Waterjetting 22a - Mining horizontal coal

Over the last few posts I have discussed some of the problems that arise in dealing with the use of waterjets in mining coal, when the material mined has to be collected and transported away from the face where the coal is extracted. I thought I would follow on that thread in a few more posts, ending up, hopefully, where I began back in the process of removing thin layers of material (such as rust) from flat surfaces.

But to get there I am first going to go back to coal mining. One of the problems with adapting what we might call conventional hydraulic mining to coal is that many of the coal seams around the world are relatively flat – it is, after all, the way in which the vegetation that became coal was laid down. Thus the gravity that can be used in a steeply dipping seam as a way of carrying away the coal and the water together, is not initially that helpful.

There are several different ways that have been suggested over the years to solve the problem. Initially these were based on existing mining machines, and methods for mining the coal, but with the teeth of a conventional machine replaced with high-pressure waterjets. One such, as I have written earlier was the MS&T Hydrominer, where the cutting teeth along the edge of a coal plow were replaced with oscillating dual-orifice waterjets to cut a kerf around the coal being mined.

Figure 1. Artist’s impression of the initial Hydrominer, with jets cutting a slot one foot deep ahead of the wedge shape of the plow.

The water used was less than that conventionally used on a mining machine to suppress the dust generated as coal is mined from the solid, and the coal loads onto the armored face conveyor on which it rides down the face.

That particular design was based on an earlier mechanical machine, the Meco-Moore, which I had previously seen working on a longwall in the United Kingdom.

Figure 2. Meco-Moore mining machine set up to mine coal. The cutter jibs cut slots and the coal then collapses onto the transverse conveyor.

However this concept required a considerable investment in the supporting longwall equipment both to hold up the roof and to remove the coal. An alternative approach was to continue to conventional roof-and-pillar mining which is the most popular method of underground coal mining in the United States, but again replacing the cutting teeth with waterjets. The first of these was conceived by IIT Research Institute in Chicago, under Dr. Madan Singh.

Figure 3. A high-pressure waterjet continuous miner.

Unfortunately in this configuration the system did not work well. The jet pressures used were too high, and in consequence the volumes of the jets too low to achieve a deep penetration into the coal.

When the jets were replaced with a combination similar to that of the Hydrominer, and in a device we called RAPIERS, a slightly better performance was achieved, but the demand for innovation had, by that time passed for a spell, even though this particular machine was developed with considerable technical input and financial assistance from the Jet Propulsion Laboratory in Pasadena.

Figure 4. Progression of the RAPIERS machine in room-and-pillar mining.

Both of these machines required that a second set of machines sit behind the excavator and carry away the coal that had been mined, again at significant cost, and they also required machines to support the roof.

There is a different type of machine that is often used at the edge of the productive limit of surface mining. As seams near the surface get deeper so the cost of removing the overlying material becomes too expensive to justify continued mining. At that point companies may bring in an auger which can drill long holes into the coal, and remove the material as with conventional smaller augers that might be used for drilling in dirt (or even drilling holes in wood).

Figure 5. Conventional auger mining (Rosamine )

Because the auger drills a hole to the size of the following scroll, it is relatively easy to carry the coal back out of the horizontal hole, which might exceed 300 ft in depth. But there is a problem with the machine, in that the cutting force to push the auger teeth into the coal at the face of the machine has to be carried through the entire string of augers.

Because of the string of segments this becomes more difficult to control with longer depths, and in addition there is a friction loss due to the continual rubbing of the scrolls against the floor and sides of the hole. Together these act to limit the machine range, since there is little to steer the machine other than the direction of the hole, as it deepens.

Figure 6. Picks on the face of the auger, with early jets mounted in the center of the head to cut a central hole.

If, however, the picks on the face of the auger are largely replaced with waterjet nozzles, particularly at the outer edge of the auger, and with the flow directed there, rather than, as shown in Figure 6, towards the center, then an outer free face – up to a foot deep, can be cut ahead of the cutting head. With larger auger heads the nozzles can be placed across the face, to break the rib of coal, should it start to get too large – especially since the coal needs to be fragmented somewhat to feed down the auger.

Figure 7. Waterjets across the face of an auger (courtesy W.A. Summers)

The reduction in the amount of force that this allows on moving the auger into the coal can be illustrated by example. In developing a version of the machine we built an artificial coal face, made up of coal pieces and cement. It is a little more resistive than conventional coal, however the student, Chris Cannon, had little difficulty pulling the machine into the face with a come-along, even though he only had one uninjured arm at the time of the test.

Figure 9. Chris pulling the 2-ft diameter auger into the artificial coal seam.

By confining the coal and water it was possible to recover both, so that the water could, if needed be recycled.

I’ll continue the thread next post.

T7. On Augering, coal reserves and EROI.

