Page 281 - Wastewater Solids Incineration Systems

Incinerator Operations 243

Variations in oxygen content in the exhaust gases also result from changes in the
cake feed rate, moisture content, combustible content, and heat content. Minor varia-
tions in these variables do not typically cause the oxygen content to vary so signifi-
cantly that adjustments are required. If the incinerator has an oxygen trim package
installed, the controls will automatically adjust the fluidizing airflow rate. An over-
ride prevents the fluidizing air rate from being reduced below the minimum allow-
able rate. However, this type of automated adjustment is rare because the oxygen
instrumentation may require up to five minutes to sense changes. Therefore, manual
adjustments are typically sufficient.
To maintain the incinerator at the highest thermal efficiency point, the feed rate
should be steady but at the maximum incineration rate to enhance burning. Cake
feed rate depends on the dewatering operation; a smooth dewatering operation pro-
vides a relatively constant cake feed rate. A decrease in feed cake load only slightly
lowers the electrical energy consumption for the fluidizing air blowers. Therefore,
overall incinerator efficiency is also at its highest point when the feed cake is fed at
the maximum incineration rate.

3.3.1 Temperature Control
The temperature control system for a fluid bed incinerator ensures safe, steady incin-
erator operation. The temperature control system consists of bed temperature super-
vision and freeboard temperature control. The bed temperature supervision system
controls the auxiliary fuel feed rate, prevents fuel and cake from being fed to the
incinerator if the bed temperature is too low or too high, and generates alarms for
operator notification.
The auxiliary fuel feed rate is controlled by a temperature controller that receives
signals from one or more bed temperature thermocouples. If the bed temperature
increases, the controller reduces the auxiliary fuel rate by modulating a fuel valve or
varying the speed or stroke of a fuel pump. If the bed temperature decreases, the con-
troller increases the auxiliary fuel rate in a similar manner. If the feed cake is autoge-
nous, no auxiliary fuel is required.
Temperature interlocks allow auxiliary fuel to be fed to the bed only if the tem-
perature is higher than 621°C (1150°F) for fuel oil or higher than 760°C (1400°F) for
natural gas. The interlocks also allow cake solids to be fed if the bed temperature is
higher than 704°C (1300°F), although this temperature varies depending on the type
and consistency of cake solids. Finally, the interlocks shutdown both auxiliary fuel
and cake feeds if the bed temperature rises higher than the range of 871 to 982°C