Page 264 - Wastewater Solids Incineration Systems

226 Wastewater Solids Incineration Systems

equilibrium, the operator must control the feed cake rate, the incinerator tempera-
ture, and the turbulence between the hot gases and the fuel. Numerous incinerator
controls may regulate these variables, which are discussed more fully below.
2.6.1 Center Shaft Speed
Some of the controls, such as center shaft speed, affect more than one of the parame-
ters required to achieve equilibrium. The center shaft rotation controls the speed at
which the cake solids are conveyed through the incinerator.
An increase in the center shaft speed will turn over the cake solids more rapidly
resulting in a shorter retention time in the incinerator, higher moisture evaporation
rate within the cake, quicker ignition of the cake solids, and a higher temperature
within the incinerator. Combustion then moves upward in the incinerator.
If the center shaft speed is decreased, the cake solids furls (i.e., rolls of cake solids
between the center rabble arm teeth) become larger, thereby reducing the frequency and
surface area of the solids exposed to the hot gases. This decreases the moisture evapora-
tion rate, slowing combustion. Combustion then moves downward in the incinerator.
In general, varying the center shaft speed can only provide a short-term correc-
tion, and without changing another variable, the fire will reestablish itself in approxi-
mately the same location, frequently only moving half a hearth. Because changes in
center shaft speed take time for the results to be observed, the operator should make
changes in 1/10 rpm increments and then wait at least 10 to 15 minutes to see the
result of the change. However, the operator should not wait too long to avoid losing
control. Most operators indicate that the center shaft speed is used only to determine
required retention time in the incinerator. A limited number of MHF operators never
change the center shaft speed.

2.6.2 Combustion Air
The combustion air fans and ambient air drawn in by the induced draft fan provide
the oxygen required for the combustion process. This air also provides the cooling
necessary to keep the combustion reaction at equilibrium. Auxiliary burner combus-
tion air is provided to the auxiliary burners. The combustion fans are typically shut
down when the burners are shut down. To prevent destruction of the burner tip,
however, some burner manufactures recommend that a limited amount of air con-
tinue to pass through the tip of the burner even if the burner is shut down.
If the incinerator experiences high temperature because of an uncontrolled
burnout, the operator can start the combustion air fans to lower the incinerator
temperature. Additionally, ambient air may be drawn into the incinerator by the