Computational fluid dynamics 

modeling of spontaneous heating 

in longwall gob areas

L. Yuan and A.C. Smith

Research engineer and supervisory physical scientist, respectively, Pittsburgh Research Laboratory, 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania

Abstract

To provide insights for the optimization of ventilation systems for U.S. underground coal mines facing 

both methane control and spontaneous combustion issues, a computational fluid dynamics (CFD) study 

was conducted to model the potential for spontaneous heating in longwall gob areas. A two longwall 

panel district using a bleeder ventilation system with a stationary longwall face was simulated. The permeability and porosity profiles for the longwall gob were generated from a geotechnical model and were 

used as inputs for the CFD modeling. In this study, the effect of methane emissions from the mined coal 

seam, including the longwall face and overlying rider seam reservoirs on the gob gas distribution was 

considered. The spontaneous heating is modeled as the low-temperature oxidation of coal in the gob 

using kinetic data obtained from previous laboratory-scale spontaneous-combustion studies. Unsteady 

state simulations were conducted, and the effects of the coal’s apparent activation energy and reaction 

surface area on the spontaneous heating process were also examined.1

Introduction

In the United States, approximately 17% of the 87 reported 

fires in underground coal mines for the period 1990 through 

1999 were caused by spontaneous combustion (DeRosa, 2004). 

The risk of an explosion ignited by a spontaneous combustion 

fire was also present in those mines with appreciable levels of 

accumulated methane. In fact, three of the mine fires from the 

reported period resulted in subsequent methane explosions. 

The incidence of such fires and resulting explosion hazard is 

expected to increase with the projected increased mining of 

lower-rank coals, deeper mines (more methane) and the growth 

in the dimensions of longwall panels (ventilation strain). 

Spontaneous combustion occurs when the heat produced 

by the low-temperature reaction of coal with oxygen is not 

adequately dissipated by conduction or convection, resulting in 

a net temperature increase in the coal mass. Under conditions 

that favor a high heating rate, the coal attains thermal runaway 

and a fire ensues. The spontaneous combustion potential of 

coals can be evaluated qualitatively in a laboratory using 

one of four commonly used methods: adiabatic calorimetry, 

isothermal calorimetry, oxygen sorption and temperature differential methods. Although laboratory results are valuable, 

their extrapolation to the mining environment has not been 

completely successful because of complicated scaling effects 

that cannot be reproduced in small-scale experiments. In actual 

spontaneous heating events in coal mines, much larger coal 

masses may be involved. 

The spontaneous heating of coal in mines often occurs in 

a gob area and is not easily detected. The amount of coal that 

accumulates in these areas and the degree of ventilation can 

combine to give optimum conditions for spontaneous combustion. Although much research has been done in experimental 

study and mathematical modeling of spontaneous combustion 

of coals (Schmidt and Elder, 1940; Nordon, 1979; Brooks and 

Glasser, 1986; Smith and Lazzara, 1987; Smith et al., 1987; 

Edwards, 1990; Arisoy and Akgun, 1994; Carras and Young, 

1994; Krishnaswamy et al., 1996; Rosema et al., 2001), most 

of the research was mainly focused on small size coal stockpiles. Saghafi and co-workers performed numerical modeling 

of spontaneous combustion in underground coal mines with a 

back-return U-ventilation system (Saghafi et al., 1995; Saghafi 

and Carras, 1997), but their work was limited to two dimensions. Balusu et al. (2002) conducted a CFD study of gob-gas 

 1 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.
flow mechanics to develop gas and spontaneous combustion 

control strategies for a highly gassy mine. 

To prevent the occurrence of spontaneous combustion in a 

gob area, it is important to investigate the spontaneous heating 

of coals under realistic mine ventilation conditions with realistic 

methane generation and coal chemistry. In previous research 

carried out by NIOSH, computational fluid dynamics (CFD) 

modeling was used to describe the ventilation pathways through 

the immediate gob under different ventilation schemes, and 

the possible location of critical velocity zones where the gob 

is most liable to spontaneous heating was discussed (Yuan et 

al., 2006). In this paper, preliminary three-dimensional CFD 

modeling of spontaneous heating of coals in a two-panel gob 

area using a bleeder ventilation system and a stationary longwall face is presented. 

