Continuous wave laser ignition thresholds of coal dust clouds

Thomas H. Dubaniewicz, Jr.,a) Kenneth L. Cashdollar, and Gregory M. Green

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, P.O. Box 18070,

Pittsburgh, Pennsylvania 15236

~!

Laser-based instruments are used in areas where coal dust ignition presents a safety hazard. The

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory ~NIOSH,

PRL! is conducting a study to help determine when an optical beam may be considered a potential

ignition source in underground coal mines or coal storage facilities. Researchers conducted

experiments to determine threshold igniting powers for Pittsburgh seam bituminous coal dust clouds

using an 803 nm continuous wave laser. For fine sized coal dust dispersed in air, concentrations

ranging from 600 to 2000 g/m3 were the most easily ignited. A heated layer of coal dust that

deposited on the fiber tip during dispersion ignited the dust clouds. Minimum observed igniting

powers for laser beams delivered by 200, 400, and 800 mm core fiber optic cables and directed into

coal dust clouds were 2.0, 3.0, and 5.0 W, respectively. Threshold igniting power was proportional

to beam diameter, and threshold igniting power density decreased with larger fiber diameters.

Ignition delay times averaged 0.6 s and did not vary significantly with laser power under initially

turbulent test conditions and with flammable targets. Comparisons are made with the results of other

researchers.

Key words: laser ignition, coal dust ignition, nonbeam hazard

I. INTRODUCTION

Laser beams can present fire or explosion hazards. Explosion hazards can be divided into ignition of explosive

materials ~explosives! or flammable atmospheres. Most flammable atmospheres can be further subdivided into flammable

gases, vapors, mists, or dusts mixed with air. Flammable

concentrations of dust in air can form in places such as grain

storage silos, processing plants, and underground coal mines.

Laser-based instruments such as level monitors are being introduced into coal dust–air atmospheres.1 Laser ignition of

flammable dust-air mixtures has not been studied extensively. As a result, this important hazard category has not

been addressed significantly in safety standards or recommended practices.2,3 For example, the National Fire Protection Association ~NFPA! Recommended Practice on Laser

Fire Protection NFPA 115 addresses flammable gases, but

does not address flammable dust–air atmospheres.3

Minimum requirements of ignition sources for causing

dust cloud explosions are usually defined in terms of minimum ~electrical! ignition energy ~MIE! and minimum autoignition temperature ~MAIT!.

4–6 MAITs are measured in specially designed furnaces, where the volume inside the

furnace is heated to a high temperature before a burst of air

disperses the dust into the furnace.5 Capacitive electrical

spark discharges are typically used for determining dust

cloud MIE.6 Neither the MIE nor the MAIT directly relates

to dust cloud ignitability by continuous wave ~cw! lasers,

which are characterized by beam power and power density.

The National Institute for Occupational Safety and

Health, Pittsburgh Research Laboratory ~NIOSH, PRL! is

conducting a study to determine when an optical beam may

be considered a potential ignition source where coal is mined

or processed. This is relevant not only to lasers already in use

in underground mines and processing plants but also to possible future use of higher-power lasers. One phase of the

study involved experiments to determine ignition thresholds

of a continuous wave laser beam penetrating coal dust

clouds. Ignition of coal dust in air6–9 is a function of several

variables, including composition, particle size and shape, and

moisture content. Small, dry coal particles with a high volatile matter content generally ignite most easily. Very fine

size, dried, Pittsburgh seam high volatile bituminous coal

dust was used in this study. Laser beam spatial dimensions

and duration also affect threshold igniting powers.10,11 The

spatial dimension of the beam was varied by using fiber optic

cables with different core diameters. Beam durations can

range from short pulses, to trains of pulses, to cw. Laserinduced spark ignition processes and thermal ignition processes can occur with very high peak power laser

pulses.10,12,13 Laser-induced photochemical ignition processes can occur with ultraviolet or shorter wavelengths as

demonstrated in studies reviewed by Tran12 and Ronney.13

Ignition mechanisms using cw lasers with wavelengths in the

visible or near infrared are primarily thermal processes.10–13

Strongly absorbing particles facilitate the thermal ignition

process.14 A cw laser with a wavelength centered at 803 nm

was used in this study. Since coal dust is highly absorbing

over the visible to near-infrared spectral region ~as discussed

later in the paper in Sec. III!, this 803 nm laser would be

representative of thermal ignition hazards posed by a wide

range of laser wavelengths. The goal of the work reported

here was to find igniting power thresholds as function of coal a!

