An Unseen Fire
Deep beneath the surface of Shandong province, a fire burned for years. This was not a fire of wood or brush, but of coal—a sprawling, subterranean inferno consuming the very rock it was embedded in. Weishan Lake is located in one of China's major coal-producing regions, where such fires represent a persistent environmental and economic problem.
Coal seam fires are a global phenomenon, often triggered by mining activities that expose underground coal to the air. Coal has the ability to self-heat through oxidation; as it reacts with oxygen, its temperature rises. This process can accelerate until the coal reaches its spontaneous combustion point, which can be as high as 750-800 degrees Fahrenheit. Once ignited, these fires are difficult to extinguish. They can burn for decades or even centuries, supported by the vast, interconnected seams of coal and a steady supply of oxygen from the surface through cracks and fissures. The fires release immense quantities of greenhouse gases like carbon dioxide and methane, as well as toxic pollutants including sulfur dioxide, arsenic, mercury, and lead.
Choking the Inferno
To extinguish the Weishan Lake fire, engineers turned to an element that makes up 78% of our atmosphere: nitrogen. The strategy relies on a combination of chemistry and engineering engineering. The first step involved drilling approximately 200 boreholes from the surface directly into the burning coal seams below.
Through these boreholes, massive quantities of liquid nitrogen were pumped into the ground. Liquid nitrogen, at a temperature of -320°F (-196°C), has two primary effects. First, its extreme cold absorbs immense heat from the surrounding rock and coal, drastically lowering the temperature below the point of combustion. Second, as the liquid nitrogen vaporizes, it expands into a gas, displacing the oxygen in the underground voids. Fire requires oxygen to burn, and nitrogen injection is to reduce the oxygen concentration to a level where combustion cannot be sustained, effectively suffocating the flames. The process is slow and requires a continuous injection of nitrogen, sometimes for weeks or months, to ensure the fire is fully inert and does not reignite.
Thermal Monitoring
Once the active firefighting phase is complete, a new challenge begins: ensuring the fire stays out. The rock underground can retain heat for a very long time, and any reintroduction of oxygen could lead to re-ignition. To monitor the cooling process, scientists and engineers rely on thermal imaging technology.
Sensitive thermal cameras, sometimes mounted on drones or satellites, continuously scan the surface above the extinguished fire zone. These instruments can detect minute variations in surface temperature, revealing lingering hotspots deep underground. Some advanced thermal cameras can detect temperature differences as small as 0.05°C. This data allows engineers to create a detailed map of the cooling process, tracking the gradual dissipation of heat from the rock. If a particular area is not cooling as expected or begins to heat up again, it is an early warning, allowing for a rapid response before a new fire can take hold.