TM 5-810-15
required for wet bottom boilers to prevent solidifi-
Table 3--5. Chemical Constituents of Coal Ash.
cation of the ash during low load operation. Fur-
nace volume must be increased for coals producing
ash with high fouling and slagging potentials, or to
counteract the erosive effects of large quantities of
ash or very abrasive ash. The greater furnace
volume results in both lower exit gas temperatures-
reducing fouling and slagging-and lower exit gas
velocities, reducing tube erosion. The relationship
between coal classification and furnace volume is
shown in figure 3-6.
3-6. Combustion technology selection.
a. Exclusionary factors. Gas and oil fired boilers
are available over the entire size range. Their use is
limited to areas where these fuels are economically
available. Stoker-fired boilers are available for the
entire load range covered by this manual.
b. Ash characteristics and boiler design. The
Pulverized coal (PC) boilers are available in capac-
characteristics and quantity of ash produced by a
ities of 100,000 pph and above. Atmospheric circu-
specific coal strongly influence several aspects of
lating fluidized bed (ACFB) boilers are available in
pulverized coal, ACFB and stoker boiler design,
capacities of 80,000 pph and above. PC fired units
including the selection of a bottom ash handling
were used in capacity ranges below 100,000 pph
system and furnace sizing. Ash with a high (2400
prior to the advent of package boilers, but with the
degrees F and above of a reducing basis) fusion
new designs it became more difficult to evaluate
temperature is most suitable for dry bottom boilers,
PC firing as a preferred method of firing coal and
while lower (1900 degrees F to 2400 degrees F on
hence have essentially become obsolete. When
a reducing basis) ash fusion temperatures are
3-9
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