Mesoscale transport and sustained cooling of dual-network geopolymer foam for coal-fire control
This study combines experimental rheology with mesoscale modeling to demonstrate that while the rheology of dual-network geopolymer foam primarily dictates injection pressure into irregular coal-fire voids, continuous injection is essential for sustained cooling by replacing warmed foam, thereby significantly reducing the time required to lower coal temperatures compared to static placement.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep underground, where coal is extracted, the earth is left with a chaotic landscape of broken rock and leftover coal fragments. This area, known as the goaf, is a maze of gaps and tunnels. While mining brings us energy, it also leaves behind a hidden danger: the leftover coal can slowly catch fire on its own. This happens when the coal reacts with oxygen in the air, generating heat that builds up until it ignites. To stop this, engineers often pump a special foam into the ground to fill the gaps, block the oxygen, and cool the hot spots. But there is a catch. The foam must be thick enough to stay put and block the air, yet fluid enough to squeeze through the tiny, winding cracks between the rocks. If it is too thick, the pumps cannot push it far enough; if it is too thin, it might leak away or fail to seal the cracks. Finding the right balance is difficult because the underground world is hidden and impossible to see directly.
A team of researchers set out to solve this puzzle by creating a new type of foam and testing how it moves through these hidden cracks. They developed a material called a dual-network geopolymer foam, which is made from fly ash—a waste product from burning coal—mixed with water and other ingredients to form a stable, heat-resistant gel. In the lab, they proved this foam could withstand high temperatures and stop coal from burning. However, knowing the foam works in a jar is different from knowing how it behaves when forced through a jagged, irregular network of rocks. To bridge this gap, the scientists combined real-world experiments with advanced computer simulations. They built a digital model based on a photograph of packed rocks to mimic the actual underground voids, then watched how their new foam flowed through it on a computer screen.
The researchers discovered that the way this foam flows is more complex than simple liquids. Unlike water, which flows easily, this foam has a "yield stress," meaning it acts like a solid until enough pressure is applied to make it move. Once it starts moving, it becomes thinner and flows faster, a behavior known as shear thinning. When the team simulated injecting the foam into their digital rock maze, they found that this thick, sticky nature did not stop the foam from filling the space. In fact, whether the foam behaved like a thick gel or a simple liquid, it filled nearly the same amount of the empty space in the same amount of time. The difference was not in how much space it covered, but in how hard the pumps had to work. The thick, gel-like foam required significantly more pressure to push through the cracks than a thinner liquid would. This means that while the foam can reach the dangerous spots, the machinery used to inject it must be powerful enough to overcome that extra resistance.
Once the foam is in place, the next challenge is cooling the hot coal. The researchers tested two strategies: stopping the flow of foam once the cracks were filled, or keeping the pumps running to constantly push fresh, cool foam into the area. The results showed that stopping the flow was far less effective. When the injection stopped, the foam sitting around the hot coal slowly warmed up, acting like a blanket that trapped the heat. The temperature difference between the coal and the foam shrank, and the cooling slowed to a crawl. In contrast, when the researchers kept the pumps running, the fresh, cool foam constantly replaced the warmed-up foam. This kept a steady stream of cold material against the hot coal, maintaining a strong temperature difference that pulled heat away efficiently.
The simulations revealed that keeping the injection going cut the time needed to cool the coal by more than half. While the stopped-flow method took 274 seconds to reduce the coal's excess heat by half, the continuous flow achieved the same result in just 132 seconds. Over a longer period, the continuous method removed significantly more total heat, preventing the coal from reigniting. The study highlights that the success of fire control depends on two distinct phases: the initial push to get the material into the cracks, which requires powerful pumps to handle the foam's thickness, and the subsequent cooling phase, which relies on constantly refreshing the material to keep the heat away. By understanding these two separate needs, engineers can design better systems to keep underground mines safe, ensuring that the foam not only reaches the fire but stays effective long enough to put it out.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.