A coal fire is the underground smouldering of a coal seam or coal mine. They are emerging as a global threat with significant economic, social and ecological impacts.

Coal seam fires        Prevention Extinction

Coal seam fires may be categorized into near surface fires in outcropping seams that are supported by oxygen from direct contact to the atmosphere and mine fires supported by oxygen from the artificial mine ventilation.

Spontaneous coal fires

The final reason for all fires is the chemical reaction of the hydrocarbon molecules of the fuel with oxygen of the air. This exothermal reaction can take place at any temperature. The reaction velocity however is strongly temperature dependent and increases with temperature nearly exponential. If the fuel is broken up in small particles or porous, oxygen has access everywhere and the entire volume may act as a heat source.
The situation is not critical if the heat energy transported to the surface by either conduction or convection and finally lost to the environment is larger then the heat produced by the reaction. If the heat produced by the reaction however is over longer time larger then the heat loss to the environment the system will turn to be critical. Temperatures will rise continuously, the reaction will accelerate and finally the fuel will start burning spontaneously.

Two factors will finally be responsible: The surrounding temperature and the volume of fuel involved:

• If the surrounding temperatures are higher the oxidation processes will run faster and thus the heat production is higher within the fuel volume.

• If the fuel volume is larger the heat produced inside can hardly flow to the surface and into the environment, the fuel is more likely to start burning. In addition material broken up in small part or porous material usually has a low heat conduction coefficient and may act like thermal isolators.

The most important parameter characterizing self ignition is the self ignition temperature. This is not a material constant, but dependent on volume and shape, more specific on the relation of volume and surface. Self ignition temperatures decrease strongly with volume. Furthermore this temperature is dependent on many fuel material parameters as caloric value, heat conduction coefficient, particle size. In case of coal it depends further more on the coal type and rank; for hard coal its generally higher the for brown coal or lignite.

If the fuel volume is sufficient spontaneous ignition may happen at room temperatures or at temperature at the yearly average temperature. The time needed for the fire to develop may be month or even years.

Brown coal or lignite may start burning at 40 °C to 60 °C whereas anthracite will start (under the same conditions!) at 140 °C. The smouldering will usually start several decimetres below the surface in a depth where the permeability of the coal allows the access of enough air but the air flow is slow enough to not extract the produced heat by convection. Due to the low heat conductance coefficient of coal heat extraction by conduction alone is not sufficient.

Factors influencing spontaneous induction are beside others:

Air circulation
Climate (arid, semiarid)
Coal quality coal type (carbon content, gas content, ash content, rank)
Particle size (small particles have larger surface/volume relation)
Geological geomorphological settings
Mining influence (Openings, fractures, subsidence)
Hydro geological settings (moisture content)

Spontaneous ignition needs time. How much depends on many factors, as temperature, volume, particle size. Finally the time to ignition is another parameter to describe the addiction of coal to burn. For larger volumes the temperature needed is smaller but the time needed larger. Normally it will take months before coal will start smouldering.
If coal seams outcrop to the surface, air has access for long times. At those location seams will start to burn spontaneous and continue burning for decades. Globally at least 20 to 30 million tons are burned by those fires. The coal being made inaccessible for further mining is about the times more.

Heat producing reactions

There are two know heat producing adsorption reactions:

Physisorption of oxygen. This takes place at temperatures up to 50 °C and delivers 42 kJ/mol.

Chemisorption of oxygen. This produces several chemical compounds after overcome the activation energy of the coal surface. From carbon-, hydrogen- and oxygen atoms peroxides are formed and about 100 kJ/mol of heat energy is produced. The newly built molecules may oxidise further and produce heat with increasing temperatures and finally exhaust as carbon dioxide, carbon monoxide and water (vapour).

The most important reactions are:

C und O2 form CO2 (394 kJ/mol)
2C und O2 form 2CO (170 kJ/mol)

Coal seam fires not spontaneously ignited

Nearly all coal seam fire started spontaneously. In some case however external ignition is possible. Finally we may not see if a certain fire started spontaneous or not. This is in any case true for fires in deep mines, but also for fires close to the surface as long as mining is involved. Possible sources for ignition are electrical machinery, bad maintained bearings as well as handling of explosives or wrong application of welding or bevelling.
In many reported cases leaving back coal in mining application or accumulation of coal dust was the final reason for a fire. Consequent acceptance of mining regulation may avoid most of those fires.
Mine fire may interact with methane explosions or coal dust explosions in mines. Near surface coal fires may interact with forest fires. This was reported from the USA and especially from Sumatra, Indonesia.

