ing During Fu 

roscopic and Ana 

By Ronald S. Conti, Isaac A. Zlochower, 

and Michael J. Sapko 
Report of Investigations 91 92 

ing During Fu 

roscopic and Ana 

By Ronald S. Conti, Isaac A. Zlochower, 

and Michael J. Sapko 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

T S Ary, Director 
Library of Congress Cataloging in ]Publication Data: 

Conti, Ronald S. 

Rapid (grab) sampling during full-scale explosions. 

(Bureau of Mines Report of investigations; 9192) 

Bibliography: p. 16 

Supt. of Docs. no.: 1 28.23:9192. 

1. Mine explosions. 2. Mine gases-Measurement. 

I. Aochower, Isaac A. 11. Sapko, M. J. Ill. Title. IV. Series: Report of 

investigations (United States. Bureau of Mines); 9192. 

TN23.U43 [TN306] 622 s [622'.8] 88- 14520 
CONTENTS 

Page 

Abstract .........................................................................

Introduction ........................................................................ Acknowledgments .................................................................... Samplingsystem ..................................................................... Triggermechanism ................................................................... Scanning electron microscope studies ..................................................... Lake Lynn experimental mine ............................................................ LLEMtest110 ................................................................... LLEMtestln ................................................................... LLEMtest112 ................................................................... Gasanalyses ........................................................................ Thermodynamiccalculations ............................................................ Conclusion ......................................................................... References ......................................................................... Appendix.-List of symbols .............................................................. 

Perspective view of mine-scale version of rapid-sampling system mechanism .................... Cross-sectional view of rapid-sampling device. in quiescent and activated states .................. Perspective view of triggering mechanism .............................................. Electrical schematic of triggering circuit ................................................. Typical SEM photomicrographs of burned and unburned Pittsburgh coal and 65% rock dust particles . . X-ray spectra of Pittsburgh coal and rock dusts ......................................... Illustration of typic4 sampling cycle with respect to flame arrival and to trailing edge-of flame for 

three Lake Lynn experiments ...................................................... Plan view of Lake Lynn underground mine ............................................. Plan view of face area in D drift showing configuration of nominal dust loading for a single entj .... Wave diagram of single-entry coal dust 65% rock dust explosion LLEM test 110 D drift .............. Complete sampling system installed in D drift ........................................... Photomicrographs of dust collected with sampling device during LLEM 110 explosion test in D drift . . Photomicrograph of large residue collected with sampling system ............................ Photomicrographs of partially devolatilized coal particles, sampled before and during the explosion ... Photomicrograph of partially devolatilized coal particle .................................... 

TABLES 

Performance of sampling system .................................................... Actual burned gas compositions from LLEM tests 110. 111. 112. with corresponding flame 

arrivaltimes .................................................................. Average dry gas composition ....................................................... 
UNIT OF MEASURE ABBREVIATlONS USED IN THIS REPORT 

atm 

cal 

atmosphere 

Calorie 

centimeter 

cubic centimeter 

foot 

gram 

gram per cubic meter 

kelvin 

kilo electron volt 

kilogram 

kilohm 

liter 

meter 

meter per second 

square meter 

m 3 

mg 

mg/ms 

mm 

ms 

PP" 

psi 

psk 

S 

Pm 

vac 

Vdc 

~01% 

w/em2 

cubic meter 

milligram 

milligram per millisecond 

millimeter 

millisecond 

part per million 

pound per square inch 

pound per square inch, gauge 

second 

micrometer 

volt, alternating current 

volt, direct current 

volume percent 

watt per square centimeter 
SAMPLING DURING FULL-SCALE 

EXPLOSIONS-MICROSCOPIC AND ANALYTICAL EVALUATION 

By Ronald S. conti,' Isaac A. ~lochower: and Michael J. sapko3 

ABSTRACT 

The Bureau of Mines has developed a system using a high-speed electropneumatic mechanism for 

the rapid (grab) sampling of dusts and gases during an explosion. The sampling system consists of an 

aluminum housing that incorporates two 30-cm3 preevacuated glass vials with rubber septums. Upon 

actuation, the sampling probe needle is driven through the septum with a pressurized air pulse, filling 

the tube with gas and dust from the mine explosion. After a predetermined time, the sampling probe 

needle is retracted by a second high-pressure air pulse to its normal (quiescent) state, allowing the 

sampling tube to reseal. The onset and the duration of sampling are independently variable and 

controlled by a time-delay relay package. This technique enables the monitoring of pyrolysis-charring 

in coal particles and the generation and combustion of the pyrolysis vapors in both large- and smallscale explosions. The following results were obtained from full-scale dust explosion tests at the Bureau 

of Mines Lake Lynn Laboratory: (1) Gas sampling of the leading edge of the flame front shows the 

large concentration changes, characteristic of the flame front; (2) gas samples taken entirely in the flame 

zone consist of pyrolysis and combustion products with very low residual oxygen; and (3) the particles 

collected in the flame zone show signs of extensive pyrolysis and charring. 

