Published in the December 2009 Coal USA Magazine

The company said the N-300 CMI, created and tested with the help of the National Institute for Occupational Safety and Health, was capable of producing 300 cubic feet per minute of N2 gas.

“The objective of that effort was to create a safe, reliable in-mine mobile plant that would extract N2 gas from the mine atmosphere to inert sealed mine areas,” Guy Hatch and Michael Thibou said in a recent findings report.

They noted the design used pressure swing adsorption technology and could be easily transported by a standard shield car or dolly.

Following the successful evaluation at the NIOSH Safety Research Coal Mine, a major coal company identified an opportunity to use the system to inert a sealed area that encompasses millions of cubic feet in an active mine.

The particular sealed area for this project borders on compliance during barometric pressure changes, and the company was seeking technology to maintain an inert atmosphere in the sealed area.

Hatch and Thibou said the technology was developed for situations when direct underground access could not be achieved via a borehole due to factors such as difficult ground conditions at the surface. In those cases, underground pipelines must be installed to deliver the nitrogen from the surface to needed locations underground.

For that reason NIOSH and OSG felt it may be beneficial and desirable to put a nitrogen generation plant underground or near the area of concern.

In its active mine testing, three troublesome seals, A, B and C, were selected for continuous N2 injection. An On Site model N-300 CMI nitrogen generator was used to inject approximately 300cf/m of gas at a rate of 95% purity or more into several sealed areas of the operation.

Evaluations completed two weeks prior to the injection test revealed four occurrences when the atmosphere behind the seals dropped into MSHA’s defined explosive range, resulting in three evacuations.

“Injection of N2 gas to the areas behind the seals A, B and C resulted in a net effect of lowering and maintaining the lower O2 concentration of the atmosphere behind the seal with zero evacuations or non-compliances,” the company found.

“Once nitrogen inerting started, Seal C stabilized below 10 per cent, and seal B stabilized below 8 per cent though there were a few readings over 10 per cent.”

Seal A, it turned out, was the most troublesome; despite most readings below 10% several went above that. In all but two cases, the cause was negative pressure differential causing an inflow of air.

OCG noted that seal geometry, location and total number are all important considerations, particularly in the case of the mine tested. There, it noted, geometry appeared to amplify air inflow and outflow at seal A during changes in barometric pressure.

“Nitrogen flow from the N-300 CMI proved sufficient to maintain an inert atmosphere behind the seals and prevented recurrences of mine evacuation as evidenced by similar barometric pressure changes before N2 injection which, in fact, did affect the atmosphere behind the seals to the point of falling into the explosive range and necessitating evacuation,” Hatch and Thibou noted.

“Injecting N2 near the seal provides an inert atmosphere, prevents work stoppages due to evacuations, improves safety and enhances mine productivity.”