Rapid Sampling of Products During Coal Mine Explosions 

RON'ALD S. CONTI, ISAAC A. ZLOCHOWER and MICHAEL J. SAPKO 

Abstract-This paper describes a U.S. Bureau of Mines investigation of large-scale coal dust explosions in 

an experimental mine using a high speed electropneumatic mechanism for the rapid grab-sampling of gases 

and dusts. This technique enables the monitoring of pyrolysis and charring in fuel dust particles, and the 

collection of gaseous combustion products, in both large and small-scale explosions. Data obtained from 

full-scale dust explosion tests at the Bureau of Mines Lake Lynn Test facility show the following: Rapid 

sampling appears to "freeze" the burned gas compositions at the flame temperature values. Gas samples 

taken entirely in the flame zone consist of pyrolysis and combustion products with very low residual 

oxygen. The particles collected in the flame zone show signs of extensive pyrolysis and charring. Measurements of gas concentrations and particle flame temperatures suggest that char burning may have occurred 

in the flame zone. 

INTRODUCTION 

Coal dust explosion research has been conducted for many years at the Bureau of 

Mines to gain an understanding of the requirements for ignition, propagation, and 

inhibition of dust explosions. Considerable technical and practical information has 

been gained from this research, which was used in establishing the present safety 

standards in the coal mining industry. Nonetheless, dust explosion studies in experimental mines continues to be predominantly empirical due to the absence of a detailed 

theoretical understanding of the initiation and propagation of combusion waves in 

mine configurations. Some of the difficulty in developing a theoretical basis is associated with the limitation of the experimental techniques for studying the chemical 

aspects of full-scale mine explosion processes. This report describes a rapid (grab) 

sampling system (Conti et al., 1988) for collecting small samples of gases and dusts 

from the rapidly moving flame front and hot gas zone'. This system should promote 

the collection of accurate chemical data and help expand our knowledge of the 

flammability behavior of dusts, gases, and inhibitors, which is essential for a realistic 

appraisal of the explosion hazards involved in coal mining. 

EXPERIMENTAL SAMPLING SYSTEM 

An array of novel collector devices is used, Each device consists of an aluminum 

housing which incorporates two pre-evacuated glass vials with rubber septums. Upon 

actuation, the sampling probe needle is forced through the septum with a pressurized 

air pulse, filling the tube with gas and dust from the mine explosion. After a predetermined time, the sample probe needle is retracted by a second high pressure air pulse 

to its no~al (quiescent) state, allowing the sampling tube to reseal. The onset and 

duration of sampling are independently variable and controlled by a time-delay relay 

package. 
R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

Collection tubes 

for hot gases and dusts 

Scale, cm - 

FIGURE 1 Perspective view of mine scale version of rapid-sampling system mechanism. 

The mechanical device used to obtain rapid gas and dust grab samples is similar in 

concept to the previously reported laboratory (Conti et al., 1983) version. In the 

previous version, however, the probe needles were fixed to the laboratory chamber, 

and the sealed evacuated tubes were air driven into and out of the needles. In the mine 

version, the tubes are fixed and the needles move with the piston. The current version 

is shown in a sectional perspective in Figure 1. The main components of the system 

are an electropneumatic, double-acting air cylinder to power the sampling system, a 

piston section to which the sampling probe needles are attached, and the housing for 

the evacuated glass collection tubes. The 1 1-gauge (2.39 mm i.d.) hypodermic (sampling probe) needles used are modified by plugging the beveled point and drilling a 

1.6mm hole through the side to minimize septum tearing and core boring by the 

needle. The sampling system is shown in more detail in Figure 2. The main body is 

essentially a rectangular aluminum housing with two cylindrical cavities that contain 

the 30cc glass collection tubes. The housing cover serves a three-fold purpose, it 

allows access to the collection tubes, holds the sealed collection tubes in place, and 

serves as a guide for the sampling probe needles. The two sampling probe needles are 

mounted on a support bar, which is attached to the piston rod of the double-acting 

air cylinder. When the system is in the activated state shown in Figure 2, the cornbustion products are drawn into the evacuated collection tubes. When the preset sampling time is reached, the probe needle returns to its initial position or quiescent state, 

allowing the rubber septum of the collection tubes to reseal. This predetermined 

sampling time is controlled by a series of time-delay relays that send electrical pulses 

to a high speed, direct-solenoid-actuated valve. The initial pulse directs high-pressure 

(1 50 psi) air or N, from the solenoid valve to a port of the double-acting air cylinders, 

which forces the sampling probe needles forward into the collection tubes. A similar 

pulse applied to the opposite port of the double-acting air cylinder returns the 

sampling probe assembly to its quiescent state, thus completing the sample cycle. 

