• Factors causing coal fires

• Spontaneous combustion

• Conveyor fires

• Fire Detection

• Fire fighting techniques

Factors causing coal fires

Coal as a fossil fuel can catch fire by both natural and man-made causes. Some causes of coal fires can be found in:

• Spontaneous combustion 

• Forest fires close to coal seams 

• Traveling fire from one seam to the other 

• Natural hazards (e.g. thunderstorms) 

• Old mining techniques (e.g. insufficient ventilation of the mine shafts, old generators and lighting systems that spray sparks) 

• External heat sources (e.g. illegal distillation of alcohol in Indian coal mines) 

• Human induced (careless acts of mine workers) 
  

Spontaneous combustion:

Spontaneous combustion is one of the most frequent reasons for coal fires. The term 'spontaneous combustion' means that coal can start to burn without any recognizable outer influence. It is caused by coals ability to react with oxygen contained in the air. As a result of the oxidation process the temperature of the coal starts to rise. If the temperature reaches a certain temperature noxious gases are produced such as carbon dioxide. Finally, if the temperature still continues to rise the coal reaches the flash point and starts to burn. 

Spontaneous combustion depends on:

• Coal type

• Temperature 

• Availability of oxygen 

• Exposure to surface

• Thickness of coal seam

• Methods of mining
  

Conveyor fires

In the conveyor structure, material (such as coal) that falls from the moving belt, or an accumulation of settled dust from the transported substance, provides a ready source of combustible matter.   

A mechanical fault in the bearing of a roller, or friction caused by the build up of coal around a roller, or a friction fault on the belt itself, may result in a heat source sufficient to initiate combustion (fire).   

Tests have shown that hot objects such as metal falling from welding or cutting operations do not readily ignite coal. It is also evident from tests that even a mass of several kilograms of red hot metal when placed on top of or into coal dust or 'fines' will not impart sufficient energy to cause a smoldering combustion process (smoldering is considered to be the increase in temperature of a slowly increasing volume of coal over a long period). Many kilowatts of energy are required to raise the temperature of coal to the point where smoldering occurs leading to gases being generated which subsequently ignite. 

Fire Detection

Many different types of fire detectors have been tried for coal conveyors over the years including:

• Collective reflectors (thermo-couple hoods)

• Ionization & optical point smoke detectors

• Point Heat Detectors

• Linear Heat Detecting Cable

• Point Flame Detectors

• Obscuration detectors employing infra-red beams

• Pneumatic detectors with fusible bulbs   
  

Fire fighting techniques

Coal fires are reported from coalfields in China, India, Indonesia, Venezuela, South Africa, USA, Australia, Germany and many other countries. But while coal fires in developed countries are mainly under control, the technique for effective fire fighting is lacking in newly industrializing countries.

Depending on the situation, e.g. location, depth, size or age of the coal fire there are different techniques existing to extinguish them. Some of the methods are listed below. They can be operated alone or used in combination:

• Cooling down with water or injecting liquid nitrogen 

• Taking out burning coal by bulldozing or blasting 

• Cut-off air supply by covering and/or filling cracks with soil or slurry 
  

Normally the direct method is to bring the fire under control through the use of suitable extinguishing agents or coolants (for example water). Loading out the fire can also be tried if the fire is not large and is approachable. If the above attempts are not effective and are likely to endanger the safety of the personnel - than in such cases sealing off the fire is advisable. In the majority of endogenous fires in mining shafts, the method of "sealing off" is applied.

Fire & Safety Group does not warrant or assume any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process presented here.