Mining Fires and Explosions

Overview

Explosions and fires are a major source of injuries and fatalities in the mining industry. After the signing of the landmark 1969 Federal Coal Mine Health and Safety Act, fatalities due to underground coal mine explosions decreased. However, since 2001 they have become more frequent. From 2006 to 2011, mine explosions accounted for nearly one-quarter of mining-related deaths.

Many of these recent explosions have been due to methane ignitions that started in abandoned work areas and extended through mine seals into active work areas. Inadequate rock dusting practices may have contributed as well.

Advances in mining technologies may be responsible for this resurgence in fatalities as well. Mines are being dug deeper than ever before, leaving large areas that must be monitored.

Many underground coal mines cover vast areas where workers are not located. Monitoring the ventilation system status is critical for detecting and correcting problems in the mine in their earliest stages.

While much progress has been made in preventing explosions in coal mines, explosions still occur, sometimes producing multiple fatalities.

Fire and ignition risks

Fire in an underground coal mine is extremely hazardous. This is because of the confined space, possible effects on the ventilation system, and limited and long evacuation routes.

Mine fires in conveyor belt entries can produce smoke and high levels of toxic gases. The ventilation system can then carry these throughout the mine.

Additionally, a fire in an underground coal mine with higher levels of methane can be especially dangerous, potentially causing an explosion.

Causes

Mine explosions are caused by accumulated flammable gas and/or combustible dust mixed with air in the presence of an ignition source. The most common sources of explosions are:

• Methane buildup and ventilation issues
• Inadequate rock dusting

Methane and ventilation systems

Methane is an odorless, tasteless, colorless, flammable, lighter-than-air gas. Methane ignites easily, resulting in a violent explosion. This may result in further explosions in the presence of combustible coal dust. Many explosions are due to methane ignitions in abandoned work areas that breached the mine seals and extended into active areas.

A mine's primary ventilation system is designed to dilute and control these accumulations. However, increased production from larger longwall panels challenges the limits of accepted ventilation system designs to maintain safe working conditions.

Ventilation issues

Federal regulations require mines to have machine-mounted methane monitors to detect potentially dangerous concentrations of methane from coal mining. These monitors provide continuous readings of methane levels near a work area.

These methane monitors must provide a warning whenever methane levels are 1% or higher and before they reach 5%. One problem with the use of methane monitors is their response time. While the mining machine is cutting coal, methane levels can rise and fall rapidly. The response time is critical because it gives workers enough warning to either fix the issue or evacuate before the methane buildup becomes fatal.

Severe changes in atmospheric pressure may also cause increased interaction with sealed areas of the mine. This can overwhelm the normal ventilation system.

Inadequate rock dusting

Accumulations of combustible coal dust in coal mines create the risk of large-scale explosions that can result in multiple deaths and traumatic injuries. The explosion hazard can be effectively controlled through the application and mixing of quality rock dust, such as limestone dust, to render inert the combustible coal dust generated during the mining and transport of coal.

However, explosions can propagate with as little as 0.01 inches of float coal dust on top of pure rock dust, suggesting the need to simultaneously apply rock dust as the mining face advances. Another potential complication is rock dust caking, where rock dust becomes wet and dries, forming a hardened cake that will not disperse.

Recent studies have uncovered a number of questions related to rock dusting, including:

• How much rock dust effectively makes coal dust inexplosive.
• How researchers can better determine the potential explosibility of coal and rock dust mixtures.
• How rock dust properly meets the quality standards to prevent coal dust from being explosive.
• How mine operators can keep rock dust from caking.

What NIOSH is doing

NIOSH conducts research to identify and reduce the causes of underground coal mine explosions. Research strategies include:

• Eliminating ignition sources
• Minimizing methane concentrations and coal dust accumulations
• Applying proper dispersible rock dust in sufficient quantity at all locations
• Properly managing sealed areas

Mitigating coal dust hazards

After NIOSH research revealed that there are finer coal particles in intake airways, the Mine Safety and Health Administration (MSHA) revised the Title 30 Code of Federal Regulations 75.403 for maintenance of incombustible content of rock dust by increasing the incombustible requirement from 65% to 80% in intake airways.

NIOSH developed and commercialized the Coal Dust Explosibility Meter to assess the potential explosibility of a coal dust-rock dust mixture. Mine operators can use the Coal Dust Explosibility Meter to manage their day-to-day rock dusting practices.

Preventing rock caking hazards

If rock dust absorbs moisture, it forms a hardened cake that will not disperse and mix with the coal dust. To combat this, NIOSH worked with manufacturers to develop anti-caking rock dusts.

Small amounts of additives are mixed into the rock dust and water slurry before spraying onto the exposed mine surfaces. The additives allows the rock dust to maintain dispersibility.

The anti-caking treated rock dusts are as effective as non-treated rock dusts. They may also be most appropriate for application in areas near seals and in tailgate entries.

Improving methane detection and ventilation

The greatest effect on methane monitor response time is a dust cap over the sensor of the monitor. All sensors have dust caps to protect the sensor head from the harsh influences of the mining environment.

Instruments do exist with faster response times that provide more accurate measurements of methane concentrations. However, the sensor heads on these devices must be continuously protected from exposure to excessive amounts of dust and water.

Through numerical modeling, NIOSH Mining identified that airflow distribution around the continuous miner face affects the methane concentration. NIOSH Mining also found that the gas accumulation zone is less dependent on the emission location.

Through testing, NIOSH Mining found that mounting the methane monitor on the return side of a continuous mining machine provides the most accurate results. However, monitors closest to the face would likely encounter damage. Those furthest from the face would not be accurate enough.

Laser technology

Additionally, lasers can detect dangerous gases. However, laser power must be kept at a minimum threshold to prevent igniting the gases. A strong laser could also cause smoldering in coal dust layers, leading to the possibility of large-scale burning.

Research found that the amount of laser power needed to create explosions was proportional to the laser beam diameter. Making the beam diameter large enough to reduce the beam intensity can prevent explosions. Using these findings, researchers demonstrated the effectiveness of a commercial fiber optic power limiter for preventing ignitions.

Sealing mines

Sealing is often a safer alternative to ventilation. Continued ventilation of abandoned areas requires continued inspection and may divert air away from where mine workers are active.

An area is inert (inactive) when the atmosphere in that area will not sustain combustion, including from ignitions. Reducing the oxygen in the atmosphere makes a sealed mine area inert. This is done by adding inert gases such as nitrogen or carbon dioxide. Alternatively, increasing the amount of existing flammable gas (methane) until it is noncombustible relative to the oxygen concentration is possible.

Fire prevention

Fire-resistant belt materials and belt fire suppression systems can prevent fires in conveyor belt entries in underground coal mines.

Additionally, ventilation air velocity has a significant effect on the detection, activation, and suppression capabilities of fire detection and suppression systems.