After the experiments, the sampling probe needles are removed from the support bar 

and cleaned. The housing cover is unfastened and the collection tubes are removed for 

a complete analysis. The use of a number of test devices mounted on a post securely 
RAPID SAMPLING OF PRODUCTS 

fAluminum housing Rubber septum, 

Air 

out - 

Mounting 

holes (4) - 

Activated state o 4 

Air in 

Scale, cm 

[Rubber inserts Sornpling probe needles 

Support bar-' 

Quiescent state 

FIGURE 2 Cross-sectional view of rapid-sampling device, in the quiescent and activated states. 

fastened to the floor and roof permits the collection of samples at different sampling 

times and intervals. In principle, the background gas and dust concentration can be 

obtained before flame arrival, while a brief sampling at flame arrival will give the 

conditions at the flame front. The contents of the collection tubes are used for 

duplicate gas analysis, or one is used for gas analyses and the other for microscopic 

(SEM) analysis of the dust residue. 

There are several advantages of this system over previous sampling systems. Sampling starts when the forward stroke of the sampling probe needle penetrates the 

rubber septum of the sampling tube. The length of sample time and amount of sample 

are controlled by the time difference in the forward stroke and return stroke of the 

sampling probe needle. The sample time relative to the combustion process can be 

measured exactly by observing the sampling probe movement with optical sensors. In 

previous systems, one monitored the current to the solenoid valves to indicate the 

approximate start of the sample. The difference between the electrical time, as indicated by the current rise in the solenoid valve, and the actual movement of the 

sampling probe needle obtained by the optical sensors is about 20ms. The times 

reported in this paper are the actual sample times as measured by the movement of 

the sampling probe needle, using an optical sensor. 

LAKE LYNN EXPERIMENTAL MINE 

The studies were conducted with the sampling system installed in the D-drift of the 

Lake Lynn Experimental Mine. The Lake Lynn Laboratory, formerly a limestone 

mine, is now a multipurpose mining research laboratory operated by the Bureau of 
R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

'~0s-mixing stub rk%:~ 

FIGURE 3 Plan view of the Lake Lynn Experimental Mine. 

Mines (Mattes et al., 1983). The underground layout of Lake Lynn (shown in 

Figure 3), allows full-scale research of explosion propagation and suppression as 

encountered in modern U.S. coal mining. A plan view of the face area in D-drift is 

illustrated in Figure4 and shows the configuration for a single entry dust explosion. , 

The average dimensions are 2.1 and 5.8 meters for an average cross-sectional area of 

12m2. 

Instrumentation to monitor pressure development, dust concentration (Conti et al., 

1982; Cashdollar et al., 198 l), and flame arrival times is provided at 10 stations along 

229 meters of the entry, starting at the bulkhead. Flame temperatures are monitored 

with 3 wavelength pyrometers at several stations. A detailed descrption of the physical 

arrangement and instrumentation for dust explosion studies at the Lake Lynn Experimental Mine has been published (Sapko et al., 1987; Weiss et al., 1988). A flame 

D DRIFT 

0 

E do !2.2L 117b 

DRIFT Scale, m 

FIGURE 4 Plan view of face area in D-drift showing the configuration of a nominal dust loading for a 

single entry. 
RAPID SAMPLING OF PRODUCTS 

20 1 I I I I 1 

I I I I I 

0 I 2 3 4 5 

TIME AFTER IGNITION, s 

FIGURE 5 Wave diagram of single-entry coal dust, 65% rock dust explosion: LLEM (test 110) D-drift. 

sensor trigger device (Liebman et al., 1979a,b) is used to activate the time-delay relays 

and to control the sampling sequence at the station 30m further downstream. The 

device was designed to activate the relays after the initial shock wave has passed and 

flame radiation has reached the sensor station. 

