Three sizes of fiber optic cable were used. The smallest had an output core diameter of
200 pm. This was a Fiberguide* Anhydrous plastic clad silica (PCS) 400 to 200 wm fiber-optic
taper. The next size was a Spectran 400 ym core, 430 ym clad diameter Hard Clad Silica cable.
The third cable was a Fiberguide Anhydrous PCS, 800 xm core, 900 pm clad diameter cable.
Targets were selected to match the surface area of the fiber tips, thus providing a convenient way
to vary target surface area while ensuring most of the laser energy was absorbed. Krytox, a
nonflammable material, was used to adhere targets to the fiber tips.

Scale, cm

i
Laser (_]

Figure 4. Cross section of 20 liter chamber methane-air ignition test setup

Figure 5 shows a summary of minimum observed igniting powers as a function of fiber optic
core diameter under various test conditions. Out of over 150 ignitability experiments, 45
produced ignitions. Minimum igniting powers with iron oxide-based targets were 0.6, 1.1, and
2.2 W with 200, 400, and 800 um core diameter fibers, respectively. Similar tests with PC
consistently required slightly higher incident powers to cause ignition. Results reported by Hills
et al’! are included for comparison. Hills found minimum methane-air igniting powers of about
300 mW when 38-45 zm diameter coal particles placed in a glass dish were irradiated by single

* Mention of any company name or product does not constitute endorsement by the
National Institute for Occupational Safety and Health


include laboratory and large-scale dust and gas explosion hazard evaluation and the development
of instrumentation for explosibility testing. He is the author or coauthor of over 80 publications
and has one U.S. patent.

William D. Monaghan received the B.S. degree in Electrical Engineering from The Pennsylvania
State University in 1984. He joined the U.S. Bureau of Mines, Pittsburgh, PA in June 1990.

His current research is focused on intrinsic safety guidelines for lasers used in underground
mining. William has performed research in the following areas: Infrared Imaging, Ground
Penetrating Radar, Proximity Detection, and Teleoperated Equipment design. William holds two
U.S. patents.


Discussion

The high temperatures measured for targets on the fiber tips are on the order of temperatures
required to ignite methane-air with electrically heated wires. For example, in previous research,
the temperature of a 1-mm diameter nichrome wire had to be 1,220°C in order to ignite methane-
air'', This is considerably higher than the 600 to 630°C autoignition temperature (AIT) for
methane-air in a furnace'"?. Figure 5 shows that small, inert, strongly absorbing targets ignited
methane-air more easily than similar sized coal particles under certain test conditions. These
results are in agreement with trends observed by others® *'* . Progressively larger coal particles
required higher incident powers to ignite methane-air, indicating heat of combustion
contributions from the coal were negligible. Smaller targets require less power to reach high
temperatures. Nonflammable, small targets may remain intact longer than comparably sized
flammable targets, allowing them enough time to reach temperatures high enough to cause
ignition. One important implication of these observations is the need to consider nonflammable
targets as well as flammable targets in assessing potential ignition hazards.

Minimum igniting power densities for iron oxide-based targets calculated by dividing the
igniting power by the surface area of the fiber core produces values of 1920, 870, and 440 W/cm’
for 200, 400, and 800-1m fibers, respectively. Comparing these calculations with igniting
powers of figure 5 shows that smaller core fibers required lower incident powers for ignition than
larger core fibers, but larger power densities. Power limits appear appropriate for small beam
diameters, while power density limits appear appropriate for larger diameter beams.

Igniting powers and power densities observed for methane-air are much higher than those
observed for some other flammable atmospheres. For example, Welzel® reported ignition of a
carbon disulfide-air atmosphere when a layer of black iron oxide on the tip of a 50 pm core
optical fiber was heated by 50 mW from a Nd:YAG laser. Carleton® reported ignition of a
carbon disulfide-air atmosphere when a 15 mm diameter, 0.934 W/cm’, Argon ion laser beam
was directed onto a layer of coal dust. Similar disparities are seen when comparing minimum
spark ignition energies (MIE) and AITs of these two gases. MIEs are 0.015 and 0.3 mJ, and
AITs are about 90° and 600 to 630°C for carbon disulfide and methane, respectively''. Carbon
disulfide test data were used as a partial basis for optical power and power density safety
recommendations by McGeehin®. Using carbon disulfide test data as a basis for safety
recommendations would be overly conservative for methane-air atmospheres.

