Steels_ Metallurgy and Applications, Third Edition

362 Steels: Metallurgy and Applications

Pendant

Superheaters

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Bumers [

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Ash

collection

Pendant

reheater

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Figure 4.38 Schematic illustration of coal-fired boiler

generation of carbon monoxide promoting sulphidation and the formation of
hydrogen chloride. 33
At the top of the combustion zone, the hot gases are turned through an angle
of 90 ~ and over banks of superheater tubes which are suspended from the
boiler roof. In this region of the boiler, the steam is heated progressively to
maximum temperatures of 540"C in the case of oil-fired boilers and 565"C in
coal-fired stations. These temperature limits in UK power stations are dictated
very largely by the aggressive environmental conditions described above and the
need to maintain an adequate balance between operating efficiency and excessive
corrosion/oxidation of the boiler tubing. In the superheater section, the first few
rows of tubing are subjected to very arduous conditions due to the impingement
of hot gases and fly ash.
Beyond the superheater section of the boiler, the hot gases turn through a
second angle of 90* and over the reheater tubes. Being in the same part of the
boiler, the temperatures in the superheater and reheater are similar but, due to
the fact that they operate on different steam circuits, the pressure of steam in the
reheater is significantly lower than that in the superheater.
Before leaving the boiler, the gases pass over a final bank of tubes that consti-
tute the economizer. At this stage, the gas temperature has fallen to about 300"C
and the economizer operates at a steam temperature of about 250~
The flow of steam/water through the boiler is shown in Figure 4.39. Feedwater
is first preheated in the economizer before passing into the evaporator tubing in
the furnace walls. From the various outlets in the evaporator, the steam/water
mixture then passes into a large collection vessel, known as the steam drum. In
this vessel, the steam is separated from the water, the steam being passed to the