Page 210 - Wastewater Solids Incineration Systems

176 Wastewater Solids Incineration Systems

Multiple-hearth furnaces (MHFs) and fluid bed incinerators use either wet or dry
conveyance systems. Ash is abrasive and sometimes nonuniform, making it difficult
to convey as a bulk material. Fugitive ash emissions often cause problems with the
equipment; steps need to be taken to seal all leaks. Fugitive ash can also have detri-
mental effects on ancillary or surrounding equipment. Controlling discharges of
fugitive ash is also often required by the air emissions permit for the facility.
Working in an ash-laden environment can present challenges. An improperly
designed or maintained system may create severe environmental conditions that
require continuous use of particulate masks by operating personnel. In such cir-
cumstances, maintaining a clean facility can be problematic. A clean facility typi-
cally indicates that equipment is being properly operated and maintained, which is
an important consideration for visitors.
Ash historically has been sent to landfills. In recent years more emphasis has
been placed on finding beneficial uses for the ash. Ash has been used as landfill
cover, a soil amendment, a substitute for fly ash in concrete, a fine aggregate in
asphalt, flowable fill material, and an additive in brick manufacturing. This chapter
does not cover all the possible configurations and equipment available for handling
ash. It does, however, present several techniques and systems of ash conveying and
its application to MHFs and fluid bed incinerators.

2.0 SOURCES OF ASH
The multiple-hearth incineration process produces two types of ash: bottom ash and fly
ash. “Bottom ash” is discharged from the bottom of the incinerator and constitutes the
largest portion of ash generated. “Fly ash” is discharged from the incinerator in exhaust
gases and typically is captured in a wet scrubbing process and returned to the plant in a
recycle stream. If a waste heat boiler follows the incinerator, the ash may accumulate
and need to be removed periodically with purge air or soot blowers. The waste heat
boiler will have a hopper at the bottom where the ash accumulates and is then combined
with the bottom ash system. Bottom ash will sometimes contain “clinkers.”
Clinkers are chunks of fused or partially fused ash or chunks of refractory. A
grinder can be installed to crush the clinkers. The grinder is often referred to as a
“clinker breaker” and consists of two counter-rotating, motor-driven rollers that
grind clinkers into smaller particles. The clinker breaker is typically installed in the
ash discharge chute at the bottom of the MHF. The grinder should be installed before
mechanical conveying equipment and before sluicing, if applicable. After grinding,
the ash is discharged to the conveying system.