'~lectronics engineer. 

'~esearch chemist. 

3~upe~visory chemical engineer. 

Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA. 
INTRODUCTION 

Coal dust explosion research has been conducted for 

many years by the Bureau to gain an understanding of the 

requirements of 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 

mining industry. Nonetheless, dust explosion studies in 

experimental mines continue to be predominantly 

empirical because of the absence of a detailed theoretical 

understanding of the initiation and propagation of 

combustion waves in mine configurations. Much 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 gas and dust 

explosion processes. This report describes a rapid (grab) 

sampling system for collecting small samples of dusts and 

gases from the rapidly moving (90 to 300 m/s) flame front 

and hot gas zone. This sampling system should promote 

the collection of accurate chemical data to expand the 

knowledge of the flammability behavior of dusts, gases, 

and inhibitors, which is essential for the realistic appraisal 

of the explosion hazards involved in coal mining. 

ACKNOWLEDGMENTS 

The authors wish to thank the important contributions their assistance with the installation of the sampling system 

of the following members of the Bureau's Pittsburgh (PA) at the Lake Lynn experimental mine (LLEM); and 

Research Center: J. Leong, electronics technician, for his R. Thomas, engineering technician, for his assistance in 

initial assistance in the preparation of the sampling system; data collection. 

R. Pro and G. Green, physical science technicians, for 

SAMPLING SYSTEM 

The mechanical device used to obtain rapid gas and 

dust grab samples is shown in a sectional perspective 

in figure 1. The main components of the system are an 

electropneumatic, double-acting air cylinder (Bimba 

Manufacturing ~0.1~ 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. A time-delay relay package controls a 

high-pressure air solenoid valve that directs the air pulses 

to the double-acting air cylinder. 

The sampling system is shown in more detail in 

figure 2. The main body is essentially a rectangular 

aluminum housing with two cylindrical holes that contain 

the 30-cm3 glass collection tubes. To guard against the 

mechanical shock caused by the forward thrust of the 

sampling probe needles, rubber inserts are fitted into the 

bottom of each hole to serve as shock absorbers for the 

collection tubes. The center portion of the main body 

(back side) is milled parallel to the collection tubes, to 

provide mounting space for the double-acting air cylinder. 

The housing cover serves a threefold 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. The sampling probe needle is 

a modified 11-gauge, hypodermic needle (stainless steel). 

The beveled point of the needle is plugged and then silver 

soldered, and a 1.6-mm-diam hole is drilled through 

the side close to the point. This modification minimizes 

the risk of septum tearing or core boring by the needle 

penetration, thereby allowing the septum to reseal 

(reducing leakage of the collection tube) during the return 

cycle of the air cylinder. 

When the system is in the activated state (fig. 2), the 

combustion products are drawn into the evacuated 

collection tube. 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 tube 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 

Collection tubes 

,,,-for hot gases and dusts 

Scale, cm 

4~eference to specific products does not imply endorsement by the Figure 1. - Perspective view of mine-scale version of rapidBureau of Mines. sampling system mechanism. 
Air 

out - 

[Alum inum housing Rubber septum, 

Mounting 

holes (4). 

/ Rubber inserts 

Air 

sampling 

inlets, 

Activated state 0 

u 

4 

Scale, cm 

Sampling probe needles 

\ 

Quiescent state 

Figure 2. - Cross-sectional view of rapid-sampling device, in quiescent and activated states. 
12 V dc 

IIO Vdc 

Pressure I switch 

Figure 4. - Electrical schematic of triggering circuit. 

SCANNING ELECTRON MlCROSCOPE STUDIES 

The microscopic structure of coal dust-rock dust 

' mixtures was analyzed by a scanning electron microscope 

(SEM) system. A detailed description of the SEM, and 

fue and explosion residues for various dusts can be found 

in reference 6. A typical photomicrograph of a 65% rock 

dust (CaC03) Pittsburgh pulverized coal mixture, both 

burned and unburned is shown in figure 5. The unburned 

coal particles (fig. 54) collected prior to the explosion have 

distinct characteristics, sharp edges and angular features, 

as opposed to the smooth rounded edges of the burned 

coal particles shown in figure 5B, which were collected 

during flame passage of one of the experiments (LLEM 

110). 

