MODELING THE EFFECT OF BAROMETRIC PRESSURE CHANGES 

ON SPONTANEOUS HEATING IN BLEEDERLESS LONGWALL 

PANELS 

Liming Yuan, NIOSH, Pittsburgh, PA 

Alex C. Smith, NIOSH, Pittsburgh, PA 

Abstract 

Barometric pressure changes affect air density, leading 

to change in the mass of the gas in the gob. When the 

barometric pressure decreases, the volume of gas in the gob 

expands, while the volume of gas contracts when the 

barometric pressure increases, causing the gob to breathe 

out and in. Although the concept of gob “breathing” is 

simple, its effect on spontaneous heating of coal in the gob 

area is not clear. In this study, computational fluid dynamics 

(CFD) simulations were conducted to model the 

spontaneous heating of coal in longwall gob area under 

measured barometric pressure changes. A single longwall 

panel using a bleederless ventilation system was simulated. 

If there is no barometric pressure change, the intake airflow 

rate is equal to the return airflow rate. When the barometric 

pressure changes, these two airflow rates are no longer 

equal, and the difference between the two airflow rates 

represents the airflow rate the gob breathes in and out. The 

effect of inflow and outflow of gas on the potential 

spontaneous heating in the gob was investigated using the 

CFD model developed in our previous study. The effect of 

barometric pressure changes on the spontaneous heating was 

found to be dependent on the gob permeability and the coal 

oxidation rate. The effect of barometric pressure changes on 

oxygen concentrations in the gob was also examined. 

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. 

Introduction 

Barometric pressure changes have long been known as 

a factor that may influence conditions in underground 

mines. For an underground coal mine, when the barometric 

pressure changes, it can affect the gas emission not only 

from the active mining seam, but also from the overlying 

and underlying strata into the mined coal seam. On the other 

hand, the barometric pressure changes can also cause 

leakage into and out of a sealed area or outflow and inflow 

of an unsealed gob area. In the latter case, the inflow of 

fresh air into the sealed area or gob area may increase the 

possibility of a fire or explosion behind the seal or in the 

gob. The barometric pressure changes can be caused by 

weather and atmospheric heating and cooling on a daily 

basis. The weather caused barometric pressure changes are 

created by storm fronts that are associated with decreasing 

barometric pressures as they approach, followed by an 

increasing barometric pressure after they pass. The pressure 

changes caused by regular atmospheric heating and cooling 

are responsible for localized changes in barometric pressure, 

called diurnal pressure changes. These changes occur at 

approximately the same time each day for a given location. 

Generally speaking, the barometric pressure changes caused 

by weather are much bigger than the diurnal pressure 

changes. Little research has been done to thoroughly study 

the effect of barometric pressure changes on the gas outflow 

and inflow in a sealed area or an unsealed gob area. Francart 

and Beiter [1] investigated the barometric pressure influence 

in mine fire sealing. They compared the diurnal pressure 

changes with the measured oxygen concentrations behind a 

seal and found that the rate of airflow is directly affected by 

the rate of change in the barometric pressure. They also 

discussed using the National Weather Service data as a tool 

for predicting barometric pressure changes. A computer 

program called Gob Assistant was developed by the U.S. 

Bureau of Mines to predict air flows into and out of a sealed 

mine area [2]. The application of this program was limited 

because the gas concentration was assumed uniform in the 

gob behind the seal. Fauconnier [3] analyzed 59 major 

explosions in South African gold and coal mines over a 
period of 20 years and concluded that diurnal fluctuations in 

pressure do not have a major influence on the accumulation 

of gas in the underground workings, but that pressure drops 

associated with cyclonic weather systems are the major 

factor contributing to gas explosions in mines. Hemp [4] 

measured barometric pressure over a period of 26 months to 

examine the manner in which the barometric pressure 

changes in the Highveld region of South Africa. He also 

described the leakage flow into and out of a sealed area 

based on the observed changes in barometric pressure. 