Global coal seam fires

Coal fires are reported from coal mining districts all over the world. The most important are the following:

India

Besides the areas of Ranigani and Singareni coal seam fires rage in Jharia (North West India). In an area of about 700 km2 about 160 fires are burning. As a consequence of the fires hang slides, sink holes and subsidence is reported. As this area is densely populated pollution is severe.
Coal mining supports the development of fires it give air better access. On the other hand coal fires imitate the mining and may even stop it. It is estimated that in India 70% of the fires are due to mining.

USA

Many coal mining areas in the USA suffer from spontaneous coal seam fires. The Federal Office of Surface Mining (OSM) provides a data base (AMLIS) that lists 150 fire zones (1999). Those are not only in Kentucky, Pennsylvania and West Virginia in the east of the Appalachian-coal district, but also in  Colorado and the Rocky Mountains.
In Pennsylvania 45 fire zones are reported. The most known is Centralia Mine, in the anthracite- coal area of Columbia County. This fire burn since 1962 and develop below the city. There was some effort to extinguishing the fires but finally the city was lost.
In Colorado some spontaneous coal fires are due to annual changes in the water table. Those changes may rise water temperatures by 30 °C, and thus start the self ignition process.
In Powder River Basin in Wyoming und Montana about 800 billion tons of lignite is known. Already the Lewis-und-Clark-Expedition (1804 to 1806) reported on coal fires in that region. Here we have also coal fires from geological times. They are three million years old and shaped the landscape to a certain extend. An area of 4.ooo km2 is covered with clinker or scoria, part of the laying in the c. Well known and spectacular is the outlook from the scoria point on an extended area of fire red clinker.

Germany

In Planitz near Zwickau a coal seam burned from 1476 and was finally not extinguished before 1860. Ernst August Geitner started in 1479 a green house with tropical plants above the known Planitz fire zone and was possible the first in using energy from coal fires commercially. In Dudweiler (Saar) 1668 a coal seam started to burn and developed to a tourist attraction named 'Burning Mountain', even visited and described by Goethe. Equally known is the so called 'Smelling Wall' at the east slopes of 'Hohe Meissner', where after closing the lignite mining some seams started burning centuries ago and exhaust gases escaped to the surface causing the 'smell'. The hard rock mining was accompanied all time by coal- mainly mine fires. Reported are about two fires per year on average. As the coal mining concentrated in Germany on the Ruhr- and Saar Area, fire prevention technologies were developed in those areas. Today most of the coal fires here are due to unwanted ventilation in abandoned parts of the mines. Those measures were principally successful and heavy mine fires with loss of human life did not occur.
After closing the last deep lignite mines in Hirschberg close to Grossalmerode in Hessen in 2003, lignite is mined in Germany in open pits only, in the Rheinische Revier, in the  Mitteldeutschen Revier and in the  Lausitzer Revier. In the last years no coal fires were reported from these areas as complex strategies of prevention are successful.

Rest of Europe and Russia

Reported coal fires in those areas are decreasing parallel to the decrease of mining activity in general. Some burning is reported from Poland, Czech Republic and Ukraine. In Ukraine 2.000 million tons are laying on dumps and 74 fire zones are reported. This is mainly in the basins of Kuzbass, Petschora and Donezk.
In Kosovo (Serbia) and Bosnia-Herzegovina coal seams are burning close to open pit or deep mining.

Africa

The big coal mining districts of Africa are in the south of the continent, in South Africa, Zimbabwe, Botswana, Mozambique und Zambia. Coal fires are reported from all that regions.

Australia

Five kilometres north of the city of Wingen in New South Wales (NSW) the Burning Mountain is a tourist place since thousands of years. Actually the fire is 30m below surface and advances about 1m per year. Overall it moved about 6 kilometres. Many more fire zones are reported in Australia.

China

China is the larges coal producer (and consumer) world wide: It produces about 1.8 billion tons in 2006. As a result coal fires are a severe problem in China. It is estimated that 10-20 million tons are directly burned by coal seam fires and 100-200 million tons of coal are lost for the mining industry. The fire zones are located in a belt covering the entire north of China. More then 100 burning areas are known divided in several burning zones each. Concentrations are in the provinces (autonomous regions) Xinjiang, Inner Mongolia and Ningxia. Besides the loss of energy resources those fires cause air and water pollution and emit enormous amounts of green house gases (carbon dioxide and methane). This mainly causes the international interest in those fires. China is the only country in the world starting and performing enormous activities for extinction. Several fires are already extinguished. New methods are developed within a Sino-German Research Initiative.

Other areas

The Indonesian Forest Fire Prevention and Control Project (FFPCP) reported coal fires burning more then four years. Two new fires were reported from the region Suban Jeriji. Coal fires were also reported from Venezuela, but no detailed information is available.

