Page 111 - Wastewater Solids Incineration Systems

80 Wastewater Solids Incineration Systems

burners located at various points in the furnace. Burners may operate either continu-
ously or intermittently and on all or selected hearths.
A measure of the quantity of water evaporated from the cake during burning is
the drop in temperature of the hot gases as they pass between the combustion zone
and the gas outlet. In an MHF, gas temperatures in the combustion zone may exceed
925°C (1700°F). These gases sweep over the cold, wet cake fed into the drying zone,
giving up considerable portions of their heat in evaporating water. While the temper-
ature of the solids is only marginally increased in the drying zone, the gas tempera-
ture is drastically reduced, typically to approximately 315 to 480°C (600 to 900°F).
Exhaust gas temperatures should be maintained at less than 480°C (900°C) by con-
trolling airflow to prevent distillation of odorous greases and tars from the drying
solids. However, afterburning is still likely to be required to meet hydrocarbon limits
(U.S. EPA, 1979).
Excess air of 100 to 125% must be provided to ensure adequate cake burnout.
Some 10 to 20% of the ash is airborne, and gas cleaning equipment must be provided
for its capture. Occasional odor problems may require installation of afterburning
equipment.
Chapter 4 discusses combustion theory and practice for the MHF; Chapter 10
provides information on operating a MHF.

9.2 Description of Major Components
The MHF has been supplied in diameters ranging from approximately 2 m (6 ft 6 in)
o.d. to 7.85 m (25 ft 9 in) o.d., with anywhere from 6 to 12 hearths. Some furnaces
have been supplied with purpose-built internal afterburning hearths, typically the
top hearth, which has a larger volume than typical hearths. Other MHFs have been
modified to provide internal afterburning by removing hearths to create a single
system with larger volume or by dedicating more than one hearth.
Multiple-hearth incinerator systems consist of the MHF and the air pollution
control system. Figure 5.13 illustrates a typical multiple-hearth incinerator system
process flowsheet. The system consists of an MHF with an internal afterburner, a
waste heat boiler, a Venturi scrubber, and a wet electrostatic precipitator (ESP). Com-
bustion air is supplied to the burners and the furnace by a combustion air burner
blower. Shaft cooling air is supplied to the central shaft by a shaft cooling air blower.
The furnace draft and pressure drop through the exhaust gas components is pro-
vided by an induced draft fan located upstream of the stack. An emergency bypass
stack (not shown) is provided for emergency relief.