CO dispersion from a coal fire in a mine entry

J.C. Edwards, R.A. Franks, G.F. Friel & L. Yuan

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, USA

ABSTRACT: Five mine fire experiments were conducted in a 2.08 m high and 2.90 m wide, ventilated mine

entry in the National Institute for Occupational Safety and Health (NIOSH)’s Safety Research Coal Mine (SRCM)

to determine the effect of the dispersion of carbon monoxide (CO) on mine fire detection. CO measurements

were made at distances from 7.6 m to 45.2 m downwind from the fire with diffusion mode CO sensors positioned

near the roof. For small intensity fires, less than 30 kW heat release rate, generated by 14 kg coal in a 0.61 m

square tray, it was determined that air flow and sensor spacing were significant for fire detection at the 10 ppm CO

alarm level. Within 15.0 m downwind distance from the fire, 10 ppm CO alarm values occurred for volumetric

air flows less than 11.5 m3/s. However, the 10 ppm CO alarm value did not occur 30.0 m downwind from the fire

for air quantities greater than 6.2 m3/s due to dilutive mixing of the CO in the air stream. The criterion that the

mine fire alarms occur within 15 min of the onset of flaming combustion could not be consistently met with the

10 ppm CO alarm. This suggests the use of lower CO alarm values, or reduced CO sensor spacings for mine fire

protection. It is demonstrated how computational fluid dynamics (CFD) can be used to model the CO dispersion

downwind from the fire in support of a plan to optimize sensor spacings.

Disclaimer:The findings and conclusions in this report are those of the authors and do not necessarily represent

the views of the National Institute for Occupational Safety and Health.

1 INTRODUCTION

The deployment of carbon monoxide (CO) sensors in a

mine entry to achieve early and reliable fire detection

is important for miner safety.The Code of Federal Regulations (2004) specifies CO sensor spacing distances

for a belt air-course. The fire source location for a fire

in a belt air-course typically occurs on the entry floor,

or near the entry half-height associated with the conveyor belt structure. Fires could occur due to frictional

heating at the belt drive and along the belt. Coal which

has accumulated on a belt drive can be heated by roller

slippage. Another possible fire source is the ignition

of spilled coal on the floor from cutting and welding

activity. Previous research (Litton, et al., 1991) has

established that approximately 15 min (average time

for 15 experiments was 14.3 min with a standard deviation of 7.3 min) after a smoldering coal fire evolves

to the flaming stage, a conveyor belt in contact with

the fire can be ignited. Early and reliable fire detection

is important within this time frame. When a plume of

combustion products rises towards the mine roof due

to the thermally induced density differences between

the hot products-of-combustion (POC) and the ambient air, the POC will also be convected with the bulk

air flow downwind from the fire source. The POC will

be dispersed by the dilutive mixing with the fresh air

over the entry cross-section downwind from the source

fire. This dispersion process is enhanced by the thermal equilibration of the POC with the ambient air. It

is important to know the expected distribution of CO

near the mine roof along the entry in the smoldering and flaming coal fire stage to provide guidance

for sensor site location, and the effect of volumetric

air flow upon the CO concentration. The objective of

this research is to analyze experimentally and computationally the dispersion of CO from small coal fires

in a mine entry and determine the sensitivity of CO

concentration to ventilation and sensor site location.

2 EXPERIMENTAL CONFIGURATION

The experimental location selected for the fire experiments to determine the effect of air flow and sensor

spacing upon fire detection was an entry in NIOSH’s

SRCM at the Pittsburgh Research Laboratory (PRL).

