USE OF CFD MODELING TO STUDY INERT 

GAS INJECTION INTO A SEALED MINE AREA 

Michael A. Trevits, NIOSH, Pittsburgh, PA, 

Liming Yuan, NIOSH, Pittsburgh, PA, 

Michael Thibou, On Site Gas Systems, Newington, CT and 

Guy Hatch, On Site Gas Systems, Newington, CT 

Abstract 

Since the promulgation of the MINER Act and the 

follow-up changes to the regulations governing mine seal 

construction and maintenance, mine operators must be 

acutely aware of the atmosphere in sealed mine areas and 

prepared to deploy control technologies when the 

conditions are warranted. On Site Gas Systems1

1 Mention of a company name or product does not constitute an 

endorsement by NIOSH. 

 in 

Newington, Connecticut was awarded a contract by the 

National Institute for Occupational Safety and Health 

(NIOSH) to construct a novel in-mine nitrogen (N2) 

generation plant using pressure swing adsorption (PSA) 

technology. PSA technology utilizes carbon molecular 

sieve material and pressure to adsorb oxygen (O2) 

molecules while allowing N2 molecules to pass through 

the sieve material. The prototype PSA unit was tested at 

the NIOSH Safety Research Coal Mine (SRCM) where a 

62,000 ft3 area of the mine was rendered inert during a 

series of two tests. The resultant data were then used to 

construct a CFD simulation of both injection tests. In 

addition, O2 depletion and gas leakage rates were 

quantified in the model and were compared to actual 

values. Once the model was calibrated, the sealed mine 

area was doubled and simulations were made with various 

injection rates and injection site locations to determine the 

impact on the time needed to render this area inert. This 

paper presents an overview of the PSA plant, the details 

of the gas injection tests and CFD modeling work. 

Disclaimer: The findings and conclusions in this report are those 

of the authors and do not necessarily represent the views of 

NIOSH. 

Introduction 

The concept of monitoring, sampling, and 

maintaining an inert atmosphere in sealed mine areas is a 

relatively new practice for the US coal mining industry. 

Subsequent to the enactment of the Mine Improvement 

and New Emergency Response Act of 2006 (PL 109-236) 

also known as the MINER Act, the Mine Safety and 

Health Administration (MSHA) issued mandatory health 

and safety standards relating to the sealing of abandoned 

areas in underground coal mines. On May 22, 2007, 

MSHA issued an Emergency Temporary Standard (ETS) 

addressing the strength, construction, maintenance, and 

repair of mine seals [1]. The ETS also included 

requirements for sampling and controlling the atmosphere 

behind seals. Because of the requirements of the ETS, 

mine operators had to seek technology to render and 

maintain a sealed mine area inert. On April 18, 2008, 

MSHA published a final rule that superseded the ETS. 

The final rule addressed seal strength design, 

construction, maintenance and repair of seals and 

monitoring and control of the atmosphere behind seals to 

reduce the risk of seal failure and the risk of explosions in 

abandoned areas of underground coal mines [2]. Under 

current US federal law (30 CFR §75.336 - except as 

provided in §75.336(d)), the atmosphere in the sealed 

mine area is considered inert when the O2 concentration is 

less than 10% or the methane (CH4) concentration is less 

than 3% or greater than 20% [3]. When it is determined 

by sampling that the atmosphere in a sealed area is greater 

than 10% O2 or CH4 gas concentration is between 3% and 

20%, the mine operator must, by law, take immediate 

action to restore an inert sealed atmosphere. Furthermore, 

when additional sampling indicates that the O2
concentration is 10% or greater and CH4 is between 4.5% 

and 17%, persons shall be withdrawn from the affected 

area which is the entire mine or other affected area 

identified by the operator and approved by the MSHA 

District Manager [4]. 

 A sealed mine area can be rendered inert through the 

natural accumulation of CH4, over time, to levels beyond 

the explosive range, oxidation of the coal and possibly 

other materials (thereby removing O2 and releasing 

carbon dioxide (CO2) and the injection of N2 into the 

sealed mine areas to quickly reduce the O2 content to a 

level that will not support combustion. In the US, the 

available sources of N2 gas for use by mine operators 

includes liquid or gaseous N2 that is trucked to a mine site 

in tankers, on site cryogenic plants that separate the 

components of air through rectification, or technologies 

that separate and extract N2 gas from the atmosphere 

using hollow fiber membranes or pressure swing 

adsorption (PSA). 

