The effect of ventilation on spontaneous heating of coal

Liming Yuan*,

*	 Corresponding author. Tel.: þ1 412 386 4961; fax: þ1 412 386 6595.
 

E-mail address: Lcy6@cdc.gov (L. Yuan).
 

Alex C. Smith

Office of Mine Safety and Health Research, National Institute for Occupational Safety and Health, P.O. Box 18070, Cochrans Mill Road, Pittsburgh, PA 15236, USA

abstract

Ventilation plays an important role in the spontaneous heating of coal in an underground coal mine. If

the ventilation rate is too high, heat is carried away by convection. If the ventilation rate is too low, the

reaction rate becomes oxygen-limited. The effect of ventilation on the spontaneous heating of coal was

investigated in an isothermal oven in this study. Experiments were conducted on three U.S. coal samples

with ventilation 3 rates ranging from 100 to 500 cm /min. Experiments under ventilation were conducted

to determine the critical ambient temperature, which is the minimum oven temperature required for

a coal sample to achieve thermal runaway. Spontaneous heating tests were then conducted at various

ventilation rates at the critical ambient temperature and the results were compared with spontaneous

heating tests without ventilation. It was found that there is an optimum ventilation flow to produce the

maximum rate of temperature rise at the critical ambient temperature. When the coal sample particle

size was increased, a higher critical ambient temperature was required. The results in this study have

application in the prevention of spontaneous combustion in underground coal mines.

1. Introduction

Spontaneous heating has long been a problem in the mining,

storage, and transport of coal. Spontaneous heating is a low

temperature coal oxidation reaction which takes place when coal is

exposed to air. If the heat produced by the coal oxidation is not

adequately dissipated by conduction or convection, it results in

a net temperature increase in the coal mass. Because coal oxidation

is an irreversible exothermic reaction and its reaction rate increases

with temperature, the increase in temperature leads to a higher

oxidation rate. If not averted with appropriate action, this process

results in thermal runaway and a fire ensues.

Much research has been done to understand the physical and

chemical processes involved in spontaneous heating and the effects

of different parameters on the spontaneous heating process (Akgun

& Arisoy, 1994; Beamish & Arisoy, 2008; Carras & Young, 1994;

Nugroho, McIntosh & Gibbs, 2000; Ren, Edwards & Clarke, 1999;

Smith & Glasser, 2005; Smith, Miron & Lazzara, 1991; Wang,

Dlugogorski & Kennedy, 2003). However, the chemical reaction

between coal and oxygen at low temperatures is complex and still

not well understood. Generally, three types of processes are

believed to occur (Carras & Young, 1994): (i) physical adsorption of

oxygen; (ii) chemical adsorption which leads to the formation

of coaleoxygen complexes and oxygenated carbon-species; and

(iii) oxidation in which the coal and oxygen react with the release of

gaseous products, typically carbon monoxide (CO), carbon dioxide

(CO2), and water vapor (H2O).

In laboratory experiments, a number of methods has been used

in attempting to predict the spontaneous combustion tendencies of

coals, as reviewed by Wang, Dlugogorski and Kennedy (2006). In the

U.S., the Bureau of Mines developed the minimum self-heating

temperature (SHT) method using an adiabatic heating oven to

evaluate the relative self-heating tendency to spontaneous

combustion for U.S. coals (Smith & Lazzara, 1987). Although these

evaluation methods can be useful for predicting the spontaneous

heating potential of coal, there are difficulties in scaling the results

to the large-scale conditions found in underground coal mines.

The spontaneous heating of coal in underground mines often

occurs in mined-out areas (gobs) and is not easily detected. Computation fluid dynamics (CFD) modeling on spontaneous heating of coal

in longwall gob areas has been conducted (Yuan & Smith, 2008). In

mines that ventilate these mined-out areas, the amount of coal that

accumulates in these areas and the degree of ventilation can combine

to give optimum conditions for coal self-heating to occur. However,

little research is available about the effect of ventilation on the

spontaneous heating of coal. To better prevent the spontaneous

combustion of coal in underground coal mines, it is important to

understand the effect of ventilation on the reaction mechanisms of

the low temperature oxidation of coal. In this paper, laboratory-scale

experiments were conducted to investigate the effect of ventilation
on spontaneous heating. The experimental results in this study

provide insight into the interaction between ventilation and coal

oxidation and improve the CFD modeling. The analysis of CO and CO2

data will be presented in a separated paper.

