Waterjetting 21a - Of peat, coal and New Zealand

During the historical development of waterjets there was not a lot of change from the time of the Ancient Greeks, who found water flow being used to uncover and move minerals such as gold, and the Northern British miners of the 1800’s who were still using this tool in a form known as “hushing” as a way of helping to get lead ore out of the ground. The use of a directed higher-pressure stream (even though only powered by gravity) through a nozzle, as a way of mining the gold ores of California was the first step in a major change.

In part this was because the higher pressures sped the process up considerably, and in part it was because there was more method in the way that the jets were applied, so that the operators were kept safe and away from the area where the jets were mining, and where the surrounding rock was unstable. This had advantages in other applications.

Consider that peat is a fuel that is still mined and used in various parts of the world. (As a young man I helped my father dig peat in the spring using a shovel, and then carry it back to my Grandma’s house where it would dry over the summer and be burned the following winter.) But many peat bogs are relatively unstable places, so that bringing in heavy equipment is difficult given the soft ground. Modern practice is to cut large drainage channels around the mining area, allow the water to drain out and then mine the dried peat, which is more often these days used in gardening.

However, before such large operational processes were around peat miners had used water jets (along the lines of those used in California) to break out the fibrous material, and then to move the liquefied slurry in pipelines which are easier and lighter on the land than road beds. Back in 1979, for example:

Western Peat Moss Ltd in Vancouver is currently using hydraulic mining and pumps a peat moss slurry of a concentration varying beteen 0.75 and 1.5% by mass, a distance of 3.65 km in a pipeline 0.3 m in diameter, at a rate of 340 cubic m/hour (1,500 gpm). A similar operation is being used in Alberta by Hood Manufacturing Ltd, for the mining and transport of Peat moss.

Figure 1. A peat mining operation (the peat is transported away in the pipeline. (SRS Crisafulli)

Given that peat covers about 3% of the world land surface it is sometimes surprising that more has not been made of the resource, although it is physically strenuous to mine manually, and the high water content means that it must be dried before it can be used. This is why the largest market in America has become the garden market where the costs can be more easily absorbed by the market, and modern vacuum mining of the dried material has become the more common modern method of extraction.

Figure 2. Peat distribution around the world (SRS Crisafulli).

The first mention of hydraulic mining of peat came from Prussia, and by 1914 it was used as the primary source of fuel for the Electroperedacha Electric Power Station in Russia. After the war that usage had grown to mining roughly a third of all the peat mined in the Soviet Union. (Yufin A.P Hydromechanization, State Scientific Technical Press of Literature in Mining, Moscow 1965). There was, however, another place where remote mining can have a considerable advantage.

There are many places in the world where coal beds, originally laid down horizontally, have over the course of time been tilted to steep angles. These mines are very difficult to work in, since the steep slopes can be dangerous to workers, and productivity is generally quite slow. It appeared to dawn on mining engineers in both Russia and New Zealand at about the same time, that using water to mine and move the coal might get away from many of the problems of moving men, material and supports to a working face that isn’t that stable or needed for very long. A simple schematic shows the process:

Figure 3. Early New Zealand hydraulic mining operation (New Zealand Ministry of Culture and Heritage.

In the initial operations the coal was first weakened by blasting it with explosive, and as pressures and flow rates were increased this was found to be unnecessary, and the coal could be mined from the sold. Further, although the illustration shows that a man is directing the stream at the coal face, earlier methods had the stream being directed along a face, carrying the coal from one entry to another, where it could be flumed and carried away, by the same water flow.

By a suitable choice of geometry this did not need an operator to guide the jet stream, and so the practice arose back then of firing the face, and turning on the water flow and going home for the evening. When the miners arrived back the following day the coal had been moved down to storage (or in some cases out through a lower adit and out of the mine) and the face was cleaned off. All that was needed was to insert new roof supports in the exposed part of the seam, move the hydraulic channel over to the new face, drill it, and then, at the end of the shift, blast down the coal and go home. I will confess this is a much less strenuous way of mining than the 15 yards length of blasted coal some 6 ft deep that I had to physically move onto a conveyor each day with a shovel and pick (to break up the big lumps and to drive wedges above the props to hold the roof up) back when I was an Indentured Apprentice.

There are a number of different ways in which the coal was mined, with the pattern of extraction changing with the slope of the coal seam, the thickness of the seam and the strength of the overlying rock (since as the jet mining distances grew greater the area mined was no longer supported) and I will discuss some of these in more detail over the next couple of posts or so.

Waterjetting 2d - Adding cracks to Nature

In the last few weeks I have focused on demonstrating, with examples, that water effectively removes material by penetrating into natural cracks in the material and causing them to grow. But what happens when there are not enough cracks to remove material at an economic rate? The modern approach has been to raise the pressure of the water so that smaller cracks grow faster, thus providing the production rates needed, but that option wasn’t available in the past.

