Effect of longwall face advance 

on spontaneous heating 

in longwall gob area
� L. YUAN AND A.C. SMITH 

Liming Yuan, member SME, and Alex C. Smith are lead 

research engineer and senior research chemist, respectively, 

with the National Institute for Occupational Safety and Health 

(NIOSH), Pittsburgh, PA.

Introduction 

In underground longwall coal 

mines, some coal may be left in the 

gob, either from the unmined roof or 

floor coal or from an overlying coal 

seam that caves into the gob. When 

exposed to the air, the coal may undergo spontaneous heating, a low 

temperature coal oxidation. When 

the heat produced by the low temperature reaction of coal with oxygen 

is not adequately dissipated by the airflow, it will result 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. For the period 1990 – 2006, a 

total of 25 reported fires for underground coal mines in the 

U.S. were caused by spontaneous combustion (DeRosa, 

2004). Although much research has been done in experimental studies and mathematical modeling of spontaneous combustion of coals (Arisoy & Akgun, 1994; Brooks 

Abstract 

To reduce fire hazards from spontaneous combustion 

of coal in longwall gob areas, a series of computational 

fluid dynamics (CFD) simulations were conducted by the 

National Institute for Occupational Safety and Health 

(NIOSH) to model the spontaneous heating of coal in longwall gob areas.The previous modeling results demonstrate 

that spontaneous heating of coal usually occurred behind 

the longwall shields and along the face with a bleeder 

ventilation system. Assuming a stationary longwall face, 

the spontaneous heating could turn to a spontaneous fire 

in several days for the most reactive coal under favorable 

conditions. When the longwall face advances, the spontaneous heating process will be significantly affected. In this 

study, the effect of longwall face advance on the spontaneous heating in the gob area is investigated using the CFD 

model developed in previous studies. One longwall panel 

with a bleeder ventilation system is simulated. The width 

of the panel is 300 m (984 ft), while the length of the panel 

is changed between 1,000 to 2,000 m (3,280 to 6,560 ft). 

The same permeability and porosity profiles are used for 

gobs with different lengths. The spontaneous heating first 

develops in the gob when the longwall face is stationary. 

Then,the face advances at a certain rate.The face advance is 

simulated as a series of discrete movements,and the effect of 

the face advance on the maximum temperature developed 

during the face stoppage is examined. 

& Glasser, 1986; Carras & Young, 

1994; Edwards, 1990; Nordon, 1979; 

Krishnaswamy et al., 1996; Rosema 

et al., 2001), most of the research is 

mainly focused on small coal stockpiles without considering the effect 

of ventilation air flow. Saghafi et al., 

(1995, 1997) did numerical modeling of spontaneous combustion in 

underground coal mines with a back 

return U-ventilation system, but their 

work was limited to two dimensions. Balusu et al., (2002) 

conducted a computational fluid dynamics (CFD) study 

of gob gas flow mechanics to develop gas and spontaneous combustion control strategies for a highly gassy mine. 

These strategies include new gob hole designs to ensure 

that oxygen concentration in the holes was below 4 to 5%, 

immediately sealing of the crosscut behind the face, and 

reduction in air velocity on the intake side of the gob. 

In order to reduce the fire hazard from spontaneous 

combustion of coal in gob areas, the National Institute 

for Occupational Safety and Health (NIOSH) has conducted a series of CFD simulations using coal kinetic data 

from the former U.S. Bureau of Mines laboratory-scale 

experimental results (Smith and Lazzara, 1987). Previous 

CFD models were developed to simulate the spontaneous 

heating of coals in a two-panel gob area using a bleeder 

ventilation system with a stationary longwall face (Yuan 

and Smith, 2007). Parametric studies were then conducted 

to examine the effects of coal’s activation energy, coal surface area, heat of reaction, different ventilation conditions 

and gob permeability distributions on the spontaneous 

heating process (Yuan and Smith, 2008a). CFD simulations 

were also conducted to model the spontaneous heating in 

longwall gob area using a bleederless ventilation system 

with a stationary longwall face (Smith and Yuan, 2008). In 

this paper, CFD modeling of the effect of longwall face advance on the spontaneous heating of coals in a two-panel 

gob area using a bleeder ventilation system is presented. 