2. Experimental procedure

Experiments were conducted in a programmable isothermal

oven. The oven temperature can be held constant from 40 to 200 °C,

within an accuracy of ±0.5 °C. In each experiment, the oven is first

turned on and preset at a certain temperature. After about 30 min,

the isothermal condition is reached in the oven. Then, a stainless

steel container with the coal sample is placed in the center of the

oven. During testing, the coal sample is contained in a 6-cm

diameter by 10-cm high brass wire mesh basket enclosed in

a stainless steel container. Three 0.5-mm-diameter type K thermocouples are placed at the center of the coal sample, 3 cm above

and below the coal center, respectively, to measure the coal

temperatures. The ventilation gasdair or nitrogendis introduced

into the container through ports in the bottom and exits from the

top, allowing the gas to flow through the coal sample. The gas is

preheated to the oven temperature by being passed through heat

exchange coils in the oven. Initially, the coal sample is heated to

a selected temperature while exposed to a flow of dry nitrogen.

When the coal sample reaches the oven temperature, the nitrogen

flow is switched to the same flow rate of dry air. The exhaust gas

exiting the container passes through a water trap before going

through oxygen, carbon monoxide (CO), and carbon oxide (CO2) gas

analyzers. The CO and CO2 gas concentrations are measured

continuously using an infrared gas analyzer, while the oxygen

concentration is monitored continuously by a paramagnetic oxygen

analyzer. Tests are terminated when the coal temperature begins to

decrease or the temperature reaches 250 °C. A schematic of the

experimental setup is shown in Fig. 1.

Water trap CO 

air N2 

Thermocouples 

Exhaust gas 

O2 

analyzer 

CO2 

analyzer 

analyzer 

Data 

acquisition 

Oven 

Fig. 1. Schematic of experimental setup and measurement.

Coals from three U.S. mines were evaluated in these experiments. The proximate and ultimate analyses for the three coals are

given in Table 1.

Table 1

Proximate and ultimate analyses of the coal samples.a

Coal sample A B C

Proximate analysis

Volatile matter (%) 36.88 38.40 44.03

Fixed carbon (%) 55.52 54.46 47.44

Ash (%) 7.60 7.14 8.53

Ultimate analysis

Hydrogen (%) 5.52 5.83 5.47

Carbon (%) 81.88 82.72 77.79

Nitrogen (%) 1.49 1.57 1.48

Sulfur (%) 0.81 0.49 2.61

Oxygen (by difference) 10.30 9.39 12.65

a Proximate analysis is on dry basis, and the ultimate analysis is on dry ash-free

basis.

The coal samples are freshly mined and shipped in

airtight containers. To be sized for the experiment, the coal sample

is first passed through a jaw crusher and then pulverized and sieved

to the 100 x 200 mesh (75 x150 mm) size fraction. This coal is dried

in the oven with a ow rate of 200 cm3 fl /min nitrogen at 67 °C prior

to the experiment. 150 g of coal sample is used in each experiment.

3. Experimental results and discussion

3.1. Determination of critical ambient temperature

Initial experiments were conducted to determine each coal’s

critical ambient temperature and activation energy, shown in

Table 2.

Table 2

Activation energy and SHT of coal samples.

Coal sample Critical ambient Measured activation Calculated SHT, °C

temperature, °C energy (kcal/mol)

A 110 22.3 72

B 115 20.2 78

C 100 9.5 57

The critical ambient temperature is defined as the minimum

oven temperature at which a thermal runaway is reached (Sujanti &

Zhang, 1999). Here, thermal runaway is defined as a continuous

temperature rise at the center of the coal sample. To determine the

ambient temperature, experiments are conducted at an initial oven

temperature with different airflow rates. If there is no thermal

runaway, the oven temperature is increased by 5 degrees. Experiments are repeated until a thermal runaway is reached. This

minimum oven temperature at which the thermal runaway occurs

is the critical ambient temperature for the coal sample at this flow

rate. Experiments were conducted on coal sample A at an initial

oven temperature of 95 °C with airflow rates ranging from 100 to

500 cm3

/min. Eventually, a thermal runaway was reached for the

sample A with the oven temperature at 110 °C at an airflow rate of

300 cm3

/min. With this oven temperature at the airflow rates of

100 and 200 cm3

/min, no thermal runaway occurred. With this

oven temperature at the airflow rate of 500 cm3

/min, the thermal

runaway also occurred. Similar tests were conducted for coal

samples B and C. For coal sample B, thermal runaway occurred at an

oven temperature of 115 °C and an airflow rate of 200 cm3

/min.