Figure 6 compares the X-ray spectra of rock dust (fig. 

6A) with those of coal dust (68). Notice the strong 

calcium lines, Ka at 3.7 keV and K/l at 4.01 keV. The 

other elements shown reflect the copper sample substrate, 

interface media of sample, and the gold and platinum 

coating of the sample. A collection of all the calcium Xray photons produced during the electron beam scanning 

of the samples shown in figures 54 and 5B were used to 

make the X-ray elemental maps in figures 5C and 5D, 

respectively. The maps can be used to identlfy the rock 

dust particles and how they interact with the coal particles 

before and after flame passage. 

A sampling cycle representing six sampling stations is 

shown in figure 7. The pulses shown for three tests, 110, 

111, and 112, illustrate when the sampling device is 

activated and when the device returns to its quiescent 

state. The shaded portion represents the radiation zone, 

from flame arrival to trailing edge of flame. The numbers 

at the top of each cycle boundary indicate the distance 

between the sampling station and the leading edge of the 

flame. 
valve. The initid puke directs ]high-pressure (150 psi) air 

or Nz from the solenoid valve to a port of the doubleacting air cylinder, which forces the sampling probe 

needles fornard into the collection tubes (the activated 

state). A.similar pulse applied to the opposite port of the 

double-acting air cylinder return 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 to be cleaned in an 

ultrasonic bath. The housing cover is unfastened and the 

collection tubes are removed for a complete analysis. The 

contents of one collection tube is used for gas analysis and 

the other, typically, for microscopic analysis of the dust 

residue. This mine-scale version of the rapid sampling 

system is similar in concept to the previously 

reported laboratory version (1)' in the ut 

double-acthg air cyhders, the glass co]iP.ection tubes, and 

the smphg probe needles. The mlllin diierence between 

the two systems is essentially in the operation of the 

sampling probe needles. In the laboratory version, the 

sampling probe needles are stationary (fastened to the test 

apparatus) and the glass collection tube moves with the 

piston. It was especially designed to interface with the 

Bureau's ignitability furnace (2). The timing of the 

sequences in the laboratory version is controlled by a 

microprocessor. A detailed description can be found in 

reference 1. 

- - 

'italic numbers in parentheses refer to items in the list of references 

preceding the appendix. 

Figure 3 is a perspective drawing of the device used to 

trigger the sampling system. The flame sensor-trigger 

device (3-4) was developed for use with barriers to 

suppress coal dust explosions in underground mines (5). 

The principal feature of the device is a dual infrared flame 

sensor combined with a pressure-arming element. This 

combination prevents false and premature triggering. The 

device is mounted to the rib approximately 1 m from the 

floor, and 61 m from the face in the LLEM D drift. 

A schematic of the basic circuit is shown in figure 4. 

During explosions the pioneer wave activates the pressure 

switch, energizing relay A to supply power to the dual 

infrared sensors. Once fired, relay A remains energized 

for a minimum of 3 s, the time being controlled by the 

capacitance of capacitor C1 and the inductance and 

resistance of relay A. This keeps the device armed for 

sufficient time for the flame to reach the trigger device. 

The view field of one of the flame sensors across the main 

entry is displaced from the other by a horizontal angle of 

25'. Each sensor must detect radiation simulltaneously to 

energize the firing of relay B. 

Once fired, the output pulse of relay B activates relay 

C, allowing the preset time-delay relays to control the 

signal to the high-speed, direct-solenoid actuated valves, 

which control the sequential sampling at the station 

located 30 m downstream. A typical sample time is on the 

order of 100 ms, which is quite adequate for fiUing the 

collection tube (I). A detailed description of the trigger 

device can be found in reference 3. The system allows Scale, cm 

sampling of the leadling edge of the flame where the 

combustion process is most intense and critical for further Figure 3. - Perspective view of triggering mechanism. 

flame propagation. 
A Unburned Ou 50 

Scale, pm 

CX-roymap,rockdust , 5P 

Scale, pm 

B Burned o - 

Scale, ym 

50 D X-ray map,rock dust 0 do 

Scale, pm 

Figure 5. - Typical SEM photomicrographs of burned and unburned Pittsburgh coal and 65% rock 

dust particles, X-ray maps made using calcium line. 
ENERGY, keV 

Figure 6. - X-ray spectra of Pittsburgh coal and rock dusts. 

