Coal mine fire from Wikipedia, the free encyclopedia
A coal fire is a fire which has broken out in a coal seam or in stored coal. They often occur spontaneously when coal comes into contact with oxygen in the air. In the case of coal seam fires, this can happen by natural means when the seam comes to the earth surface as a result of orogenesis and erosion. However, fires of this kind are also caused by underground mining when oxygen sucked in by ventilation reaches seams at deep levels. Fires frequently arise underground in abandoned workings where coal has been left behind. Coal fires can also be caused when coal is shifted around from one storage locality to another. Fires in coal and coke heaps, fires in waste tips which still contain coal remains, particularly in odd spots with higher concentrations, and fires during coal transport in ships and railway wagons are well known. While the reaction temperature is around 150ºC to 500 ° C, this is known as incomplete combustion or a smouldering fire. This type of combustion takes place with a lack of oxygen. Above 500ºC, fed with sufficient oxygen, the fire becomes a Glimmbrand[German distinguishes 2 kinds of smouldering fires] (up to approx. 1000ºC) or a flaming fire (up to 1200 °C) in which complete combustion takes place. This article does not deal with deliberate incineration of coal in combustion plants.
Detailed view into a crevice above a coal fire in China
List of contents
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Coal seam fires
Coal fire in Xinjiang, China
Coal seam fires can be subdivided into near-surface fires, in other words seams exposed at the surface where oxygen in the air is available, and fires in mines at greater or lesser depth. In this case the oxygen comes from the ventilation system.
Spontaneous coal fires [ .]
The cause of spontaneous coal fires is the flammability of coal. In a coal fire the oxygen reacts with the molecules of the fuel . Such exothermic reactions take place at all temperatures. However, the speed is highly dependent on temperature and thus the rate of reaction increases almost exponentially as temperature rises. If a fuel is present in comminuted form such as solid particles or when the solid matter is porous, then oxygen is available in the whole system and the entire agglomeration can be seen as a heat source.
A sub-critical position is reached when the rate of heat energy released from the interior of the aggregate is less than the rate at which conductivity transports the heat to the edge of the aggregate and dissipates it to the surroundings from the surface of the aggregate. If inside the aggregate more heat energy is released than can be dissipated to the exterior, then the system enters a critical condition. The temperature in the interior increases which leads to a further increase in the reaction rate and finally to spontaneous combustion.
Two factors are decisive in whether spontaneous combustion takes place or not: the ambient temperature and the aggregate size.
• The higher the ambient temperature is, the more rapidly the oxidation reactions proceed and the greater the rate of heat release inside the aggregate.
• The larger the aggregate is, the more difficult it becomes to dissipate the heat to the exterior, which means the more quickly a fire caused by spontaneous combustion will occur. The poor heat dissipation is caused by the fact that the porous or comminuted solids mostly have low thermal conductivity- they have an insulating effect.
The most commonly used parameter for the start of this process is the auto ignition temperature. This is not a constant but always depends on the aggregate size and geometry, in particular the relationship of volume to surface area and decreases as aggregate volume increases. Of course the auto ignition temperature also depends on the characteristics of the fuel in question. These include among others, the calorific value, its thermal conductivity and grain size. Determining it is rather difficult since the exact specifications must be known. Under equal conditions, the auto ignition temperature of bituminous coal will be higher than that of lignite.
