Flammable and toxic gases in open cut coal mines

Recent incidents have occurred in several Bowen Basin open cut coal mines where flammable and toxic gases have been found in areas where normal mining activities are being undertaken. Flammable and toxic gases have been found emitting from blast holes, post blast areas and strata, including spoil dumps and accumulating in the bottom of pits during temperature inversion events. Ignitions of methane have occurred during blasting operations and evidence of spontaneous combustion has been identified with the accompanying noxious gases. Methane gas has also been detected in product coal recovery tunnels, including in the cavity surrounding the coal chutes.

Hazards are evident when personnel and equipment operate in close proximity to these sources of flammable and toxic gases. Mine workers undertaking duties in and around the areas nominated should be made aware of the potential risks, their measurement and controls.

The following information may be useful in alerting mine workers in the characteristics of some the more commonly occurring gases found in open cut mines and their current recognised exposure standards.

Open cut personnel should be aware of the risks of the toxic risks and/or explosion resulting from adverse concentrations of some or all of the following gases:

Methane (CH₄)

Methane is a colourless, flammable, non toxic gas that has no odour in its pure state. Methane is the major component of natural gas and is found in varying levels in most coal and shale deposits. It is a simple asphyxiant due to its ability to displace oxygen from air where sufficient quantities exist. Oxygen deficiency causes headaches, nausea, dizziness and is likely to result in death when the oxygen concentration falls below 6%. Methane's non specific odour provides no warning of its presence in potentially dangerous concentrations.

With a density relative to air of 0.55, it is lighter and will tend to accumulate at the roof level of enclosed spaces where limited ventilation exists. Methane mixtures in air in the range 5.0 - 15.4% are flammable. The most explosive methane-air mixture is 9.46%. The most easily ignited methane-air mixture is 7.5%. Care should be taken to eliminate ignition sources from environments where there is a potential for methane to be present. Possible areas include the cabin of drill rigs working in ground with high methane content or underneath machinery parked over cracks in ground above the coal seam.

Methane can be monitored using a wide range of portable gas detectors.

Hydrogen sulphide (H₂S)

Hydrogen sulphide is a colourless gas that has a sweet taste and a pungent odour resembling that of rotten eggs. Hydrogen sulphide is produced from the decay of organic materials and is a naturally occurring seam gas in some coal and shale deposits.

It can be detected by smell at small concentrations as low as 1ppm but this is unreliable as a warning characteristic as nasal sensitivity decreases with exposure and increased concentration levels.

Hydrogen sulphide is a highly toxic gas that irritates the mucous membranes and eyes and has narcotic effect on the nervous system. The gas acts on the nervous system causing headache, dizziness, excitement and staggering gait. Massive acute exposures levels produce anoxia (absence of oxygen in arterial blood and tissues) resulting in death. Exposure to concentrations greater than 500ppm can be fatal. Excessive exposure is often characterised by depression at low concentrations, then stimulation, followed by paralysis of the respiratory system at higher levels (1000ppm).

Hydrogen sulphide has a density of 1.19 relative to normal air and as such, will tend to pool and stagnate in wells and poorly ventilated areas. It forms flammable mixtures in air in the range of approximately 4.5 - 45%. At these concentrations human life will not survive.

Hydrogen sulphide can be measured using portable gas detector fitted with an electrochemical cell or by using indicator stain tubes.

The current exposure standards specified for hydrogen sulphide are:

Time-weighted average (TWA) of 10ppm.
Short-term exposure Limit (STEL) of 15ppm.

Carbon monoxide (CO)

Carbon monoxide is a colourless, odourless, tasteless gas with a density relative to normal air of 0.97. Carbon monoxide is formed during the incomplete combustion of any carbonaceous material. It is always present in proximity to coal fires. Due to its general association with heat it is usually found in spaces tending toward roof level even though it has a specific gravity similar to air.

Carbon monoxide is both flammable and explosive. The flammable limits in air are 12.5 to 74% with the most explosive concentration being 29%.

Carbon monoxide is recognised to be the most dangerous toxic gas found in coal mines as it is impossible to detect by smell or taste. Carbon monoxide poisons the body by being absorbed into the bloodstream and preventing the blood from taking up and transporting the necessary oxygen to the various cells and organs (i.e. red blood cells will attach to carbon monoxide molecules forming carboxyhemoglobin rather than attaching to oxygen molecules and forming oxyhaemoglobin). As exposure is continued the blood becomes saturated with carbon monoxide until the blood can no longer absorb oxygen.

Carbon monoxide poisoning is also cumulative. A person can be exposed for a number of short terms to the gas with no apparent ill effect. However, on each occasion, the blood has absorbed some of the gas and the victim will become more and more susceptible to its effect. This occurs because the half life of carboxyhemoglobin is between 4 and 5 hours.

Acute carbon monoxide poisoning is a reversible process. In fresh air or oxygen, the blood will gradually rid itself of the carbon monoxide and regain its usual oxyhemoglobin level. The blood itself suffers no apparent ill effect and neither do the lungs and air passages.

Carbon monoxide can be measured using portable gas detector fitted with an electrochemical cell or by using indicator stain tubes.

The current exposure standards specified for carbon monoxide are:

TWA of 30ppm.
There is no STEL or peak limitation specified for carbon monoxide.

