Waterjetting 22b - Steep seams and shrouds

It was not until we started to move the auger that I described last time, that I realized how heavy and cumbersome it remained. It is true that we could lighten it considerably (and we did in the UK version), but it was still largely a platform mounted device that was relatively easy to move on the surface, but which would be much more difficult to move around in the confined spaces underground.

In this regard it is worth comparing two photographs.

Figure 1. Coal being transported in the pipes at the Hansa hydraulic mine in Germany.

Notice in this first one how the coal is confined, there is no dust, and the tunnel is relatively clean and clear. Contrast this with the typical mining operation where the coal is carried from the working face to at least the main haulage drifts, and often all the way out of the mine using conveyor belts. (The method of transport that would also likely be used in a typical underground auger section).

Figure 2. Conventional belt conveyor carrying coal underground. (Famur )

Note that in the second photo the belt occupies most of the space in the roadway making passage more difficult, and that the coal is openly exposed. The problem that this occasionally generates is that the coal transfers from one belt to another as it moves through the mine by falling off the end of one belt onto the next. This puts dust into the air, and if not properly maintained coal dust can accumulate under the belt and around the support rollers and drives. If not cleaned this can create heat through friction, and can lead to disastrous fires.

Figure 3. Studying a belt fire underground (Office of Mine Safety and Health Research )

Confining the coal, and using the water that mined it as a transport fluid – or at least part of such – has many advantages.

The Russians were aware of this as they developed some of their different mining machines. One of these was a small monitor that could be pushed up a seam, from a lower drift (without the need of a higher one), by adding segments as the unit was jacked forward.

Figure 4. Russian GVD monitor

The monitor could be advanced up the seam, cutting a channel about 3-ft wide, and to the height of the coal. This relatively narrow channel confined the water and the coal produced, so that both ran back down to the drift, where they could be either enclosed in a pipe, or run into an open flume, that would carry the coal away. Once the drive had reached the end of the section, then the two hydraulic cylinders that sat under the monitor could be turned, so that, from a protected section down-slope, the monitor could successively ream out strips on either side of the entry, until the roof collapsed, or the operation holed through to the previous panel.

Figure 5. Schematic showing the sequence of extraction in a) a coal seam with a relatively strong roof b) a seam with a weak roof, where a pillar of coal is left between successive lifts, typically around 30 ft, so as to provide additional support to the roof as the coal is mined out.

Production from these machines averaged about 76 tons/hour from seams that were in the range from 2 to 4 ft thick, this was more than double the production output achieved by more conventional means, with significantly less manpower.

However while there are some conditions where gravity helps to bring the coal and water back together, there are many cases where this is not possible due to other constraints. This is where it becomes necessary to use a shroud to confine the jets, debris and water so that they can be extracted together, often using a vacuum to assist in the process.

Figure 6. Rendition of the combination of three cutting jets rotating around a vacuum tube to slice into material and remove it.

The particular device shown in Figure 6 was developed as a way of remotely slicing into high-level radioactive waste for the Pacific Northwest National Laboratory. The waste is held in underground storage tanks, where the levels are too high for human access. As a result the waste had to be broken into pieces and removed remotely.

The initial problem was that the tanks, though holding perhaps half-a-million gallons of waste, had only small (18-inches or less) ports through which they could be accessed. Thus a relatively small tool was required, yet the mining rate had to exceed 4-cu. ft/minute. The situation required an excavation system on the end of a robotically controlled arm. But the arm would have to be more than 60-ft long (in the end a cousin of the arm used on the Space Shuttle was used). Any mechanical force applied at the end of such an arm would have tremendous leverage on the holding fixture, and a relatively low overall force would have to be found, yet one capable of cutting material perhaps as strong as a weak cement.

The answer came in the form of the device shown in Figure 6. By placing three cutting jets to rotate around a central tube, connected to a vacuum line, one could cut into the material and break out relatively small pieces, which could be aspirated away with the cutting water. (The requirements were that there would be no water left in the tank for any significant amount of time). Because the bottom of the tank was some 40 or 50 ft below ground level a small, high-pressure (10,000 psi) jet pump was developed by Michael Mann, capable of drawing particles of up to an inch in size into the line, and then projecting them up with sufficient energy to carry them out of the tank.

Figure 7. Basic system conceived to mine high-level radioactive waste and remove it from a storage tank.

While this description of the system makes the tool seem to be relatively simple to build and operate there are a number of features to the design that are fairly critical in order for it to work well, and I will cover some of those next time.

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.