Page 218 - Wastewater Solids Incineration Systems

184 Wastewater Solids Incineration Systems

Some advantages of the dense-phase system include substantially reduced amounts of
air, lower energy cost, smaller piping, and reduced abrasive wear and tear. Disadvan-
tages include the requirement for a high-pressure air system conditioned to –40°C
(–40°F) dewpoint, and the potential need for booster stations along the pipeline.
All components of pressure systems must be thoroughly sealed to reduce the
amounts of fugitive ash dust released to the surrounding areas of the facility. Even
small leaks can result in substantial amounts of dust. To avoid accumulation, an
extensive housekeeping program must be implemented. In addition, reduced airflow
caused by system leaks can reach a point where insufficient motive force exists to
transport the ash load. This can result in system plugging.
Depending on facility and system design, the vacuum system operates either
continuously or is sequenced between pick-up points using vacuum pumps located
on top of the ash storage bins. These vacuum pumps draw ash from collection cham-
bers at the bottom of the incinerator to the storage bins. Vacuum systems also include
bag filters upstream of the pumps that must be cleaned periodically to maintain
system performance. Bag house filters may be equipped with automatic cleaning
controls that initiate cleaning based on differential pressure across the filters or on a
time clock.
The vacuum system (Figure 8.4) has the advantage of avoiding fugitive ash dust
emissions. It is subject, however, to reduced capacity from undetected leaks. This
reduced capacity can result in system plugging and failure. An additional advantage
of vacuum systems is that the vacuum pumps are located on the roof of the building
or ash bins, well away from water and dust that can accumulate on the lower floors
of incinerator facilities. A properly sized vacuum system fitted with appropriate
access and connection ports can be used in housekeeping operations to remove any
ash dust that accumulates in the incinerator area during maintenance activities. In
contrast to vacuum systems, the pressure system is less susceptible to capacity reduc-
tion and plugging associated with system leaks. In a pressure system, leaks are
quickly located because ash dust discharges and accumulates near the leak opening.
Equipment for all of these systems must be constructed of materials that can
withstand the abrasion of dry ash. In fluid bed incinerators, the ash may contain sand
from the fluid bed and potentially increase abrasiveness. To achieve abrasion resis-
tance, several different techniques are used depending on manufacturer or designer.
These techniques include use of hardened materials such as Ni-Hard, and special lin-
ings such as glass, ceramic, basalt, and concrete. Material selection is best accom-
plished through testing of ash material by the manufacturer for specific abrasion