A flammable mixture of methane and air, typically 10% CH,, is formed by 

introducing CH, in the gas zone and stirring to provide a uniform mixture. Electrically activated pyrotechnic igniters are used to produce the methane explosions in the 

gas zone. In most experiments, a mixture of 65% rock dust and 35% pulverized (PPC) 

Pittsburgh seam coal (80%-200mesh), is spread equally on the floor and on roof 

shelves in the dust zone to provide a nominal PPC dust concentration near 200 g/m3, 

and a total nominal dust concentration of 370g/m3 if the dust were uniformly 

dispersed throughout the dust zone volume by the methane-air explosion. In some 

experiments, pure PPC is placed only on the shelves to give a nominal concentration 

near 60 g/m3 in the dust zone. The flammable dust mixture is dispersed and ignited by 

the methane-air explosion causing rapid flame propagation down the entry. 

The series of explosions involving coal dust mixtures that is described here consisted of a 107 m dust zone containing 60 or 65% pulverized limestone at a nominal 

PPC concentration of 200g/m2, that was ignited by a 12 m gas zone consisting of a 

10.3% methane-air mixture. Figure 5 is a wave diagram of one such explosion. It plots 

the change in static pressure with time at the various stations, and includes data on 

sampling times as well as flame arrival and decay. 

The complete sampling system set-up at one of the monitoring stations consists of 
200 R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

FIGURE 6 Photo of complete sampling system installed in D-drift. 

six sampling devices mounted on two vertical stands. The two stands were mounted 

vertically 9 1 meters from the face and secured to the mine floor and roof by bolts. The 

photo in Figure 6 is a cross sectional view of D-drift and shows the sampling system, 

including the relay control package. The roof shelves or dust trays are in place as are 

the dust barrels. The latter are removed once their contents are spread out on the floor 

and shelves of the dust zone. 

A sampling cycle comprising the six samplings in a typical test is shown in Figure 7. 

It indicates when the sampling device is activated and when the device returns to its 

quiescent state. The shaded portion represents the time in flame or, correspondingly, 

the length of the flame zone. The numbers at the top of each cycle boundary, indicate 

the distance between the sampling station and the leading edge of the flame. Thus, 

Sample B in LLEM-110 was taken starting 7 m ahead of the flame front and concluding 16 m behind the front, but still in the flame zone. 

PARTICLE COLLECTION 

Particle collection data of the sampling system in test LLEM-110, which featured the 

timing sequences shown in Figure 7, are presented in Table I. The starting time, t,, of 

each sampling device is measured with respect to the initiation of the explosion. The 

total sample time is A, and tj is the duration of sampling of the burned gases behind 

the flame front. The residue (m,) collected for each sample was weighed using a 

microbalance and then microscopically analyzed. Sample A, taken after the passage 

of the methane explosion shock front and just prior to flame arrival, shows the largest 

amount collected. Sample B was drawn before and during the flame, whereas C 

through F were taken entirely within the flame. The mass sampling rate was taken as 

the arithmetic average over the injection interval. It is noted that both the amount of 
RAPID SAMPLING OF PRODUCTS 

DISTANCE, m 

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 

TIME, s 

FIGURE 7 Illustration of a typical sampling cycle with respect to flame arrival and the trailing edge of 

flame for three Lake Lynn experiments. 

solids and the collection rate generally decrease with time due, possibly, to char 

burning and/or coal volatilization. Sampling bias in favor of small particles was not 

expected in view of previous results with the laboratory version of the rapid sampling 

device (Conti et al., 1983). No significant change in the trimodal distribution of a 

mixture of 3 classified coal dusts having mass weighted means of 7.5, 18, and 60 pm 

was noted after passage through the same needle. The 1.6mm diameter orifice will, 

however, prevent passage of any stable agglomerate much larger than 1 mm. Moreover, the fluctuations in local dust concentrations and in flow rate through the needles 

mitigates against obtaining reliable quantitative information on particle concentration from such mass sampling. 

TABLE I 

Performance of sampling system; LLEM test No. 110,35% PPC/65% Rock Dust - 200 g/m3 nominal PPC 

concentration, flame arrival at 1.16 1 s. 

Sample Sample start Sample Interval Mass residue Collection 

No. 4, ms duration in flame collected rate 

At, ms ',, ms m,., mg r, mglms 
202 R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

ELECTRON MICROSCOPE STUDIES 

The microscopic structure of coal dust-rock dust mixtures was analyzed by a scanning 

electron microscope (SEM) system. Ng et al. (1983) has provided a detailed description of the SEM and of fire and explosion residues for various dusts. A typical 

photomicrograph of a 65% rock dust (CaCO,), 35% Pittsburgh pulverized coal 

(PPC) mixture, both burned and unburned, is shown in Figure 8. The unburned coal 

particles in Figure 8A collected prior to the explosion have sharp edges and angular 

features, as opposed to the smooth rounded edges of the burned coal particles shown 

in Figure 8B that were collected during flame passage. A collection of all the calcium 

x-ray photons produced during the electron beam scanning of samples 8A and 8B 

were used to make the x-ray maps in Figures 8C and 8D, respectively. These maps can 

be used to identify the rock dust particles and how they interact with the coal particles 

before and after flame passage. 