DISTANCE, m 

0 0.4 0.6 0.8 1 .O 1.2 1.4 1.6 1.8 2.0 

TIME, s 

Figure 7. - illustration of typical sampling cycle with respect to flame arrival and to trailing edge of flame for three 

Lake Lynn experiments. 
MKE LYNN EXPERIMENTAL MINE 

The studies were conducted with the sampling system suppression as encountered in modern U.S. coal mining. 

installed in the D drift of the Lake Lp Laboratory The new entries dimensions range from 1.8 to 2.3 m high 

experimental mine. The Lake Lynn Laboratory, formerly by 5.3 to 6.7 m wide. The average dimensions are 2.1 and 

a limestime mine Q, is now a multipurpose mining 5.8 m for an average cross-sectional area of 12 m2. A plan 

research laboratory operated by the Bureau. The view of the face area in D drift is given in figure 9 and 

laboratory underground layout, shown in figure 8, allows shows the configuration for a single entry dust explosion. 

full-scale research of explosion propagation and 

 as-mixing stub Control 

building 

Figure 8. - Plan view of Lake Lynn underground mine. 

E 

DRIFT 

u 

Scale, m 

Figure 9. - Plan view of face area in D drift showing configuration of 

nominal dust loading for a single entry. 
A flammable mixture of methane and air, typically 

10% CH4, is introduced in the gas zone, whose length 

varies from 3.7 to 12 m to provide weak to strong ignitions, 

respectively. A mixture of rock dust and/or pulverized 

(80% minus 200 mesh) Pittsburgh seam coal (PPC) is 

spread on the floor and on shelves in the dust zone 

(typically 107 m) to provide a PPC dust concentration near 

200 g/m3, if the dust were uniformly dispersed throughout 

the dust zone volume by the methane-air explosion. 

A flammable dust mixture is dispersed and ignited by 

the methane-air explosion, causing rapid flame propagation 

down the entry. Instrumentation to monitor pressure 

development, dust concentration (8-9), and flame arrival 

times is provided at 11 stations along 229 m of the entry, 

ead. A detailed description of the 

physical arrangement and instrumentation for dust 

explosion studies at the Lake Lynn experimental mine are 

given in references 10 and 11. 

The series of explosions involving coal dust mixtures 

that is described here (LLEM 110, 111, 112) consisted of 

a 107-m dust zone containing 60% or 65% pulverized 

limestone with PPC, at a nominal concentration of 

200 g/m3, that was ignited by a 12-m gas zone consisting 

of a 10.3% methane-air mixture. Figure 10 is a wave 
diagram of one such explosion (LLEM 110). 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 setup at one of the 

monitoring stations consists of six sampling devices 

mounted on two vertical stands. The two stands were 

mounted vertically 91 m from the face and secured to the 

mine floor and roof by bolts. Figure 11 is a cross-sectional 

view of D drift and shows the sampling system and 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. 

LLEM TEST 110 

Particle collection data of the sampling system in this 

explosion, which featured a dust mixture containing 65% 

rock dust, are presented in table 1. The starting time, t,, 

of each sampling device is measured with respect to the 

initiation of the explosion. The total sample time is At, 

and tf is the duration of sampling of the burned gases 

behind the flame front. The residue, mo collected for 

each sample was weighed using a microbalance and then 

microscopically analyzed. Sample A, taken just prior to 

flame arrival, shows the largest amount collected. Sample 

B was drawn before and during the flame, where~s C 

through F were taken entirely within the flame. The mass 

sampling rate was taken as the ordinary average over the 

injection interval. Actually, the collection rate decays 

exponentially with time, with most of the sample being 

collected within the frrst 30 ms (see figure 3 in reference 

1). Samples collected from within the flame sampled a 10- 

to 23-111 flame length, depending on the speed of flame 

propagation. Generally, as the flame progressed down the 

entry, the linear rate of residue collected decreased with 

increasing distance behind the flame front. 

The photo micrographs of the unburned and burned 

dust particles from the explosion (LLEM 110) are shown 

in figure 12. 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 

figure 1 1. - Complete sampling system installed in D drift. 
Figure 12. - Photomicrographs of dud collected with sampling device during 

LLEM 110 explosion test in D drift. 
TABLE 1. - Performance of sampling system 

(LLEM test 110, flame arrival 1.161 s) 

flame 

Station Sample Sample interval Mass residue Collection length 

start Duration In flame collected rate sample 

(t,), ms (At), ms (tf), ms (m,), mg (r), mg/ms (I,), m 

A,.. 1,021 99 0 50.85 0.51 0 

B . . . 1,120 130 89 15.43 .12 15.9 

C . . . 1,250 110 110 8.58 .08 19.7 

D... 1,470 130 130 7.91 .06 23.3 

E... 1,600 60 60 4.41 .07 10.7 

F... 1,660 130 130 6.26 -05 23.3 

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 outgassing of 

volatiles, which are then burned. This outgassing causes 

the softened coal particle to swell and eventually leads to 

the formation of blowholes of various sizes. 

