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How to effectively cool down extraction regions in mines

A mining corridor. A tunnel-boring machine in action.

Photo: Dreamstime

How to effectively cool down extraction regions in mines

The first cooling system in the world with a capacity of more than 9MW using ice slurry pressure-piping has been operating for more than a year at the Lubelski Węgiel ‘Bogdanka’ mine. It is considerably more efficient from water systems, says its creator, Dr hab. Eng. Łukasz Mika, Associate Professor at the AGH University from the Faculty of Energy and Fuels.

If the temperature in a mine goes beyond a certain threshold value (we can round it up to 33°C), all work, with the exception of rescue operations, should cease immediately. With reduced air circulation and high humidity, the conditions in the tunnel would pose a serious threat to the health and life of miners, if they continued their work. Meanwhile, when we go deeper, the temperature begins to rise, and at the extraction level in a coal mine, it can reach more than 40°C. Lowering the temperature markedly increases the working comfort of miners; however, until now, maintaining it within optimal ranges for the miners, or at least around 28°C, was not always possible, and even if it was achieved, it required significant energy input.

The solution developed by an AGH University scientists, Associate Professor Łukasz Mika, facilitates the maintenance of a considerably lower temperature in the extraction regions with reduced energy consumption compared to the cooling systems used to date. Furthermore, the solution can be implemented in many different mines without the need to modify the underground infrastructure, which would otherwise require a stoppage of extraction work for the time of the cooling system modernisation.

Associate Professor Łukasz Mika controlling the production of ice. Photo: private archives of Ł. Mika

Łukasz Mika controlling the production of ice. A man in an industrial-looking hall with yellow railings and blue and black pipes.

The deeper and farther from the shaft, the warmer

Today in mines, the most advanced air-cooling system in operation is central air conditioning. In simple terms, it cools the water above ground (to about 1–2°C) and transfers it via a vertical pipeline underground, where it travels through horizontal pipelines to various areas of the mine. Then, the water fuels the air coolers that cool down the air in the parts of the mine in which the extraction is carried out. In the early days of any mine, this solution can be quite efficient; however, the longer the mining lasts, the longer the horizontal pipelines get, and therefore, the cooled water has a longer distance to travel. This causes trouble because the rest of the mine gets really hot and, despite the good thermal insulation of the pipes, the water that passes through those hot spots gets warmer with every kilometre. If it starts at the top with 2°C, before it reaches the cooler located in the extraction spot, after travelling several kilometres underground, it can heat up to even 8°C, sometimes even 12°C. This significantly reduces the efficiency of air coolers, and therefore, in mining areas, the temperature increases.

We cannot further cool down the water above the ground, first of all because it would simply freeze, making it impossible to transfer it down the pipeline. This could be prevented by creating a special mixture – adding alcohol or salt to the water, the temperature of the solution could drop below 0°C, simultaneously staying liquid. However, the application of such a solution is prohibited by regulations – it is forbidden to introduce liquids below 0°C into mines.

What is the solution then? We can build a new shaft, that is, we can drill a hole, build new above-ground infrastructure, and new vertical and horizontal pipelines. We can replace all currently used pipes and pumps for bigger ones, we can replace the entire infrastructure above and underground, so that we can transfer more water to the mine, so it won’t have time to warm up so much on the way. However, this requires us to stop the extraction, which is quite expensive. We can also apply this type of solution, which was developed for the ‘Bogdanka’ mine', explains Dr hab. Eng. Łukasz Mika, Associate Professor at the AGH University, the author of the innovative cooling system project implemented in the aforementioned mine.

Melting ice can cool it down

While working on the new cooling system, the scientist from the AGH University was aware of the limitations placed on such solutions by human and physical laws. However, with the system he had developed, he managed to harness the phenomenon of freezing and melting of ice, turning it into an advantage, and, observing all the regulations, smuggle some ice particles down below. The system designed by the scientist has been operational in the ‘Bogdanka’ mine for more than a year, helping to maintain an optimal temperature for the miners to work. In his project, Prof. Mika used the knowledge which he acquired during the work on his doctoral dissertation and the postdoctoral qualifications procedure at the Cracow University of Technology.

It should be noted that the basis of the modernised system is the existing central air conditioning system of a mine. Then it is expanded with ice slurry generators. Subsequently, the ice, in the form of ice flecks or wet snow, travels to a mixer, where it is combined with the cooled water already present in the mine’s infrastructure, and in the form of a slurry flows down through the pipelines’, says Łukasz Mika about his cooling system. ‘Hence, we use the existing pipeline to pump down not only water, but water with ice particles; thus, the temperature at the top is zero degrees Celsius. The slurry is delivered to the mine via pipelines, and, as a result of the high temperature of the surroundings, the pipe is hit by heat (transfer losses). But, the temperature of the water doesn’t rise because it’s the ice particles that melt in the first place, and only when they’re gone, the water in the pipe starts to heat up. When you design and use the system appropriately, you can pump water with a temperature of 1 to 2°C, or even near 0°C to distant parts of the mine. And this is a dream come true for all those responsible for cooling mines: with the water systems, we had 8–12°C water reaching the coolers; with the ice slurry, we’re close to 0°C. When we fuelled the coolers with 8–12°C water, their efficiency is reduced by several dozen per cent compared to that declared by the producer, which makes it impossible to effectively cool the air. However, the effectiveness of coolers powered by the ice slurry surpasses the catalogue efficiencies, as they are calculated with 4°C water in mind. With shorter distances, coolers might receive unthawed ice slurry, but this won’t do them any harm; in contrast, it might even be beneficial, as they get more “chill”, which they can transfer to the air in the region where the miners work’.

