GAS-FIRED COMBINED-CYCLE POWER PLANTS HOW DO THEY WORK? A company of

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1 GAS-FIRED COMBINED-CYCLE POWER PLANTS HOW DO THEY WORK? A company of

2 Cover picture: Gas turbine compressor with combustion chamber

3 CONTENTS At a glance How does a gas-fired combined-cycle power plant work? The gas-fired combined-cycle power plant in model form The gas turbine: the heart of the power plant The water/steam cycle: more than just hot air The high-voltage system The control station: control and security What are the positive aspects of a gas-fired combined-cycle power plant? The issue of efficiency Cost matters Modern combustion technology: a plus EGL projects in Italy (inserts)

4 1 1 Model of an EGL gas-fired combined-cycle power plant

5 AT A GLANCE A gas-fired combined-cycle power plant, also known as a Combined Cycle Gas Turbine Power Plant, combines the strengths of two thermal processes in ideal fashion: electricity production using a gas turbine together with a steam turbine. The acronym normally used to describe this system is CCGT. Around two thirds of the electrical power generated is produced by the gas turbine. In a similar way as an aeroplane engine, a mixture of compressed air and fuel is combusted. The hot gases that this process creates drive the turbine and, with it, the generator that is coupled to it. The rest of the electrical power generated, roughly a third, is produced by the steam turbine using the hot exhaust gases leaving the gas turbine. In the heat recovery steam generator (HRSG) the exhaust gases transfer their heat to the circulating water: the pressurised water vaporises, causing the temperature in the system to rise. The steam drives the steam turbine and, with it, the generator that is coupled to it. Gas-fired combined-cycle power plants are technologically advanced and used throughout the world. Compared with other types of power plant, they are highly efficient: the state-of-the-art EGL plants in Italy achieve a percentage in the region of 56, meaning that the energy supplied in the form of fuel is converted into electricity as efficiently as technically possible. Investment costs are comparatively low because the main components are largely standardised. The extremely powerful gas turbine allows for a compact power plant, which minimises construction time to around two and a half years. Gas-fired combined-cycle power plants are built modular, consisting of several blocks. Each block is practically a self-contained power plant that can be operated independently. The standardised EGL power plants all have two blocks with a combined yield of around 760 megawatts. One of these power plants could cover about ten percent of Switzerland s annual energy needs. With modern combustion engineering, the emission of harmful exhaust gases can be controlled and minimised. Compared with all other conventional thermal plants, such as coal-fired power plants, carbon dioxide and nitric oxide emissions are lowest when plants are fired with natural gas. 4 5

6 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? 1 2 The gas-fired combined-cycle power plant combines two thermal processes so well that the resulting electricity generation is as efficient as possible. Around two thirds of the electricity is produced by the gas turbine, and the remaining third by the steam turbine coupled to it. Powered by the expanding fuel gases (gas turbine) or the steam (steam turbine), a mechanical rotation takes place in the two turbines, which is then converted by the generators into electrical energy. The EGL power plants are designed as multi-shaft installations. This means that the two turbines are coupled to separate generators. This is in contrast to single-shaft installations, where both turbines power the same generator. The EGL power plants in Italy consist of two blocks each, so they each have four drive machines (two gas and two steam turbines) and four driven machines (generators). 1 Gas turbine 2 End winding of generator

7 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? 1.1 The gas-fired combined-cycle power plant in model form high-voltage lines steam lines 4 medium and low pressure steam turbine 6 generator high pressure steam turbine 5 condenser exhaust gas stack 1 air intake feed-water pump steam high-voltage lines 2 gas turbine air intake water 6 generator compressor gas supply combustion chamber turbine heat recovery steam generator 3 1. Ambient air is drawn in via filter stages and compressed in the compressor. 2. Gas turbine: air is compressed, natural gas is mixed in. Combustion takes place, generating hot gases under high-pressure. The turbine powers the generator and the compressor. 3. Heat recovery steam generator: water is vaporised using the hot exhaust gases from the gas turbine. 4. Steam turbine: the steam powers the turbine. The resulting mechanical energy is transferred to the generator. 5. Condenser: here the exhaust steam from the steam turbine is converted back into water by means of air cooling. 6. Generators: here the mechanical energy from the turbines is converted into electricity. 6 7

