Page 280 - Wastewater Solids Incineration Systems

242 Wastewater Solids Incineration Systems

generated from feed cake, fluidizing air, and fuel, if required. Matching heat losses
are generated from offgases, ash, and radiation and convection from the incinerator
shell and ducting. Heat loss because of radiation and convection is nearly constant
regardless of cake feed load rates. If feed cake loading is reduced, the corre-
sponding heat input is decreased although the total heat losses remain the same. In
this case, incinerator heat loss is greater than the heat gained through incineration,
decreasing thermal efficiency.
To offset this decrease in thermal efficiency, additional heat can be obtained from
additional preheating of fluidizing air or from adding auxiliary fuel. The operator
can reduce the fluidizing air rate to maintain the same level of excess air, but a min-
imum quantity of fluidizing air is required to keep the bed fluidized. If the operator
instead introduces excess fluidizing air into the incinerator, additional auxiliary fuel
will be required.

3.3 Combustion Control
Good combustion is achieved when all of the combustible elements in the feed cake
react with the oxygen in the fluidizing air to form carbon dioxide and water vapor.
Combustion occurs only if there is sufficient heat, oxygen (air), turbulence, and reten-
tion time (i.e., sufficient time to complete the combustion reaction).
In a fluid bed incinerator, the bed temperature must remain higher than the
autoignition temperature of the cake solids, approximately 649°C (1200°F). The free-
board temperature must be higher than 760°C (1400°F) to destroy hydrocarbons and
burn out carbon monoxide. Oxygen levels in the exhaust gases should be between 4
and 10% and the gases should remain in the freeboard for at least three to five sec-
onds. Turbulence is provided by the fluidizing motion of the sand bed.
Turbulence in the bed and retention time in the freeboard both vary with flu-
idizing airflow and temperature. An increase in fluidizing airflow at constant tem-
perature increases bed turbulence but decreases freeboard retention time. Con-
versely, a decrease in fluidizing airflow reduces bed turbulence but increases
freeboard retention time. An increase in temperature at constant airflow has little
effect on bed turbulence but decreases freeboard retention time.
The bed temperature can vary according to the cake feed rate, moisture content,
combustible content, and heat content. Minor variations in these variables are
masked by the fluidized sand bed, which provides a heat sink. Large variations in
any of these variables will manifest as changes in bed temperature.