In contrast to a tangible source of combustion such as an open flame or a hot surface, the process of spontaneous combustion occurs without any outside influence. If an aggregate is sufficiently large, this process can even occur at temperatures which are within the annual average temperature range of a locality. This was the case for example in what is known as the Senate coal reserve [coal stored in W Berlin in the event of a 2nd Blockade by the Soviet Union]. The coal kept in store there was partly supercritical, ie: stored above its auto ignition temperature and thus ignited itself. Here it should be noted that the induction times, that is the times from the start of storage to ignition become longer, the larger the aggregate and thus the lower the auto ignition temperature are. In the case of the Senate coal reserve, the induction time was several years. Lignite can begin to burn spontaneously at between about 40ºC and 60ºC, whilst this does not occur with the highest value anthracite until a temperature of 140ºC is reached. The fire usually occurs at a depth of a few decimetres within the coal, at a depth where the permeability of the coal permits access to air but where on the other hand the passage of air does not generally dissipate the heat created. The low thermal conductivity of coal does not permit heat dissipation without fluid circulation. Factors which influence spontaneous ignition, among others, are: • air circulation • climate (arid or semiarid) • quality and type of coal (carbon content, proportion of volatile substances, rank of coal) • particle size (with smaller granulation the surface is greater and thus so is the danger of auto ignition or ignition by outside influences) • geological and geomorphological conditions • influences of mining (open pits, cracks, subsidence) • hydrogeological influences (humidity content) The spontaneous ignition of coal requires time. It depends on many factors, among other things the ambient temperature, but also on the volume of coal, and therefore in the laboratory on the size of the test sample. The time necessary to reach spontaneous combustion is a further measurement of how prone coal is to self-ignite. With large amounts of coal, for instance in situ, the necessary minimum ambient temperature is lower than in the case of small amounts of coal; on the other hand it takes a very long time, usually months until a coal sample ignites spontaneously. Where a seam lies close to the earth's surface, air has direct access for a longer period. In these cases fires occur spontaneously and can burn for decades. In the world as a whole, at least 20 to 30 million tonnes of coal are consumed annually by fires of this type. At least ten times as much coal is rendered useless for mining , because the remaining coal cannot be reached or is no longer economically extractable. Heat-producing reactions [ .]
There are 2 heat-producing adsorption processes:
• The physisorption of oxygen is possible up to about 50ºC and provides a quantity of energy of 42 kJ/mol. • The chemisorption of oxygen creates a whole series of chemical bonds after overcoming activation energy in the surface of the coal. For instance peroxides are produced from the carbon, hydrogen and oxygen atoms which are present during energy transfer of more than 100 kJ/mol. These newly created molecules oxidise and release heat as the temperature rises further and then are given off mainly in the form of carbon dioxide, carbon monoxide or steam. The principle reactions of carbon with oxygen in the air are: • C and O2 form CO2 and release 394 kJ/mol. • 2C and O2 form 2CO and release 170 kJ/mol.
Coal seam fires caused by extraneous activity [ .]
Coal seam fires occur for the most part due to processes of spontaneous combustion. In a minority of cases, they may be caused by outside activity. Afterwards, it is impossible to see how the fire was started. This applies in particular to fires underground, but also to near-surface fires, insofar as they are related to near-surface mining. Possible causes are electric machinery in poor condition, rollers on conveyor belts which have run hot and lack of proper attention during explosive, welding or grinding jobs. As a rule, leaving behind coal remains in abandoned seams, or the accumulation of large quantities of coal dust play a role. Consistently adhering to mining regulations excludes the possibility of ignition caused by other means. Underground coal seam fires may interact with methane explosions or coal dust explosions. Near-surface coal seam fires frequently ignite forest fires and vice-versa. There are reports of this from the USA, but especially from the Indonesian Island of Sumatra. Coal seam fires worldwide [ .] Coal burns in all coal mining districts throughout the world. The most important countries which have reported coal fires are the following: India [ .] Besides the coal-mining districts of Ranigani and Singareni fires rage especially in the Jharia Region in the northwest of India, west of Calcutta. In 1997, in an area of 700 square kilometres there were 160 individual fires. Related to the fires, there are reports of land slips, sinkholes and subsidence. As these areas are densely populated, the subsequent local effects on the environment are considerable.
Bituminous coal mining favours coal fires, because it gives the oxygen in the air easier access to the coal , on the other hand, the fires have a negative effect on or may even prevent mining activity altogether. It is estimated that in India around 70 per cent of the fires are a result of mining.
USA [ .]
Crevice giving a view of the fire
Many coal mining districts in the USA are affected by spontaneous coal fires. The Federal Office of Surface Mining (OSM) keeps a data bank (AMLIS), which for 1999 listed 150 fire areas. Not only Kentucky, Pennsylvania und West Virginia in the eastern part of the Appalachian coal fields are affected, but also Colorado and the Rocky Mountains.
In Pennsylvania, 45 fire zones are registered. The best-known is the fire at the Centralia Mine in the anthracite coal district of Columbia County. These fires have been burning since 1962 and are spreading under the town. In Centralia attempts were made at extinguishing the fires, but in the end the town was abandoned.
In Colorado coal fires have occurred as a result of fluctuations in the groundwater. With fluctuations of this kind, the coal temperature can increase by up to 30 ° which can set off spontaneous combustion.
