Hydraulic Systems. Hydraulic bottom-ash systems are used on
boilers of all sizes and are particularly useful in slagging type furnaces
where removal of molten ash would otherwise be a problem. With this type of
system, however, fly-ash must be handled separately by steam or mechanical
exhausters. The hydraulic system uses either jet pumps or centrifugal pumps
to transport an ash/water slurry from the wet ash hopper. A clinker grinder
reduces ash size to ensure unhampered transport through the pipeline. De-
watering tanks are then used to prepare ash for removal from the plant.
Mechanical Systems. For plants up to 50,000 lbs of steam per hour
(6.3 kg/s), mechanical handling systems are sometimes used. These systems may
include any combination of screw conveyors, flight conveyors, and scraper
conveyors to transport ash from hoppers to the ash silo. Wet bottom-ash
hoppers can also be emptied by mechanical systems.
Ash Silos. Provide ash silos with a minimum capacity of 72 hours
at maximum boiler plant capacity. Although ash may set up in about three days
and proper operation includes daily removal of ash, some plants may require 7
to 10 days storage because of 4-day weekends and problems with weather and ash
disposal contractors. Size system for 2 times the ash rate. Use rotary ash
conditioners for all heating plants. In areas of freezing climates, the
rotary ash conditioner area should be enclosed and the equipment at the top of
the silo should be protected with an enclosure. The ash storage silo shall be
constructed of steel and be provided with explosion relief to conform with
NFPA 68, Venting of Deflagrations.
Ash Disposal. Disposal of ash from the rotary conditioner should
be considered at the time of design. Wood ash may be disposed of in local
land fill areas or used as a fertilizer if not contaminated with oil fired
soot. Coal ash may require disposal in a hazardous waste landfill area if not
used commercially in cement of concrete block manufacturing.