This critical ambient temperature is slightly higher than the critical

ambient temperature for sample A. Tests on coal sample C resulted

in a critical ambient temperature of 100 °C with the airflow rate at

200 cm3

/min or greater.

The measurement of the coal’s activation energy is based on the

heat release method (Jones, Chiz, Koh & Matthew, 1996) using the

same isothermal oven. In this method, coal samples were placed in

a 10-cm high, open-topped cubical brass wire mesh basket. This

was placed in the oven without any airflow. Two thermocouples

were placed at the center and at half the distance between the

center and the surface of the sample. When the temperatures

recorded at two thermocouples are equal, the thermal conduction

within the sample becomes zero. Therefore, the one-dimensional

energy balance for the self-heating for the sample becomes

(

vT

) kQ ( E

exp vt T

1⁄4 Cp 0-RT0

)

where T is the sample temperature, t is the time, k is the preexponential factor, Q is the heat of reaction, Cp is the specific heat
capacity, E is the activation energy, R is the gas constant and T0 is

the temperature at the point when the temperatures from two

thermocouples cross. This equation enables the estimation of the

kinetics of the oxidation reactions. Plotting the logarithm of the

transient heating lnðvT=vtÞT0 at the temperature T0 versus the

reciprocal temperature 1/T0, the activation energy can be determined from the slope of the resulting straight line. This method has

been demonstrated to produce comparable results as other

methods (Malow & Krause, 2004).

Also shown in Table 2 is each coal’s minimum self-heating

temperature (SHT). The SHT is a measure of a coal’s reactivity,

and is calculated based on the method established by Smith and

Lazzara (1987) using coal analysis data.

Table 3

Rate of temperature increase for coal sample A (

°C/h) at 110 °C ambient

temperature.

Ventilation rate 1st stage 2nd stage 3rd stage

300 cm3

/min 28 109 16.7

500 cm3

/min 40 140 e

“In open” 21.7 466.7 e

The differences in the

apparent relative reactivity between the three coal samples were

also reflected in their different SHTs. Based on calculated SHT data,

sample C is the most reactive coal, while sample B is the least

reactive coal.

3.2. Effect of ventilation rate

Fig. 2 shows the temperature-time and exit oxygen concentration histories for the test of coal sample A at its critical ambient

temperature of 110 °C at different locations under the airflow rate

of 300 cm3/min.

0

3

6

9

12

15

18

21

0

50

100

150

200

250

300

350

Oxygen concentration (%) 

Temperature (°C) 

top center bottom oven

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (hr) 

Fig. 2. Coal temperatures at different locations and oxygen concentration under the

airflow rate of 300 cm3

/min for sample A.

The bottom temperature exhibited a clear threestage increase. The first stage is a slow increase, the second one is

a fast increase, and the third one is a slow increase again. The center

temperature decreased slightly but increased slowly again in the

third stage. The top temperature was much lower than the center

and bottom temperatures. The center temperature increased

slightly faster than the bottom in the first stage, while the bottom

temperature increased faster than the center in the second stages,

probably because not enough oxygen was available at the center.

Fig. 3 shows the center coal temperatures of coal sample A under

different ventilation rates at the critical ambient temperature of

110 °C.

400

350

300

Temperature (°C) 

250

200

150

100

50

0

0 1 2 3 4 5

Time (hr) 

Fig. 3. Center coal temperatures under different ventilation rates with the oven

temperature at 110 °C for sample A.

With ventilation rates of 100 and 200 cm3/min, the center

temperatures first increased then decreased. Under these conditions, no thermal runaway was reached. Although a thermal

runaway was reached with the flow rates of 300 and 500 cm3

/min,

the center coal temperature decreased a little, then increased very

slowly after reaching over 200 °C. To better understand the effect of

ventilation on spontaneous heating, an experiment was then conducted for coal sample A without any ventilation, designated as “in

open”. During the test, a coal sample was first placed in the same

brass wire mesh basket. The basket was then put into the oven

without being enclosed in the stainless steel container. The oven

temperature was first set at 90 °C and the center coal temperature

was monitored continuously. If there was no thermal runaway at

a given oven temperature, the oven temperature was increased by

5 °C. Tests were repeated until a thermal runaway was reached.