Prevention of coal seam fires

Most of near surface coal seam fires start spontaneously without artificial ignition. They are observed from geological time for instance by mapping pyromethamorphic clinkering of surrounding rock. Nevertheless most of the coal seam fires burning today are under the responsibility of mining activities. Avoiding fires or fire prevention is therefore primarily the responsibility of the mining industry. The final aim is 'mining without fires'. Many suggestions have been made, as change of the mining technology or mining methods, alternative mine lay out, different ventilation methods. Extremely important is the complete extraction of coal and to avoid leaving back coal or coal dust in the goafs. Finally mine closure is an issue and should be made strictly along the regulations.
Mine fires finally can only develop it regulations are not strictly followed. This may happen especially if the min is under strong production pressure as in the energy hungry China. But also in the USA and Germany mine fires are not that seldom. Often entire mines had to be given up due to mine fires.

Exploration of fire zones. Measuring technologies

For planning extinction of near surface coal seam fires it is essential to have information about depth, extension and shape of the fire zones. Besides recherché geological, geographical and infrastructural settings further information from direct measurements are needed. This may be:

Temperature measurements at the earth's surface or in fractures or drill holes using contact or radiation thermometers.

Gas measurements at the earth's surface to describe the ventilation system connected to the fire. This includes gas quantities and composition and gives some insight in the burning reactions

Geophysical measurements as well on the earth's surface as from fix wing planes or helicopters to map the electrical conductivity of the subsurface or other parameters. Doing this, conductivity may show the changes in moisture content and magnetic parameters may show changes in the surrounding rock due to temperature effects (pyromethamorphosis)

Remote sensing techniques as well from planes as from satellites. Besides high resolution optical scanners thermal imaging and hyper spectral scans are helpful. Cola seam fires burning with temperatures of several hundred or even thousand degrees Celsius may cause thermal anomalies at the earth's surface from just a few degrees. This may be within the same range as temperature anomalies caused by insolation on sun oriented slopes

Mine fires in deep mines are monitored by permanently installed sensor systems within the mine. They may monitor Temperature, pressure, air velocity and different parameters describing gas composition (carbon monoxide, methane). Those sensors are connected to the mine monitoring system and are mainly used to rescue miners in time.

Extinction of coal seam fires

Burn triangle

A fire needs to exist continuously:
Fuel (coal)
Oxygen (air)
Energy (heat)

The influence of these three parts on the fire is known as the 'burn triangle'.
Al possible extinction methods may be derived from the burn triangle. It is possible to separate a fire from fuel, as using aisles or barriers. Some fires, normally located in steep slopes may be excavated entirely.
The access of oxygen (air) to the fire may be suppressed by covering the fire zone with impermeable material. Also vertical barriers may isolate the fire from air access. Alternative the exhaust gases may be hindered to flow away and this will finally choke the fire.
Extraction of energy (heat) is normally done by cooling by injecting large amount of water. It is essential that the heat is not stored in the coal but in the surrounding rock. Cooling down the rock volume may take years. Most of the energy is extracted by vaporizing the water (latent heat extraction).
Practically all known methods may be combined. Often the availability of resources influences the decisions, this may be availability of water in arid areas or the availability of clay to cover the fire zones.
Actually coal seam fires are only extinguished in China. The Chinese mining industry developed guidelines which are (simplified) as follows:

1.    Levelling the earth's surface to give access to heavy machinery.

2.    Drilling wells as close as possible to the fires into the hot area on a regular grid of about 15-20m.

3.    Injection of water with enough quantity over a time of 1-2 years.

4.    Covering the entire area with impermeable soil (clay) with a thickness of up to one meter.

5.    Re-vegetation of the area.

The water used for cooling may be enriched with additives. This is still a topic of research.
Mine fires in deep mines are usually extinguished using inter gas as nitrogen. This will be filled in parts of the mine separated by dams especially built for the extinction procedure.

Monitoring of fire zones

Areas with actually burning fires close to the earth's surface but as well potential fire risk areas need permanent supervision and monitoring. This allows early notice about new fires and finally extinction in an early not fully developed phase. Already extinguished fire zones have to be monitored as well, as remaining small hot islands may remain there for years cause to the low heat conductivity of coal. Large scale supervision is possible using satellites with repeated readings. Isolated fire zones may me supervised by repeated control, using adequate measuring technology (temperatures, gas measurements, geophysical measurements). Those visits may be every six month as coal fires develop slowly.
The risk of mine fires in deep mines is usually monitors by permanently installed sensor systems.