Page 344 - Wastewater Solids Incineration Systems

Appendix B Incineration Subsystems 305

the exhaust gas is at a temperature between 30 to 80°C (100 and 180°F), saturated
with water, and abrasive because of the small amount of particulate matter that may
pass through. The water that condenses can form acids that attack the fan blades. As
a result, the casing is typically constructed of 316L stainless steel, Hastelloy, Inconel,
or other corrosion- and abrasion-resistant material. Designers should consider pro-
viding trapped drains to remove condensate and an inspection port to view the con-
dition of the blades and housing.

3.2.3 Recirculation Fans
Recirculation fans have been used to recirculate hot gases from the exhaust breeching
of MHFs to the lower hearths. This recirculation has the advantage of providing a
more even heat distribution throughout the incinerator and reduces the oxygen con-
tent within the incinerator. As a result, these fans operate at temperatures of 427 to
649°C (800 to 1200°F) and are subject to severe soot and abrasive loadings. The high
temperature needs to be considered both from a material expansion and personnel
safety perspective.

4.0 AUXILIARY FUEL SYSTEMS

Auxiliary fuel systems provide fuel to burners or to fluid bed injection guns. For
MHFs, the auxiliary fuel is fed to burners and, for fluid bed incinerators, auxiliary
fuels may be fed to burners or to fuel guns that inject the fuel directly to the bed. Fuel
oil and natural gas are the most common types of auxiliary fuel, but digester gas and
coal have also been used successfully.

4.1 Fuel Oil

Fuel oil systems consist of a storage tank day tank, positive-displacement pump(s),
piping and flow measurement, and control and relief valves.
Burner pumps are typically constant speed with an oil return system. The oil
injection pumps supplying the oil guns are typically variable speed to control the bed
temperature. Regulations require the bed temperature to be higher than 620°C
(1150°F) before direct injection of oil is permitted.
For fluid bed incinerators, oil guns are typically installed in the middle of the
sand bed to ensure complete mixing. The oil gun is an injection nozzle that extends
into the bed to evenly distribute the heat. The number of guns is dependent on the
size and can be as high as 12 guns for large units. Each injection nozzle typically has