In the Powder River Basin in Wyoming and Montana there are reserves of some 800 billion tonnes of lignite. Even the Lewis and Clark Expedition (1804 to 1806) reported fires. These have been occurring naturally there for about three million years and have shaped the landscape. For instance there lies an area of approximately 4000 square kilometres of clinker (clinker or scoria) , partly inside the Theodore Roosevelt National Park. For example the view from Scoria Point out over the firey red clinker is spectacular.[1]
Germany [ .]
In Planitz (Municipality of Zwickau) a coal seam burnt from 1476 onwards which was only able to be extinguished in 1860. In the year 1837 Ernst August Geitner planned and started a nursery on the site of the Planitz underground fire where tropical plants were grown. [2] In Dudweiler in the Saarland, a coal seam caught fire around 1668 which is still burning today. What was known as the Burning Hill turned into something of a tourist attraction which was visited by Goethe himself. Equally well known is the Stinksteinwand in the Schwalben Valley on the eastern slope of the Hohe Meißner where, after lignite mining was abandoned several centuries ago, some seams ignited and the combustion gases nowadays reach the surface. In connection with bituminous coal mining, there have been and there still are today up to two coal fires per mine per year. As a result of the concentration of bituminous coal mining in the Ruhr (7 mines), the Saar (1 mine), and the Ibbenbüren coal-mining district (1 mine), in these coal-mining districts advanced methods of preventing and fighting coal fires have been developed. Nowadays the causes of underground coal seam fires can mostly be found in what is known as "creeping ventilation" or defective machinery. "Creeping ventilation" are air currents in a mine ("ventilation") which manage to get unintentionally into isolated or closed off areas, for instance into those with fine-grained coal. Extensive prevention, control and rescue measures have meant that serious mine accidents with loss of life rarely happen in Germany nowadays. As well, self-extinguishing conveyor belt materials, fixed and portable CO, CO2 and other gas measuring instruments are used. Just as important are the Mines Rescue Services and their regular practices in preventing and extinguishing fires.
After the closing of the last underground lignite mine at Hirschberg near Großalmerode in Hessen in 2003, in Germany lignite is only mined open cast in the Rhine, in the Central German and in the Lausitz coal-mining districts. Here too, fires can occasionally occur when the earth overlying the lignite coal seam has long been removed and the coal has been in contact with oxygen in the air for a long time. This can be avoided by a suitable programme of excavation and coal sorting. For these cases too there are control measures and Mines Rescue Service to recognise and combat hazard situations early on.
The rest of Europe and Russia [ .] The number of coal fires in Europe has gone down in accordance with the reduction in mining activity. There are still reports of fires from Eastern Europe, namely Poland, the Czech Republic and the Ukraine. The last-mentioned has 2,000 million tonnes of coal stored in 2100 coal tips. Of these, 140 tips are burning. In Russia in 1998, 74 coal fires were reported. The coal mining districts affected are those of Kusbass and the basins near the cities of Petschora und Donetsk (Donets basin). On the Kosovo plain, huge coal seams of up to 20 metre deep burn in the open cast mines and in the areas where coal was formerly mined underground. Africa [ .] Africa's large coal mining districts are in the south, in South Africa, Zimbabwe, Botswana, Mozambique and Zambia, where fires are most often found in coal stores. Australia [ .] Five kilometres to the north of the town of Wingen in New South Wales, Burning Mountain has been burning for some 6 000 years, the oldest coal seam fire in the world. At present the fire is 30 metres below the surface and advancing at a speed of 1 metre per year. Up to today it has covered a distance of 6 kilometres. [3]Other coal fires are recorded. China [ .] In China, with annual production of around 2.5 billion tonnes the world's largest coal producer, coal fires represent a serious problem. It is estimated that each year in Northern China around 10 to 20 millions tonnes of coal go up in flames, and 100 to 200 million tonnes are made inaccessible for mining. The coal fires stretch along a belt throughout the whole of the north of China, in which over one hundred large fire areas can be named, each of which contains a multitude of individual fire zones. The main foci are in the provinces of Xinjiang, Inner Mongolia, and Ningxia. Apart from the losses due to the burnt and wasted coal, these fires contribute to air pollution as well as considerably increased emission of greenhouse gases and are thus a problem of international concern. In China at the same time is where the most intense action is being taken at world level to extinguish fires. New extinguishing methods are being developed in the coal fires research project within the Sino German Coal Fire Research Initiative. Other areas [ .] The Indonesian Forest Fire Prevention and Control Project (FFPCP) gave reports of coal fires which burned for four years. Besides these, two new fires were reported in the Suban Jeriji Region. There have also been reports of coal fires from Venezuela , however, without precise information. Avoiding coal seam fires [ .]