I mentioned last time that miners in the Caucasus Mountains of what is now Georgia used the power of mountain streams to erode gold deposits over 3,000 years ago. Perhaps learning from that, when the Romans came to Las Médulas in Spain, some 2,000 years ago, they though of water again as a way of mining the gold-bearing sandstone of the local hills. And though they had to modify the initial idea, the result became the most important gold mine in the Roman Empire. It is now a World Heritage Site.

Figure 1. Location of Las Médulas in Spain. (Google Earth)

The sandstone was more resistant than soil, and so the Romans came up with two ideas to improve the rate at which the gold ore could be removed. The first idea was to run galleries into the sides of the hills, creating large chambers underground, with support for the roof from wooden supports that were left in place.

Figure 2. Tunnel driven into the bottom of the hill at Las Médulas.

Figure 3. Underground room at Las Médulas.

At the same time that the mining preparations were going on local streams were being diverted and dammed so that a large volume of water was held in reservoirs and then carried by manmade channels to a point over the mining chambers. With the water ready, the timbers were set on fire, which initially weakened the overlying rock so that it began to fail, falling into the opening, and as the support burned away more rock fell into the opening until the cavity worked its way up to the surface. At this point the reservoir gate was opened and water flooded down the channel to fall into the cavity. As the water fell it further broke the rock into grain-sized pieces, and carried these down and out through the original opening in the hillside.

Figure 4. A Collapsed cavity, not the two figures at the arrows to get a sense of scale.

The water and debris flow was directed into flumes, in much the same way as modern miners in Alaska practice today, except that where carpet is used to catch the gold particles in Alaska, in Spain the Romans used plant stems (silex) to catch the gold. After drying the plant could be burned, easing to recovery of the gold. (In more modern times Spanish miners have lined the flumes with oxen hides.)

Figure 5. Artist sketch of the troughs used to capture the gold particles at the Spanish mines.

The use of heat to weaken rock before using water pressure for cutting has been tried with a couple of interesting wrinkles both by researchers at Rolla, and at the then U.S. Bureau of Mines and in Colorado, among others. But those more modern trials will be described later in the series. Using water streams to erode surface outcrops of mineral survived as “hushing” in the North of England and elsewhere until fairly recently.

Move forward some 1800 years or so from Roman Spain, and at the turn of the 19th Century miners in both Russia and New Zealand had a problem in mining coal. In both countries there were good quality coal seams, but they sloped at a steep angle that made it difficult to move men around without their slipping and falling. It was also difficult to support the roof, which was achieved at the time by sawing wooden props to length and wedging them between the roof and floor. Both nations had the idea of modifying the Roman idea of using water to remove the mined coal, but coal was thought to be somewhat stronger and more resistant than the Spanish sandstone.

In the New Zealand case the mountainous countryside makes it expensive to drive roads and as early as 1891 wooden flumes were being used to carry coal to the consumer. However it was then realized that the water could be used to also remove the mined coal, particularly that which was left in regions of the mine where it was not safe for men to go. The coal was therefore initially blasted, and then the flow from the nearby streams was directed at the debris pile. The volume of water, and the slope of the mine combined to remove all the mined coal, often overnight, so that a new area could be worked the following day. It was not until 1947 that pumps began to be used to drive the water at greater pressures. At this point, with the higher pressures that pumping brought, it was no longer necessary to pre-crack and break the coal with explosives.

While the New Zealand coal seams outcropped at the surface in very hilly ground, the situation was somewhat different in the Donets coal seams in the Soviet Union, where the seams were thinner, and production was barely economic. The seams in these mines were much deeper than in New Zealand, and so jet pressure could be provided from the drop in height from the mine surface to the location of the large nozzle or monitor that was used to aim the water flow at the coal. As with the New Zealand experience the Soviet miners (at the Tyrganskie-Uklony mine) initially blasted the coal with explosives to weaken it with a high density of cracks, before applying the water. However the miners found that not only did the water double production (to 600 tons/shift) the streams were powerful enough that it wasn’t necessary to pre-blast the coal. The nozzle diameters of the time were up to 2-inches in diameter, and could throw a jet up to 60 ft.

Figure 6. Early Soviet underground coal miner

It was from these small beginnings that hydraulic mining began, it was, in its time the most productive method of mining gold in California, and was used for many years around the world for mining coal, and other minerals. But that again is a subject for more detailed discussion at a later time.

The combination of explosives and water power remains in use in harder rocks, particularly in South Africa in the gold mines. Here again the seams of gold are very narrow and can slope or dip at a steep grade, the working area is thus kept very cramped and difficult to work. By blasting the ore with explosive, it can again be moved with water pressure, although there is an additional advantage to water here that I will further explain when I write about cleaning rust from plates.

