Mine Gases: Types, Hazards and Detection

Mine gases are among the oldest and deadliest hazards in mining. Some are released from the strata, others are produced by blasting, diesel equipment, oxidation and fires. This article explains each important gas, the traditional “damp” names, where the gases collect, how they are detected and how mine ventilation systems keep them below safe limits.

Normal air and oxygen deficiency

Fresh air at the surface contains about 78% nitrogen, 20.9% oxygen, 0.9% argon and about 0.04% carbon dioxide by volume. Underground, oxygen can fall when it is consumed by oxidising coal, sulfide ores and timber, by people and engines, or when it is displaced by other gases such as methane or carbon dioxide.

US regulations require at least 19.5% oxygen in active workings (30 CFR 75.321 for coal, 30 CFR 57.5015 for metal and nonmetal mines). Oxygen deficiency gives no warning to the senses, so it must be measured.

Types of mine gases and their hazards

The table summarises the main mine gases. Relative density is the molar mass divided by that of air (28.96 g/mol), which tells you whether a gas tends to rise to the roof or sink to the floor in still air. Exposure values are from the NIOSH Pocket Guide.

Methane (firedamp)

Methane is the main mine gas in coal mining. It is colourless, odourless and lighter than air, and it is held in the coal by adsorption and released when the seam is mined. Its explosive range in air is 5 to 15% by volume, with the most violent explosions near 9.5%. Below 5% the mixture is too lean to burn and above 15% it is too rich, but rich mixtures become explosive again when diluted by fresh air.

US coal mine rules in 30 CFR Part 75 set clear action levels. At 1.0% methane in a working place, all work stops except correcting the ventilation, and at 1.5% people are withdrawn and electric power is cut off. Methane monitors on face machines must give a warning at 1.0% and de-energise the machine at 2.0%.

Carbon monoxide

Carbon monoxide is colourless, odourless and nearly the same density as air, so it mixes through the airstream. It binds to haemoglobin far more strongly than oxygen, starving the body of oxygen. CO is the main killer after mine fires and explosions, and a rising CO level on a conveyor belt or in a return airway is often the first sign of a developing fire.

Carbon dioxide (blackdamp)

Carbon dioxide is heavier than air and collects in low areas, sumps and old workings. Miners called oxygen deficient air rich in carbon dioxide and nitrogen blackdamp or chokedamp.

Hydrogen sulfide (stinkdamp)

Hydrogen sulfide is heavier than air, flammable and extremely toxic. Its rotten egg smell earned it the name stinkdamp, but high concentrations paralyse the sense of smell, so a fading smell can mean rising danger.

Nitrogen oxides, sulfur dioxide and hydrogen

Nitrogen oxides (NO and NO₂) come from blasting and diesel engines. NO₂ attacks the lungs, sometimes with delayed effects hours after exposure, which is why re-entry after blasting waits until fumes clear. Sulfur dioxide forms when sulfide ores are blasted or burn, and is strongly irritating. Hydrogen is released at battery charging stations and in fires, and is the lightest gas of all, so charging stations are ventilated directly to the return.

The damps: firedamp, afterdamp, whitedamp and stinkdamp

Traditional mining terms are still used in training and regulations:

• Firedamp: flammable gas, mainly methane.
• Afterdamp: the gas mixture left after an explosion or fire, low in oxygen and rich in carbon dioxide, nitrogen and carbon monoxide.
• Whitedamp: carbon monoxide, or air containing it.
• Blackdamp: oxygen deficient air rich in carbon dioxide and nitrogen.
• Stinkdamp: hydrogen sulfide.

Where gases collect: layering

Download diagram (WebP)

Once gases are thoroughly mixed with air, they do not separate again under gravity. Layering happens near the point of release, where air is slow or still. Methane emitted from the roof or a face can form a flammable layer along the roof, especially in high spots, cavities and the tops of dead-end headings. Heavy gases such as carbon dioxide and hydrogen sulfide pool in dips, sumps and abandoned areas. Good ventilation keeps the air velocity and turbulence high enough to break up layers, and detectors are placed where each gas is expected: methane sensors near the roof, CO₂ and H₂S checks low down.

How mine gases are detected

• Catalytic (pellistor) sensors: methane burns on a heated catalyst bead, changing its resistance in a bridge circuit. They read in the 0 to 5% range, need oxygen to work, and can be poisoned by silicones and some sulfur compounds, so they are bump tested and calibrated regularly. Above the explosive range a catalytic sensor can give a falsely low reading.
• Infrared sensors: gases such as methane and carbon dioxide absorb infrared light at specific wavelengths. Infrared sensors work without oxygen, are not poisoned, and can read up to 100% methane.
• Electrochemical cells: the gas reacts at an electrode and produces a current proportional to concentration. They are standard for oxygen, carbon monoxide, hydrogen sulfide, nitrogen dioxide and sulfur dioxide in portable and fixed monitors.
• Flame safety lamp: historically the main detector. A methane cap forms above the flame and grows as methane increases, while the flame dims or goes out in oxygen deficient air. It is now largely replaced by electronic instruments.
• Detector tubes: a measured sample is drawn through a glass tube of reagent, and the length of the colour stain gives the concentration. They are simple spot checks for many gases.

Modern mines combine portable multigas detectors carried by miners, machine mounted methane monitors and fixed sensors linked to a surface control room. Fixed CO sensors along conveyors work alongside the systems described in types of fire detectors.

Controlling mine gases by ventilation

The main control is dilution with enough fresh air, delivered where gas is released. Other controls include methane drainage before and during mining, sealing old workings, removing ignition sources with permissible electrical equipment, stone dusting, controlled blasting practice and diesel emission limits. These fit within the wider risk controls in mine hazards and controls.

For a steady gas emission q mixed into an airflow Q, the gas concentration is

$$c = \frac{q}{Q + q} \approx \frac{q}{Q}$$

so the airflow needed to hold the concentration at a chosen limit is

$$Q = \frac{q}{c}$$

The exact fresh air needed, allowing for the gas volume itself, is

$$Q_a = \frac{q\,(1 - c)}{c}$$

Key formulas for mine gases

Mine gases interview and viva questions

1. What is the explosive range of methane in air? Methane is explosive between about 5 and 15% by volume in air, with the most violent explosions near 9.5%.
2. What is afterdamp and why is it dangerous? Afterdamp is the gas left after an explosion or fire. It is low in oxygen and contains carbon monoxide, so it can kill miners who survive the blast itself.
3. Why can a catalytic methane sensor give a false reading? It needs oxygen to oxidise methane on the bead, so it reads low in oxygen deficient air and above the explosive range, and it can be poisoned by silicones.
4. Where should you test for hydrogen sulfide and carbon dioxide? Near the floor, in dips and sumps, because both are heavier than air and collect in low areas when air is still.
5. What airflow dilutes 0.15 m³/s of methane to 0.5%? Q = q ÷ c = 0.15 ÷ 0.005 = 30 m³/s.

Related reading

• Mine ventilation systems
• Mine hazards and controls
• Surface vs underground mining