Steam Power Stations for Electricity and Heat Generation
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant
4.4.1 Increases in Thermal Efficiencies
4.4.1.2 Influence of Feed Water Preheating
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant 147 Fig. 4.55 Influence on the
efficiency of reheater spraying (Baehr 1985)
As reported in Sect. 4.3.5.5, controlling the reheater temperature by a spray attemperator diminishes the efficiency, because the high-pressure range of the steam generator is bypassed by doing so, and steam is produced at a low pressure and tem- perature. Figure 4.55 shows the influence on the efficiency of the reheater attemper- ator mass flow (Baehr 1985). In the case of the reference power plant, the spraying mass flow at full load is about 0.9% of the feed water mass flow. New power plant designs limit the temperature-controlling spraying mass flow to 0.2% of the feed water mass flow (Breuer et al. 1995).
The measures described above have an effect only on the thermal and on the turbine efficiency, but not on the energetic steam generator efficiency. They are included in the exergetic steam generator efficiency rate, though (see Sect. 3.2).
148 4 Steam Power Stations for Electricity and Heat Generation Nowadays, six to nine feed water heaters with feed water outlet temperatures between 250 and 300◦C are commonly used for large thermal power plants. Higher feed water outlet temperatures are chosen as live steam pressures increase. The heater configuration of the reference power plant without raised live steam con- ditions is shown in Fig. 4.28. Four LP heaters and two HP heaters preheat the feed water up to 250◦C, with the feed water tank and pumps necessary parts of the pro- cess.
The last, upper-most HP heater is usually heated by extracted steam from the cold reheat line behind the HP turbine. The reheat pressure, derived from optimisation calculations for the entire cycle, thus defines the feed water outlet temperature (see Fig. 4.56) (Rukes et al. 1994).
The feed water heating temperature can be further raised by inserting an addi- tional preheater, heated by extraction steam from the high-pressure section of the turbine. Such additional extraction from the HP turbine section uncouples the reheater pressure and feed water outlet temperature. Figure 4.57 shows a heat flow diagram, with stages, where the feed water is preheated to 300◦C.
A feed water heating temperature increase from 250 to 290◦C, by additional extraction of steam from the HP turbine section, results in an efficiency increase of 0.7% (Billotet and Joh¨anntgen 1995); the result of an increase from 270 to 300◦C is an absolute improvement of 0.75% (Kotschenreuther et al. 1993). Figure 4.58 shows the impact of an increase in the feed water temperature – a relative decrease of the heat rate, which is dependent on the pressure level (Klebes 2007).
The rise of the feed water outlet temperature comes up against limiting factors with regard to the steam generator design. It is imperative to prevent boiling in the economiser in order to avoid flow instabilities and to ensure a steady charge of the evaporator tubes. For this reason, the economiser must be designed to be smaller for higher outlet temperatures of the regenerative feed water heating. Increasing feed water temperatures entering the steam generator make the transferable flue
40 60 80 100 120 140 160
250 270 290 310 330 350
Feedwater temperature [°C]
Reheat pressure [bar]
Fig. 4.56 Feed water temperature as a function of the reheat pressure (Rukes et al. 1994)
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant 149
Boiler
4.3 bar 600°C
285 bar
620°C 52 bar
HP IP LP1 LP2
300°C Desuper heater
HD-Pre- heater 8
HD-Pre- heater 7 HD-Pre- heater 6 Feed- water pump
LP-pre- heater 4 LP-pre- heater 3 LP-pre- heater 2 LP-pre- heater 1
193°C 357 bar
45 mbar Condensator 187°C
800 MW G
273 MW
Feed- water tank
Condensate pump
Fig. 4.57 Heat flow diagram of a thermal power plant with advanced steam conditions and nine- stage feed water heating (data from Tremmel and Hartmann 2004)
gas heat in the economiser decrease, which can then be used only to preheat the combustion air.
In designing a power plant, after the feed water heating outlet temperature is defined, further optimisation is only possible within the feed water heating chain.
The design should, in this process, provide for the smallest possible temperature difference between the heating medium, i.e. the extracted steam, and the feed water to be heated. By increasing the number of heaters while keeping the same outlet temperature, smaller temperature rises for the individual stages result. This helps to achieve a better adaptation of the temperatures of the heat-absorbing to the heat- dissipating heat transfer medium – water flow and extraction steam flows – and thus to minimise the exergy losses. The improvement in efficiency of each additional stage, as shown in Fig. 4.59, is positive but decreasing, so that a point is reached where installation of yet another stage cannot be justified economically (Eichholtz et al. 1994).
150 4 Steam Power Stations for Electricity and Heat Generation
Fig. 4.58 Effect of the live steam pressure and the feed water temperature on the heat rate (Klebes 2007)
Fig. 4.59 Influence of the number of stages on the net efficiency, at constant outlet temperature (Eichholtz et al.
1994)
4.4 Possibilities for Efficiency Increases in the Development of a Steam Power Plant 151 The temperature differences between the heat-dissipating and the heat-absorbing flows in a preheating stage are characterised by the so-called terminal temperature difference (TTD), which is defined as the smallest temperature difference between the two mediums. At the transition to small TTDs, larger heating surfaces and hence heavy and expensive plant components are required. A compact construction is the result when counterflow heat exchangers are used.
Preheaters are usually designed as shell-and-tube heat exchangers. The extracted steam and the feed water are segregated from each other by a heat exchanger sur- face, which consists of tube bundles. The sensible heat of the steam can be utilised in so-called desuperheaters. The sensible heat of the condensate can be utilised in condensate coolers, which can be mounted either by integration into the preheaters or separately. The desuperheater, with respect to the feed water, is mounted after the preheater(s). This way, the feed water can be heated to a higher temperature than is possible with the condensing preheater. The condensate cooler, with respect to the feed water, is mounted before the preheater.
The most reasonable solution in terms of thermodynamics is to mix, without cooling, the condensate in the preheater with the feed water. This method is not used for HP preheaters because the high feed water pressure requires a complex system of pumps, pipes and fittings. Thermodynamically, it is therefore a compromise to subcool the condensates and to let them flow into the next lowest preheat stage. In configurations with multistage LP preheaters, it is usually economical to pump the condensates of one or several preheat stages back into the condensate flow.
In a direct-contact heater, the heat of the extracted steam is transferred to the feed water by mixing and condensation of steam in water. Given its low terminal temperature difference, the direct-contact heater has thermodynamic advantages.
However, because the container is under the pressure of extraction, the entire condensate flow has to be pumped to reach the corresponding pressure level.
Because of the necessary pumps, direct-contact heaters are only used in the feed water tank for deaeration.
The common values for the terminal temperature differences of regenerative heaters of modern hard coal power plants are (STEAG 1988) as follows:
• Desuperheater 25 K
• Condensation equipment 2 K
• Condensate cooler 7 K
4.4.1.3 Lower Heat Dissipation Temperatures – Optimisation