Coal Storage

SPONTANEOUS HEATING/IGNITION

Spontaneous Heating and Pyrophoricity

DOE-HDBK-1081-94

OF HYDROCARBONS AND ORGANICS

exposed per unit of weight (specific area) and the greater the oxidizing potential, all

other factors being equal.

Many times, segregation of the coal particle sizes is the major cause of heating. The

coarse sizes allow the air to enter the pile at one location and react with the high

surface area fines at another location. Coals with a large top size

[e.g., 100 mm ( 4 in.)], will segregate more in handling than those of smaller size

[50 mm ( 2 in.)].

It is generally believed that the rate of reaction doubles for every 8 to 11 C

(15 to 20 F) increase in temperature.

Freshly mined coal has the greatest oxidizing characteristic, but a hot spot in a pile

may not appear before one or two months. As the initial oxidization takes place, the

temperature gradually increases and the rate of oxidization accelerates.

There is a critical amount of airflow through a portion of a coal pile that maximizes

the oxidation or heating tendencies of coal. If there is no airflow through a pile, there

is no oxygen from the air to stimulate oxidation. If there is a plentiful supply of air,

any heat generated from oxidation will be carried off and the pile temperature will

reach equilibrium with the air temperature; this is considered a ventilated pile.

When there is just sufficient airflow for the coal to absorb most of the oxygen from

the air and an insufficient airflow to dissipate the heat generated, the reaction rate

increases and the temperatures may eventually exceed desirable limits.

Coal Storage

Coal should be stored in properly designed bunkers, silos, bins, or in outside piles. The most

important aspects of coal storage are minimizing the flow of air through the pile, using the

"first-in, first out" rule of thumb, and minimizing the amount of finely divided coal in the pile.

"Hot spots" should be removed or exposed to the atmosphere to allow cooling. Coal should

be compacted if possible to reduce the amount of air in the pile. Water may be used to cool

hot spots, but should be used with caution on large areas of hot coal to present

accumulations of hazardous amounts of water. Coal should not be stored in outside piles

located over utility lines (water, gas, etc.).

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Pyrophoricity