Electronic mail: tcd5@cdc.gov
dust concentration and laser beam diameters, under the

above mentioned experimental conditions.

II. LITERATURE REVIEW

Proust conducted two recent studies of ignition of dust

clouds by cw lasers.15,16 A number of combustible dusts were

studied, including lignite coal. The ignition chamber consisted of a vertical glass column, with the dust clouds generated by a fluidized bed of dust at the bottom of the column.

The setup could maintain a dust cloud for up to several minutes. A fixture placed in the column held selected targets. A

Nd–yttrium–aluminum–garnet laser beam was directed

through a window onto the target. Proust’s reported igniting

laser powers for the dust cloud tests were the calculated powers reaching the target through the dust cloud, and took into

account the loss due to attenuation by the dust cloud. The

powers directed into the dust clouds at the window were

significantly higher than the reported igniting powers at the

target. Laser ignition experiments included:

~i! ignition of a layer of combustible dust with no cloud

present;

~ii! ignition of a combustible dust cloud by heating a layer

of the combustible dust;

~iii! ignition of a combustible dust cloud by heating a layer

of inert particles; and

~iv! ignition of a dust cloud with no target present.

A partial list of Proust’s findings include:

~i! combustible dust clouds ignite at the lowest powers

when a target is present;

~ii! strongly absorbing, inert iron oxide targets generally

produced lower dust cloud igniting laser powers than

when using targets made of the flammable dust itself;

~iii! lignite coal layers with no dust cloud ignited ~appearance of flame! at 4.07 W minimum laser power;

~iv! lignite dust clouds with an iron oxide layer target ignited at 2.5 W minimum laser power;

~v! a range of laser powers ~2.5–4 W! was reported as the

minimum igniting power for lignite dust clouds ignited with a lignite layer target;

~vi! ignition time delays at minimum igniting powers were

on the order of seconds for selected dust clouds and

tens of seconds for selected combustible dust layers;

~vii! beam diameters smaller than about 1 mm generally

required higher laser powers to ignite selected dusts;

~viii! varying the flow velocity of selected dust clouds from

0 to 0.5 m/s did not affect minimum igniting powers;

and

~ix! ignition delay of selected dust clouds increased with

flow velocity.

For his experimental conditions, Proust observed that beam

diameters smaller than 1 mm required higher laser powers to

cause ignition of dust clouds. However, studies of laser ignition of flammable gases show minimum igniting powers

typically occur with smaller beam diameters.10,11,17,18 In

studies of ignition of methane–air mixtures by laser-heated

targets on optical fiber tips, igniting powers were lower for

beam diameters below 1 mm, and were approximately proportional to the beam radius over the range of about 0.1–1

mm.18,19 In the current study, optical fibers with core diameters ranging from 0.2 to 0.8 mm were used to deliver the

beam into the dust cloud to determine if more conservative

safe power recommendations are appropriate for beam diameters below 1 mm.

Proust’s optical attenuation estimates for the intervening

dust cloud were greater than 75% for the high dust concentrations that produced the lowest igniting powers. The PRL

method of beam delivery by optical fibers ~with the target on

the end of the fiber! overcomes uncertainties in estimating

the amount of power reaching a remote target through a

dense dust cloud and contributing to ignition. The fiber optic

method also simulates situations where dust can contact

beam delivery optics.

Towle20 contends a dust cloud is unlikely to be composed of particles of ideal size and absorptivity, which will

obligingly remain stationary within the light beam. Dust

clouds generally consist mostly of fine particles as larger

particles are more difficult to entrain into a cloud and precipitate out of the cloud faster. Fine size coal dust ignites

more easily than coarse dust.6–9 A fine size coal dust was

selected for the PRL ignition testing for these reasons. To

allow the beam to interact freely with an initially turbulent

dust cloud, PRL researchers did not attach targets to the fiber

tip, and only the coal dust which deposited on the fiber was

used as an absorbing target. This method of ignition testing

simulates situations where a dust cloud is dispersed and contacts initially clean beam delivery optics.

