MIL-HDBK-1003/6
Piping Connections for Boiler Maintenance. The boiler, economizer,
10.4.7
and air preheater are periodically washed down with high pressure hot water.
Soot blowers are often used for the wash down. A high pressure (pumped) water
connection shall be provided to the soot blowing manifold. A shut
off/isolation and check valve shall be placed upstream of the tie-in point to
protect the system. The high pressure water connection shall allow easy
hookup of the high pressure water system.
10.5
Fly Ash Reinjection on Coal-Fired Units (Cinder Return)
10.5.1
Advantages. Collection and reinjection into the furnace of cinders
carried over with fly ash may increase fuel consumption efficiency by 3 to 5
percent. Actual savings are doubtful because of operating difficulties,
increased maintenance and electrical power consumption. Collection equipment
shall be from the rear convection pass, economizer hopper, and air heater
hopper.
10.5.2
Disadvantages
a) Increased maintenance of fans, dust collectors and reinjection
piping because of the abrasive nature of dust.
b)
Increased flyash loading of fines in baghouse.
c) Health hazard. (The flue gas fines are so small they make the
dust carcinogenic.)
d) EPA claims the dust is so fine it goes right through the
pollution control devices.
e)
Tube erosion is increased because of additional fines in flue
gas.
f) Interface with combustion.
Reinjection of cinders can cause
slagging and clinker formation.
Operation. Economizer hoppers and air heater hoppers collect about
10.5.3
5 percent of the flyash. The cinders from these hoppers contain an unburned
carbon which may be reinjected into the furnace for combustion. Cinders from
low efficiency collectors also contain unburned carbon and may also be
reinjected for combustion. Cinders from high efficiency collectors should not
be used because of excessive fines.
Cinders are collected in hoppers and fed directly into a high
pressure air stream and blown into the furnace. The high pressure blower
supplies air to both the cinder return system and the over-fire air jets. To
prevent ash buildup in the convection section, ash must be continually removed
and reinjected into the furnace.
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