CO and CO2 emissions from spontaneous heating of coal under different

ventilation rates

Liming Yuan ⁎, Alex C. Smith

⁎	 Corresponding author. Tel.: +1 412 386 4961. 

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

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

abstract

Carbon monoxide (CO) and carbon dioxide (CO2) emissions during a spontaneous heating event in a coal

mine are important gases to monitor for detecting the spontaneous heating at an early stage. However, in

underground coal mines, the CO and CO2 concentrations and their related fire ratios may be affected by mine

ventilation. In this study, CO and CO2 emissions from spontaneous heating of a U.S. coal sample were

evaluated in an isothermal oven under different airflow ventilation rates ranging from 100 to 500 cm3

/min.

Laboratory experiments were conducted at oven temperatures of 70, 90, and 100 °C. The temperature at the

center of the coal sample was continually monitored, while the CO, CO2, and oxygen (O2) concentrations of

the exit gas were continually measured. The results indicate that CO was generated immediately after the

airflow passed through the coal, while CO2 was generated in a late phase. The amounts of CO generated under

different airflow rates were approximately the same at the initial temperature of 70 °C, while the amounts of

CO generated increased significantly as the airflow rates and initial temperatures increased. The ratio of

CO/CO2 was found to be independent of airflow rate and initial temperature, approaching a constant value of

0.2 quickly ifthere was no thermal runaway. The value tended to decrease when a thermal runaway took place.

The CO/O2 deficiency ratio was dependent on both airflow rates and the initial temperature. The experimental

results are in qualitative agreement with some large-scale test and field monitoring results.

1. Introduction

Coal mine fires caused by spontaneous heating generally occur by

slow oxidation in the coal seams or gob areas and happen most

frequently with low-rank coals. Spontaneous heating is a lowtemperature coal oxidation reaction which takes place when coal is

exposed to air. Coal oxidation is an irreversible exothermic reaction

and its reaction rate increases with temperature. When the heat

produced by the coal oxidation is not adequately dissipated by

conduction or convection, the temperature in the coal mass increases.

This increase in temperature leads to an increase in the coal oxidation

rate. If not averted with appropriate action, this process results in

thermal runaway and a fire ensues.

Spontaneous heating has long been a problem in the mining,

storage, and transport of coal. Much research has been done to

understand the effects of different parameters on the spontaneous

heating process (Akgun and Arisoy, 1994; Beamish and Hamilton,

2005; Carras and Young, 1994; Kucuk et al., 2003; Monazam et al.,

1998; Nugroho et al., 2000; Ren et al., 1999; Smith and Glasser, 2005;

Smith and Lazzara, 1987; Wang et al., 2003a). The petrographic and

geochemical properties of coal waste during self-heating process were

investigated by Misz et al. (2007) and Misz-Kennan and Fabianska

(2010). Some works have been done to study the gas emissions from

spontaneous combustion of coal and their impact on the environment

(Carras et al., 2009; Pone et al., 2007). However, the chemical reaction

between coal and O2 at low temperatures is complex and still not well

understood. Generally, three types of processes are believed to occur

(Carras and Young, 1994): (i) physical adsorption of O2; (ii) chemical

adsorption which leads to the formation of coal–O2 complexes and

oxygenated carbon compounds; and (iii) oxidation in which the coal

and O2 react with the release of gaseous products, typically CO, CO2,

and water vapor (H2O).

Although a number of methods have been proposed and used in

laboratory experiments in an attempt to predict the spontaneous

combustion tendencies of coals, spontaneous heating events still

occur in underground coal mines because of complicated parameters

such as ventilation airflow, geological conditions, and mining

practices that are not considered in laboratory experimental conditions. However, it is well understood that if a spontaneous heating

event is detected early enough, some prevention measures can be

taken to control and suppress the heating. Therefore, early detection is

critical in the prevention and control of fires caused by spontaneous

combustion in underground coal mines.