8 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? The gas turbine: the heart of the power plant The gas turbine is the first stage in the process of producing electricity. The gas turbine compressor draws in air from the environment via a filter (1. on the model). This air is compressed in the compressor (2. on the model), which means that it is elevated to a higher pressure, and then directed into the combustion chamber. Fuel is fed into this chamber in the form of natural gas, and combustion takes place. This process produces hot gases that are allowed to relax in the turbine, which means that they are brought to virtually ambient pressure. The gas spreads out and expands. The energy that this releases is converted into a mechanical rotation just like a toy balloon when air is escaping from it. Here too the pressure compensation (expansion) brings about the motion, the action of the balloon darting off. The mechanical rotation powers the compressor and the generator. The generator converts this energy into electricity. When the hot gas exits the turbine as exhaust gas, it has a temperature of around 600 C. This heat energy is then transfered to the water in the heat recovery steam generator. The second stage of electricity generation then takes place: the water/steam cycle where the pressurised water is heated and vaporised. We regard the gas turbine as the heart of the power plant. Not only does it produce two thirds of the electricity, it also gives the power plant its name. Diesel oil 1 Heat recovery steam generator 2 Gas turbine compressor

9 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? could also possibly be used as fuel, but the EGL plants in Italy have not been designed for this. Natural gas has lower exhaust gas emissions compared to diesel oil. In addition, the cost and effort needed to maintain and service turbines powered with natural gas is usually lower. A general overhaul, where heavily used parts have to be replaced, tends to be necessary only once every three years. During such maintenance inspection times, it pays for a power plant to have more than one block. One part of the plant can carry on producing electricity normally even if the other is undergoing inspection and is therefore out of action for some time. All EGL power plants use type V94.3A2 gas turbines. They are manufactured by the Italian company Ansaldo, under a licence provided by Siemens. The turbine weighs roughly 300 tons and yields a nominal electrical output of 260 megawatts. By way of comparison, around 3,500 VW Golfs together would be needed to produce this kind of output. 1.3 The water/steam cycle: more than just hot air A third of the total electrical output comes from the steam turbine (4. on the model). The water/steam cycle uses the heat energy of the exhaust gases that would otherwise go to waste in the gas turbine process. This heat is used to generate water vapour, which produces electricity with the help of a steam turbine. The water/steam cycle is closed, which means that it is always the same water that is heated, vaporised and then converted back into water in the condenser. The heat recovery steam generator (3. on the model) is a large and complex configuration consisting of bundles of pipes and drums. It has three areas, each with a different pressure level: one high, one medium and one low. By dividing into these three levels, it is possible to harness an impressive amount of the energy contained in the exhaust gas. In all EGL power plants the boiler is roughly 45 metres high and 17 metres wide. The steam turbine (4. on the model), like the boiler, is divided into three pressure levels: one high, one medium and one low. The respective area of the steam boiler supplies the steam turbine with the correct steam. This is then allowed to relax in the turbine; in other words, it releases pressure. The turbine converts the steam energy into a mechanical rotation that is then transferred to the generator. There, it is transformed into electricity. The steam turbines for all EGL power plants in Italy are manufactured by Ansaldo under a licence provided by BBC (ABB). The generator s nominal output is 132 megawatts. If you again compare this output with a VW Golf, you would need another 1,800 vehicles to produce this level of output. 8 9

10 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? Rotor of high pressure steam turbine 2 Ventilators of condenser

11 1. HOW DOES A GAS-FIRED COMBINED-CYCLE POWER PLANT WORK? The condenser (5. on the model), on account of its size, is the most striking component of the power plant. The steam exits the turbine under vacuum, i.e. negative pressure. It flows through pipes a few metres in diameter into the condenser, which is cooled by air. Large ventilators add ambient air, which cools the steam right down until it becomes water again: it is condensed. The feed-water pump then returns this condensate to the boiler. And the cycle starts afresh. 1.4 The high-voltage system The alternating current generated in the plant cannot be stored. Power plants therefore need an electrical system that reliably conveys the electricity produced to consumers. The power plant s own transformers convert the electricity produced so that it can be fed directly into the high-voltage network. In the event of a disruption to the high-voltage network, the plant automatically begins to run down in a safe mode. As soon as the electricity grid is available again, and the corresponding demand exists, the power plant automatically connects to the grid again and supplies the required output. 1.5 The control system: control and security The control system in the plant s central control room steers, controls and monitors all the processes and operations in the power plant. It records all the major sequences and process variables, and assists human intervention. It is also able to draw comparisons between target and actual conditions by itself and to respond to them. In addition, the continuous feedbacks sent by the many sensors in the power plant trigger automatic responses. Normally speaking, all standard operation sequences within the power plant are fully automated from start to finish. If necessary, operating staff can purposefully intervene and make improvements. The system collects and stores a large volume of operational data that is important for specific analyses. The historical data record also makes it possible to determine maintenance and inspection times precisely