Several standards organizations are pursuing these and other issues pertaining to lasers used in
hazardous (classified) locations. ISA - The International Society for Measurement and Control
has established the SP12.21 Fiber Optics committee, concerned primarily with fiber optic
systems routed through hazardous locations. The International Electrotechnical Commission,
Technical Committee 31, has chartered a working group to develop safety recommendations for
optical equipment used in hazardous locations (IEC TC 31\ WG 8). These findings may also be
of interest for the ANSI Z136.1 standard. Flammability hazards for gases exist below power
levels of 0.5 W (i.e. in class 3) and power densities of 10 W/cm’, indicating existing guidelines
in the standard related to fire hazards are not directly applicable.



mode and 50 pm core diameter optical fibers. Ignition of an 8% methane-air mixture was also
reported when a 40 im particle attached to a 100 jm core diameter optical fiber was heated by
360 mW.

In the most recent work, an Agema 550 thermal imaging camera recorded temperatures of
particles on optical fiber tips heated by laser powers that produced methane-air ignition. The
Agema camera system measures the infrared (IR) radiation at wavelengths of 3.6 to 5 um and
shows the calculated temperatures as a false-color display on a monitor. The maximum
temperature in the area of interest is displayed as a numerical value. The IR camera was
calibrated up to 1500°C by the manufacturer. A 30/80-mm close-up lens allowed very high
spatial resolution. Prior to observations of the optical fibers, the temperature calibration and
spatial resolution of the IR camera were confirmed by using small apertures placed in front of a
blackbody source. Temperature measurements were within manufacturer’s specifications down
to the smallest aperture on hand - a 340-pm hole drilled through a thin metal sheet.

To measure the temperatures of targets on the ends of optical fibers, both the IR camera and
optical fiber were positioned on an optical bench (outside the 20-L chamber). The camera was
positioned about 5 cm from the end of an 800-1m core diameter fiber coated with an iron oxide
target. The fiber was tilted approximately 40° from vertical (50° from the camera axis) to
maximize the viewing area of the fiber tip while avoiding direct exposure of the camera to the
laser beam. In preliminary measurements, laser powers of 2.2 W produced target temperatures
in the range of 1400-1500°C in three separate tests. Additional tests under these and other
conditions are in progress.

3 T T T T T T T T
Key

rF ®@ Iron oxide-Krytox
4 PC (3 yum)-Krytox
29 e Fiber sized PC-cyanoacrylate

@ Coal particles: Hills
et al

POWER, W

n r 1 i Hl i i at
ie) 100 200 300 400 500 600 700 800 900

FIBER-OPTIC CORE DIAMETER, sm

Figure 5. Minimum igniting power vs. Fiber-optic diameter, various targets.



International Symposium on Hazards, Prevention, and Mitigation of Industrial Explosions:
Colloquim on Gas, Vapor, Hybrid, and Fuel-Air Explosions. Schaumburg, IL. September 21-25,
1998. Safety Consulting Engineers Inc., 2131 Hammond Drive, Schaumburg, Illinois USA
60173. pp. 218-236.

8. Cashdollar, K. L., Hertzberg, M. “20-Liter Explosibility Test Chamber for Dusts and Gases.”
Rev. of Sci. Instrum. Vol: 56 (1985) pp. 596-602.

9. Hills, P. C., Samson, P. J. “Explosion Hazards of Optical Fibres in Combustible
Environments.” Proc. 7th Optical Fibre Sensors Conference. Sydney, New South Wales. Dec. 2-
6, 1990. The Institution of Radio and Electronics Engineers Australia, Commercial Unit 3,
Edgecliff Court, 2 New McLean Street, Edgecliff, NSW 2027. pp 63-65.

