Environmental Engineering Reference
In-Depth Information
will require a source of heat to reach ignition temperature, a solids feed
system, and ash handling equipment. It is important to note that the sys-
tem must also include all required equipment (e.g., scrubbers) to achieve
compliance with air pollution control requirements. Solids are pumped
to the incinerator. The solids are dried and then ignited (burned). As
they burn, the organic matter is converted to carbon dioxide and water
vapor and the inorganic matter is left behind as ash or fixed solids. The
ash is then collected for reuse of disposal.
10.7.5.1 Process Description
The incineration process first dries then burns the sludge; the pro-
cess involves the following steps:
1. The temperature of the sludge feed is raised to 212°F.
2. Water evaporates from the sludge.
3. The temperature of the water vapor and air mixture increases.
4. The temperature of the dried sludge volatile solids rises to the igni-
tion point.
Note:
Incineration will achieve maximum reductions if sufficient fuel,
air, time, temperature, and turbulence are provided.
10.7.5.2 Incineration Processes
10.7.5.2.1 multiple hearth furnace
The multiple hearth furnace
consists of a circular steel shell sur-
rounding a number of hearths. Scrappers (rabble arms) are connected to
a central rotating shaft. Units range from 4.5 to 21.5 feet in diameter and
have from 4 to 11 hearths. Dewatered sludge solids are placed on the
outer edge of the top hearth. The rotating rabble arms move them slowly
to the center of the hearth. At the center of the hearth, the solids fall
through ports to the second level. The process is repeated in the oppo-
site direction. Hot gases generated by burning on lower hearths are used
to dry the solids. The dry solids pass to the lower hearths. The high
temperature on the lower hearths ignites the solids. Burning continues
to completion. Ash materials discharge to lower cooling hearths, where
they are discharged for disposal. Air flowing inside the center column
and rabble arms continuously cools internal equipment.
10.7.5.2.2 fluidized Bed furnace
The fluidized bed
incinerator consists of a vertical circular steel
shell (reactor) with a grid to support a sand bed and an air system to pro-
vide warm air to the bottom of the sand bed. The evaporation and incin-
eration process takes place within the super-heated sand bed layer. Air
is pumped to the bottom of the unit. The airflow expands (fluidizes) the
sand bed inside. The fluidized bed is heated to its operating temperature
