Steam Power Stations for Electricity and Heat Generation
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant
4.4.2 Reduction of Losses
4.4.2.1 Internal Turbine Efficiency and Losses
In the expansion process in the turbine, the steam is accelerated, and its kinetic energy converted into mechanical work by impulse transfer onto the rotating blades.
The measure for the quality of the conversion into mechanical work is the internal turbine efficiency ηi,T, which indicates the difference between real and loss-free isentropic expansions. For the real thermal cycle efficiency ηth, the following applies:
ηth=ηth,0·ηi,T (3.31)
whereηth,0represents the thermal cycle efficiency at loss-free expansion.
About two thirds of the total losses occur over the blade stages. The HP first stages (of the turbine), in particular, and the LP last stages (of the turbine) are the areas of the turbine incurring the highest losses. The losses arise through fluid fric- tion in the channels, friction of the rotating blades in the surrounding steam, steam leakages from rotating and fixed parts and through steam moisture in the last stages (Strauß 2006).
The greatest single loss, in the order of magnitude of about one tenth to one third of the total loss, is the outlet loss. It comes about because of the kinetic energy of the exhaust steam. Further losses occur in the inlet valves and in the cross-over pipes (Adrian et al. 1986).
An exhaust steam diffuser partly recovers kinetic energy from the exhaust steam exiting at high velocity from the last blading. The kinetic energy is converted into pressure energy in the diffuser, which is located between the last turbine blades and before the condenser, and partially compensates for the pressure losses arising on the way to the condenser. With a constant condenser pressure, an exhaust steam diffuser brings about a lower pressure after the last turbine stage and in consequence more power is produced in the turbine than would be the case without the diffuser (Schr¨oder 1968).
Besides by an exhaust steam diffuser, it is possible to influence the outlet loss by the exhaust steam velocity, which, at a given steam mass flow and a given condenser
162 4 Steam Power Stations for Electricity and Heat Generation Fig. 4.69 Development of the
internal efficiencies of steam turbines (Billotet and Joh¨anntgen 1995)
pressure, can only be varied by means of the cross-sectional area of the turbine outlet. Because of the limited blade length of the last LP stage, the outlet surface can only be enlarged by the number of the LP turbines. The last stages and the exhaust steam cross-section are designed in combination with the heat extraction (see Sect. 4.4.1.3).
Turbine improvements have contributed substantially to the increases in effi- ciency of modern power plants. Three-dimensional calculations (i.e. computer mod- elling) of flow processes reveal the potential for reducing the flow losses, and modern manufacturing technologies make it possible to build complex blade geome- tries (Nowi and Haller 1997; Oeynhausen et al. 1996). The modernisation of the turbine of existing power plants is an effective means to increase the efficiency.
Figure 4.69 shows the internal turbine efficiencies for existing and planned power plants (Billotet and Joh¨anntgen 1995).
4.4.2.2 Steam Generator Losses
In the steam generator, or boiler, the chemically bound energy of the fuel is con- verted into thermal energy of the flue gas and then transferred to the steam – water cycle. The efficiency of the energy conversion is designated as the steam generator or boiler efficiencyηB, and the arising losses are called the boiler losses. Referring to the calorific value of the fuel, the steam generator efficiency of modern hard coal fired furnaces amounts to 94%, while brown coal fuelled furnaces have an efficiency of around 90%.
The losses consist of the following:
• Loss through unburned matter (κU)
• Loss through sensible heat of the slag (κS)
• Flue gas loss (κFG)
• Loss through radiation and convection of the external surfaces of the boiler (κRC)
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant 163 Losses through unburned matter are well below 1%. Of these losses, a differen- tiation is made between unburned gas, unburned matter in the slag and unburned matter in the fly ash. Where a carbon content of less than 5% is required for the use of the fly ash as a by-product, the respective maximum loss of unburned matter in the fly ash will be, for instance, 0.5% for a coal type with an ash content of 10%.
Typical losses through unburned matter range around 0.3% (Riedle et al. 1990).
Heat losses due to radiative, conductive and convective transfer to the environ- ment by the steam generator are below 1% and diminish further as the power rating of the steam generator increases. The losses of brown coal fuelled furnaces are sig- nificantly higher than hard coal fuelled furnaces because, at the same output, the steam generator has a considerably larger external surface area. Hard coal fuelled furnaces typically have heat losses around 0.3% (Billotet and Joh¨anntgen 1995).
Ash is predominantly removed in the electrostatic precipitator (ESP) as fly ash, though part of it stays in the furnace as slag and is typically removed while in a hot state. The sensible heat of the slag, when unused, results in a portion of the boiler losses. In dry-bottom furnaces, the amount of the so-called hopper ash is about 10%
of the total ash mass flow, and the respective loss is below 0.4% of the calorific energy input in hard coal fuelled furnaces. In slag-tap furnaces, the loss by sensible heat is higher, because either all or a large portion of the ash (depending on the degree of retention and the fraction of the re-injected ash) runs off as liquid slag with a high temperature.
The principal loss of the steam generator occurs because the flue gas cannot be cooled down to ambient temperature. After the exhaust steam heat loss, this is the most major loss in a power plant.
Efforts to increase the steam generator efficiency concentrate on reducing the flue gas heat loss. This loss depends both on the flue gas outlet temperature of the steam generator (after the air heater) and on the flue gas mass flow. Figure 4.70 shows the
Fig. 4.70 Boiler loss as a function of the boiler exit temperature and air ratio, for hard coal firing (Riedle et al. 1990)