The entry height and width at the location of the fire

pan were 1.96 m and 2.79 m, respectively. Since in a

mine environment the source fire could occur on a belt

drive at entry mid-height, the experiments included

both floor and mid-height fire source locations. Coal

was selected as the fire fuel source. Five small coal fire

experiments were conducted. The CO concentration

was measured with diffusion mode Conspec Controls

Inc CO monitors near the roof at locations 7.6, 15.0,
30.0, and 45.2 m downwind from the fire pan. (Reference to a specific product is for informational purposes

and does not imply endorsement by NIOSH.) At the

7.6 and 15.0 m locations two CO sensors each were

located at equally spaced intervals across the entry

near the roof. At the 30.0 and 45.2 m stations a single CO sensor was positioned at the entry center near

the roof. Calibration of the CO monitors was conducted before each experiment with hydrocarbon free

air and 25 ppm CO in air. The entry average height

and width were 2.08 m and 2.90 m. The entry associated hydraulic diameter was 2.42 m. Table 1 lists the

fire source location and entry air flow for each experiment. Sensor data was collected every 2 seconds with a

mine monitoring system. For experiment nos. 1–3 the

fire pan was positioned on the mine entry floor, and for

experiment nos. 4 and 5 the fire pan was positioned on

a set of blocks 0.89 m above the floor, which is approximately half the 1.96 m entry height at the fire zone. For

each experiment about 14 kg of run-of-the-mine Pittsburgh Seam coal contained in a 0.61 m square tray was

heated by embedded electrical strip heaters. A small

quantity, about 0.5 kg, of pulverized Pittsburgh Seam

coal was added to the coal. Heating of pulverized coal

provided an early source of CO. Electrical power was

supplied to the strip heaters over a 30 min period with

an increase from 1.7 to 2.8 kW. The slow increase in

heating was to produce incipient CO emissions at the

lowest power source available, and to simulate the slow

heating to be expected from a heated element, such as

at a belt drive or along the belt.The linear air flow measurements were made with a Solomat Neotronics hot

wire anemometer for experiment nos. 1–4. An average linear air flow was determined from a five point

average measurement over the entry cross-section.The

Solomat was not temperature compensated. A correction to the Solomat measured air flow was made based

upon a calibration of the Solomat with an Edra 6 Air

flow Developments Ltd vane anemometer over a temperature range from 25 to 73 deg F. For the four temperature evaluations a linear fit was made which had an

R-square value, coefficient of determination, equal to

0.9973. The corrected air flow values at the fire pan

are listed in Table 1. The volumetric air flow refers to

the entry cross-section at the fire pan. For experiment

no. 5 an ultrasonic flow monitor from El-Equip Inc

was used. The ultrasonic flow monitor measures the

average air flow along a diagonal between the entry’s

opposing ribs.

3 RESULTS

For all five experiments the CO measurements at the

7.6 and 15.0 m stations indicated the CO dispersion

was uniform over the midsection near the roof. Consequently, the CO concentration at these two locations

Table 1. Mine fire experimental conditions.

Volumetric Average air

Experiment air flow Q, velocity, Fire source

number m3/s m/s location

1 9.34 1.71 Floor

2 2.74 0.50 Floor

3 6.19 1.13 Floor

4 11.5 2.10 Mid-height

5 5.84 1.07 Mid-height

0

5

10

15

20

9:30 9:43 9:56 10:10 10:23

TIME

CO, PPM

7.6 m

15.0 m

30.0 m

45.2 m

EVENT

FIRST SMOKE

FIRST FLAME

POWER OFF

15.0 m

7.6 m

Figure 1. CO concentration at sensor stations in response

to coal fire on entry floor for experiment no. 2 at 7.6, 15.0,

30.0, and 45.2 m distance from fire.

was represented as an average. Experiment nos. 2 and

5 provide a comparison of the time dependent CO

concentration at the sensor stations with a fire on the

mine entry floor and at entry mid-height. These two

experiments are representative of the two experimental configurations for source fire location. Figures 1

and 2 show the measured CO concentration at each

station for experiment nos. 2 and 5, respectively. For

experiment no. 2 the heat source was increased from

1.7 kW to 2.8 kW over the time period 9:32 to 10:02.

