Coal-seam fire
A coal-seam fire is a burning of an outcrop or underground coal seam. Most coal-seam fires exhibit smoldering combustion, particularly underground coal-seam fires, because of limited atmospheric oxygen availability. Coal-seam fire instances on Earth date back several million years. Due to thermal insulation and the avoidance of rain/snow extinguishment by the crust, underground coal-seam fires are the most persistent fires on Earth and can burn for thousands of years, like Burning Mountain in Australia. Coal-seam fires can be ignited by self-heating of low-temperature oxidation, lightning, wildfires and even arson. Coal-seam fires have been slowly shaping the lithosphere and changing atmosphere, but this pace has become faster and more extensive in modern times, triggered by mining.
Coal fires are a serious health and safety hazard, affecting the environment by releasing toxic fumes; reigniting grass, brush, or forest fires; and causing subsidence of surface infrastructure such as roads, railways, pipelines, electric lines, bridge supports, buildings, and homes. Whether started by humans or by natural causes, coal-seam fires continue to burn for decades, centuries, or even millennia, until one of the following occurs: either the fuel source is exhausted, a permanent groundwater table is encountered, the depth of the burn becomes greater than the ground's capacity to subside and vent, or humans intervene. Because they burn underground, coal-seam fires are extremely difficult and costly to extinguish, and are unlikely to be suppressed by rainfall. There are strong similarities between coal fires and peat fires.
Across the world, thousands of underground coal fires are burning. The problem is most acute in industrializing, coal-rich nations such as China. Global coal fire emissions are estimated to cause 40 tons of mercury to enter the atmosphere annually, and to represent three percent of the world's annual CO2 emissions.
Origins
Coal-seam fires can be divided into near-surface fires, in which seams extend to the surface and the oxygen required for their ignition comes from the atmosphere, and fires in deep underground mines, where the oxygen comes from ventilation.Mine fires may begin as a result of an industrial accident, generally involving a gas explosion. Historically, some mine fires were started when bootleg mining was stopped by authorities, usually by blowing the mine up. Many recent mine fires have started from people burning trash in a landfill that was in proximity to abandoned coal mines, including the much-publicized Centralia, Pennsylvania, fire, which has been burning since 1962. Of the hundreds of mine fires in the United States burning today, most are found in the state of Pennsylvania.
Some fires along coal seams are natural occurrences. Some coals may self-ignite at temperatures as low as 40 °C for brown coal in the right conditions of moisture and grain size. The fire usually begins a foot or two inside the coal at a depth in which the permeability of the coal allows the inflow of air but in which the ventilation does not remove the heat which is generated. Self-ignition was a recognised problem in steamship times. One well known source of fires is mining breaking into a high pressure cavity of methane gas which on release can generate a spark of static electricity to ignite the gas and start a coal explosion and fire.
Two basic factors determine whether spontaneous combustion occurs or not, the ambient temperature and the grain size:
- The higher the ambient temperature, the more quickly the oxidation reactions proceed.
- The grain size and structure determine its surface area. Kinetics will be limited by availability of reactant, which in this case is carbon exposed to oxygen.
Rural Chinese in coal-bearing regions often dig coal for household use, abandoning the pits when they become too deep, leaving highly combustible coal dust exposed to the air. Using satellite imagery to map China's coal fires resulted in the discovery of many previously unknown fires. The oldest coal fire in China is in Baijigou and is said to have been burning since the Qing Dynasty.
