MODERN THERMAL POWER PLANTS

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T-Q diagram in Figure 9. The cycle in Figure 9 is typical for a dual-pressure CCPP with no superheating of the low-pressure steam. The steam produced in the lowpressure cycle is injected into the steam turbine in the crossover pipe between the intermediate-pressure and low-pressure turbine cylinder. The cycle now has the advantage of recovering a large amount of energy, associated with a low pressure level, while the overall temperature difference and entropy generation are limited. The higher the cycle pressure, the greater the scope for additional pressure levels. If reheating is used and the cycle pressure is very high the mean temperature difference in the HRSG can be further reduced if a third pressure level is introduced. The amount of heat recovered is also somewhat increased as the total mass flow will be increased and the optimum low-pressure level will be slightly reduced. In today’s market it is economically justifiable to use three pressure levels in a large combined cycle. It was mentioned in Section 2.2 that the turbine is limited to 3 turbine cylinders and, therefore, the reheating pressure of a triple-pressure CCPP will be set to the intermediate pressure level so that it can be inserted at the same position. A four-pressure CCPP would require a fourth turbine cylinder, which would lead to an unrealistically high initial cost of the plant. 3.4 The deaerator To avoid corrosion of the drum walls and piping of the system the feedwater must be deaerated (see Section 2.1.2). This can be done in a number of ways. If deaeration is performed in the low-pressure drum, no separate deaeration unit is required. To do this, the drum approach temperature must be higher than normal and all the water must be passed through the unit. The higher the approach temperature to the deaerator, the more gases can be removed. The energy consumed in the deaeration process will be supplied from the evaporator, which results in less low-pressure steam being available in the turbine. However, no additional deaerator tank is required, which reduces the initial cost and saves space. From a thermodynamic perspective, it is more beneficial to use energy at a lower temperature for deaeration. For instance, part of the flow from the economizer outlet can be flashed and used in the deaerator. To do this a separate deaeration tank is required. 22
4 Thermodynamic Power Plant Modelling Through calculations it is possible to determine the critical parameters of a power plant. In this work, such calculations were carried out to evaluate the impact of changes in the boundary conditions, the introduction of a new component or the modification of a component. One-dimensional steady-state modelling is suitable to study the effects on the whole power cycle. This type of modelling does not provide any details on the dynamic events taking place in the components, but is excellent in giving a more global view of the performance. The principle is that each component has a set of inlets and outlets at which the stagnation properties are calculated using thermodynamic equations. When designing a computer-based model, the key is to maintain a good balance between the number of inputs and the accuracy. With too many inputs, the model becomes difficult to use and the requirements on the user’s experience increase. Also, if information is not available for all the inputs, imprecise estimates may make the model less accurate than a simpler version. The number of inputs needed is generally decided by the operational window of the unit. The most complex unit to model is one that employs different working media under wide variations of temperature and pressure. It is important to evaluate every parameter that is left out and the user should be well aware of the limitations of the model. Heat and mass balance models can be based on equations or characteristics, i.e. tabulated values or graphs, or both. Simple units are preferably based on equations, but more complex units become too complicated to model with equations only. 4.1 Model components A large number of equations containing an even larger amount of variables are required to model a power plant. To make modelling more practical, the equations are collected into groups representing separate units or components of the power plant such as heat exchangers, pipes, condensers, and turbine stages. Each component depends on the others and in order to model the complete cycle the 23
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
Page 12 and 13: 4.6.1 Dry efficiency ..............
Page 14 and 15: ρ [kg/(m 3 )] Density φ [-] Flang
Page 16 and 17: is higher than it would be if direc
Page 18 and 19: 1.5 Outline of this thesis This the
Page 20 and 21: of two thermodynamic cycles and inc
Page 22 and 23: temperatures of 1400-1500 °C and s
Page 24 and 25: additional evaporators working at l
Page 26 and 27: Figure 3 shows the layout of a trip
Page 31: very low pressure will be beneficia
Page 36: components should be linked togethe
Page 49: condensational shock does not take
Page 57 and 58: extracted fluid. A more accurate pa
Page 61 and 62: 5 Post-Combustion Carbon Capture Th
Page 63 and 64: Equation (37), this results in a sm
Page 65 and 66: egenerator or stripper. The CO 2 is
Page 67 and 68: for instance, for enhancing oil rec
Page 69 and 70: In an attempt to reduce the amount
Page 71 and 72: If condensing steam, which is extra
Page 73: MEA carbon capture unit is a dual-p
Page 77 and 78: educed the pressure ratio will fall
Page 79 and 80: 7 Low Calorific Fuels in CCPPs Reso
Page 81: 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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