The primary method used for the detection of spontaneous

combustion in underground coal mines is the analysis of the gaseous

products of coal oxidation using a gas analyzer or a gas chromatograph.
The gaseous products of low-temperature oxidation at the very early

stages are CO, CO2, and H2O. Methane (CH4), hydrogen (H2), and other

low-molecular hydrocarbons are released as the temperature rises.

Chamberlain and Hall (1973) found that CO is the most sensitive

indicator of the early stages of coal oxidation, and the continuous

monitoring of this gas provides the earliest detection of self-heating.

Other gases have also been investigated, suchasCO2, CH4, H2, and higher

hydrocarbons (Xie et al., 2011). CO2 production increases with

increasing temperature and is useful in determining the state of a fire.

However, several sources of CO2 can be present in mines, making its use

unreliable. CH4 is usually present in large background quantities and, as

with H2 and other hydrocarbons, is not produced until much higher

temperatures are reached. Since many of the combustion product gases,

including CO, are encountered in the normal mining of the coal seam,

and because these gas concentrations fluctuate with changes in

ventilation, the reliability of these gases as indicators of spontaneous

combustion diminishes.

To better detect spontaneous heating in underground coal mines,

somefire ratios, such asGraham's ratio (CO/O2 deficiency) and Trickett's

ratio (CO/CO2) are used. These ratios were originally developed and

are often used for assessing the status of a sealed-off fire with the

gas samples usually collected behind the seal, as investigated by Singh

et al. (2007). The application of fire ratios to detect a spontaneous

fire in underground coal mines has had limited success because the

gas concentrations are dependent not only upon the state of development of the heating but also upon the mass of coal involved in the

heating, airflow flux through the heating zone, and the flow of air into

which the gaseous products of the heating are mixed. Different fire

ratios have also been used to detectthe spontaneous heating atthe early

stage.

Cliff et al. (2000) conducted large-scale tests for detection and

monitoring of spontaneous combustion of coal. They found that the

best indicators of spontaneous combustion are those that are

independent of airflow, such as the amount of CO released and

Graham's ratio, but that even these indicators have some limitations.

Chakravorty and Woolf (1979) found that the absolute level of CO in

the mine air, whether high or low, is not of great significance but that

an increasing trend is indicative of heating. In the U.S., the Graham's

ratio (CO/O2 deficiency) in the mine atmosphere has become the most

widely used indicator of the occurrence of spontaneous combustion

(Mitchell, 1996).

In this paper, laboratory-scale experiments were conducted to

investigate the effect of ventilation on the CO and CO2 emissions from

a spontaneous heating. The influence of ventilation on two commonly

used fire ratios is also examined. The experimental results have

implications for the early detection of the spontaneous heating in

underground coal mines.

2. Experimental

Experiments were conducted in an isothermal oven. The oven

temperature can be set from 40 to 200 °C, within an accuracy of±0.5 °C.

Hereafter, the oven temperature is referred to as the initialtemperature.

Fig. 1 shows a schematic of the experimental setup. During the test, the

coal sample is contained in a brass wire mesh basket, 6-cm-diameter by

10-cm-high, which is enclosed in a brass container.

Water trap 

air N2 

Thermocouples 

Exhaust gas 

Oven 

Fig. 1. Schematic of experimental setup and measurement.

CO 

analyzer 

CO2 

analyzer 

O2 

analyzer 

Data 

acquisition 

The container has

ports in the bottom and top to allow gas to flow through the coal sample.

The container is placed at the center of the oven. The ventilation gas (air

or nitrogen) is introduced into the container from a compressed gas

cylinder. The gas is pre-heated to the oven temperature by passing

through heat exchange coils in the oven. Initially, the coal sample is

heated to a selected temperature while exposed to a flow of pre-heated

nitrogen. The gas flow is then switched to the same flow rate of air.

With air flowing through the coal sample, coal oxidation takes place.