 Gob layout and ventilation system

A typical longwall district in an underground coal mine may 

consist of multiple panels. These panels are typically ventilated 

using bleeder fans to ventilate the active and mined-out panels. 

In coal seams with a high spontaneous combustion potential, 

bleederless ventilation systems may be employed. In the United 

States, only two mines now utilize the bleederless system. In 

this work, this situation is simulated with two panels, one as a 

mine-out panel and the other one as an active panel, utilizing a 

bleeder system and a bleeder fan. The layout of the two panels 

and the ventilation system is shown in Fig. 1. Each simulated 

gob area is 2,000 m (6,560 ft) long, 300 m (980 ft) wide and 

10 m (33 ft) high, starting from the bottom of the coal seam. 

The ventilation airways are 2 m (7 ft) high and 5 m (16 ft) 

wide. Panel A represents the completed panel, while Panel B 

represents the active one. The ventilation scheme includes a 

three-entry bleeder system. This scheme and the panel dimensions are typical of longwall mines operating in the Northern 

Appalachian Coal Basin of the Pittsburgh coal seam. In the 

model, it is assumed that the middle entry between Panels 

A and B and an entry on Panel A’s tailgate side are partially 

open. All crosscuts between the first and second entries on the 

tailgate side of Panel B are open. The bleeder entries at the 

back end of the gob are represented as one entry connecting 

to the bleeder fan for modeling purposes. Four regulators are 

located at the end of the second and third intake entries and 

two tailgate entries, respectively, for controlling the bleeder 

ventilation. Major assumptions used to construct the model 

are summarized as follows: 

• Gobs A and B are modeled as two porous rectangular 

volumes, each with dimensions of 2,000 × 300 × 10 m 

(6,560 × 980 × 33 ft). 

• Gobs A and B have the same permeability and porosity 

distributions. The permeability and porosity varies with 

the panel length and width, but not the height. 

• Between Panels A and B, only the middle entry is open 

and is characterized by the porosity and permeability 

values. Crosscuts on Panel B tailgates and Panel A 

headgate are assumed crushed into the gob areas.

• Right above the middle entry between Panel A and 

Panel B is the original rock with a porosity of 0.05 and 

a permeability of 0.1 millidarcies (md).

• The entry at Panel A tailgate is modeled the same way 

as the middle entry between Panel A and B. The rock 

volume right above this entry is not modeled.

• Crosscuts on the Panel B headgate are open, while crosscuts connecting each panel and bleeder entries at the back 

end of each panel are assumed completely closed.

Low temperature coal oxidation

The chemical reaction between coal and oxygen at low temperatures is complex. Generally, the following three types of 

processes are believed to occur (Carras and Young, 1994): 

• physical adsorption; 

• chemical adsorption, which leads to the formation of 

coal-oxygen complexes and oxygenated carbon-species; 

and 

• 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). 

The moisture content of coal can play an important role in 

the low-temperature coal oxidation. The interaction between 

water vapor and coal can be exothermic or endothermic, depending on whether the water condenses or evaporates. Sondreal 

and Ellman (1974) reported that for dried lignite, the rate of 

temperature increase due to the adsorption of water increased 

with the moisture content up to a value of 20% water (by mass) 

and then decreased with further increasing moisture content. 

Smith and Lazzara (1987) found that the effect of the moisture 

content of the air on self-heating process was also dependent 

on coal rank and temperature. 

