MIL-HDBK-1003/6
the oxidation becomes spontaneous. This occurs when temperatures reach 430
degrees F (221 degrees C) which is the beginning of the fourth stage. The
last stage begins at 660 degrees F (349 degrees C) at which point the coal
spontaneously ignites. The high rank coals are the easiest to store as they
contain less oxygen and oxidize slower. The iron disulfides react with water
and oxygen forming sulfuric acid, iron sulfate, and heat resulting in
increasing oxidation rates and slacking. This is the reason the washed low-
sulfur coals tend to have the best storage properties. It has been found that
spontaneous combustion is much more likely with coals having greater than 1
percent sulfur, high ash content and low fusion temperatures. When these type
coals are stored, provisions should be made to monitor the conditions within
the stockpile. For those locations where excessive losses will result from
the storage of poor quality coal, it may be justifiable to purchase a more
stable higher rank coal from another location. Without oxygen, the oxidation
process cannot take place; so it is important that the total coal surface
exposure to air be as low as possible. Coal should be stored so that air
cannot infiltrate or move through the storage pile. Spontaneous combustion
usually results from careless storage procedures. Where coal is properly
stored, this likelihood is remote.
b) Outside Storage. Where outside storage serves only as a
reserve, normal practice is to take part of an incoming shipment and transfer
it directly to live storage within the plant, and to divert the remainder to
the outside pile. The reserve stockpile should be large enough to last a
minimum of 30 days at peak steam demand. Whenever possible, additional coal
should be stockpiled in preparation for interruptions in coal delivery (such
as possible mine or freight strikes). The method of storing and reclaiming
coal in an outside storage pile should be determined to satisfy regulatory
environmental restraints. The pile should be on a concrete surface with an
treatment or neutralization of effluent should be included in design. The
storage pile should be built up in successive compacted layers no more than 2
ft (610 mm) thick. The top should be slightly crowned to prevent water from
penetrating the pile. A surface layer of fines, which can be created by
compacting with a bulldozer, will prevent oxidation of lower layers.
c) Inside Storage. The active coal stockpile is almost always
retained in a covered structure such as a bunker or silo. Quite frequently,
these structures are used for reserve storage as well. Although enclosed
storage of reserve coal is more expensive than an outdoor pile, some of its
advantages include minimized losses, good coal condition, and less labor-
intensive handling.
1.
Bunker Design. Bunkers should be sized for a minimum 30
hour supply at maximum boiler capacity. The interior surfaces of the bunkers
must be kept clear of obstructions that would impede the flow of coal.
Interior ladders should be at least 1 foot off the walls and discharge gates
should be self cleaning. The vertical bunker walls should be lined with a
corrosion resistant liner such as a resin with hardener and graphite
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