Thermal–oxygen evolution and atmospheric pollutant emission characteristics during the vertical propagation of underground coal fires
This study utilizes a physical similarity simulation to characterize the stage-dependent and depth-dependent coupling of thermal–oxygen evolution and atmospheric pollutant emissions during the vertical propagation of underground coal fires, revealing distinct temporal release patterns for different pollutants and identifying CO as the primary contributor to moderate-to-severe pollution.
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
Beneath the earth's surface, hidden within layers of rock and soil, coal seams can sometimes catch fire without a single spark from a match or a lightning strike. This phenomenon, known as spontaneous combustion, occurs when coal slowly reacts with oxygen in the air, a process that generates heat. If that heat cannot escape, it builds up until the coal ignites, creating a fire that can smolder for decades, moving through the ground in ways that are difficult to see or track. These underground blazes are more than just a geological curiosity; they are a persistent environmental hazard. As the fire burns, it consumes oxygen and releases a toxic mix of gases and fine dust particles into the air above, threatening the health of nearby communities and the stability of the land. Understanding how these fires move vertically through the earth, how they change the air around them, and what they release at different stages is crucial for keeping mines safe and protecting the environment.
To uncover the secrets of this hidden process, a team of researchers from several universities and research institutes in China built a controlled model of an underground coal fire. They constructed a large, insulated box divided into eight horizontal layers. The bottom six layers were filled with coal particles, while the top two were packed with soil to mimic the ground and rock that sit above a real coal seam. By heating the bottom layer of coal to a specific temperature, the scientists triggered a self-sustaining fire that began to spread upward through the coal and into the soil above. Throughout the experiment, which ran for over 1,100 hours, they continuously monitored the temperature, the amount of oxygen, and the concentration of various gases and particles at every level, creating a detailed, hour-by-hour record of the fire's life cycle from its first spark to its final extinguishment.
The researchers discovered that the fire does not behave the same way at every depth. As the heat moved upward, the process unfolded in five distinct stages: a quiet incubation period, a rapid heating phase, an intense reaction zone, a quick cooling phase, and finally, a burnout stage. The layers closest to the surface heated up and cooled down quickly, essentially burning out before they could sustain a long fire. However, the middle layers, buried deeper, became the heart of the blaze. These deeper sections retained heat much better and maintained a steady supply of oxygen, allowing them to sustain a powerful, long-lasting reaction that lasted for hundreds of hours. The deepest layer, at the very bottom of the coal section, struggled to keep the fire alive. It received heat from the layers above, but the path for fresh oxygen to reach it was too long and restricted, causing the fire to weaken and eventually die out in that specific zone.
The gases released by the fire told a similar story of changing conditions over time. In the very early hours, the fire released large amounts of methane and ethane, which are hydrocarbon gases formed when coal breaks down under heat. These gases appeared quickly but vanished just as fast, dropping to background levels within about 80 hours. As the fire matured and the temperature rose, the chemical reaction shifted. The release of these hydrocarbons faded, and the fire began to produce massive quantities of carbon dioxide and carbon monoxide. These two gases became the dominant pollutants for the remainder of the experiment, with carbon monoxide reaching levels that would be classified as severe pollution in the air above. The study also tracked the release of tiny solid particles, known as particulate matter. Like the hydrocarbon gases, these particles were released in a concentrated burst during the first 40 to 60 hours, when the combustion was incomplete, before dropping off significantly as the fire burned more cleanly.
One of the most striking findings was the delay between what happened deep underground and what appeared at the surface. The temperature spikes and gas surges recorded in the coal layers took time to travel through the soil and rock above. By the time the heat and pollutants reached the ground surface or the atmosphere, they were weaker and arrived later than they did in the coal itself. This lag means that if you only look at the air above the ground, you might miss the true intensity of the fire happening below. The researchers found that the air quality above the fire was dominated by severe pollution for most of the experiment. While the early stage was marked by high levels of dust and particles, the middle and later stages were defined by a persistent, dangerous cloud of carbon monoxide. This shift highlights that the danger from an underground coal fire is not static; it evolves, changing from a dust-heavy hazard to a gas-heavy threat as the fire matures.
The study concludes that the life of an underground coal fire is a complex dance of heat, oxygen, and chemical reactions that changes depending on how deep you are and how long the fire has been burning. It is not a simple, uniform event but a layered process where the middle sections of the coal seam often become the most dangerous and persistent zones. The findings provide a clearer picture of how these fires develop, offering a roadmap for better monitoring. By knowing which gases appear at which stages and how they move through the ground, scientists and engineers can develop more effective ways to detect these hidden fires early and manage the risks they pose to the surface world. The research confirms that while the fire may start deep underground, its effects are felt clearly in the air we breathe, shifting from a mix of dust and hydrocarbons to a long-lasting cloud of toxic gas.
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