When barometric pressure changes in the unsealed gob 

area, the absolute pressure in the gob also changes leading 

to the change of air density. If the longwall face is stationary 

the gob volume is fixed. The air density change results in a 

total mass change in the gob. With the barometric pressure 

decreasing, the density of gas in the gob decreases, while 

the density of gas increases with the barometric pressure 

increasing, causing the gob to breathe in and out. When 

there is no barometric pressure change, the intake airflow is 

equal to the return airflow, assuming no gas inflows from 

strata and no gas emission inside the gob. With the 

barometric pressure changing, these two airflows are no 

longer equal, and the difference between the two airflows 

represents the flow rate gob breaths in and out, designated 

as barometric-induced flow. The outflow and inflow caused 

by barometric pressure changes affect gas concentrations in 

the gob, thus affect a potential spontaneous heating in the 

gob. In this study, the CFD model developed in our previous 

studies for simulating the spontaneous heating of coals in 

longwall gob area is used to quantify the outflow and inflow 

of the gob and examine the effect of barometric pressure 

changes on the spontaneous heating of coal in the gob area 

[5, 6]. 

CFD Modeling of Spontaneous Heating in the 

Gob 

In this study, an active longwall panel using a 

bleederless ventilation system was simulated. The layout of 

the panel and the ventilation system, shown in Figure 1, are 

the same as simulated in authors’ previous study with the 

bleederless system [6]. 

gob

headgate

tailgate

intake

return

face

Figure 1. Layout of longwall panel and ventilation system 

used in simulations 

 The simulated gob area is 2,000 m 

long, 300 m wide, and 10 m high starting from the bottom 

of the coal seam. The ventilation airways are 2 m high and 

5 m wide. The ventilation scheme is a simple “U” 

bleederless ventilation system. In the model, all entries inby 

the longwall face are treated as though they are collapsed. 

The face is assumed stationary during the simulations. 

The chemical reaction between coal and oxygen at 

ambient temperatures is complex and still not well 

understood. In this study, the chemical reaction between 

coal and oxygen is simplified such that one mole of coal 

reacting with one mole of oxygen generates one mole 

carbon dioxide and 0.1 mole carbon monoxide plus the heat 

of coal oxidation, based on 1988 U.S. Bureau of Mines 

experimental study [7]. The dependence of the rate of 

oxidation on temperature and oxygen concentration is 

expressed in the form [5]: 

Rate = A[O2]

n

 exp(-E/RT) 

where the chemical reaction rate is defined as the rate of 

change in the concentrations of the reactants and products, 

A is the preexponential factor (K/s), E is the apparent 

activation energy (kJ/mol), R is the gas constant, n is the 

apparent order of reaction, T is the absolute temperature 

(°K), and [O ] 3 2 is the oxygen concentration (kmol/m ). 

 In the simulation, a 1-meter thick rider coal seam less 

than 1 m above a 2-meter thick main coal seam is 

considered. The coal source in the model is this rider coal 

seam that is assumed to cave into the bottom of the gob after 

the main coal seam is completely mined out. An average 

coal particle diameter of 10 cm, with a surface-to-volume 

ratio of 36 m-1, is used in the simulations [5]. Coal oxidation 

is an exothermic reaction and the heat generated from coal 

oxidation is dissipated by conduction and convection, while 

the oxygen and oxidation products are transported by 

convection and diffusion. A typical reactive bituminous coal 

is used in this study with its physical and kinetic properties 

of this coal listed in Table 1 [6]. 

Table 1. The physical and kinetic properties of coal layer 

 

Coal density 1300 kg/m3

Coal specific heat 1003.2 J/kg-K 

Coal conductivity 0.1998 W/m-K 

Heat of reaction 300 kJ/mol-O2

Activation energy 73.6 kJ/mol 

Pre-exponential factor 1.1×107 K/s 

Coal particle diameter 0.1 m 

Initial coal temperature 300 (27) K (°C) 
The permeability and porosity distributions of the gob 

are based on geotechnical modeling of longwall mining in 

the Pittsburgh coal seam and the associated stress-strain 

changes using FLAC (Fast Lagrangian Analysis of 

Continua) code [8]. For a Pittsburgh coal seam longwall 

panel, the permeability values in the gob area are estimated 

to vary from 3.0x104

 to 8.5x105 millidarcies (md), while the 

porosity value varies from 0.17 to 0.41 based on the 

modeling result from FLAC. Around the perimeter of the 

gob and immediately behind the face shields, the 

permeability and porosity values are the largest, while near 

the center of the gob; these values are the smallest due to 

compaction. The porosity profile in the gob is similar to the 

permeability profile. It is assumed that these permeability 

and porosity files do not change with the gob height. 

