Page 259 - Wastewater Solids Incineration Systems

Incinerator Operations 221

2.3 Steady-State Process Control

After startup procedures have been completed, the incinerator is ready for feed cake.
As the incinerator is fed, the operator must make decisions based on characteristics of
the site-specific wastewater solids (e.g., percent solids, percent volatile solids, heat con-
tent), equipment design, and operational experiences. As wastewater solids are intro-
duced to the incinerator, its internal temperature will decrease. If the temperature
decreases too much, the cake will not ignite. Proper startup temperatures ensure suffi-
cient heat to commence the combustion process. If the temperature within the inciner-
ator drops below the combustion temperature and the upper hearth burners are on,
feed cake should be stopped and a burner on a lower hearth started. This process will
assist with combustion of the cake solids that are already in the incinerator.
The optimal startup feed rate varies depending on the moisture content of the
feed cake. As a general rule, the feed rate at startup should be 25% or less of the
designed maximum feed rate.
The moisture content of the feed cake also dictates center shaft speed. When the
cake has a high moisture content and the incinerator is near maximum feed capacity,
the operator should increase the center shaft speed. A faster center shaft speed
spreads the cake more quickly on the top hearths, thereby increasing the evaporation
rate. However, if the center shaft speed is too high, unburned cake can be forced past
the burn zone. Conversely, a slower center shaft speed can help feed with a higher
moisture content burn when the incinerator is not at maximum capacity.
If the feed cake has a high solids content (28% or greater), it may be desirable to
directly feed the cake to a lower hearth within the incinerator.
The combustion process is a chemical reaction; as the feed starts to burn and pro-
duce heat, the evaporation of moisture in the cake on the upper hearths is acceler-
ated. Chapter 4, Combustion Theory, contains additional details. If the fire is allowed
to work its way up through the incinerator until all of the cake solids have been
burned, the reaction may increase exponentially, which is referred to as an uncon-
trolled burnout. An uncontrolled burnout produces high temperatures within the
incinerator and should be avoided as it can result in refractory damage.
Understanding the effects of the various operating practices can provide more cost
effective and enhanced operations. The various factors which affect operations are

• Autogenous operations,
• Nonautogenous operations,