Mine Gases and Firedamp Explained: Questions and Answers for Coal Mining Students || Indian Minerology

Mine Gases

1. Which gases are found in mines?

Ans. Nine kinds of gases can occur in a mine:

No.	Gas (Hindi miners' name)	Formula
1	Fire Damp (Garam Gas)	CH₄
2	Black Damp (Thandi Gas)	Mixture of CO₂ and N₂
3	White Damp (Khooni Gas)	CO
4	Stink Damp (Pocha/Sadi Gas)	H₂S
5	Nitrous Fumes (Khansi Gas)	NO, NO₂, N₂O₃, etc.
6	Gob Stink (Aag ki Gas)	Mixture of some gases
7	After Damp (explosion-generated gas)	Mixture of several gases
8	Sulphur Dioxide	SO₂
9	Hydrogen	H₂

Water vapour is also present in mine air.

Fire Damp (Firedamp)

2. Is "garam gas" really hot?

Ans. No. It is methane, which is not hot by itself, but it catches fire not only from a flame or electric spark but also on contact with heated surfaces. That is why miners call it "garam gas" (hot gas). If it is present in the air in a particular proportion, such contact can even cause an explosion.

In English this inflammable gas is called Fire Damp.

3. Why is firedamp found in coal mines?

Ans. Hundreds of millions of years ago the earth was covered with huge dense forests. Earthquakes and similar events brought the forests down, and the trees and plants were washed into low-lying places and collected under water. Over time, layers of soil and sand settled on top and later turned into rock strata. Without air, and under the weight above, the plant matter turned into coal, and as leaves and other matter rotted in the water, firedamp was also formed. This gas stayed trapped under pressure in the tiny pores of the coal and in some cavities and cracks. That is why firedamp is found in coal mines.

4. Can firedamp be found in every coal mine?

Ans. Yes, firedamp can be found somewhere in any coal mine. In India, until 1965 the law treated some mines as non-gassy and others as gassy. In 1965 firedamp emerged in Dhori Colliery, which was considered non-gassy, and the open lights then in use caused an explosion in which 375 persons died. It then became clear that gas can occur anywhere in any coal mine, and since then all underground mines are classed as "gassy mines".

5. Is a mine called a "gassy mine" if any gas at all is found in it?

Ans. No. In a coal mine, "gas" ordinarily means firedamp. Since firedamp may be met anywhere in every coal seam, every underground coal mine is called a gassy mine.

6. Can opencast mine coal not contain firedamp?

Ans. Opencast coal can also contain gas, but as soon as the coal is exposed or cut, the gas mixes with the air and escapes. So such a mine is not called a gassy mine.

7. How many types of coal seams are there according to the quantity of gas?

Ans. Three types:

First Degree Gassy Seam – In the underground workings, no more than 0.1% firedamp is found anywhere in the general body of the air, and no more than 1 cubic metre of gas is emitted per tonne of coal produced. Seams with no gas at all are also counted as first degree.
Second Degree Gassy Seam – More than 0.1% firedamp is found somewhere in the general body of the air, or the gas emitted is more than 1 but up to 10 cubic metres per tonne of coal produced.
Third Degree Gassy Seam – More than 10 cubic metres of firedamp is emitted per tonne of coal produced.

8. How does firedamp come out into the workings?

Ans. Firedamp can be released in five ways:

Slow seepage – while coal is being cut, the trapped gas escapes slowly. This is called exudation.
As a gas blower.
As a gas outburst.
From a fall of the roof.
When the atmospheric pressure drops suddenly, gas stored in the goaf comes out onto the face.

9. What are a gas blower and an outburst?

Blower – In some places in a coal seam gas is held under high pressure. When coal cutting meets a fissure or crack, the compressed gas comes out as a continuous stream with a hissing sound, and sometimes a gurgling sound. This is called a gas blower. If a lot of gas is coming out, you can feel it like a puff of air when you hold your hand there.

Outburst – In some places where gas is held under high pressure, the layer of coal becomes thin as cutting proceeds, and the compressed gas inside suddenly breaks through the coal and rushes out. This is called a gas outburst.