Three 0.5-mm-diameter type-K thermocouples are placed at the center

of the coal, 3 cm above and below the coal center, respectively, to

measure the coal temperatures. The exhaust gas exits the container and

travels through a water trap before passing through O2, CO, and CO2 gas

analyzers. The CO and CO2 concentrations are measured continuously

using an infrared gas analyzer, while the O2 concentration is measured

by a paramagnetic O2 analyzer. The measurement range is 0 to

5000 ppm for CO, 0 to 10,000 ppm for CO2, and 0 to 25% for O2. Tests

are terminated when the coal temperature is decreasing or the coal

temperature approaches 250 °C.

The coal sample used in the study is a bituminous coal from the

western United States. The proximate and ultimate analyses for the

coal sample are given in Table 1. To prepare the coal for testing, lumps

of fresh coal are first passed through a jaw crusher and then sieved. A

100-gram sample of the 100 × 200 mesh (75× 150 μm) size fraction is

then dried in the oven under nitrogen at 67 °C with a flow rate of

200 cm3

/min prior to the experiment.

3. Experimental results and discussion

3.1. Time difference between CO and CO2 emission onset

Experiments to measure CO and CO2 emissions were conducted

under various ventilation rates at oven temperatures of 70, 90, and

100 °C.Although CO appears at a temperature well below 70 °C for some

coals, CO concentrations for this coal at a temperature below 70 °C

under the experimental conditions in this study were very low,

especially with higher ventilation rates, and the values became

unreliable and less repeatable. At 100 °C, this coal experienced a

thermal runaway. Therefore, 90 °C was selected as 10 °C below its

criticaltemperature, 100 °C. Fig. 2 shows CO, CO2, and O2 concentrations
Table 1

Proximate and ultimate analyses of the coal sample.a

Proximate analysis (%) Volatile matter 44.0

Fixed carbon 47.5

Ash 8.5

Ultimate analysis (%) Hydrogen 5.5

Carbon 77.8

Nitrogen 1.5

Sulfur 2.6

Oxygen 12.6

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

700 21 

600 18 

CO 

CO 

O 

2 3 4 5 6 7 

15 

O2 (%)

CO & CO2 (ppm) 

12 

9 

6 

100 3 

0 0

Time (hours) 

Fig. 2. CO, CO2, and O2 concentrations at initial temperature of 70 °C and ventilation rate

of 200 cm3

/min.

200 

300 

400 

500 

for the coal sample at an initialtemperature of 70 °C with the ventilation

rate of 200 cm3

/min. Results indicate that the CO was generated earlier

than CO2. The concentrations of both gases increased to a maximum

value quickly, then began to decrease. The maximum concentration of

CO2 was about 5 times the concentration of CO. The CO2 concentration

also decreased more rapidly than the CO concentration. In all of the

experiments, CO was detected earlier than CO2. The results indicate that

CO was generated immediately after the airflow passed through the

coal, while CO2 was generated in a late phase. This was also observed in

other experiments in this study.

Fig. 3 shows the difference in CO and CO2 emission onset time for

tests at the initial temperatures of 70, 90, and 100 °C and flow rates of

100, 200, 300, and 500 cm3

/min. These airflow rates were selected to

generate a significant spontaneous heating but without a thermal

runaway at lower flow rate and a thermal runaway at the higher flow

rate based on the experimental conditions and the amount of coal

sample used in this study.

45

40 70 °C 

90 °C 

100 °C 

0 100 200 300 400 500 600 

Time difference (min) 35 

30 

25 

20 

15 

10 

5 

0 

Flow rate (cm3/min) 

Fig. 3. Time difference between CO and CO2 emission onset.

Experimental results show that the time

difference between CO and CO2 emission onset depends on both the

initial temperature and the ventilation rate. At the same initial

temperature, the time difference between the CO and CO2 emission

onset decreased with the increasing ventilation rates. The time

difference also decreased with the increased temperature at the same

ventilation rate. The maximum time difference was 40 min at 70 °C with

the flow rate of 100 cm3

/min, while the minimum time difference was

1 min at 100 °C with the 3 flow rate of 500 cm /min.