III. EQUIPMENT AND MATERIALS

NIOSH PRL experiments were conducted in a 20 L test

chamber ~Fig. 1! designed for explosion testing of dusts,

gases, and hybrid mixtures.21 It can be used to measure lean

and rich limits of flammability, explosion pressures, and rates

of pressure rise, minimum ignition energies, minimum oxygen concentrations for flammability, and amounts of inhibitor

necessary to prevent explosions. Sapphire windows at the top

and sides allow viewing inside the chamber. The chamber

can be used at initial pressures that are below, at, or above

atmospheric as long as the maximum explosion pressure is

less than 21 bar, which is the rated pressure of the chamber.

For these tests, chamber instrumentation included a pressure

transducer, and high-speed ~200 frames/s! video camera. The

video camera was positioned at the sapphire window view

port. The 20 L chamber dust dispersion uniformity has been

extensively studied using optical dust probes.7,21

The laser was a SDL model 8110-B Integrated Laser

System ~ILS!. The laser diode wavelength was centered at

803 nm in the near infrared. The ILS output power is variable up to 10 W out of a 400 mm diam aperture. An SMA

bulkhead adapter provided fiber optic coupling. The laser

system also included a low-power visible ~red! aiming laser,

which proved useful in setting up the experiments.

Three sizes of fiber optic cable directed the laser beam

into the 20 L chamber. The smallest optical fiber had an

output core diameter of 200 mm. This was a Fiberguide An-
hydroguide plastic clad silica ~PCS! 400–200 mm fiber–

optic taper. The next size was a Spectran 400 mm core, 430

mm clad Hard Clad Silica cable, 0.4 numerical aperture

~NA!. The third cable was a Fiberguide Anhydroguide PCS,

800 mm core, 900 mm clad diameter, 0.4 NA cable.

A Scientec power meter ~model D200PC! with attached

calorimeter ~model AC2501! measured the laser powers. The

manufacturer documented the meter accuracy annually,

traceable to National Institute of Standards and Technology

standards. The accuracy was reported to be within 64% over

a 0.265–1.2 mm spectral range during the period covering

these experiments.

The coal dust used for this ignitability research was

Pittsburgh seam, high volatile bituminous coal mined from

the NIOSH PRL Safety Research Coal Mine ~SRCM!. Pittsburgh coal from the SRCM has been used as a standard at

PRL for many years under the former U.S. Bureau of Mines,

and the coal’s properties are well documented.7 The volatility

is 37% and the ash content is 6%. The Pittsburgh coal is

representative of the highest volatile bituminous coals mined

in the U.S. Fine size dust is more easily ignited6–9 than

coarse dust, and dust clouds generally consist mostly of finer

particles, so a fine size of Pittsburgh coal was chosen for

these tests. The Pittsburgh coal known as SRCM fines consists of float dust retrieved from the fines collector of a coal

pulverizing process. SRCM fines particles have irregular

shapes and different methods used to characterize the particle

sizes produce slightly different results. The size of SRCM

fines has been studied previously and is 100% less than 75

mm with a volume or mass median diameter of 11 mm, a

volume mean diameter of 14 mm, and a surface mean diameter of 9 mm.7 The moisture content of dust samples measured during the current study averaged 1.4% and ranged

from 0.9% to 2.2%.

Labsphere Inc. measured the reflectance of a sample of

Pittsburgh coal dust from 250 nm to 5 mm, in 50 nm increments. Absorptance was calculated as unity minus the reflectance. Pittsburgh coal absorptance at the 803 nm laser wavelength was about 87%. The absorptance was between 87%

and 88% throughout the visible wavelengths, decreasing to

about 75% at 2.5 mm in the infrared. The absorptance was

greater than 87% from 2.75 to 3.85 mm, corresponding with

a range of resonant vibration frequencies of several chemical

bonds found in coal.22 The highest absorptance was 95.5% at

3.4 mm. Since laser ignition is a thermal ignition process and

since coal dust is highly absorbing over the visible to nearinfrared spectral region, the 803 nm laser is representative of

ignition hazards posed by a wide range of laser wavelengths

in the visible to near-infrared spectral region. Ignition tests

with the 803 nm laser may not be representative of coal dust

clouds ignited by laser wavelengths from about 2.75 to 3.85

mm.