Under a program created by the MINER Act, the 

National Institute for Occupational Safety and Health 

(NIOSH) awarded a contract to On Site Gas Systems in 

Newington, Connecticut to design and construct an inmine N2 generation plant. The objective of this effort was 

to build a high volume, high purity PSA N2 generator that 

could operate in an underground mine. PSA technology 

was selected because these systems are 12.5% more 

efficient than the alternative membrane systems in terms 

of the ratio of feed air required to N2 gas produced. PSA 

systems operate over a broad range of incoming feed air 

temperatures without impacting the efficiency, thus 

external air heaters are not required. Electrical 

requirements for a PSA system are minimal and thus 

operating costs are less than a comparable membrane 

system. Also, the lifespan of a N2 generating system 

using PSA technology is indefinite with regular 

maintenance and avoidance of oil and water 

contamination. 

PSA technology uses a carbon molecular sieve 

material and pressure to adsorb O2 molecules while 

allowing N2 molecules to pass through the sieve material. 

A typical PSA unit has at least two adsorber beds filled 

with the carbon molecular sieve material. During each 

half cycle, one adsorber bed produces N2 while the other 

is purged of O2. Compressed air (feedstock gas) is used 

to pressurize the adsorber sieve beds and, during this 

process, the smaller O2 molecules of the feedstock gas are 

adsorbed by the sieve material while the larger N2

molecules float free (figure 1). 

Figure 1. Conceptual drawing of PSA Technology. 

 Once an adsorber bed is 

saturated with O2, a bit of the gas pressure is released in 

the bed to draw off the N2 molecules. The N2 molecules 

are then collected in a surge/storage tank for use. A valve 

is then opened in the saturated adsorber bed which 

releases all of the pressure, forcing out the captured 

molecules of the unwanted gases, and cleanses the sieve 

for the next cycle (the molecules of the released gas 

immediately diffuse back into the atmosphere at 

essentially the ambient percentages). This cycle repeats 

continuously and with the use of multiple adsorber beds, 

working at opposing ends of the cycle, and a 

storage/surge tank, a consistent flow of N2 gas is achieved 

[5]. 

Under this effort, On Site Gas Systems conceived and 

designed a completely new concept in PSA sieve bed 

design which enabled a substantial decrease in sieve bed 

heights while still maintaining sufficient N2 gas 

production. The N2 generating system is capable of 

producing up to 300 scfm of N2 at 50 psig (figure 2). 

Currently, the electrical requirement for the N2 generating 

system is 110 Vac and approximately 10 A, but the unit 

could be reconfigured to operate on some other type of 

power supply. The feed air requirement is 600 scfm air at 

minimum 90 psig with a temperature between 32° F and 

100°F and a dew point of less than 40oF non-condensing. 

In order to facilitate transportation and movement 

within a mine, it was determined that the N2 generating 

system had to fit within the confines of a standard-sized 

shield car transport and also within height of typical 

entries of mines operating in the Pittsburgh Coalbed 

(about 6 ft). These constraints resulted in overall unit 

dimensions of 206 in (length) by 84 in (wide) by 44 in 

(height). A complete description of the development of 

the N2 generating system is available in [6]. 
Figure 2. Novel PSA N2 generating system. 

On Site Gas Systems also designed and built a skidmounted air dryer/filter that modifies the feed air for the 

PSA plant for circumstances in which feedstock air is 

unable to meet required input temperatures and dew point 

(figure 3). 

Figure 3. N2 generating system with air dryer.

 The air dryer/filter was also designed to meet 

dimensional constraints mentioned above and would be a 

separate, ancillary part of the N2 generating system. 

Currently, the air dryer/filter design calls for 240 V, 3-

phase power, but it could be reconfigured to operate on 

single-phase 110 Vac or some other available power 

supply. 