Figs. 4 and 5 show the CO, CO2, and O2 concentrations for the

experiments with the flow rate of 200 cm3

/min and the initial

temperature of 90 and 100 °C, respectively. At 90 °C, the O2 concentration was 15.6% after the initial phase, while the CO concentration

increased to 460 ppm during the same time period.

7000 21

6000 18 

CO 

CO 

O 

4.5 5 5.5 6 6.5 7

5000

CO & CO2 (ppm) 

4000

O2 (%) 

3000

2000 

1000 3

0 0

Time (hours) 

Fig. 4. CO, CO2, and O2 concentrations at initial temperature of 90 °C and ventilation rate

of 200 cm3

/min.

6 

9 

12 

15 

7.5 

Then, both

concentrations increased slowly. At 100 °C, the O2 concentration was

13.2% after the initial phase, while the CO concentration increased to

950 ppm during the same period. Then, the O2 concentration decreased

quickly, while the CO concentration increased quickly, indicating that a

thermal runaway was occurring. Results indicate that no CO2 was

generated during the initial phase; CO2 was generated only after the

initial phase. As the initial temperature increased from 70 to 90 and

100 °C, the oxidation process started earlier, leading to the smaller time

difference between CO and CO2 emission onset. There are no similar

experimental results in the literature for comparison because most gas

emission experiments were conducted using a gas chromatography and

were not measured continually.

3.2. Effect of ventilation on CO and CO2 emission rates

If the same amount of a gas is produced at the same initial

temperature under different flow rates, the gas concentration at the

exit would decrease proportionally with the increase of ventilation

rate. To examine the effect of ventilation on the CO and CO2 gas

emission rates, the amounts of gas generated during the heating

process were plotted in Figs. 6 and 7 for the coal sample at an initial

temperature of 70 °C under different ventilation rates. At 70 °C, the CO

and CO2 emission rate curves for 200, 300, and 500 cm3

/min flow

rates nearly overlap, indicating a pure dilution effect of the

ventilation. Experimental results also show that there is sufficient

O2 available at the lowest airflow rate to sustain the coal oxidation

reaction. At 100 cm3

/min flow rate, the CO emission rate curve was

slightly higher than those at larger ventilation rates, while the CO2

emission rate curve was lower than those at larger ventilation rates.

The coal sample center temperatures under different ventilation

rates are shown in Fig. 8. It is interesting to note that the highest

temperature occurred at the flow rate of 100 cm3

/min, while the

lowest temperature occurred at 200 cm3

/min. The ventilation not

only provides O2 for the coal oxidation (heating) but also carries away

the heat through convection. Higher ventilation provides more O2 for

the heating, but also carries away more heat. The coal temperature is
4000 

5000 

6000 

7000 

10000 

CO 

CO 

O 

25 

9000 

8000 20 

CO & CO2 (ppm) 

15 

O2 (%) 

10 

3000 

2000 5 

1000 

0 

5 5.5 6 6.5 7 0 7.

Time (hours) 

Fig. 5. CO, CO2, and O2 concentrations at initial temperature of 100 °C and ventilation

rate of 200 cm3

/min.

3.5 

100 cm3/min 200 cm3/min 

300 cm3/min 500 cm3/min 

3 

2.5 

CO (cm3/min) 

2 

1.5 

1 

0.5 

0

0 0.5 1 1.5 2 2.5 3 3.5 4 

Time (hours) 

Fig. 6. CO emission rates under different ventilation rates—initial temperature 70 °C.

4 

16 

14 

12 

0 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO2 (cm3/min) 10 

8 

6 

4 

2 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 7. CO2 emission rates under different ventilation rates—initial temperature 70 °C.

5

80 

78 

Temperature (°C) 76 

74 

72 

70 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Time (hours) 

Fig. 8. Coal center temperatures under different ventilation rates—initial temperature:

70 °C.

controlled by this combined effect of ventilation on the spontaneous

heating.

Figs. 9 and 10 show the CO and CO2 emission rates for the coal

sample at 90 °C initial temperature under different ventilation rates.