Measurement of radiation temperature
Near-surface coal fires occur for the most part spontaneously and unintentionally, in other words without anybody directly igniting them. Coal fires are proven to have occurred in previous geological ages, for instance by the presence of clinkerised surrounding rock. Nevertheless, nowadays nearly all known coal fires are in the end caused by humans. The most important factor here is mining. Avoiding coal fires thus means carrying on mining operations without fires. To achieve this there are many suggestions, ranging from changes in mining methods, changes in design or alternative digging techniques to different ventilation methods. Particularly important is the complete removal of coal, in other words avoiding leaving coal remains in particular in the form of small chunks of coal or coal dust. Properly monitoring very small near-surface workings is another matter which needs to be looked at.
Underground coal seam fires occur only when existing regulations are disregarded. This occurs especially frequently in the Chinese mines which are under pressure to produce, but even in other coal-mining districts including the Ruhr area, coal fires are not infrequent. Mines have often been abandoned as a result of such fires.
Reconnoitring fire zones, measuring techniques [ .]
Measuring the distribution of electrical conductivity with a sensor suspended from a helicopter.
When extinguishing a near-surface coal seam fire, it is an advantage to know as accurately as possible its position and how far it stretches underground. Apart from research on the geographical, geological and infrastructural surroundings, further data can be gathered by direct measurements. The main focus is on:
• temperature measurements at the earth's surface, in cracks and boreholes, for example using a pyrometer.
• gas measurements in order to determine the fire's ventilation system (quantity and speed) and the gas composition in order to determine the reaction.
• Geophysical measurements both on the ground, as well as from aircraft and helicopters in order to determine the distribution of conductivity or other parameters below the surface. In the process, the conductivity measurements chart changes in humidity in areas surrounding the fires. For instance magnetic measurements can demonstrate changes in the magnetic characteristics of the surrounding rock as a result of the effects of heat.
• methods of remote sensing from aircraft, but especially from satellites. Apart from high resolution optical mapping, thermal imaging and hyperspectral data are important. Coal fires which can range in temperature from several hundred up to over a thousand Celsius degrees may be revealed by a temperature increase at the earth's surface of only a few degrees. This is of the order of magnitude of the temperature difference between the sunny or the shaded slope of a waste tip or of a sand dune.
In mines, underground coal fires are monitored by a permanently installed system of sensors. Besides temperature, pressure and forced air ventilation speeds, this keeps a check on various gas concentrations. A fire starting up is indicated practically in real time in the mine's central control station. The system serves above all for early warning and to save miners who might be in danger.
Extinguishing coal seam fires [ .]
Combustion chart
In order to be propagated, a fire needs fuel (coal), oxygen (air) and energy (heat). The interaction of these three elements is usually represented as a Combustion chart.
These three elements allow deductions to be made about firefighting (extinguishing) methods. The fire can be separated from additional fuel, for instance by means of firebreaks or fireproof barriers. Some fires, particularly on slopes, can be completely dug out.
In the case of near-surface coal seam fires the oxygen in the air can be prevented from reaching the fire by covers or gas-tight barriers. Alternatively, the evacuation of burnt gases can be prevented so that the fire is stifled by its own waste gases.
The removal of large amounts of energy is achieved by cooling, mostly by injecting large quantities of water. The remains of coal which has kept dry may however generate absorption heat as a result of taking up water so that an already extinguished fire may re-ignite itself. Consequently, more energy must be removed than the fire generates.
In practice, these methods are combined. A decisive factor in the choice of methods is the availability of resources. This is particularly true of water, for instance in arid zones and for covering material s such as loess or clay.
At present the only near-surface coal seam fires which are extinguished as a matter of routine are in China. There, a standard method has been developed, which basically consists of the following phases:
1. Levelling of the surface above the fire area with heavy machinery in order to ease wheeled access.
2. Boring holes in the fire zone in a regular pattern with holes roughly 20 metres apart down to the depth of the focus of the fire.
