Page 104 - Wastewater Solids Incineration Systems

Combustion Technology 73

auxiliary fuel directly to the fluidized bed. This mode of operation is much simpler
than with the preheat burner and is typically preferred to such an extent that a bed
once heated will be kept hot for an extended period by reheating. Another reason for
keeping the bed hot at all times is the extension of the refractory life.
Oil injection takes place through oil guns located at the periphery of the bed at
approximately 0.3 m (1 ft) above the distributor. To prevent fouling of the injector
guns, the oil is mixed with purging air at the supply end of the gun and blown into
the bed in the form of a coarse mist. Natural gas injection takes place through gas
guns located at the distributor level. To homogenously distribute the gas throughout
the sand bed, gas guns are delivered in different lengths.
It is important to maintain airflow on the oil or gas guns whenever they are
inserted to the bed. Therefore, the air supply line is equipped with a flow indicator.

7.5 Water System

To protect the heat exchanger from excessive temperatures in a hot or warm wind
box, the fluidized bed is equipped with water quench spray nozzles. The nozzles are
installed through the roof of the furnace above the freeboard. The nozzles operate at
a high gauge pressure (approximately 2100 kPa or 300 psig) to create a fine water
mist that will evaporate and quench the exhaust gas as fast as possible. The evapora-
tion takes place close to the freeboard exhaust gas duct, limiting the cooling effect of
the flue gas leaving the incinerator. The nozzles are used in sequence, depending on
the temperature of the gas at the heat exchanger inlet.
The spray nozzles have small orifices and swirl grooves. To maintain clear
internal passages and provide some cooling for the nozzle when it is not spraying, it
is purged with air from the compressed instrument air system at a gauge pressure of
approximately 410 kPa (60 psig). Both air and water supply have check valves
installed to prevent backflow of one medium into the pipes of the other.
The water supply includes the water pump, pressure regulator, filter, and
relief valve.

7.6 Ductwork and Expansion Joints
All hot gas and air ducts are furnished with internal refractory lining and internal
thermal insulation and expansion joints. The refractory lining must resist abrasion
because the hot gas contains abrasive ash and elutriated sand. Internal thermal insu-
lation is selected and sized for a typical steel temperature of 93°C (200°F).