MODERN THERMAL POWER PLANTS

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of two thermodynamic cycles and incorporate them into one cycle with high efficiency, power output and flexibility. These cycles are referred to as combined cycles, and are typically defined as any power-producing unit consisting of two or more power-producing cycles. They consist of a topping cycle and a bottoming cycle, where the waste heat from the topping cycle constitutes the heat input to the bottoming cycle. The most common combined cycle consists of a gas turbine (Brayton cycle) as the topping cycle and a steam-turbine-based steam cycle (Rankine cycle) as the bottoming cycle. Plants employing this combination are common, and are normally referred to as a combined-cycle power plant (CCPP). The CCPP is a popular power producing unit that has a number of desirable features such as high efficiency, low initial cost, relatively short construction time, small footprint and a short start-up time. These desirable features have made the CCPP a common choice for the power industry, and the number of CCPPs has thus increased around the world, from 5% to almost 20% of the total capacity in the past 10 years [9]. The CCPP does have some disadvantages, the major one being that its fuel flexibility is rather limited. For instance, solid fuels require gasification before they can be combusted in a gas turbine. The key feature is the high efficiency which, for a state-of-the-art unit of today is very close to reaching 60%. Because of its popularity, many books and scientific articles have been written on the CCPP. Three publications giving good scientific descriptions of the CCPP are the book by Kehlhofer [9], the doctoral thesis by Bolland [10] and the book by Horlock [11]. These three authors have also published numerous articles on the subject. 2.1 The components of a CCPP Figure 1 shows a schematic illustration of a CCPP and its three main components: a gas turbine, a steam turbine and a heat-recovery steam generator (HRSG). The gas turbine operates at a high temperature and pressure where it produces power which is fed to the electric grid. The flue-gases from the gas turbine are relatively hot and contain a large amount of energy. This energy is fed to a HRSG where the flue-gases heat, boil and superheat water to a high temperature under high pressure. The hightemperature steam is delivered to a steam turbine where it expands and produces work. The energy in the exhaust gases, which is by far the major loss in the gas turbine, is thus utilized to produce additional power from a second cycle. The following sections will briefly describe the different parts of the CCPP. More details can be found in Sections 4.6 and 4.7. 8
HRSG Gas Turbine Steam Turbine Figure 1: Schematic illustration of a single-pressure CCPP 2.1.1 The gas turbine The gas turbine is the topping cycle, which is a normal open Brayton cycle with some special features. A simple cycle gas turbine, i.e. a gas turbine not operated in a combined cycle, has maximum efficiency at a high pressure ratio. A high pressure ratio results in a large expansion and hence the temperature of the exhaust gas is low. The low exhaust gas temperature means a reduction in stack loss and therefore a high efficiency. Too high a pressure ratio results in the compressor consuming a large amount of energy in comparison to the energy that can be added with the fuel without exceeding the maximum turbine inlet temperature. When a gas turbine is to be designed for combined-cycle operation it is no longer desirable to have a low flue-gas temperature. A low flue-gas temperature would give a low steam admittance temperature in the bottoming steam cycle, which limits the efficiency and power output. Therefore, the pressure ratio of the gas turbine should be lower than for a single-cycle unit to ensure that the bottoming cycle receives energy at a suitable temperature for steam production. On the other hand, too low a pressure ratio will lead to poor gas turbine efficiency. In today’s plants, with combustor outlet 9
Page 1 and 2: MODERN THERMAL POWER PLANTS Aspects
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Page 6 and 7: Key words: Combined cycle, CO 2 , c
Page 8 and 9: Metoderna för att minska koldioxid
Page 10 and 11: Paper VI K. Jonshagen, M. Genrup Im
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If condensing steam, which is extra
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MEA carbon capture unit is a dual-p
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educed the pressure ratio will fall
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7 Low Calorific Fuels in CCPPs Reso
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7.1 Summary of the results on low c
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9 Summary of papers Paper I K. Jons
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References [1] Le Treut, H., R. Som
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[39] Mimura, T., Shimojo, S., Suda,
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Proceedings of ASME Turbo Expo 2011
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[20] and uses fully dimensionless p
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The dual-pressure chilled ammonia C
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[3] Woollatt, G., and Franco, F., 2
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Table 1 Reduction in isentropic exp
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Fig. 1 Schematic figure of the refe
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deaeration is performed in the LP-d
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III
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Fig. 1 Schematic figure of the refe
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Table 3 Settings for the CO 2 compr
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Fig. 4 Schematic figure of the sing
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Acknowledgment The authors wish to
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SCOC-CC Semi-closed oxy-fuel combus
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a given stage loading parameter ψ.
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[11] Wennerstrom A. J., 2000, “De
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Mass flow m [kg/s] Pressure p [Pa]
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The inlet pressure drop is simply m
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cannot run on 100% air blown gasifi
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and firing temperature, increase th
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[2] Eriksson, P., Genrup, M., Jonsh
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K. Jonshagen, M. Genrup / Energy 35
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VII
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Proceedings of ECOS 2009 Copyright
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Proceedings of ECOS 2009 Copyright
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