The gas emission rate curves no longer overlapped, but became

distinct. The highest CO and CO2 emission rates both occurred at the

flow rate of 200 cm3

/min, while the lowest CO2 emission rate

occurred at the flow rate of 300 cm3

/min. The coal sample center

temperatures under the different ventilation rates are shown in

Fig. 11. At 90 °C initial temperature, the highest temperature also

occurred at 200 cm3

/min. At this flow rate, enough O2 was provided

for spontaneous heating and not too much heat was lost, leading to

the highest temperature. Thus it can be seen that the CO and CO2

emission rates were following the same pattern as the coal center

temperatures under different ventilation rates, implying that coal

temperature greatly influences the CO and CO2 emission rates

compared to dilution. Because the rate of coal reaction increases

with the temperature exponentially, the CO and CO2 emission rates

increase quickly with the temperature. At both 70 and 90 °C, no

thermal runaway was reached, and CO and CO2 emission rates

increased first, then decreased.

Figs. 12 and 13 show the CO and CO2 emission rates for the coal

sample at 100 °C initial temperature under different ventilation rates.

The gas emission rate curves became clearly distinct. Both CO and CO2

emission rates increased with time, eventually beyond the measuring

scale. With the increase of ventilation rate, the CO and CO2 emission

rates also increased. At this initial temperature, a thermal runaway

was achieved with the flow rate of 200, 300, and 500 cm3

/min. The

coal sample center temperatures are shown in Fig. 14. With the

increase of ventilation rate, the coal temperature also increased. This

finding is different from the experimental results at initial temperatures of 70 and 90 °C. This is probably because the heat carried away

by ventilation, even at 500 cm3

/min, was much less than the heat

generated by the coal oxidation at 100 °C initial temperature. As with

the initial temperature of 90 °C, the CO and CO2 emission rates

were controlled by the reaction rate, as evidenced by the center

temperature of the coal. After the thermal runaway was achieved with

the flow rate of 200 cm3

/min and above, the CO and CO2 emission

rates increased very quickly. The dilution effect of the ventilation

became minimal, and temperature had a major effect on the CO and

CO2 emission rates during the heating process.

3.3. Effect of ventilation on fire ratios

Fire ratios have been used for the purpose of early detection of

spontaneous heating. Results indicate that the CO and CO2 concentration are affected by the ventilation in situations where thermal

runaway has not occurred. Once a condition of thermal runaway

occurs, the emission rate is reaction rate controlled. Therefore, fire

ratios are more important in early detection since thermal runaway

has not occurred. For the coal sample used in this study, and because

the CO was always generated earlier than the CO2, the CO/CO2 ratio

and the CO/O2 deficiency ratio are the potential ratios to use in mine

applications. Figs. 15, 16, and 17 show the CO/CO2 ratios under

different ventilation rates at the initial temperatures of 70, 90, and

100 °C, respectively. During the test, the CO2 concentration increased

beyond the CO value very quickly after CO2 was detected. The CO/CO2

ratio was initially high, then decreased quickly to the value of 1.

Therefore, the CO/CO2 ratio was plotted starting from the value of 1.

The ratios under different ventilation rates decreased very quickly and

leveled off around the value of 0.2 at the initial temperatures of 70

and 90 °C as shown in Figs. 15 and 16. These results imply that the

ventilation rate and initialtemperature have little effect on the CO/CO2
0 

5 

10 

15 

20 

25 

30 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO (cm3/min) 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 9. CO emission rates under different ventilation rates—initial temperature 90 °C.

140 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO2 (cm3/min) 120 

100 

80 

60 

40 

20 

0 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 10. CO2 emission rates under different ventilation rates—initial temperature: 90 °C.

160 

140 

120 

100

Temperature (°C) 

80 

60 

40 

100 cm /min /min 

300 cm /min 500 cm 

200 cm 

/min 20 

0 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 11. Coal center temperatures under different ventilation rates—initial temperature

90 °C.

600 

500 

400 

300 

200 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO (cm3/min) 100 

0 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 

Time (hours) 

Fig. 12. CO emission rates under different ventilation rates—initial temperature 100 °C.

