Page 262 - Wastewater Solids Incineration Systems

224 Wastewater Solids Incineration Systems

When the combustion process can continue without auxiliary burners the
burners should be shut down; the combustion process is now autogenous. Returning
hot center shaft cooling air to a lower hearth may assist in obtaining autogenous con-
ditions. However, some insurance companies will not allow the incinerator to
operate without at least one burner being on. Other insurance companies may allow
the burners to be shut off but require that temperatures remain higher than 760°C
(1400°F) somewhere in the incinerator.

2.4.2 Nonautogenous Operation
The goal of nonautogenous incinerator operation is to maintain a stable combustion
process with the addition of a minimal amount of auxiliary fuel. Auxiliary fuels are
traditionally natural gas or no. 2 fuel oil. In limited instances, coal or municipal solid
waste have been used as auxiliary fuel. However, the amount of municipal solid
waste used cannot exceed 30% by weight (U.S. EPA, 1993).
As in autogenous operation, the feed rate to the MHF under nonautogenous
operation depends on the moisture, solids, volatility, and heat content of the feed
cake. As a general rule, the feed rate at startup should be less than the optimum
steady-state feed rate, as determined by the manufacturer and operators. The initial
rate of feed takes into account auxiliary burner capacity as described in the previous
section regarding startup for autogenous incineration.
As the moisture content of the feed cake evaporates and ignition occurs, the feed
rate may be increased. Once the feed rate reaches the optimum steady-state rate, the
combustion reaction will continue if the total heat available from the auxiliary
burners and the feed solids combustion is equal to or greater than the heat required
for evaporation.
If the feed cake solids pass through the hearth equipped with ignited burners
without reaching combustion, they likely will pass through the entire incinerator
unburned. If ignition does not occur before cake solids drop down from the upper
hearth with ignited burners, the operator should immediately start the burners on
the next burner hearth to ensure ignition. Through careful monitoring, the operator
can prevent passage of unburned cake solids.
The feed cake may dry out too quickly if auxiliary burners are allowed to stay on
too long. This may lead to hot spot creation, leading to clinker formation. This can
result from a small burnout in a limited area of the incinerator. However, if the auxil-
iary burners are shut down too soon the feed cake in the immediate area can burn