Carbon dioxide (CO₂)

Is a colourless gas with a slight pungent or acrid smell and a soda water taste. It has a density relative to normal air of 1.53 and will accumulate in low lying and unventilated areas. Carbon dioxide plays a major role in the body's respiration and cerebral circulation systems. At low concentrations, it acts as a respiratory and central nervous system stimulant. At high concentrations, it depresses the central nervous system causing unconsciousness and narcosis. It can be found in some mines as a seam gas and also in diesel engine exhaust emissions, mines fires, explosions and blasting operations.

Carbon dioxide can be measured using portable gas detector fitted with an electrochemical cell or by using indicator stain tubes.

The current exposure standards specified for carbon dioxide are:

TWA of 12500ppm (in coal mines).
TWA of 5000ppm (in all other industrial applications).
STEL of 30,000ppm (in all applications).

Other toxic gases

Sulphur dioxide (SO₂)

Sulphur dioxide is a colourless gas with a strong pungent odour that can be detected when the concentration reaches approximately 3ppm. It is heavier than air with a density relative to air of 2.26. Sulphur dioxide is produced by the combustion of sulphur compounds and is usually found in the vicinity of heatings in coal containing sulphur.

Sulphur dioxide is non flammable and incombustible.

Sulphur dioxide is highly irritating to the mucous membranes of the eye and respiratory tract. In low concentrations the gas produces tearing, sneezing and coughing. Sulphur dioxide is extremely poisonous but poisoning rarely occurs because it is intolerable to breath for any length of time at high concentrations. Concentrations greater than 500ppm are dangerous to life and health after short exposures.

Sulphur dioxide can be measured using a portable gas detector fitted with an electrochemical cell or by using indicator stain tubes.

The current exposure standards specified for sulphur dioxide are:

TWA of 2ppm.
STEL of 5ppm.

Nitrogen dioxide (NO₂)

Nitrogen dioxide belongs to a group of gases collectively referred to as the oxides of nitrogen. Other gases in this group include nitric oxide (NO), nitrous oxide (N₂O) and nitrogen peroxide (N₂O₄). Of these gases, nitrogen dioxide represents the greatest risk to mine workers due to its prevalence and the serious health effects that may present after exposure to low concentrations.

Nitrogen dioxide is reddish brown in colour with an acrid smell and an acid taste. It has a density relative to air of 1.6. Nitrogen dioxide is non flammable and incombustible, but it will support combustion.

Nitrogen dioxide is produced as a component of diesel equipment exhaust or as a result of the detonation or burning of explosive. Usually found in working areas immediately after shot firing. The amount of gas increases during incomplete detonation of explosives or from poorly maintained engines.

Nitrogen dioxide is extremely poisonous gas that is highly irritating to the respiratory system. Dangerous concentrations may be inhaled without causing significant discomfort to the worker. More severe symptoms may then ensue several hours later after exposure has ceased. Concentrations of 100ppm may seriously irritate the respiratory system and concentrations of up to 200ppm are dangerous to life and health if breathed only for a few minutes.

Nitrogen dioxide can be measured using a portable gas detector fitted with an electrochemical cell or by using indicator stain tubes.

The current exposure standards specified for Nitrogen dioxide are:

TWA of 3ppm.
STEL of 5ppm.

Note: TWA for nitric oxide is 25ppm.

Recommendations

With reference to the Coal Mining Safety and Health Regulation 2001 s142 Flammable or toxic gas, it is recommended that all open cut coal mine operators regularly review the mine Safety and Health Management System specifically related to the management of toxic and/or flammable gases on the mine site.

More specifically this review should focus on the effective implementation of the Standard Operating Procedure required under the Coal Mining Safety and Health Regulation 2001 s142 (2)

The training of supervisors and all mining personnel who may be exposed to these gasses is recommended to assist in achieving an acceptable level of risk. Training should include but not be limited to:

• Gas awareness training programs, including detection methods and the use of monitoring devices.
• Locations of where potential for flammable and toxic may accumulate eg. dragline tubs, coal reclaim tunnels, machine operator cabins, downwind from post blast sites, sewerage pits, workshop pits etc.
• How to safely control the hazards (i.e. exclusions zones, continuous monitoring, condition reporting etc).

Definitions

(as referenced from the document 'National Occupational Health and Safety Commission NOHSC: Exposure Standards for atmospheric contaminants in the Occupational Environment'):

• Exposure Standards: Exposure standards represent airborne concentrations of individual chemical substances which, according to current knowledge, should neither impair the health of nor cause undue discomfort to nearly all workers. Additionally, the exposure standards are believed to guard against narcosis or irritation that could precipitate industrial accidents.
• Time Weighted Average (TWA): The exposure standards for airborne contaminants are expressed as a TWA concentration over an entire 8 hour working day. However, during this 8 hour averaging period, excursions above the TWA exposure standard are permitted providing these excursions are compensated for by equivalent excursions below the standard during the working day. However, because some substances can give rise to acute health effects even after brief exposures to high concentrations, it is evident that excursions above the TWA concentration should be restricted.
• Short Term Exposure Limit (STEL): Are expressed as airborne concentrations or substances, averaged over a period of 15 minutes. This short term TWA concentration should not be exceeded at any time during a normal 8 hour working day. Workers should not be exposed at the STEL concentration continuously for longer than 15 minutes, or for more than four such periods per working day. A minimum of 60 minutes should be allowed between successive exposures at the STEL concentration.

Authorised by Brian Lyne - Chief Inspector of Mines

Contact: minesafetyandhealth@dnrm.qld.gov.au

Issued by Queensland Department of Mines and Energy