USE OF PRESSURE SWING ADSORPTION TECHNOLOGY TO 

INERT SEALED MINE AREAS WITH NITROGEN 

Michael A. Trevits, NIOSH, Pittsburgh, PA, 

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

Thomas P. Mucho, Thomas P. Mucho and Associates, Washington, PA, and 

Guy Hatch, On Site Gas Systems, Newington, CT 

Disclaimer: The findings and conclusions in this report are those 

of the authors and do not necessarily represent the views of 

NIOSH. 

Abstract 

Mine seals are used in underground coal mines 

throughout the United States to isolate abandoned mining 

areas from the active mine workings. The objective of 

adding inert gas to a sealed mine area is to quickly reduce 

the oxygen concentration in the sealed area to a level that 

will not support combustion. In the US, this process is 

typically conducted from the ground surface and because 

surface-to-mine access is constrained by so many 

variables, it is very difficult to provide a source of inert 

gas to the place where it is needed underground. To 

alleviate this problem On Site Gas Systems, under a 

NIOSH contract, built a nitrogen (N2) generation plant 

that operates in the underground environment. The 

purpose of this work was to create a reliable in-mine 

mobile plant that would extract N2 gas from the mine 

atmosphere and use the gas to create and maintain a safe 

sealed mine area. The plant is based on a novel design 

using pressure swing adsorption technology and is sized 

to fit on a standard shield car or mine dolly for easy 

transport in and around an underground coal mine. The 

plant is capable of producing about 900 scf/min of N2 gas 

at ambient mine conditions. After an exhaustive series of 

laboratory tests, the plant was field tested at the NIOSH 

Safety Research Coal Mine (SRCM) where a portion of 

the mine was sealed and rendered inert. During the test, 

the movement of the injected gas was monitored at 

several underground points to observe the progress and 

extent of the inerting process. This paper describes the 

plant design and the results of the field test at the SRCM. 

Introduction 

Mine seals are used in underground coal mines 

throughout the United States to isolate abandoned mining 

areas from the active mine workings. Sealing is 

sometimes a more economical and possibly a safer 

alternative to ventilation since continued ventilation of 

abandoned areas of active mines is costly and may divert 

air away from other, more productive underground uses. 

Without sealing, large mined-out areas still require 

regular inspections and can expose miners to a variety of 

underground hazards [1]. Current federal mining laws 

permit sealing of mine areas providing the seals meet 

certain standards and the laws also require sampling of 

the atmosphere in the sealed area [2-4]. 

It is commonly known that in an underground coal 

mine, a methane/air mixture becomes explosive when a 5 

to 16% methane gas concentration is present with at least 

12% oxygen (O2) concentration [5]. The homogeneity of 

the methane gas/air mixture can vary depending on the 

elevation, the methane gas liberation of the sealed area 

and outside factors such as the temperature and 

barometric pressure [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 10.0% or the 

methane concentration is less than 3.0% or greater than 

20.0% [3]. When it is determined by sampling that the 

atmosphere in a sealed area is greater than 10% O2 or 

methane gas concentration is between 3% and 20%, the 

mine operator must, by law, take immediate action to 

restore an inert sealed atmosphere. Furthermore, when 

the additional sampling indicates that the O2 

concentration is 10% or greater and methane 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 [3]. 

Inerting of Sealed Mine Areas 

When a coal mine area is sealed, the atmosphere in 

the sealed area will change as coal, and possibly other 

materials, oxidizes (thereby removing O2 and releasing 

carbon dioxide (CO2) into the sealed mine atmosphere). 

Methane gas may also accumulate in the sealed area and 

will eventually enter and pass through the explosive 

range. According to Zipf et al, the time required for the 

atmosphere in the sealed area to pass beyond the upper 

explosive limit and become inert ranges from about 1 day 

to several weeks or more depending on the mine’s 

methane gas liberation rate. Shallow mines, mines that 

have breached into old workings, and sealed areas with 

high leakage rates may never become inert [1]. Other 

strata gas emissions, such as CO2, may also impact the 

atmosphere in the sealed area. 

