Research and applications of gas distribution space form under the synergistic effect of injection and extraction in goaf of high gas and thick coal seam

Deep beneath the earth, where coal seams are thick and rich, lies a hidden danger that has plagued miners for centuries: the accumulation of invisible, explosive gas. When a coal seam is mined, the roof above it collapses, creating a vast, chaotic space filled with broken rocks and rubble known as a goaf. This rubble is not solid; it is a porous sponge of gaps and cracks where gas, primarily methane, seeps out from the surrounding rock and coal. Because this gas is lighter than air, it naturally rises and can become trapped in pockets near the ceiling of the mine or in the upper corners of the working area. If enough of this gas builds up and meets a spark, it can cause a catastrophic explosion. For decades, engineers have tried to solve this by sucking the gas out with powerful pumps, but in massive, thick coal seams, the gas often hides in deep, hard-to-reach pockets that standard pumps cannot clear effectively.

To tackle this persistent problem, a team of researchers from Xi'an University of Science and Technology and other institutions in China turned to a different strategy: instead of just trying to suck the gas out, they decided to push it. They investigated a method that combines two actions: injecting nitrogen gas into the mine and simultaneously extracting the mixture with a vacuum pump. Nitrogen is an inert gas, meaning it does not burn or explode, and it is heavier than methane. By pumping nitrogen into the bottom of the collapsed rock zone, the researchers aimed to create a rising current that would displace the lighter methane, forcing it upward and away from the workers below. The goal was to understand exactly how these two forces—the push of the nitrogen and the pull of the vacuum—work together to move gas through the complex, rubble-filled underground space.

The researchers built a detailed digital model of a typical mine goaf, representing a space roughly 400 meters high, 300 meters wide, and 130 meters long, filled with broken rock. In this virtual environment, they simulated the injection of nitrogen at various speeds and the extraction of gas at different levels of suction power. They found that the speed at which nitrogen was injected was the most critical factor. When nitrogen was pumped in slowly, it barely moved the gas. However, as the flow rate increased, the nitrogen acted like a rising tide, pushing deep into the rubble and displacing the methane. This displacement was so effective that it increased the range of gas movement by more than 30 percent. The heavier nitrogen settled at the bottom, creating a cushion that forced the lighter methane to float up to the very top of the mine space. There, the gas accumulated in a dome-like shape, far away from the working face where miners operate.

While the nitrogen injection moved the gas, the vacuum pump was needed to catch it. The researchers discovered that simply increasing the suction power did not always help; if the suction was too strong, it could pull the nitrogen out of the mine before it had a chance to do its job of displacing the methane. The key was finding the right balance. By using a mathematical approach called response surface optimization, the team identified a specific combination of settings that worked best: a nitrogen injection speed of 1.20 meters per second paired with an extraction suction pressure of 20 kilopascals. At this specific setting, the nitrogen pushed the gas deep into the mine, and the vacuum pulled the mixture out efficiently, preventing the gas from getting stuck in dangerous pockets. The model showed that this combination could reduce gas concentration significantly more than using either method alone.

To prove that this digital solution worked in the real world, the team applied these findings at a coal mine in Xinjiang. They installed a system that injected nitrogen and extracted gas using the optimized settings they had discovered. Over an 80-day period, they monitored the results and compared them to the mine's previous performance. The data showed a clear improvement: the amount of pure gas being extracted increased by 36 percent. More importantly, the system remained stable over time. Even after two months of operation, the extraction volume stayed consistently above 50 cubic meters per minute, a level that was difficult to maintain with traditional methods. The nitrogen successfully pushed the gas out of the deep, hidden areas of the mine, and the vacuum pump kept the air in the working area safe.

The study concludes that this "push-and-pull" method offers a reliable way to manage gas in the most challenging mining conditions. By understanding how nitrogen and vacuum pressure interact, engineers can now predict exactly how to set their equipment to keep mines safe. The research demonstrates that by carefully matching the speed of injection with the strength of extraction, it is possible to clear gas from the deepest parts of a mine and guide it safely to the surface. This approach does not just remove the immediate danger; it provides a systematic way to prevent gas from ever building up to critical levels again, offering a practical solution for the safe mining of thick, gas-rich coal seams.