3. Injecting water or mud into the boreholes for a long period, mostly 1 to 2 years.
4. Covering the entire area with a 1 metre impermeable layer, for example loess.
5. Planting vegetation, as far as the climate will allow.
They are working on refinements to this method, for example by means of additives in the extinction water or using alternative extinction media
Underground coal seam fires as a rule are extinguished by the mine fire service by inertising [link?]. To do this, the affected area is closed off by earth mounds in the galleries . Then the inert gas, generally nitrogen, is fed in for a considerable length of time usually using already existing pipework.
Monitoring fire zones [ .]
Areas where coal seams near the earth's surface are burning, and also potential fire zones (high-risk areas), must be regularly monitored. This allows new fires to be spotted early and to fight them from the beginning. Even extinguished fire zones need monitoring, as they can start to burn again at any time. The principal cause is that isolated patches of heat remain after extinguishing, and may remain for years due to the low thermal conductivity of the rock. Large-scale monitoring is only manageable with saletilite-based methods. Monitoring individual fire zones can also be carried out by repeatedly going over them with appropriate measurement instruments (for temperatures, gas measurements, geophysical measurements) since fire zones move only relatively slowly.
The risk of underground coal seam fires is limited by a permanently installed system of sensors so that countermeasures can be taken in time.
Fires in coal stores [ .]
Excavated coal as well as coke is often kept for months in huge tips. In this case there are legal regulations on how to avoid spontaneous combustion. For example, for coal and coke heaps which come under the Mining Supervisory Authority in North Rhine-Westphalia it was laid down that coal is to be stored [in horizontal layers] to a maximum height of 6 metres. [ORIGINAL NOT WELL EXPRESSED] For each type of coal a maximum height of pile is laid down.[4]
In older waste tips, there may still be considerable quantities of coal. Great Britain had considerable problems as a result of burning slag heaps. In the mean time, these have been dug over and re-vegetated. Those slag heaps in Germany which are still burning are in part monitored from the air by thermal imaging. These relatively small fires are suppressed by injecting cement or anhydrite.
In a powdered coal mill,[NO LINK] coal is processed to a grain size 0.5 mm. The cause of a fire is rarely spontaneous combustion, but rather a dust explosion as a result of extraneous activity. Consequently the fire prevention regulations of the Professional Association concerned are inflexible. Plants of this kind are operated using inert gas to reduce the amount of oxygen available. Regulations to prevent sparks, high temperatures (max. 80º C) and the carbon monoxide concentrations which indicate a smouldering fire must be observed.
Coal fires during transport [ .]
The USS Maine
During the age of steam shipping, fires occurred while transporting coal. In the early days, the coal for the few existing steam ships was transported by wooden sailing ships. As a result, in the period from 1871 to 1880, 152 British ships were damaged, 68 were a total loss and 84 badly damaged[5]. The first scientific investigations revealed the connection between the number of fires which increased as the length of the journey and the quantity being transported increased. The first iron ship was built as early as 1838 but this type of vessel did not become generally accepted. Only after 1881 when the first steel ships were built in Great Britain did transporting coal lose some of its dangers, as coal could now be transported in unburnable steel bunkers.
A historically important incident on board the US battle ship, the USS Maine may have been caused by a coal fire. The ship exploded on 15 February 1898 in the harbour of Havana; this unleashed the Spanish-American War. Research and material experiments in 2000 and 2001 support the supposition that spontaneous combustion of the coal in the forward coal bunker led to a fire , as a result of which the steel bulkhead separating it from the adjoining munitions bunker became heated and thus ignited the gunpowder and shells.
In the year 2004 Robert Essenhigh came up with the theory that a smouldering fire in the coal bunker of the Titanic, which is documented by the Harbour Fire Brigade in Southampton, caused the Captain, despite the danger of icebergs, to sail faster than was wise. The fire on the starboard side between boiler rooms 5 and 6 could have been fought by shovelling the coal from the coal bunker affected quicker than usual into the boiler in order to reach the burning coal. This was common practice with smouldering fires in coal bunkers.