When the oven temperature was below 120 °C, coal temperatures

always increased first, then decreased slowly. When the oven

temperature was at 120 °C, a thermal runaway was reached. This

temperature was defined as the self-ignition temperature (SIT) for

combustible materials by Krause and Schmidt (2001). This SIT was

higher than the critical ambient temperature,110 °C for coal sample

A, at which the thermal runaway was reached with the flow rates of

300 and 500 cm3

/min, indicating that with the forced ventilation,

the coal sample could reach a thermal runaway at a lower

temperature than its self-ignition temperature.

The center coal temperature without ventilation was also

compared with the center coal temperatures with ventilation in

Fig. 3. It is interesting to note that without ventilation, the center

coal temperature exhibited a two-stage increase. The first stage was

a slow increase, followed by a fast increase. Compared with center

coal temperatures with ventilation, the significant difference is that

the center coal temperature without ventilation increased over

400 °C very quickly in the second stage. However, the center coal

temperature with ventilation increased faster in the first stage than

that without ventilation, indicating that ventilation might have

enhanced the spontaneous heating at this stage. The temperature

increase was then much slower than that without ventilation. In

the third stage, the temperature increase became nearly flat. Table 3

lists the rates of temperature rise for coal sample A under different

ventilation rates. It can be seen that the rate of temperature

increase in the second stage “in open” was 3e4 times faster than

the temperature increase with ventilation. This is likely due to the

fact that under ventilation conditions the amount of oxygen in the

airflow was not enough to sustain the fast coal oxidation reaction in

the late stage, and the temperature rise became ventilation

controlled; while without ventilation, the coal sample was exposed

to the ambient air, and enough oxygen was available for the coal

oxidation reaction.
Fig. 4 shows the oxygen concentration at the exit of the oven

under different ventilation rates for sample A.

5

10

15

20

25

Oxygen concentration (%)

0

0 1 2 3 4 5

Time (hr) 

Fig. 4. Oxygen concentration at the exit of the oven under different ventilation rates

for sample A.

With the airflow rate

of 300 and 500 cm3

/min, the oxygen concentration also exhibited

three stages. In the first stage, the oxygen consumption rate was

small, while it became significantly larger in the second stage.

However, in the third stage, the oxygen concentration actually

increased slowly. With the airflow rate below 30 3 0 cm /min, the

oxygen concentration only decreased slowly at the beginning of the

first stage, but then increased without experiencing a fast decrease

stage.

Fig. 5 shows the center temperatures for coal sample B at its

critical ambient temperature of 115 °C under different ventilation

rates.

300

250

Temperature (°C) 

200

150

100

50

0

Time (hr) 

0 1 2 3 4 5

Fig. 5. Center coal temperatures under different ventilation rates with the oven

temperature at 115 °C for sample B.

As the airflow rate increased, the rate of temperature rise in

the first stage also increased. The oxygen concentrations at exit

under different ventilation rates for this test are shown in Fig. 6.

20
 

18
 

Oxygen concentration (%)

16

14

12

10

8

6

4

2

0

0 1 2 3 4 5

Time (hr) 

Fig. 6. Oxygen concentration at the exit of the oven under different ventilation rates

for sample B.

Compared to Fig. 4, sample B had a higher oxygen consumption rate

(reaction rate) in the first stage than sample A at the same airflow

rate, probably because of its lower activation energy.

Fig. 7 shows center coal temperatures for sample C under

different ventilation rates at its critical ambient temperature of

100 °C.

Fig. 7. Center coal temperatures under different ventilation rates with the oven

temperature at 100 °C for sample C.

At this oven temperature, thermal runaway was reached 3 with the airflow rate of 200 cm /min or greater. Fig. 8 shows the

oxygen concentrations at the exit for coal sample C under the

different ventilation rates. It is clear that at the flow rate of

100 cm3

/min, thermal runaway did not occur, even though the

lowest oxygen concentration was below 6%. It should be pointed

out that the oxygen concentration for sample C was lower at

200 cm3

/min than at 300 cm3/min, which is different from samples

A and B. This is probably because sample C already adsorbed much

more oxygen initially (oxygen concentration at 13.2%) at 200 cm3

/

min than sample A and B (oxygen concentrations: 18.7% and 17.4%,

respectively). Therefore, at the higher flow rate of 300 cm3

/min, the

oxygen adsorbed by samples A and B increased significantly, while

it did not change significantly for sample C. This led to a higher

oxygen concentration for sample C at 3 300 cm /min. At 500 cm3

/

min, the oxygen adsorbed by sample C increased significantly as it

did with samples A and B.

