Page 263 - Wastewater Solids Incineration Systems

Incinerator Operations 225

itself out, resulting in too little heat to evaporate the moisture and ignite the fresh
feed cake entering the incinerator.
During nonautogenous operation, incinerator operators should ask the dewa-
tering equipment operator to increase the solids content of the feed cake, if possible.
The goal is to use as little auxiliary fuel as possible to maintain steady-state opera-
tions, thus reducing operating costs.

2.5 Excess Air Reduction Techniques

The air introduced to the incinerator in excess of the theoretical amount of air
(oxygen) required for complete combustion of the cake solids is excess air. The
amount of excess air in the incinerator can be calculated using the oxygen reading in
the incinerator after combustion has occurred. A simplified equation for the per-
centage of excess air is

[(% O measured)/(20.9% % O measured)] 100 (10.3)
2 2
The oxygen concentration in the incinerator exhaust gases before the scrubber or
any dilution air is found to be 6%.

% excess air [6/(20.9 6)] 100
40% excess air
The reduction of excess air substantially affects the operating economics and has
a positive effect on air emissions from MHFs. By reducing the excess airflowing
through the incinerator, less heat energy is required to heat the makeup air and thus,
flue gas energy losses are reduced. Substantial auxiliary fuel savings occur when the
level of excess air is properly controlled in both autogenous and nonautogenous
operations. The observed incinerator excess air rate should be tracked and compared
against the design excess air rate. To determine the actual energy saved by reducing
the excess air in the incinerator, a mass balance must be calculated. In the mass-bal-
ance calculation, analytical data from incinerator operations relative to the site-spe-
cific feed cake are needed.

2.6 Combustion and Temperature Control
After the initial startup, the combustion process must be stabilized. Because the com-
bustion process is a chemical reaction which increases with higher temperatures, the
reaction rate must be controlled to allow the process to reach equilibrium. To achieve