Mine Fire Detection In The Presence of Diesel Emissions

J.C. Edwards, R.A. Franks, G.F. Friel, C.P. Lazzara, and J.J. Opferman
National Institute for Occupational Safety and Health
Pittsburgh Research Laboratory
P.O. Box 18070, Cochrans Mill Road
Pittsburgh, PA 15236
(jce9@cde.gov. rdf3@cdc.gov, gcfl @cdc.gov, cgl9@cde.gov, jdo2@cde.gov)

INTRODUCTION

A series of four coal combustion experiments (1)! was conducted at the National Institute for
Occupational Safety and Health, Pittsburgh Research Laboratory in the Safety Research Coal Mine to
evaluate the response of optical and ionization smoke and CO sensors to a small 0.61 m square
smoldering coal fire which transitions to flaming combustion in the presence of diesel emissions. With the
increased utilization of diesel engines in underground coal mines, it is important to be able to discriminate
fire products-of-combustion (POC) from diesel emissions. One proposed method to accomplish this is
the deployment and interpretation of multiple sensors.

EXPERIMENTAL PROCEDURES

A schematic of the airways of 2 m height and 4 m width in which the experiments were conducted is
shown in figure 1. Electrical strip heaters were used to heat a mixture of pulverized and 6 cm diameter
coal slowly through smoldering to a flaming combustion transition with a CO production rate of
approximately 0.001 ppm/s in ventilation air of 5 cu m/s. The fire sensors were positioned near the
airway roof 148 m downwind from the fire. Diesel engines were operating for three of the four
experiments 79 m upwind of the sensors. A spatially averaged path measurement of the POC was made
with an optical smoke sensor (SA) which consisted of an infrared transmitter and receiver separated by an
optical path of 9.65 m. Point measurements of the POC were made with a chemical cell, CO sensor, and
an ionization smoke sensor (SB). Smoke optical density was determined from the measured optical
obscuration of light in the 0.4 - 0.7 micron range over a 1 m path.

RESULTS

The optical fire sensor SA and the ionization sensor SB were quite dissimilar in their response to the
initiation of diesel emissions. For experiment No. 4, figure 2 shows the response of smoke sensor SA
and the CO sensor to diesel emissions from two diesel operated pieces of mining equipment, and figure 3
shows the response of smoke sensor SB and the CO sensor to the same diesel emissions. Time zero
refers to the initial heating of the coal. For the three experiments with diesel emissions present, the
optical sensor SA responds rapidly to the diesel emissions when the diesel engines were started with
maximum signal changes between 4.7 and 19.2 pct, but returns to an asymptote within 1.5 pct of the
ambient signal in the steady state diesel emissions. The ionization sensor SB responds with a signal
change to an asymptote offset between 10.8 and 26.7 pct from the ambient pre-diesel signal. The
characteristic response of the sensors to diesel emissions is due to the greater responsiveness of optical
sensors to smoke particle size, and of ionization sensors to smoke particle concentration.

‘Italic numbers in parentheses refer to items in the list of references at the end of this report.



The response of smoke sensors SA and SB to the coal fire POC in the presence of diesel emissions for
experiment No. 4 is shown in figures 4-5. The sensors respond continuously to a change in the POC
associated with the increase in the measured CO concentration. Smoke sensors SA and SB respond
similarly to the fire, as opposed to their characteristically different response to the diesel emissions in
figures 2 and 3. The rate of increase in diesel emissions associated with starting the engine was much
greater than the POC rate of increase associated with the coal combustion. In experiment No. 4, the
average rate of increase in CO associated with the diesel engine’s startup was 0.0716 ppm/s, and the
average rate of rise in CO associated with the coal combustion was 0.00141 ppmv/s. For the slowly
developing coal fires in these experiments, sensor SA’s signal had a small rate of increase, or remained
constant, or continued to decrease for experiment Nos. 2, 3, and 4, respectively when the 5 ppm CO alert
level was reached. However, a significant reduction in SA’s signal, between 25.4 and 38.0 pct for
experiment Nos. 2-4, had occurred in the case of coal combustion at the 5 ppm alert value as compared
to the diesel engine’s startup. Based upon previous considerations (2) for the definition of the alarm
value for a smoke sensor with a continuous analog output signal as a 10 standard deviation change in the
signal from its ambient value, the alarm times of the fire sensors were compared for the coal combustion
processes. In the presence of steady diesel emissions, the ambient value is the time average in the diesel
emissions background. The optical path smoke sensor alarmed earlier than the point type diffusion mode
ionization smoke sensor, which alarmed prior to a CO alert value of 5 ppm above ambient for each of the
coal combustion experiments. For the three coal combustion experiments with background diesel
emissions, the average optical density at alarm time was 0.0025 m~ for the optical sensor, 0.012 m “for
the ionization point sensor, and 0.024 m + for the CO sensor.

A commercially available multiple sensor consisting of a NO and a CO chemical cell with an algorithm to
use the history of the CO and NO produced by an operating diesel engine to discriminate from the CO
produced by a fire was unable to distinguish a smoldering coal fire with a rate of increase of CO of
0.0014 ppm/s in a volumetric flow of 4.63 cu m/s. The sensor had been shown to be applicable to higher
intensity fires.

CONCLUSIONS

The results of the experiments demonstrated that multiple sensors, which include an optical path smoke
sensor and a CO sensor, can be used to detect a slowly developing coal fire in the presence of diesel
emissions. This result is limited to the experimental conditions considered. An optical path smoke sensor
and an ionization point smoke sensor alarmed earlier than the 5 ppm CO alert level for smoldering coal
combustion in the presence of diesel emissions, Current research focuses upon a decision making
process, such as a neural network, which can be applied to fire sensor response to a wide variety of fire
and diesel emission conditions.

ACKNOWLEDGMENTS

The authors acknowledge the miners who maintain the Safety Research Coal Mine at the Pittsburgh
Research Laboratory for their assistance in preparation of the experiments.

REFERENCES

1. Edwards, J.C., R.A. Franks, GF. Friel, C.P. Lazzara, and J.J.Opferman. “Mine Fire
Detection in the Presence of Diesel Emissions.” Proceedings of the 8* U.S. Mine Ventilation Symposium,
Univ. of Missouri-Rolla, Rolla, MO, June 11-17, 1999, Univ. of Missouri-Rolla Press, pp. 295-301.

2. Edwards, J.C. “Overview of Mine Fire Detection.” Second International Conference on Fire
Research and Engineering, August 3-8, 1997, NIST, Gaithersburg, MD, Society of Fire Protection
Engineers, pp. 489-499.


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