The objective of adding inert gas to a mine area is to 

quickly reduce the O2 level to below 10% as mandated by 

law. Gas can be injected into an area at any time (prior to 

sealing, immediately after sealing, or long after the area 

has been sealed) depending upon underground conditions. 

In general, monitoring, sampling, and maintaining an inert 

atmosphere in sealed mine areas is generally a new 

concept and practice for the US coal mining industry. 

Exchange of the atmosphere between a sealed mine 

area and the active mine workings regularly occurs [6]. 

The common pathway for atmosphere exchange, or 

leakage, is around a mine seal, through fractures, bedding 

planes, and through other common or anomalous geologic 

features in the adjacent rock strata. Changes, in both the 

magnitude and the rate of change of barometric pressure 

affect how an atmosphere exchange occurs. In addition, 

the mine ventilation pressure difference between inert 

sealed areas and adjacent ventilated areas, at any point in 

time, will dictate the direction and rate of the atmosphere 

exchange as the pressures strive to equilibrate. For this 

reason, many mine ventilation engineers attempt to 

minimize this pressure differential. However, normal 

changes in barometric pressure will upset the balance 

between the sealed and ventilated area. The absolute 

pressure in the ventilated area is a function of both the 

ventilation system (with its controls) and barometric 

pressure changes. The pressure in the sealed area is a 

function of its pressure relative to the reservoir pressure 

of strata gases and any relief mechanisms that it may 

have, such as exchange through strata or seals with a 

ventilated area, the ground surface in very shallow 

circumstances, or boreholes to the surface [7]. 

Inerting Technology 

The typical available sources of inert gas for use by 

mine operators in the US 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). All of these sources of 

N2 are injected from the ground surface through boreholes 

or pipelines to the underground mine. Use of these 

sources requires surface road access (good surface roads 

in the case of trucked gas and cryogenic plants), sufficient 

lead time to obtain permission from surface property 

owners, time to build roads, install pipelines, construct 

surface locations and drill boreholes. When direct 

underground access cannot be achieved through a 

borehole because of difficult surface topography or other 

reasons, underground pipelines must also be constructed 

to deliver the gas from a borehole to the underground 

location. Of late, separation technologies have received 

favorable interest by the mining industry because these 

plants are more amenable to field deployment using 

earthmoving equipment or a helicopter. Because surface 

access to the underground mine is constrained by so many 

variables, it is very difficult to provide a source of inert 

gas to the place where it is needed underground. 

Therefore, it became very desirable to place a N2 

generating plant underground at or near the point of need. 

Under a program created by the Mine Improvement 

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

also known as the MINER Act, the National Institute for 

Occupational Safety and Health (NIOSH) awarded a 

contract to On Site Gas Systems in Newington, 

Connecticut (in conjunction with Thomas P. Mucho and 

Associates, Washington, Pennsylvania) to design and 

construct a novel in-mine N2 generation plant. The 

objective was to build a high volume, high purity PSA 

nitrogen generator for placement in an underground mine. 

PSA technology uses a carbon molecular sieve 

material and pressure to adsorb oxygen molecules while 

allowing N2 molecules to pass through the sieve material. 

Compressed air is used to pressurize a vessel filled with 

this carbon sieve material, which sifts the air molecules 

by physical composition or structure. By forcing air into 

this pressure vessel or sieve bed, O2 molecules are trapped 

in the sieve, while N2 floats free (figure 1). After a period 

of time, a bit of the pressure is released in the sieve bed to 

draw off the N2 molecules, and collect them in a 

surge/storage tank for use in the application. A valve is 

then opened in the sieve 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. With the 

use of multiple sieve beds, working at opposing ends of 

the cycle, and a storage/surge tank, a more consistent flow 

of N2 gas is achieved. 

Figure 1 Conceptual drawing of PSA technology. 