A commercial CFD software, FLUENT from Ansys, 

Inc., was used in this study to simulate the gas flow and 

spontaneous heating in the longwall gob areas. The gas 

flow in the longwall mine gob area was treated as laminar 

flow in a porous media using Darcy’s law, while the gas 

flow in the ventilation airways was simulated as fully 

developed turbulent flow. Typical ventilation pressures for 

the bleederless ventilation system were used in the 

simulation. The intake airflow rate was 30 m3

/s (64,000 

cfm). Without barometric pressure change, the pressure was 

-0.747 kPa (- 3.0 inches water gauge) at the intake inlet, -

0.872 kPa (-3.5 inches water gauge) at the return outlet. 

The recorded typical barometric pressure variations over a 

22-day period in western Pennsylvania area, shown in 

Figure 2, were used in the simulations.

29.0

29.2

29.4

29.6

29.8

30.0

30.2

30.4

30.6

30.8

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Baromeric pressure (in. Hg)

Figure 2. Barometric pressure changes used in simulations 

It represents the 

combined effects of both storm fronts and daily natural 

heating and cooling. The barometric pressure increase and 

decrease over the standard atmospheric pressure at sea level, 

101.3 kPa (29.92 in Hg), were superimposed to both intake 

inlet pressure and return outlet pressure. 

Simulation Results and Discussion 

A base case CFD simulation was conducted using the 

barometric pressure data shown in Figure 2. Figure 3 shows 

the barometric-induced flow under the barometric pressure 

changes. 

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Barometric-induced flow (kg/s)

Figure 3. Barometric-induced flow for the base case 

The positive value indicates the inflow while the 

negative one indicates the outflow. From the beginning to 

Day 4, the barometric pressure decreased from 30.4 to 29.4 

in Hg causing the maximum outflow rate of 0.26 kg/s (450 

cfm). The maximum inflow rate was about 0.15 kg/s (260 

cfm). The simulated maximum temperature from 

spontaneous heating of coals in the gob is shown in Figure 

4. 

300

301

302

303

304

0 5 10 15 20 25

Time (days)

Maximum temperature (K)

w barometric change w/o barometric change

Figure 4. Maximum temperatures (K) in the gob versus 

time with and without barometric pressure changes 

To demonstrate the difference caused by the barometric 

pressure changes, the maximum temperature simulated 

without barometric pressure change is also shown in Figure 

4. It can be seen that the maximum temperature change 

followed closely with barometric pressure variations: the 

former decreased as the pressure decreased; vise versa is 

also true. 
As the barometric pressure decreased, the gob breathed 

out, and therefore less oxygen was available for coal 

oxidation. On the other hand, as the barometric pressure 

increased, the gob breathed in, more oxygen was available 

for coal oxidation. Figure 5 shows the contours of oxygen 

concentration in the gob at 19.5 days and 22.5 days, 

respectively.

face

headgate

tailgate

100 m

(a) 

tailgate

face

headgate

100 m

(b) 

Figure 5. Oxygen concentration distribution in the gob at 

two different times: (a) 19.5 days (b) 22.5 days 

 From 19.5 days to 22.5 days, the barometric 

pressure increased from 29.33 to 30.33 in Hg, causing the 

gob to breathe in. It is evident that at 22.5 days, higher 

oxygen concentration was available at the headgate corner 

of the gob as compared to 19.5 days. 

To quantitatively examine the oxygen concentration 

variation caused by the barometric pressure changes, 

oxygen concentrations along a line 3 m from the gob 

perimeter on the headgate side are plotted in Figure 6 for 

19.5, 20.5, 21.5 and 22.5 days, respectively.

 

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 10 20 30 40

Distance from face (m)

O2 concentration (1=100%)

50

19.5 days 20.5 days 21.5 days 22.5 days

Figure 6. Oxygen concentration along the 3-m line in the 

gob from 19.5 to 22.5 days 

50

 

 It is clear that 

as the barometric pressure increased, more oxygen was 

breathed into the gob, which resulted in the higher 

temperature. Figure 7 shows oxygen concentrations along 

the 3-m line for 18, 18.5, 19 and 19.5 days, respectively. 

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0 10 20 30 40

Distance from face (m)

O2 concentration (1=100%)

18 days 18.5 days 19 days 19.5 days

Figure 7. Oxygen concentration along the 3-m line in the 

gob from 18 to 19.5 days 

From 18 days to 19.5 days, oxygen concentration at the 

headgate corner decreased as the barometric pressure 

decreased from 29.96 to 29.33 in Hg. As the coal oxidation 

was taking place at the same time, this oxygen concentration 

decrease was a combined effect of both barometric pressure 

decrease and coal oxidation. 