Sample A, collected just before flame arrival, shows no 

evidence of burned or charred particles (fig. 124). The 

composition of the corresponding gas sampled is that of 

background air mixed with 0.6% pyrolysis gases (table 

1-2). The contribution of combustion products from the 

methane-air zone to this sample is thus minimal, as will be 

explained in the "Gas Analyses" section. Sample B, shown 

in figure 128, was collected primarily before flame arrival 

but with continued sampling into the flame front. A 

microscopic evaluation of this dust at several view fields 

indicates that about 10% of the particles are melted or 

charred, and several particles even have blowholes. The 

largest charred coal particle measures 150 to 200 pm in 

diameter. The corresponding gas sample is a mixture of 

air and pyrolysis-combustion gases. 

The remaining photo micrographs of the dust residues, 

samples C through F, (figs. 12C-12F) show that practically 

every particle has reacted. The angular features are totally 

absent, indicating that the particles have undergone 

considerable heating and devolatization. The largest 

particle collected from this test measured 400 pm in 

diameter. Furthermore, agglomeration is quite evident. 

In a few cases, the original particles have agglomerated to 

huge masses with diameters of the orders of 1,000 pm. 

These large agglomerates were most likely formed in the 

collection tube, while the particles were still plastic. The 

corresponding gas samples consist almost entirely of 

combustion-pyrolysis products as shown in t~ble 2. 

Residual oxygen was only 0.6% and there was 17% C02 as 

well as 5% coal pyrolysis products. 

LLEM TEST 11 1 

Microscopic evaluations and gas analyses were made of 

a similar test of a 60% limestone-PPC dust mixture 

(LLEM 111). SEM results indicate that 10% of the dust 

particles collected at station A, just before the apparent 

flame arrival, are melted. The largest unburned coal 

particle measures 200 to 225 pm in diameter and the 

diameter of the largest coal particle that exhibits signs of 

charring is 125 pm. The largest rock dust particle 

observed is in the 200- to 325-pm range. The associated 

gas sample is predominantly background air with only 

0.5% pyrolysis-combustion products. 

The sample taken at station B just before the flame 

front arrival and continuing afterwards, indicates that 90% 

of the coal is melted or charred. In addition, large 

agglomerates are formed and some of the coal particles 

show evidence of blowholes and large areas of molten 

bitumen. The associated gas sample and those taken at 

stations C through D contain predominantly combustionpyrolysis products with only 0.6% residual oxygen. In the 

residue collected entirely in the flame zone (stations C-D), 

there are several large (1,100 pm) mineral particles, as 

shown in figure 13, but very few coal particles. The largest 

burned coal particles ranged from 300 to 550 pm in 

diameter. The sampling of these large particles indicate 

that the sampling system is capable of collecting dust 

particles over a wide range of sizes. The greater 

turbulence associated with this test may be responsible for 

the earlier and more intensive particle pyrolysis previously 

noted. 

TABLE 2. - Actual burned gas compositions from LLEM tests 110, 11 1, 112, with corresponding flame 

arrival times of 1,161 s, 1.021 s, and 1 .1 19 s, respectively 

Sampling Vol % PPm 

LLEM Start Inflame N2 A, O2 C02 CO H2 CH, C2H2 C2H4 C2H6 C3H6 C3H, 

test (t,), ms OF), ms 

110~ I... 1,021 0 77.87 0.93 20.60 

1108 ... 1,120 89 77.76 .93 5.41 

llOC.. . 1,250 110 75.67 .91 .74 

110D ... 1,470 130 75.01 .90 .48 

110E ... 1,600 60 76.31 .91 .78 

11OF ... 1,660 130 77.36 .93 .33 

111~ l... 897 0 78.15 .94 20.10 

111B ... 996 105 74.12 -89 .65 

111 C.. . 1,127 116 72.79 .87 .28 

111 D.. . 1,354 116 74.62 .89 9.12 

111 F . . . 1,536 130 73.32 .88 2.21 

1128.. . 1,101 115 76.10 .91 1.49 

112C.. . 1,234 115 73.27 .88 1.44 

112F .. . 1,642 30 73.37 .88 5.87 

lSamples contained background air and trace burned gas. 
The following photomicrographs (figs. 14-15) reveal 