PSA N2 generating systems are 12.5% more efficient 

in terms of the ratio of feed air required to N2 gas 

produced than membrane systems. They operate over a 

broad range of incoming feed air temperatures without 

impacting the efficiency, thus no external air heaters are 

required and electrical requirements for the PSA system 

are minimal. Operating cost is less than a comparable 

membrane system. The lifespan of a N2 generating 

system using PSA technology is infinite with regular 

maintenance and avoidance of oil and water 

contamination. 

Novel PSA Plant Construction 

Under this effort, 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. One of the 

most stringent criteria for the design of the underground 

PSA N2 generating system was the overall 

dimensions/footprint of the unit. In order to facilitate 

transportation and movement within a mine, it was 

determined that the PSA system should fit within the 

confines of the width and length of standard-sized shield 

car as well as the height of typical mine entries operating 

in the Pittsburgh Coalbed. That resulted in an overall 

dimension of the unit of 206 in (length) by 84 in (wide) 

by 44 in (height). The system built is capable of 

producing up to 300 scfm of N2 at 50 psi (approx. 1,000 

scfm at ambient pressure) (figure 2). Currently, the 

electrical requirement for the prototype PSA N2 

generating system is 110V AC and approximately 

10 amps, but the unit could be reconfigured to operate on 

some other type power supply. The feed air requirement 

is 600 scfm air at minimum 90 psi with a temperature 

between 32° F and 100°F and a dew point of less than 

40oF non-condensing. 

Figure 2. Novel PSA N2 generating system. 

On Site Gas Systems also designed a skid-mounted 

air dryer/filter that modifies the feed air for the PSA plant 

for cases when the available feed air is unable to meet 

required input temperatures and dew point. The air 

dryer/filter was also designed to meet dimensional criteria 

regarding height and width limitations and would be a 

separate, ancillary part of the PSA N2 generating system. 

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

phase power, but could be reconfigured to operate on 

110V AC or some other available power supply. 

In-House and Field Testing of the PSA Unit 

at the NIOSH Safety Research Coal Mine 

Once the PSA unit was built, On Site conducted 

testing to ensure that the design requirements, as 

identified through interviews with mining experts, were 

achieved. The N2 system prototype was tested at specific 

maximum and minimum design parameters. Extended, 

continuous operation at maximum conditions was also 

completed and verified. The system was able to maintain 

approximately 300 scf/min at purity of below 5% O2 

content with air input of 600 scf/min across an input 

pressure range of 90-140 PSIG. From the test data, subtle 

changes were made to maximize performance as well as 

simplify operation and maintenance. The prototype N2 

system was then delivered to the NIOSH Safety Research 

Coal Mine (SRCM) located near Pittsburgh, Pennsylvania 
 

 

 

 

 

 

for a series of two tests. The Safety Research Coal Mine 

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 [8]. 

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

to be sealed and rendered inert by the PSA plant 

(figure 3). This area included two long entries with 6 

intervening cross-cuts. The area was isolated from the 

rest of the mine by an existing 3 ft concrete seal 

constructed some time in the past and 2 newly constructed 

ventilation seals. To observe the progress of the inerting 

process, a gas sampling array was installed in the area to 

be sealed. Some of the gas sample points were located in 

open entry areas or in an open equipment door while 

others were located in areas closed by ventilation seals or 

a mine seal. A gas sample point was placed outside the 

sealed mine area to monitor the mine atmosphere and a 

sample line was attached to the N2 injection line to 

monitor the quality of the injection gas. Gas from the 

PSA plant was injected into the sealed mine area during 

the tests through a 1-in diameter line that was placed into 

and through the water trap. The PSA 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 PSA plant and the dryer 

insured that the feedstock air was free of oil (from the 

compressor) and moisture. Gas from the PSA plant was 

piped to the sealed mine area using 1.5-in diameter fire 

hose. The PSA plant was not placed in the mine because 

a shield car 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. 

Test 1 

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 3). 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 36 hrs of sealing, it was determined 

that the sealed area would not self-inert quickly and N2 

injection operations were initiated. The PSA plant 

operated continuously for about 23.3 hrs, producing on 

average approximately 900 scf/min at ambient mine 

conditions, or about 1.28 million scf of N2 (figure 4). 

