Wednesday, December 1, 2010

GAG - Jet Engine to kill the New Zealand Coal fire.

The tragic end to the mine fire in New Zealand, in which 29 miners lost their lives has moved to a new level, with the evidence now being clear that the coal within the mine is, and has been on fire. It is quite possible, on a small local level, that this became true from a time shortly after the initial explosion, and would explain the reluctance of the authorities to send in mine rescue teams. The gases produced by the burning coal are similar (because the combustion is not complete within the underground environment) to those that would be generated in an in-situ combustion process. The gases (mainly carbon monoxide and methane) remain flammable and highly explosive, as has been shown by the sequence of four major blasts after the initial explosion.

In order to ultimately put the fire out the New Zealand authorities have obtained a specially designed jet engine from Australia. While it may seem at first a little odd, and perhaps overkill to use a jet engine at a coal mine the idea is, at its heart, a good one, and it has already been shown to work, both in Australia and at a mine in West Virginia.

Australian jet engine being unloaded in New Zealand to fight the mine fire

Once the coal seam has started to burn it becomes very difficult to extinguish by conventional means. The different curtains and permanent walls that have been built into the mine to control the path of air through it have been blasted down and broken, and air can move throughout the mine after the blast.

The fire needs a continuous supply of fuel and oxygen to continue to burn. Given that the fire is in the coal, it has the fuel, and the different paths that air can get to the site(s) of the fire mean that it can burn intensely along a number of different entries. In order to put the fire out, the oxygen supply must be removed, and this is the purpose of the jet engine. However, because of the scale and nature of the fire, it took some time to decide that the GAG would be deployed.

When the concept of using a jet engine was first developed the engine that was used was from the Soviet Union and was the GAG A3.
This system is based upon a Soviet designed agricultural jet engine which consumes aviation fuel with oxygen (O2) from the intake air and exhausts combustion gases, primarily carbon dioxide (CO2) and water (H2O), along with the nitrogen (N2) from the air and small amounts of carbon monoxide (CO) and hydrogen (H2). The system is designed to approach stoichiometric combustion (ideally, pure burning that scavenges all the oxygen from the intake air). Therefore, these exhaust gases are almost entirely “inert gases”- i.e., gases which do not contribute to, and in fact, can suppress the combustion process due to the lack of oxygen.

The system has been used to fight fires at the Loveridge Mine in West Virginia where it was used to put out a fire that had burned for two months, taking ten days to fill the mine with inert gas and to ensure that the fire had been put out. Prior to that the system had been used to extinquish a mine fire in Australia that had been burning at the Blair Atholl mine for over 50 years. One of the problems at that mine was that the coal is capable of self-igniting, a condition that is called “spontaneous combustion,” particularly when the coal was exposed to sufficient quantities of oxygen. When the mine was being redeveloped as a surface mine, some of these underground workings were exposed, providing that access to the oxygen in the air. The problems were not just that the coal was on fire, but that the extensive nature of the underground fire was generating enough carbon monoxide to be dangerous, even at the surface. Recommended levels of exposure are 30 ppm, if sustained for 8 hours; 200 ppm for 15 minutes; and 400 ppm as an absolute limit. Gas levels of up to 1,400 ppm were being detected around the burning section, and in parts of the underground workings levels of up to 10% CO.

The mine had to put out the burning coal seam and attacked it both from the surface, and from underground, where the GAG engines were used to produce up to 20M^3/sec of inert gas (much higher than viable alternatives) . To produce the gas the engine needs some 530 gallons of fuel an hour and up to 10,000 gallons of water (both to cool the engine and to generate steam which is co-injected into the mine). The process is not without some difficulties due to the need to keep the engines running continuously for a period of days, but it is gaining growing acceptance as a way of putting underground fires out.

Back at the Pine River Mine in New Zealand, the unit has now been started (December 2nd) and has begun to inject gas and steam into the mine. However, even after the fires are all extinguished it will take some time before the mine is cool enough to allow rescue parties to enter.

Our thoughts and prayers remain with the families of those who were lost.

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