 As demonstrated in the previous study [6], the gas flow 

inside the gob was significantly affected by the gob 

permeability. To examine the effect of gob permeability on 

the barometric-induced flow, CFD simulation was 

conducted with the gob permeability increased 100 times. 

Figure 8 shows the barometric-induced flow for this case. 

The maximum outflow rate was about 1.0 kg/s (1,730 cfm), 
while the maximum inflow rate was about 0.90 kg/s (1,557 

cfm).

-1.5

-1

-0.5

0

0.5

1

1.5

0 2 4 6 8 10 12 14 16 18 20 22

Time (days)

Barometric-induced flow (kg/s)

Figure 8. Barometric-induced flow for the case with 

permeability increased 100 times 

 These maximum outflow and inflow rates were much 

higher than those in the base case because of smaller 

resistance in the gob. The maximum temperature in the gob 

with and without barometric pressure changes under 

increased gob permeability is shown in Figure 9. 

300

350

400

450

500

550

0 5 10 15 20

Time (days)

Maximum temp (K)

with barometric change w/o barometric change

Figure 9. Maximum temperatures (K) in the gob versus 

time with and without barometric pressure changes: gob 

permeability increased 100 times 

As 

discussed in the previous paper [6], when the gob 

permeability was increased 100 times, the maximum 

temperature from spontaneous heating reached 500 °K in 

about 22 days without barometric pressure changes. 

However, with barometric pressure changes, the maximum 

temperature became slightly lower most of the time, not 

following the trend of barometric pressure changes. This 

indicates that the effect of barometric pressure on 

spontaneous heating is different with the increased gob 

permeability. Figure 10 shows the oxygen concentrations 

along the 3-m line for the case with the gob permeability 

increased 100 times at 19.5, 20.5, 21.5, and 22.5 days, 

respectively. 

0

0.05

0.1

0.15

0.2

0.25

0 50 100 150 200 250 300

Distance from face (m)

O2 concentration (1=100%)

19.5 days 20.5 days 21.5 days 22.5 days

Figure 10. Oxygen concentration along the 3-m line in the 

gob for the case with permeability increased 100 times 

As the barometric pressure increased from 19.5 

days to 22.5 days, oxygen concentration at the headgate 

corner decreased. This may be caused by a faster coal 

oxidation rate. Although air was breathed in during this 

period, more oxygen was consumed by the coal oxidation 

than that added by air breathed in. In the base case, 

however, oxygen consumed by coal oxidation was less than 

that breathed in, so oxygen concentration at the headgate 

corner increased with time. This faster coal oxidation rate 

was supported by the higher oxygen concentration. Figure 

11 shows the oxygen concentration along the 3-m line at 

19.5 days for both cases. 

0

0.05

0.1

0.15

0.2

0.25

0 50 100 150 200 250 300

Distance from face (m)

O2 concentration (1=100%)

base case increased permeability

Figure 11. Comparison of oxygen concentrations along 

the 3-m line in the gob for the base case and the case with 

permeability increased 100 times 

It is evident that the oxygen 

concentration was higher in the case with increased gob 

permeability compared with the base case. The higher 

oxygen concentration resulted in faster coal oxidation rate, 

and thus higher temperature at 19.5 days as shown in Figure 

12. At 19.5 days, the maximum temperature along the 3-m 

line reached nearly 350 K, while it was only about 302 K in 

the base case. 
290

300

310

320

330

340

350

360

0 50 100 150 200 250 300

Distance from face (m)

Temperature (K)

base case increased permeability

Figure 12. Comparison of temperatures along the 3-m line 

in the gob for the base case and the case with permeability 

increased 100 times 

These simulation results indicate that the influence of 

barometric pressure changes on spontaneous heating was 

influenced by the gob permeability that determines the 

outflow and inflow rates. With the higher inflow rate caused 

by the increased gob permeability, the coal oxidation 

reaction rate was faster, leading to higher temperature. The 

spontaneous heating then consumed more oxygen than 

added by the gob inflow. The effect of barometric pressure 

changes on spontaneous heating was a slightly lower 

maximum temperature. The reason for this lower maximum 

temperature is that during the first decrease period (4 days), 

the maximum temperature with the barometric pressure 

change became slightly lower than that without the 

barometric pressure change. As the barometric pressure 

increased after first four days, the continuous inflow of air 

had no effect on the spontaneous heating, probably because 

enough oxygen was already available for coal oxidation. As 

the barometric pressure decreased again, the maximum 

temperature decreased slightly again. This trend continued 

with time, resulting in apparently lower maximum 

temperature most of the time. 

