Normal Air vs Mine Air: Composition, Gas Properties, and Mine Ventilation
If you are a mining engineering student or preparing for gas testing examinations, understanding the difference between Normal Air and Mine Air is essential. Atmospheric air on the surface differs significantly from the air inside underground coal mines due to various chemical and physical processes.
In this article, we break down the composition of air, the physical properties of common mine gases, the biological effects of oxygen depletion, and the statutory requirements for mine ventilation.
1. What are the Main Components of Air?
Dry atmospheric air is primarily a mixture of three main gases:
- Oxygen (O2): 20.93%
- Nitrogen (N2): 79.04%
- Carbon Dioxide (CO2): 0.03%
- Total: 100.00%
Note: Broadly speaking, air consists of roughly 21% Oxygen and 79% Nitrogen. This means approximately 1/5th (one-fifth) of the atmospheric air is oxygen.
Composition of Air: Surface Air vs. Mine Air
While the atmospheric composition remains uniform across the Earth’s surface, it changes inside underground mines:
| Gas Name | Surface Air (%) | Mine Air (%) |
|---|---|---|
| Oxygen (O2) | 20.93% | 20.28% |
| Nitrogen (N2) | 78.10% | 78.90% |
| Argon & Inert Gases | 0.94% | – |
| Carbon Dioxide (CO2) | 0.03% | 0.36% |
| Methane (CH4) | – | 0.46% |
2. Main Properties of Oxygen, Nitrogen, and Carbon Dioxide
A. Oxygen (O2)
- Essential for Life: Required for respiration by all living beings; hence termed Pranvayu.
- Physical Characteristics: Colorless, odorless, and tasteless.
- Weight: Approximately 1.1 times heavier than air.
- Combustion & Reactivity: Essential for combustion (nothing can burn without oxygen). It is non-explosive. Its chemical reaction with other elements produces heat, a process known as Oxidation.
- Solubility: Slightly soluble in water (3 parts oxygen per 100 parts water), supporting aquatic life.
B. Nitrogen (N2)
- Inert Gas: Chemically inactive under normal conditions.
- Physical Characteristics: Colorless, odorless, and tasteless.
- Weight: Slightly lighter than air (Specific Gravity: 0.97).
- Physiological Effect: Non-flammable, non-explosive, and non-toxic. It has no direct harmful effect on human health; it enters and exits the lungs unchanged during respiration.
C. Carbon Dioxide (CO2)
- Fire Extinguishing Property: Extinguishes open flames.
- Weight: 1.5 times heavier than air (Specific Gravity: 1.53).
- Physical Characteristics: Colorless and odorless, with a slightly sour taste.
- Solubility: Highly soluble in water (100 parts water can dissolve 100 parts CO2).
3. Physiology of Respiration and Oxygen Deficiency
When humans inhale, oxygen from the air binds with Haemoglobin in red blood cells to form an unstable compound called Oxihaemoglobin. Exhaled air contains approximately 16% Oxygen, 4% Carbon Dioxide, and 80% Nitrogen.
Effects of Oxygen Deficiency on the Human Body:
- 21% Oxygen: Normal atmospheric concentration.
- Below 19%: Adverse health effects begin to appear.
- At 17%: Breathing rate increases, fatigue sets in, and cognitive functions decline.
- At 2%: Causes unconsciousness and death within 1 minute.
Statutory Requirement (CMR ’57 Reg No. 130): In underground mines, oxygen levels must not fall below 19% in any place where persons work or pass.
4. Detection of Oxygen Deficiency and Associated Hazards
Oxygen deficiency in mines is traditionally detected using a Flame Safety Lamp or modern Oximeters:
- Every 1% drop in oxygen reduces the illumination of a safety lamp by 25% to 30%.
- If oxygen drops below 17%, the flame safety lamp extinguishes completely.
Danger: Entering areas where a safety lamp extinguishes is extremely dangerous. If oxygen levels drop below 2%, a person can die within a minute without experiencing prior warning symptoms.
5. Effects and Statutory Limits of Carbon Dioxide (CO2)
| CO2 Concentration (%) | Physiological Effect |
|---|---|
| 0.5% | Deeper breathing begins |
| 3% | Breathing rate doubles |
| 10% | Suffocation and unconsciousness within 30 minutes |
| 20% | Fatal within 30 minutes |
Legal Limit (CMR ’57 Reg No. 130): The concentration of Carbon Dioxide in mine air must not exceed 0.5% in any working or traveling area.
6. Causes of Air Pollution in Mines & Mine Ventilation
When fresh surface air enters an underground mine, its composition alters due to:
- Human respiration and open flames (safety lamps).
- Blasting operations and coal oxidation (which consumes O2 and generates CO2).
- Emission of hot gases like Methane (CH4) from coal seams and toxic gases from explosives or decaying timber.
Solution: Mechanical Ventilation
To prevent health hazards and gas explosion risks, foul air must be continuously extracted and replaced with fresh surface air. This process is known as Mine Ventilation, which is maintained using heavy-duty ventilating fans installed on the surface of every mine.
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