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Concept of a thermal power plant
CHAPTER SEVEN
Concept of a thermal power plant Contents 7.1. Introduction 7.1.1 Thermal power generation within global power mix 7.1.2 Thermal power as a transition technology 7.2. Steam power plant 7.2.1 Steam turbine cycle 7.2.2 Heat balance diagram and its optimization towards higher efficiency 7.2.3 Support systems and equipment areas 7.3. Gas turbine powered energy 7.3.1 Gas turbine simple cycle 7.3.2 Combined cycle as a way to increased efficiency 7.3.3 Major components and systems References
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7.1. Introduction Traditional thermal power is based on a historically established and proven technology of burning fossil fuels — solid, liquid, or gaseous, when they release their chemical energy in the form of heat and radiation, justifying the name of the technology. Properties of the fuels, their availability, and cost in many cases define the approach that is used in a conventional thermal power station. Various plants have been built throughout the world, usually powered by two major kinds of machines: steam turbines and gas turbines. Recently, the combination of both has proven to be among the most efficient in delivering power out of natural gas. Moreover, the overall plant configuration has been subject to multiple modifications in the race towards higher efficiency through eliminating energy waste in the process and equipment. The overall arrangement of plant’s major systems evolved in the direction of modularization and following the same approach for all types of power plant: steam and/or gas turbine driven. All of them claim their own ways to design with some similarities, though still following the process of burning fuel to deliver heat. Sustainable Power Generation Copyright © 2019 Elsevier Inc. https://doi.org/10.1016/B978-0-12-817012-0.00018-9 All rights reserved.
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7.1.1 Thermal power generation within global power mix Thermal power generation from fossil fuels, namely coal, natural gas, and oil, accounts for more than 65% of the global electricity production [1,2]. Coal power is the most mature and common power generation technology in the world. With more than 2550 GW of installed capacity, it provides roughly 38% of world electricity generation, which proves the fact that it has served as a reliable, cost-effective, and durable baseload power source to utility scale networks [1,3–6]. Coal-fired power plants account for 8.5–10 trillion kWh of electricity production, which is expected to increase in the next decades [5]. The global coal power generation mix is dominated by Asia and especially China, as well as Africa and North America. China and India alone account for 69–75% of the projected worldwide increase in coal-fired generation, while the OECD nations continue to reduce their reliance on coal-fired electricity generation [3,5]. While natural gas-fired power generating technology accounted for roughly 21% of the global energy mix in 2016 [7], it is expected to grow for the next decades as well, however, with lower rate. In the United States, natural gas-fired generation is encouraged by low prices and favorable greenhouse gas emission characteristics compared to coal-power plants, however, there are some trends of revitalizing the coal industry and coal-fire generation. The fastest growing regions for natural gas-fired efficient technologies are Mexico, the Middle East, and some of the Asian countries, especially Japan, which imports LNG for these purposes. Remark 7.1. In some countries, like Russia, many coal-fired power plants have been refurbished from solid fuel combustion to employ natural gas as a primary fuel and crude oil as a backup fuel. These power plants have proved to emit lower levels of CO2 , but provide the same or even higher efficiency range compared to those operating with coal. These plants shall not be mixed with modern gas turbine based combined cycle technology which can have much higher efficiency. Fuel oil occupies roughly 4% of the global electricity generation mix and is mostly employed in the areas where oil is cheap enough to be burnt. Its use is supposed to steadily decline in the next decades, also due to the introduction of new energy policies.
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7.1.2 Thermal power as a transition technology Thermal power plants generate electricity with the economically feasible and historically established technology, which has been developed during the years of thermal cycle optimization, material research, and technology development. Still, these facilities burn fossil fuels, which has to deal with waste and releases large amounts of GHG. Environmental concerns notwithstanding, fossil-fired power generation would continue to play a significant and perhaps a dominating role, however, may not be considered as a sustainable technology in the long run. Still, it is impossible to replace this large share of base load generation in a reasonably short time: even though it may be technically feasible to replace all existing fossil-fired units with sustainable energy sources within few decades, such a conversion would almost certainly take longer due to the difficulty in implementing all necessary policies [8]. Since it may be much easier and cheaper to provide at least all newly introduced energy for the next 20 years from renewable sources, it would still stay as a favorable forecast rather than the real picture due to multiple factors, which shall be discussed in further detail in Part 7. Thus, thermal power is supposed to play a crucial role in the world power generation mix and would bridge the gap between the current power generation system and the future sustainable energy system. Moreover, recent advancements in the development of more environmentfriendly technologies, which at least eliminate some of the emissions to the atmosphere and increase efficiency, make it possible to replace older capacities with the newer technology at reasonable costs, hence maintaining certain balance during this transition period. However, we shall clearly state that average coal-fired power plants are the dirtiest power generating facilities accounting for 650–1200 g of CO2 per generated kWh [9,10]. This significant range is mostly due to two reasons: 1. There are several technologies in burning coal and converting its chemical energy into thermal energy, which differ in the physical parameters of the process like temperature and pressure, and therefore in different levels of efficiency, which finally result in certain decrease of emissions per kWh; and 2. Even though there are efficient technologies to eliminate some of the emissions, they are expensive to install and operate, so that it results in higher price of electricity for the final consumer. For this reason, some companies either do not install all the required systems or shut them
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down during operation to decrease maintenance costs. This happens mostly in the regions with limited or more liberalized regulation. Contrary to coal, which always incorporates multiple volatiles and hazardous components (for more details refer to Chapter 2), burning natural gas, which consists mostly of methane, is much cleaner in terms of CO2 equivalent emissions with the levels almost twice lower than those for coal per kWh generated. Still, being relatively dirtier than most of the renewable energy sources, natural gas fired power generation can bring two major benefits: 1. Decrease the levels of emissions through cleaner combustion process and much higher efficiency of the thermal cycle with more than 62% for modern combined cycle configuration; and 2. Provide high capacity base load, which can also serve more flexibly in increasing and decreasing its output if required by the grid, which is extremely beneficial in the current power mix and may justify the higher cost of technology. It is still debatable whether or not we shall shut down all thermal power plants. Despite obvious negative impact to the environment, the amount of energy these plants deliver cannot be replaced in a moment, and they would still be with us in the next decades, either newly built, upgraded or still operated. According to this logic, we shall first draw our focus to the technological development of a steam power plant and go through the basics of the cycles and their optimization up to the modern level. This idea is twofold: first, we will see the possible areas of improvement to increase the level of friendliness of the thermal power, and secondly, we will understand the foundations of this established technology as it became fundamental for other types of power plants, including emerging sustainable technologies. Next, we will outline key equipment areas and discuss in much detail overall plant configuration. The same approach will be applied to both steam and gas turbine driven power plants. With the short overview of the systems, we will get the idea of the common features of major types of plant, allowing for better understanding of the power generation business, as well as the EPC and OEM business approach. The logic here is obvious: due to the historic development, thermal power turns out to be most established, therefore providing a knowledge base for other power generating facilities, most of which have a lot in common and employ thermal energy conversion. This way, we may put much of the effort to understand
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this technology and further refer back to this discussion while investigating other power generating approaches.
7.2. Steam power plant As one of the most historically established types of power plant, a steam power plant provides a reliable energy conversion technology. It is based on the Rankine thermodynamic cycle (for more details one can refer to any of the engineering books, for instance, [11–13] and many others). Its gradual optimization allowed coming up with the relatively efficient water and steam cycle supported by an established technology of the major equipment. Given that, we focus on the concept of a steam power plant in detail.
7.2.1 Steam turbine cycle Consider a simplified cycle of a steam turbine unit as shown schematically in Fig. 7.1. To start with, fossil fuel is fed to the special equipment called boiler where it burns. To ensure that the combustion process goes continuously, it requires constant supply of oxygen from the air, which is taken from the ambient environment and which can also be considered as a resource. The chemical reaction of fuel combustion is exothermic and therefore releases heat and is accompanied by various combustion products — a mix of different gases and particles (that are released back to the environment as
Figure 7.1 Simplified cycle of a steam turbine unit. Boiler burns fuel and generates steam that is fed to the steam turbine (ST), which rotates electric generator (Gen). Cold steam goes to the condenser and turns back into water, which is pumped back to the boiler to close the cycle.
