Sealing, Monitoring and Low Flow Goaf Inertisation

This project tested the feasibility of the Low Flow Inertisation concept and successfully demonstrated that the Tomlinson Inert Gas Generator has considerable potential for the elimination of potential explosion hazards which may exist in some circumstances when areas of a mine containing flammable gases are sealed.   This summary details the work undertaken on the various phases of the project and provides reliable information on the observations made with a view to promoting thought, discussion, training and further research.

Aims and Objectives

The aims and objectives of this project were:

• To develop, test and refine a Sealing Management Plan. 
• To identify gas monitoring protocols which occur in a goaf, before, during and after sealing. 
• To demonstrate that low flow inertisation provided by a Tomlinson Inert Gas Generator, of a sealed area will:

• prevent the atmosphere behind the seals from entering the explosive range;
• be achieved without interruption to the normal production cycle;
• be cost effective. 

• To develop a computer model which will enable future predictions for the inertisation of mine areas using either external inert gas generation, natural processes or a combination of both.

Conclusions and Recommendations

The Low Flow Inertisation concept was successfully demonstrated at Cook Colliery during May / June 1997 and the overall result is very encouraging.

CRM is confident that the Tomlinson Inert Gas Generator is capable of eliminating potential explosion hazards and business interruption, which may exist in some circumstances, when areas of a mine containing flammable gas are sealed.

It must be recognised that the make or volume of flammable gas found in the 9 West Waste Workings at Cook and the 2 South District at Laleham Collieries, prior to the inertisation trials was relatively low, however in the case of Cook, the make or liberation of methane into the workings, should have been sufficient for an explosive mixture to occur, before the zone could self inert.

In both cases, the make or liberation of methane was much lower than expected and this raises a number of interesting points:

The Tomlinson Inert Gas Generator produced a positive pressure in the sealed area of about 300 pa at Cook and about 650 pa at the seals for the 3 South District at Laleham.

• It was noticed that this pressure increase was maintained regardless of diurnal variations in the barometer of up to 900 pa and it would appear that provided the ventilation pressure across the seals has been balanced effectively, then the barometric pressure has little if any effect.
• The overpressure indiced by the Tomlinson Intert Gas Generator is very small, being equivalent to 0.044 and 0.096 psi respectively or 1.22 and 2.66 inches of Water Gauge.  To put this into perspective, this force would be similar to that acting on a  man door in a stopping located between intake and return airways of an underground mine.
• There is no evidence to support a hypothesis that this overpressure suppressed the release or desorptioon of seam gas and in particular methane, but it is an interesting thought worthy of due consideration and research by the scientific community.
   

There is also no evidence to support a hypothesis that the methane was in fact, displaced. Rescue Team personnel could find no evidence of layering or stratification of gases and this was confirmed by numerous spot tests and bag samples taken from the floor, roof and mid seam heights in a number of roadways. It is probable that the make of seam gas and in particular methane, is much lower than that calculated by the accepted method.

• Evidence for this may be found in the high standard deviation recorded for the weekly, air free gas determinations.  CRM could not explain the wide variation in gas makes recorded over a long period for both places.  These variations could not be attributed to barometric variations however, there is some evidence to indicate that changes or reductions in the ventilation flow, do in fact reduce the make or liberation of seam gases.
• In support of this observation, the results from Cook and Laleham were compared with the ventilation rates and gas analysis results supplied by on of the larger longwall mines.
• These mines are from the longwall return, prior to sealing.

This raises the following questions:

• What is the true make of methane from this area ?
• What happens when we reduce the air flow further ?
• What happens when we stop the air flow altogether and induce a positive pressure in the goaf ?

We would recommend that additional research be conducted with a view to answering these very relevant questions and to prove that the Tomlinson Inert Gas Generator will in fact eliminate potential explosion hazards in those mines with a high flammable gas make.

Sealing Issues

Good sealing practice dictates that ventilation should be maintained throughout the panel until the intake and return airways are blocked off or sealed simultaneously.

In the past, we have attempted to achieve this by sealing non-critical intake and return airways first and then coordinate the sealing of the main intake and return airways.

The erection of seals that are designed and constructed in accordance with the Approved Standard require a great deal more preparation, materials and resources. Seals must be erected without a cold joint, and once started, they must be completed.

In reality, when two seals of this type are to be erected simultaneously, we face major challenges. In addition to the timing and resource problems, these seals require some time to cure, before they are subjected to the low pressure produced by an Inert Gas Generator.

We are of the opinion that for the inertisation process, in any form, to be successful, we need to maintain a ventilation circuit until the latest possible time and this may in fact, be some time after the actual inertisation process or injection of inert gas has commenced.

Industry needs the ability to close off the mine ventilation rapidly and to remove the mine's ventilation pressure from the seals. These needs cannot be satisfied using the sealing method employed at Laleham. CRM recommends that additional research be carried out with a view to identifying the most appropriate seas and sealing methods for inertisation.

The inertisation of the 9 West Waste Workings at Cook Colliery was a success, in that:

• the oxygen in the sealed area atmosphere was reduced to a level below 12%;
• the methane level in the sealed area atmosphere did not reach the lower explosive limit for that gas.

There is no doubt that this project and in particular the Inertisation trials, has broken new ground and the overall results are very encouraging. CRM is confident that Low Flow Inertisation has considerable potential for the elimination of potential explosion hazards and business interruptions in some mines which contain flammable gas.

The trial at Cook Colliery lasted about 236 hours with 181 hours of effective pumping or inertisation time and this equates to an overall unit efficiency of about 77%. It should be recognised that the Inert Gas Generator was new and in fact, commissioned on site during the early days of the trial.

The methods employed here were new and the operators were to a degree, self trained on the job. CRM is extremely pleased with this result and the performance of the dedicated people who worked tirelessly to ensure that this phase of the project was given every chance of success.

There is clear evidence in the analysis results for in Sample Point No. 6, that the delays caused some regression and there appears little doubt that the goal would have been achieved much sooner, had the Inert Gas Generator continued to operate without major downtime.