Combined cycle
A gas -steam combined cycle or combined- cycle power plant ( CCPP short ) is a power plant in which the principles of a gas turbine power plant and a steam power plant combined. A gas turbine is used as a heat source for a downstream heat recovery boiler, which in turn acts as a steam generator for the steam turbine.
With this combined procedure, a higher efficiency is achieved than with gas turbines in open mode or in conventional fired steam power plants in the thermodynamic cycle. Combined cycle power plants are electrical efficiencies of up to 60% of the most efficient conventional power plants ( see gas engine ).
Combined cycle power plants are very flexible power plant management: Due to a fast start times and the possibility of rapid load changes they are ideal mid-load power plants. Primarily, these power plants are operated even in the medium load range and, if required in the area of the peak current. Theoretically, the operation as a base load power plant is possible, but this is not economically and therefore not common in Germany because of the gas price.
Nomenclature
The short- term combined cycle power plant has long been a protected designation of Siemens; since end of September 2009 but no longer. Also known as GUD was still a brand, this name was often used as a generic term. Generally, this type of power plant is referred to as combined cycle power plant, with a likelihood of confusion exists, for example, with combined heat and power generation. Other trade names include Steam and gas ( STEAG ) of General Electric or combination system (KA ) of Alstom. In the English -speaking world it is called Combined Cycle Power Plant ( CCPP ), or combined cycle gas turbine ( CCGT ).
Operation
In the combined cycle power plant is produced with one to four gas turbines and one steam turbine electricity, either each turbine each drives a generator ( multi-shaft system, Eng. Multi imprisonment) or a gas turbine with the steam turbine ( uncoupled ) on a common shaft generator ( single shaft, English. single detention). The hot exhaust gases from the gas turbines are used in a waste heat boiler for generating steam. The steam is then expanded through a conventional steam turbine process. It accounts for about 2/3 of the electric power to the gas turbine and 1/3 of the steam process.
If in addition to the use of the exhaust heat of the gas turbine by additional firing ( supplemental ) of the boiler, the steam power and thus the electrical output of the steam turbine is increased, it is called a combined process. For the operation of the gas turbine gas or liquid fuel such as natural gas, bio gas or fuel oil are used. For the operation of the burner in the boiler and other fuels can be used.
Performance
The benefits of combined cycle power plants are in the range 80-845 (eg Irsching ) megawatts per unit gas turbine / steam turbine, a power plant may consist of several units. In comparison, a block of a nuclear power plant has a capacity 400-1600 MW.
Combining the two types of turbines, very high power plant efficiencies arise.
Efficiency
The extremely high efficiency of the combined cycle power plant is accomplished by the heat from the flue gas is fed into the process at a high temperature level. The stoichiometric ( λ = 1) combustion temperature of natural gas with air at atmospheric pressure is about 2200 ° C. Due to excess air inlet temperatures of gas turbines are however reduced. The current maximum possible gas inlet temperature is 1600 ° C. The outlet temperatures are about 650 ° C. The gas turbines have (based on electric power supplied heat ) depends on the power efficiency of 35 % (10 MWe ) to 40 % (100 MWe ).
The exhaust gas is used in a boiler to produce superheated steam with a temperature of> 500 ° C. A large part of heat ( enthalpy difference ) is, however, needed for the evaporation at the pressure-dependent saturation temperature. At 100 bar, for example, is this only 311 ° C. This means that in a system with only a vapor pressure stage (see first diagram on the right) the temperature difference between the flue gas and the water is very large in the evaporator at the inlet of the flue gas, and are accordingly high exergy losses in the heat transfer. If the flue gas is cooled to a few degrees above the vaporization temperature, its residual heat content can only be used for feed water preheating (see heat transfer diagram and Ts diagram right). The computationally active original file, which is taken from the diagram shown at right, calculated an efficiency of 52 % based on the net calorific value. Based on the calorific value of the efficiency is 52 % / 1.11 = 47 %.
The water-side connection of the boiler can be very different depending on operational requirements. In addition to the forced circulation boilers shown here also natural circulation boilers and once-through boiler ( Benson ) are common.
In order to avoid higher exergy losses, that is to say to improve the efficiency, a plurality of pressure levels are used. The second diagram shows a two-stage system. The feed water is taken from the feedwater tank either with separate feed water pump and fed to two separate feedwater heaters, or - as in the picture - the pressure stages connected in series. The evaporation and overheating is thus at different temperature levels.