For experiment no. 5 the heat source was increased

1.7 kW to 2.8 kW over the time period from 9:39 to

10:09. In each experiment the first flames occurred

approximately when the increased electrical power to

the strip heaters reached 2.8 kW. The measured CO

concentration at the 30.0 m and 45.2 m stations tracked

each other closely for all five experiments. These stations are approximately 12 and 19 hydraulic diameters

downwind from the source fire.The implication is that,

beyond 10 hydraulic diameters, the source fire product

emissions are well mixed over the entry cross section.
0

10

20

30

40

50

9:40 9:56 10:13 10:30 10:46

TIME

CO, PPM

7.6 m

15.0 m

30.0 m

45.2 m

EVENT

FIRST

SMOKE

FIRST FLAME

REMOVE HEATERS

APPLY H O2

7.6 m

15.0 m

Figure 2. CO concentration at sensor stations in response

to coal fire at entry mid-height for experiment no. 5 at 7.6,

15.0, 30.0, and 45.2 m distance from fire.

The CO concentrations at the 7.6 m and 15.0 m stations, which were three and six hydraulic diameters

downwind from the fire, had more divergent CO concentrations than those at the 30.0 and 45.2 m stations.

The instability of the CO production due to fluctuations in the fire intensity was not averaged out by

the air flow in the 15 m zone immediately downwind

from the fire. This was observed for all five experiments. In each experiment an approximately linear

increase in CO concentration occurred in the smoldering stage, with an increased rate in the flaming stage.

For experiment no. 1 there was an electrical power disruption after initial flaming combustion. This resulted

in a cooling of the coal and the extinguishment of an

early flaming combustion. Power was reapplied, and

steady flaming combustion followed.

3.1 CO alert and alarm

For each experiment the 5 ppm CO alert value was

attained at each of the four sensor stations. Only for

experiment no. 2 did the CO alert occur in the smoldering combustion stage. This was a consequence of

the lowest volumetric flow rate of 2.74 m3/s for experiment no. 2. To assure early fire detection in the

smoldering stage for volumetric air flow rates greater

than 2.74 m3/s for these small coal fires, either the

CO alert values could be lowered or the sensor spacing could be decreased. Reference to volumetric air

flow rate must be understood with caution. The air

velocity can affect the fire production rate of CO. In

addition, the turbulent mixing of the fire POC plume

will be affected by the confinement height of the tunnel. However, for early detection of small fires, these

effects should be relatively small.

Table 2. Measured values of maximum CO, and CO and

smoke optical density, OD, 15 min after flaming combustion

45.2 m downwind from the fire source.

Exp. MAX. HR CO∗, H∗

R,

No. CO, ppm kW, ppm OD∗, m−1 kW

1 6.8 17 6.84 —– 17

2 15.8 11 14.06 —– 10

3 7.8 13 6.64 0.024 11

4 7.2 22 2.54 0.0072 8

5 18 28 7.03 0.022 11

∗ 15 min after visible flames

At the 7.6 m and 15.0 m stations the 10 ppm CO

alarm value occurred for all five experiments. The CO

10 ppm alarm value was not achieved at the 30.0 m

and 45.2 m stations for experiment nos. 1, 3, and 4.

These latter experiments are associated with the air

quantity greater than 6.19 m3/s, and consequently, CO

is subjected to much greater dilution.

3.2 Fire heat production

The heat release for coal combustion can be estimated

from the CO generated in the flaming combustion

mode. The CO generated by the flaming combustion

of coal is 4.8 mg/kJ (Egan, 1990). Formally, the heat

release HR (kW) is determined from

where [CO] is the CO concentration in ppm, and Q is

the volumetric air flow in m3/s.

Since eq (1) depends upon the volumetric air flow,

more complete mixing in a mine entry results in a better estimate of the heat production rate when a CO

sensor near the roof is used to detect a concentration

representative for the cross-section. The observation

that the measured CO concentration at the 30.0 m and

45.2 m stations tracked each other provides the CO

concentration at the 45.2 m station as an estimate of the

heat production rate based upon uniform POC mixing.

