Aerial view of the Počerady Steam-Gas Power Station
Between the Ore Mountains and the Central Bohemian Uplands
The Počerady Power Station lies near the village of Počerady in the Ústí nad Labem region, in the centre of a triangle marked by the towns of Louny, Žatec and Most. Thanks to the elevated panoramic shot, the flat landscape offers spectacular views, especially in the north-west where the main ridge of the Ore Mountains is clearly outlined 25 kilometres away. The noticeable towns before the ridge are Chomutov and (in the north) Most. The hilltops extending from the east to the north-east are the protected landscape area of the Central Bohemian Uplands with the dominant hills of Oblík (509 MASL) and Milá (510 MASL), where you can obtain a hilltop stamp, which also depicts the Počerady Power Station. The town of Postoloprty with the Rakovník Highland and the Plaská Highland lie in the background, 5 km to the south. About 10 kilometres southeast, behind Lenešický pond, the viewer's eye falls on the former royal town of Louny.
Počerady Power Station and Gas-Steam Plant
The Počerady plant has been a reliable producer of electric power and heat for more than 40 years. The history started with a coal-fired power station with four 200 MW units, which started operation between 1974 and 1975. The Počerady Power Station was built as the first 200 MW plant and, after the first construction stage, the first four units were commissioned between 1970 and 1971. In 1977, the last two units of the second construction stage were commissioned. In the first years of operation, until 1980, the power station received coal from the Třískolupy mine, which was located in its immediate vicinity. Since 1980, the power station has been supplied by the Vršany mine 7 km away. The power station does not supply the surrounding area with heat (except the actual Počerady location). Interestingly, this power station was originally to be installed in Egypt. However, the project was abandoned in the 1950s and the African project was finally used in Počerady. This fact results in several interesting features – the turbine halls assembled from reinforced concrete have no cellar. The actual turbines and alternators are located in individual boxes separated from the other turbo units.
The arrival of the 21st century has also brought about diversification of the sources. One of the preferred forms in the first decade of the year 2000 is the steam-gas cycle. Steam-gas power stations represent a highly flexible source, which is able to stabilise the electrical grids, thus rapidly equalising consumption of the electric power with its production. Their operation covers peaks in power consumption. This source can be connected to the network within several minutes after startup. Only hydroelectric power stations are faster. The steam-gas cycle boasts almost double efficiency in power production compared with the older classic coal-fired plants, with minimum ecological load to the neighbouring area.
Let us look at an animation of the steam-gas cycle.
The ČEZ group feels responsible for the future of the power in the region, so it became involved in several projects involving construction of steam-gas plants. Finally, only the Počerady project has been realised. The Počerady steam-gas cycle (PPC) with the generating capacity of 838 MWe is located behind Unit 6 of the classic coal-fired power station and is the first of its kind in the Czech Republic. During its design, the basic steam-gas cycle concept was chosen, i.e. a multi-shaft arrangement with two gas turbines (GT) and one steam turbine (ST).
Watch a video from the start of the Počerady steam-gas power station construction.
The combined cycle consists of two gas turbines (each with the capacity of 284 MW) and one steam turbine with the capacity of 270 MW. In 2009, ŠKODA PRAHA Invest, the general contractor, signed a contract with the Siemens Company for delivery of two gas turbines. ŠKODA PRAHA Invest also signed a contract for delivery of a steam turbine with ŠKODA POWER and an exhaust-heat boiler with SES Tlmače. Construction of the steam-gas plant started in spring 2011. The commissioning was terminated by certification of support services to the electrical grid in November 2014.
According to the original expectations, the power station was to be operated in a semi-peak mode, i.e. on weekdays from 6:00 until 22:00, or during shutdowns of other plants, and overnight and weekend shutdowns. However, the entire European power engineering environment experienced dynamic changes during the construction. The market mechanisms, distorted by several phases of regulations, ceased to fulfil their roles as reliable price creators of electric power. In a situation when only renewable sources can be built and operated without risk thanks to support from European grants, the steam-gas power stations moved to a category of sources with excessive production costs. Consequently, utilisation of PPC Počerady is now lower than originally expected. In the Czech energy system, it works as an operative backup capable of a relatively rapid startup.