IV. IGNITABILITY TEST METHODS

An adequate length of fiber was pulled through a feedthrough in the 20 L chamber ~Fig. 1! to allow preparation of

the fiber tip. A fiber optic cleaver ~York model FBK 11C!

provided a flat, perpendicular, optical surface for each ignition test with the 200 and 400 mm core fibers. The 800 mm

core fiber required a manual cleaving tool. After each cleave,

the red aiming laser was directed onto a flat surface. The

cleave was determined to be acceptable when the aiming

laser produced a circular spot on the surface. The power

emanating from the cleaved end of the fiber was measured

before each test. All igniting or nonigniting powers reported

for the NIOSH experiments were those measured emanating

from the cleaved end of the fiber prior to each test. Excess

fiber was then pulled back through the feed-through until the

fiber tip was placed about mid height in the chamber, and

about 5 cm from the sapphire window for viewing with the

high speed camera. The fiber tip was positioned vertically,

pointing upwards, for most tests. In two series of tests, the

fiber was placed in a similar position but pointed downwards. For the downward-pointing tests, the fiber was positioned through the feed-through above the sapphire window

shown in Fig. 1. No attempt was made to place an artificial

target on the optical fiber tip. The only target was the coal

dust that was deposited on the tip during the dust dispersion

process. This coal dust deposit was heated to incandescence

by the laser and became the ignition source for the dust

cloud.

The dust ignitability test methods were adapted from

standard operating procedures developed under extensive

previous explosibility research.7,8,21 Each ignition test was

conducted at a fixed laser power and fixed dust concentraFIG. 1. 20 L chamber laser ignition test setup.
tion. A preweighed sample of dust was placed over a dispersion nozzle in the bottom of the 20 L chamber ~Fig. 1!. The

chamber was sealed and partially evacuated, and a pressurized burst ~0.3 s! of dried and hydrocarbon-free air dispersed

the dust into the laser beam. After dispersion, the chamber

pressure was about 0.9–1.0 bar absolute for the test. The

laser beam was turned on at least 30 s prior to dispersing the

dust. The pressure transducer detected ignitions in conjunction with the high-speed video camera. The pressure signal

was recorded using a personal computer ~PC! based highspeed data acquisition system. The criterion for ignition was

a pressure rise of at least 0.5 bar. There was a clear distinction between ignitions and nonignitions. Ignitions produced a

pressure rise of greater than 4 bar, and nonignitions produced

a pressure rise of less than 0.2 bar. In a few tests, some

localized flames were observed with the high speed camera

without producing a pressure rise of greater than 0.2 bar.

These tests were not counted as ignitions.

The ignition delay time was taken as the time between

the start of dispersion of the coal dust and a detectable explosion pressure rise at ignition. The delay times were measured from the PC data plots of the pressure and rate of

pressure rise (dP/dt) traces versus time.

V. RESULTS

A. Igniting power versus dust–air concentration

Combustion can take place over a wide range of dust–air

concentrations.7,8 Several series of experiments were conducted using the 400 mm core diameter fiber ~pointing upwards! over a range of dust concentrations and laser powers

to determine how the minimum igniting power varies with

concentration. When a test resulted in an ignition, another

test was conducted at a lower power. This process was repeated at a particular concentration until the test resulted in a

nonignition. At each dust concentration and laser power, the

tests were repeated until there was an ignition or until there

was no ignition in five tests, as shown in Fig. 2. This process

was repeated for different dust–air concentrations ranging

from 200 to 2000 g/m3

. It should be noted that the dust

concentrations shown in Fig. 2 are the nominal concentrations ~mass per chamber volume!. For the highest dust concentrations and for long ignition delay times, the actual concentrations may have been less because some of the dust may

have settled. In Fig. 2, the plotted powers and concentrations

of some of the data points were adjusted slightly to avoid

overlap of data points. No ignition was observed for a dustair concentration of 200 g/m3 at any power level. No ignition

was observed for a laser power of 2 W at any dust concentration. Researchers determined 800 g/m3 was a representative worst case dust concentration. Additional tests were then

conducted using 2.5 W laser power at 800 g/m3 to confirm

the limiting laser igniting power. The boundary between laser powers that ignite a particular dust concentration and

laser powers that do not ignite is shown as the dashed line in

Fig. 2. There is a fairly large amount of data scatter within

the ignitable region of Fig. 2. This shows the variability in

the ignition process, which was probably dependent on how

the dust was deposited on the optical fiber and heated to

become the ignition source. Despite this variability, the regions of ignitability and nonignitability are discernable in

Fig. 2. Ignition was never observed to the left of and below

the dashed boundary curve. Within the ignitable region

~above and to the right of the dashed curve! ignition is possible, but does not always occur.