The maximum CO concentration 45.2 m downwind

from the fire is used to represent the maximum heat

release rate in the flaming stage. The CO maximum

concentration and the associated heat release rate are

shown in Table 2. For experiment nos. 1–3 with the

fire source on the entry floor the highest maximum CO

concentration is associated with the lowest ventilation

of experiment no. 2. The maximum CO concentration

at the 45.2 m station occurred after the onset of flaming combustion for each experiment. For experiment

nos. 1–2 the time at which the maximum CO occurred

at the 45.2 m station was approximately 15 min after

flaming combustion. The maximum CO at the 45.2 m

station occurred at least 30 min after flaming combustion for experiment nos. 3–5. As shown in Table 2
the maximum fire intensities were less than 30 kW for

the fire sources used in these experiments. These fire

intensities are indicative of a small fire source.

3.3 CO and smoke 15 min after flaming

combustion

Previous research (Litton, et al., 1991) established the

significance for fire detection within 15 min of flaming combustion based upon ignition of a belt fire by

a small flaming coal fire. For these experiments, the

measured CO concentration 45.2 m downwind from

the source fire where the fire POC are well mixed is

listed in Table 2 at 15 minutes after flaming combustion. Only for experiment no. 2 with a volumetric air

flow of 2.74 m3/s was the CO concentration greater

than the 10 ppm alarm level 15 minutes after flaming

combustion. In experiment no. 5, the other experiment

in which a CO alarm was reached, the CO did not reach

alarm level until 24 min after flaming combustion.

In proximity to the fire, at the 7.6 m station, where

the contaminants are less diluted, the CO alarm value

was reached for experiment nos. 1 and 5 within 15

min after flaming combustion. For experiment no. 2

the CO alarm value occurred within the smoldering

combustion stage. For experiment nos. 3 and 4 the CO

alarms did not occur until 34 and 65 min after flaming combustion. This inconsistency of the CO alarm

time with air flow further supports the need to lower

the CO alarm level or place the sensor closer to any

in-mine locations identified as high risk for a potential

fire source.

A light obscuration monitor was located 1.6 m

upwind from the 45.2 m CO sensor location for experiment nos. 3–5.The light obscuration monitor consisted

of an incandescent light source and a photovoltaic cell

separated by one meter. The measured light transmission through the light monitor was used to determine

the smoke optical density. Table 2 shows for experiment nos. 3 and 5 that 15 minutes after visible flames

the CO concentration was in excess of the 5 ppm alert

value and the smoke optical density value was greater

than the smoke sensor optical density alarm value of

0.022 m−1. The higher ventilation in experiment no. 4

resulted in CO alert and smoke alarm values occurring

65 min and 51 min after visible flames. For this latter

case the greater POC dilution suggests that a lower CO

or smoke sensor alarm value should be used for earlier

fire detection, or the placement of the smoke sensor

should be closer to any location identified as high risk

for a fire source.

Figure 3 shows the CO concentration 15 min after

flaming combustion at the four sensor locations for

the five cases. The separation in CO concentration values is more significantly dependent upon ventilation

velocity than whether the fire source is on the floor or

at mid-height. The lowest CO concentration is associated with the highest air flow of 2.1 m/s. The highest

0

5

10

15

20

0 10 20 30 40 50

DISTANCE, m

CO, PPM

1.71 m/s 0.50 m/s

1.13 m/s 2.10 m/s

1.07 m/s

Figure 3. CO concentration 15 min after flaming combustion dependence upon sensor location for average air flows

of 0.50, 1.07, 1.13, 1.71, and 2.10 m/s.

0

5

10

15

20

0 4 8 12 16

VOLUMETRIC AIR FLOW, m3

 / s

CO, PPM

30 m 45.2 m

15 m 7.6 m

Linear fit

Figure 4. CO concentration 15 min after flaming combustion at 7.6, 15.0, 30.0, and 45.2 m distance from fire.

CO concentrations are associated with the lowest air

flow of 0.5 m/s. The intermediate CO concentrations

are associated with the range of air flows from 1.07 m/s

to 1.71 m/s. There is not a significant variation in the

CO concentration between the 30.0 and 45.2 m sensor

stations for a specific air flow. Because of the small

fire intensities, the air flow has little effect upon the

CO production.