During the first stage of this study, the effect of water vapor 

is not considered, and the chemical reaction between coal and 

oxygen is simplified as 

Coal + O2 → CO2 + 0.1CO + heat (1)

The detailed chemical structure of coal is not clear and varies 

with the rank and origin of coal. According to experimental 

data (Smith and Lazzara, 1987), one mole of coal reacting with 

one mole of oxygen generates one mole of carbon dioxide and 

roughly 0.1 mole of carbon monoxide plus heat at the early 

stage of coal oxidation. The chemical reaction rate is defined 

as the rate of change in the concentrations of the reactants and 

products. The dependence of the rate of oxidation on temperature 

and oxygen concentration can be expressed by

	 Rate = A[O2]n exp(-E/RT) (2)

where 

A is the pre-exponential factor, 

E is the apparent activation energy that is the energy needed 

to initiate a chemical reaction, 

R is gas constant, 

n is the apparent order of reaction, 

T is the absolute temperature and 

[O2] is the oxygen concentration.

The apparent activation energy (E) of different coals can 

very between 12 and 95 kJ/mol. It is about 90 kJ/mol for Pittsburgh coal, which is considered to have a moderate potential 

to spontaneous heating (Smith and Lazzara, 1987; Babrauskas, 

2003). The pre-exponential factor (A) depends more on coal 

rank and measurement method and has a typical value between 

1 and 7×105 per second. The value of the apparent order of the 

reaction (n) in low-temperature oxidation studies of coal and 

other carbonaceous materials has been shown to vary from ~0.5 

to 1.0 (Smith and Lazzara, 1987; Carras and Young, 1994) and 

is about 0.61 for some U.S. coals (Schmidt and Elder, 1940). 

The heat of reaction of coal oxidation is approximately 300 

kJ/mol-O2 for Pittsburgh coal.

To simulate the spontaneous heating of coal in longwall gob 

areas, the source of coal needs to be defined. The coal source 

can be coal left from the mined coal seam or other overlying or 
underlying coal seams. In this study, the Pittsburgh coal seam 

was considered with a 1-m- (3.3-ft-) thick rider coal seam less 

than 1 m (3.3 ft) above the 2-m- (6.6-ft-) thick main coal seam. 

The rider seam was modeled as caving into the gob after the 

main coal seam was completely mined out. 

Figure 2 shows the cross section of two longwall panels 

with the caving coal layer at the bottom of the gobs. The coal 

pillars left along the perimeter of the gob were also considered 

as a coal source. The oxidation of coal will occur on any available coal surface, including both external and internal pore 

surfaces. The reaction rate of spontaneous heating of coal in 

coal stockpiles was found to be related to the external surface 

area for nonporous coal particles with small pore diameters 

and weakly related or not related to particle size for small 

porous coal particles with larger pore diameters (Carras and 

Young, 1994). It is difficult to define a coal particle size distribution in the coal layer in the gob area because of the large 

gob size. The parameter that affects the heat generation and 

dissipation during the spontaneous heating process is the coal 

surface area available in a unit volume or surface-to-volume 

ratio. If an average coal particle diameter of 50 mm (2 in.) 

is assumed, then the surface-to-volume ratio would be 72/m 

using a typical porosity value of 0.4 within the uncompacted 

gob (Esterhuizen and Karacan, 2007). The heat generated from 

oxidation will be dissipated by conduction and convection 

while the oxygen and oxidation products are transported by 

convection and diffusion. 

Estimation of gob permeability 

The permeability and porosity distributions of the gob areas 

were based on geotechnical modeling of longwall mining and the 

associated stress-strain changes using Fast Lagrangian Analysis 

of Continua2 (FLAC) code (Esterhuizen and Karacan, 2007). In 

FLAC modeling, mining was simulated in increments, starting 

from one side of the grid and advancing to the other side. Extraction of the coal bed was modeled by removing elements over 

the height of the coal bed. The process of gob formation was 

modeled by first deleting rock elements in the roof of the coal 

bed, so that they are stress relieved, followed by inserting gob 

properties in these elements. Gob properties were also inserted 

in previously mined coal bed elements, so that the gob filled 

the mined void. In the gob caved area, the recompaction of the 

caved rock has a significant effect on its permeability. Stress 

changes in the rock in the gob cause changes in the fracture 

apertures, which then impact the permeability. 