B. Fiber orientation

The high speed video camera showed that ignition typically occurred at the tip of the upward-pointing fiber. The

fiber tip glowed brightly prior to ignitions, presumably due

to a layer of coal dust forming on the tip. The fiber was

pointed downward in two series of tests to determine if the

orientation of the fiber may affect the formation of a layer on

the tip, and therefore influence dust cloud ignitability. In ten

tests at 9 W laser power and 800 g/m3 coal dust, no ignition

was observed with the fiber pointing downward. Likewise, in

ten tests at 5 W and 800 g/m3

, no ignition was observed with

the fiber pointing downward. Only brief flashes were observed on the video of the downward pointing fiber tip, indicating that the coal dust did not adhere to the downward

pointing fiber tip or had fallen off as it was heated. This was

in contrast to sustained incandescence until ignition in the

previous tests with the fiber pointed upwards. These tests

showed the coal dust did not adhere well to the downward

pointing fiber, and that formation of a layer on the fiber tip

was key to igniting the dust cloud.

C. Igniting power versus beam diameter

Next, the relationship between laser igniting power and

beam diameter was investigated in order to compare with

similar data for laser ignition of methane.18 The fiber was

returned to its original upward pointing position. Beam diameter was varied by using three different fiber optic cables

with core diameters of 200, 400, and 800 mm. The core

diameter was taken as the beam diameter. From Fig. 2, 800

g/m3 was determined to be a representative worst case dust

concentration, and was used for all tests with the 200 and

800 mm core diameter fibers. Nonignitions were repeated ten

times before ending each series of tests at a particular beam

FIG. 2. Coal dust clouds ignited by a laser beam delivered by a 400 mm core

diameter optical fiber.
diameter. The results listed in Table I are plotted in Fig. 3.

The data for the 400 mm fiber in Fig. 3 are taken from Fig. 2.

The threshold igniting power density for each beam diameter

was calculated by dividing the threshold igniting power by

the surface area of the optical fiber core. The linear regression equation and trend line in Fig. 3 show the threshold

igniting power was proportional to beam diameter.

D. Ignition delay time

Summary statistics and histogram for all ignition delay

times with all three fibers are shown in Table II and Fig. 4.

The data show some deviation from the normal distribution.

The normal Q – Q plot was also generated and compared to

Q – Q plots generated for the Weibull, Gamma, and lognormal distributions. Deviations of similar magnitude were

found for these distributions as well. The Q – Q plots were

generated by SPSS for Windows statistical software package,

version 10, with distribution parameters estimated from the

data.23

VI. DISCUSSION

A. Ignition delay times

Ignition delay times for the 400 mm fiber at low laser

igniting powers ~3–4 W! were compared to delay times at a

high igniting power ~9 W! to see if delay times varied measurably with power. In general, higher laser powers would be

expected to have shorter ignition delay times, as observed

with flammable gases.14,18 Ignition delay times from the 9 W

tests were taken as a sample from a population with mean m1

and variance s1

2

. Likewise, delay times from the power limiting 3 and 4 W tests were taken as a sample from another

population with mean m2 and variance s2

2

. Summary statistics for the two samples are listed in Table III. The two

populations are independent and were assumed to be approximately normally distributed based on the parent distribution and reducing the power variability. The null hypothesis was m15m2 , and the alternative hypothesis was m1

,m2 . The hypothesis that m1.m2 was not considered as

ignition delay times should not increase with increasing

power. A test of equality of variances suggested equal variances could be assumed.

For the test, the alternative hypothesis would be accepted if

T,2t~a,n11n222!, ~1!

where T is the t-test statistic with pooled sample variances.

The alternative hypothesis was rejected at the a50.1 significance level. Given the assumptions made, the statistical

analysis shows that ignition delay times did not increase

measurably with decreasing power, and the deviations from

normality in the overall ignition delay time sample were not

attributable to variations in laser power.

In previous research on the laser ignition of methane, the

delay time was shorter at higher laser powers for a 400 mm

fiber.18 It is reasonable that the time to heat the target to the

ignition temperature would be shorter at higher laser powers.

Since no such effect was observed for the dust cloud ignition

delay times, different processes must have been dominant.

For the dust cloud ignitions, the time for dispersion of the

dust and for the accumulation of a target layer on the end of

the optical fiber probably dominated over the time to heat the

target. The times for dust dispersion and target formation

FIG. 3. Laser igniting power and power density as a function of beam

diameter, for Pittsburgh coal dust clouds ignited by laser heating of dust on

optical fiber tips. FIG. 4. Histogram of delay times for all ignitions.

