Page 295 - Wastewater Solids Incineration Systems

256 Wastewater Solids Incineration Systems

maximum temperature differential the operator can help prevent the ash from
cooling too quickly and bridging the drop holes, which could cause the incinerator
to go into a positive draft condition below that point. However, the exact tempera-
ture differential required can vary depending on the composition of the feed cake
and the size and configuration of the drop holes.
The addition of auxiliary ambient air through air ports or the visual access
doors on or just below the combustion hearth can help reduce operating tempera-
tures and slag formation. The addition of forced air through the combustion air
lines of inactive burners on or just below the combustion hearth also will reduce
operating temperatures. However, the forced air is not as effective for temperature
control because it is warmer than ambient air, a result of passing through a pressur-
izing fan or blower. In addition, the air can also place an additional load on the
induced draft fan motor.
Reducing the center-shaft speed to a minimum level to limit the combustion rate
also can reduce operating temperatures. However, caution must be exercised when
slowing down the center-shaft speed. If the center-shaft speed is too slow, the cake
will make contact with the rabble arms in the hearth where the feed cake is intro-
duced to the incinerator. This contact can cause an overload on the center-shaft drive
or destabilize the combustion process.
A long-term defensive measure to help minimize the problem of blockages in
out-hearth drop holes is to inspect the cleaner teeth on the ends of the rabble arms
and replace those that are worn or missing. These teeth often are taller than most
teeth by 50 to 100 mm (2 to 4 in). The extra depth provides the reach necessary to
compensate for the self-supporting arch of the brick work in the hearth.
As the incinerator ages and hearths are replaced, converting the original, small
rectangular or trapezoid drop holes with arch-shaped holes will create larger open-
ings that are less likely to catch clinkers or allow slag bridging and blockage (refer to
information regarding drop holes elsewhere in this chapter).
If the flame pattern is flared out, slag buildup around the burner is likely. The
burner should be readjusted to reduce the flare angle. It may even be necessary to
replace the burner with a “pencil” flame type (see Figure 11.1).
Open register burners have flared flame patterns. They also have high flame tem-
peratures (higher than 1650°C [3000°F]) because of inadequate mixing of secondary
air. The burners could generate excessive oxides of nitrogen (NOx). This type of
burner has often has severe slag buildup problems.
Closed register burners can be adjusted to have narrow flame patterns. The sec-
ondary air mixing is excellent and results in actual flame temperatures of 1093°C