Today I thought I would digress a little from the more methodical passage through history that I was taking in regard to coal mining to talk a little about coal reserves, coal resources and technology. It is a little focused towards some of the work that we developed some years ago, and so I give you that warning up front. I am going to try and keep it simple, and so those who know about what I write should bear that in mind.

When you find a seam of coal, if it is near the surface then the soil and rock can be removed from over the coal, the coal removed, and then the rock and soil are replaced. This is surface mining and I will cover that in a specific post later. At the same time there is a point where the coal is too deep for that process to be economical, and so underground mining takes place. There are two main methods of mining coal, room and pillar mining and longwall mining, and I’ll talk about them in separate posts also. Today however I want to cover that point where the seam has just become too deep to make it profitable to take any more of the cover from the coal. One method of mining at this point has been to send a small mining machine known as an auger in to mine out the coal, that is exposed at the edge of the mine.

The auger works in the same way as a wood bit that chews into a piece of wood, when you want to drill a large hole through it. There is a cutting head on the front of the machine that cuts into the coal, and then behind the head is a scroll feed that carries the coal out of the hole, to a point where it can be collected and taken away.

Auger cutting head (Cutting head. (Source BryDet Augers Note the head is shown without the picks that would be placed in the sockets on the face of the two perimeters.

Auger scroll

There is a small video of the process here

Typically the auger holes are placed relatively close together, and they are drilled on the order of 100 ft deep.

Auger holes (Source Lukhele )

One of the major reasons for the limitation that the auger will drill into the coal is related to the way in which the auger works. In just the same way as when you drill a hole with a wood auger, you have to push hard to get the bit to cut into the coal. But the push has to be transmitted down the flights of the spiral sections of the auger assembly. These are not very strong, and they rub against the walls of the hole that has been drilled, so that as the drill goes further into the coal, more of the push is used up in the rubbing friction between the scrolls and the wall of the hole. Also, if you push too hard after the auger is in the coal some distance then the scroll shafts can slightly buckle and this can thrust the auger head out of alignment so that it drills into either the roof or the floor.

The auger is thus a tool with a relatively limited role, though in that role it can be quite effective. Now here is the change that we made. If you take two or three small (0.04 inch diameter) nozzles and attach them to the front of the auger head, so that two cut on the outer edge of the hole, and one is on the inner diameter, then the jets of water that come out of the nozzles will cut into the coal. The jets should operate at around 7 – 10,000 psi, depending on what other rock is found to be in the coal. Typically these jets will cut into the coal about 6 – 9 inches ahead of the auger body as the head rotates. This breaks the central core of the coal free from the confinement of the surrounding coal, and when the head contacts the coal, it will break outwards in tension, in handle-able sized pieces. The push now required to move the machine into the coal is much lower (we had a student with one arm in a cast use a come-along to pull a 2-ft diameter machine into the face). Because the coal breaks so easily, and the force is so much lower, the scroll sections do not have to be so large, and without the need for the high thrust a smaller scroll, that does not contact the walls all the way around the hole, can be used. With this combination, since there is now no frictional limitation to the push, and the force required to pull the auger head into the coal is so much lower, the range of the machine can be extended from under a hundred feet, to several hundred feet.

The small blocks across the auger face show where the nozzles are mounted. (The sawdust is because we cut plywood to see the cutting pattern).

However there is an additional advantage to this change in design, and that comes about because the coal is now being cut with a water stream instead of with a mechanical tool. The pressure and cutting pattern of the stream can be adjusted so that the jet cannot continuously cut into the rock that overlies and underlies the coal. When the cutting pattern is controlled in that way, and with the lowered push being applied to move the auger head forward, the head cannot cut into the rock, but is held to follow the coal seam. The machine becomes inherently self-steering, and thus can drill further into the coal than its predecessor.

I have described this machine (which we only tested in the lab before the energy crisis of the 80’s went away) to make a relatively simple point. With very little change in the design of the auger – using existing pumps and other parts, we built a machine that could extend the range of mining from the highwall of the surface mine into coal that would otherwise be uneconomic to mine, for a distance of probably more than half-a-mile. (That is based on other research I’ll talk about another day). The innovation is but one of many that could be made to transition equipment being used today to allow it to mine coal that is only counted today as a resource and which people are quite quick to discount as being un-minable.

The economic need for coal is going to be such, however, remembering that all the solar and wind energy being currently used in the country adds up to the power output of only one medium sized coal mine, that we will be mining for a long time. As we do, and the easily recoverable coal goes, then in exactly the same way as innovation has made more difficult gas and oil resources into reserves, so we will see the same change occur with coal resources. And of those we have enough to see us through until new sources of power come along in the right scale and from that they will replace coal.

Jets on the auger face cutting into artificial coal

(Please note I have originally used photos and sketches from the internet for this piece, as I get more skilled in modeling I will replace them with my own. I will also more jet pictures later).