Photomicrographs of samples A-F collected from this explosion (LLEM-1.10) are 

shown in Figure 9. After the explosion, the particles are noticeably larger than before 

the explosion. They exhibit rounded smooth surfaces in contrast to angular and sharp 

edges of the particles before the explosion. Many of the particles in the explosion show 

blowholes and bubbly masses, and some have formed cenospheres. This is due to 

rapid heating of the particles that leads to out-gassing of volatiles which causes the 

softened coal particle to swell, and eventually leads to the formation of blowholes of 

various sizes. 

GAS ANALYSES 

Gas samples were collected both before and after flame arrival at the sampling station, 

as described above. These were dried and analyzed by gas chromatography for N,, 

Ar, O2, C02, Co, CH4, C2H2, C2H47 C2H6, C3H6, C3H8, C4Hi07 and C5Hi2. 

Samples collected after ignition and prior to flame arrival gave an average composition which is very close to standard dry air as shown in Table TI. The small excess 

of CO, detected together with the small concentrations of CO and CH, that were also 

found are probably due to residual gases from previous explosion. 

The confirmation that the test zone atmosphere in front of the flame is basically air 

is not as obvious as it may seem. Consider that, typically, a 12 m zone from the D-drift 

bulkhead filled with stoichiometric methane-air is ignited. If the combustion products 

from the resulting methane explosion are mixed with the remaining air in the 119 m 

zone to the sampling station, then significant concentrations of CO, (1.4%) from the 

explosion should be detected. The fact that the CO, concentration is only increased, 

on average, from the 0.03% of standard air to 0.09% indicates that longitudinal gas 

mixing is minimal. The hot burned gas acts primarily as a piston, pushing the coal 

dust laden air ahead of it. Richmond and Liebman (1975 and 1978) had reached the 

same conclusion on the basis of explosion measurements made in the experimental 

mine at Bruceton. 

Also shown in Table I1 is the average composition of the samples collected after 

flame arrival. These results do not include the few samples whose compositions gave 

evidence of air dilution (leaking). The average "Flame Zone" composition consists of 

both coal vapors and their combustion products. The former consists of the hydrocarbon gases (CH,, C2H2, C2 H,, C2 H6, C3 H6, etc.) as well as H, and some of the CO 

and CO, produced. The fact that acetylene is the most abundant hydrocarbon after 

methane is both evidence for the high temperatures produced in the explosion, and 
RAPID SAMPLING OF PRODUCTS 203 

A Unburned 9 , 50 C X-ray map rock dust 9 , 50 , 

Scale, pm Scale, pm 

8 Burned o 

w 50 D X-ray map rock dust 0 50 

Scale, pm Scale, pm 

FIGURE 8 Typical SEM photomicrographs of burned and unburned Pittsburgh coal and 65% rock dust 

particles, X-ray maps made using the calcium line. 

the effectiveness of the sampling technique in preserving such an unstable species by 

means of rapid cooling. Possible contribution, however, to the concentration of the 

other hydrocarbons by collected particles that continue to devolatilize cannot be ruled 

out at this point. 
204 R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

FIGURE 9 Micrographs of dust collected with sampling device during the explosion test LLEM-I 10. 

DUST EXPLOSION MODELS 

The coal dust made airborne by the turbulent pressure wave or shock front formed 

from the methane-air explosion is subject to a high heat flux from the radiation and 

hot gases in the rapidly approaching flame front. The coal particles heat up rapidly 
RAPID SAMPLING OF PRODUCTS 205 