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flue gases). These hot gases possess enough energy, which is transferred to the water flowing inside the tubes called heat exchangers located within the boiler. This water evaporates, so that the pressure and temperature of the steam go up. The superheated steam is fed to the inlet of the machine called steam turbine (ST). To ensure that the steam flows through and does not condense inside the turbine, the special vessel called condenser is located at the exhaust of the steam turbine. This device is kept under a pressure much lower than the atmospheric to make the steam go through the turbine straight into it and condense back into water. This so-called vacuum of the condenser requires constant supply of cold water to cool down the coming steam and maintain low pressure. While flowing through the steam turbine to condenser, the steam cools down and expands. It gets kinetic energy during the flow and expansion, and hits the blades, which are mounted on the rotor, so that the rotor starts to spin. Therefore, the kinetic energy of the steam is converted into the mechanical energy of the rotating shaft. The rotor of the turbine is coupled to the rotor of the electrical generator (Gen): in its windings the electrical current is generated and then delivered to the grid. On the other hand, exhaust steam releases its heat to the environment inside the condenser and therefore condenses back into water. Finally, this water is pressurized and fed with the pump back to the boiler where it receives heat from the burning fuel. Major heat losses of a thermal power plant occur inside the condenser when the heat of the exhaust steam is taken away by cold water leading to around 50% of energy waste. Another part of energy is used for auxiliary needs of the process to provide its functionality. These include powering of special equipment like air ventilators, fuel supply, and water pumps for continuous operation and may result in 3–5% of energy losses. Certain part of energy cannot be transferred to the water inside the heat exchangers due to technological reasons and hence this part leaves the boiler through the exhaust stack into the atmosphere. These products are called exhaust gases and normally have the temperature range of 100–160◦ C. The amount of heat wasted to the atmosphere can be around 5–15% of the initial chemical energy of fuel and depends on the technology, type of fuel, and plant operating conditions. The temperature of the exhaust gases, as well as their chemical composition, is limited by the regulation to minimize environmental impact.
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What is finally left is called the net capacity of the power plant. This amount of power can be then delivered to the grid and transferred to the consumer. The net efficiency of a thermal power plant is the ratio of the energy supplied to the grid within a certain period of time and the heat energy of the burnt fuel. The average efficiency of a steam power plant can be up to 40% with the losses due to the energy transformation process and the present state of the technology. In other words, within the traditional steam power plant, we can use roughly 40% of energy stored in fossil fuel.
7.2.2 Heat balance diagram and its optimization towards higher efficiency A simplified steam power plant schematically shown in Fig. 7.1 employs a steam turbine, which generates only electricity with all the steam from the turbine going into the condenser. Such a power plant is therefore called a condensing power plant. In some cases, customers require not only electricity but also steam or hot water for district heating or industrial needs. A power plant that supplies not only electricity but also heat is called cogenerating. Cogeneration normally increases overall plant efficiency as it uses “waste” heat from the process. In a cogenerating power plant, as shown schematically in Fig. 7.2, some steam is extracted from the steam turbine and guided to the consumer supplying heat for residential and/or industrial needs. This allows for less steam in a condenser and hence less waste of heat in the process. Then the cold water from the consumers is pumped back to the cycle.
Figure 7.2 Simplified steam power plant with cogeneration. Schematic diagram of the process with cogeneration: steam is extracted from the steam turbine and delivered to some process (district heating, industrial use, etc.). Cold water is then delivered to the cycle.
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Figure 7.3 Simplified steam power plan with a two-pressure steam turbine. Schematic diagram of the process employing a two pressure steam turbine to increase efficiency.
This way, both types of steam power plant would have the following major equipment as shown in Fig. 7.2: • steam generator, or boiler, which consumes water, air, and fuel to produce high pressure and high temperature steam; • steam turbine with electrical generator; • condenser; • feedwater pump. The above presented simplified process still has quite low efficiency due to multiple losses of energy. For instance, the hot high pressure steam goes through the steam turbine and loses some of its energy, but can remain hot enough to be reused and therefore guided to another steam turbine section called the low pressure steam turbine (LP ST) (see the schematic in Fig. 7.3). Such a configuration would allow to increase the plant efficiency by utilizing as much energy of the steam as possible. On the other hand, the amount of energy from the burning fuel is high enough to not only evaporate water and heat up the initial steam for the high pressure steam turbine (HP ST), but also reheat the used steam and increase its temperature and pressure. This means the hot combustion products go through the first heat exchanger and evaporate the water, and then, instead of being released (wasted) into the atmosphere, they are guided through the second heat exchanger to reheat the steam. Fig. 7.4 shows the schematics of the respective steam reheat cycle: the cold steam from the HP ST is guided to another heat exchanger in the boiler called reheater where its temperature and pressure increase. Finally, this reheated steam is delivered to the LP ST and further to condenser. Such a cycle allows for better energy utilization and hence increased efficiency of the plant.
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Figure 7.4 Simplified steam power plan with a two-pressure boiler with reheater. Schematic diagram of the process employing a two pressure boiler with reheater.
Assume now that we require to further increase the efficiency of the plant. One of the ways to do so is to optimize the burning process inside the boiler and provide additional heat exchangers to ensure we get the most out of the combustion process. This can possibly be delivered by increasing the temperature of the combustion products through better mixing the air and fuel, gas flow path optimization, furnace geometry correction, etc. The overall approach is only limited with two major factors: 1. Physical properties of the materials, which are capable to withstand very high temperatures and pressures, and 2. Economical factors or the cost of such technology to ensure that the generated net power is cheap enough. By further increasing the pressure through the boiler engineering and optimization, we would produce steam of better quality and excess heat. This would allow us to go from two pressure (high and low) to three pressure cycle shown in Fig. 7.5. Here the steam with the highest energy is first admitted to the HP steam turbine and, after flowing through this section, it is delivered back to the boiler for reheating. After its temperature and pressure are recovered from the energy of the boiler, it is further fed to the IP steam turbine section and then further to LP. The water that comes from the condenser back to the boiler is cold enough (due to the condensing conditions) and would require significant energy from the chemical reaction of the burning fuel to be evaporated. In order to save some energy and optimize the process, this water can be heated up and come to boiler already preheated. This is normally done through the special heat exchangers called feedwater preheaters or economizers. They use some of the hot steam extracted from the steam turbine and guided to this heater as schematically shown in Fig. 7.6. Depending on
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Figure 7.5 Simplified steam power plan with a three-pressure steam cycle. The threepressure cycle allows employing a more efficient and larger steam turbine with three cylinders: high pressure (HP), intermediate pressure (IP) and low pressure (LP).
Figure 7.6 Simplified steam power plant with preheater. Employing a feedwater preheater within the schematic cycle of a steam power plant allows reusing waste heat to warm up water, so that it would require less energy of the fuel to evaporate it.
the configuration of the plant, there may be several preheaters to ensure that most of the energy is captured within the process and not dissipated as waste. After the preheater, colder steam is guided back to the process, for instance, into the condenser. Despite proper insulation and the closed cycle, there are still some inevitable losses of steam and water inside the process. This can take place due to multiple leakages in the steam turbine, drainage within the pumps, tanks, and pipes, emergency situations or even quality issues which would require maintenance. To maintain a proper amount of water inside the cycle, it has to be properly prepared and constantly added to the process. This
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is normally done within the water treatment plant where water undergoes thorough cleaning to ensure that it does not damage the heat exchangers, pipes, steam turbine, and other equipment. On the other hand, the condensed water may contain dissolved gases, especially oxygen, due to the vacuum inside this vessel as it would suck in not only steam from the steam turbine but also air from the outside, mixing it with water. This would require special equipment inside the cycle to deaerate the water and make sure it does not cause corrosion in the system. This equipment is called deaerator and serves not only for removing oxygen (and other gases) from the water but also as a storage for boiler feedwater. It means that extra water coming from the treatment plant goes first to the deaerator tank as shown in Fig. 7.7. Such a scheme would require a second condensate pump to guide water from the condenser to the deaerator.
Figure 7.7 Simplified steam power plant with deaerator. Use of deaerator within the cycle of a steam power plant allows to remove oxygen that is inevitably sucked into the cycle and decrease corrosion of the system.
In spite of the historically developed optimization solutions, the efficiency of a fossil-fired condensing steam power plant does not normally exceed 40% and is subject to the technological process as shown above. However, it can be also influenced by the load regime of the plant or conditions of the equipment. This way, the optimization of the steam power plant requires not only efficient and modern equipment properly integrated into the plant, but mostly a well-designed and optimized heat balance. Fig. 7.8 shows a simplified diagram of a real three-pressure condensing steam power plant. The preparation and optimization of such a cycle shall be done by an application engineer within the special software on an iterative basis.