Immediately behind the advancing face, the caved rock is 

loosely stacked and has an estimated porosity of approximately 

0.4. As the mining face advances away from this caved rock, the 

weight of the overburden gradually increases and recompacts 

the caved rock. This loading or stress in the caved material 

increases exponentially until the caved material supports the 

full overburden load. The recompaction, estimated with the 

FLAC model’s assigned strength and mechanical properties 

of the gob material, results in a reduction in the void space 

within the broken rock rubble and, therefore, significant changes 

in permeability. A simple relationship was used to estimate 

the changes in permeability in the caved rock based on the 

Kozeny-Carman equation

k f n

n = −













3

2 1( )

(3)

where 

n is the porosity and 

k is the permeability. 

For a Pittsburgh coal seam longwall panel, the permeability 

values in the gob area were estimated to vary from 3.0x104 to 

8.5x105 md, while the porosity value varies from 0.17 to 0.41. 

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. A detailed description of permeability calculation is given in Esterhuizen and Karacan (2007). 

These permeability and porosity distributions were then used 

in FLUENT as input. 

Figure 3 shows the permeability variation with the panel 

length and width for Panel A. The same profile was also applied to Panel B. The porosity profiles in the two panels were 

similar to those permeability profiles except the maximum and 

minimum values are 0.41 and 0.17, respectively. It is assumed 

that these permeability and porosity files do not change with 

the gob height.

Numerical modeling

A commercial CFD software, FLUENT (from Fluent 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. The physical 

model and mesh for the CFD simulation were generated using 

the mesh generator software, GAMBIT (from Fluent Inc.). The 

cell size varies from 1 to 4 m (3.3 to 13.1 ft) in the model with 

larger size cells located near the center of the gob. The total 

cell number was more than 1.5 million.

 2 References to a specific product is for informational purposes 

and does not imply endorsement by NIOSH.

Figure 1 — layout of gob areas and ventilation system

Figure 2 — Cross section of two longwall panels with caving coal layer.
Methane emission was also considered in the simulation 

because it affects the oxygen concentration distribution in the 

gob. Methane is always liberated from the face and inside 

the gob from the coal remaining within and around the gob. 

Under normal mining conditions, most of the methane from 

the gob coal is released in the first several hours, after which 

the release becomes very slow (Mucho, 2006). Because the 

time period for spontaneous combustion is typically at least 

several days, the gob coal methane emission is not considered 

in this simulation. Another source of continuous gob methane 

emission is from any overlying rider coal seam reservoirs. The 

methane from the overlying coal seam reservoir is assumed to 

be released uniformly along the border between the gob and the 

reservoir. Using ventilation data from a local Pittsburgh coal 

seam mine, the amount of methane released from the reservoir 

in this simulation is 0.133 m3/s (281 cfm) for Panel B and 0.024 

m3/s (50 cfm) for Panel A. The methane emission rate from 

the face to Panel B is 0.014 m3/s (29 cfm). These data were 

also entered into FLUENT using C subroutines. 

The boundary conditions for ventilation pressures used in 

the simulation were also obtained from a local Pittsburgh coal 

seam mine’s ventilation data. The pressure was -747 Pa (-3.0 

in.) water gauge at the intake inlet, -872 Pa (-3.5 in.) water 

gauge at the return outlet and -2,740 Pa (-11.0 in.) water gauge 

at the bottom of the bleeder shaft. The wall roughness of the 

ventilation airways was adjusted to have a total intake airflow 

rate of 41 m3/s (87,000 cfm) in the active longwall panel. 

The pressure drops through the two regulators located at the 

second and third intake entries were also adjusted to have an 

airflow rate entering onto the face of 28 m3/s (60,000 cfm). 

The pressure drop at Regulator 3 was adjusted to have a flow 

rate at the return of 24 m3/s (50,000 cfm), and the flow rate in 

the entry on Panel A’s tailgate side was 3.3 m3/s (7,000 cfm) 

by adjusting the pressure drop at the Regulator 4. 