TABLE I. Threshold laser igniting powers and calculated power densities

for coal dust clouds as a function of laser beam diameter.

Beam diameter

~mm!

Maximum

nonigniting power,

ten attempts

~W!

Minimum

igniting power

~W!

Minimum igniting

power density

~W/mm2!

200 1.5 2.0 64

400 2.5 3.0 24

800 4.5 5.0 10

TABLE II. Ignition delay time statistics for all ignitions.

Mean

~s!

Sample variance

~s

2

!

Minimum

~s!

Maximum

~s!

Count

~n!

0.62 0.0177 0.46 0.92 38
would be independent of laser power. The dust was dispersed

throughout the chamber by the 0.3 s air burst. However, the

dust concentration was probably not uniform on a scale less

than 1 mm, based on previous observations using an optical

dust probe.21 Therefore, there would be variability in the

depositing of the coal dust onto the end of the fiber to form

a target. In addition, the video for some tests showed incandescence at the fiber tip followed by a decrease in incandescence, and then an increase in incandescence until ignition

occurred. The initial target apparently burned off of the fiber

tip followed by new target formation leading to ignition. The

initial turbulence appeared to affect the target forming on the

fiber tip in all tests, based on incandescence intensity fluctuations observed at the fiber tip. Target formation processes

likely obscured a relationship between laser power and igniting delay times.

B. Threshold igniting powers

ASTM Standard Test Method E 2019-99 recommends

procedures for determining the minimum spark ignition energy of a dust–air mixture.6 Although the standard does not

apply directly to laser ignition sources, it does provide some

guidance on a reasonable number of tests that should be conducted. The standard calls for producing ten nonignitions at a

given energy level and dust concentration, and at least 30

nonignitions total over a range of concentrations, before a

given energy level may be considered insufficient to ignite

the dust. The minimum laser igniting power was essentially

constant for dust concentrations above 600 g/m3 in tests using the 400 mm fiber ~Fig. 2!. Fifty nonignitions were recorded at 2–2.5 W over this range. Therefore, 2 W through

the 400 mm fiber may be considered as nonigniting under

these test conditions, based on the ASTM recommendations

for minimum number of nonignition tests.

In comparison, 2 W readily ignited the dust cloud when

delivered by the 200 mm fiber. Also, the calculated power

density of 2 W through the 400 mm fiber ~16 W/mm2

! exceeds the igniting power density ~10 W/mm2

! for the larger

800 mm fiber. The values for the intermediate beam diameter

exceeded both the minimum igniting power of the smaller

beam and the minimum igniting power density of the larger

beam without causing ignition.

The linear regression equation for minimum igniting

power as a function of beam diameter ~Fig. 3! suggests that

smaller beam diameters than those tested will produce even

lower igniting powers. Further research using smaller beam

diameters is warranted to find the absolute limiting igniting

power ~independent of beam size!. A linear relationship between threshold igniting power and beam diameter appears

to hold over a range of beam diameters below 1 mm. A

similar relationship was observed for methane–air

mixtures.18,19

An inverse relationship between igniting power and igniting power density as a function of beam diameter as

shown in Fig. 3 has several implications for preventing dust

cloud ignition. For small diameter cw beams, limiting the

beam power to a value below the minimum igniting power

can prevent ignitions without considering the beam power

density. For large diameter cw beams, limiting the beam

power density below the minimum igniting power density

can prevent ignitions without considering the beam power.

Limiting a cw beam of any diameter below either the lowest

minimum igniting power or the lowest minimum igniting

power density of all beam diameters can prevent ignitions.

cw beams of intermediate dimensions can exceed both the

minimum igniting power of smaller diameters and the minimum igniting power density of larger diameters without

causing ignition, provided the beam power and power density remain conservatively below their respective ignition

curves such as shown in Fig. 3. The observation that power

was the limiting coal dust cloud ignition criteria for the

smaller diameter cw beams, whereas power density was the

limiting coal dust cloud ignition criteria for the larger diameter cw beams, parallels observations made in studies of

flammable gas ignition by cw lasers.24,25

Proust16 observed that target diameters below about 1

mm generally required higher powers for ignition. This is in

contrast to results of this study where significantly smaller

beam diameters were more hazardous. The difference is

probably due to the different experimental techniques. Proust

observed that smaller beam diameters drilled a hole into his

targets, possibly interfering with the ignition process and

rendering ignition less likely. Other differences in experimental techniques are discussed below.