Gold, as is shown by the way it can be collected in flumes, is very heavy, and part of the problem in the South African mines is that small pieces can get trapped in small pockets on the floor of the seam. The higher pressure water flows can flush out these pockets driving the gold particles down to a common collection point. In that particular the practices haven’t changed that much in three thousand years.

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.

P53. Pick Points

Half-a-dozen or so stories that might be of interest:

There does appear to be a little recognition out there now that oil prices have hit a floor, and may perhaps be bounding up a little. I suppose if I was that kind of blogger I would point to the post where I said so, but let’s be a little cautious a week or so longer. Ecuador thinks that the price should really be $80 (per barrel) but would be happy with $60. Although Shell admitting they weren’t replacing their withdrawals from reserves, might also have helped. With some of the excess oil that has been held in tankers now coming onto the market perhaps others are seeing the sort of signal that says we may now see a crawl back up in price. It was only a month ago that something like 80 million barrels was being held in these vessels, given that a VLCC (Very Large Crude Carrier) can hold up to 2 million barrels, and with 45 tankers having been used that way, there was a lot to ease back into the market. Shell sold their first two tanker loads (some 1.2 mb) back at the end of January and it seems that others are now also finding a sale.

The lower supply price for natural gas is now reaching the point (as winter demand dies) that supply companies are starting to pass on their savings to the customer. For example up in Canada, Enbridge Gas Distribution has just go permission to drop their price from 30.4 cents per cu.m to 23.5 cents. For a household using 3,000 cu m per year, this will save some $230. (That price converts to a drop from $8.60 to $6.65 per kcf). Similar things are happening in New Hampshire with the utility there, Unitil Corp, is getting a new rate of 69 cents per therm, (or $6.90 per kcf), which is down 26% on recent prices, and 56% from last summer’s peak ($15.50 per kcf). There are some out there, however, that have picked up the message I have mentioned here earlier, that as rigs drop off, so availability will again become tight, and thus prices could double again by next year. Next January’s futures are up 49% on April. However, while I was looking at a 20% shortfall some time into early next year, with the current fall in production, some are seeing 5% drops by the fourth Quarter. And looking back in history (which I favor)

The last time drillers stopped rigs at this pace was seven years ago, when futures advanced 86 percent. The world's biggest hedge funds have already started to close bets on a drop in prices, government data show. Natural gas tumbled 30 percent this year, the worst start since 2006, as sales weakened with the recession.

I usually only just look at the weekly EIA numbers for crude, gasoline and natural gas, (and those comments may be a few hours delayed since I am working in Sweden) but it is worth having a quick peak at the coal forecasts, which come out on Monday’s. For reference here are the current spot prices for coal:

Source EIA

In case you were wondering why most utilities are buying Powder River Coal from Wyoming. The amount of coal being produced and used is remaining fairly stable.

Source EIA

The blue line for last year shows record production levels, that are, at this time, not anticipated to occur this year because of the economy. However, when one looks at the international market, where last year saw record prices of up to $300 a tonne, (sometime I will start correcting for the difference between short tons (US) and metric tonnes (most others)), the market is currently looking at prices of around $115. Of course that view came from New Zealand, where a new coal offering was fully subscribed. Australia is hoping to settle, for the moment, at around $70. But those who think that the global slowdown will seriously reduce consumption, might want to consider that China’s imports were at the highest level in 22 months in February, at 4.88 mill tons, and with prices being bruited of $62.10 per ton in Newcastle, Australia, they may not be the only ones that come calling. (But part of the demand relates to internal Chinese politics over the price utilities will have to pay the mines for coal). It might also be worth noting that in order to sustain their economies both China and India are pouring money into infrastructure, and that means steel, and steel means iron, and iron means coal. India is going into elections this year, in case you had forgotten. However the number of ships lined up to take coal at Newcastle has dropped from 70, eighteen months ago, to 15.

Well having just skimmed around the big three tonight, I thought I’d leave room for a couple of pictures. Back when we went to Cork for the ASPO Conference , Colin Campbell laid on a piper to lead us in to dinner. Well I was led to where I was ended up deciding to eat tonight by pipers* in the Stockholm Gamla Stan.


Pipers in Stockholm

And then when I wandered back to the hotel, I found that the Royal Palace had been surrounded by a belt of snow about a street wide, and some 20 cm (8 inches or more) thick of artificial snow. Maybe they thought I missed it, or was expecting it (it was snowing when I arrived). Anyway, not a good picture in the light, but just to show, these are normally the steps up to the Royal Palace.

Snow covering the stairs into the Royal Palace

(it’s artificial, and 20 cm plus deep)
* I actually dined on moose, and cloudberries, just around the corner.