In-House Testing 

The N2 system was tested in-house at the On Site Gas 

System facility at specific maximum and minimum design 

parameters. Testing involved extended, continuous 

operation at maximum input and output rates. The tests 

showed that the system was able to maintain gas flow at 

approximately 300 scf/min with a purity of below 5% O2

content given feedstock air input of 600 scf/min across an 

input pressure range of 90-140 psig. From the in-house 

tests subtle changes were made to maximize performance 

of the system as well as to simplify operation and 

maintenance. 

Field Testing – NIOSH Safety 

Research Coal Mine 

The N2 generation plant was delivered to the SRCM 

located near Pittsburgh, Pennsylvania for a series of tests. 

The SRCM is a room-and-pillar operation approximately 

the size of a working section of a coal mine and is utilized 

for mine health and safety research in areas such as 

ground control, ventilation, fires, explosives use, 

materials handling, and environmental monitoring [7]. 

A 62,000 ft3 area of the SRCM was selected as the 

area to be sealed and rendered inert by the N2 generation 

plant (figure 4). 

Figure 4. Layout map showing sealed mine area gas 

and CFD calibration simulation sample points. 

 This area included two long entries with 

6 intervening cross-cuts. The area was isolated from the 

rest of the mine by using an existing 3-ft thick concrete 

seal and 2 newly constructed ventilation seals. To 

observe the progress of the inerting process, a gas 

sampling array was installed in the area being sealed. Gas 

from the N2 generation plant was injected into the sealed 

mine area during the tests through a 1-in diameter line 

that was placed into and through a water trap. The N2

generation plant was positioned outside of the mine on a 

flat bed tractor trailer along with a dryer and compressor. 

The compressor provided the feedstock air for the N2

generation plant and the dryer insured that the feedstock 

air was free of oil (from the compressor) and moisture. 

Gas from the N2 generation plant was piped to the sealed 

mine area using 450 ft of 1.5-in diameter fire hose. The 

N2 generation plant was not placed in the mine because a 

shield car transport was unavailable at the time of the 

tests. Also, the SRCM does not produce much gas so the 

components of the mine atmosphere are essentially the 

same as the normal atmosphere, thus operating the unit 

inside or outside of the mine would produce similar 

results. 
The first test was designed to determine if the PSA 

plant could render the sealed mine area inert and if the 

inert environment could be maintained. The test area was 

set up to inject inert N2 gas through ventilation seal 1 

(refer to figure 4). The first portion of Test 1 was 

designed to determine if coal in the sealed mine area 

would oxidize and start to self inert by using up the 

available O2. After being sealed for 36 hrs, it was 

determined that the sealed area would not self-inert 

quickly, and N2 injection operations were initiated. The 

PSA plant was then operated continuously for 23.3 hrs 

(figure 5). 

Figure 5. Average O2 concentration in the sample 

array during test 1. 

Shortly after N2 injection was initiated at ventilation 

seal 1, it was noticed that the pressure within the sealed 

area was not increasing, but remained constant; indicating 

that gas was escaping from the sealed area at the same 

rate it was being injected. This leakage was confirmed 

during an underground inspection and showed that gas 

was escaping from the mine roof areas above each 

ventilation seal but not through the seal itself, the ribs or 

the mine floor. After 23.3 hrs of injecting gas at an 

average rate of 280 scfm at 50 psig, the O2 content over 

entire sealed area was reduced to an average of 5.2% 

(maximum value 5.4%) over the array which was near the 

O2 level being produced by the PSA unit. The test was 

completed and the sealed mine area was opened and 

ventilated. 

