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Clean and Efficient Coal-Fired Power Plants: Development Toward Advanced Technologies

Heinz Termuehlen
Heinz Termuehlen
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Werner Emsperger
Werner Emsperger
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ASME Press
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For large steam generators in the power-generation industry, mainly 3 types of steam generators are used, the distinguishing differences of which are characterized by their evaporator systems as illustrated in Figure 3-1 :

• Natural circulation system

• Forced circulation (La Mont) system

• Once-through system

In the natural circulation-type boiler, the water in the evaporator section circulates naturally due to gravity because it is warmed up and partially evaporated in the heated evaporator tubes, creating a difference in density between the fluid in the heated tubes and the unheated connecting pipes (downcomers). The downcomers connect the boiler drum with the inlet headers of the heated tubes, which form the furnace walls of the steam generator. About 40% of the heated water evaporates, and the saturated steam produced is separated in the drum and flows to the boiler superheater. The remaining water is circulated. That means a circulation rate of about 2.5 ensures sufficient cooling of the evaporator tubes. The operating line [2] in the enthalpy/pressure diagram of Figure 3-2 illustrates typical conditions for such a 2400 psia/1000°F (166 bar/538°C) drum-type boiler.

With increasing main steam pressure, the driving density difference and consequentially the cooling of the evaporator tubes becomes smaller, which limits the natural circulation principle to a main steam pressure of about 2500 psia (172 bar). For a further main steam pressure increase, forced circulation can be applied by installing a circulation pump in the downcomer system. With this forced circulation system, the main steam pressure can be increased to about 2900 psia (200 bar).

In the once-through boiler design, all feedwater entering the once-through system is warmed up, evaporated, and superheated in a once-through mode (Benson principle). The location of the endpoint of evaporation is not fixed like in the drum-type boiler design. It automatically moves between one or more heating surfaces in such a way that, depending on the load, fouling of the heating surfaces and the amount of excess air, the main steam temperature always remains constant. The once-through principle can be adopted for subcritical and supercritical steam conditions, even far above the critical pressure of 3206 psia (221 bar). Furthermore, the once-through steam generator design allows subcritical as well as supercritical units to operate in a sliding pressure mode, which increases the operating flexibility and prolongs the lifetime of highly pressurized components. The operating line [1] in Figure 3-2 shows the conditions for the supercritical advanced unit at 100% load and line [3] the conditions of this unit operating in a variable pressure mode at 30% part load at subcritical pressure.

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