Page 279 - Wastewater Solids Incineration Systems

Incinerator Operations 241

• Adjust the airflow rate to maintain sufficient excess air, typically 4 to 10% O .
2
• Adjust the fuel firing rate to maintain a constant bed temperature.
If the feed cake is autogenous the operator should decrease the fuel
firing rate.
If the feed cake is nonautogenous the operator should increase the fuel
firing rate.

• Set all controllers to the automatic position when the operating feed cake rate
has been established and the incinerator operation has stabilized.

3.2 Autogenous versus Nonautogenous Operations
Autogenous operations in a fluid bed incinerator may be assisted by recovering heat
from the incinerated offgases and heating the fluidizing air. A hot wind box design
accomplishes this heat recovery by passing the fluidizing air and offgases through a
gas-to-air heat exchanger. Thus, offgases are cooled and the fluidizing air is heated.
In general, autogenous incineration can be achieved without preheating the flu-
idizing air if the feed cake contains 70% volatile solids with a heating value of
23 260 kJ/kg (10 000 Btu/lb) and 35% or more dry solids by weight. This varies
with both the solids’ volatile content and heating value. However, if similar feed
cake contains at least 26% dry solids by weight, autogenous incineration can gener-
ally be achieved if the fluidizing air is preheated using the hot wind box design. If
the feed cake contains lower than 26% solids, fuel addition is required and autoge-
nous incineration is not possible.
Because fuel addition can significantly increase the operating costs of the inciner-
ation system, operators should minimize fuel addition while still maintaining good
combustion control. Methods that operators can use to minimize fuel addition
include
• Optimizing the dewatering operation to get as dry a cake as possible,

• Operating at the design excess air rate while maintaining the minimum
required fluidizing air rate for feed cake rates below design, and

• Using the maximum preheat of fluidizing air for hot wind box designs.
Fluid bed incinerators are designed to provide the best thermal efficiency at
design loading. Therefore, autogenous operations, nonautogenous operations, and
fuel addition are affected by feed cake loading to the incinerator. Heat inputs are