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Figure 7.8 Simplified steam power plant with optimized cycle. A modern steam power plant with optimized cycle employs all of the discussed solutions: several pressure levels, cascades of feedwater preheaters, deaerator, etc.
Depending on the objective of the power plant and its required capacity (electrical and thermal), the application engineer would choose several pressures, type of boiler and auxiliary equipment (heat exchangers, pumps, condenser and cooling, etc.), and optimize the flows of water and steam. The above mentioned diagram, together with the values of temperature, mass flows, and pressure, defines the overall heat balance of a power plant. Based on these calculations, the steam turbine application engineer would make the turbine configuration, leading to the fact that most of the steam turbines are adjusted or even specially designed for each project.
7.2.3 Support systems and equipment areas The heat balance defines the configuration and the parameters of the water and steam cycle, performances and the major equipment with some auxiliaries. However, it does not give a complete picture of the overall plant configuration, equipment arrangement, and supporting systems to allow for proper functionality.
7.2.3.1 General process description and equipment areas We now summarize the overall process of a fossil fuel fired steam power plant shown in Fig. 7.9. The process starts with the fuel supply from fuel handling system where it is properly prepared to ensure that the combustion process delivers most of the energy. The fuel is fed to the furnace of a boiler
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Figure 7.9 Simplified blocks of a steam power plant process. Simplified process to illustrate major steps within a steam turbine power plant (many steps and equipment units are omitted intentionally).
where it is ignited and burnt. The combustion gases follow the hot gas path of a steam generator (colored in yellow (light gray in print version)) transferring their energy to steam, water, and air through the system of heat exchangers. As these gases contain combustion products, they go through the flue gas treatment equipment before being released to the atmosphere through the chimney. To maintain the burning process of the fuel, air is supplied to the furnace through the so called air path. To increase plant efficiency, the air is preheated with the hot flue gas inside the heat exchanger called air preheater and guided to the boiler by the fan. In order to deliver steam and to minimize the damages inside the equipment, water shall be prepared within the water treatment station and then preheated with the hot flue gases inside the water preheater to optimize the efficiency of the plant. A number of heat exchangers inside the boiler transfer heat to the water to evaporate it and then heat the steam to increase its energy and decrease water content inside to supply hot steam to the steam turbine. The exhaust steam then goes to the condenser where it turned back into water. To ensure that the condenser is operating at the required capacity, the heat rejection loop is employed. The gathered condensate is then pumped back to the boiler through the feedwater preheaters. The steam turbine is powering the electrical generator. The electricity is evacuated from the generator terminals through the main transformer into the grid. The overall control and operation of the plant is ensured via
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Figure 7.10 Equipment areas of a steam power plant. Simplified representation of the equipment areas of a fossil-fired steam power plant.
the control system. The description of the process, together with the heat balance diagram, gives an overview of how a power plant looks like. The described process of a fossil-fired steam power plant allows for breaking down the facility into a number of parent systems. These equipment areas, as shown in Fig. 7.10, include: • Boiler island, which supplies hot pressurized steam. Subject to definition, boiler island can consolidate all equipment around the boiler related to the steam production cycle, including fuel treatment and waste handling, i.e., everything required to transfer chemical energy of the fuel into the energy of hot pressurized steam, considering also the waste and its proper treatment. • Turbine island that transforms the heat of the steam into the mechanical energy of the rotating shaft of the steam turbine coupled to the generator. • Mechanical balance of plant, including all supporting systems like heat rejection, water preparation piping, etc. • Electrical balance of plant, including the equipment that is responsible for transferring the power from the generator terminals to the grid, as well as for supplying power for all systems of the plant. This simplified breakdown of the facility into the major areas or subsystems is helpful for two reasons. First, it generally corresponds to the scope of supply or the major manufacturers of equipment or companies that do engineering, procurement and construction (EPC) of a power plant. For example, there are companies that supply equipment only to the boiler island, or there are those, which act only in the steam turbine or generator market. Second, it gives the basis for comparing steam power plants to other power generating facilities and defining similarities not only in the smaller pieces of equipment but also in the major areas and systems. Finally, it allows for modularization of the power plant, which is mostly applicable for larger utilities.
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7.2.3.2 Modular configuration of a steam power plant The simplified process presented in Fig. 7.9 assumes one boiler feeding steam to one turbine. In many cases, however, the required capacity of a power plant cannot be fulfilled by only one boiler or only one steam turbine. This means that the overall configuration of both the boiler and power island can vary. Here several options are available: • Several similar blocks or units, each of them having their own boiler island and turbine island. Such an option would require duplication of most of the systems, complicated routing of cables and pipes, and larger footprint, which leads to a higher cost of electricity generated at this plant. On the other hand, such a configuration may give certain flexibility, but practice shows this is usually not feasible economically. • One boiler supplying steam for several steam turbines located in one power island, and each turbine employing its own electrical generator and steam condenser. Such a structure would optimize the boiler island equipment (e.g., single fuel and emissions treatment system) but would lead to complicated steam piping and sophisticated control system. However, from an economic point of view, this configuration can be favorable. • Several boilers supplying steam to the main steam collector, from where it is distributed to a number of steam turbines. This configuration is applicable in large power plants or, for example, in case of adding new capacities to the existing plant. Depending on the output equipment arrangement, this can be a preferred option compared to the previous two.
7.3. Gas turbine powered energy The development of gas turbines primarily for the aircraft propulsion has raised a question of their implementation for stationary power generation in a thermal power plant. Gas turbines have proved to be more flexible in operation and therefore could be used within a broader range of grid applications, i.e., both for constant load and when extra load is quickly required by the market. On top, combination of both technologies under one umbrella has enabled significantly increasing efficiency of a thermal power plant.
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Figure 7.11 Simplified cycle of a gas turbine unit. Fuel is fed to the combustion system, and the air is compressed by compressor.
7.3.1 Gas turbine simple cycle We will first focus on a simplified gas turbine cycle, which is schematically represented in Fig. 7.11. The idea of the process is based on the combustion of gaseous or liquid fuel and is fairly simple. The ambient air is taken from the atmosphere at the inlet of the compressor section of the turbine. The rotating blades of the compressor suck this air and compress it to a much higher pressure while pushing it through in the axial direction, so that its temperature increases. Hot pressurized air flows to the combustion chamber where it is mixed with the natural gas and burnt under constant pressure. The temperature of the combustion products, which are a mixture of various gases, rises up to 1200◦ C. Under this temperature and constant pressure, the hot combustion products are released to the turbine section where they tend to expand to the atmospheric pressure and therefore acquire kinetic energy. The energy of the hot exhaust gases is used to spin the shaft of the turbine, which is coupled to the shaft of the generator. After loosing some of their energy, the exhaust gases are finally released to the atmosphere. The shaft of the turbine section is connected to the shaft of the compressor section, and the rotation of the former will spin compressor and maintain the constant air flow and its compression to maintain combustion. However, to start the process, there should be an external rotating machine to start the spinning of the shaft and start the flow of the air to the combustion. Then, part of the energy of the rotation that comes from the energy of the hot gases will be used for rotation and air compression. Remark 7.2. A simple gas turbine cycle is a steady repetition of the Brayton cycle. In this simple cycle, combustion and exhaustion occur at constant pressure and compression, and expansion occurs continuously. This means
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that a gas turbine generates power continuously rather than in pulses like in an internal combustion engine. This process, usually called an open or simple cycle of a gas turbine, uses hot gases as the main working medium. The environment, or simply the atmosphere, is considered as an infinite supply of cold ambient air fed to the compressor, while the energy is supplied to the system with the fuel. It is the chemical energy of the fuel that is transferred first into the energy of the hot pressurized gases (combustion products), and then into the mechanical energy of the rotating shaft. Part of this energy is used to rotate the compressor and therefore suck the air and compress it before delivering into the combustion chamber. As the gas turbine cycle is exposed to the ambient conditions, its performance and flexibility characteristics depend on these conditions. Therefore the cycle is usually designed for particular conditions, and the performance calculations are conducted at both standard or ISO conditions and site average conditions. Similarly to a steam turbine, gas turbine cycle has quite low efficiency in transferring the chemical energy of the fossil fuel into electricity compared to a theoretical limit. This is mostly due to multiple points in the cycles, where energy is wasted or dissipated within the environment. Concerning the efficiency of a gas turbine cycle, there are several sources of energy losses: • Friction within the rotating equipment (compressor and turbine shaft); • Exhaust gases of the gas turbine, which are released to the atmosphere. They are still hot enough, however, the energy is simply wasted. A thermal power plant based on such a gas turbine cycle is called a simple cycle power plant and has efficiency around 30%. Among the approaches to increase efficiency of this type of power plant, one of the simplest and most evident ways is to recover the heat from the hot gas turbine exhaust and utilize it for other purposes. Here two major options are possible: 1. Use special heat recovery equipment to boil water and produce steam for industrial utility purposes. A power plant, which produces both electricity and heat, is called a cogeneration plant (Cogen) or combined heat and power plant (CHP). 2. Implement a heat recovery steam generator to supply steam to the steam turbine. Combining gas and steam turbine cycles together in one power plant allows for increasing the efficiency considerably. Such a power plant is called combined cycle power plant (CCPP).
