Modeling the effect of seal leakage on spontaneous heating in a 

longwall gob area 

A.C. Smith & L. Yuan 

Office of Mine Safety and Health Research, National Institute for Occupational Safety and Health 

(NIOSH), Pittsburgh, PA, USA 

ABSTRACT: Federal regulations provide that mines with a demonstrated history of spontaneous combustion or 

that are located in a coal seam determined to be susceptible to spontaneous combustion may use a bleederless 

ventilation system as a spontaneous combustion control measure. Currently, three coal mines in the U.S. are 

utilizing a bleederless system to prevent spontaneous combustion. In a bleederless ventilation system, one of the 

headgate entries is used as the tailgate entry of the succeeding panel and is isolated from the gob of the active 

panel by gob seals constructed in each crosscut inbye the active face. Air is coursed across the face area and 

outbye in the tailgate entry into a return entry instead of inbye through the gob. As the longwall progresses, some 

seals may leak. When the headgate entry is ventilated, such as in a Y-type bleederless system, the air leakage 

through the seals into the gob may lead to spontaneous heating in the gob behind the seals. In this study, a 

computational fluid dynamics (CFD) model developed in our previous work is used to model the effect of seal 

leakage on spontaneous heating of coal in longwall gob behind the seal. A single longwall panel using a Y-type 

bleederless ventilation system was simulated. With typical bleederless ventilation conditions, the simulation 

results demonstrate that the effect of seal leakage on the spontaneous heating process depends on both seal 

leakage rate and gob permeability. 

1 Introduction	 

In a longwall gob, spontaneous heating occurs when the 

heat that is produced by the low temperature reaction of 

coal left in the gob with oxygen is not adequately 

dissipated by conduction or convection, resulting in a net 

temperature increase in the coal mass. Under conditions 

that favour a high heating rate, the coal attains thermal 

runaway and a fire ensues. The spontaneous heating of 

coal in the gob area may not be easily detected. The 

amount of coal that accumulates in the gob area, the degree 

of ventilation, and the lack of heat dissipation can combine 

to give optimum conditions for spontaneous combustion. 

Although the number of spontaneous combustion fires has 

remained nearly constant for the last 35 years in the U.S., 

there is the possibility that the number of spontaneous 

combustion fires could increase due to growth in the 

dimensions of longwall panels and due to the exhausting of 

easily-mined coal beds in the U.S., which results in 

increased mining of lower rank coals and deeper coal beds. 

In the U.S., bleederless ventilation systems may be 

approved by the Mine Safety and Health Administration 

(MSHA, 2002) to serve as a spontaneous combustion 

control method in mines with a demonstrated history of 

spontaneous combustion. Currently, three U.S. coal mines 

are utilizing bleederless ventilation systems. For a typical 

bleederless ventilation system, one of the headgate entries 

is used as the tailgate entry of the succeeding panel and is 

isolated from the gob of the active panel by gob seals 

constructed in each crosscut inbye the active face (MSHA, 

2002). Air is coursed across the face area and outbye in the 

tailgate entry into a return entry instead of inbye through 

the gob. Under a typical bleederless system, the areas most 

susceptible to spontaneous combustion with this design are 

immediately behind and around the seals and behind the 

face shields, due to pressure differentials. Each leakage 

source can provide oxygen and increase the possibility of 

spontaneous combustion occurring. 

It is difficult to obtain seal leakage rate data from 

actual underground coal mines because of the difficulty in 

measuring leakage rates. The Code of Federal Regulation 

(CFR) requires that all permanent ventilation controls, 

including explosion-resistant seals, be maintained to serve 

the purpose for which they were built. NIOSH and MSHA 

conducted a joint program that evaluated the strength 

characteristics of proposed explosion-resistant seal designs 

(Weiss et al., 1993; Stephan & Schultz, 1997). Full-scale 

seals were constructed and tested in NIOSH’s Lake Lynn 

Laboratory Experimental Mine (LLEM). Post-explosion 

air leakage characteristics that MSHA deemed acceptable 

for explosion-resistant seals tested at LLEM are as 

follows: a) for pressure differentials up to 0.25 kPa (1"wg ) 

leakage must remain below 0.05 m3

/s (100 cfm), b) for 

pressure differentials up to 0.50 kPa (2"wg) leakage must 

remain below 0.07 m3

/s (150 cfm), c) for pressure 

differentials up to 0.75 kPa (3"wg) leakage must remain 

below 0.10 m3

/s (200 cfm), and d) for pressure 

differentials exceeding 0.75 kPa leakage must remain 

below 0.12 m3

/s (250 cfm). Since these studies, new seal 

regulations have been adopted for seal strength and 

construction requirements. However, no new seal leakage 

criteria were included in these regulations. For the purpose 
of this study, the leakage rates from the previous work 

were used as guidelines for the model. 