12 2. WHAT ARE THE POSITIVE ASPECTS OF A GAS-FIRED COMBINED-CYCLE POWER PLANT? The issue of efficiency A plant s availability is the key to its cost efficiency: to what extent is the power plant ready for operation (after scheduled and unscheduled downtimes)? Gas-fired combined-cycle power plants can achieve very high values upwards of 95 percent if maintained well. This means that they are connected to the grid roughly 345 days of the year on average. Compared with other types of power plant, gas-fired combined-cycle power plants boast a high level of efficiency. They convert the energy introduced in the form of natural gas into electricity with the least amount of loss that is technically feasible. It is not possible to achieve zero loss given that friction and heat transfer always cause some energy to be lost. The gas turbine manages a level of efficiency of around 35 percent. The water/ steam cycle coupled to it significantly raises the efficiency for the entire plant by using the heat energy emitted by the exhaust gases from the gas turbine to produce steam and, therefore, to generate electricity. This way, the EGL plants in Italy achieve excellent values of roughly 56 percent. Coal-fired power plants tend to be 10 to 15 percentage points below this level. While it is true that plant efficiency is slightly lower if cooling is done with air, as opposed to cooling with water, this does offer greater independence for plant operation and protects natural water resources. But it is not just the features of the plant itself that are key to its cost efficiency. Other decisive factors include the location and the professionalism of plant operation. An important consideration in the choice of location is 1 Rotor of low pressure steam turbine 2 Condenser

13 2. WHAT ARE THE POSITIVE ASPECTS OF A GAS-FIRED COMBINED-CYCLE POWER PLANT? the proximity to a high-voltage power line (380 kilovolts). Short distances mean that transmission losses can be limited. In addition, it is important that line capacities to consumer centres are adequate. Like connection to the high-voltage network, connection to the natural gas supply is also very important. Therefore, an ideal location for a power plant is one that is close to the high-voltage network and to the natural gas pipeline. Well trained, experienced personnel are a basic requirement for reliable operation of a power plant, and one on which EGL places special emphasis. It recruits its staff early and offers continuous training for their specialist duties. The EGL employees responsible for operation on site ensure the availability of the plant. The EGL specialists at head office make decisions regarding the use of the power plant and production volumes. 2.2 Cost matters The investment costs for a gas-fired combined-cycle power plant are lower than those for coal-fired power plants and other conventional thermal plants, standing at around 0.6 million euros per megawatt of installed capacity, whereas for coal-fired power plants, for example, twice that amount can be expected. The reasons for this are the largely standardised main components of this type of power plant and the short construction time of just two and a half years or so. Depending on the design of the plant, it is even possible to go into operation with just the gas turbine first and to complete the water/steam cycle in parallel. That way, the investor can start generating in- come from the sale of electricity earlier, thereby achieving a better return. A gas-fired combined-cycle power plant has a useful life of 25 to 30 years, which is comparable with other types of power plant. Production costs for electricity, measured in euros per megawatt hour, depend very heavily on fuel costs. With today s natural gas prices (as at autumn 2006), their share can account for up to 70 percent. This heavy dependency suggests that it is better to use gas-fired combinedcycle power plants as medium load electricity generating plants: this means taking advantage of their flexibility and only operating them when electricity market prices are attractive. The block size that has been chosen for all EGL power plants (380 megawatts each), guarantees attractive economies of scale, measured on production costs. Smaller units require more specific investment expenditure and therefore generate fewer profits. 2.3 Modern combustion technology: a plus Gas-fired combined-cycle power plants emit greenhouse gases. Compared with all other fossil-thermal plants, that are all plants with combustion processes, the lowest emissions of carbon dioxide and nitrogen oxide (NOx) are from plants operated with natural gas. The values for coal-fired power plants, for example, are roughly twice as high. Natural gas is regarded as a relatively clean fuel. Continuous monitoring of the combustion also helps to control and minimise the emissions

14 Editor/contact EGL Elektrizitäts-Gesellschaft Laufenburg AG Corporate Communications Lerzenstrasse 10 CH-8953 Dietikon Tel Photos: EGL May only be reproduced with written permission of the editor

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