Page 297 - Wastewater Solids Incineration Systems

258 Wastewater Solids Incineration Systems

It is suggested that ash at each wastewater treatment plant (WWTP) practicing
incineration be thoroughly analyzed to determine the temperatures at which the dif-
ferent types of slag are formed. The standard test method used to determine slag for-
mation temperature is ASTM test procedure D-1857.

1.3.1 Slag-Related Problems
Excessive slag buildup can cause several operating problems by plugging burners
and drop holes, shielding thermocouples and pressure-sensing openings, plugging
rabble teeth, and plugging the ash removal system. Each hearth should be visually
inspected periodically for slag buildup. When excessive buildup is detected, it
should be removed immediately.

1.3.2 Drop Hole Plugging
Drop holes on combustion hearths are prone to slag plugging. The soft slag that
forms at the spherical and hemisphere temperatures bridges over drop holes. This
bridging restricts upward movement of secondary air and downward movement of
biosolids. Part of the incinerator can develop positive air pressure when drop holes
plug; this must be prevented. Puffs of fly ash and smoke can be seen from door open-
ings when there is plugging and positive air pressure.
The operator should periodically check for positive pressure and drop hole plug-
ging and immediately correct any problems by rodding-out the drop holes. Modifi-
cations to drop holes that reduce plug ups will be discussed elsewhere in this chapter.

1.3.3 Slag Removal
Care should be taken when removing slag from the refractory to minimize damage
to the refractory and maintain the integrity of the hearths, sidewalls, and burner tiles.
As a result, guidelines should be prepared for the operators based on the refractory
manufacturer’s recommendations.

1.4 Other Maintenance-Related Problems
1.4.1 Burner Flame Impingement
Flame impingement on a rabble arm can result in high rabble arm temperatures. If
the arm is properly insulated with refractory, the effect will be minimal. Arms
without any insulation or broken insulation are prone to thermal stress, cracking, and
sagging. Slag buildup also occurs at high-temperature locations. When large chunks
of slag fall off of the arm they cause plugging of the spaces between the rabble teeth.