. TABLET1 

Average gas composition (dry basis) of 6 runs with 35% PPC/65% Rock Dust at 200 g/rn3 nominal PPC 

concentration 

Sampling Percent, % 

N2 Ar O2 C02 CO Hz CH4 C2H2 C2H4 C2H6 C3H6 

Standard 78.08 0.936 20.95 0.032 0 0 0 0 0 0 0 

Air 

Before flame 78.07 0.933 20.77 0.087 0.0024 0 0.0016 0 0 0 0 

a,,, , + 0.06 0.002 0.10 0.016 0.0012 0 0.0004 0 0 0 0 

In flame 74.82 0.896 0.51 15.95 5.59 1.77 0.232 0.043 0.025 0.0013 0.0014 

a,", $. 0.37 0.004 0.06 0.52 0.47 0.68 0.035 0.006 0.005 0.0003 0.0003 

Note: The error limits are the standard deviation of the mean values, a,, = a/\/n. 

as the thermal wave progresses inward. First the adsorbed low weight species are 

desorbed from the pores, followed by thermal degradation of the polymeric material 

composing the bulk of the coal. The surface of the bituminous coal particle softens, 

becomes plastic and "skin-like." The volatiles produced form bubbles under this skin, 

and vent intermittently as the vapor pressure exceeds the tensile strength of the film. 

The particles so transformed are surrounded by a vapor halo. The halo also contains 

soot formed from the polymerization of some reactive unsaturated species. Oxygen 

diffusing in these hot vapors causes ignition. The burning halo atmosphere surrounding the particles releases heat energy to advance the flame front and engage coal 

particles nearby and further down stream. The mass loss of these particles is appreciably greater than the loss from a bed of particles placed in a closed crucible in a 

furnace, which is the standard volatility measurement procedure (ASTM D3175). In 

the latter case, there is both slow particle heat up and ample opportunity for condensation and polymerization of the vapors. Thus, the mass loss observed for a separated 

small array of Pittsburgh seam coal particles having diameters < 100 micrometers and 

subject to very rapid heating by a strong CO, laser flux of 100 or 200 w/cm2 is about 

50%, whereas it is only 37% in the ASTM procedure. For Pocahontas coal, the 

discrepancy is more striking. The ASTM volatility is only about 18%, whereas it is 

33% with the laser technique. The laser flux used is roughly comparable to the heat 

flux in a well-developed explosion, taking into account the undirectional heating by 

the laser beam and the heat loses to both the cold sample substrate and the cold 

ambient gas (Cashdollar et al., 1988). 

There is some controversy in the literature over the possible involvement of 

heterogeneous burning of the charred coal particles in the explosion process. We can 

consider the two extremes; either char burning does not occur, or it occurs readily. In 

the former case, we may consider the coal to consist of a reactive volatile portion and 

an inert char. In the latter case, the entire coal reacts. Even here, the questions 

remains, "Does the char burning occur at a rate which can affect flame propagation?" 

In coal dust explosions conducted in a 20-L laboratory chamber (Cashdollar et al., 

1985), only the volatiles appear to contribute significantly to flame propagation, and 

the char in fact, appears to act as a thermal inertant. The greater turbulence and flame 

residence time in a mine explosion, however, can lead to some char involvement. We 

will therefore consider both possibilities. Typical proximate and ultimate analyses of 

Pittsburgh and Pocahontas seam coals are given in Table111 together with the 

ultimate analysis of their "fixed carbon," or char fractions. Empirical formulas of the 
206 R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

TABLE I11 

Proximate and ultimate analysis of Pittsburgh and Pocahontas seam coal and their components 

Coal Seam Proximate analysis Elements 

(mass pct - as received) (mass pct - moisture 

ash free basis) 

Volatiles Fixed Carbon Ash Moisture 

C H 0 NS 

Pittsburgh 

Coal 37.9 54.5 6.6 0.94 82.67 5.62 8.74 1.79 1.18 

0.50 Char 95.05 0.76 1.26 1.85 1.08 

0.50 Volatiles 70.30 10.48 16.22 1.73 1.28 

Pocahontas 

Coal 18.82 75.40 5.40 0.38 88.85 4.50 4.56 1.17 0.92 

0.67 Char 96.70 0.93 0.36 1.14 0.87 

0.33 Volatiles 73.14 11.64 12.96 1.23 1.02 

coals and chars are derived and heats of formation, AHf, are calculated based on the 

stoichiometric reactions and the experimental heats of combustion (moisture, ash-free 

basis). Taking the volatile fractions to be 0.50 and 0.33 for Pittsburgh and Pocahontas, respectively, one can deduce their elemental compositions heats of formation, 

and empirical formulas. The heats of formation of the volatiles are derived, as above, 

from deduced heats of combustion, AHC, based on energy conservation. The formulas, AH,, and AHJfor Pittsburgh and Pocahontas coals, and their components are 

