Page 126 - Wastewater Solids Incineration Systems

94 Wastewater Solids Incineration Systems

The temperature of the exit gas varies depending on the temperature and quantity of
the dilution gas, but is typically in the range of 370 to 870°C (700 to 1600°F).
Exhaust gases are ducted into a heat exchanger to recover thermal energy and
generate low temperature dilution gas. Several options are available for recovering
the heat, including steam generation, thermal oil, and hot gas heat exchangers. After
the exhaust gas exits the heat recovery unit, it is split into two directions. Approxi-
mately 70% of the exhaust gas flow enters the exhaust gas recirculation (EGR) fan
and is injected back into zone 3. The remaining 30% is directed to a fabric filter to cap-
ture and remove particulate from the exhaust stream.
After exiting the fabric filter, the exhaust is further cooled and water vapor, pro-
duced during combustion, is condensed in a packed tower condenser. The exhaust
gas is cooled to 32 to 49°C (90 to 120°F) and directed to the gas recycle header where
a portion of the exhaust gas—approximately 10%—is vented out of the process to
advanced air pollution control equipment. The remaining 20% of recycled gas is
boosted in pressure through a recycle fan and enriched with oxygen. The end result
is synthetic air which is injected back to zone 1.
Heat energy recovered from the system can be transferred directly to a feed
drying circuit. In most cases the dry feed cake provides enough thermal energy to
eliminate the need for an additional energy source for drying. Figure 5.19 illustrates
the thermal energy balance for the process installed at the North Shore Sanitary Dis-
trict’s Sludge Recycling Facility, Zion, Illinois.
Oxygen can be provided to the process from either on-site liquid storage or gen-
eration with either adsorption or cryogenic air separation technologies. Liquid
oxygen is vaporized and warmed to ambient conditions to meet process demands.
Markets for the glass aggregate product are large and diverse. More than 2.25
mil. metric tonnes (2.5 mil. tons) per year of the material is currently produced in a
similar industry known as slag marketing. The process of water quenching the
molten glass as it exits the system results in the formation of an environmentally inert
aggregate. During quenching, heavy metals that may be present are physically
sequestered in the glass matrix resulting in low leaching.

10.2 Miscellaneous Technologies
10.2.1 Gasification
Gasification is a process that uses heat, pressure, and steam to convert materials to a
gas composed primarily of carbon monoxide and hydrogen (California Integrated