0 

200 

400 

600 

800 

1000 

1200 

CO2 (cm3/min) 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

0 0.2 0.4 0.6 0.8 1 

Time (hours) 

Fig. 13. CO2 emission rates under different ventilation rates—initial temperature 100 °C.

250 

200 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

Temperature (°C) 150 

100 

50 

0

0 0.5 1 1.5 2 2.5 

Time (hours) 

Fig. 14. Coal center temperatures under different ventilation rates—initial temperature

100 °C.

ratio when there is no thermal runaway occurring. It should be noted

that the CO/CO2 ratio decreased quickly even as the coal temperature

continually increased.

Although the CO/CO2 ratio values under different ventilation rates

also decreased at the 100 °C initial temperature, the ratios did not level

off around the value of 0.2 but instead decreased continually below 0.2.

Because ofthe limitation ofthe gas monitor, not many data points below

the value of 0.2 were obtained. However, the trend of the data indicates

that when the thermal runaway took place, the ratio tended to decrease

quickly below the value of 0.2. This is consistent with field monitoring

results from Timko and Derick (1995), predicting spontaneous heating

in underground coal mine pillars. In their study, several boreholes were

drilled into pillars. Each hole was 3.8 cm in diameter. These holes were

aligned at right angles to the coal cleat and drilled horizontally. A 0.95­

cm outside diameter gas sampling line stopped just inside the borehole

opening. The end of each hole was sealed with a urethane foam plug.

The gas data were analyzed by a gas chromatograph. The CO data

gradually increased—an indication of accelerating oxidation. However,

the CO/CO2 ratio remained essentially unchanged untilthe fire occurred.

This unchanged value was about 0.08, and the ratio value decreased

quickly as the thermal runaway took place.

In a large-scale spontaneous combustion experiment conducted by

Smith et al. (1991), similar results were obtained. In their experiment,

approximately 11,800 kg of Wyoming No. 80 seam coal was tested in a

1.8-m-high by 1.8-m-wide by 4.5-m-long coalbed chamber. Initially, a

50-L/min airflow was established. O2 concentrations quickly fell to

less than 2% in the center of the coalbed and to 15% near the front of

the coalbed. Meanwhile, temperatures rose steadily over the entire

coalbed. Corresponding increases in CO across the coalbed ranged

from 0.1% to 0.3% while CO2 was near the detection limit, indicating
0 

0.2 

0.4 

0.6 

0.8 

1 

CO/CO2 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

0 0.5 1 1.5 2 2.5 3 3.5 4 

Time (hours) 

Fig. 15. CO/CO2 ratio under different ventilation rates—initial temperature 70 °C.

0 

0.2 

0.4 

0.6 

0.8 

1 

0 0.5 1 1.5 2 2.5 3 3.5

CO/CO2 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

Time (hours) 

Fig. 16. CO/CO2 ratio under different ventilation rates—initial temperature 90 °C.

0 

0.2 

0.4 

0.6 

0.8 

1 

0 

CO/CO2 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 

Time (hours) 

Fig. 17. CO/CO2 ratio under different ventilation rates—initial temperature 100 °C.

CO generation from the O2 adsorption. At day 7.6, the airflow was

increased to 150 L/min. Temperatures continued to rise, O2 concentrations continued to decrease, and a slight increase in the CO2

concentration was observed. CO concentrations were stable or slightly

decreasing, indicating the onset of coal oxidation. The CO/CO2 ratio in

the experiment was 0.25 at day 21.7 and 0.22 at day 25. This is

qualitatively consistent with the results in this study.

Wang et al. (2002, 2003b) conducted experiments to examine CO,

CO2, and H2O formation during low-temperature oxidation of an

Australian bituminous coal. At temperatures between 60 and 90 °C,

they found that the most important carbon-containing gaseous

product of oxidation is CO2. CO is emitted in smaller amounts, with

a molar ratio of CO2/CO production of about 3. They also found that

the ratio of the rates of production of CO2 and CO has a rather high

value at the beginning of the experiment, but decreases with time,

and that the ratio evidently depends on temperature. This finding is

contradictory to the result in this study, probably because of different

coal properties.