Short implementation periods and high thermal efficiency favour the steam-gas power stations over other plants operating on fossil fuels. However, these benefits are sacrificed to the need of high-grade expensive fuel for operation of the combustion turbine. Most frequently, steam-gas power stations are powered by natural gas or, less frequently, oil. There are other potential fuels such as gases from coal gasification, biomass or from various technological processes, but these are exceptional. There is perhaps a certain future in steam-gas plants with integrated coal gasification (IGCC – Integrated gasification combustion cycle), which belongs to the so-called Clean Coal Technologies. New trends in technical parameters or investment costs can be followed on the Power Technology website.
The actual term "steam-gas cycle" is simplified because, from the technical perspective, its involves two cycles, a steam and a gas cycle, mutually interconnected by an exhaust-heat boiler, where the residual energy of exhaust gases leaving the gas turbine is used for production of steam for the steam turbine. The steam-gas cycle is a modern and broadly used and proven source of electric power, which has a higher thermal efficiency compared to the coal-fired units thanks to the basic configuration of the main components, i.e. combustion turbines whose exhaust heat contained in the exhaust gases is used in exhaust-heat exchangers for production of steam for the steam turbine.
Together with usage of a more ecological fuel, i.e. natural gas, the load on the environment is thus significantly lower compared with the classic coal-fired units.
It is thus an option that involves combined circulations in which the input heat is multiply utilised. The assets of combined circulations include better utilisation of the input energy and lower emissions related to the produced MWh.
The steam-gas circulation started to develop in connection with the development of jet engines at the end of the 1930s. Since the 80s, the steam-gas circulation has flourished, supported by an effort to bring ecology to industrial power engineering and by the growing global natural gas deposits.
Over the past 10 years, hundreds of new steam-gas facilities have grown in the global power network; the total number of installed units with a capacity exceeding 400 MWe lies in thousands.
The leading tandem in installation of new steam-gas plants is the USA and Japan; the European leaders are Italy, Spain and Great Britain.
The steam-gas cycle in Počerady works with the following indicators:
- Gross efficiency 59%
- Gross capacity on the generator's terminals 838 MWe
- Internal consumption 10,5 MWe
- NOx, CO emissions up to 20 mg/Nm3
Počerady Power Station
The Počerady steam-gas power station is standing next to the Počerady coal-fired power plant. The power plants are connected by a steam, water and electric distribution system. Steam from the coal-fired power station is used for feeding glands of the PPC plant during startup and then for heating in winter; the electric power ensures emergency feeding; demineralised water for the steam-gas plant is conducted from an adjoining chemical water treatment plant.
Look at the operation of a steam-gas power station.
It is the only power station of its kind in the Czech Republic (the ČEZ Group operates a similar power station with the generating capacity of 872 MW in Egemer, Turkey). The Počerady steam-gas station has operated since November 2014, but its operation corresponds to the price of gas. The construction started in 2011 and took about two years. Testing and commissioning proceeded for the following two years.
When the power station is out of operation, its boilers, steam pipelines and the condensate are dried, the pumps are filled with a preservation solution of demineralised water and hydrazine, constant pH and O2 values are maintained in the circuits and moisture in the drained components is monitored, because it must not exceed 30%. The equipment is thus preserved. However, in case of need, the power station can start up within two days from this preserved condition or even within ca 20 hours in the fastest possible case.
From cold condition, the power station can reach full performance in 4 hours, but from the hot condition, it is less than 2 hours. In fact, gas power stations might be able to start working immediately, but it is necessary to keep the temperature trends in the boiler and the steam turbine, which must first be heated.
In the Czech Republic, it is new technology. In the world, however, steam-gas plants substitute the oldest nuclear power plants and are standard technology. Steam-gas power stations also cover the failures of large plants.
