1 October 1997
operations crew should be aware that wire and items containing wire (e.g.,automobile tires) are a
primary cause of problems with drag conveyor jamming.
(a) Conveyor Arrangement. Because of the persistent problem of jamming, each
combustion chamber must have its own ash conveyor; however, they may share a common water
(b) Maintenance Considerations. Since the ash conveyor is known for its maintenance
problems, the designer must make provisions to ensure ease of maintenance and repair. Proper
design requires sufficient space be provided on the sides for access, but not excessive room so
that ash will accumulate in these areas. The chain must not pass through the falling ash before
turning to pick up the ash from the bottom of the tank because metal objects (e.g., cans, bolts,
scrap steel) may get caught, jam the conveyor, and break the flights. Provisions (e.g., portable
pumps, filter, and floor drains) will have to be made for removing the water from the water pit for
repairs and overhaul. The conveyor must lift the ash high enough outside the plant to allow a
dumpster to be placed underneath to receive the ash. The ash has a higher density than the
original waste, thus dumpsters have to be emptied half full not to exceed truck capacities.
(2) Back-Hoe Conveyor. Some manufacturers prefer the back-hoe type conveyor to
remove ash from the quench tank. This type of system allows the chain mechanism to remain out
of the water. A mechanism on the chain flips a metal plate (hoe) into the tank, pulls the hoe out of
the tank and up the chute, drops the ash into the dumpster, and then flips the hoe back into the up
position for the return to the quench tank. This design requires less maintenance and allows for a
much steeper rise as the ash is withdrawn.
c. Ash Cooling. Some manufacturers do not quench the ash. Instead, a spray system may be
adequate for cooling the ash if a large enough volume of water (deluge) is used. For small
incinerators (less than 10 tpd), some manufacturers may dump the hot ash directly into the
dumpster. In that case, the operators have to watch for fires in the dumpster from incompletely
burned waste. In very small units, ash removal may be done manually after a cool-down.
d. Ash Disposal. In all cases, provisions have to be made in the design for adequate ash
(1) Dewatering. Dumpsters must have drain provisions because most landfills require the
ash to be dewatered; in addition, drained dumpster water must be collected in a sump rather than
discharged into the storm sewer.
(2) Treatment. In some cases, if other precautions are not taken to reduce the source of
heavy metals, the ash may have to undergo some form of treatment before disposal to minimize
leaching of heavy metal. If an MRF is used, the removal and collection of all batteries, electronic
equipment, and other sources of lead, cadmium, and mercury from the waste sent to the incinerator
will greatly reduce the need for special treatment and/or special disposal of the ash. If the ash does
contain excess heavy metals, ash stabilization techniques such as combining the ash with portland
cement may be required. Ultimate disposal will usually be in a monofill (i.e., a landfill specially
dedicated to receiving incinerator ash). If such a commercial landfill is not available, one will have
to be constructed on the base.