10. Hills, P. C., Zhang, D. K., Sampson, P. J., Wall, T. F. “Laser Ignition of Combustible Gases
by Radiative Heating of Small Particles”. Combustion and Flame. Vol: 91 (1992). pp. 399-412.

11. Kuchta, J. M. “Investigation of Fire and Explosion Accidents in the Chemical, Mining, and
Fuel-Related Industries - A Manual.” U. S. Bureau of Mines Bulletin 680. 1985, 84 pp.

12. Conti, R. S., Hertzberg, M. “Thermal Autoignition Temperatures for Hydrogen-Air and
Methane-Air Mixtures.” Journal of Fire Sciences. Vol: 6 (1988). pp. 348-355.

13. Adler, J., Carleton, F. B., Weinberg, F. J. “Ignition of Flammable Atmospheres by Radiation
Heated Fibrous Agglomerates.” Proc. R. Soc. Lond. A. Vol: 440 (1993). pp. 443-460.

14. Carleton, F. B., Weinberg, F. J. “Radiative Ignition of Loose Agglomerates of Fine Fibers.”
Proc. Fiber Optics ‘90. SPIE-The International Society for Optical Engineering. Vol: 1314
(April, 1990). pp. 298-306.

Thomas H. Dubaniewicz, Jr. received the B.S.E.E. from the University of Pittsburgh. After
graduation, he joined the Pittsburgh Research Laboratory, U.S. Bureau of Mines, now under the
direction of NIOSH®. He has developed fiber optic sensors for methane and strain, and systems
for optical powering of remote instrumentation. He is currently working with the IEC and ISA to
develop safety guidelines for laser diodes used in hazardous locations. Tom is a registered
professional engineer in the state of Pennsylvania, and is a member of ISA and Eta Kappa Nu.

Kenneth L. Cashdollar has been a Research Physicist for 25 years at the Pittsburgh Research
Center of the Bureau of Mines and now at the Pittsburgh Research Laboratory of NIOSH. He has
an M.S. degree from the University of Wisconsin - Madison. His primary areas of research

>The Pittsburgh Research Center was part of the U.S. Bureau of Mines until 1996, when it
became the Pittsburgh Research Laboratory of the National Institute for Occupational Safety and
Health.



Laser Ignition of Flammable Gas

Thomas H. Dubaniewicz, Jr., Kenneth L. Cashdollar, and William D. Monaghan
National Institute for Occupational Safety and Health (NIOSH)
Pittsburgh Research Laboratory
Pittsburgh, Pennsylvania, USA

Abstract

Emerging laser technologies are quickly gaining acceptance in the industrial workplace.
Besides the risk of human exposure, one safety concern with the more intense lasers is the
potential for ignition of flammable gases, vapors, dusts, fibers, or flyings often found in
hazardous (classified) industrial settings. In underground coal mines, lasers are being considered
for remote measurement of explosive methane gas. Current federal safety regulations for
underground mines contain no specific guidance for evaluating the safety of optoelectronic
components such as diode lasers. The National Institute for Occupational Safety and Health is
conducting research to help provide a scientific basis for developing appropriate safety guidelines
for optical equipment in the presence of flammable methane gas and/or coal dust. One phase of
the study is an experimental evaluation of methane-air ignition by laser heated small particles.
Minimum observed igniting powers for laser energy delivered by 200-, 400-, and 800-m core
diameter fiber optic cables and directed onto selected targets in methane-air atmospheres were
0.6, 1.1, and 2.2 W, respectively. Comparisons are made with other ignition phenomena,
including heated wires.

Introduction

Today, laser diodes capable of producing several hundred milliwatts or more are found in
industrial measurement and control applications'’?. Equipment that may contain intense laser
sources include closed or open path optical analytical devices, fiber-optic links for
communication or power transmission, and others. One application of laser technology in
underground coal mines currently under evaluation is the remote measurement of explosive
methane gas (figure 1). Methane gas is often released during the mining process. Federal
regulations require periodic methane measurements at the mining face, and abatement measures
should methane exceed a specified concentration. The process of making methane measurements
in the roof-fall prone face area can require extensive safety precautions to prevent injury. The
possibility of such injuries occurring has been cited as a safety concern by the United Mine
Workers of America. A simple-to-use remote methane monitor would therefore enhance miner
safety in the roof-fall prone face area.