GOB layout and ventilation system In this study, a single longwall panel, 2,000-m- (6-

560-ft-) long and 300-m- (984-ft-) wide, with a three-entry 

bleeder ventilation system is simulated. The layout of the 

panel and the ventilation system is shown in Fig. 1. The 

original coal seam is 2-m- (6.5-ft-) high, and the gob is assumed to be 10-m- (33-ft-) high starting from the bottom 

of the coal seam. The ventilation airways are 2-m- (6.5-ft-) 

high and 5-m- (16-ft-) wide. This scheme and the panel 
dimensions are typical of longwall mines operating in the 

Pittsburgh coal seam in Northern Appalachian Basin. The 

bleeder entries at the back end of the gob are modeled as 

one entry connecting to the bleeder fan. Three regulators 

are located at the end of the second and third headgate 

entries and the tailgate entry, respectively, for controlling 

the bleeder ventilation. 

The longwall face is originally at location #1, 1,000 m 

(3,280-ft-) from the start line of the panel. It is assumed 

that the face advances in a rate of 20 m/d (65 ft/day) from 

location #1. The face advances 100 m (328 ft) in five days 

to reach locations #2, #3 and #4, respectively. The face advances 200 m (656 ft) in 10 days to reach location #5. 

FIGURE 1 

Layout of longwall panel and ventilation system. 

Low temperature coal oxidation 

The chemical reaction between coal and oxygen at low 

temperatures is complex and still not well understood. 

Generally, three types of processes are believed to occur in 

low temperature coal oxidation (Carras and Young, 1994), 

including 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 process. 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. 

In 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 

The detailed chemical structure of coal is not clear 

and is believed to vary with the rank and origin of coal. 

According to experimental data (Smith et al., 1991), one 

mole of coal reacting with one mole of oxygen generates 

one mole carbon dioxide and 0.1 mole carbon monoxide 

plus heat at the early stage of coal oxidation. The dependence of the rate of oxidation on temperature and oxygen 

concentration can be expressed in the form: 

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 (in K/s), E is the apparent activation energy (in kJ/mol) that is the energy 

needed to initiate a chemical reaction, 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). 

The value of apparent activation energy, E, of different coals can very between 12 and 95 kJ/mol. The preexponential factor, A, depends more on coal rank and 

measurement method, and has a typical value between 1 

and 7×105 /s. 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 (Carras and Young, 1994), and is about 0.61 for 

some U.S. coals (Schmidt and Elder, 1940). 

The coal source in the gob can be coal left from the 

mined coal seam or other overlying 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 coal source in 

the model is this rider coalbed that is assumed to cave into 

the gob after the main coal seam is completely mined out. 

FIGURE 2 

Temperature distribution (K) after 20 days with longwall face 

at location #1. 

FIGURE 3 

Temperature distribution (K) after five days with longwall face 

at location #2. 
 

 

 

 

 

 

 

 

The oxidation of coal occurs on both external and internal 

micro pore surfaces. 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. The effect of coal surface area 

on spontaneous heating was investigated in the previous 

study (Yuan and Smith, 2007). In this study, an average 

coal particle diameter of 10 cm (4 in.), with a surface-tovolume ratio of 36 m-1, is used in the simulations. The heat 

generated from oxidation is dissipated by conduction and 

convection, while the oxygen and oxidation products are 

transported by convection and diffusion. 

It is well known that the kinetic properties of coal 

oxidation have a major effect on the spontaneous heating 

process. Because the effect of coal kinetic properties on 

the spontaneous heating has been studied in the previous 

work (Yuan and Smith, 2008b), only a typical reactive bituminous coal was presented 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 coal layer. 

