Ask any old miner about "cold gas" and he'll tell you it's the one that doesn't announce itself. No smell, no colour, no warning. Your lamp simply starts to sink, and so do you. In English, we call it black damp. This post walks through the basics in a simple question-and-answer format: what it is, how it forms, how to detect it, and how mines get rid of it.
1. What is black damp?
Black damp is a mixture of gases found in mines that put out flames. It is made up of roughly 11 to 13 percent carbon dioxide and 87 to 89 percent nitrogen (an excess of nitrogen, in other words). Because it smothers fire, miners in India call it thandi gas, or "cold gas".
The English name has a simple origin. Back when miners worked with open flame lamps, this gas would snuff them out and leave them in total darkness. Hence, "black" damp.
2. How is black damp formed?
It is a blend of carbon dioxide and nitrogen. A small amount of carbon dioxide seeps out of the coal itself, but most of it comes from the slow oxidation of coal, timber and other carbon-based material underground.
Several other things add to it:
- People breathing out
- Lamps burning
- The use of explosives
- Spontaneous heating (self-combustion) in the mine
In some mines, acidic mine water reacts with limestone or iron stone and produces a surprising amount of carbon dioxide as well.
So why so much nitrogen? In all of these processes, oxygen gets used up while nitrogen is left behind. The more oxygen is consumed, the larger nitrogen's share becomes.
3. What are the properties of black damp?
- Heavier than air. Its relative density is about 1.5, so it behaves a bit like water and flows down to the lowest parts of the mine, such as the dip side.
- Found near the floor. You will usually meet it close to the bottom of a roadway. But if an entire gallery is filled with it, it can reach up to roof level.
- Does not burn. It neither burns nor helps anything else burn. It puts fires out.
- Dissolves in water. That is why it can collect in water sumps and in old, flooded workings.
- Not poisonous, but deadly all the same. It makes the body sluggish because of the lack of oxygen. If there is no oxygen left at all, a person who walks in can suffocate, much like drowning.
- No colour or smell. You may notice a faintly sharp, sour taste, but that is about it.
- Slow to mix with air. This is why it tends to hang around in old workings where the airflow is poor.
4. How does black damp affect human health?
Because the air in a black damp area is short of oxygen, a person first feels tired. Breathing gets faster, a headache sets in, and the body goes limp. If the concentration is high enough, the person can suffocate and die.
5. Is it good or bad to have black damp in the goaf?
It cuts both ways. A large amount of black damp sitting in the goaf actually helps, because it protects against spontaneous heating. The trouble starts when the air pressure drops. Then the gas can push out of the goaf and spill into the working area, and that can be dangerous.
6. How does black damp affect an oil safety lamp?
When black damp is around, the flame of an oil safety lamp becomes shorter and dimmer. For every 5 percent of black damp, the oxygen level drops by about 1 percent and the light falls by roughly 30 percent. Once black damp reaches 20 percent, oxygen is down to about 17 percent, and the lamp goes out.
7. How is black damp tested for?
The traditional method uses an oil safety lamp. At the place to be tested, lower the lamp slowly from above towards the floor. While lowering it, keep your head above the lamp at all times and watch the flame from a distance.
The flame will shrink as the concentration of gas rises, and if there is plenty of gas, it will go out. The moment the flame begins to fade, lift the lamp straight back up and carry it to fresh air.
- Take an air sample and analyse it in a laboratory.
- Measure the amount of carbon dioxide with an instrument made for the purpose.
8. At what percentage of carbon dioxide does an oil safety lamp go out?
When the air contains 3 to 4 percent carbon dioxide, the oil safety lamp will be extinguished.
9. What is the maximum carbon dioxide allowed in mine air under the regulations?
Under Coal Mines Regulation 130, enough air must be sent into the mine, and ventilation must be arranged so that carbon dioxide does not exceed 0.5 percent in any place where people work or travel.
10. What can be done to remove black damp?
- Increase the quantity of air going through the mine.
- Use burnt limestone. Soaked in water or scattered dry, it absorbs carbon dioxide from the black damp.
- Build a wall along the edge of the goaf. This keeps the gas inside the goaf so it cannot enter the workings. It is called wax walling, and it should be made of clay.
Quick takeaway: black damp does not burn, explode or poison. It simply takes the oxygen away, which is exactly why it is so easy to underestimate. Test before you enter, ventilate well, and never work in a low-lying, poorly aired area without checking first.
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