The second test was designed to determine if the PSA 

plant could render the sealed mine area inert, maintain the 

inert environment, and to observe inerting frequency and 

duration as well as the time period between cycles 

(periods of gas injection followed by a periods of 

equilibration). During this test all gas injection was 

through ventilation seal 2 (refer to figure 4). Once the 

mine area was re-sealed, inerting operations were 

initiated. For the first part of Test 2, the PSA plant was 

operated continuously for 14 hrs at an average rate of 272 

scfm at 49 psig. Injection operations were then stopped 

and the sealed area was allowed to equilibrate (through 

the inward leakage at ventilation seal 2) until the O2 level 

reached 8% in the sealed area 10.5 hrs later. Injection 

operations were then re-started and continued for 5.8 hrs 

at an average rate of 276 scfm at 49 psig. At that point, 

the O2 level throughout the sealed mine area had been 

reduced to below 7%. Injection operations were then 

stopped again and the O2 content of the sealed area 

increased (through inward leakage) to 7.5% some 5.5 hrs 

later. Injection operations then resumed and continued 

for 2 hrs at an average rate of 269 scfm at 49 psig. At that 

point, the O2 level in the sealed mine area had been 

reduced to below 7%. The sealed area was then allowed 

to equilibrate (through inward leakage) for 10 hrs when 

the test was completed. The maximum average O2 value 

observed in the sample array at the time of completion 

was 8.3% (figure 6). A detailed description of the tests 

and discussion of the test results can be found in [6]. 

Figure 6. Average O2 concentration in the sample array 

during test 2. 

Computational Fluid Dynamics (CFD) 

Modeling 

In order to estimate the capability of the N2

generating system without the expense and time required 

to conduct a large scale underground experiment at an 

operating coal mine, it was decided to construct a CFD 

model of the same dimensions and geometry as the field 

test at the SRCM. Once the model was calibrated, using 

the data from the field test, then the size of the mine could 

be expanded and the volume of inert gas injected changed 

to measure the resultant effects at two selected monitoring 

points in the model. 

CFD is a sophisticated computationally-based design 

and analysis technique that enables users to simulate 

flows of gases and liquids, heat and mass transfer, 

multiphase physics, chemical reaction, and fluid-structure 

interaction through computer modeling. Using CFD, the 

user then builds a 'virtual prototype' of the system or 

device and then applies real-world physics and chemistry 
to the model to generate images and data, which predict 

the performance of that design [8]. 

The injection and migration of N2 in the SRCM was 

modeled using the Fire Dynamics Simulator (FDS), a 

computational fluid dynamics (CFD) program developed 

by National Institute of Standards and Technology. FDS 

is a three-dimensional, large eddy simulation model that 

was developed to study the transport of smoke and hot 

gases during a fire in an enclosed space. It was believed 

by the authors that this model would be useful to analyze 

inert gas movement. FDS is the most widely used large 

eddy simulation model in the fire science field and has 

demonstrated good agreement with experimental data in 

numerous validation studies. The model uses finite 

difference techniques to solve the Navier-Stokes 

equations numerically for fluid flow with a mixture 

fraction combustion model. 

 In the simulations, N2 gas leakage at the ventilation 

seals (as observed during the SRCM field tests) was 

replicated by creating a very small rectangular opening in 

the seal. The size of the opening was reduced to the point 

where leakage through the seals matched the N2 gas 

injection rate and the measured differential pressure 

across the seals. The gas leakage rate was held constant 

in the model. Also, no attempt was made to simulate the 

periods in the tests at the SRCM where the N2 injection 

was stopped. Figures 7 to 10 show a comparison between 

the actual field data from the test at the SRCM and the 

CFD model for two select points. 

Figure 7. Comparison of SRCM Test 1 data and CFD 

model for sample point A. 

Figure 8. Comparison of SRCM Test 1 data and CFD 

model for sample point B. 

Figure 9. Comparison of SRCM Test 2 data and CFD 

model for sample point A. 

Figure 10. Comparison of SRCM Test 2 data and CFD 

model for sample point B. 

 The gas injection rate 

and injection point for the first simulation was the same as 

that used in Test 1 (ventilation seal 1) and similarly the 

rate and injection point was the same as used in Test 2 

(ventilation seal 2) for the second simulation. Sample 

point A was located in a closed cross-cut while sample 

point B was in the preferred N2 gas flow path as shown in 

figure 4. These points were selected for comparison with 

the model because they were far away from the N2 gas 

injection points. Note there is close agreement between 

the SRCM data and the output of the model. 
In the next series of simulations, the mine volume 

was approximately doubled (figure 11) and the N2 gas 

injection rate at ventilation seal No. 1 was incrementally 

increased in the model from 1 to 5 times (from 280 to 

1,400 scfm) to observe the time needed to reduce the O2

level in the simulated mine to below the 10% statutory 

limit. 

Figure 11. Map of simulated mine in CFD model. 

