SPOTANEOUS COMBUSTION Spontaneous heating Certain materials have tendency

SPOTANEOUS COMBUSTION Spontaneous heating • Certain materials have tendency to self heat due to oxidation process, if the heat produced is faster than the rate at which it is dissipated to environment, self heating results. • Unlike exogenous fires, endogenous fires are not spontaneous. They involve incubation period for initiation -that may vary from few hours to months. • The most important example is spontaneous combustion of coal. • Oxidation of coal is exothermic process and heat liberated varies from 2. 0 to 4. 0 cal/ml of oxygen consumed. • The heat generated by oxidation process increases the temperature which in turn accelerates the oxidation process until self ignition temperature is reached.

SPONTANEOUS COMBUSTION Spontaneous combustion continued………. • During early stage of heating Co and Co 2 are evolved indicating incipient heating. Further rise in temperature results in the formation of Hydrogen and other hydrocarbons like ethylene and propylene • A typical air sample may show serious proportions of the following: • Firedamp • Carbon dioxide • Carbon monoxide • hydrogen

SPONTANEOUS COMBUSTION Factors effecting endogenous fire of coal: Mining operations Adiabatic compression of air due to sudden roof falls Heat generated due to mining machines Crushing of coal and production of fines during mining Sluggish ventilation and other adverse environmental conditions like presence of adjacent fires, etc.

SPONTANEOUS COMBUSTION Mine Fires Seam characteristics Thick seams (thickness greater than 5 m) would have higher risk Overlying seams have also higher risk Overburden over 300 m may produce more fractures in seams thereby increasing risk of spontaneous heating too little burden below 50 m is also risky as it allows entry of air from surface to coal seam. Fractured and crushed gateroad, parting are also associated with higher risk of combustion

SPONTANEOUS COMBUSTION Mine Fires Seam characteristics continued…. . Geologically disturbed seams with more cracks and seams with high friability are liable to spontaneous heating. Presence of any fault, floor heaving, dykes result more cracks in the vicinity thus complicating the spontaneous heating problems.

SPONTANEOUS COMBUSTION Mine Fires Bacterial action: Heating due to action of bacteria is possible in wood at least up to 70 degree Celsius. Presence of pyrite Ø It is a common phenomenon in pyrite mines, coal contains pyrite in the form of iron pyrite. Ø Heat is generated due to oxidation of pyrite and as such it assists the spontaneous combustion of coal Ø It causes heating when present in considerable amounts and in very finely divided stage.

SPONTANEOUS COMBUSTION Mine Fires Effect of Methods of extraction and ventilation In case of longwall method, complete stowing helps in reducing the risk of spontaneous combustion. In room and pillar method corners of pillars due to highly stressed areas pose risk. Accumulation of powdered coal: accumulation of coal in goaves, near pillars, etc increase the chances of self-heating Ventilation: Too little ventilation would not support aerial oxidation, and too high ventilation would not allow accumulation of heat. The optimum ventilation level for a particular mine would depend on the heat transfer conditions.

SPONTANEOUS COMBUSTION Mine Fires Temperature: High ambient temperature conditions caused due to geothermal gradient, presence of hot springs or fire in adjoining areas increase the risk of spontaneous heating. Humidity: Higher humidity increases the risk of spontaneous heating but highly wet seams are safer than partially humid seams. Gas emission: Higher gas emission rates are favorable in reducing the content of oxygen in goaves thus reducing oxidation rate.

SPONTANEOUS COMBUSTION Mine Fires Detection of Heating Physical symptoms: Sound Typical cracking sound indicative of advancement of fire Haze During early stages of heating, the moisture is released and water vapors come in contact with cooler ventilation air and condensed making haze like formation. Sweating of the strata Smoke can be observed when there is active fire It follows the haze like situation and droplets of water are produced that can be seen on the roofs, walls and timber supports. Gob stink It is tarry smell found in coal fire area, it is a warning sign about spontaneous heating.

Fire indices: Graham’s Ratio It is rate of production of Co or Co 2 with that of oxygen consumed. 1. Co/O 2 deficiency or 2. Co 2/O 2 deficiency (Young’s ratio) Graham’s ratio increases with the intensity of heating Fresh coal: Graham’s ratio 0. 5 to 1 represents and 2 -10 represents a fire Old coal: 1 -2 represents a heating 10 -20 represents a fire