower condensation temperatures may be achieved. When using air condensers ( Luko ) the condensation temperatures are higher, since the heat transfer to the air without the aid of evaporation is worse. The condensation temperatures are bar depending on the method and time of year is between 25 ° C and 40 ° C, the corresponding condensation pressures of 0.026 to 0.068, so that the capacitor is always driven to vacuum.

Modern steam power plants have an efficiency of up to 45 %. That is: More than 55 % of the energy in the form of heat can not be used industrially for the time and go - mainly via the cooling tower - lost. If one assumes, when technically feasible overheating of 700 ° C heat supply exclusively at this temperature ( which is unrealistic ), so the comparison Carnot cycle achieved an efficiency of 70%. The heat loss of 30 % would be for physical reasons and could not be less technical.

Next to the highest possible inlet temperature of the live steam, the lowest possible outlet temperature of the exhaust steam, and the dual reheat steam turbine of the regenerative feed water plays a role to improve the efficiency. With this method, the feed water is preheated with tapped steam from the steam turbine before it is returned to the steam generator. In practice, up to six Turbinenanzapfungen be provided; in this way one avoids the mixing of the cold feed water to the water standing on the evaporation temperature of the steam generator capacity, thus a significant saving of fuel associated.

After by the Federal Ministry of Economics in order COORETEC study this today efficiencies can be increased by steam power plant processes through consistent development up to 2010 to about 51 percent from 2020 and higher efficiencies are anticipated.

Combined heat and power (CHP )

The use of primary energy used can be improved by a so-called combined heat and power generation. The turbine is driven with back pressure or it will be a Turbinenanzapfung set up at a for heating purposes for the production of local or district heating suitable temperature steam and sum (eg 100 ° C / pressure = 1 bar ( abs) ). Due to the higher exhaust steam pressure, the efficiency of electricity generation (smaller Carnot factor of the incoming combustion heat) decreases. In sum, however, significant primary energy savings are achieved ( less waste heat in power plants and reduced use of primary energy for heating ). Steam power plants without combined heat and power are referred to as condensing power plants.

The reason for the reduced exergy losses by CHP is the low exergetic portion of the heat that is normally needed for space heating ( the temperature level based on the ambient temperature is low). If the heat for heating a room ( at 20 ° C) provided by combustion, so lost at an ambient temperature of 0 ° C for about 93% of the exergy of the fuel. If the heat from the power plant is coupled out at 100 ° C, the exergy loss is only about 27%.

The efficiency, which is achieved in the generation of electric current from the fuel used should not be confused with the thermal efficiency of the heat from the fuel used.

In comparison to the combustion of fuel to the evaporation of the water a much higher efficiency of power generation can be achieved by using hot exhaust from a gas turbine. Such consisting of gas and steam turbine power plants is called combined cycle power plants or combined cycle power plants (gas and steam power plants). These plants are operated as CHP plants (CHP).

Developments

In the past, consideration has been given again and again to complete the work by other means water evaporating substances in the steam power plant. In the first place, the metal mercury is to be called, which circulates in its own steam cycle, expands in a separate steam turbine and then its condensation gives off heat in a separate capacitor to a steam cycle. The highest values ​​for the mercury cycle were in the running in the United States from 1914 installations 10 bar and 500 ° C.

Further studies were commissioned in 1980, which included analogously triple cycle systems of potassium vapor, diphenyl and water. Each of these work means acts on its own steam turbine. Despite the high efficiency of such processes has been to dispense with the execution of such a steam power plant due to the high costs.

New meaning gained alternative work equipment by geothermal power plants, because there rarely are temperature levels above 150 ° C is available.