Spontaneous Oxidation and Heating of Coal

SPONTANEOUS HEATING/IGNITION

OF HYDROCARBONS AND ORGANICS

DOE-HDBK-1081-94

Spontaneous Heating and Pyrophoricity

raise the material to the temperature range of 50 to 75 C (122 to 167 F), where the living

organisms die. Beyond this point, oxidation reactions must take over if ignition is to occur.

The existence of biological heating requires careful control of moisture, air supply, and

nearby combustible or flammable materials. If a "hot spot" in a pile of organic material

comes in contact with a highly flammable liquid or gas, a fire or explosion may occur. Heat

generated by biological action may also act as a catalyst for other reactions which occur only

at elevated temperatures.

The likelihood of biological heating may be reduced by the following measures:

Provide adequate ventilation of the organic material to remove moisture, heat, and

dust particles.

Limit the storage time of the organic material using a "first in, first out" rule of

thumb.

Circulate large quantities of organic materials to disperse areas of localized heating.

Appendix A lists many organic materials known to heat spontaneously and measures to

prevent their ignition.

Spontaneous Oxidation and Heating of Coal

Coal presents hazards between the time it is mined and its eventual consumption in boilers

and furnaces. Below are listed some of the characteristics of spontaneous fires in coal.

These characteristics can be used to evaluate the potential for coal fires and as guidelines for

minimizing the probability of a fire.

The higher the inherent (equilibrium) moisture, the higher the heating tendency.

The lower the ash free Btu, the higher the heating tendency. The higher the oxygen

content in the coal, the higher the heating tendency.

Sulfur, once considered a major factor, is now thought to be a minor factor in the

spontaneous heating of coal. There are many very low-sulfur western subbituminous

and lignite coals that have very high oxidizing characteristics and there are high sulfur

coals that exhibit relatively low oxidizing characteristics.

The oxidation of coal is a solid/gas reaction, which happens initially when air (a gas)

passes over a coal surface (a solid). Oxygen from the air combines with the coal,

raising the temperature of the coal. As the reaction proceeds, the moisture in the coal

is liberated as a vapor and then some of the volatile matter that normally has a distinct

odor is released. The amount of surface area of the coal that is exposed is a direct

factor in its heating tendency. The finer the size of the coal, the more surface is

Pyrophoricity

Page 10

Rev. 0