NUMERICAL STUDY ON SPONTANEOUS COMBUSTION OF 

COAL IN U.S. LONGWALL GOB AREAS 

Liming Yuan1

1

National Institute for Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Road, Pittsburgh, PA 15236, Lcy6@cdc.gov

and Alex C. Smith2 

2

National Institute for Occupational Safety and Health, P.O. Box 18070, 

626 Cochrans Mill Road, Pittsburgh, PA 15236, Aos6@cdc.gov

ABSTRACT 

In order to reduce fire hazards caused by spontaneous combustion in longwall gob 

areas, a computational fluid dynamics (CFD) study is being conducted by National 

Institute for Occupational Safety and Health (NIOSH) to simulate the spontaneous 

heating of coal in longwall gob areas. A CFD model was first developed to model 

the spontaneous heating process in a two longwall panel district using a bleeder 

ventilation system with a stationary longwall face. 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. Heat 

generated from coal oxidation is dissipated by convection and conduction, while 

oxygen and oxidation products are transported by convection and diffusion. CFD 

simulations were then conducted to model the spontaneous heating in longwall gob 

area with a bleederless ventilation system. A single longwall panel with a bleederless 

ventilation system was simulated. The effect of gob permeability on the spontaneous 

heating was also investigated. 

1. INTRODUCTION 

Spontaneous combustion continues to pose a hazard for U.S. underground coal 

mines, particularly in western mines where the coal is generally of lower rank. 

Approximately 15% of the 164 total reported fires for underground coal mines for the 

period 1978 – 1990 were caused by spontaneous combustion. For the period 1990 – 

2006, totally 25 reported fires for underground coal mines were caused by 

spontaneous combustion (DeRosa, 2004). The spontaneous heating of coal in 

underground mines often occurs in a gob area and may not be easily detected. 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. Although 

much research has been done in experimental study and mathematical modeling of 

spontaneous combustion of coals, most of the research was mainly focused on small 

size coal stockpiles. 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 CFD study of gob gas flow mechanics to develop gas and spontaneous combustion 

control strategies for a highly gassy mine. 

In order to reduce the fire hazard from spontaneous combustion of coal in gob areas, 

NIOSH has conducted a series of computational fluid dynamics (CFD) simulations 

(Yuan and Smith, 2007; Smith and Yuan, 2008). A CFD model was first developed 

to simulate the spontaneous heating of coals in a two-panel gob area using a bleeder 

ventilation system with a stationary longwall face. CFD simulations were also 

conducted to model the spontaneous heating in longwall gob area using a bleederless 

ventilation system with a stationary longwall face. In this paper, CFD modeling 

results for both bleeder and bleederless ventilation systems are presented and the 

difference between two systems is discussed. 

2. GOB LAYOUT AND VENTILATION SYSTEMS 

2.1 Bleeder Ventilation System 

In the U.S., most underground coal mines utilize a bleeder ventilation system to 

ventilate the mined out panels and the gob area of the active panel. In our study, this 

situation is simulated with two panels, one as a mined-out panel and the other one as 

an active panel, utilizing a bleeder system ventilated by a bleeder fan. The layout of 

the two panels and the ventilation system is shown in Figure 1. Each 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. Panel A represents the 

completed panel, while panel B represents the active one. The ventilation scheme 

uses a three-entry gateroad system. In the model, it is assumed that the middle entry 

between panel A and B and an entry on panel A’s tailgate side are partially open. For 

modeling purposes the bleeder entries at the back end of the gob are represented as 

one entry connecting to the bleeder fan. Four regulators are located at the end of the 

second and third entries inby the longwall face and the two tailgate entries, 

respectively, for controlling the bleeder ventilation. 

The ventilation conditions used in the simulation were based on a local Pittsburgh 

coal seam mine’s ventilation data. The pressure was -3.0 inches water gauge at the 

intake inlet, -3.5 inches water gauge at the return outlet, and -11.0 inches 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 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 60,000 

cfm. The pressure drop at regulator 3 was adjusted to have a flow rate at the return 

of 50,000 cfm, and the flow rate in the entry on panel A’s tailgate side was 7,000 cfm 

by adjusting the pressure drop at the regulator 4. 

 

 

regulator 4 bleeder fan 

return 

regulator 1 

Panel A (mined out) regulator 3 

Panel B (active) 

regulator 2 

face 

intake 

A Tailgate 

A Headgate 

B Headgate 

B Tailgate 

Figure 1: Layout of two longwall panels with a bleeder ventilation system. 

