TM 5-810-15
segregation. Approximately 25 to 50 percent of the
should lean toward spreader stokers. This type of
coal is burned in suspension by this method. This
stoker is more flexible in its capability to efficiently
spreader feed has a larger grate heat release rate
burn a wider range of coals.
than the cross feed type; requires a smaller furnace
envelope; and has a quicker response time for load
3-13.
Fly ash reinjection for coal firing.
changes. This type of fuel feed must have a uniform
a. General. A fly ash reinjection system for coal
air flow through the grates due to the large amount
fired boilers is used to return coarse, carbon
of suspension burning. For best results, the fuel fed
bearing particulate back into the furnace for further
to this type of unit should be properly sized. Refer
combustion. This is only economically justified in
to Figure 3-12 illustrating coal size.
stoker fired boilers with steam capacities over
d. Stoker selection considerations. Table 3-6
70,000 pph. Fly ash reinjection from the boiler,
provides a summary of factors to be considered
economizer, air heater and dust collector hoppers
when selecting a stoker for a boiler within the
can improve boiler thermal efficiency by 3 to 5
range of 20,000 to 250,000 pph of steam. Prior to
percent. However, fly ash recirculation within the
submitting a set of specifications to the boiler or
boiler is significantly increased.
stoker manufacturers, the type of coal that is to be
b. Equipment sizing. Tube erosion and other
burned must be known. Selection of the design coal
maintenance costs in addition to requiring an
is required so that these manufacturers are able to
increase in the capability of the air pollution control
guarantee their equipment performance. When the
equipment are to be expected and must be taken
coal is not known, or when the possibility exists
into consideration when sizing the air pollution
that many different types of coals will be burned
control equipment.
over the life of the stoker, the selection emphasis
3-18
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