Underground » Detection and Prevention of Fires and Explosions
Inertisation is a technique that has been used around the world to enhance the safety of underground coal mine areas either to avoid the potential for a combustion event or to stabilise a situation after an ignition, fire or heating. The term normally refers to the fact that the atmosphere in the area is such that it cannot sustain combustion, including ignitions, and is therefore “inert”.
The primary objective of the study was to review coal mine inertisation in Australia. In particular, to focus on the use of the Polish mine fire simulation software “VENTGRAPH” to gain better understanding of how inertisation (GAG, Mineshield, Nitrogen Pressure Swing Adsorption (Floxal) and Tomlinson Boiler) units interact with the complex ventilation behaviour underground during a substantial fire. Most emphasis has been given to understanding the behaviour of the GAG unit because of its high capacity output. Critical aspects targeted for examination include location of the unit for high priority fire positions, size of borehole or pipe range required, time required for inertisation output to interact with and extinguish a fire, effects of seam gases on fire behaviour with inertisation present and main fan management. The project aims to increase understanding of behaviour of mine fires in modern mine ventilation networks with the addition of inert gas streams.
A second aim of the project has been to take findings from the simulation exercises and develop inertisation related modifications to the program in conjunction with the Polish program authors.
Computer simulation of mine fires and effects on ventilation networks has been introduced with considerable success. This has already put a significant number of mines in an improved position to preplan for mine fires and of possible emergency incidents. Mine exercises have been built around the use of the fire simulation computer program “VENTGRAPH” and modelling of fire scenarios across the mine layouts.
Simulation software provides a dynamic representation of a fire’s progress in real time and utilises a colour-graphic visualisation of the spread of combustion products, O2 and temperature throughout the ventilation system. During the simulation session the user can interact with the ventilation system (e.g. hang brattice or check curtains, breach stoppings, introduce inert gases and change fan characteristics). These changes can be simulated quickly allowing for the testing of various fire control and suppression strategies.
Inertisation has been accepted as having an important place in Australian mining emergency preparedness. The two jet engine exhaust GAG units purchased from Poland by the Queensland government in the late 1990s for the Queensland Mines Rescue Service have been tested and developed and mines made ready for their use in emergency and training exercises. Their use in real and trial mine fire incidents has underlined the need for more information on their application.
The NSW Mineshield (liquefied nitrogen) apparatus and has been actively used a number of goaf heating incidents. The Tomlinson (diesel exhaust) boiler has been purchased by a number of mines and is regularly used as a routine production tool to reduce the time in which a newly sealed goaf has an atmosphere “within the explosive range” and for goaf spontaneous combustion heatings. Nitrogen Pressure Swing Adsorption (Floxal) units are available and in use both for reducing time in which goafs are “within the explosive range” and for goaf spontaneous combustion heatings. Each of these facilities puts out very different flow rates of inert gases. Each is designed for a different application although there is some overlap in potential usages.
Simulation exercises undertaken with a range of Australian mines confirmed that many potential underground mine fire sources cannot be successfully inertised with the GAG docked at the current specified point. Two important conclusions are:
- Successful delivery of GAG output from units on the surface must consider other (that is alternative to Mains Travel or Conveyor Heading portals) delivery conduits directly into workings near the fire through existing or purpose drilled boreholes.
- During a fire the stopping of the main surface fan or fans will lead to rebalancing of pit ventilation and in some cases potential explosions through air reversals bringing poorly diluted explosible seam gases or fire products across the fire site.
Another section explained inertisation and dilution issues in Mains headings. These present a complex ventilation network and with additional interference from a fire, maintaining control of the movement of inert gas is more difficult than elsewhere in the mine. Even good quality segregation stoppings allow significant dilution of inertisation flows over relatively short distances.
A calibration exercise on the VENTGRAPH software has occurred in two parts. Back analysis of the gas monitoring data during a fire at the US Pattiki Mine showed that a VENTGRAPH model could be established to simulate this incident. The inertisation exercise during part sealing of the Newlands South highlighted a number of findings. The GAG quantity measured exhausting from the mine area being sealed was at first considered to be unrealistically low. However further analysis indicates that temperature and moisture mass changes explains any differences. The hypothesis that some of the GAG exhaust, with diurnal pressure changes, will flow into and out of goafs is of interest and needs to be accounted for. Further monitoring of mine site GAG exercises are warranted to give greater understanding to this complex system.
A brief overview of the VENTGRAPH simulation software is given. It has highlighted the new features that have been added to the software and in particular the ability to use up to four different types of inertisation gases (at varying flow rates) across a mine layout simultaneously and the ability to include carbon dioxide and nitrogen seam gases as well as methane.
Exercises based on Oaky North and Oaky No 1 mines have involved “evaluation or auditing” of ability to deliver inert gases generated from GAG units to high priority underground fire locations. These exercises have been built around modelling of fire scenarios across the mine layouts.
The fire simulation exercises at Oaky North and Oaky No 1 mines demonstrated that it is possible to efficiently evaluate possible inertisation strategies appropriate to a complex mine layout extracting a gassy seam and determine which approach strategy (if any) can be used to stabilise a mine in a timely fashion.
The final chapter focused on borehole design parameters. Analyses have been established applicable to Australian conditions based on the complex fluid flow theory that describes the dynamic, hot, pressurised exhaust carrying a superheated vapour. Determinations have been made of the relationships between borehole back pressure and GAG thrust relationships and the best approach to vary the jet engine thrust to overcome this base pressure. These mathematical relationships can now be applied to investigate the possibility of using GAG in small diameter boreholes for either production inertisation or fire fighting purposes. This would be a verification exercise taking the equations describing GAG exhaust fluid behaviour based on the steady flow energy equation and comparing the theoretical predictions of GAG exhaust fluid behaviour with actual measurements of pressure, quantity and temperature at various locations downstream from GAG exhaust trials proposed.
It is proposed that a study on production or proactive use of inertisation and particularly the GAG inertisation unit should be undertaken. The study should examine the possibility of a wider and proactive application of GAG in Australian mines responding to or recovering from mine fires or inertisation of sealed mine workings or spontaneous combustion heatings or elimination of the potential explosibility of newly sealed goafs.