Detection
Before attempting to extinguish a near-surface coal-seam fire, its location and underground extent should be determined as precisely as possible. Besides studying the geographic, geologic and infrastructural context, information can be gained from direct measurements. These include:- Temperature measurements of the land surface, in fissures and boreholes, for example using pyrometers
- Gas measurements to characterize the fire ventilation system and the gas composition, so that the combustion reactions can be described
- Geophysical measurements on the ground and from aircraft to establish the extent of conductivity or other underground parameters. For example, conductivity measurements map humidity changes near the fire; measuring magnetism can determine changes in the magnetic characteristics of the adjacent rock caused by heat
- Remote sensing from aircraft and satellites. High resolution optical mapping, thermal imaging and hyperspectral data play a role. Underground coal fires of several hundred to over a thousand degrees Celsius may raise the surface temperature by only a few degrees. This order of magnitude is similar to the temperature difference between the sunlit and shadowed slopes of a slag heap or sand dune. Infrared detecting equipment is able to track the fire's location as the fire heats the ground on all sides of it. However, remote sensing techniques are unable to distinguish individual fires burning near one another and often lead to undercounting of actual fires. They may also have some difficulties distinguishing coal-seam fires from forest fires. Combining in-situ data with remote sensing data does allow for monitoring of coal fire intensity over longer periods using time-series analyses.
Environmental impact
Besides destruction of the affected areas, coal fires often emit toxic gases, including carbon monoxide and sulphur dioxide. China's coal fires, which consume an estimated 20 – 200 million tons of coal a year, emit 6 to 43 million tonnes of greenhouse gas each year.One of the most visible changes will be subsidence. Another local environmental effect can include the presence of plants or animals that are aided by the coal fire. The prevalence of non-native plants can depend upon the fire's duration and the size of the affected area. For example, near a coal fire in Germany, many Mediterranean insects and spiders were identified in a region with cold winters, and it is believed that elevated ground temperatures above the fires permitted their survival.
Extinguishing coal fires
In order to thrive, a fire requires fuel, oxygen, and heat. As underground fires are very difficult to reach directly, fire fighting involves finding an appropriate methodology which addresses the interaction of fuel and oxygen for the specific fire in question. A fire can be isolated from its fuel source, for example through firebreaks or fireproof barriers. Many fires, particularly those on steep slopes, can be completely excavated. In the case of near-surface coal-seam fires, the influx of oxygen in the air can be interrupted by covering the area or installing gas-tight barriers. Another possibility is to hinder the outflow of combustion gases so that the fire is quenched by its own exhaust fumes. Energy can be removed by cooling, usually by injecting large amounts of water. However, if any remaining dry coal absorbs water, the resulting heat of absorption can lead to re-ignition of a once-quenched fire as the area dries. Accordingly, more energy must be removed than the fire generates. In practice these methods are combined, and each case depends on the resources available. This is especially true for water, for example in arid regions, and for covering material, such as loess or clay, to prevent contact with the atmosphere.Extinguishing underground coal fires, which sometimes exceed temperatures of 540 °C, is both highly dangerous and very expensive.
Near-surface coal-seam fires are routinely extinguished in China following a standard method basically consisting of the following phases:
- Smoothing the surface above the fire with heavy equipment to make it fit for traffic.
- Drilling holes in the fire zone about 20 m apart down to the source of the fire, following a regular grid.
- Injecting water or mud in the boreholes long term, usually 1 to 2 years.
- Covering the entire area with an impermeable layer about 1 m thick, e.g., of loess.
- Planting vegetation to the extent the climate allows.
Underground coal-seam fires are customarily quenched by inertisation through mine rescue personnel. Toward this end the affected area is isolated by dam constructions in the galleries. Then an inert gas, usually nitrogen, is introduced, usually making use of available pipelines.
In 2004, the Chinese government claimed success in extinguishing a mine fire at a colliery near Urumqi in China's Xinjiang province that had been burning since 1874. However, a March 2008 Time magazine article quotes researcher Steven Q. Andrews as saying, "I decided to go to see how it was extinguished, and flames were visible and the entire thing was still burning. ... They said it was put out, and who is to say otherwise?"
A jet engine unit, known as Gorniczy Agregat Gasniczy, was developed in Poland and successfully used for fighting coal fires and displacing firedamp in mines.
Putting out the fires can be expensive for local government, so may be funded by central government. ''Time'' magazine reported in July 2010 that less expensive alternatives for extinguishing coal-seam fires were beginning to reach the market, including heat-resistant grouts and a fire-smothering nitrogen foam, with other innovative solutions on the way.