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Figure 7.12 Simplified diagram of a combined cycle power plant. Hot exhaust gases from the gas turbine transfer their waste energy to the water within the heat recovery steam generator, which acts as a boiler. Stem is then fed to the steam turbine. Both turbines can transfer their mechanical energy into electricity.
7.3.2 Combined cycle as a way to increased efficiency In a combined cycle power plant, two thermodynamic cycles are employed: 1. Brayton cycle for a gas turbine, the same as for a simple cycle power plant. This cycle is also known as topping cycle. 2. Rankine cycle of a steam turbine, where the energy for evaporation is taken not from the burning fuel but from the hot exhaust gases of a gas turbine. The steam turbine cycle is called bottoming cycle. The implementation of this cooperation is fairly straightforward. Consider the simplified diagram of the combined cycle power plant schematically represented in Fig. 7.12. The gas turbine follows the simple cycle schematic with the only difference: the exhaust energy is fed to the heat recovery equipment or a special type of boiler called heat recovery steam generator, or HRSG. This unit transfers the energy of the hot gases to the water that circulates through the tubes of the heat exchangers and evaporates. The steam is admitted to the steam turbine where it expands and is released to the condenser. The steam cycle is similar to that in a steam turbine power plant, thus being limited by the exhaust parameters of the gas turbine, namely temperature and flow. For instance, if the exhaust gases are hot enough (especially for bigger units), the steam turbine performance can be optimized in the same way as for a steam power plant, for instance: • High pressure steam from the HRSG can be fed to the HP steam turbine section and, after being released, can be brought back to the boiler as cold reheat;
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This cold reheat can be further heated in a boiler’s reheater section and, with recovered temperature, be fed to the IP section of the steam turbine; • Finally, released from IP ST section, steam can be fed to the LP ST section. However, even the largest available gas turbines still have a limited exhaust gas flow and temperature, which is definitely much lower than the temperature inside the conventional fossil-fired boiler. Therefore, in order to optimize the output of a combined cycle power plant, multiple approaches are used: 1. The above discussed combined cycle power plant with one gas turbine coupled to the HRSG and supplying steam to one steam turbine. Each turbine has its own generator (two in total in this case) coupled to a respective turbine shaft. Therefore such a configuration is called multi-shaft and shown in Fig. 7.13(A). 2. If we are not able to increase the output of a single gas turbine, we are still left with the option of using multiple gas turbines with their own •
Figure 7.13 Combined cycle power plant configurations. Multi-shaft configuration (A); two gas turbines with their own HRSGs feeding steam to a single steam turbine (B); single-shaft configuration (C) with gas and steam turbines coupled to a single generator.
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heat recovery steam generators. The steam is then collected from all of them and supplied to one large steam turbine to increase its output and efficiency. In practice, usually two or three gas turbines are combined with one steam turbine, as shown in Fig. 7.13(B). In some literature, such configurations are called, for example, “two on one” or “three on one” meaning that there are two or three gas turbines operating with one steam turbine, respectively. 3. If we want to increase the output of the plant, we can also use only one generator, which is coupled to both gas turbine and steam turbine. Such a configuration is called single-shaft and is represented in Fig. 7.13(C). It is normally implemented for higher capacities and base load operation at top levels of efficiency. All of the above mentioned configurations of the combined cycle power plant can be implemented in either a single plant or in modular configuration. In general, configuration of a combined cycle power plant depends on many factors. To start with, the major defining factor an investor considers is the MW capacity of the plant and his capability to sell energy to a consumer. The capacity here covers not only electrical megawatts but also thermal power in the form of steam or hot water. The latter can bring much more profit compared to electricity, for instance, when the power plant produces high quality steam of required properties for industrial site, e.g., for refinery or chemical plant. Another example may be a cogeneration power plant where the major output parameters of the station are electricity and hot water for district heating needs. The electrical efficiency of a combined cycle power plant (without considering possible heat production) is evidently higher that of a simple cycle or conventional coal-fired power plant. The incoming chemical energy of the fuel is used for the topping cycle, however, the major waste of the simple cycle — the exhaust — is reused in the process. Remark 7.3. The term combined cycle is used to define a type of a power plant. In some cases it is appropriate to name it a gas turbine combined cycle (GTCC) power plant, specially pointing out that this power plant is based on a gas turbine cycle combined with steam turbine. Also the name combined cycle gas turbine (CCGT) plant is widely used. For simplicity, however, it is recommended to use the term combined cycle power plant, or CCPP, as it immediately assumes gas and steam turbines.
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Figure 7.14 Equipment areas of a combined cycle power plant. Simplified representation of the major equipment areas of a CCPP includes turbine island with a topping cycle (gas turbine-generator) and a bottoming cycle (HRSG and steam turbine with a possible generator) supported by the BOP systems.
7.3.3 Major components and systems The schematic process diagrams and heat balances discussed above define the key parameters of the gas and steam and do not fully specify all necessary equipment, systems and site layout. Similarly to the steam power plant, every simple and combined cycle power plant can be segmented into several major systems, with the heart being always gas turbines, steam turbines, and generators. A gas turbine driven power plant can be broken into the following major equipment areas (Fig. 7.14): • Turbine island or power island that transforms the chemical energy of the fuel into the mechanical energy of the rotating shaft of the turbines coupled to the generators. It normally includes: • Topping cycle that incorporates a gas turbine with its generator and dedicated systems; • Bottoming cycle, which includes heat recovery steam generator, steam turbine with its generator (if applicable), condenser, essential valves and supporting systems (applicable only for combined cycle); • Mechanical balance of plant, including all supporting systems like heat rejection, water preparation piping, etc.; • Electrical balance of plant including the equipment that is responsible for transferring the power from the generator terminals to the grid, as well supplying power for all systems of the plant. This breakdown allows for the customer to clearly see the potential scope of supply and define the configuration of a power plant. On top of this, such a modular approach allows customers to implement upgrades and power plant extension projects in a simpler way. The typical configurations can be derived one from the other and include:
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1. Mechanical drive gas turbine, when a unit is used to power other machinery, for instance, a compressor for a gas pipeline or LNG facility; 2. Simple-cycle power plant with a gas turbine and generator operating in an open cycle; 3. Simple cycle with cogeneration mode, when the power island consists of a gas turbine, generator and heat recovery equipment; 4. Combined cycle power plant. Depending on the engineering approach and historical trends, the breakdown into equipment areas may differ. Various market players prefer to have their own itemization, which does not necessarily coincide with the given here. However, in most cases the approach is similar to what has been discussed here and can be easily adopted to the market conditions.