In our previous studies, a CFD model was developed to 

simulate the spontaneous heating of coal in a longwall gob 

area using a bleederless ventilation system (Smith & Yuan, 

2008). The CFD model was also used to simulate the effect 

of barometric pressure changes on spontaneous heating in 

bleederless longwall panel (Yuan & Smith, 2010). Those 

simulations were conducted without any seal leakages, and 

the results demonstrate that the area most susceptible to 

spontaneous heating occurred immediately behind the face 

shields. The potential for air exchange across a seal due to 

pressure differential has long been a concern. If methane is 

present behind the seal, a spontaneous heating could cause 

a methane ignition and/or explosion. Much research has 

been conducted on the potential for methane accumulation 

behind seals, but little research is available in 

understanding how the seal leakage affects the 

spontaneous heating of coal in longwall gob area. In this 

study, the previously developed CFD model is used to 

model the effect of seal leakage on the spontaneous 

heating in a longwall gob. 

2 CFD Modeling of Spontaneous Heating in the 

Gob 

In this study, an active longwall panel using a Y-type 

bleederless ventilation system is simulated. Compared with 

the typical U-type bleederless system, the Y-system keeps 

the headgate entry ventilated for utilization as the tailgate 

for the next panel and seals are installed in the headgate 

entry as the face advances. The seals are accessible for 

inspection and maintenance but are more exposed for 

leakage and differential pressure across the seals to 

develop (Smith et al., 1994). The layout of the panel and 

the ventilation system, are shown in Figure 1.

intake face 

gob 

to return return 

Figure 1 Layout of longwall panel and ventilation system 

used in simulations. 

 The 

simulated gob area is 1,000 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. In the 

model, the headgate entry is ventilated and eventually goes 

to return after passing the back end of the panel. Seals are 

built between this entry and the active gob. The face is 

assumed stationary during the simulations. 

The chemical reaction between coal and oxygen at low 

temperatures is complex and still not well understood. In 

the study, the chemical reaction between coal and oxygen 

is simplified so that one mole of coal reacts with one mole 

of oxygen to generate one mole of carbon dioxide and 0.1 

mole of carbon monoxide plus the heat of coal oxidation 

(Smith et al., 1991). The heat generated from coal 

oxidation is dissipated by conduction and convection, 

while the oxygen and oxidation products are transported 

by convection and diffusion. The dependence of the rate of 

oxidation on temperature and oxygen concentration is 

expressed in the form: 

Rate = A[O n 2] 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 pre-exponential factor (in K/s), E is the apparent 

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

apparent order of reaction, T is the absolute temperature 

(in K), and [O2] is the oxygen concentration (in kmol/m3

). 

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

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

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-tovolume ratio of 36 m-1, is used in the simulations. A 

typical bituminous coal with a high spontaneous 

combustion potential was modeled in this study. The 

physical and kinetic properties of this coal are listed in 

Table 1. 

Table 1 The physical and kinetic properties of the rider 

seam 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 

were based on geotechnical modeling of longwall mining 

in the Pittsburgh coal seam and the associated stress-strain 

changes using a FLAC (Fast Lagrangian Analysis of 

Continua) code (Esterhuizen & Karacan, 2007). For a 

Pittsburgh coal seam longwall panel, the permeability 

values in the gob area were estimated to be in a range of 

3.0 x 104

 to 8.5 x 105 millidarcies (md), while the porosity 

value is in a range of 0.17 to 0.41 based on the modeling 

result from FLAC. These values are used as base values in 

the simulations. Around the perimeter of the gob and 

immediately behind the face shields, the permeability and 

porosity values were the largest, while near the center of 

the gob, these values were the smallest due to compaction. 

The porosity profile in the gob was similar to the 
permeability profile. It is assumed that these permeability 

and porosity profiles do not change with the gob height. 