A simulation was conducted first without coal oxidation 

to obtain steady state flow field and gas distributions. Then, 

the unsteady simulation with coal oxidation was conducted 

using the steady state solution as the initial conditions. For 

the unsteady simulation, all the boundary conditions were 

kept constant, and the chemical reaction term was initialized. 

The face was assumed stationary during the simulation. The 

physical and kinetic properties of the coal layer are listed in 

Table 1. The kinetic properties of coal oxidation were obtained 

from the previous laboratory-scale spontaneous combustion 

studies conducted by Smith and Lazzara (1987). 

Flow patterns inside the gob 

The flow patterns inside a gob will have a 

significant effect on the spontaneous heating of coals because the oxygen needed for 

the oxidation is provided by the gas flow, 

and the heat generated from the oxidation 

may be carried away by the gas flow. The 

gas flow inside a gob is expected to be three 

dimensional with the flow in the vertical 

direction, but weaker than in the other two 

directions due to reduced permeability and 

pressure gradients. To visualize the flow 

patterns inside the gob, a virtual horizontal 

reference surface was created 1 m (3.3 ft) 

from the bottom of the mined seam floor to 

compare the results with respect to this horizontal reference surface. Figure 4 shows the 

steady state pressure distribution over the 

two gob areas. A pressure differential was 

established between the intake and return 

and between the face and the bleeder fan 

with higher pressure at the face and lower 

Coal density 1,300 kg/m3

Coal specific heat 1,003.2 J/kg-K

Coal conductivity 0.1998 W/m-K

Heat of reaction 300 kJ/mol-O2

Activation energy 89.9 kJ/mol

Pre-exponential factor 6×104 /s

Porosity 0.4 -

Initial coal temperature 300 K

Table 1 — The physical and kinetic properties of the 

coal layer.

Property Value Units

Figure 3 — Permeability distributions for the gob.

Figure 4 — Contours of static pressure (pascal) over gob areas.
pressure at the bleeder fan. Figure 5 shows the flow path lines 

colored by velocity magnitude in the two gob areas. The path 

lines indicate that the flow was mainly from the headgate side 

to the tailgate side in the two panels. At the face, air entered 

into the gob through the face shields and flowed to the middle 

entry between the two panels. The higher gas velocity was 

between 1.5x10-4 to 2.5x10-4 m/s (0.03 to 0.05 fpm) near the 

back end of Panel B. Figure 6 shows the oxygen distribution 

in two panels. The oxygen concentration was at 21% in most 

of mined-out Gob A. In the active gob (Gob B), the oxygen 

concentration close to the intake entry was below 21% because 

of methane emission from the overlying coal seam reservoir. 

The opening of crosscuts between the first and second intake 

entries appears to be very important to dilute the methane 

concentration near the perimeter of the gob on the headgate 

side of the active panel. 

Simulation results and discussion

Simulations of spontaneous heating of coals were conducted 

with different values of apparent activation energy and coal 

surface area to investigate the effects of activation energy and 

coal surface area on the spontaneous heating of coal in gob areas 

as shown in Table 2. Case 1 represents a typical Pittsburgh coal 

seam, while Case 2 represents a coal with lower rank values 

typical of a Western coal. Case 3 represents the Western coal 

with a lower surface-to-volume ratio. The pre-exponential 

Factor A was kept the same at 6x104/s for the three cases.