3.3. Effect of coal properties

Fig. 9 compares the center coal temperature rise for three

samples under the airflow rate of 300 cm3

/min. The oxygen

concentrations for the three samples are compared in Fig. 10. For

the center coal temperature rise, sample A clearly exhibited a threestage temperature increase with the slowest increase rate in the

first stage. Sample C exhibited a two-stage temperature increase

with the fastest increase rate in the first stage. The oxygen

concentrations in Fig. 10 show that the lowest oxygen concentration was reached for coal sample C, implying that for the more

reactive coal, the smaller lowest oxygen concentration is required
to sustain the coal oxidation. Compared to the activation energy

and SHT data in Table 2, the experimental results for the three coal

samples also indicate that the critical ambient temperature is

related to the SHT, while the oxidation rate, especially in the first

stage, is related to the activation energy of the coal. Therefore, for

a certain type of coal to reach a thermal runaway under ventilation,

the reaction rate in the first stage must be higher than that “in

open”.

It is interesting to note that no thermal runaway was reached for

all three coal samples with the airflow rate of 3 100 cm /min, indicating that there exists, for ambient temperatures lower than the

SIT, a minimum ventilation rate below which a thermal runaway

can be prevented. Although no thermal runaway was achieved at

this airflow rate, the oxygen concentrations were still reduced at

exit because of physical and chemical adsorption. The exit oxygen

concentrations at 0.5 h for coal samples A, B, and C were 17.2%,

14.0% and 6.9%, respectively. These concentrations imply that

a more reactive coal adsorbs more oxygen and that the amount of

oxygen adsorbed initially determines whether a thermal runaway

can be reached. If enough oxygen is adsorbeddas in the cases for

sample B and C with the airflow rate of 200 cm3

/min or above, and

in the case for sample A with the airflow ra 3 te of 300 cm /min or

abovedthen thermal runaway could be reached. Thus, the amount

of oxygen adsorbed by a certain coal mass will depend not only the

physical and chemical properties of the coal, but also the oven

temperature (ambient temperature).

Fig. 8. Oxygen concentration at the exit of the oven under different ventilation rates

for sample C.

Fig. 9. Center coal temperature rises for three samples under the airflow rate of

300 cm3

/min.

Fig. 10. Oxygen concentrations for three samples under the airflow rate of 300 cm3/min.

3.4. Effect of ventilation with initial coal temperature below the

critical ambient temperature

It is important to detect spontaneous combustion in its early

stages, so that appropriate measures can be taken to prevent the

spontaneous heating from developing into a fire. Therefore,

experiments were conducted to investigate the effect of ventilation

on the spontaneous heating with the oven temperature below the

critical ambient temperature of the coal sample. Coal sample C was

used in the experiments, and the oven temperature was set at

90 °C. At this temperature, a thermal runaway could not be reached.

The center coal temperatures and oxygen concentrations at the exit

at this oven temperature under different ventilation flow rates are

shown in Figs. 11 and 12. With the oven temperature at 90 °C, the

maximum temperature rise, about 48 °C, was reached at the flow

rate of 200 cm3

/min, as shown in Fig. 11.

Fig. 11. Center coal temperatures under different ventilation rates with the oven

temperature at 90 °C for sample C.

The maximum temperature at the flow rates of 300 and 500 cm3

/min are nearly the same,

but lower than that at the rate of 100 cm3 /min. This is significantly

different from the case with the oven temperature at 100 °C, as

shown in Fig. 7, in which the coal temperature increased with the

airflow rate. Oxygen concentrations shown in Fig. 12 clearly show

that no significant chemical reaction took place. At this oven

temperature, no thermal runaway occurred at any airflow rate.