A commercial CFD software, FLUENT from Ansys, 

Inc., was used in this study to simulate 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 

a U.S. bleederless ventilation system were used in the 

simulation. The intake airflow rate was 30 m3

/s 

(64,000 cfm). The pressure was -747 Pa (-3.0"wg) at the 

intake inlet, -872 Pa (-3.5"wg) at the return outlet. 

3 Simulation Results and Discussion 

The simulation was first conducted without any seal 

leakage using base values for gob permeability and 

porosity. The temperature distribution in the gob after 15 

days is shown in Figure 2. 

 

Figure 2 Temperature distribution (K) in the gob after 15 

days without any leakage through gob seals. 

A small temperature increase of 

2 K occurred at the intake corner in the gob. Along the 

face and immediately after the face shields, there was a 

slight temperature rise of about 1 K. Simulations were then 

conducted using seal leakage rates of 0.0047, 0.012, 0.024 

and 0.047 m3

/s (10, 25, 50 and 100 cfm ). For the purpose 

of this study, all seals were assumed to have the same 

leakage rate. 

Figure 3 shows temperature distribution in the gob 

after 15 days with a leakage rate of 0.0047 m3

/s (10 cfm). 

Figure 3 Temperature distribution (K) in the gob after 15 

days with a leakage rate of 0,0047 m3

/s. 

The maximum temperature was 308 K at the corner of the 

back end on the headgate side. Because of leakage at each 

seal, there was a slight temperature rise of about 2 K 

behind each seal. Figure 4 shows the temperature 

distribution in the gob after 15 days with a leakage rate of 

0.047 m3

/s (100 cfm). 

Figure 4 Temperature distribution (K) in the gob after 15 

days with a leakage rate of 0.047 m3

/s. 

The maximum temperature in this 

case was 325 K, also observed at the corner of the back 

end on the headgate side. Because of higher leakage at 

each seal, the area of temperature rise behind each seal 

increased, and the magnitude of temperature rise increased 

to about 15 K above ambient. The maximum temperature 

histories in the gob with different seal leakage rates are 

shown in Figure 5. 

 

330 

100 cfm 50 cfm 25 cfm 10 cfm No leakage 

Maximum temperature (K) 325 

320 

315 

310 

305 

300 

0 2 4 6 8 10 12 14 16 

Time (days) 

Figure 5 Maximum temperature historie

different seal leakage rates. 

s in the gob with 

It is clear that with the increase of the 

leakage rate, the maximum temperature increased, 

increasing the risk of fire hazard. 

3.1 Effect of Gob Permeability 

Gob permeability has a strong effect on gas flow in the 

gob. It is important to examine the effect of gob 

permeability on the spontaneous heating of coal with 

different leakage rates. CFD simulations were conducted at 
the same seal leakage rates with the gob permeability 

increased 100 times from its base value. Figure 6 shows 

temperature distribution in the gob after 15 days without 

any leakage through the gob seals.

Figure 6	 Temperature distribution (K) in the gob with 

permeability increased 100 times after 15 days 

without any leakage through gob seals. 

 The maximum 

temperature was 319 K. Temperature increases were 

observed at the intake corner in the gob and immediately 

behind the face shields. Figures 7 and 8 show the 

temperature distributions in the gob after 15 days with the 

seal leakage rates of 0.0047 and 0.047 m3

/s (10 and 

100 cfm), respectively. 

Figure 7	 Temperature distribution (K) in the gob with 

permeability increased 100 times after 15 days 

with the seal leakage rate of 0.0047 m3

/s. 

Figure 8	 Temperature distribution (K) in the gob with 

permeability increased 100 times after 15 days 

with the seal leakage rate of 0.047 m3

/s. 

At the leakage rate of 0.0047 m3

/s 

(10 cfm), the maximum temperature was still 319 K at the 

inbye corner of the intake in the gob. Because of leakage at 

each seal, there was again a slight temperature rise behind 

each seal. At the leakage rate of 0.047 m3

/s (100 cfm), the 

maximum temperature was 324 K, but at the outbye corner 

of the back end on the headgate side. The temperature 

increases were similar to the results with the base value 

permeability. Because of higher leakage at each seal, the 

area with a slight temperature rise behind each seal 

increased. It should be pointed out that the major 

difference between the two leakage rates with the gob 

permeability increased 100 times is the location of the 

maximum temperature. The maximum temperature 

occurred at the intake corner at the leakage rate of 

0.0047 m3

/s (10 cfm), while it occurred at the corner of the 

back end on the headgate side at 0.047 m3

/s (100 cfm). The 

maximum temperature histories in the gob with 

permeability increased 100 times under different seal 

leakage rates are shown in Figure 9. It is clear that with the 

increase of the leakage rate from 0 to 0.023 m3

/s (50 cfm), 

the maximum temperature increased insignificantly. Only 

with the leakage rate of 0.047 m3

/s (100 cfm) did the 

maximum temperature rise become significant. 