Compared with coal, it is an ecological power station, which can reach the gross efficiency of 59% measured in full performance at the ambient temperature of 10°C. Both boilers are followed by chimneys, where the residual temperature of exhaust gases reaches about 90°C. Compared with a coal-fired power station, the NOx and CO emissions are one-tenth, the CO2 emissions are one-third. Sulphur oxides and solid pollutants are only present in the exhaust gases in trace amounts. Thus, continuous measurement of these emissions is not necessary.
Gas for the gas section of the power station is extracted from a transit gas pipeline. The gas is conducted to a regulation station from a branch near Bečov. It is the only high-pressure inlet in the Czech Republic; the other facilities do not have such high gas pressure. At maximum performance, up to 150,000 m3 per second is supplied to the power station.
Steam turbine
The steam turbine's thermal circuit incorporates the heating of pressure water to the boiling temperature, evaporation, superheating of steam to the working temperature and subsequent expansion of steam in the steam turbine. The circuit is terminated by condensation of steam into water.
The 270 MW Škoda KT-270 – 12.8 steam turbine is an integral component of the Počerady CCGT power station, which consists of two gas turbines and two non-reheat HRSG, with reheating on three pressure levels. The steam turbine is a double-case impulse stage turbine with one combined HP-MP section and one double flow LP section, condensing, without regeneration and with reheating. It has three steam inlets: HP (superheated steam), MP (reheated steam) and LP steam. The outlet from the LP sections is conducted to a surface condenser. Cooling proceeds in a natural draft wet cooling tower.
| Rated generator capacity measured on the terminals | 272,532 MW |
| Minimum permanent TG capacity | 83,5 MW |
| Rated turbine speed | 3000 ot/min |
| Rated generator speed | 3000 ot/min |
| Nominal pressure of superheated steam before HP stop valve | 12,8 MPa |
| Nominal superheated steam temperature before HP stop valve | 550 °C |
| Nominal amount of superheated steam | 141,4 kg/s |
| Nominal pressure of reheat steam before MP stop valve | 2,772 MPa |
| Nominal temperature of reheat steam before MP stop valve | 548 °C |
| Nominal amount of reheat steam | 169,8 kg/s |
| Nominal pressure of LP steam | 0,3995 MPa |
| Nominal temperature of LP steam | 288 °C |
| Pressure in condenser (depending on temperature of cooling water) | 3 ÷ 7 kPa(a) |
| Nominal temperature of cooling water | 16,5 °C |
| Minimum temperature of cooling water | 12 °C |
| Maximum temperature of cooling water | 26,3 °C |
| Nominal amount of cooling water in main condenser | 10500 kg/s |
Oil management of the steam turbine
The turbo unit oil management ensures:
- lubrication and cooling of turbine and generator bearings
- oil supply for the turning gear
The next function is continuous flush of impurities from the oil system and oil filtration.
The turbo unit oil management consists of the following main parts:
- Main oil tank
- Auxiliary oil tank
- Main oil pump (on turbine shaft)
- Feed injector
- Startup oil pumps (≈)
- Emergency oil pump (=), (≈)
- Auxiliary oil pump (≈)
- Oil vapour fan
- Oil coolers
- Thermostatic control valve AMOT
- Duplex oil filter for lubrication oil
- Duplex oil filter for jacking oil
- Jacking oil pumps (≈)
Main steam turbine hall
The steam turbine is located in the main steam turbine hall at a level of 12.5 m. The oil distribution system, condenser (large, green) and vacuum pumps (orange) are located below the turbine. To ensure the correct operation of the condenser and to maintain efficiency of the turbine, it is necessary to constantly exhaust air and non-condensed gases from the steam space of the condenser. Air penetrates into the vacuum system through leakage or with the steam.