2.2 Bleederless Ventilation System 

In the U.S., bleederless ventilation systems may be approved by the Mine Safety and 

Health Administration (MSHA) 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. In a bleederless 

ventilation system, the previously mined-out panels are usually isolated from the 

active gob and the remainder of the mine. In our study, only the active panel was 

simulated. The layout of the panel and the ventilation system is shown in Figure 2. 

 

return 

intake 

face gob 

headgate 

tailgate 

Figure 2: Layout of single longwall panel with a bleederless ventilation system. 

The simulated gob area is also 2,000 m long, 300 m wide and 10 m high. 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 

were treated as though they were collapsed. 

Typical ventilation pressures for the bleederless ventilation system were used in the 

simulation. The pressure was -3.0 inches water gauge at the intake inlet, -3.5 inches 

water gauge at the return outlet. To control the air flow quantity to the longwall face, 

the wall roughness was adjusted to have a realistic intake airflow rate of 64,000 cfm. 
3. MODELING LOW-TEMPERATURE COAL OXIDATION 

A detailed description of the low temperature coal oxidation process used in these
 

simulations is described in our previous study (Yuan and Smith, 2007). Here, only a
 

brief description is provided. 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 varies 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 roughly 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 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 activation energy and pre-exponential factor were
 

measured using an adiabatic heating oven (Smith and Lazzara, 1987). 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, and is about
 

0.61 for some U.S. coals. 

The coal source for spontaneous heating can be coal left from the mined coal seam or 

other overlying or underlying coal seams. In our study, a 2-m-thick main coal seam 

was considered, with a 1-m-thick rider sequence 1 m above the main coal seam. The 

rider seam was modeled as caving into the bottom of the gob after the main coal 

seam was completely mined out. The oxidation of coal will occur on any available 

coal surface. Surface area available for coal oxidation in a unit volume, or surfaceto-volume ratio, is calculated based on coal particle size and gob porosity. The 

physical and kinetic properties of the coal used in the simulation are listed in Table 1. 

Table 1. The physical and kinetic properties of the coal layer 

Coal density 1300 3 kg/m

Coal specific heat 1003.2 J/kg-K 

Coal conductivity 0.1998 W/m-K 

Heat of reaction 300 kJ/mol-O2 

Activation energy 66.5 kJ/mol 

Pre-exponential factor 6×104 /s 

Coal particle diameter 10 cm 

Initial coal temperature 300 K 
 

4. NUMERICAL MODELING 

A commercial CFD program, FLUENT1

1 Reference to a specific product is for informational purposes and does not imply endorsement by NIOSH 

, 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 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 permeability and porosity distributions of the gob area were 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. For a Pittsburgh coal seam longwall panel, the permeability values in the gob 

area were 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 were the largest, while near the center of the gob, 

these values were the smallest due to compaction. A detailed description of the 

permeability calculation is given in Esterhuizen and Karacan (2007). The porosity 

profile in the gob was similar to the permeability profile except the maximum and 

minimum values are 0.41 and 0.17, respectively. 

5. SIMULATION RESULTS AND DISCUSSION 

The flow patterns inside a gob have a significant effect on the spontaneous heating of 

coals because the oxygen needed for the oxidation is provided by the air flow, and 

the heat generated from the oxidation may be carried away by the air flow. The air 

flow inside a gob is expected to be three dimensional with the flow in the vertical 

direction being weaker than in the other two directions due to reduced permeability 

and pressure gradients. In order to visualize the flow patterns inside the gob, a 

virtual horizontal reference surface was created 1 m from the bottom of the mined 

seam floor. Steady-state simulations were first conducted without coal oxidation to 

determine the flow patterns in the gob. Figure 3 shows the flow path lines colored by 

velocity magnitude in the two gob areas with the bleeder ventilation system. 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 

the tailgate area and flowed to the middle entry between the two panels. The higher 

velocities of air traveling through the gob itself were between 1.5x10-4 to 2.5x10-4 

m/s (0.03 to 0.05 fpm) near the back end of panel B. 