References [1] World Coal Association (WCA). Coal and electricity generation, vol. 3. World Coal Association; 2012. [2] World Energy Council. World energy resources 2016. World Energy Council. ISBN 9780946121588, 2016. [3] GE. Powering the world. Steam power systems product catalog. GE; 2015. [4] IEA. Energy supply security. Emergency response of IEA countries. International Energy Agency 2014. [5] Energy Information Administration (EIA). International energy outlook 2016. US Energy Information Administration; 2016. [6] IEA. Key world energy statistics 2018. International Energy Agency; 2018. [7] IEA. Key world energy statistics 2016. International Energy Agency 2016. [8] Jacobson MZ, Delucchi MA. Providing all global energy with wind, water, and solar power, part I: technologies, energy resources, quantities and areas of infrastructure, and materials. Energy Policy 2011:1154–69. https://doi.org/10.1016/j.enpol.2010.11.040. [9] Teske S, European Renewable Energy Council, Greenpeace International. Energy [r]evolution. A sustainable world energy outlook 2015. Greenpeace International; 2015. [10] WCA. Coal facts 2014. World Coal Association; 2014. [11] Gupta M. Power plant engineering. PHI Learning. ISBN 9788120346123, 2012. [12] Kehlhofer R, Rukes B, Hannemann F, Stirnimann F. Combined-cycle gas and steam turbine power plants. [13] Sarkar D. Thermal power plant: design and operation. Elsevier Science; 2015. 612 pp.
Concept of a thermal power plant Contents 7.1. Introduction 7.1.1 Thermal power generation within global power mix 7.1.2 Thermal power as a transition technology 7.2. Steam power plant 7.2.1 Steam turbine cycle 7.2.2 Heat balance diagram and its optimization towards higher efficiency 7.2.3 Support systems and equipment areas 7.3. Gas turbine powered energy 7.3.1 Gas turbine simple cycle 7.3.2 Combined cycle as a way to increased efficiency 7.3.3 Major components and systems References
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7.1. Introduction Traditional thermal power is based on a historically established and proven technology of burning fossil fuels — solid, liquid, or gaseous, when they release their chemical energy in the form of heat and radiation, justifying the name of the technology. Properties of the fuels, their availability, and cost in many cases define the approach that is used in a conventional thermal power station. Various plants have been built throughout the world, usually powered by two major kinds of machines: steam turbines and gas turbines. Recently, the combination of both has proven to be among the most efficient in delivering power out of natural gas. Moreover, the overall plant configuration has been subject to multiple modifications in the race towards higher efficiency through eliminating energy waste in the process and equipment. The overall arrangement of plant’s major systems evolved in the direction of modularization and following the same approach for all types of power plant: steam and/or gas turbine driven. All of them claim their own ways to design with some similarities, though still following the process of burning fuel to deliver heat. Sustainable Power Generation Copyright © 2019 Elsevier Inc. https://doi.org/10.1016/B978-0-12-817012-0.00018-9 All rights reserved.
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7.1.1 Thermal power generation within global power mix Thermal power generation from fossil fuels, namely coal, natural gas, and oil, accounts for more than 65% of the global electricity production [1,2]. Coal power is the most mature and common power generation technology in the world. With more than 2550 GW of installed capacity, it provides roughly 38% of world electricity generation, which proves the fact that it has served as a reliable, cost-effective, and durable baseload power source to utility scale networks [1,3–6]. Coal-fired power plants account for 8.5–10 trillion kWh of electricity production, which is expected to increase in the next decades [5]. The global coal power generation mix is dominated by Asia and especially China, as well as Africa and North America. China and India alone account for 69–75% of the projected worldwide increase in coal-fired generation, while the OECD nations continue to reduce their reliance on coal-fired electricity generation [3,5]. While natural gas-fired power generating technology accounted for roughly 21% of the global energy mix in 2016 [7], it is expected to grow for the next decades as well, however, with lower rate. In the United States, natural gas-fired generation is encouraged by low prices and favorable greenhouse gas emission characteristics compared to coal-power plants, however, there are some trends of revitalizing the coal industry and coal-fire generation. The fastest growing regions for natural gas-fired efficient technologies are Mexico, the Middle East, and some of the Asian countries, especially Japan, which imports LNG for these purposes. Remark 7.1. In some countries, like Russia, many coal-fired power plants have been refurbished from solid fuel combustion to employ natural gas as a primary fuel and crude oil as a backup fuel. These power plants have proved to emit lower levels of CO2 , but provide the same or even higher efficiency range compared to those operating with coal. These plants shall not be mixed with modern gas turbine based combined cycle technology which can have much higher efficiency. Fuel oil occupies roughly 4% of the global electricity generation mix and is mostly employed in the areas where oil is cheap enough to be burnt. Its use is supposed to steadily decline in the next decades, also due to the introduction of new energy policies.
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7.1.2 Thermal power as a transition technology Thermal power plants generate electricity with the economically feasible and historically established technology, which has been developed during the years of thermal cycle optimization, material research, and technology development. Still, these facilities burn fossil fuels, which has to deal with waste and releases large amounts of GHG. Environmental concerns notwithstanding, fossil-fired power generation would continue to play a significant and perhaps a dominating role, however, may not be considered as a sustainable technology in the long run. Still, it is impossible to replace this large share of base load generation in a reasonably short time: even though it may be technically feasible to replace all existing fossil-fired units with sustainable energy sources within few decades, such a conversion would almost certainly take longer due to the difficulty in implementing all necessary policies [8]. Since it may be much easier and cheaper to provide at least all newly introduced energy for the next 20 years from renewable sources, it would still stay as a favorable forecast rather than the real picture due to multiple factors, which shall be discussed in further detail in Part 7. Thus, thermal power is supposed to play a crucial role in the world power generation mix and would bridge the gap between the current power generation system and the future sustainable energy system. Moreover, recent advancements in the development of more environmentfriendly technologies, which at least eliminate some of the emissions to the atmosphere and increase efficiency, make it possible to replace older capacities with the newer technology at reasonable costs, hence maintaining certain balance during this transition period. However, we shall clearly state that average coal-fired power plants are the dirtiest power generating facilities accounting for 650–1200 g of CO2 per generated kWh [9,10]. This significant range is mostly due to two reasons: 1. There are several technologies in burning coal and converting its chemical energy into thermal energy, which differ in the physical parameters of the process like temperature and pressure, and therefore in different levels of efficiency, which finally result in certain decrease of emissions per kWh; and 2. Even though there are efficient technologies to eliminate some of the emissions, they are expensive to install and operate, so that it results in higher price of electricity for the final consumer. For this reason, some companies either do not install all the required systems or shut them
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down during operation to decrease maintenance costs. This happens mostly in the regions with limited or more liberalized regulation. Contrary to coal, which always incorporates multiple volatiles and hazardous components (for more details refer to Chapter 2), burning natural gas, which consists mostly of methane, is much cleaner in terms of CO2 equivalent emissions with the levels almost twice lower than those for coal per kWh generated. Still, being relatively dirtier than most of the renewable energy sources, natural gas fired power generation can bring two major benefits: 1. Decrease the levels of emissions through cleaner combustion process and much higher efficiency of the thermal cycle with more than 62% for modern combined cycle configuration; and 2. Provide high capacity base load, which can also serve more flexibly in increasing and decreasing its output if required by the grid, which is extremely beneficial in the current power mix and may justify the higher cost of technology. It is still debatable whether or not we shall shut down all thermal power plants. Despite obvious negative impact to the environment, the amount of energy these plants deliver cannot be replaced in a moment, and they would still be with us in the next decades, either newly built, upgraded or still operated. According to this logic, we shall first draw our focus to the technological development of a steam power plant and go through the basics of the cycles and their optimization up to the modern level. This idea is twofold: first, we will see the possible areas of improvement to increase the level of friendliness of the thermal power, and secondly, we will understand the foundations of this established technology as it became fundamental for other types of power plants, including emerging sustainable technologies. Next, we will outline key equipment areas and discuss in much detail overall plant configuration. The same approach will be applied to both steam and gas turbine driven power plants. With the short overview of the systems, we will get the idea of the common features of major types of plant, allowing for better understanding of the power generation business, as well as the EPC and OEM business approach. The logic here is obvious: due to the historic development, thermal power turns out to be most established, therefore providing a knowledge base for other power generating facilities, most of which have a lot in common and employ thermal energy conversion. This way, we may put much of the effort to understand
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this technology and further refer back to this discussion while investigating other power generating approaches.
7.2. Steam power plant As one of the most historically established types of power plant, a steam power plant provides a reliable energy conversion technology. It is based on the Rankine thermodynamic cycle (for more details one can refer to any of the engineering books, for instance, [11–13] and many others). Its gradual optimization allowed coming up with the relatively efficient water and steam cycle supported by an established technology of the major equipment. Given that, we focus on the concept of a steam power plant in detail.