 Figures 12 and 13 show the reduction of O2 levels 

in the simulated mine at sample points C and D (figure 

11). 

Figure 12. Multiples of the N2 injection rate and model 

results at sample point C. 

Figure 13. Multiples of the N2 injection rate and model 

results at sample point D.

A horizontal red line was added to plots to signify 

the 10% O2 level. The simulations were stopped at an O2 

concentration of 9.2%. Note the reduction in the time 

needed to reduce the O2 level in the simulated mine is not 

substantially reduced with each incremental increase in 

rate of N2 gas added. 

Figure 14 shows a graph of the average decrease in 

the time needed to reduce the O2 level for sample points C 

and D to just below 10% as a function of multiples of the 

base injection rate (280 scfm). 

Figure 14. Time needed to reduce the O2 level to below 

10% in the simulated mine as a function of increasing the 

N2 gas injection rate above the base rate used in the 

CFD model (280 scfm). 

 As can be observed in the 

graph, the reduction in time is not linear and the benefit of 

increasing the injection rate diminishes. It should be 

noted, that under the conditions studied in the model, a 

50% reduction in the time needed to reduce the O2 level 

to just below 10% (from 15 to 16 hrs to around 8 hrs) was 

achieved from a fivefold increase in the injection rate 

(from 280 to 1,400 scfm. 
Figure 15 shows a comparison of simulations where 

the mine volume was doubled and the N2 gas was injected 

at both ventilation seals simultaneously versus a two fold 

increase in the injection rate (from 280 to 560 scfm) only 

at ventilation seal 1 (for sample points C and D).

Figure 15 Comparison of N2 injection at both ventilation 

seals simultaneously compared to a two fold increase in 

the injection rate only at ventilation seal 1 

(for sample points C and D). 

 As can 

be observed from the plot, the time to lower the O2

concentration in the sealed area below 10% was decreased 

by about 2.5 hrs when the gas was pumped into both 

seals. Most likely the injection of N2 simultaneously at 

two different locations altered the gas flow pattern in the 

model resulting in a more rapid decrease in the O2 gas 

concentration. 

Summary and Conclusions

On Site Gas Systems conceived and built a 

completely new concept in PSA sieve bed design which 

enabled a substantial decrease in sieve bed heights yet 

still maintain sufficient N2 gas production. The N2 gas 

generating system is intended for use in the underground 

mine environment. The design of the unit included 

considerations for minimizing power requirements, 

sturdiness of construction, ease of use, ease of 

maintenance, reliability and transportability. 

The N2 gas generation system was tested in-house by 

On Site Gas Systems at the NIOSH SRCM where the 

atmosphere in a 62,000 ft3

 area of the mine was rendered 

inert during a series of two tests. Furthermore it was 

demonstrated that the system could successfully maintain 

an inert gas environment. 

A CFD model of the NIOSH SRCM test layout was 

constructed and a good match was achieved with the field 

data collected from the SRCM tests. The most difficult 

challenge in the model design was gas leakage in the 

vicinity of the ventilation seals. Once the model was 

calibrated, the volume of the model mine was doubled 

and simulations were made using successive increases in 

the rate of gas injection. 

The results of the CFD simulations show that with 

multiple increases in the N2 gas injection rate, the average 

percent decrease in the time needed to reduce the O2 level 

to just below 10% in the simulated mine is not linear and 

the benefit of increasing the injection rate diminishes as 

the N2 injection rate is increased. Furthermore, 

simulations show that N2 gas injection at both seals 

simultaneously reduces the O2 content in the sealed area 

below the statutory limit more quickly as compared to 

doubling the injection rate at one seal location. 

Acknowledgments 

The authors would like to recognize and thank Paul 

Stefko, Richard Thomas, John Soles, Joseph Sabo, Jack 

Teatino, James Addis, Frank Karnak, and Don Sellers (all 

from NIOSH) and John Garro and Scott Fitzgerald from 

On Site Gas Systems for the valuable help and assistance 

during the set up and conduct of the tests at the NIOSH 

SRCM. The authors would also like to acknowledge the 

valuable input and knowledge of Sanh Phan, Sean 

Haggerty, Robert Wolff and Glenn Scheffler (all from On 

Site Gas Systems) in the design of the system. 

References

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