Figs. 18, 19, and 20 show the CO/O2 deficiency ratio under different

ventilation rates at the initial temperatures of 70, 90, and 100 °C,

respectively.

0.1 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO/O2 deficiency ratio 0.08 

0.06 

0.04 

0.02 

0 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 18. CO/O2 deficiency ratio under different ventilation rates—initial temperature

70 °C.

4 

0.08 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

CO/O2 deficiency ratio 0.06 

0.04 

0.02 

0 

0 0.5 1 1.5 2 2.5 3 3.5 

Time (hours) 

Fig. 19. CO/O2 deficiency ratio under different ventilation rates—initial temperature:

90 °C.

Results indicate that the ratio was strongly affected by the

ventilation and the initial temperature. At 70 °C, the ratio tended to be

leveled off at the 100 and 200 cm3

/min flow rates after its initial rise,

while it increased quickly at the 300 and 500 cm3

/min flow rates. At

90 °C, the ratio increased slowly at the 100, 200, and 300 cm3

/min flow

rates, while it rose faster at the 500 cm3

/min flow rate. At 100 °C, the

ratio increased very rapidly except at the 100 cm3

/min flow rate. These

results imply that the CO/O2 deficiency ratio changed very little at the

lower initial temperature and the lower ventilation rate. However, with

the higher ventilation rate, this ratio rose even at the lower initial

temperature. When thermal runaway was reached, the ratio rose

quickly.In the Timko and Derick (1995)field study,the CO/O2deficiency

ratio wasunchanged (around 0.01) untilthe fire occurred. This finding is

qualitatively consistent with the experimental results in this study,

showing the lower ventilation rate at the lower initial temperature.

4. Conclusions

Laboratory-scale experiments were conducted to investigate the

effect of ventilation on the CO and CO2 emission during the

spontaneous heating process of a U.S. coal sample. The experimental

results indicate that CO was generated immediately after the airflow

passed through the coal, while CO2 was generated in a late phase

under the experimental conditions. The time difference between CO
CO/O2 deficiency ratio 0.08 

0.06 

0.04 

0.02 

0 

100 cm /min 200 cm /min 

300 cm /min 500 cm /min 

0 0.5 1 1.5 2 2.5 

Time (hours) 

Fig. 20. CO/O2 deficiency ratio under different ventilation rates—initial temperature:

100 °C.

and CO2 emission onset decreased with the increase of the initial

temperature or the ventilation rate.

At low initial temperature, the ventilation had a pure dilution

effect on the CO and CO2 emission rates. At high initial temperature,

the dilution effect from ventilation became insignificant for CO and

CO2 emission rates, and these emission rates became dominated by

the reaction rate. Without a thermal runaway occurring, there was an

optimum ventilation rate for the highest CO and CO2 emission rates.

With a thermal runaway, the CO and CO2 emission rates increased

with the increase of ventilation rate.

The CO/CO2 ratio was found to be unaffected by the initial

temperature and the ventilation flow rate, leveling off around the

value of 0.2 in this study when no thermal runaway occurred. When a

thermal runaway occurred, the value of the ratio continually

decreased below 0.2. The CO/O2 deficiency ratio was significantly

affected by the ventilation rate except at low initial temperature and

low ventilation flow rate.

The results from this study indicate that it is helpful to monitor

CO/CO2 ratio in addition to monitoring CO and CO2 concentration to

detect and prevent spontaneous heating of this particular coal in

underground coal mines. It should be noted that the value of the

CO/CO2 ratio indicating thermal runaway is dependent on the coal

type. Finally, in case of significant ventilation rate changes, the CO/O2

deficiency ratio may become unreliable.

Acknowledgements

The authors wish to thank Richard A. Thomas and John Soles of

Office of Mine Safety and Health Research for conducting all

experiments.

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.

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