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 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) 

Estimation of GOB permeability 

The permeability and porosity distributions of the 

gob areas are based on geotechnical modeling of longwall 

mining and the associated stress-strain changes using fast 

lagrangian analysis of continua (FLAC) code. A simple relationship is used to estimate the changes in permeability 

in the caved rock based on the Kozeny-Carman equation:

 (1) 

where n is the porosity, k is the permeability, and f is 

a function. 

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. The permeability and porosity distributions in the gob are not uniform. 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. A detailed description of permeability calculation is given in Esterhuizen and Karacan 

(2007). These permeability and porosity distributions are 

used as input for the CFD program. The porosity profile 

in the gob is similar to the permeability profile, except the 

maximum and minimum values are 0.41 and 0.17, respectively. It is assumed that these permeability and porosity 

profiles do not change with the gob height. 

Simulation of face advance 

When the face is continuously moving, the gob volume is expanding. Since the gob permeability and porosity 

distributions are non-uniform and affected significantly 

by the face advance, the continuous gob advance will result in continuous changes of permeability and porosity 

distributions in the gob. Although it is possible to model 

the continuous face movement, it is difficult to model this 

face movement process with the continuous changing gob 

permeability and porosity. In this study, the face advance 

is simulated as a series of discrete movements. Instead of 

simulating the continuous face movement from location #1 

to location #2 in five days, it is assumed that the face moves 

instantaneously to location #2 and remains there for five 

days. This method is repeated for the moves to location 

#3, #4 and #5. At location #5, the face remains for 10 days 

because the distance between location #4 and #5 is 200 m 

(656 ft). Each time the face moves to a new location, the 

similar permeability and porosity profiles are applied to 

the new gob. Although this approximation method does 

not simulate the exact face movement, it still captures the 

nature of the dynamic face movement. 

FIGURE 4 

Temperature distribution (K) after five days with longwall face 

at location #3. 

Numerical modeling 

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. The physical model and mesh 

for the CFD simulation were generated using the mesh 

generator software, GAMBIT, from Ansys, Inc. 

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 -0.75 kPa (-3 in. water gauge) at the intake inlet 

at the headgate side, -0.87 kPa (-3.5 in. water gauge) at 

the return outlet at the tailgate side and -2.2 kPa (-9 in. 

water gauge) at the bottom of the bleeder shaft. Typical ventilation airflow rates for the Pittsburgh coal seam 

mines were used. 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 panel. The pressure drops 

through the two regulators located at the second and 
third headgate 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 three was adjusted to have 

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

Results and discussion 

The longwall face was assumed to be stationary at 

location #1. Simulations of spontaneous heating of coals 

with the stationary face at location #1 for 20 days were 

conducted, and the temperature distribution in the gob 

area is shown in Fig. 2. After 20 days, the maximum temperature was 476° K (203° C), occurring in the gob area 

at the tailgate corner of location #1. As shown in the previous simulations (Yuan & Smith, 2007 and 2008a), the 

temperature rise in the gob mainly occurred immediately 

behind the longwall shields and along the face. There was 

also a small temperature rise at the corner of the gob near 

the bleeder fan, approximately 350° K (77° C), because of 

higher airflow velocity. The face was then moved to location #2 and stayed there for five days. Figure 3 shows the 

temperature distribution in the gob five days after the face 

arrived at location #2. The maximum temperature at the 

tailgate corner of location #1 decreased from 476° K (203° 

C) to 345° K (72° C). Because of face advance to location 

#2, the permeability and porosity along the face area of 

location #1 decreased, leading to less airflow through this 

area.Therefore,the temperatures along the face of location 

#1 decreased. It is worthwhile to note that a temperature 

rise also occurred along the new face at location #2. When 

the face was moved to location #3, the temperatures along 

the face area of location #1 continued to decrease, as shown 

in Fig. 4, with the maximum temperature at 325° K (52° C) 

after five days.This occurred because of a further reduction 

in permeability and porosity in that area.At the same time, 

the temperatures increased several degrees along the new 

face at location #3. Figure 5 shows the temperature distribution in the gob five days after the face moved to location 

#4. The maximum temperature along the face of location 

#1 further decreased to 320° K (47° C), while temperature 

rise occurred along the new face. 