The flow path lines colored by velocity magnitude in the gob area for the bleederless 

ventilation system are shown in Figure 4. The path lines show that flow through the 

gob itself was mainly concentrated behind the shields. At the headgate side, air 

leaked through the shields but some flowed back into the face again through the 
shields near the tailgate side. The air velocity ranged between 1.0x10-5 to 3.0x10-5 

m/s (0.002 to 0.006 fpm) in the gob near the shields, and there was nearly no flow 

farther away from the shields into the gob. 

 

 

Figure 3: Flow path lines colored by velocity magnitude (m/s) in gob areas for the 

bleeder ventilation system. 

 

tailgate 

face 

headgate 

Figure 4: Flow path lines colored by velocity magnitude (m/s) in gob area for the 

bleederless ventilation system. 
Unsteady state simulations were then conducted to model coal oxidation. Figure 5 

shows the temperature distribution in the two panels with the bleeder ventilation 

system after about 9 days.

 

Area I 

Area III 

Area II 

Figure 5: Temperature distribution (K) in gob areas with the bleeder system after 9 

days. 

 It is apparent that temperature increases only occurred in 

three areas. Area I is the area close to the active tailgate and around the return. Area 

II is the area nearby the crosscuts close to the back end of the active panel B, and 

area III is around the middle entry at the back end of the mined-out panel A. There 

was very little or no temperature rise in areas other than areas I, II and III. This is 

because nearly all the oxygen was consumed by coal oxidation at the periphery of the 

gob areas and little or no oxygen was available inside the gobs. At any given time 

during the simulation, there existed a maximum temperature somewhere in the gob 

area. From a fire safety point of view, this maximum temperature represents the 

worst-case scenario. Hereafter, this temperature is referred as the maximum 

temperature in the gob at a given time. 

Figure 6 shows the temperature distribution in the gob with the bleederless 

ventilation system after 20 days. The maximum temperature at that time was only 

302.8 K. The maximum temperature rise, 2.8 degree K, occurred in the corner of the 

gob between the face and the intake entry. There was very little or no temperature 

rise in the other areas. This is because the oxygen was nearly all consumed by coal 

oxidation at the intake corner of the gob. This is much different from the case with a 

bleeder ventilation system where a temperature increase was observed along the face 

and behind the shields in the gob. Figure 7 shows the maximum temperature versus 

time histories from the simulation for both bleeder and bleederless ventilation 

systems. With the bleederless system, the maximum temperature was only 302.8 K 

after 20 days, while the maximum temperature reached 500 K in 8 days with the 

bleeder ventilation system. 
 

headgate 

face 

tailgate 

Figure 6: Temperature distribution (K) in gob area with the bleederless system after 

20 days. 

The gob permeability also has a major effect on the spontaneous heating in the gob 

because it affects the quantity of air flowing into and through the gob. In order to 

examine the effect of the gob permeability on the spontaneous heating, simulations 

were conducted with the permeability increased 100 times for both ventilation 

systems. Figure 8 shows the maximum temperatures versus time histories for the 

increased permeability simulations for both bleeder and bleederless systems. With 

the increased permeability, the maximum temperature reached 500 K in about 22 

days compared to only 2.8 degree K temperature rise in the previous simulation for 

the bleederless system. Additionally, the induction time for the bleederless system 

simulation was reduced by 14 days compared with the bleeder system simulation. 

6. CONCLUSIONS 

CFD simulations were conducted to investigate the spontaneous heating of coals in 

longwall gob areas with both bleeder and bleederless ventilation systems. The 

spontaneous heating hazards of different coals were strongly dependent on mine 

ventilation conditions. Under typical three-entry bleeder ventilation conditions, 

simulations 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. Simulation results demonstrate that under typical 

ventilation conditions, the bleederless system would greatly reduce the rate of 

temperature rise in the gob compared with the bleeder system, and the temperature 

rise occurred only at the corner of the intake entry with the bleederless system. For 
the bleederless system with the increased permeability, the induction time to thermal 

runaway was reduced significantly. 

 

500 

Maximum temperature (K) 450 

400 

350 

300 

bleederless 

bleeder 

0 2 4 6 8 10 12 14 16 18 20 

Time (day) 

Figure 7: Maximum temperatures (K) versus time histories for both ventilation 

systems. 

 

 

500 

Maximum temperature (K) 450 

400 

350 

300 

bleederless 

bleeder 

0 5 10 15 20 25 

Time (day) 

Figure 8: Maximum temperature (K) versus time histories with gob permeability 

increased 100 times for both systems. 
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