 To examine the effect of coal oxidation on the oxygen 

concentration in the gob, additional simulation was 

conducted by turning off coal oxidation at 19.5 days in the 

increased gob permeability case. Figure 13 shows the 

oxygen concentration along the 3-m line in the gob at 19.5, 

20.5, 21.5 and 22.5 days, respectively. 

 

0

0.05

0.1

0.15

0.2

0.25

0 50 100 150 200 250 300

Distance from face (m)

O2 concentration (1=100%)

19.5 days 20.5 days 21.5 days 22.5 days

Figure 13. Oxygen concentration along the 3-m line in the 

gob for the case with permeability increased 100 times from 

19.5 to 22.5 days: reaction turned off at 19.5 days 

It is obvious that the 

oxygen concentration increased quickly at the headgate 

corner form 19.5 days to 22.5 days when the gob was 

breathing in. Because there was no oxygen consumption by 

the coal oxidation, the oxygen concentration increased 

quickly with the time, indicating that the net effect of the 

barometric pressure changes also depends on the coal 

oxidation rate. 

Conclusions 

CFD simulations were conducted to investigate the 

potential effect of barometric pressure changes on 

spontaneous heating of coal in a bleederless longwall gob 

area. Simulation results demonstrate that under typical 

bleederless ventilation conditions, the maximum 

temperature from the spontaneous heating in the gob was 

affected by the barometric pressure changes, although not 

significantly. As the barometric pressure decreased, the 

oxygen concentration at the headgate corner was reduced 

and the maximum temperature became lower, while as the 

barometric pressure increased, the oxygen concentration at 

the headgate corner was increased and the maximum 

temperature became higher. However, this effect became 

quite different when the gob permeability was increased 100 

times. Under the increased gob permeability, the barometric 

pressure changes resulted in higher outflow and inflow 

rates. As the barometric pressure increased, the oxygen 

concentration at the headgate corner continually decreased, 

probably because of faster rate of coal oxidation. The 

maximum temperature became slightly lower than that 

without barometric pressure changes. The net effect of 

barometric pressure changes on the spontaneous heating 

depends on the gob permeability and the coal oxidation rate. 

Because of the complexity of the problem and lack of 

field data for gob permeability and porosity distribution, the 

results reported here are valid only for the permeability and 

porosity data used in this study with the longwall panel 

setup and ventilation conditions stated in the paper. 

Cooperation with U.S. coal mines to validate our modeling 

on spontaneous heating in the longwall gob area is greatly 

needed in our future study. 
References 

1. Francart, W. J. and Beiter, D.A. (1997), “Barometric 

pressure influence in mine fire sealing”, Proceedings of 

6th International Mine Ventilation Congress, 

Pittsburgh, PA, May 17-22, pp. 341-342. 

2. Mining Research Contract Report, (1988), “Improved 

ventilation of sealed mine gob”, U.S. Bureau of Mines, 

Contract J0308029. 21 pp. 

3. Hemp, R. (1994), “The effect of changes in barometric 

pressure on mines in the Highveld of South Africa”, 

The Journal of The South African Institute of Mining 

and Metallurgy, June 1994, pp. 133-146. 

4. Fauconnier, C.J. (1992), “Fluctuations in barometric 

pressure as a contributory factor to gas explosions in 

South African mines”, Journal of the South African 

Institute of Mining and Metallurgy, May, pp.31-147. 

5. Yuan, L. and Smith, A. C. (2007), “Computational fluid 

dynamics modeling of spontaneous heating in longwall 

gob areas”, 2007 SME Annual Meeting, Feb. 25-28, 

2007, Denver, Colorado, Preprint 07-101. 8 pp. 

6. Smith, A. C. and Yuan, L (2008), “Simulation of 

spontaneous heating in longwall gob area with a 

bleederless ventilation system”, 2008 SME Annual 

Meeting, Feb. 24-27, Salt Lake City, UT, Preprint 08-

043. 

7. Smith, A.C., Miron, Y. and Lazzara, C.P. (1991), 

“Large-scale studies of Spontaneous combustion of 

coal”, Report of Investigations 9346, U.S. Bureau of 

Mines. 30 pp. 

8. Esterhuizen, G. S. and Karacan, C.Ö. (2007), “A 

methodology for determining gob permeability 

distributions and its application to reservoir modeling 

of coal mine longwalls”, SME 2007 Annual Meeting, 

Denver. Preprint 07078; 6 pp.