what happens when the devolatization process of a coal 

particle is interrupted. Normally, as the explosion 

propagates down the entry, the pyrolysis process of the 

coal continues to near completion and the reacting gases 

continue to supply fuel to the fire ball. When particles are 

"grabbed from the explosion environment and collected, 

their devolatization process is quenched. The two partially 

devolatilized coal particles shown in figure 14 were 

collected just prior to (fig. 14A) and just after (fig. 14B) 

flame arrival in LLEM test 112. In figure 14A, notice the 

charred residue or liquid bitumen in the central portion of 

the particle, clearly showing the unreacted coal residue 

with its angular features near the bottom. This bubbly 

mass or char is more pronounced in the coal particle 

collected during the initial flame passage (fig. 14B). Again 

the char layer is quite evident in the top portion of 

Partially burned 

Scale, pm 

Figure 13. - Photomicrograph of large residue collected with 

sampling system. 

LLEM TEST 1 1 2 

Scale, pm 

Partially burned 

Scale, pm 

Figure 14. - Photomicrographs of partially devolatilized coal 

particles, sampled before and during the explosion. 
Port idly burned 

Scale, prn 

Figure 15. - Photomicrograph of partially devolatilized coal 

particle. 

that particle. Figure 15 shows another partially 

devolatilized coal particle. Here the molten bitumen is 

centrally located and the'ends expose the angular features 

of the unreacted coal. 

The partially devolatilized coal particles from LLEM 

test 112 that appear to precede the flame front support the 

nonisothermal, flux driven volatilization model of 

Hertzberg (14, whose dnidirectional laser heated solids 

show similar structure. Those particles were about twothirds devolatilized after an exposure ta a laser flux of 

100 w/cm2 for 1 s. The net absorbed flux in the laser 

experiment (75 w/cm2) is not much higher than the 

calculated flux for a stoichiometric hydrocarbon flame. 

The similarities of the particle structures are striking, and 

may reflect unidirectional radiative heating in advance of 

the flame front. 

GAS ANALYSES 

Gas samples were collected both before and after flame 

arrival at the sampling station, as previously described. 

These were dried and arialyzed by gas chromatography for 

N29 4, 02, C02, CO, CH4, qH2, qH4, qH6r c3H6, 

C3H8, C4H10, and C5y12. Samples collected prior to 

flame arrival gave an average composition that is very 

close to standard dry air, as shown in table 3. 

This finding is not as obvious as it may seem. Consider 

that typically a 12-m zone from the D drift bulkhead faed 

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 that explosion should be detected. The fact 

that the CO, concentration is increased on average from 

the 0.03% of standard air to 0.09% indicates that 

longitudinal gas mixing is minimal and that the hot burned 

gas acts primarily as a piston, pushing the coal-dust-laden 

air ahead of it. Richmond and Liebman had reached the 

same conclusion on the basis of explosion measurements 

made in the experimental mine at Bruceton (13-14). 

Also shown in table 3 is the average composition of the 

samples collected after flame arrival. These results do not 

include the few samples whose compositio~s gave evidence 
TABLE 3. - Average gas composition (dry basis), percent 

Sampling N2 Ar 0, CO, CO H, CH4 C7 H7 C2H4 C2H6 C3H6 

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

am (2) . . . . ... . . .06 .002 .10 .016 .0012 0 .0004 0 0 0 0 

After flame . . . . . . 74.82 .896 .51 15.95 5.59 1.77 .232 ,043 ,025 ,0013 0.0014 

am (2) . . . . . . . . . .37 .004 .06 .52 .47 .68 ,035 ,006 .005 .OOO3 ,0003 

NOTE.-The error limits are the standard deviation of the mean values, am = a/= 

of air dilution (leaking). The average Flame zone The gas compositions for the individual samples taken 

composition consists of both coal vapors and their at the sampling stations for LLEM tests 110, 111, and 112 

combustion products. The former consists of the are shown in table 2. The large gradient in gas 

hydrocarbon gases (CH4, C2H2, C2H4, C2H6, C3H6, etc.) concentrations just within the leading edge of the flame is 

as well as H2 and some of the CO and C02 produced. apparent. 