given in Table IV. A prior analyses (Conti et al., 1988) of Pittsburgh seam coal and 

vO1atiles gave CHO.XOX O0.082 N0.017 S0.006 and .64 O0.15 N0.015 S0.006 . In view of the uncertainties in the analyses and the "true" volatilities, one can 

simplify the formulas and heats of formation as shown in TableV. The heats of 

formation listed are somewhat arbitrary but have only a small eRect on calculated 

flame temperature and product concentrations. The values chosen for the volatiles 

model give a reasonable limiting flame temperature (1400 K) for Pittsburgh seam coal 

dust explosions conducted in a laboratory chamber. The CaCO, component of the 

rock-dust/coal-dust mixtures is treated either as chemically inert or fully reactive at 

or above the decomposition temperature of 1 170 K. 

Thermodynamic calculations of the adiabatic flame temperature and product 

TABLE IV 

Formula and energy for Pittsburgh and Pocahontas seam coal and their components 

Coal Seam Empirical Formula Combustion Heat of 

Heat (MAF) Formation 

(callg) (cal/mol) 

Pittsburgh 

Coal 

Char 

Volatiles 

Pocahontas 

Coal 

Char 

Volatiles 
RAPID SAMPLING OF PRODUCTS 207 

TABLE V 

Simplified Empirical Formulae and Heats of Formation (cal/mol). 

Whole Coal Volatiles Char 

Pittsburgh Coal CHo,,Oo~o,, AH0, = 0 CHl,,00.,5, AHo/ = -5000 "C", AH0, = 0 

Pocahontas Coal CHo,,Oo,,, AH0/ = 0 CHl,900:13r AH0, = 0 "C", AH0, = 0 

concentrations for methane and coal dust explosions were performed on a VAX 111 

780 computer using the CEC-80 fortran code for complex equilibria calculations 

developed at the NASA-Lewis Research Center (Gordon and McBride, 1976). This 

program uses data taken primarily from the JANAF compilations of enthalpy, free 

energy, and other thermodynamic properties as a function of temperature (JANAF, 

1985). The free energy of the system is minimized considering all possible species and 

concentrations derivable from the reactants within the constraint of conservation of 

energy and atoms. The enthalpy of this product mixture as a function of temperature 

then determines the final temperature. The pressure in a constant volume calculation 

is determined from the temperature and number of moles of gas by the ideal gas law. 

The temperatures, pressure, and concentrations are assumed to be uniform throughout the volume. 

RESULTS AND. ANALYSIS 

The samples obtained from the gas zone in two experiments showed the high temperature combustion products expected from the measured initial methane concentrations of 10.63 + 0.02 and 10.45 + 0.24% in air that was measured by conventional sampling and analysis. In fact, the combustion product compositions corresponded 

to calculated starting methane concentrations of 10.84 + 0.43 and 10.73 $- 0.36%, 

respectively. The richer mixture gave the following dry gas composition: 

N, + Ar = 82.46 + 0.22%, 0, = 0.96 + 0.47%, CO, = 8.18 $ 0.36%, 

CO = 4.10 $ 0.25%, H, = 3.95 + 0.33%, CH, = 0.237 0.135% and a totalof 

0.0360 + 0.0180% higher hydrocarbons. Of these, all but the methane and higher 

hydrocarbons are the expected concentrations of the flame temperature reaction 

products. The hydrocarbons detected can be attributed to background emission from 

desorbed gases in the mine. The significant concentrations of CO and H, (4% each) 

predicted and measured is understandable in view of the high adiabatic flame temperature calculated for these mixtures (2180 I() which favors the formation of diatomic and monatomic species. 

The small concentration of free radicals such as OH, H, and 0 that were generated 

at these temperatures are too reactive to be detected by the rapid sampling technique. 

They simply recombine on the cold surfaces of the sampling probe needle and glass 

vial. The sampling, however, leads to a cooling rate which is, apparently, fast enough 

to inhibit other reactions, as evidenced by the agreement of the sampled burned gas 

composition with the flame calculation. The flame temperature composition is thus 

"frozen" by the sampling technique, which is a decided advantage. 