The coal and laser properties used in this study and

Proust’s studies15,16 are compared in Table IV. The mean

particle diameters listed are surface weighted ~also known as

D32 or Sauter mean diameters!. The autoignition temperatures ~AITs! were obtained using the Godbert–Greenwald

apparatus.16,26,27 All other factors being equal, the lower AIT

lignite would be expected to ignite at lower laser powers

than the bituminous coal. Proust measured the relative absorptance of lignite with respect to that of Fe3O4 . The absorptance of the bituminous coal at the 803 nm laser wavelength was calculated from the reflectance data measured by

Labsphere, Inc. The most easily ignitable beam diameters

listed differ by a factor of ten. The listed igniting powers are

for igniting a coal dust cloud with a coal dust target. In

TABLE III. Statistics for two independent ignition delay time samples.

9 W tests 3 and 4 W tests

Mean ignition delay time ~s! 0.662 0.668

Sample variance ~s

2

! 0.0232 0.0169

Standard deviation ~s! 0.152 0.130

Count n1510 n2510

TABLE IV. Coal dust and threshold ignition characteristics.

Coal rank

Particle

diameter

~mm!

AIT

~°C!

Absorptance

~%!

Beam

diameter

~mm!

Igniting

power

~W!

Prousta lignite 17 500 60 2–4 2.5–4

This study bituminous 9 610 87 0.2 2

a

See Refs. 15 and 16.
Proust’s dust cloud ignition study, the input laser beam was

separated from the target by the dust cloud. His reported

igniting laser powers were the calculated powers reaching

the target through the dust cloud, based on the attenuation by

the dust cloud. The powers reported for the NIOSH experiments were measured out of the cleaved fiber tip prior to

each test, and the target consisted of coal particles depositing

on the fiber tip during the test. Because of the different coals

and different experimental techniques used in the two studies, it is difficult to make a direct comparison of the data. The

threshold igniting powers in both studies are similar although

there is a significant difference between the most easily igniting beam diameters.

Heat of combustion contributions from the target layer

appeared to have little impact on dust cloud ignitability. For

example, in the NIOSH research, powers that produced ignitions on the 200 mm fiber tip did not produce ignitions on the

400 and 800 mm tips, even though there was presumably

more coal present ~Fig. 3!. The coal was observed to glow

brightly and occasional localized flames were observed on

the 400 and 800 mm tips for the nonigniting experiments

shown in Fig. 3, confirming pyrolysis. Proust observed that

strongly absorbing, inert, iron oxide targets generally produced lower igniting powers than reactive targets. These arguments point to the laser beam as the primary source of heat

in these studies. One implication is that inert targets ~such as

iron oxide placed on the optical fiber tip prior to the dust

dispersion! that absorb the laser energy well and do not dissipate heat through vaporization, may produce lower igniting

powers than those observed in this study. However, the experiments conducted here are more representative of situations where the flammable dust itself is dispersed onto initially clean beam optics.

In the NIOSH laser ignition experiments, no ignition of

the Pittsburgh bituminous coal fines was observed at 200

g/m3 ~Fig. 2! for any laser power tested. However, the reported minimum explosible concentration ~MEC! of the

Pittsburgh coal fines is 85 g/m3

, using a strong ignition

source.7 There are two likely explanations for the higher observed MEC for the laser ignitions. Leaner mixtures generally require stronger ignition sources to ignite.28 For the laser

experiments, the maximum ignition energy was less than 10

J ~9 W over 1 s or less!. In comparison, the 85 g/m3 MEC

was measured with a 2500 J chemical ignitor, and a 500 J

chemical ignitor produced an MEC of 120 g/m3 for the Pittsburgh coal fines. A more powerful laser may therefore produce ignitions at lower concentrations than reported here.

Another possible reason for the higher observed MEC may

be related to target formation on the fiber tip. Lower dust

concentrations may cover the fiber tip less efficiently, preventing formation of an adequate target for the laser.