7.2.1 Steam turbine cycle Consider a simplified cycle of a steam turbine unit as shown schematically in Fig. 7.1. To start with, fossil fuel is fed to the special equipment called boiler where it burns. To ensure that the combustion process goes continuously, it requires constant supply of oxygen from the air, which is taken from the ambient environment and which can also be considered as a resource. The chemical reaction of fuel combustion is exothermic and therefore releases heat and is accompanied by various combustion products — a mix of different gases and particles (that are released back to the environment as
Figure 7.1 Simplified cycle of a steam turbine unit. Boiler burns fuel and generates steam that is fed to the steam turbine (ST), which rotates electric generator (Gen). Cold steam goes to the condenser and turns back into water, which is pumped back to the boiler to close the cycle.
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flue gases). These hot gases possess enough energy, which is transferred to the water flowing inside the tubes called heat exchangers located within the boiler. This water evaporates, so that the pressure and temperature of the steam go up. The superheated steam is fed to the inlet of the machine called steam turbine (ST). To ensure that the steam flows through and does not condense inside the turbine, the special vessel called condenser is located at the exhaust of the steam turbine. This device is kept under a pressure much lower than the atmospheric to make the steam go through the turbine straight into it and condense back into water. This so-called vacuum of the condenser requires constant supply of cold water to cool down the coming steam and maintain low pressure. While flowing through the steam turbine to condenser, the steam cools down and expands. It gets kinetic energy during the flow and expansion, and hits the blades, which are mounted on the rotor, so that the rotor starts to spin. Therefore, the kinetic energy of the steam is converted into the mechanical energy of the rotating shaft. The rotor of the turbine is coupled to the rotor of the electrical generator (Gen): in its windings the electrical current is generated and then delivered to the grid. On the other hand, exhaust steam releases its heat to the environment inside the condenser and therefore condenses back into water. Finally, this water is pressurized and fed with the pump back to the boiler where it receives heat from the burning fuel. Major heat losses of a thermal power plant occur inside the condenser when the heat of the exhaust steam is taken away by cold water leading to around 50% of energy waste. Another part of energy is used for auxiliary needs of the process to provide its functionality. These include powering of special equipment like air ventilators, fuel supply, and water pumps for continuous operation and may result in 3–5% of energy losses. Certain part of energy cannot be transferred to the water inside the heat exchangers due to technological reasons and hence this part leaves the boiler through the exhaust stack into the atmosphere. These products are called exhaust gases and normally have the temperature range of 100–160◦ C. The amount of heat wasted to the atmosphere can be around 5–15% of the initial chemical energy of fuel and depends on the technology, type of fuel, and plant operating conditions. The temperature of the exhaust gases, as well as their chemical composition, is limited by the regulation to minimize environmental impact.
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What is finally left is called the net capacity of the power plant. This amount of power can be then delivered to the grid and transferred to the consumer. The net efficiency of a thermal power plant is the ratio of the energy supplied to the grid within a certain period of time and the heat energy of the burnt fuel. The average efficiency of a steam power plant can be up to 40% with the losses due to the energy transformation process and the present state of the technology. In other words, within the traditional steam power plant, we can use roughly 40% of energy stored in fossil fuel.
7.2.2 Heat balance diagram and its optimization towards higher efficiency A simplified steam power plant schematically shown in Fig. 7.1 employs a steam turbine, which generates only electricity with all the steam from the turbine going into the condenser. Such a power plant is therefore called a condensing power plant. In some cases, customers require not only electricity but also steam or hot water for district heating or industrial needs. A power plant that supplies not only electricity but also heat is called cogenerating. Cogeneration normally increases overall plant efficiency as it uses “waste” heat from the process. In a cogenerating power plant, as shown schematically in Fig. 7.2, some steam is extracted from the steam turbine and guided to the consumer supplying heat for residential and/or industrial needs. This allows for less steam in a condenser and hence less waste of heat in the process. Then the cold water from the consumers is pumped back to the cycle.
Figure 7.2 Simplified steam power plant with cogeneration. Schematic diagram of the process with cogeneration: steam is extracted from the steam turbine and delivered to some process (district heating, industrial use, etc.). Cold water is then delivered to the cycle.
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Figure 7.3 Simplified steam power plan with a two-pressure steam turbine. Schematic diagram of the process employing a two pressure steam turbine to increase efficiency.
This way, both types of steam power plant would have the following major equipment as shown in Fig. 7.2: • steam generator, or boiler, which consumes water, air, and fuel to produce high pressure and high temperature steam; • steam turbine with electrical generator; • condenser; • feedwater pump. The above presented simplified process still has quite low efficiency due to multiple losses of energy. For instance, the hot high pressure steam goes through the steam turbine and loses some of its energy, but can remain hot enough to be reused and therefore guided to another steam turbine section called the low pressure steam turbine (LP ST) (see the schematic in Fig. 7.3). Such a configuration would allow to increase the plant efficiency by utilizing as much energy of the steam as possible. On the other hand, the amount of energy from the burning fuel is high enough to not only evaporate water and heat up the initial steam for the high pressure steam turbine (HP ST), but also reheat the used steam and increase its temperature and pressure. This means the hot combustion products go through the first heat exchanger and evaporate the water, and then, instead of being released (wasted) into the atmosphere, they are guided through the second heat exchanger to reheat the steam. Fig. 7.4 shows the schematics of the respective steam reheat cycle: the cold steam from the HP ST is guided to another heat exchanger in the boiler called reheater where its temperature and pressure increase. Finally, this reheated steam is delivered to the LP ST and further to condenser. Such a cycle allows for better energy utilization and hence increased efficiency of the plant.
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Figure 7.4 Simplified steam power plan with a two-pressure boiler with reheater. Schematic diagram of the process employing a two pressure boiler with reheater.
Assume now that we require to further increase the efficiency of the plant. One of the ways to do so is to optimize the burning process inside the boiler and provide additional heat exchangers to ensure we get the most out of the combustion process. This can possibly be delivered by increasing the temperature of the combustion products through better mixing the air and fuel, gas flow path optimization, furnace geometry correction, etc. The overall approach is only limited with two major factors: 1. Physical properties of the materials, which are capable to withstand very high temperatures and pressures, and 2. Economical factors or the cost of such technology to ensure that the generated net power is cheap enough. By further increasing the pressure through the boiler engineering and optimization, we would produce steam of better quality and excess heat. This would allow us to go from two pressure (high and low) to three pressure cycle shown in Fig. 7.5. Here the steam with the highest energy is first admitted to the HP steam turbine and, after flowing through this section, it is delivered back to the boiler for reheating. After its temperature and pressure are recovered from the energy of the boiler, it is further fed to the IP steam turbine section and then further to LP. The water that comes from the condenser back to the boiler is cold enough (due to the condensing conditions) and would require significant energy from the chemical reaction of the burning fuel to be evaporated. In order to save some energy and optimize the process, this water can be heated up and come to boiler already preheated. This is normally done through the special heat exchangers called feedwater preheaters or economizers. They use some of the hot steam extracted from the steam turbine and guided to this heater as schematically shown in Fig. 7.6. Depending on
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Figure 7.5 Simplified steam power plan with a three-pressure steam cycle. The threepressure cycle allows employing a more efficient and larger steam turbine with three cylinders: high pressure (HP), intermediate pressure (IP) and low pressure (LP).
Figure 7.6 Simplified steam power plant with preheater. Employing a feedwater preheater within the schematic cycle of a steam power plant allows reusing waste heat to warm up water, so that it would require less energy of the fuel to evaporate it.
the configuration of the plant, there may be several preheaters to ensure that most of the energy is captured within the process and not dissipated as waste. After the preheater, colder steam is guided back to the process, for instance, into the condenser. Despite proper insulation and the closed cycle, there are still some inevitable losses of steam and water inside the process. This can take place due to multiple leakages in the steam turbine, drainage within the pumps, tanks, and pipes, emergency situations or even quality issues which would require maintenance. To maintain a proper amount of water inside the cycle, it has to be properly prepared and constantly added to the process. This
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is normally done within the water treatment plant where water undergoes thorough cleaning to ensure that it does not damage the heat exchangers, pipes, steam turbine, and other equipment. On the other hand, the condensed water may contain dissolved gases, especially oxygen, due to the vacuum inside this vessel as it would suck in not only steam from the steam turbine but also air from the outside, mixing it with water. This would require special equipment inside the cycle to deaerate the water and make sure it does not cause corrosion in the system. This equipment is called deaerator and serves not only for removing oxygen (and other gases) from the water but also as a storage for boiler feedwater. It means that extra water coming from the treatment plant goes first to the deaerator tank as shown in Fig. 7.7. Such a scheme would require a second condensate pump to guide water from the condenser to the deaerator.
