TM 5-810-15
(2) In the negative pressure pulverizer
rates over 25,000 pph. It is not recommended
operation, the exhauster fan pulls the air through
because of high costs for installation and mainte-
the mill and then forces it up through the riffle box,
nance.
c. Acceptable type stokers..
then the burner lines to the furnace. In this case,
(1) Vibrating or oscillating grate stoker.
any leakage would be into the mill. However, the
burner lines are under pressure and any leakage
This type of stoker is available for boilers with
would result in finely ground coal showing up
steam capacities between 20,000 and 150,000 pph,
around these leaks.
depending upon what feed types are used. It is
b. Coal air mixture. In either case, pressure or
available with either mass feed or spreader feed.
(2) Traveling grate stoker. This type of
exhauster type mills, the coal-air mixture is usually
at a temperature of approximately 150 degrees F
stoker moves the coal through the boiler furnace on
and a velocity 2000 feet per minute (fpm) or
a continuous belt made of the stoker grate bars.
higher, to prevent the coal from settling out in the
Combustion air passes through the grate bars to
burner lines. If the coal does settle out in these
reach the fuel bed. The combustion air pressure
burner lines, fires or explosions in the burner lines
drop across the grate due to the construction of the
or pulverizers may occur.
grate bars is evenly distributed to the fuel at all
loads. This design feature makes the traveling grate
stoker acceptable for use with spreader type coal
3-11.
Stokers.
feed. For mass burning, plenums and dampers must
a. Type. Mechanical stokers continuously supply
be incorporated into the design of the stoker. The
coal to their grates in a manner that allows for
traveling grate stoker is acceptable for boilers with
controlled combustion of the coal. There are sev-
steam capacities of 50,000 pph and above.
eral combinations of stokers and grate types here-
(3) Traveling chain grate stoker. This type
after referred to as stoker types and are available
of stoker moves the coal through the boiler furnace
for use in coal fired boilers with steam capacities of
on a continuous belt made of interlocking links or
20,000 to 250,000 pph which is the entire range
bars. Unlike the traveling grate stoker, it has a low
covered by this manual. However, not all of them
pressure drop across the chain due to the spaces
are acceptable for state-of-the-art boiler plant de-
between the links. As a result, the air flow on this
sign.
type of stoker is not evenly distributed at all boiler
b. Unacceptable stoker types.
loads. Therefore, the traveling chain grate stoker is
(1) Dump grate stoker. This type of stoker
acceptable only with a mass type feed in boilers
is not recommended because it has a high
with steam capacities between 20,000 and 75,000
particulate emission rate whenever the grates are
pph.
dumped. This factor necessitates added cost for air
pollution control equipment due to the increased
3-12.
Stoker feed.
size required to handle the dust loading.
a. Types. Two types of stoker feed are available
Maintenance costs are relatively high. However,
one use of this type of stoker that may be desirable
for use with vibrating grate and traveling grate
is in conjunction with a pulverized coal fired unit
stokers; cross-feed (or mass feed) and spreader
for the reduction of refuse derived fuel (RDF) at
feed distributor (or flipper). Selection of one type
the furnace hopper outlet. The use of this type of
or other of these stokers will depend primarily on
stoker in the described application will prevent
a comparative analysis of the capital and operating
large particles of refuse that fall to the bottom of
costs associated with the pollution control equip-
the furnace from being dumped into the furnace ash
ment needed for these feed types. With the travel-
pit before having been completely consumed in the
ing chain grate, only a mass fuel feed type will be
combustion process.
used.
(2) Single retort underfeed stoker. This type
b. Cross feed. This type of stoker feed is a mass
of stoker is not recommended to be used in boilers
fuel overfeed in which coal is placed directly on the
with steam capacities above 25,000 pph which is at
grates from a coal hopper. Continuous feed is
the low end of the size range addressed by this
automatic as a fuel bed moves away from the coal
manual. Because of its limited application, the
hopper. This type of fuel feed must have adjustable
single retort underfeed stoker will not be consid-
air dampers under the fuel bed to control combus-
ered for this manual.
tion zones. The depth of the fuel bed is generally
(3) Multiple retort underfeed stoker. This
controlled by a gate.
c. Spreader feed. This type of stoker feed throws
type of stoker is a grouping of the single retort
underfeed stokers to increase potential applications
coal to the rear of the furnace and evenly distrib-
of the underfeed retorts for boilers with steaming
utes coal from side to side with a small degree of
3-17
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