Several other aspects of laser ignition hazards applicable to coal mines and other industries
that use or process combustible dusts are being considered in the present NIOSH study. Coal dust
suspensions in air are another explosion hazard besides methane-air. Larger accumulations of
coal dust on surfaces represent a smoldering fire hazard. The next phases of the current study
will examine laser ignition of coal dust suspensions using the 20-L ignitability/explosibility
chamber, as well as smoldering ignition of coal dust layers.

Conclusions

Temperatures required to ignite methane by laser heated small particles were on the order of
temperatures required by electrically heated wires, and much higher than the AIT. Igniting
powers with iron oxide targets were proportional to fiber optic diameter over the range of 200 to
800 pm. Igniting powers and power densities observed for methane-air are significantly higher
than those observed for some other flammable atmospheres.

References
1. Cucci, G. R. “Remote Measurement and Environmental Monitoring Using Transmitters

Powered By Fiber Optics.” Proc. Of the 41st Annual ISA Analysis Division Sym. Vol. 29:
(April, 1996). pp. 165-169.

2. Werthen, J. G., Andersson, A. G. “Optically Powered Sensors.” Proc. Of the 41st Annual ISA
Analysis Division Sym. Vol. 29: (April, 1996). pp. 145-152.

3. Dubaniewicz, Jr., T. H., Chilton, J. E. “Optically Powered Remote Gas Monitor.” U.S.
BuMines RI 9558. 1995, 9 pp.

4. Magison, E. C. (1998). Electrical Instruments in Hazari cations, 4th ed. ISA- The
International Society for Measurement and Control, Research Triangle Park, NC. Chap. 12, pp.
493-519.

5. “Criteria for the Evaluation and Test of Intrinsically Safe Apparatus and Associated
Apparatus.” (1995). CDS No. ACRI2001. US Dept. of Labor, Mine Safety and Health
Administration, RR 1 Box 251 Industrial Park Rd., Triadelphia WV 26059. 57 pp.

6. McGeehin, P. “Optical Techniques in Industrial Measurement: Safety in Hazardous
Environments.” European Commission Report EUR 16011 EN. 1994. 15 pp.
Annex I: Carleton, F. B., Weinberg, F. J. Final Summary Report, Imperial College.
Annex II: Welzel, M. M., Bothe, H., Cammenga, H. K. Final Report, PTB.
Annex III: Tortoishell, G. Summary Report, SIRA Test and Certification Ltd.
Annex IV: Scott, S. K. University of Leeds.
Annex V: Proust, Ch. Final Report, INERIS.

7. Dubaniewicz, Jr. T. H., Cashdollar, K. L., Green, G. M., with appendix by Chaiken, R.F.
(1998). “Ignition of Methane-Air Mixtures by Laser Heated Small Particles.” Proc. of the



agreement in the literature that strongly absorbing targets facilitate ignition®. The role of target
surface area, volatility, and reactivity can be somewhat convoluted. For example, small, volatile
or combustible targets may vaporize, dissipating the laser energy before igniting the surrounding
gas. Larger combustible targets have sufficient mass to contribute significant heat of combustion
to ignite methane-air more easily than a similar sized inert target. Also, larger heated surfaces
can ignite methane-air at lower temperatures than smaller surfaces, but require higher powers to
attain similar temperatures as small targets. Small targets that vaporize near appropriate ignition
temperatures may ignite gases more readily than other small targets by achieving a minimum
volume (Carleton‘). All of these issues are relevant to coal targets that may intercept the laser

beam.
-—~
—TM
V4 Heat

( Explosive
f gas

>

Figure 3. Flammable gas ignition by laser heated small particle.