Figure 3. Layout map showing the gas sample points in 

the sealed mine area. 

Figure 4. Average O2 concentration in the sample 

array during test 1. 

Shortly after N2 injection was initiated, 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. 

Figure 5 shows a plot of O2 concentration reduction as a 

function of elapsed inert gas injection time. In the figure, 

the two important O2 thresholds mentioned earlier are 

highlighted. The first threshold shows that 4 hrs of inert 

gas injection time was needed to reduce the sealed area 

from the starting concentration of 19.6% to below 12% O2 

and that about 7.2 hr of gas injection time was needed to 

reduce the O2 content below 10%. After about 23.3 hrs of 

injecting gas, the O2 content over entire sealed area was 

reduced to 5.4% which was near the O2 level being 

produced by the PSA unit. It was also determined that 

because the O2 content could be reduced to such a low 
 

 

 

 

 

 

 

 

 

value relatively easily, maintaining the inert environment 

in the sealed area would not be difficult. The test was 

completed and the sealed area was opened and ventilated. 

Figure 5. Plot of O2 reduction in the sealed area for the 

initial gas injection segment of Test 1. 

Test 2 

The second test was designed to again determine if 

the PSA plant could render the sealed mine area inert, 

maintain the inert environment, and to also 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. 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 producing an average of 865 scf/min (at 

ambient mine conditions) and injecting about 

735 thousand sfc of N2. Pressure in the sealed mine area, 

similar to Test 1, did not increase indicating leakage was 

reoccurring. Figure 6 shows a plot of O2 concentration 

reduction as a function of elapsed inert gas injection time 

for the first part of Test 2 (note this data is very similar to 

that observed in Test 1, refer to figure 5). 

Figure 6. Plot of O2 reduction in the sealed area for the 

initial part of Test 2. 

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. This occurred at about 10.5 hrs 

later. Injection operations were then re-started and 

continued for 5.8 hrs, producing an average of 882 

scf/min (at ambient mine conditions) and injecting about 

303 thousand sfc of N2. 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 was allowed to 

increase to 7.5%. This occurred about 5.5 hrs later. 

Injection operations then resumed and continued for 2 hrs, 

producing an average of 865 scf/min (at ambient mine 

conditions) and injecting about 104 thousand sfc of N2. 

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 leakage) for about 10 hrs when the 

test was completed. The maximum O2 value observed in 

the sample array at the time of completion was 8.3%. 

Figure 7 shows the average O2 concentration in the 

sample array during test 2. 

Figure 7. Average O2 concentration in the sample 

array during test 2. 

Summary 

The objective of this effort was to build a high 

volume, high purity PSA nitrogen generator to provide 

mine operators with the ability to inert a sealed mine area 

for as long as needed without the potential obstacles that 

are associated with injection of N2 from the ground 

surface through a borehole. The PSA system was 

designed and built to operate at or near an underground 

sealed mine area. Under this effort, 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 PSA N2 system constructed under this 

effort was intended for use in the underground mine 

environment taking into account factors such as minimal 

power requirements, sturdiness of construction, ease of 

use, ease of maintenance, reliability and transportability. 

After undergoing a series of exhaustive tests at the On 

Site Gas Systems facilities, the unit was moved to the 

NIOSH SRCM for a series of two inerting tests, where it 
 

 

 

 

was demonstrated that the PSA N2 system could 

successfully reduce the oxygen concentration in the 

sealed mine area and maintain an inert gas environment. 

To fully understand the capacity and limitations of this 

novel inerting system, it is recommended that the system 

be further tested at an actual mine site, over a longer 

duration of time, with a larger sealed mine area. 

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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Explosion Pressure Design Criteria for New Seals in 

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18, 2008, Volume 73, Number 76, pages 21181­

21209. 

3.	 Code of Federal Regulations (CFR), 2008. Title 30, 

Part 75, Subsection 336. 

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HAActionsEnhanceMineSafety.asp 

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