In the experiments using low nominal concentrations of Pittsburgh seam coal dust 

(50-70g/m3), the average concentrations found were as follows: 

N2+Ar= 80.64+0.55%, 0,=6.75$0.45%, C02= 12.4+0.1%, and 

CO = 0.20 $- 0.14%. The concentrations were best matched by a calculated coal 

concentration of 75 + 7 g/m3 for the reactive coal model, or 160 + 16 g/m3 for the 

50 volatilities, 50% inert char model. The same results were obtained on comparing 
208 R. S. CONTI, I. A. ZLOCHOWER AND M. J. SAPKO 

the 1930 + 20 K peak temperature of the incandescent solids measured by a 3-wavelength near IR pyrometer (Cashdollar and Hertzberg, 1982 and 1983) stationed in the 

dust zone, with the results of the calculation. The agreement between the particle temperatures and gas concentrations suggest that the particle temperatures are close to that 

of the gas, and may be a measure of the temperature of submicron soot particles. The 

6-wavelength pryometer (Cashdollar and Hertzberg, 1982 and 1983) should give a better average value of particle and gas temperatures and it would be more desirable, although more difficult for mine use. The 0-50% excess in calculated coal concentration 

over the nominal value in the reactive coal model is easily rationalized in terms of a 

non-uniform dust dispersion. That is, dust falling from the roof shelves will tend to produce a greater concentration in the upper region. The 200-300% excess concentration 

predicted by the volatiles model is more difficult to rationalize. 

Particle temperature of 19 10 K were also previously measured in mine explosions 

of fine Pocahontas coal particles at a nominal concentration of 40-50 g/m3, that were 

dispersed and ignited as above, and compared with the adiabatic equilibrium predictions of the reactive and volatiles models for this coal. The reactive coal model gave 

a matching coal concentration of 70 g/m3, while the 33% volatiles model predicted a 

concentration of 235 g/m3. The latter concentration is 400-600% in excess of the 

nominal value and appears to be unrealistic. Based on these 3-wavelength pyrometer 

measurements, we may conclude that either char burning occurs in a mine explosion 

or that we have underestimated the true volatile yield. It should be noted, however, 

that the pyrometers in these experiments had not been calibrated, at that time, against 

a black-body source, and thus have an unknown accuracy. 1 

A similar conclusion is reached in analyzing the gas concentration and particle 

temperatures in entrained mixtures of 35% PPC and 65% rock dust. The average dry 

gas composition is: N, + Ar = 76.0 + 1.5%, 0, = 0.84 + 0.54%, 

CO, = 16.2 + 1.g0/0, CO = 5.4 + 2.0°/0, H, = 1.7 $- 0.7%. Using the reactive 

coal model and a chemically inert CaCO,, the above concentrations are best matched 

by a 125 g/m3 PPC concentration. This calculated value is 40% below the nominal 

200 g/m3 concentration. However, half of the dust in the PPC-Rock dust mixture was 

spread on the floor and may not have been lifted efficiently into the flame zone by the 

prior methane explosion. The measured temperature of 1810 + 75 K is similarly 

fitted best by a 155 g/m3 PPC concentration using the reactive coal-unreactive rock 

dust model. The corresponding reactive CaCO, model gives 125 and 130 g/m3 as the 

best matching PPC concentrations for the experimental gas concentrations and 

temperatures, respectively. The 50% volatiles model gives 230 g/m3 as the best matching PPC concentration. The best match to the experimental temperature was 200 g/m3 

PPC, which is the nominal value, for the unreactive CaCO,. With reactive CaCO, the 

calculated temperatures are too low. Thus, either the 50% volatiles model with inert 

CaCO, or the reactive coal models account for the mine results. For 35% Pochahontas-65% CaCo, mixtures with a nominal coal concentration of 200 g/m3 the 

previously measured 3-wavelength pyrometer temperature of 1900 K could however, 

be matched only by the reactive coal models which give 115 and 135 g/m3 for the 

reactive and inert CaCO, models, respectively. Again, however, we must note the 

absence of a current calibration of the pyrorneter, which mitigates against drawing 

firm conclusions based solely on temperature measurements. 

CONCLUSIONS 

Immediately prior to flame arrival, the background gas is virtually that of standard 

air with few particles showing signs of tar, volatiles or char formation. Gas samples 
RAPID SAMPLING OF PRODUCTS 209 

taken entirely in the flame zone consist of pyrolysis and combustion products with 

very little residual oxygen. The particles show signs of extensive pyrolysis and charring. The correlation between the experimental values of the gas composition and the 

particle temperatures with that calculated for models consisting of reactive Pittsburgh 

or Pocahontas coals is suggestive, but not conclusive evidence of char burning in mine 

explosions. These experimental results, demonstrate the utility of the high-speed gas 

add dust sampling systems, in analyzing large scale explosions. 