Prior art combined cycle power plants to pure electricity, without further use of the waste heat that is for heating or as process heat, is the three-pressure reheat process. In this case, usually a gas turbine, the so-called F- class is used. The electrical capacity of these plants is about 400 MW. The pressures are from about 130 bar (high pressure), 30 bar ( medium pressure ) and 8 bar ( low pressure). The high pressure steam is superheated to about 570 ° C. After expansion in the high pressure part of the turbine, the steam is returned to the boiler, is mixed with the medium pressure steam and superheated to approximately 570 ° once more C. Theoretically even more pressure levels to better adapt the steam characteristic would be conceivable to the flue gas, but the additional investment loss is in proportion to the heat technical improvement too high. The three- printing process with a reheat is currently the economic optimum dar. With an efficiency of the gas turbine to generate electricity of 40 % and an efficiency of waste heat utilization from 18.4 % ( in each case based on the lower calorific value ) is the best ever achieved total efficiency at 58.4 %.
At E.ON Irsching site is currently being built a testing ground for the world's largest and most powerful SGT5 -8000H (375 MW ), which was newly developed by Siemens Power Generation. After completion of the test phase, the gas turbine in a modern combined cycle is integrated with an overall efficiency of 60%. On May 11, 2011, the power plant unit 4 reached in Irsching in trial operation an efficiency of 60.75 %, which is a new world record holder.
Waste heat utilization
By using a back-pressure turbine, or by a Turbinenanzapfung the thermal energy of water vapor can be used for district heating. Due to the higher back pressure, but then reduces the mechanical work produced and the exergetic efficiency of the overall process. Looking at the bottom of the power plant cycle in the Ts diagram (condensation ) may be approximated as a Carnot cycle (see steam power plant, Section Ts diagram ) with a counter- pressure instead of 0.037 bar (= 30 ° C condensing temperature ) is then reduced at 1 bar back pressure ( = production of district heating at 100 ° C) of the Carnot efficiency based on the achieved efficiency of 17%. Thus, only a efficiency of the generation of the mechanical work of 48 % is reached instead of 58%. After the steam extraction at 1 bar 52% of the fuel energy are at a temperature level of 100 ° C is available. This enthalpy of the steam is made of the condensation or evaporation enthalpy at 1 bar / 100 ° C; it is
And the remaining sensible heat in the condensate is no longer usable by
.
Without heat, the enthalpy of use are dissipated via the cooling tower to the environment in the condensation temperature of 30 ° C. This is reduced in case of a complete heat utilization on the 292 kJ / kg. Then stand next to the conversion of 48 % of the fuel energy into mechanical work for 45 % of the input energy as heat energy at 100 ° C is available. It should be noted that the energy in the heating water is only about 19% of exergy and the remaining 81 % of anergy. Therefore, the different efficiencies may not be added together. But the sum of both efficiency levels can not exceed 100%.
The total emissions are therefore generally very low compared to the power generated. In addition to the high efficiency are further advantages of a combined cycle power plant, the short construction period and the short startup time of the gas unit, which is why this type of power plant can be used to offset peak loads. However, a combined cycle power plant requires relatively expensive fuels, why deal with recent research combined cycle coal power plants.
The construction of combined cycle power plants is tax- favored in Germany, if the efficiency of the plant is about 57.5%. The State waived its rightful gas tax of € 5.50 per megawatt hour, if this efficiency is exceeded and the total availability of the power plant is more than 70 %. In other countries there are similar benefits.
Costs
Combined-cycle power plants can be built relatively quickly and inexpensively. The construction and investment costs amount to only about half of a coal power plant of the same capacity. In base load with 8,000 operating hours per year, however, coal-fired power plants can compensate for the higher construction costs due to lower fuel costs. The flexible CCGT power plants are therefore used primarily in the peak and medium load range with 4000 operating hours per year.
Combined cycle coal plants
An operation of the gas turbine with the pulverized coal is not possible at first, since the combustion of coal ash is formed, which would destroy the blades of the gas turbine by quickly wear. Deposition of ash from the hot gas stream is large and technically difficult part of current research. However, the possibility of coal provides first in a coking plant at least partially transform into coke oven gas and to use in a combined cycle power plant. The remaining solid coke can be converted into a conventional steam power plant into electricity.
Another form is the coal -fired combined cycle power plant, which presents itself as a coal-fired steam power plant combined with a gas turbine. The gas turbine is operated with natural gas and drives itself a generator. The exhaust gases leaving the gas turbine having a high temperature and a high content of oxygen, so that they can still serve as supply for a fired coal boiler, which in turn provides the steam for a steam turbine with a coupled generator. This method uses the example Gersteinwerk in Werne an der Lippe, which operates with an overall efficiency of 42%.
Another variation on an industrial scale represent coal plants gasify the coal under oxygen deficiency. The heat - almost a third of total energy - is converted into electricity in a normal steam power plant, while the resultant carbon monoxide burned after the removal of the ash in a gas turbine - that is converted to CO2 - is. The overall efficiency is also higher than the simple steam power plants.