Effect of apparent activation energy. Figure 7 shows the 

temperature distribution in the two panels after about 14 days 

for Case 1. The maximum temperature was 305 K (32oC), a 5 

degree increase. The maximum temperature increases occurred 

near the tailgate corner of Panel B, in the return split between 

Panel A and B, and just inside the crosscuts near the back of 

Panel B. For Case 2, shown in Fig. 8, where the coal had a 

lower activation energy, the maximum temperature was 564 K 

(291oC), a 264 degree increase, after only five days. According 

to Babrauskas (2003), when the coal temperature reaches above 

503 K (230oC), the spontaneous heating mechanism changes 

to rapid combustion. So under the conditions of Case 2, a fire 

would likely occur given a sufficient oxygen supply. Case 1 

can be viewed as representative of a coal with a moderate selfheating tendency, such as Pittsburgh coal. For this kind of coal, 

the spontaneous combustion hazard is usually moderate under 

normal mining conditions. Case 2 represents a coal with a high 

self-heating tendency. For this kind of coal, the spontaneous 

heating hazard is considered high according to Smith and 

Lazzara (1987) under normal mining conditions. For Case 1, 

although the temperature rise was lower, it was observed over 

most of the gob except the center area of the gob. In Case 2, 

the much higher temperature rise was limited to some very 

narrow areas behind the face shields and close to the middle 

entry between the two panels. In Panel A, the temperature rise 

occurred close to the return airway and the middle entry on 

Figure 5 — Flow path lines colored by velocity magnitude 

(m/s) in gob areas: (a) over the entire gob areas, (b) near 

the face area and (c) in a vertical cross section 1,000 m 

(3,280 ft) from the face.

1 89.9 72

2 52.7 72

3 52.7 12

 Surface-to-volume

 Activation energy, ratio, 

Case kJ/mol /m

Table 2 — Simulation scenarios.

its tailgate side. In Panel B, the temperature rise occurred on 

the tailgate side near the back end of the gob and Regulator 3. 

It is interesting to see that most of the gob area was still at its 

initial temperature. It should be pointed out that in a real situation similar to Case 2, a bleederless ventilation system may 

be needed instead of a bleeder ventilation system. 
everywhere, indicating that the oxygen was nearly all consumed by the oxidation of the coal near the active mining face. 

These oxygen distributions indicate that spontaneous heating 

is mainly controlled by oxygen availability when significant 

temperature rise occurs. This explains why the temperature 

rise only occurred in the area behind the shields and the area 

close to the middle entry, because oxygen was only available 

in these areas. Under the ventilation conditions studied here, 

not enough oxygen could be carried to the compressed gob 

area through convection and diffusion. 

Effect of coal surface area. The effect of coal surface area on 

the spontaneous heating was also examined by using a lower 

surface-to-volume ratio of 12/m, equivalent to an average 

coal particle diameter of 12 in. (300 mm), and an activation 

energy of 52.7 kJ/mol, while the other parameters were kept the 

same as in Case 2. Figure 10 shows the temperature distribution over the two panels after five days. Compared to Fig. 8, 

the temperature distribution was very similar, except that the 

maximum temperature was 370 K (97oC), a 70 degree rise. This 

is much lower than the value for the higher surface-to-volume 

ratio in Case 2, indicating that a larger particle size caused a 

smaller temperature rise in the same period of time. It was 

further found that after 25 days, the maximum temperature 

was 226oC (439°F), still below 230oC (446°F), indicating that 

decreasing the coal reaction surface area will greatly reduce 

the spontaneous combustion fire hazard.

Figure 6 — Contours of oxygen concentration (1 = 100%) 

in gob areas.

Figure 7 — Temperature distribution (K) for Case 1 after 

14 days: (a) over the entire gob areas and (b) near the 

face area.

Figure 8 — Temperature distribution (K) for Case 2 after 5 days: 

(a) over the entire gob areas and (b) near the face area.

The oxygen concentration distributions for two cases 

are shown in Fig. 9. For Case 1, oxygen was still available 

throughout most of the gob and 21% oxygen was available at a 

short distance behind the shields and close to the middle entry 

and return airway. Deeper into the gob, oxygen concentration 

dropped to about 3%. For Case 2, there was no oxygen left even 

immediately behind the shields or close to the middle entry. 

Deeper into the gob, the oxygen concentration was near zero 
CO concentration distribution. The carbon monoxide concentration distribution for Case 3 after five days is shown in Fig. 