The system can be further upgraded by interfacing a microprocessor or computer 

to enhance timing capabilities and eliminate the relay control package. For example, 

data from a set of flame sensors upstream of the sampling system could be used by 

a microprocessor to calculate the expected time of flame arrival and thereby insure 

sampling of the flame zone or even its leading edge. A more definitive test of the 

reactive coal and CaCO, vs inert char and CaCO, models could then be obtained. 

REFERENCES 

Cashdollar, K.L., Liebman, I., and Conti, R.S. (1981). Three Bureau of Mines Optical Dust Probes. U.S. 

Bureau of Mines RI 8542, 26. 

Cashdollar, K.L. and Hertzberg, M. (1982). Infrared Pyrometers For Measuring Dust Explosion 

Temperatures. Optical Engineering, 21, 82. 

Cashdollar, K.L. and Hertzberg, M. (1983). Infrared Temperatures of Coal Dust Explosions. Combustion 

and Flame, 51, 23. 

Cashdollar, K.L. and Hertzberg, M. (1985). 20-L Explosibility Test Chamber for Dusts and Gases. Review 

Scientific Instruments, 56, 596. 

Cashdollar, K.L., Hertzberg, M., and Zlochower, I.A. (1988). Effect of Volatility on Dust Flammability 

Limits for Coals, Gilsonite, and Polyethylene. Twenty-second Symposium (International) on 

Combustion, The Combustion Institute, Pittsburgh, pp. 1757-1765. 

Conti, R.S., Cashdollar, K.L., and Liebman, I. (1982). Improved Optical Probe for Monitoring Dust 

Explosion. Review of Scientific Instruments, 53, 3 1 1. 

Conti, R.S., Hertzberg, M., Duda, F.T., and Cashdollar, K.L. (1983). Rapid-Sampling Systems for Dusts 

and Gases. Review of ScieiztiJic Instrunzents, 54, 104. 

Conti, R.S., Zlochower, I.A., and Sapko, M.J. (1988). Rapid (Grab) Sampling During Full-Scale 

Explosions-Microscopic and Analytical Evaluation. U.S. Bureau of Mines RI 9192, 17. 

Gordon, S. and McBride, B.J. (1976). Computer Program For Calculation of Complex Chemical 

Equilibrium Composition. National Aeronauatics and Space Administration, SP-273, 251. 

JANAF, Thermochemical Tables, 3rd Edition, M.W. Chase Jr., et al, Eds. (1985). Journal of Physical and 

Chemicn! Reference Data, Vol 14, Suppl. 1. 

Liebman, I., Duda, F., and Conti, R.S. (1979a). Sensor Trigger Device for Explosion Barrier. Review 

Scientific Instruments, 50, 144 1. 

Liebman, I., Duda, F., and Conti, R.S. (1979b). Sensor Trigger Device for Explosion Barrier. U.S. Pat 4, 

173, 140. 

Mattes, R.H., Bacho, A., and Wade, L.W. (1983). Lake Lynn Description: Construction, Physical 

Description, and Capability. U.S. Bureau of Mines IC 891 1, 40. 

Ng, D.L., Cashdollar, K.L., Hertzberg, M., and Lazzara, C.P. (1983). Electron Microscopy Studies of 

Explosions and Fire Residues. U.S. Bureau of Mines IC 8936, 63. 

Richmond, J.K. and Liebman, I. (1975). A Physical Description of Coal Mine Explosions. Fifteenth 

Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, pp. 115-126. 

Richmond, J.K., Liebman, I., Bruszak, A.E., and Miller, L.F. (1978). A Physical Description of Coal Mine 

Explosions. Part 11. Seventeenth Symposium (International) on Combustion, The Combustion 

Institute, Pittsburgh, pp. 1257-1268. 

Sapko, M.J., Weiss, E.S., and Watson, R.W. (1987). Size Scaling of Gas Explosions: Bruceton 

Experimental Mine Versus the Lake Lynn Mine. U.S. Bureau of Mines R19136, 23. 

Weiss, E.S., Greninger, N., and Sapko, M.J. (1988). Recent Results of Dust Explosion Studies at the Lake 

Lynn Experimental Mine. Presented at the Twenty-third Conference (International) Safety in Mines 

Researclt Instiutes, Washington, DC, pp. 843-854.