The PPC used was mined at Bruceton and had the 

following average proimate (weight) analysis (ASTM 

D3173, 3174, 3175): volatiles, 37%; fixed carbon, 55%; 

ash, 6%; and moisture 2%. The following moisture and 

ash-free elemental, mass analysis has been obtained for 

this coal: C, 84%; H, 5.5%; 0, 7.5%; S, 1.3%; and N, 

1.7%, corresponding to the empirical formula, 

cH0.7800.06~0.006N0.017~ 

The coal produces a char in the ASTM D3175 analysis 

for volatiles, which has the elemental analysis 95% C, 

1% H, 0.5% 0, 1.5% S,and 2% N. The corresponding 

empirical formula is CH0~1300~,So~,6No,,,. 

It is generally recognized (15) that the ASTM D3175 

analysis for volatiles in bituminous coals underestimates 

the volatile yields under high heat flux conditions, such as 

in a flame front. For example, using uniform C02 laser 

irradiation (10.6-pm wavelength), at flux levels of 100 to 

400 w/cm2, gave volatile yields of 45% to 55% for this 

coal (16). Taking the true volatility as 50% 25% gives the 

following: 47.5 g C, 0.5 g H, 0.25 g 0, 0.75 g S, and 1.0 g 

N per 50 g char, and 36.5 g C, 5 g H, 7.25 g 0, 0.55 g S, 

and 0.7 g N per 50 g volatiles. This leads to the empirical 

formulas 

CH0.12O0.004S0.,N0.0,, (for char) 

and CH1~~00~150So~~No~016 (for volatiles). 

To sirnpllfy the thermodynamic calculations, Pittsburgh 

coal is taken to consist of an inert carbon char (50%) and 

volatiles having the formula, CH1.700.15, in view of the 

uncertainties in the analysis and in the true volatility. The 

heat of formation of the reactants is -5,000 cal per formula 

weight for the 'volatiles and 0 for the inert carbon. The 

NASA-LEWIS thermodynamic program (CEC-80) then 

predicts an adiabatic flame temperature at 1 atm of 

1,381 K for the lean limit concentration (90 g/m3) of the 

coal. This flame temperature is the same as that given by 

a detailed model of the volatiles, tar, char, and ash 

produced from pyrolyzing Pittsburgh seam coal (17), as 

well as that of polyethylene at its limit concentration of 

35 g/m3. 

The addition of rock dust to the coal results in a 

dissipation of the combustion energy in heating the inert 

dust. The dissipation is strcmger, however, if the CaC03 

dust is allowed to decompose to gaseous C02 and a solid 

residue of CaO. The predicted equilibrium gas 

concentrations for a coal dust-rock dust mixture containing 

65% CaC03 contains 16% C02 (dry basis) if the CaC03 

is inert, and 24% C02 (dry basis) if CaC03 is allowed to 

react. The measured, average C02 concentration in the 

flame zone is 16%, in agreement with the inert rock dust 

model. The predicted rock dust concentration required to 

bring the flame temperature down to the 1,380-K limit is 

87% for this model. Reactive CaC03, however, requires 

only 75%, in agreement with measurements made in the 

experimental mine at Bruceton as well as in the 20-L 

explosion chamber (18). However, results to date at the 

Lake Lynn Laboratory indicate an inerting level of 80% 

rock dust (19). The overall results suggest that only part 

of the rock dust actually reaches a decomposition 

temperature of 1,180 K to form C02, and not all pyrolysis 

products are oxidized to C02 in the presence of a limited 

O2 concentration. 
Immediately prior to flame arrival, the background gas 

is virtually that of standard air and few particles show signs 

of tar, volatiles, or char formation. Gas samples in the 

leading edge of the flame show the large concentration 

changes characteristic of the flame front. Gas samples 

taken entirely in the flame zone consist of pyrolysis and 

combustion products. The particles show signs of 

extensive pyrolysis and charring. The correspondence 

between the value of the principal gases (N2, 02, and C02) 

as calculated for inert (nondecomposing) CaCOgcoal 

(PPC) mixtures burning in air and the experimental value, 

suggests a limited or inefficient decomposition of the rock 

dust. These experimental results demonstrate the utility of 

the high-speed gas and dust sampling system 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. 

REFERENCES 

1. Conti, R S., M. Hertzberg, F. T. Duda, and K L. Cashdollar. 

Rapid-Sampling System for Dusts and Gases. Rev. Sci. Instrum. , v. 54, 

1983, pp. 104-108. 

2. Conti, R S., K. L. Cashdollar, M. Hertzberg, and I. Liebman. 

Thermal and Electrical Ignitability of Dust Clouds. BuMines RI 8798, 

1983,40 pp. 

3. Liebman, I., F. Duda, and R. S. Conti. Sensor Trigger Device 

for Explosion Barrier. Rev. Sci. Instrum., v. 50, 1979, pp. 1441-1444. 