To create the initial vacuum in the main condenser and keep the specified level of vacuum in the condenser, two evacuation stations with water-ring vacuum pumps are installed in the system. Condensate pumps extract condensate from the collector of the main condensate, which is produced by condensation of emission steam entering the condenser from the outlet of the turbine's LP section or condensate from hot reheat steam and LP steam, which enters the main condenser from the MP and LP bypass stations. This condensate is pumped by condensate pumps 1° through a condensate polisher into the inlet of condenser pumps 2°, which pump the condensate through a disc valve into a feed tank, or back to the main condenser by a recirculation pipeline.
The capacity of the steam turbine is 270 MW and is generated by a Siemens generator, which is connected with the turbine by a coupler. The speed of the turbine is 3,000 rpm.
Central control room – control system
The central control room controls the entire PPC Počerady operation and part of the communal facilities, which are used by the PPC cycle together with the classic Počerady Power Station.
PPC Počerady is controlled by the Siemens SPPA-T3000 system and consists of three levels:
- the operating level consists of an application busbar and operating, i.e. engineering and diagnostic centres;
- the application level consists of an application server, which forms a central core of the system and ensures distribution and archiving of data between both other levels;
- the automation level consists of automation processors.
Condenser
The unit has one main condenser located below the turbine LP section. The surface condenser is used for condensing steam extracted from terminal stages of the turbine's LP casing or steam from the MP and LP bypass stations. The condenser consists of a metal sheet casing, which delimitates the steam space and is connected to water chambers. The condenser is firmly connected with the turbine, housed on elastic supports for dilatation, with expansion bolts for relief of the turbine springs during a pressure test of the steam space by demineralised water.
The heat-exchanging surface consists of tubes rolled into the tube sheet on both sides, through which the cooling water is running. On the water inlet and outlet sides, the tube sheet is firmly connected to the casing; dilatation of the tube sheets is allowed on the return chamber side. The condenser tubes are supported by support walls along the entire length. The support walls are distributed so that they protect the tubes against vibrations and deformations. Inner surfaces of the water chambers are painted with a plastocor protective coat.
The condenser is a twin-pass condenser, separated on the water side, which allows operation of one half of the condenser when the turbine performance is lowered. The condenser is protected against excessive pressure rise by safety membranes located in the LP section of the turbine.
Gas turbine
The gas turbine is housed in a compact casing, equipped with a highly efficient compressor where the air is compressed. The gas fuel is then mixed with air in a circular combustion chamber equipped with hybrid burners, where it is ignited and combusted. The combustion chamber is equipped with ceramic thermal plates, because of high temperatures, which can reach up to 1,500°C. This arrangement minimises demands for cooling by air. In the following turbine section, the created hot air expands and rotates the turbine by means of mono-crystallic blades of top-quality alloys with a ceramic layer and highly efficient internal blade cooling, and converts thermal energy into mechanical energy, which is used for driving the compressor and the generator in which the mechanical energy is converted into electric energy. The latest generation of turbines is equipped with hydraulic control of the blade-end allowances to optimise radial clearance and reach maximum performance. The compressor has optimised flow and a controlled diffusion profile for efficient operation. The robust structure of the compressor can even operate under conditions of insufficient or excessive speeds and ensures reliable operation of the turbine even in networks with heavy frequency fluctuations. The circular combustion chamber is equipped with a hybrid burner with cylindrical burner adaptors and optimised gas flow, which ensures stable combustion with a low noise level.
Blades of the first and second turbine stages must endure high thermal load; thus, they are made of a heat-resistant alloy, which solidifies in a mono-crystallic structure. In addition, they are coated with another ceramic layer. The gas turbine is a combustion engine with a continuous flow of air and exhaust gases.
Generator
The three Siemens generators in the Počerady steam-gas cycle are remarkable for their cooling. Instead of common hydrogen cooling, they use air for direct cooling of the rotor winding and indirect air cooling for the stator winding. Three phases are conducted from the generator in sheathed conductors under the voltage of 19 kV. The sheathed conductors are air-insulated and are conducted along steel structures to block transformers, where the voltage of 19 kV is transformed to 400 kV.
