MODERN THERMAL POWER PLANTS

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Figure 3 shows the layout of a triple-pressure CCPP with reheating. When comparing Figure 2, showing a single-pressure plant, with Figure 3 it is obvious that the triple-cycle is more complex. The number of steam drums, the number of heat exchangers and the piping of the triple-pressure cycle makes the initial cost higher than for a single-pressure plant. A steam turbine normally consists of a high-pressure, an intermediate-pressure and a low-pressure cylinder. This means that only two natural induction points are available (excluding the admittance). For a triple-pressure CCPP with reheating this means that the intermediate pressure steam produced in the HRSG should be mixed and superheated with the reheating flow. Single-pressure CCPPs are used when a short start-up time is important, or if the heat remaining in the flue-gas can be recovered for an external process such as district heating (co-generation). A dual-pressure plant would typically be a mediumsized plant, where a third pressure level is not economically motivated. For a largescale plant, operated as a base load producer with many hours’ operation per year, it is easier to motivate a higher initial cost, and such a plant will include three pressure levels and reheating. Figure 4 shows the net heat-to-power efficiency for four different CCPP configurations. The plants without reheating utilize an admittance pressure and temperature of 100 bar and 565 °C, while the reheating units have a pressure of 160 bar and a temperature of 565 °C. 60 59 58 57 56 Single-pressure Dual-pressure Dual-pressure with reheating Tripple-pressure with reheating % 57.30 58.27 58.78 59.42 Figure 4: Efficiency of different CCPP configurations without parasitic or step-up transformer losses 14
3 Analysing Combined Cycle Power Plants The transfer of heat from the gas turbine exhaust to the steam cycle is vital for the overall efficiency of the CCPP. This is done by the HRSG. It is important to understand the HRSG process, and how it is affected by cycle parameters and cycle features. The process of the HRSG can be visualized in a temperature-to-heat-flux diagram (T-Q diagram). In this chapter the T-Q diagram is explained and it is used to analyse how some cycle features can improve the efficiency of a CCPP. 3.1 The T-Q diagram The HRSG should be designed with consideration to both the first and second laws of thermodynamics. The most important cycle features determining the behaviour of the HRSG are the number of pressure levels and whether reheating is implemented or not. Other considerations include how and where deaeration is performed and the heat-transfer area, i.e. the magnitude of the pinch point. The use of T-Q diagram is crucial in understanding and designing combined cycles. Figure 5 shows the T-Q diagram for a single-pressure combined cycle with 100 bar admission pressure and a turbine inlet temperature of 565 °C. The T-Q diagram in Figure 5 originates from a single-pressure CCPP, and shows the temperature profile of the heat recovery process. The smallest temperature difference in the HRSG is called the pinch-point, and is located on the cold side of the evaporator. The upper continuous line, with an almost constant slope, represents the temperature profile of the flue-gas, and the lower line represents the temperature of the water/steam. The HRSG of a single-pressure combined cycle consists of three different sections. Starting at the lowest temperature, the first section is called the economizer, and is where liquid water is heated to the saturation temperature. To avoid evaporation, which could cause steam blockage that may result in “water hammering” [12] in the economizer, the outlet temperature is always kept a few degrees below the saturated state. This temperature difference is called the approach point. 15
Page 1 and 2: MODERN THERMAL POWER PLANTS Aspects
Page 3: To Louise
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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Page 14 and 15: ρ [kg/(m 3 )] Density φ [-] Flang
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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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