4. Liebman, I., F. Duda, and R. Conti. Sensor Trigger Device for 

Explosion Barrier. U.S. Pat. 4,173,140, Nov. 6, 1979. 

5. Lieman, I., J. K. Richmond, R Pro, R S. Conti, and J. Corry. 

Triggered Barriers for the Suppression of Coal Dust Explosions. 

BuMines RI 8389, 1979, 24 pp. 

6. Ng, D. L., K L. Cashdollar, M. Hertzberg, and C. P. Lazzara. 

Electron Microscopy Studies of Explosion and Fire Residues. BuMines 

IC 8936,1983, 63 pp. 

7. Mattes, R H., A. Bacho, and L. V. Wade. Lake Lynn 

Laboratory: Construction, Physical Description, and Capability. 

BuMines IC 8911, 1983,40 pp. 

8. Conti, R S., K. L. Cashdollar, and I. Liebman. Improved Optical 

Probe for Monitoring Dust Explosions. Rev. Sci. Instrum., v. 53, 1982, 

pp. 311-313. 

9. Cashdollar, K. L., I. Liebman, and R S. Conti. Three Bureau 

of Mines Optical Dust Probes. BuMines RI 8542, 1981, 26 pp. 

10. Sapko, M. J., E. S. Weiss, and R W. Watson. Size Scaling of 

Gas Explosions: Bruceton Experimental Mine Versus the Lake Lynn 

Mine. BuMines RI 9136, 1987, 23 pp. 

11. M. J. Sapko, N. Greninger, and E. S. Weiss. Water Barrier 

Performance in a Wide Mine Entry . To be presented at Third 

(International) Colloquium on Dust Explosions (Szczyrk, Poland), 

Oct. 1988. 

12. Hertzberg, M., I. A. Zlochower, R S. Conti, and 

K L. Cashdollar. Thermokinetic Transport Control and Structural 

Microscopic Realities in Coal and Polymer Pyrolysis Explosions. Paper 

in Proceedings, ACS Fuel Division Symposium (New Orleans, LA, Aug.- 

Sept. 1987). ACS, 1988, pp. 24-41. 

13. Richmond, J. K, and I. Liebman. A physical Description of Coal 

Mine Explosions. Paper in Fifteenth Symposium (International) on 

Combustion (Tokyo, Japan). Combustion Inst., Pittsburgh, PA, 1975, 

pp. 115-126. 

14. Richmond, J. K, I. Liebman, A. E. Bruszak, and L. F. Miller. 

A Physical Description of Coal Mine Explosions. Part 11. Paper in 

Seventeenth Symposium (International) on Combustion (Leeds, 

England). Combustion Inst., Pittsburgh, PA, 1979, pp. 1257-1268. 

15. Howard, J. B. Fundamentals of Coal Pyrolysis and 

Hydropyrolysis. Ch. 12 in Chemistry of Coal Utilization, Vol. 2, ed. 

by M. A. Elliott. Wiley, 1981, pp. 665-784. 

16. Hertzberg, M., and D. Ng. A Microscopic and Kinetic Study of 

Coal Particle Devolatilization in a Laser Beam. Paper in Fundamentals 

of Physical Chemistry of Pulverized Coal Combustion, ed. by J. Lahaye 

and G. Prado, NATO AS1 Series E., No. 137. Martinus Nijhoff, 1987, 

p. 110. 

17. Hertzberg, M., I. A. Zlochower, and K L. Cashdollar. Volatility 

Model for Coal Dust Flame Propagation and Extinguishment. Paper in 

the Twenty-first Symposium (International) on Combustion (Munich, 

Federal Republic of Germany, Aug. 1986). Combustion Inst., Pittsburgh, 

PA, 1986, pp. 325-333. 

18. Cashdollar, K L., and M. Hertzberg. 20-L Explosibility Test 

Chamber for Dusts and Gases. Rev. Sci. Instrum., v. 56, 1985, 

pp. 596-602. 

19. Cashdollar, K L., M. J. Sapko, E. S. Weiss, and M. Hertzberg. 

Laboratory and Mine Dust Explosions Research at the Bureau of Mines. 

Sec. in Industrial Dust Explosions, ASTh4 STP 958. ASTM, 1987, 

pp. 107-118. 
APPENDIX.-LIST OF SYMBOLS 

Cl capacitance, micofarads 

s sample start 

At sample interval, duration 

f sample interval, in flame 

m, mass residue collected 

r collection rate 

1, length of flame sampled 

om standard deviation of the mean (016) 

% percent 

1NT.-BU. OF MINES, PGH., PA. 28784