Page 106 - Wastewater Solids Incineration Systems

Combustion Technology 75

operating range for bed temperature is typically from approximately 675 to 790°C
(1250 to 1450°F) during stable operation. The temperature of the fluidized sand bed
must be high enough to ensure that ignition of the cake and of the fuel takes place. A
sufficient part of the cake combustion must take place in the bed itself so that the heat
liberated is available for evaporation of water and subsequently to reduce supple-
mentary fuel.
The bed temperature must also be kept sufficiently high to ensure that the free-
board temperature is greater than the required minimum of 843°C (1550°F) to ensure
complete combustion of hydrocarbons. Alternatively, the bed temperature should not
be kept higher than necessary to minimize fuel consumption and to respect the upper
limit to the heat exchanger gas inlet temperature of 870°C (1600°F).
Interlocks are installed to stop cake feed if the bed temperature drops below the
ignition temperature of sludge and fuel.

7.7.2 Oxygen
An analyzer installed in the hot gas duct, which connects the heat recovery system
and the air pollution control system, measures oxygen. It is recorded in real time and
displayed in the operator console.
To ensure complete combustion, it is necessary to have excess oxygen available.
The design excess air of the fluidized bed incinerator is 40%, which corresponds to
approximately 3.5 to 4.0% oxygen (by volume on a wet basis) in the furnace offgas.
To ensure good fluidization, fluidizing airflow rate must be maintained constant
at design airflow rate. Excess air is, therefore, a function of the total amount of com-
bustible material (dewatered cake and fuel) being fed to the combustor.
If the oxygen content is high, the combustion air is not being efficiently utilized
for combustion, which can mean that the capacity of the unit is less than optimal.
If oxygen concentration in the reactor offgas falls to less than 2% (by volume wet
basis) for two minutes, alarm and interlock will be activated and interrupt cake feeding.

8.0 ADVANTAGES OF THE TECHNOLOGY

As outlined above, fluid bed incineration is the most efficient thermal technique for
wastewater solids disposal. The technique offers several advantages over other tech-
niques, including

• Flexibility, suitable to intermittent operation. The inventory of hot bed solids
acts as a thermal reservoir, thereby causing only a small change in tempera-
ture upon shutdown. This situation permits quick startup following daily or
weekend shutdown.