FIGURE 5 

Temperature distribution (K) after five days with longwall face 

at location #4. 

After the face was at location #5 for 10 days, the maximum temperature (315° K or 42° C) in the gob area was 

still at the tailgate corner of location #1, as shown in Fig. 6. 

FIGURE 6 

Temperature distribution (K) after 10 days with longwall face at 

location #5. 

The temperature decrease was just 5°, compared with the 

maximum temperature when the face was at location #4. 

This is summarized in Fig. 7, which shows the maximum 

temperature change in the gob versus time with the face 

movements. 

FIGURE 7 

Maximum temperature (K) versus time histories: longwall face 

began to advance when the maximum temperature reached 476° K. 

It should be pointed out that this maximum 

temperature was always at the tailgate corner of face location #1. The maximum temperature decrease exhibits 

three different stages.The first stage was a fast temperature 

decrease, then a moderate temperature decrease, followed 

by a slow temperature decrease. Interestingly, the change 

from the fast decrease stage to the moderate decrease stage 

occurred when the face was at location #2, while the change 

from the moderate decrease stage to the slow decrease 

stage occurred when the face was at location #3. When the 

face was at location #5, the maximum temperature was 

also in the slow decrease stage as the face was at location 

#4, indicating that the face locations had little effect on 

the maximum temperature decrease once it was in slow 

decrease stage. 

Spontaneous heating can begin at ambient temperature when coal is exposed to oxygen. As the self-heating 

proceeds, the coal temperature increases slowly. The temperature rise usually consists of two stages. The first stage is 

a slow temperature rise while the second one is a fast temperature rise. The start of the second stage is also called 

thermal runaway. The time to reach a thermal runaway is 

called induction time. When the coal temperature reaches 

about 500° K (227° C), the spontaneous heating mechanism changes to rapid combustion (Babrauskas, 2003). So 

it is important to prevent the spontaneous heating from 
proceeding into a thermal runaway. As shown in Fig. 7, 

the thermal runaway occurred when the coal temperature 

reached about 350° K (77° C) under conditions studied 

here. Additional simulations were conducted that moved 

the face to location #2 when the maximum temperature 

at the tailgate corner of face location #1 reached about 

350° K (77 °C). Figure 8 shows the maximum temperature 

change in the gob versus time. 

FIGURE 8 

Maximum temperature (K) versus time histories: longwall face 

began to advance when the maximum temperature reached 350° K. 

The maximum temperature decrease still went through three stages, indicating the 

value of the initial maximum temperature did not affect 

the pattern of maximum temperature decrease. However, 

these results show that by moving the face, which effectively changes the permeability of the coal in the heated 

area, the spontaneous combustion can be controlled. This 

also demonstrates the effect of face advance rate on the 

spontaneous combustion process. 

Conclusions 

CFD simulations were conducted to investigate the 

effect of longwall face advance on spontaneous heating of 

coals in longwall gob area. Simulation results demonstrate 

that under typical three-entry bleeder ventilation conditions, face advance can quickly reduce the maximum temperature in the gob developed during face stoppage. The 

maximum temperature decrease follows three stages: fast 

decrease, moderate decrease and slow decrease. Once the 

maximum temperature is in the slow decrease stage (below 

330° K (57° C) in this study), the face advance no longer 

has a significant effect. It is also found that the value of the 

maximum temperature prior to the face advance has no 

effect on the pattern of maximum temperature decrease. 

Although the continuous face advance was simulated as 

a series of discrete movements, the results show the main 

features of the dynamic face advance. More research is 

needed to further improve the modeling of face advance 

in the future. 

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. 

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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