For more information on laser safety contact NIOSH Mining at OMSHR@cdc.gov.
Introduction
Laser technologies are being developed for a variety of applications. In the coal mining industry, lasers can be used to detect the presence of dangerous gases, to monitor the level of coal in storage facilities, and to survey the underground mine, to name a few applications.
One of the greatest hazards in underground mines is the ignition of methane gas or coal dust. This leads to the question - just how powerful does a laser beam need to be before it can cause an explosion? NIOSH researchers took an experimental approach to help answer this question. Results of the work are referenced in the International Electrotechnical Commission standard IEC 60079-28 Protection of equipment and transmission systems using optical radiation.
How the NIOSH research was done
Researchers conducted over one thousand ignition experiments with powerful lasers, studying several variables that can contribute to an explosion. Ignition tests were conducted in a 20 liter chamber that contained the explosions (photo to the right). Explosive concentrations of methane gas or coal dust clouds were put into the chamber. The laser beam was then directed into the chamber using fiber optic cables. If the test resulted in an explosion, the laser power was reduced and the experiment was repeated. The series of tests continued until the laser beam was not powerful enough to ignite the methane or coal dust. Many such series were repeated to study the effect of gas concentration, beam size, and other variables on ignitability.
What was discovered?
Laboratories from several countries took part in studying methane-air and coal dust cloud ignitions to independently verify results. These experiments confirmed that more power is needed to ignite coal dust clouds than is needed to ignite methane-air. Researchers also observed that the amount of laser power needed to create explosions was proportional to the laser beam diameter. This suggests that explosions could be prevented even for relatively powerful beams by ensuring that the beam diameter is large enough to reduce the beam intensity.
The following graphs show "ignition curves" of laser power versus beam diameters. Explosions can be prevented by ensuring that laser systems operate well below these ignition curves. Other potential laser ignition mechanisms should be considered as appropriate. For example, lasers could cause smoldering in coal dust layers, which may lead to large scale burning.
Laser power needed to ignite methane air as a function of beam diameter.
Laser power needed to ignite coal dust clouds as a function of beam diameter.
Released publications
Continuous Wave Laser Ignition Thresholds of Coal Dust Clouds
Ignition of Methane-Air Mixtures by Laser Heated Small Particles