Note: The difference between a blower and an outburst is like that between a leak and a burst in a vehicle tyre.

10. What are the properties of firedamp?

It is the most commonly found of the inflammable gases in mines.
It burns in air or oxygen with a pale blue, non-luminous flame.
Within a certain range (about 5% to 15% methane in air) it forms an explosive mixture, through which flame spreads rapidly away from the source of ignition.
It is about half as heavy as air. Its specific gravity is 0.553, so it collects at the top: in roof cavities, near the roof, and in rise headings. Once mixed with air it does not separate easily.
Being colourless, odourless and tasteless, it cannot be detected by sight, taste or smell.
It can be found in any coal mine.
It is not poisonous, but in large quantity it reduces the amount of air at that place. One-fifth of air is oxygen, so a shortage of oxygen can affect health.

11. At which places in a mine is firedamp found?

Ans. Firedamp is found where it is emitted in greater quantity, where air flow is poor, or at high places. Such places in a mine are:

1. Working face 2. Near faults, dykes and slips 3. Rise headings 4. Near the roof 5. Roof cavities 6. Broken or cracked roof 7. Dead ends 8. Goaf edges 9. Cutter-machine kerf 10. Holes drilled for blasting.

12. Firedamp is not poisonous. Why is it still considered dangerous?

Firedamp is inflammable, so wherever it is present there is a danger of fire.
When it is 5% to 15% (precisely 5.4% to 14.8%) of the air, a flame or spark can cause an explosion.
If there is a lot of firedamp in the mine, oxygen in the air is reduced, which affects health. If a person puts his head into a place with only firedamp, death can occur immediately for lack of oxygen.

13. Why can't an explosion occur when firedamp is below 5% or above 15%?

Ans. Below 5%, firedamp burns when it meets a flame but does not explode, as there is too little gas for an explosion. Above 15%, there is no explosion because there is not enough oxygen at that place.

14. At what percentage of firedamp is the explosion most powerful?

Ans. The most violent explosion occurs when the air contains 10% (precisely 9.6%) firedamp.

15. What conditions must combine for a firedamp explosion in a mine?

Ans. Three things:

Presence of 5.4% to 14.8% firedamp in the mine atmosphere.
Presence of sufficient oxygen for an explosion.
Contact of the firedamp with a source of ignition, such as a flame, an electric spark, a heated surface, or the flash of explosives.

16. What precautions should be taken to prevent firedamp explosions?

Firedamp should not accumulate anywhere in the mine. For this:
- (a) Provide adequate ventilation, keep stoppings in good order, and carry air to the working face by coursing it.
- (b) Test for gas carefully: take special care at goaf edges, near faults, in rise headings, near the roof, and before shotfiring. If gas is found anywhere, arrange to remove it immediately.
- (c) Where more than 1.25% gas is found, cut off the electric power and arrange to remove the gas.
Maintain electrical machines, switches, cables, haulage/conveyors and other machinery, and safety lamps properly so that no spark, heat or fire is produced. Observe all precautions during shotfiring.

17. Besides firedamp, what else in a coal mine can explode?

Ans. Coal dust. If coal dust is raised in the air in sufficient quantity and meets a spark of adequate strength, it can explode. Stink damp (H₂S), white damp (CO) and hydrogen (H₂) also become explosive within certain limits, but such quantities are generally not found in mines.

18. Why can a coal dust explosion follow a firedamp explosion?

Ans. When a firedamp explosion spreads, a strong blast of air travels ahead of the flame. It raises the dry coal dust lying in the mine into a cloud of coal dust, which the flame of the firedamp explosion coming from behind can ignite.

Note: For this reason coal dust is swept away from the mine, or rendered inert with water sprays or by spreading stone dust.

19. What is the effect of firedamp on human health?

Ans. Firedamp has no direct effect on health, but its presence reduces the amount of air. One-fifth of air is oxygen, so oxygen is reduced and people are affected.

Example: If firedamp is 40%, the air is only 60% (100 − 40). One-fifth of that, i.e. 12%, is oxygen. This shortage of oxygen will affect health. If a place holds only firedamp, there will be no oxygen at all and a person will die of suffocation.