A fragment of the installation that produces the ice slurry.

A fragment of the installation that produces the ice slurry. Photo: private archives of Ł. Mika

Implementing the solution in mines where the cooling function was performed by a central air conditioning system does not even require changes to existing infrastructure. The only thing necessary is to build a hall above ground, which would house the devices that transform water into ice, that is, ice slurry makers, which is the key element in this innovative installation.

Of course, the alternative to the slurry system is an expansion of the existing central air conditioning system based on water. To build a new shaft with the necessary infrastructure, it’s about PLN 100 million, and the losses of a mine caused by all the necessary stoppages in extraction due to the extension of the system would amount to hundreds of millions. Importantly, if we wanted to modernise the system to apply the ice slurry, the alterations can run in parallel to the work in the mine; therefore, no stoppage is needed.  Extraction will not be hindered; this is what happened during the construction of the system in the ‘Bogdanka’ mine’, Łukasz Mika says about his project.

Troublesome states of matter

Although the mechanism of freezing and thawing of water can be explained in simple terms, all aspects related to the design of the ice slurry system are much more complex, creating the need to solve a myriad of technical and scientific problems. First of all, it is not simply about mixing water and ice so that it can be transmitted through a pipeline. It is necessary to prepare such a mixture in which the ice particles do not stick to one another, which could cause obstruction in the pipeline and effectively prevent the slurry from travelling down. On the other hand, a safe volume of these particles must be maintained in the slurry, their size also being of great importance. It is also significant to provide optimal flow conditions for the slurry in the pipelines. It is the depth and capacity of the shaft pipelines that cause problems. They can reach depths that go beyond 1 km; hence, they are exposed to high pressures of even 13 MPa, which is then reduced to about 3 MPa. Such conditions create an environment in which part of the particles melt, only to recrystallise later.

The slurry is therefore not that simple an agent to use as water. When we add ice particles to water, the more there are, the more the mixture becomes a non-Newtonian fluid. Therefore, we have special models that describe its behaviour during the flow. To describe it more graphically, if we added more and more ice particles to the mixture, its consistency and behaviour during the flow would start to resemble liquid concrete. Furthermore, note that an ice floe floats on the surface, that is, if we didn’t know the exact flow conditions to maintain, the mixture could split: the particles would flow up and the water down. This could happen in our pipelines. All automatic safeguards designed as part of the solution applied in the ‘Bogdanka’ mine include various malfunction scenarios: sudden blackout, partial power shortage, system stoppage, and other failures of the respective devices that make up the entire system. We have also provided for measures to regulate the system’s efficiency. Nevertheless, if for some other reasons, for instance, as a result of a human mistake, the pipeline with ice would be blocked, it would be different than in the case of an obstruction caused by sand or mud, which creates the need to disassemble the pipeline, drain it, and clean it. In the case of our ice slurry, we’re dealing with ice or wet snow particles, and it’s only a matter of time that the mixture changes into water and unblocks the pipe by itself, without the need to take it to pieces and clean it’, the scientist highlights the advantages of the solution.

Snow on a conveyor belt. Photo: private archives of Ł. Mika

Snow on a conveyor belt

Huge potential of ice particles

After the launch of the ice slurry system, the measurements taken in the ‘Bogdanka’ mine demonstrated that it is much more efficient than the old water system. Under similar conditions in the mine, the water system, supported by a compression unit, was able to cool the air down to 25.4°C; whereas the ice slurry system managed to cool down the air to a temperature below 20°C, without an additional compression unit (which also consumes energy). Due to the ice slurry, the difference in the achieved temperature was more than 5°C. This is a considerable difference even in everyday conditions above ground and even more so for the miners working hard below ground.

The solution is also energy-efficient. Reducing the temperature of the water entering the air coolers from 7°C to 3°C using only water would require us to increase the stream in the pipeline four times. This, in turn, would create the need to replace old pipelines with larger ones and to increase the surface power of the air conditioning station by additional 18 MW. Using the same pipeline, but this time with the ice slurry solution developed by Prof. Łukasz Mika, to obtain the temperature of 0–2°C in an analogous distance from the shaft, it would only be necessary to add a power element of about 3 MW – without the extra need to replace any pipelines. The ability to ‘store the chill’ by the ice particles in the slurry is simply that high, considerably increasing the efficiency of the whole system.

Until now, ice and snow have been used to cool down mines in Africa, for example (Mponeng gold mine, the Republic of South Africa), but the mechanism was totally different. The horizontal pipelines transported only water, and gravitational methods were used to deliver the ice down, where it was only used to stabilise the temperature at around 2°C in underground open reservoirs.

The solution proposed by the scientist from the AGH University has the potential to be implemented in coal mines and deep copper mines.

There is no guarantee that the system will be used in other Polish mines. New solutions that have no reliable competition on the local market are not desirable in tenders. On the other hand, several companies from Germany, Canada, and China have expressed interest in the ice slurry system’, says the scientist and adds: ‘I wish to thank all the people with whom I’ve worked on this project. Special thanks to the management and employees of the ‘Bogdanka’ mine, as well as the owners, management board, and employees of the general contractor of the installation, the DPMtech company from Rybnik. The project of the system was created at the Faculty of Energy and Fuels. The idea for the project, the technical design of the devices, the verification procedures, the design of automatics and safeguards – all this happened at the AGH University. No one has ever proposed a similar solution in the world, although many have tried. We can safely say that we’re pioneers in this field’.