Space between the two gas turbines
The space between the two gas turbines is mainly used for handling materials during shutdowns. The area is adapted for installation of stands holding the gas turbine rotor during its removal and installation. There is also equipment for cleaning the turbine's compressor section and an air-drying unit, which is automatically activated when the gas turbine is shut down. This unit drives dried air to the compressor's suction area to prevent moisture. The other equipment includes a battery of CO2 cylinders for the fixed firefighting system of the turbine and the auxiliary equipment.
Inter-machine room
The so-called inter-machine room is a separate space in PPC Počerady. It houses an outlet of exhaust gases from the turbine and a diffuser, which rotates the exhaust gases so that they are evenly distributed. From the diffuser, the exhaust gas line is conducted to the boiler. We can also find the main HP feeding pumps (blue) and the LP feeding pumps, the HP feeding pumps – two to each boiler for 100% backup, frequency converters and transformers. There are three LP feeding pumps for both boilers controlled by frequency converters.
The feeding of exhaust-heat boilers is provided by the demanded amount of degassed water for feeding both exhaust-heat boilers and supply of feeding water to exhaust-heat boilers on three pressure levels according to the boilers' power demands.
Heat recovery steam generator (HRSG)
The Počerady steam-gas cycle has a triple-pressure exhaust-heat boiler for utilisation of exhaust heat from the Siemens SGT5-4000F gas turbine, with a horizontal arrangement of individual heating pressure systems without a chimney bypass.
The exhaust gases pass from the internal combustion turbine through a turbine diffuser into an input channel of the exhaust-heat boiler. In the boiler, the exhaust gases pass their heat to a heat-carrying medium circulating in individual heating surfaces of the boiler. Gases to from the gas turbine are carried by exhaust gas pipelines (input and output ducts). Their design and reinforcement ensure minimum loss of pressure and sufficient rigidity and tightness. The boiler output is connected to a reinforced concrete chimney with a noise silencer.
When the system is out of operation, the exhaust-heat exchangers must be kept dry to prevent corrosion. This is ensured by Munters dryers. The operator also fills the pumps with a preservation solution and monitors constant values of pH and O2 in the circuits and atmospheric humidity, which must not exceed 30%. In winter, it is necessary to ensure that the air temperature does not drop below 5°C.
The preserved equipment can start up within ca 16 hours after the system operator's command.
Exhaust-heat exchanger roof
The boiler plant roof is 36 m above ground level. It holds safety valve outlets of the boilers' pressure systems and equipment for heating air in the boiler plant.
Cooling water pumping station
The pumping station is equipped with two cooling water pumps. Both pumps are operational.
These pumps are equipped with power control by rotation of the impeller blades between 70 and 140% of the rated flow, which is 19,651 m3/h.
The pumps are fitted with fixtures on the discharge (combined fixtures consisting of a DN1800 shut-off flap with the function of a non-return flap with hydraulic damping and drive).
The pumping station is also equipped with one submersible pump dimensioned for cooling a circuit incorporated in the main unit machine hall. The pump is in operation when the cooling water pumps are shut down. This pump is also used for feeding the cooling circuit.
Two submersible pumps (1+R) or (2+0) are installed for drainage of the pumping station and pumping water drained from the DN 2400 collecting pipeline.
Two firewater pumps and two feeding pumps are installed in the firewater pumping station.
The pumps are equipped with fittings on the inlet and discharge (non-return flaps, shut-off flaps with electric actuators) and fittings for connection of air chambers and to recirculation.
Cooling tower above the cooling fills
Water from the valve chamber is conducted by a concrete duct under the basin's foundation slab to two rising ducts. From the rising ducts, the water is distributed in the cooling tower area by distribution troughs and the follow-up PVC pipeline with spray jets. Water from the jets is sprayed on cooling fills that cover the entire surface of the cooling tower, where the water cools down. From the cooling fill, the water falls to the cooling tower basin and is conducted to a drain duct.