Figure 10 compares maximum temperature histories in 

the gob with different gob permeability for the seal leakage 

rates of 0.047 and 0.0047 m3

/s (100 and 10 cfm). It is 

interesting to note that the maximum temperature in the 

gob with the gob permeability increased 100 times was 

nearly the same as that with the original gob permeability 

under the 0.047 m3

/s (100 cfm) seal leakage rate. On the 

other hand, the maximum temperature in the gob with the 

gob permeability increased 100 times was much higher 

than that with the original gob permeability under the 

0.0047 m3

/s (10 cfm) seal leakage rate. This indicates that 

with a higher seal leakage rate (0.047 m3

/s), the gob 

permeability had little effect on the maximum temperature 

in the gob. Whereas with a lower seal leakage rate 

(0.0047 m3

/s), the larger gob permeability resulted in the 

higher maximum temperature in the gob. 

3.2 Effect of the Number of Leaking Seals 

Under actual mining conditions, the leakage rate for each 

seal may be different. As the longwall progresses, some 

seal leakage may increase. In order to examine the effect 

of the number of leaking seals on the spontaneous heating 

of coal in the gob, a CFD simulation was conducted with 

only one seal, the closest to the back end of the panel 

leaking. Figure 11 shows the temperature distribution in 

the gob after 15 days with a leakage rate of 0.047 m3

/s 

(100 cfm) at that seal. The maximum temperature was 

312 K behind the seal. This compares to a maximum 

temperature of 325 K at the corner of the back end of the 

panel on the headgate side with leakage behind all seals, as 

shown in Figure 4. Figure 12 shows the temperature 
distribution in the gob after 15 days with two seals leaking 

at a rate of 0.047 m3

/s (100 cfm). The maximum 

temperature was 312 K behind both seals, indicating that 

under this condition, the leakage from a neighbouring seal 

does not affect heating behind a seal. The location of the 

hot spot and the higher maximum temperature shown in 

Figure 4 was apparently caused by the combined effect of 

two seals together at the headgate side corner of the back 

end of the panel that allowed for short-circuiting of the 

ventilation flow. 

 

330 

100 cfm 50 cfm 25 cfm 10 cfm no leakage 

0 2 4 6 8 10 12 14 16 

325 

320 

315 

310 

305 

300 

Time (days) 

Maximum temperature (K) 

Figure 9 Maximum temperature histories in the gob with 

permeability increased 100 times under different 

seal leakage rates. 

 

 

 

330 

100 cfm 

100 cfm (increased permeability) 

10 cfm 

10 cfm (increased permeability) 

0 2 4 6 8 10 12 14 16 

325 

320 

315

Maximum temperature (K) 310 

305 

300 

Time (days) 

Figure 10 Maximum temperature histories in the gob with 

different gob permeability for the seal leakage 

rates of 0.047 and 0.0047 m3

/s. 

Figure 11 Temperature distribution (K) in the gob after 15 

days with the leakage rate of 0.047 m3

/s at one 

seal. 

Figure 12 Temperature distribution (K) in the gob after 15 

days with the leakage rate of 0.047 m3

/s at two 

seals. 

Simulations were also conducted with one and two 

seals leaking when the gob permeability was increased 100 

times from the base value. Figure 13 shows the 

temperature distribution in the gob after 15 days with only 

one seal leaking at the rate of 0.047 m3

/s (100 cfm) with 

the gob permeability increased 100 times. 

Figure 13 Temperature distribution (K) in the gob with 

permeability increased 100 times after 15 days 

with the leakage rate of 0.047 m3

/s at one seal. 

The maximum 

temperature was 319 K at the intake corner of the gob. The 

seal leakage caused some temperature rise, but was much 

lower than the maximum temperature at the intake corner. 