Figure 7.7 Simplified steam power plant with deaerator. Use of deaerator within the cycle of a steam power plant allows to remove oxygen that is inevitably sucked into the cycle and decrease corrosion of the system.
In spite of the historically developed optimization solutions, the efficiency of a fossil-fired condensing steam power plant does not normally exceed 40% and is subject to the technological process as shown above. However, it can be also influenced by the load regime of the plant or conditions of the equipment. This way, the optimization of the steam power plant requires not only efficient and modern equipment properly integrated into the plant, but mostly a well-designed and optimized heat balance. Fig. 7.8 shows a simplified diagram of a real three-pressure condensing steam power plant. The preparation and optimization of such a cycle shall be done by an application engineer within the special software on an iterative basis.
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Figure 7.8 Simplified steam power plant with optimized cycle. A modern steam power plant with optimized cycle employs all of the discussed solutions: several pressure levels, cascades of feedwater preheaters, deaerator, etc.
Depending on the objective of the power plant and its required capacity (electrical and thermal), the application engineer would choose several pressures, type of boiler and auxiliary equipment (heat exchangers, pumps, condenser and cooling, etc.), and optimize the flows of water and steam. The above mentioned diagram, together with the values of temperature, mass flows, and pressure, defines the overall heat balance of a power plant. Based on these calculations, the steam turbine application engineer would make the turbine configuration, leading to the fact that most of the steam turbines are adjusted or even specially designed for each project.
7.2.3 Support systems and equipment areas The heat balance defines the configuration and the parameters of the water and steam cycle, performances and the major equipment with some auxiliaries. However, it does not give a complete picture of the overall plant configuration, equipment arrangement, and supporting systems to allow for proper functionality.
7.2.3.1 General process description and equipment areas We now summarize the overall process of a fossil fuel fired steam power plant shown in Fig. 7.9. The process starts with the fuel supply from fuel handling system where it is properly prepared to ensure that the combustion process delivers most of the energy. The fuel is fed to the furnace of a boiler
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Figure 7.9 Simplified blocks of a steam power plant process. Simplified process to illustrate major steps within a steam turbine power plant (many steps and equipment units are omitted intentionally).
where it is ignited and burnt. The combustion gases follow the hot gas path of a steam generator (colored in yellow (light gray in print version)) transferring their energy to steam, water, and air through the system of heat exchangers. As these gases contain combustion products, they go through the flue gas treatment equipment before being released to the atmosphere through the chimney. To maintain the burning process of the fuel, air is supplied to the furnace through the so called air path. To increase plant efficiency, the air is preheated with the hot flue gas inside the heat exchanger called air preheater and guided to the boiler by the fan. In order to deliver steam and to minimize the damages inside the equipment, water shall be prepared within the water treatment station and then preheated with the hot flue gases inside the water preheater to optimize the efficiency of the plant. A number of heat exchangers inside the boiler transfer heat to the water to evaporate it and then heat the steam to increase its energy and decrease water content inside to supply hot steam to the steam turbine. The exhaust steam then goes to the condenser where it turned back into water. To ensure that the condenser is operating at the required capacity, the heat rejection loop is employed. The gathered condensate is then pumped back to the boiler through the feedwater preheaters. The steam turbine is powering the electrical generator. The electricity is evacuated from the generator terminals through the main transformer into the grid. The overall control and operation of the plant is ensured via
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Figure 7.10 Equipment areas of a steam power plant. Simplified representation of the equipment areas of a fossil-fired steam power plant.
the control system. The description of the process, together with the heat balance diagram, gives an overview of how a power plant looks like. The described process of a fossil-fired steam power plant allows for breaking down the facility into a number of parent systems. These equipment areas, as shown in Fig. 7.10, include: • Boiler island, which supplies hot pressurized steam. Subject to definition, boiler island can consolidate all equipment around the boiler related to the steam production cycle, including fuel treatment and waste handling, i.e., everything required to transfer chemical energy of the fuel into the energy of hot pressurized steam, considering also the waste and its proper treatment. • Turbine island that transforms the heat of the steam into the mechanical energy of the rotating shaft of the steam turbine coupled to the generator. • Mechanical balance of plant, including all supporting systems like heat rejection, water preparation piping, etc. • Electrical balance of plant, including the equipment that is responsible for transferring the power from the generator terminals to the grid, as well as for supplying power for all systems of the plant. This simplified breakdown of the facility into the major areas or subsystems is helpful for two reasons. First, it generally corresponds to the scope of supply or the major manufacturers of equipment or companies that do engineering, procurement and construction (EPC) of a power plant. For example, there are companies that supply equipment only to the boiler island, or there are those, which act only in the steam turbine or generator market. Second, it gives the basis for comparing steam power plants to other power generating facilities and defining similarities not only in the smaller pieces of equipment but also in the major areas and systems. Finally, it allows for modularization of the power plant, which is mostly applicable for larger utilities.
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7.2.3.2 Modular configuration of a steam power plant The simplified process presented in Fig. 7.9 assumes one boiler feeding steam to one turbine. In many cases, however, the required capacity of a power plant cannot be fulfilled by only one boiler or only one steam turbine. This means that the overall configuration of both the boiler and power island can vary. Here several options are available: • Several similar blocks or units, each of them having their own boiler island and turbine island. Such an option would require duplication of most of the systems, complicated routing of cables and pipes, and larger footprint, which leads to a higher cost of electricity generated at this plant. On the other hand, such a configuration may give certain flexibility, but practice shows this is usually not feasible economically. • One boiler supplying steam for several steam turbines located in one power island, and each turbine employing its own electrical generator and steam condenser. Such a structure would optimize the boiler island equipment (e.g., single fuel and emissions treatment system) but would lead to complicated steam piping and sophisticated control system. However, from an economic point of view, this configuration can be favorable. • Several boilers supplying steam to the main steam collector, from where it is distributed to a number of steam turbines. This configuration is applicable in large power plants or, for example, in case of adding new capacities to the existing plant. Depending on the output equipment arrangement, this can be a preferred option compared to the previous two.
7.3. Gas turbine powered energy The development of gas turbines primarily for the aircraft propulsion has raised a question of their implementation for stationary power generation in a thermal power plant. Gas turbines have proved to be more flexible in operation and therefore could be used within a broader range of grid applications, i.e., both for constant load and when extra load is quickly required by the market. On top, combination of both technologies under one umbrella has enabled significantly increasing efficiency of a thermal power plant.
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Figure 7.11 Simplified cycle of a gas turbine unit. Fuel is fed to the combustion system, and the air is compressed by compressor.