Coal dust cloud ignition and methane-air ignition criteria

are compared in Table V. The laser igniting powers listed for

the coal dust clouds and methane–air mixtures18 were obtained under similar test conditions, using a 200 mm diam

core optical fiber. For the methane, the fiber was coated with

a layer of coal dust before the test; and for the coal cloud, the

dust was deposited on the fiber during the test. The AIT

values listed were measured in various furnaces, including

the 0.3 L Godbert–Greenwald furnace,27 the PRL 1.2 L

furnace,29,30 and a British Gas Corp. 0.8 L stainless steel

vessel.31 The MIE values listed were obtained using electrical sparks.26,27 The laser igniting powers differ by only a

factor of 2.2, while the MIEs of the two materials differ by

more than 2 orders of magnitude. The laser igniting powers

correlate better with the AITs than with the MIEs. The best

correlation may be with a combination of AIT and MIE data,

where the AIT relates to the laser thermal ignition process

while the MIE relates to the small laser ignition volume.

In studies of pulsed laser ignition of flammable gases for

wavelengths in the visible and near infrared, minimum laser

energies needed to cause ignition were generally higher than

MIEs produced using electrical sparks.10,12,13,32–35 In

studies10 where the laser pulse duration was varied from several hundred milliseconds to below 1 ms, the pulse energy

needed to cause ignition decreased as the pulse duration decreased. Minimum igniting laser energies occurred with

pulse durations of about 70 ms, and were about a factor of 2

higher than MIEs obtained with electrical sparks.10,32 For

short laser pulse durations ~,100 ms! and small beam diameters ~,100 mm!, the energy became the limiting ignition

criteria over power or power density.10,25 That is, limiting the

beam pulse energy below a certain energy prevented flammable gas ignition even though the peak beam power and

peak beam power density were significantly higher than the

minimum igniting power and minimum igniting power density, respectively.

Although a cw laser was used for the experiments described here, ignition energies can be estimated from the

igniting power and delay times. The laser MIE was about 1.5

J ~3.0 W over a 0.53 s ignition delay time!. The MIE may

have been lower than 1.5 J since the delay time included the

time for the dust to disperse and form a target on the fiber tip.

This calculated laser MIE is much higher than the coal dust

cloud MIE obtained using electrical sparks listed in Table V.

As summarized in the preceding paragraph, gas ignition

studies suggest small diameter beams with shorter pulse durations and higher peak powers may produce lower igniting

energies in coal dust clouds than observed in this study.

Higher powers than 3 W did not produce lower calculated

ignition energies, probably another consequence of target

formation processes dominating the ignition timing.

TABLE V. Coal dust cloud and methane–air ignition characteristics.

Laser igniting power,

200 mm fiber ~W!

AIT

~°C!

MIE

~mJ!

Coal dust cloud 2.0 560–610b,c,d 60c

Methane 0.9a 600–630b,e,f 0.3b

a

See Ref. 18.

b

See Ref. 26.

c

See Ref. 27.

d

See Ref. 30.

e

See Ref. 29.

f

See Ref. 31.
VII. CONCLUSIONS

Minimum observed igniting powers for laser beams delivered by 200, 400, and 800 mm core fiber optic cables and

directed into Pittsburgh bituminous coal dust–air suspensions were 2.0, 3.0, and 5.0 W, respectively. These results

can be considered conservative for bituminous coal dust

clouds with larger particle sizes, higher moisture content, and

lower volatile matter content under otherwise similar test

conditions used in this study.

A laser-heated target ignited the dust clouds at lower

powers than a laser beam with no target ~fiber pointing

downward!, in agreement with Proust’s observation.

Coal dust–air concentrations ranging from 600 to 2000

g/m3 were the most easily ignited ~2000 g/m3 was the highest

concentration tested!. Minimum igniting powers over this

range were similar.

Threshold igniting power was proportional to beam diameter for beam diameters from 200 to 800 mm. The linear

regression model indicates igniting powers will be lower at

smaller beam diameters.

There was an inverse relationship between minimum igniting power and minimum igniting power density as a function of beam diameter. That is, the minimum igniting laser

power decreases with smaller beam diameter, while the minimum igniting power density ~power per beam area! decreases with larger beam diameter.

Ignition delay times did not vary significantly with laser

power for flammable targets forming under initially turbulent

test conditions.

Heat-of-combustion contributions from combustible target layers irradiated by small beam diameters appeared to

have little effect on igniting dust clouds, in agreement with

Proust’s observation.

ACKNOWLEDGMENTS

The authors would like to thank Ernest C. Magison ~consultant, USA! for suggesting the ‘‘upside down’’ tests. Mention of company names or products does not constitute endorsement by the National Institute for Occupational Safety

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