11. The maximum CO concentration was 1.7%. The location 

of the maximum CO concentration was nearly the same areas 

of the maximum temperature indicating the reaction products 

were not transported away quickly. CO is usually used as an 

indicating gas for spontaneous heating of coals in mine fire 

detection. Although the maximum CO concentration was 1.7%, 

it was diluted to about 4,000 ppm at the outlet to the bleeder 

fan by the air from the bleeder entry. 

Oxygen concentration near the top of the gob. It is also worth 

to note that some oxygen was still available above the coal 

layer. Figure 12 shows the oxygen concentration distributions 

at the horizontal surfaces 1 and 9 m (3.3 and 30 ft) above the 

bottom of the coal seam, respectively, for Case 3. At the 1 m 

surface, nearly all oxygen was consumed, while much higher 

oxygen concentrations existed on the 9 m surface. The model 

showed that this oxygen could not be transferred to the oxidation zone through convection and diffusion. 

Critical velocity zones for spontaneous combustion. As 

discussed in Yuan et al. (2006) and shown in this investigation, just behind the shields where the oxygen concentration is 

highest, the air velocity is also high, so that heat generated by 

self-heating is carried away. Deeper in the gob, the air velocity 

is too low to provide sufficient oxygen to support spontaneous 

combustion. This was confirmed in the simulations. Under 

conditions studied here, the simulations demonstrate that the 

area about 5 to 9 m (16 to 30 ft) behind the shields was indeed 

the critical velocity zone for potential spontaneous combustion. The critical velocity is defined as the one insufficient to 

remove the heat due to coal oxidation, but sufficient to maintain 

the oxidation process. So the temperature in a critical velocity 

zone will increase because of heat accumulation. The value 

Figure 9 — Oxygen concentration distribution (1 = 100%) 

in two gob areas: (a) Case 1 and (b) Case 2.

Figure 10 — Temperature distribution (K) in two gobs for 

Case 3: (a) over the entire gob areas and (b) near the face 

area.

Figure 11 — CO concentration (1 = 100%) for Case 2.
the reaction surface area and the maximum temperature rise 

were both significantly reduced. Simulations also confirmed 

the existence of a critical velocity zone behind the shields in 

the gob, and showed the existence of other critical velocity 

zones in both the active panel and the mined-out panel. More 

research is needed to study the effect of the longwall face 

movement on the spontaneous heating of coals in the gob areas, 

and modeling of bleederless ventilation systems. 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.

Acknowledgments

The authors wish to acknowledge the technical help from Drs. 

Gabriel S. Esterhuizen and Özgen C. Karacan with the estimation of permeability and porosity in the gob.

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Yuan, L, Smith, A.C., and Brune, J.F., 2006, “Computational Fluid Dynamics 

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of the critical velocity was not determined in this study, but 

any zone with a temperature increase will be a critical velocity zone. In the active Panel B, the critical velocity zones also 

occurred in the narrow area near the middle entry, and the 

narrow area on the tailgate side of the panel and near the back 

end of the gob, and the area near the open crosscuts between 

the first and second intake entries. In the mined-out Panel A, 

the critical velocity zone occurred in the narrow area near the 

return entry, and the narrow area near the middle entry on both 

headgate and tailgate sides. All these narrow areas were about 

several meters wide.

Conclusions

CFD simulations were conducted to investigate the spontaneous 

heating of coals in longwall gob areas with different apparent 

activation energies and reaction surface areas. Simulation results 

demonstrate that under typical three-entry bleeder ventilation 

conditions, the spontaneous heating hazard was minimal for 

coals with a higher apparent activation energy for at least two 

weeks. The hazard was high for coals with lower apparent 

activation energies after five days. When the temperature rise 

was significant, the spontaneous heating became mainly oxygen 

controlled. The temperature rise was greatly dependent on the 

available reaction surface area. For larger coal particle size, 

Figure 12 — Oxygen concentration (1 = 100%) at two horizontal surfaces for Case 3: (a) 1-m (3.3 ft) horizontal surface 

and (b) 9-m (29.5 ft) horizontal surface.