Figure 4 shows the dependency of CO concentration upon the volumetric air flow for all four sensor

stations 15 min after flaming combustion for all five

air flow conditions. The best fit of a linear relationship to the data with an R square value of 0.73

shows that the CO concentration is dependent upon

the volumetric air flow, but there is a wide degree of

uncertainty. For fires of nearly equal intensity a linear
0

10

20

30

9:40 9:56 10:13 10:30 10:46

TIME

CO, PPM

0.00

0.02

0.04

0.06

0.08

0.10

OPTICAL DENSITY , 1/m

CO

EVENT

OPTICAL

DENSITY

FIRST

SMOKE

FIRST FLAME

REMOVE HEATERS

APPLY H O2

Figure 5. Comparison of CO and smoke optical density at

45.2 m for experiment no. 5.

dependence of CO concentration upon volumetric

flow rate is expected. Extrapolation of the linear fit

of the data to non-detectable, near zero, CO concentration occurs at a volumetric flow rate of 14 m3/s. For

the 2.08 m by 2.90 m airway considered, the linear air

flow rate is 2.32 m/s. These results show the need to set

lower CO alarm values for higher air flow conditions

in a mine.

3.4 Correlation of CO and smoke

Figure 5 shows a comparison of the CO concentration

and the smoke optical density at the 45.2 m station

for experiment no. 5. The smoke optical density and

CO exhibit similar trends over the course of the experiment. An analysis of the optical density D as it is

correlated with the CO concentration at the 45.2 m

station determined the linear correlation over the combined smoldering and flaming combustion range of the

coal fire

where A is a proportionality constant.

The values for A and the coefficient of determination R2 are listed in Table 3 for experiment nos.

3–5. The R square value is not less than 0.79 for these

experiments.

The optical density D, measured in units of inverse

meters, m−1, is related to the smoke mass concentration Cm by the relationship (Mullholland, 1988)

where the specific extinction coefficient Km depends

upon the size distribution and optical properties of the

smoke. A combination of equations (2) and (3) yields

Table 3. Proportionality constant for optical density dependence upon CO concentration.

Experiment number A R2

3 0.0036 0.87

4 0.004 0.79

5 0.0045 0.86

Equation (4) provides an interpretation of the smoke

concentration as linearly dependent upon the CO

concentration. The fit of the smoke optical density

with CO concentration did not distinguish between

smoldering and flaming combustion.

The value for A can be selected to be the average

of the three values for experiments 3–5. The average value of A is 0.004. This value in equation (2)

implies that a 5 ppm CO concentration is associated

with an optical density equal to 0.02 m−1, and a 10 ppm

CO concentration is associated with an optical density

equal to 0.04 m−1. The smoke sensor optical density

alarm value of 0.022 m−1 would be associated with the

5 ppm CO alert value.

An estimate can be made of Km based upon its

definition in terms of the extinction coefficient Qe,

the smoke particulate mass density ρ, and smoke

particulate average diameter d.

For smoke particulate density 1,400 kg/m3, diameter of 0.3 micron, and extinction coefficient of 2.5,

the value of the specific extinction coefficient is

8,900 m2/kg. The specific extinction coefficient is

comparable to the value of 7,600 m2/kg (Seader &

Einhorn, 1976) for flaming combustion of wood and

plastics. The relationship between Cm and [CO] is

approximately

3.5 Simulation of CO concentration using FDS

CO concentrations from the small coal fires were

simulated using the Fire Dynamics Simulator (FDS),

an existing computational fluid dynamics program

(McGrattan, et al., 2002). The chemical structure of

the Pittsburgh coal is simplified as CH0.74O0.08. CO is

assumed to be created with constant yield at the flame

and transported with the combustion products. The

fraction of fuel mass converted into carbon monoxide is 0.078 obtained for Pittsburgh coal (Egan, 1990).

