and wind from causing the belt to run off center and prevent wind from blowing
an empty belt off its idlers. Galleries also decrease deterioration of the
belt by the sun. Belt conveyors are usually used in lieu of elevators for
system capacities over 100 tons (90 700 kg)/hour. A magnetic pulley or
suspended magnet should be located upstream of equipment to be protected.
Belt conveyors should be covered for dust control and personnel protection.
Safety pull cords must be furnished to run along the length of each conveyor
for quick shutoff of the conveyor. Backstops should be provided on an
inclined conveyor to prevent reversal of belts on loss of power.
e) Flight Conveyors. Flight conveyors use chain-drive and spaced
scrapers to push or drag coal along a trough. Poor operating and maintenance
conditions have less effect on the flight conveyor than on the belt conveyor,
but the flight conveyor uses much more power and is noisy. The maximum
economic limit for double strand flight conveyors is approximately 100 tons
(90 700 kg)/hour. Above this capacity and also where long conveyors are
required, belt conveyors are more economical to operate.
f) Screw Conveyors. Simple and inexpensive, screw conveyors can
be built into restricted spaces and can easily be made dust-tight. Screw
conveyors are used for short runs or congested locations where little or no
inclination is required. For this type of conveyor, coal must be relatively
free of tramp iron, wood, and other foreign matter. Trough loading should not
exceed 30 percent of the trough cross sectional area.
Coal Storage. Coal storage can be divided into two categories
according to purpose: active storage, which directly supplies firing
equipment, and reserve storage to guard against delays in delivery. Because
the commercial value of coal decreases in storage, it is important that this
reserve be kept to the minimum safe amount. The changes that may effect the
value of coal are loss of heating value, size reduction, and possible loss due
which results from a loss of volatile matter and elemental carbon, can greatly
effect the firing characteristics of the coal. The amount of this loss and
extent of size reduction are highest near storage-pile surfaces and are
greatest for low-ranking coals.
a) Spontaneous Combustion. Oxidation of coal begins as soon as
freshly broken coal is exposed to the air producing carbon dioxide (CO2),
carbon monoxide (CO), water, and heat. There are several characteristics and
conditions which affect the oxidation of coal. They are coal rank, size or
surface area, temperature, amount of air exposure, chemical composition, size
and amount of iron disulfide (pyrites), and moisture. Oxidation of coal takes
place in five stages. The first stage is the initial absorption of oxygen.
This is a slow process and continues until the temperature reaches about 120
degrees F (48.9 degrees C). The second stage is indicated by increasing
absorption of oxygen resulting from the continued rising of temperature to 280
degrees F (138 degrees C). At this point the third stage begins which is
marked by the water vapor and CO2 being given off. This stage continues until