7.3.1 Gas turbine simple cycle We will first focus on a simplified gas turbine cycle, which is schematically represented in Fig. 7.11. The idea of the process is based on the combustion of gaseous or liquid fuel and is fairly simple. The ambient air is taken from the atmosphere at the inlet of the compressor section of the turbine. The rotating blades of the compressor suck this air and compress it to a much higher pressure while pushing it through in the axial direction, so that its temperature increases. Hot pressurized air flows to the combustion chamber where it is mixed with the natural gas and burnt under constant pressure. The temperature of the combustion products, which are a mixture of various gases, rises up to 1200◦ C. Under this temperature and constant pressure, the hot combustion products are released to the turbine section where they tend to expand to the atmospheric pressure and therefore acquire kinetic energy. The energy of the hot exhaust gases is used to spin the shaft of the turbine, which is coupled to the shaft of the generator. After loosing some of their energy, the exhaust gases are finally released to the atmosphere. The shaft of the turbine section is connected to the shaft of the compressor section, and the rotation of the former will spin compressor and maintain the constant air flow and its compression to maintain combustion. However, to start the process, there should be an external rotating machine to start the spinning of the shaft and start the flow of the air to the combustion. Then, part of the energy of the rotation that comes from the energy of the hot gases will be used for rotation and air compression. Remark 7.2. A simple gas turbine cycle is a steady repetition of the Brayton cycle. In this simple cycle, combustion and exhaustion occur at constant pressure and compression, and expansion occurs continuously. This means
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that a gas turbine generates power continuously rather than in pulses like in an internal combustion engine. This process, usually called an open or simple cycle of a gas turbine, uses hot gases as the main working medium. The environment, or simply the atmosphere, is considered as an infinite supply of cold ambient air fed to the compressor, while the energy is supplied to the system with the fuel. It is the chemical energy of the fuel that is transferred first into the energy of the hot pressurized gases (combustion products), and then into the mechanical energy of the rotating shaft. Part of this energy is used to rotate the compressor and therefore suck the air and compress it before delivering into the combustion chamber. As the gas turbine cycle is exposed to the ambient conditions, its performance and flexibility characteristics depend on these conditions. Therefore the cycle is usually designed for particular conditions, and the performance calculations are conducted at both standard or ISO conditions and site average conditions. Similarly to a steam turbine, gas turbine cycle has quite low efficiency in transferring the chemical energy of the fossil fuel into electricity compared to a theoretical limit. This is mostly due to multiple points in the cycles, where energy is wasted or dissipated within the environment. Concerning the efficiency of a gas turbine cycle, there are several sources of energy losses: • Friction within the rotating equipment (compressor and turbine shaft); • Exhaust gases of the gas turbine, which are released to the atmosphere. They are still hot enough, however, the energy is simply wasted. A thermal power plant based on such a gas turbine cycle is called a simple cycle power plant and has efficiency around 30%. Among the approaches to increase efficiency of this type of power plant, one of the simplest and most evident ways is to recover the heat from the hot gas turbine exhaust and utilize it for other purposes. Here two major options are possible: 1. Use special heat recovery equipment to boil water and produce steam for industrial utility purposes. A power plant, which produces both electricity and heat, is called a cogeneration plant (Cogen) or combined heat and power plant (CHP). 2. Implement a heat recovery steam generator to supply steam to the steam turbine. Combining gas and steam turbine cycles together in one power plant allows for increasing the efficiency considerably. Such a power plant is called combined cycle power plant (CCPP).
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Figure 7.12 Simplified diagram of a combined cycle power plant. Hot exhaust gases from the gas turbine transfer their waste energy to the water within the heat recovery steam generator, which acts as a boiler. Stem is then fed to the steam turbine. Both turbines can transfer their mechanical energy into electricity.
7.3.2 Combined cycle as a way to increased efficiency In a combined cycle power plant, two thermodynamic cycles are employed: 1. Brayton cycle for a gas turbine, the same as for a simple cycle power plant. This cycle is also known as topping cycle. 2. Rankine cycle of a steam turbine, where the energy for evaporation is taken not from the burning fuel but from the hot exhaust gases of a gas turbine. The steam turbine cycle is called bottoming cycle. The implementation of this cooperation is fairly straightforward. Consider the simplified diagram of the combined cycle power plant schematically represented in Fig. 7.12. The gas turbine follows the simple cycle schematic with the only difference: the exhaust energy is fed to the heat recovery equipment or a special type of boiler called heat recovery steam generator, or HRSG. This unit transfers the energy of the hot gases to the water that circulates through the tubes of the heat exchangers and evaporates. The steam is admitted to the steam turbine where it expands and is released to the condenser. The steam cycle is similar to that in a steam turbine power plant, thus being limited by the exhaust parameters of the gas turbine, namely temperature and flow. For instance, if the exhaust gases are hot enough (especially for bigger units), the steam turbine performance can be optimized in the same way as for a steam power plant, for instance: • High pressure steam from the HRSG can be fed to the HP steam turbine section and, after being released, can be brought back to the boiler as cold reheat;
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This cold reheat can be further heated in a boiler’s reheater section and, with recovered temperature, be fed to the IP section of the steam turbine; • Finally, released from IP ST section, steam can be fed to the LP ST section. However, even the largest available gas turbines still have a limited exhaust gas flow and temperature, which is definitely much lower than the temperature inside the conventional fossil-fired boiler. Therefore, in order to optimize the output of a combined cycle power plant, multiple approaches are used: 1. The above discussed combined cycle power plant with one gas turbine coupled to the HRSG and supplying steam to one steam turbine. Each turbine has its own generator (two in total in this case) coupled to a respective turbine shaft. Therefore such a configuration is called multi-shaft and shown in Fig. 7.13(A). 2. If we are not able to increase the output of a single gas turbine, we are still left with the option of using multiple gas turbines with their own •
Figure 7.13 Combined cycle power plant configurations. Multi-shaft configuration (A); two gas turbines with their own HRSGs feeding steam to a single steam turbine (B); single-shaft configuration (C) with gas and steam turbines coupled to a single generator.
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heat recovery steam generators. The steam is then collected from all of them and supplied to one large steam turbine to increase its output and efficiency. In practice, usually two or three gas turbines are combined with one steam turbine, as shown in Fig. 7.13(B). In some literature, such configurations are called, for example, “two on one” or “three on one” meaning that there are two or three gas turbines operating with one steam turbine, respectively. 3. If we want to increase the output of the plant, we can also use only one generator, which is coupled to both gas turbine and steam turbine. Such a configuration is called single-shaft and is represented in Fig. 7.13(C). It is normally implemented for higher capacities and base load operation at top levels of efficiency. All of the above mentioned configurations of the combined cycle power plant can be implemented in either a single plant or in modular configuration. In general, configuration of a combined cycle power plant depends on many factors. To start with, the major defining factor an investor considers is the MW capacity of the plant and his capability to sell energy to a consumer. The capacity here covers not only electrical megawatts but also thermal power in the form of steam or hot water. The latter can bring much more profit compared to electricity, for instance, when the power plant produces high quality steam of required properties for industrial site, e.g., for refinery or chemical plant. Another example may be a cogeneration power plant where the major output parameters of the station are electricity and hot water for district heating needs. The electrical efficiency of a combined cycle power plant (without considering possible heat production) is evidently higher that of a simple cycle or conventional coal-fired power plant. The incoming chemical energy of the fuel is used for the topping cycle, however, the major waste of the simple cycle — the exhaust — is reused in the process. Remark 7.3. The term combined cycle is used to define a type of a power plant. In some cases it is appropriate to name it a gas turbine combined cycle (GTCC) power plant, specially pointing out that this power plant is based on a gas turbine cycle combined with steam turbine. Also the name combined cycle gas turbine (CCGT) plant is widely used. For simplicity, however, it is recommended to use the term combined cycle power plant, or CCPP, as it immediately assumes gas and steam turbines.
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Figure 7.14 Equipment areas of a combined cycle power plant. Simplified representation of the major equipment areas of a CCPP includes turbine island with a topping cycle (gas turbine-generator) and a bottoming cycle (HRSG and steam turbine with a possible generator) supported by the BOP systems.
7.3.3 Major components and systems The schematic process diagrams and heat balances discussed above define the key parameters of the gas and steam and do not fully specify all necessary equipment, systems and site layout. Similarly to the steam power plant, every simple and combined cycle power plant can be segmented into several major systems, with the heart being always gas turbines, steam turbines, and generators. A gas turbine driven power plant can be broken into the following major equipment areas (Fig. 7.14): • Turbine island or power island that transforms the chemical energy of the fuel into the mechanical energy of the rotating shaft of the turbines coupled to the generators. It normally includes: • Topping cycle that incorporates a gas turbine with its generator and dedicated systems; • Bottoming cycle, which includes heat recovery steam generator, steam turbine with its generator (if applicable), condenser, essential valves and supporting systems (applicable only for combined cycle); • Mechanical balance of plant, including all supporting systems like heat rejection, water preparation piping, etc.; • Electrical balance of plant including the equipment that is responsible for transferring the power from the generator terminals to the grid, as well supplying power for all systems of the plant. This breakdown allows for the customer to clearly see the potential scope of supply and define the configuration of a power plant. On top of this, such a modular approach allows customers to implement upgrades and power plant extension projects in a simpler way. The typical configurations can be derived one from the other and include:
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1. Mechanical drive gas turbine, when a unit is used to power other machinery, for instance, a compressor for a gas pipeline or LNG facility; 2. Simple-cycle power plant with a gas turbine and generator operating in an open cycle; 3. Simple cycle with cogeneration mode, when the power island consists of a gas turbine, generator and heat recovery equipment; 4. Combined cycle power plant. Depending on the engineering approach and historical trends, the breakdown into equipment areas may differ. Various market players prefer to have their own itemization, which does not necessarily coincide with the given here. However, in most cases the approach is similar to what has been discussed here and can be easily adopted to the market conditions.
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