Other important parameters for the CO simulation are

the amount of energy released per unit mass of oxygen

consumed and the fire heat release rate. The amount of

energy released per unit mass of oxygen consumed for
0

4

8

12

16

0 400 800 1200 1600 2000

TIME, s

CO, ppm

MEASURED

FDS

CO MEASURED

FDS

(a)

0 400 800 1200 1600 2000

(b) TIME, s

0

4

8

12

16

20

CO, ppm

MEASURED

FDS

CO MEASURED 

FDS

Figure 6. CO concentrations predicted by FDS for experiment no. 2 with fire source on floor (a) at 7.6 m station; (b)

at 45.2 m station.

Pittsburgh coal is 11,900 kJ/kg derived from the heat of

combustion for the coal. From the CO measurements

in the experiment, it was determined that the whole

coal combustion process can be approximately divided

into three stages: the smoldering combustion stage,

the flaming combustion stage, and the peak combustion stage. The heat release rates at these three stages

increased nearly linearly with the time. For the simulations the heat release rate in each stage was simplified

as a linear increase with time. The heat release rates

at the end of each stage were estimated by equation

(1) with the CO concentration measured at 45.2 m station. For the purpose of early CO alert and alarm, the

peak combustion stage may be not important but can

be used for the comparison between the simulation and

the experiment.

Figures 6 and 7 show the CO concentrations predicted by the FDS for experiments nos. 2 and 4,

respectively. For experiment no. 2 with the fire source

at the floor, FDS prediction was in good agreement

with the experiment at 45.2 m station while the FDS

predicted lower CO concentration than the experiment

(a)

0

15

30

45

60

0 500 1000 1500 2000 2500

TIME, s

(b)

0 500 1000 1500 2000 2500

TIME, s

CO, ppm

MEASURED

FDS

MEASURED

FDS

CO MEASURED

FDS

0

4

8

12

CO, ppm

CO MEASURED

FDS

Figure 7. CO concentrations predicted by FDS for experiment no. 4 with fire source at mid-height (a) at 7.6 m station;

(b) at 45.2 m station.

after 1,000 s at 7.6 m station. This is probably because

the fraction of fuel mass converted into carbon monoxide was not always constant in the experiment. For

experiment no. 4 with the fire source at the mid-height,

FDS prediction was much higher than the experiment

after 1,500 s at 7.6 m station while a peak occurred in

FDS simulation before 1,000 s at 45.2 m station. Generally speaking, FDS simulation agrees well for the

experiment with the fire source at the floor and can be

used to optimize the CO sensor spacing. Further modeling work is needed to improve the simulation with

the fire source at the mid-height.

4 CONCLUSIONS

For the small, less than 30 kW heat release rate intensity, coal fires in a 2.08 m high and 2.90 m wide entry,

it was determined that:

• At distances greater than 10 hydraulic diameters

from the source fire, the CO concentration was

well-mixed over the entry.
• To assure detection in the coal fire smoldering stage,

it would be beneficial to decrease the alert and alarm

CO concentration values for volumetric air flows

greater then 2.74 m3/s.

• Relatively close to the potential fire source, within

15 m of the fire, CO 10 ppm alarm values occurred

for the volumetric air flows less than 11.5 m3/s. For

air quantities greater than 6.19 m3/s, the 10 ppm

CO alarm did not occur at distances greater then

30.0 m downwind from the fire. This is consistent

with the requirement of the Code of Federal Regulations (2004) that any CO sensor be less than 30.5 m

(100 ft) downwind from each belt drive unit.

• For volumetric air flow rates greater than 2.74 m3/s,

a CO alarm was not achieved within 15 min after

the onset of flaming combustion 45.2 m distance

from the source fire. At a distance of 7.6 m from

the fire the occurrence of a CO alarm value was not

consistent with the air flows. This suggests the use

of lower CO alarm values for a CO sensor downwind

from a potential fire source, such as a belt drive.

• Extrapolation of CO concentration 15 min after

flaming combustion with air flow showed that the

CO concentration would not be detectable for air

flows greater than of 2.32 m/s at distances greater

than 7.6 m from the source fire.

• CO concentration correlates with smoke mass concentration and optical density.

• FDS simulation agreed well for the experiment with

the fire source at the floor and can be used to

optimize the CO sensor spacing. Further modeling

work is needed to improve the simulation with the

fire source at the mid-height.

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