Cooling tower
The Iterson natural draft cooling tower is used for cooling the circulating cooling water, which carries exhaust heat from the TG condenser so that the condenser is able to condense the amount of steam leaving the steam turbine. The heated water from the condenser is conducted to a valve chamber before the cooling tower by one DN 2400 fibreglass pipeline laid in the ground.
| Height of cooling tower chimney | 128,32 m |
| Height of the suction hole | 7,00 m |
| Height of the chimney outlet | 94,273 m |
| Inner diameter of the basin | 98,1 m |
| Diameter on the inlet level | 88,54 m |
| Diameter in the outlet | 56 m |
| Diameter in the tower crown | 60,6 m |
Switching station and fuel supply
SF6 gas-insulated 400 kV switching station
The main part of the 400 kV switching station is gas-insulated. It is divided into two sections connected by a 400 kV cable. The main part is located in the SO500 building. It incorporates switches, instrument current and voltage transformers, isolators and earthing switches. The outdoor section of the switching station is equipped with a pair of earthing switches, an isolator and a sheathed diverter. However, the primary role of the isolator is not a working role. It is only there for measurement on the 400 kV cable.
400 kV cable
Three 400 kV cables laid in a cable duct are used to conduct output from the PPC block transformers to a sheathed 400 kV switching station. Each of the cables is a set of three one-phase 400 kV conductors S = 1,000 mm2 arranged in a tight triangle.
PCC container – Power Control Centre for the gas turbine's turbo generator
The PCC container is a control centre for the gas turbine turbo generator. It incorporates AC, DC distributors, accumulators, autonomous control system, protections of the generator and the gas turbine and a startup converter (SFC) with a static excitation system (SEE).
Feeder transformers
All of the feeder transformers (three for the PPC) are 380 MVA three-phase transformers. The transformers have tap changing switches without load.
The generator side is connected by sheathed conductors to 19 kV porcelain insulators. On the VHV side, the 420 kV insulators are connected to sheathed conductors insulated by SF6 gas. The transformers are equipped with SERGI pressure protection.
Fuel supply
The fuel used in the Počerady steam-gas cycle is natural gas. The yellow gas line is easily distinguishable in the colour system of the lines. PPC Počerady takes the gas from a transit gas pipeline. The gas is conducted to a regulation station from a branch near Bečov. Here, it is regulated to 35 bar and conducted to a preheating plant through fine filtration. The heated and filtered gas is then separated into two gas turbines. The filters consist of micro-screens, which capture small particles of dirt.
It is necessary to provide transport of natural gas to the steam-gas source in the demanded amount and parameters from the transit pipeline to the combustion turbines.
The main parameters include the specified amount of gas, its pressure and temperature, which are specified below.
| Maximum transported amount of gas: | 190 000 m3∙h-1 |
| Minimum transported amount of gas: | 3 000 m3∙h-1 |
| Maximum gas flow for the individual GTs: | 91 000 m3∙h-1 |
| Input gas pressure in the transit point: | 41 to 63 Bar |
| Output gas pressure in the transit point: | min 33 Bar |
| Input gas temperature in the transit point: | 0 to 25 °C |
| Output gas temperature in the transit point: | max. 130 °C |
| Range of ambient temperatures: | -25 to +50 °C |
Gas regulation station
The natural gas regulation station is used to reduce gas pressure from the transit pipeline to demanded specifications for the purpose of the gas turbines.
The regulation station is designed in a two-row arrangement, each row for 100% performance. Each row has a filtration section, gas preheating and gas regulation. The machinery also includes a regulation row for internal consumption.
Gas preheating and filtration station
The gas preheating and filtration station is used for measuring the amount of gas, its preheating to the required temperature and fine filtration before feeding into the individual gas turbines.
The gas supply is located on a piping bridge; the outlet of preheated gas is also on a piping bridge. Each row is fitted with gas measuring and filtration, heat exchangers and fine filters followed by stainless steel pipelines.
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