Figure 14 shows the temperature distribution in the gob 

after 15 days with two neighbouring seals leaking at the 

rate of 0.047 m3

/s (100 cfm) with the gob permeability 

increased 100 times. The maximum temperature was still 

319 K at the intake corner of the gob. Some temperature 

increase was observed behind the seals but the temperature 

rise was similar behind both seals, indicating again that the 

leakages from neighbouring seals did not affect each other. 
Figure 14 Temperature distribution (K) in the gob with 

permeability increased 100 times after 15 days 

with the leakage rate of 0.047 m3

/s at two seals. 

4 Conclusions 

CFD simulations were conducted to investigate the effect 

of seal leakage on spontaneous heating of coal in longwall 

gob area. Simulation results demonstrate that under typical 

bleederless ventilation conditions, with the increase of 

leakage rate from 0.0047 to 0.047 m3

/s (10 to 100 cfm), 

the maximum temperature in the gob increased, and the 

maximum temperature occurred at the headgate side corner 

at the back end of the panel. When the gob permeability 

was increased 100 times, with the leakage rate of 

0.0047 m3

/s (10 cfm), the maximum temperature was the 

same as without seal leakage, while with the leakage rate 

of 0.047 m3

/s (100 cfm), the maximum temperature was 

higher and at the headgate side corner at the back end of 

the panel. With the increase of the seal leakage rate from 0 

to 0.023 m3

/s (50 cfm), the maximum temperature 

increased insignificantly. Only with the leakage rate of 

0.047 m3

/s (100 cfm) did the maximum temperature rise 

become significant. 

With a high leakage rate (0.047 m3

/s), the gob 

permeability had little effect on the maximum temperature 

in the gob, while with a low seal leakage rate 

(0.0047 m3

/s), the larger gob permeability resulted in the 

higher maximum temperature in the gob. Simulations also 

revealed that when only one or two seals were leaking, the 

maximum temperature occurred around the seal. However, 

with the gob permeability increased 100 times and only 

one or two seals leaking, the maximum temperature was 

still at the intake corner in the gob. The neighbouring seal 

leakages did not affect each other. 

These results show that complex interactions between 

pressure differentials and gob permeability at different 

locations in the gob cause ventilation pathways to change 

in reaction to these dynamic processes. These interactions 

are highly dependent on gob permeability and seal leakage 

rates. This study indicates that CFD modeling can be used 

to investigate these interactions. Future studies will be 

conducted to obtain data to verify the model results. 

Disclaimer: The findings and conclusions in this paper are 

those of the authors and do not necessarily represent the 

views of the National Institute for Occupational Safety and 

Health. 

References 

Esterhuizen, G. S. & Karacan, C.Ö. (2007). A methodology for 

determining gob permeability distributions and its 

application to reservoir modeling of coal mine longwall, 

SME 2007 Annual Meeting, Denver. 

MSHA Ventilation Specialists Training Course (2002), Bleeder 

and Gob Ventilation Systems, Course of Instruction Series 

CI 42a. 

Smith, A.C., Diamond, W.P., Mucho, T.P. & Organiscak, J.A. 

(1994). Bleederless ventilation systems as a spontaneous 

combustion control measure in U.S. coal mines, U.S. Bureau 

of Mines Information Circular 9377. 

Smith, A.C., Miron, Y. & Lazzara, C.P. (1991). Large-scale 

studies of Spontaneous combustion of coal, Report of 

Investigations 9346, U.S. Bureau of Mines. 

Smith, A. C. & Yuan, L (2008). Simulation of spontaneous 

heating in longwall gob area with a bleederless ventilation 

system, SME Annual Meeting, Feb. 24-27, Salt Lake City, 

UT, Preprint 08-043. 

Stephan, C.R. & Schultz, M.J. (1997). Construction of explosion 

resistant seals, SME Annual Meeting, Denver, Colorado, 

February 24-27, Preprint 97-83. 

Weiss, E.S., Greninger, N.B., Stephan, C.R. & Lipscomb (1993). 

Strength characteristics and air-leakage determinations for 

alternative mine seal designs, U.S. Bureau of Mines Report 

of Instigations 9477. 

Yuan, L. & Smith, A.C. (2010), Modeling the effect of 

barometric pressure changes on spontaneous heating in 

longwall gob area, SME Annual meeting, Feb. 28 – March 3, 

2010, Phoenix, Arizona.