Fuel And Combustion

Oxidation Loss

• Oxidation Loss• Absorption of oxygen from environment. This absorption is directly related to

the temperature of the coal.

• It decreases the quality of the coal, when it reaches the saturation point

further deterioration stops.

• Oxygen shares its anti calorific effect with ash. 1% of the oxygen in coal has

the same effect as 1 % of ash.

• Long time storage if the coal be discouraged.

• Wind Loss• It is estimated that wind blowing takes away .5 % to 1 % of the coal with it.

• To tackle the losses, bins/silos/enclosures be used and proper standard

operating procedures be deployed.

• Carpet Loss• Generally coal is unloaded from wagons and stored on the ground which is

prepared for the purpose.

• This results in the mixing of the ground clay and soil with the bottom most

layer of the coal.

• Storage ground should be paved to solve this particular problem.

• Loss Of Coal Due To Spontaneous Combustion• Much coal is lost due to the spontaneous combustion of the coal. Higher

atmospheric temperatures are responsible for the absorption of oxygen in

the freshly mined coal. This reaction is called spontaneous because it starts

as soon as the coal is exposed to the atmosphere, however it should be

noted here that freshly mined coal has a greater tendency to absorb as

compared to coal that has been mined for some time t.

• Spontaneous Combustion• Some important points here are:

1. Oxidation of coal is a continuous process.

2. Rate of oxidation increases with the temperature increase

and decreasing coal size.

3. Coals with textural moisture and high oxygen content have low

critical temperatures and liable to ignite spontaneously.

Spontaneous Oxidation

• Spontaneous Combustion• Some important points here are:

1. Oxidation of coal is a continuous process.

2. Rate of oxidation increases with the temperature increase

and decreasing coal size.

3. Coals with textural moisture and high oxygen content have low

critical temperatures and liable to ignite spontaneously.

• Spontaneous combustion takes place at a specifictemperature called as the critical temperature. Theinitial attainment of this temperature depends uponexternal factors. This means that spontaneouscombustion can be stopped if these externalconditions are prevented but above this criticaltemperature oxidation is independent of this criticaltemperature.

• These external conditions are:1. External source of heat rays such as sunlight, nearby steam

pipes, furnace flues etc.

2. Fineness of the size of coal which gives an increasedsurface area to come in contact with free oxygen.

• Coals with textural moisture and high oxygen contenthave low critical temperatures and liable to ignitespontaneously.

Spontaneous Oxidation

Oxidation Loss

• Coal Dust• Coal dust should be separated and removed and should be used or

disposed off as soon as possible.

• Ventilation• To prevent penetration of air into the coal heap, three sided wall storage with

hardened ground would be preferred. Ventilation must be reduced by

various means such as compacting the exposed surface.

• Location• Storage area should be away from sources of heat. Rate of oxidation

doubles for every 10 degree rise in temperature.

• Height Of The Heap• Height of the heap should not be more than 3.0 m.

• Conical Heap• Conical heaps of coal should be avoided. It increases the exposed area and

promotes fresh air into the heap.

Coal Dust

Bins/Bunkers

Conical Heaps

Conical Heaps

• Long Period Storage• Coal should not be stored for longer periods of time. Especially during

summer seasons.

• Smaller Heaps• The air on one hand supplies oxygen to the coal for auto ignition

which results in a rise in temperature, on the other hand it alsoproduces cooling effect when it is in excess and flowing and helps inheat dissipation. The balance between rise in temperature and heatdissipation depends local conditions. It is generally seen that in aparticular heap, the coal is ignited inside the heap, and burns withoutflames turning into ash, but the outside of the heap is cooler , andnot burning. Heat dissipation in smaller heaps is faster and hence anumber of smaller heaps should be preferred instead of a one largeheap.

• Flow Of Stock• Stationary coal should be moved and consumed, coal should not be

stored for longer periods of time.

• Rod Test• At least once a week temperature at different places in the stock should be

observed by rod tests. Generally a temperature of 60 degree centigrade is

considered alarming as it indicates beginning of the spontaneous

combustion, and we can expect a slow and steady rise in temperature in the

heap, leading to flame less combustion within the heap.

• Hydrant points should be provided near the coal stockpiles so that in case of

an emergency, water is readily available to drench the hot coal.

Steps To Take In Case Of A Fire

• The best step would be to remove the hot coal and use it, and

segregate the hotter stock from the unaffected one.

• The heated coal should be dug up and scattered thinly on the

ground. In most cases this brings down its temperature to

normal. In some cases drenching the burning coal with excess

water has been found to be very effective.

• Sprinkling a small quantity if water has been found to be

aggravating the fire instead of quenching it, possibly due to the

formation of carbon monoxide and hydrogen, both of which are

fuels.

References

O.P GUPTA

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