TM 5-810-15
CHAPTER 6
ASH HANDLING
b. Boiler design and configuration. The boiler
6-1. General.
determines the amount of coal to be burned, and
This chapter addresses the requirements for the ash
the percentage of fly ash to bottom ash. In a
handling system for a coal fired boiler plant.
pulverized coal-fired boiler approximately 80 per-
a. Design criteria. Ash handling systems were
cent of all ash is fly ash and the remainder 20
relatively simple prior to the enactment of stringent
percent is bottom ash. In a stoker fired boiler
environmental regulations during the past twenty
approximately 20 to 30 percent of the total ash
years. The ash was commonly quenched in wet ash
content in the coal is fly ash with the remain-
pits and hydraulically discharged through ash sluice
ing amount being bottom ash. The versatility of the
trenches to a sump pit and from there were pumped
boilers to burn a wide range of coals should be
to an ash fill area. Bottom ash, pulverizer or mill
considered to determine the highest ash produc-
pyrite rejects (pulverized coal fired plants only),
tion rate when sizing the system conveying capac-
economizer ash and fly ash are sometimes handled
ity.
by individual, independent systems in plants now
c. Disposal conditions. Disposal to an ash pond
being designed.
or, alternatively, to storage bins or silos is a factor
b. Methods. A well accepted method of handling
in selection of equipment. Ash ponds require large
bottom ash and fly ash today is by the use of
areas of land and must meet environmental regu-
pneumatic conveying systems in stoker fired boil-
latory restrictions. Ash storage bins require less
ers. Ash is pneumatically conveyed to a storage silo
space and are environmentally more compatible
without coming in contact with steam or liquid.
than ash ponds; however, the ash must ultimately
Figure 6-1 shows a typical bottom ash and fly ash
be removed from the bin and disposed.
conveying system. Ash dust control conditioners
d. Water availability. The availability of water
have been developed to mix water with dry bottom
as a source for conveying ash, its pH rating and
ash and fly ash in the proper proportions to reduce
other chemical characteristics must be considered.
the fugitive dust emissions during the transfer of
If the water is not recycled, the environmental
ash from the storage silo to either trucks or railcars.
regulations of the discharged water must be con-
Because of higher furnace temperatures and larger
sidered. In most localities, untreated overflow is
ash quantities in pulverized coal fired boilers,
not permitted.
bottom ash has been water quenched and
e. Type of coal. The type of coal to be burned, its
hydraulically conveyed. Dry bottom ash systems
ash content, sulfur content and its chemical
have been limited in quantity because of dry gravity
constituents have an effect on the selection of the
flow. Continuous removal dry bottom ash systems
ash handling system. The coal with the highest ash
are becoming available and allow reconsideration of
content at the maximum continuous boiler steam
dry bottom ash handling. Water filled bottom ash
output rating will be anticipated to assure adequate
storage hoppers have been designed to
ash handling capacity. Ash from some coals with
accommodate large ash quantities. Bottom ash is
high calcium oxide content, such as western
periodically removed from the bottom ash hopper
subbituminous coal, has a tendency to solidify when
and hydraulically sluiced to an ash pond or to
it comes in contact with water and should be
dewatering bins. Quantity and characteristics of ash
handled dry to the disposal areas where it can be
produced in a coal fired boiler, and the ratio of fly
blown underwater from a closed bed truck.
to bottom ash depends on the coal being used,
f. Design capacity. The design criteria for selec-
steaming rate, and method of burning. These
tion of conveying capacity will be made to require
factors, along with a LCCA of available ash
the system to operate no more than 50 percent of
handling systems will determine equipment selec-
the time or four hours in an eight hour shift. The
tion. This chapter will consider hydraulic, mechan-
remaining time is used for maintenance or catch up
ical and pneumatic ash handling systems.
time on the ash handling system. The conveying
time is based on the coal with the highest ash
6-2. System design.
content which can be used in the boilers and with a
a. General. There are many considerations in-
10 percent reserve margin on the estimated percent
volved in selecting an ash handling system for a
fly ash and bottom ash.
coal fired boiler plant. These are as follows:
6-1
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