Therefore, oxygen was likely consumed by chemical adsorption

and low-rate oxidation reaction. Although not much oxygen was
consumed, the oxygen concentration at the exit was still reduced to

about 14% at the air 3 flow rate of 100 cm /min because the amount of

oxygen at this flow rate is limited. However, at the airflow rate of

500 cm3

/min, the oxygen concentration at the exit was still above

20% because five times the amount of oxygen was available at this

flow rate. These results indicate that with a low ventilation rate,

100 cm3

/min, less oxygen was available, leading to less heat

generated. With a higher ventilation rate, 200 cm3

/min, more heat

was generated, which led to a higher coal temperature. However, as

the ventilation was further 3 increased to 300 and 500 cm /min, the

larger air velocity took away more heat, leading to the lower coal

temperature. Therefore, the 3 flow rate of 200 cm /min generated the

highest coal temperature. These results indicate that ventilation

plays a different role in relation to spontaneous combustion

when the initial coal temperature is below the critical ambient

temperature.

Fig. 12. Oxygen concentration at the exit of the oven under different ventilation rates

for sample C.

3.5. Effect of coal particle size

Tests were conducted with a larger size coal sample A, 150 by

300 mm, to examine the effect of coal particle size on spontaneous

heating under different ventilation rates. The initial oven temperature was set at 110 °C, the critical ambient temperature found for

this coal using 75 by 150 mm particles. When there was no thermal

runaway, the test was repeated with the oven temperature

increased in 5-degree increments. The thermal runaway was

reached when the oven temperature reached 130 °C, indicating

that a higher critical ambient temperature is required for a larger

size particle.

Fig. 13 shows the center coal temperatures under different

ventilation rates.

300

250

Temperature (°C )

200
 

150
 

100
 

50
 

0
 

0 0.5 1 1.5 2 2.5 3 3.5

Time (hr) 

Fig. 13. Center temperatures under different ventilation rates for larger size sample A

with oven temperature at 130 °C.

4

It is interesting to note that the temperature rise

exhibited a two-stage increase, compared to the three-stage

increase shown in Fig. 3. With the increased coal particle size, the

available coal surface area was reduced. Therefore, less oxygen

could be adsorbed by the coal surface. In order to adsorb enough

oxygen to increase the temperature high enough to support a fast

oxidation rate for the thermal runaway, the higher initial temperature is required. However, at the higher temperature, the effect of

kinetics becomes more significant. Even with the ventilation rate of

200 cm3

/min, thermal runaway was reached.

Fig. 14 shows the oxygen concentration at the exit of the oven

under different ventilation rates. Oxygen concentration (%) 

20

18

16

14

12

10

8

6

4

2

0

0 0.5 1 1.5 2 2.5 3 3.5 4

Time (hr) 

Fig. 14. Oxygen concentration under different ventilation rates for larger size sample A.

The oxygen concentration was

also lower at 200 cm3

/min than that at 300 cm3/min. As discussed

above for Fig. 8, this is because enough oxygen was already

adsorbed at 200 cm3

/min, probably due to the higher ambient

temperature. The oxygen concentration did not increase significantly at 300 cm3

/min.

4. Conclusions

Experiments were conducted in an isothermal oven to investigate the effect of ventilation on spontaneous heating of coal.

Experimental results demonstrate that a thermal runaway could be

reached at a lower temperature with enough ventilation airflow

than “in open” (without ventilation). With enough ventilation, the

rate of coal temperature rise in the first stage of spontaneous

heating was higher; however, the rate was slower in the second

stage compared to that without ventilation. In the third stage, the

temperature and oxygen concentration both increased slowly,

indicating that it may take a much longer time to develop a fire

compared to the unventilated state. As the airflow rate increased,

the rate of temperature rise in the first stage also increased.

Experimental results also indicate that the critical ambient

temperature at which a thermal runaway can be reached with

enough ventilation is related to the SHT of coal, while the oxidation

rate, especially in the first stage, is related to the activation energy

of the coal. For a certain type of coal, whether a thermal runaway

could be reached depends on the reaction rate in the first stage. For

the more reactive coal, the minimum oxygen concentration

required to sustain the coal oxidation was lower.
For this experiment, when the initial coal temperature was

below the critical ambient temperature, the highest temperature

was reached with a moderate ventilation rate. The lower ventilation rate would not supply enough oxygen, while the higher

ventilation rate carried away more heat, both leading to a lower

temperature. For the larger-sized coal sample A, 150 by 300 mm,

a higher critical ambient temperature was required to reach

thermal runaway.

Acknowledgments

The authors wish to thank Richard A. Thomas and Lenny An of

Office of Mine Safety and Health Research for conducting all

experiments.

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