Experimental Results

Recent PRL research’ included several series of experiments to investigate one aspect of this
ignition process - the role of laser-heated small particles in methane-air ignition. Targets were
carefully attached to the ends of optical fibers and placed in a 20-L ignitability/explosibility
chamber’ (figure 4.). The chamber was sealed and flammable mixtures of methane-air were
introduced. Laser power levels were varied in an attempt to determine minimum igniting powers
under various test conditions. The laser wavelength was centered at 803 nm. Test variables
included fiber diameter, methane-air concentration, target material, and target surface area.
Targets consisted of Pittsburgh seam bituminous coal particles (PC) and inert but strongly
absorbing black iron oxide. Methane-air concentrations were varied from 5 to 10% methane, in
1% increments.


ways. Obviously, allowing ignition capable optical beams in a potentially explosive environment
is an invitation to disaster. Conversely, overly conservative evaluation criteria resulting from a
lack of understanding of the phenomena could unnecessarily prohibit useful, potentially safety
enhancing technology.

Ignition source

Fuel O xidizer

Figure 2. Ignition triangle.

The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory
(NIOSH, PRL) is conducting a study to develop safety recommendations for lasers used in
underground coal mines. The project is limited in scope to visible and near infrared continuous
wave (CW) sources. (Very short duration, high peak power optical pulses represent a
fundamentally different hazard that is not directly applicable to anticipated undergound
applications.) Beam powers and spatial distributions required to ignite methane-air and/or coal
dust are considered. It is believed that the results of this research are also applicable to other
industries that use or produce flammable gases or combustible dusts.

Technical Approach

Several optical ignition processes have been identified in the literature. Magison* has grouped
these into four categories:

. Thermal ignition of a gas volume, where the laser wavelength matches an absorption
band of the gas.

° Photochemical ignition due to photodissociation of oxygen molecules by radiation in the
ultraviolet region.

. Direct laser induced breakdown of the gas at the focus of a strong beam, producing a
plasma.

. Thermal ignition of a gas by an intermediate target heated by optical radiation. The

intermediate target may be inert or flammable.
The first three processes require either relatively high peak powers or ultraviolet radiation, and
therefore fall outside the scope of the present study.

The fourth listed ignition process requires the conversion of optical energy to thermal energy
by absorption in an appropriate target (figure 3). The target needs to attain a minimum ignition
temperature for a given ignition volume in order to ignite the surrounding gas. Some relevant
target properties include absorbance, surface area, volatility, and reactivity with air. There is



Figure 1. Concept drawing of remote methane monitor.

In this scenario, the open laser beam passes through an area where both methane gas and coal
dust are normally present. How does one ensure the risk of ignition is not significant? Most
methods for reducing explosion hazards due to the presence of energized equipment are based on
the ignition triangle shown in figure 2. Eliminating any leg of the triangle will prevent ignition
from occurring. (Notable exceptions to this general rule are techniques that allow combustion to
occur under known or controlled conditions, such as explosion-proof enclosures.) One
protection method prevents the formation of combustible mixtures near the ignition source. An
example is the pressurization technique, where a small overpressure is applied to an enclosure to
prevent the entry of flammable gas. Another protection method isolates the ignition source from
the flammable atmosphere. An example is encapsulation, where a physical barrier is placed
between the ignition source and combustible mixture. Another popular approach limits the
amount of energy introduced into the combustible mixture, effectively removing the ignition
source. An example is the intrinsic safety technique, where electrical circuits are designed so
that under both normal and fault conditions they cannot release sufficient thermal or electrical
energy to cause ignition. Very careful consideration of a particular situation is required to ensure
proper application of any of these protection techniques’.

Referring again to the open-beam remote methane monitor (figure 1), the protection method
that appears most relevant is the energy limitation approach applied to beam strength. Federal
regulations for underground coal mines contain very strict guidelines for equipment intended for
use in gassy underground mines where permissible equipment is required. However, little or no
consideration has been given to optical ignition mechanisms because typical lasers used thus far
in mines have very low power. For example, the Mine Safety and Health Administration
(MSHA) document “Criteria For The Evaluation and Test of Intrinsically Safe Apparatus and
Associated Apparatus” contains no specific guidance for apparatus containing optoelectronic
components such as diode lasers*. Inadequate safety evaluation criteria are problematic in two