Nowadays, coal fires on bulk carriers carrying coal can still break out as a result of spontaneous combustion , as happened in Bremen in 2003. A smouldering fire occurred in a section where 5000 tonnes of coal were carried on a five-and-a -half week voyage and which was discovered during unloading in Bremen. For loads of this kind there are safety regulations, such as the Canadian Notice To Shipmasters Loading Coal[6] which lays down safety instructions for the Captain.
Effects on the environment [ .]
Spontaneous coal seam fires have considerable effects on the environment, from a global, as well as from a regional and local point of view by producing greenhouse gases Apart from the release of toxic gases, the landscape-forming consequences of subsidence are particularly relevant.
Global effects on the environment [ .]
Besides the loss of fuels, during coal seam fires greenhouse gases such as carbon dioxide, carbon monoxide, sulphur dioxide and methane are produced in the same magnitude as the emissions from German road traffic. Clearly, spontaneous and uncontrolled coal fires looked at from this point of view are an environmental problem of some significance. The German Ministry of the Environment replied in answer to a Parliamentary Question ( Paper 15/3740)[7] in June 2005, that it is looking into the question of whether the costs for extinguishing coal fires which have been going on for years could not be partly re-financed via the mechanism of "avoided greenhouse gas emissions" with the aid of the Clean Development Mechanism (CDM) in the Kyoto Protocol.
Regional and local effects on the environment [ .]
The strategies that animals and plants use to adapt to coal fires depend on the duration and extent of the affected area. Furthermore, geomorphological modifications can be expected. For instance, cracks may appear on the earth's surface; as a result of the reduction in volume of the coal seam, cave-ins and landslips may occur. The remains of burnt coal in large-scale coal fires such as at the Powder River Basin in the USA may shape the landscape.
Centralia, Pennsylvania [ .]
The biologist Tobin-Janzen[8] did research into the effects on the quantity of bacteria in the earth of the near-surface fire which broke out in Centralia, Pennsylvania in 1962 in an anthracite coalmine. At present about 1.2 square kilometres are affected. The surface investigated is of particular interest due to the hot gases released by the moving fire area. This leads to rapid change in ground temperature and chemistry.
The diversity of bacteria in the earth decreased markedly, as was expected, as the temperature rose (47 °C to 75.7 °C). On the other hand the presence of the thermophile bacterium Geobacillus thermoleovorans, which lives in the temperature range of 45 °C to 85 °C with optimum growth at 70 °C was demonstrated. In addition there were indications of sulphur bacteria and bacteria which cause the nitrification of ammoniac (NH3) or ammonia ions (NH4+) to nitrate ions (3-) Slag heaps Anna 1 and Anna 2 [ .] The slag heap Anna 1 in Alsdorf of the disused Anna mine covers an area of 41 hectares, is up to 75 metres high and has been in use since 1850. The peculiarity of the heap is that there have been erratic smouldering fires there since the middle of the last century. In the Anna 2 heap of similar age, but covering only 26 hectares, there are also hidden fires. This has led to the existence on the heaps of a particular microclimate which combined with the danger of fires has made the heaps an untouched and rare biotope. On Anna 1 many species of insects and spiders originating from the Mediterranean have been observed. These survive the cold winters in the region because the ground temperature is raised by the fires. Among them for example are the wasp spider and the blue-winged grasshopper.[9] Peat fires [ .] Moor fires [ .] Out in the open, in other words in bogs, peat fires caused by spontaneous combustion are extremely rare. Although the surface temperature on an upland moor may reach 77 °C as a result of insolation, and could catch fire as a result of escaping gases produced by fermentation eg: methane, there are no reliable records of moor fires being caused by spontaneous combustion. The reduced storage thickness and good ventilation of dry peat turves do not permit accumulation of heat. Even a short distance below the surface, the ground temperatures even in summer do not reach spontaneous combustion temperatures. In addition, the temperature fluctuations throughout the day with radiation at night due to clear skies are considerable and can create ground frosts from September to June. In the tropics and sub-tropics, however, smouldering fires in deep strata have been recorded. In particular in the periodic dry seasons or long periods of drought, in ground which is becoming bog or in boggy soil or certain sediments in West Africa, abnormally increased ground temperatures may occur. This overheating of up to 600 °C may occur where sediment lies above layers of peat and allows the creation of a hot spot. Spontaneous combustion at the surface of soil with high organic content has not been recorded, although it cannot be entirely excluded that surface fires could be caused by an exothermic reaction of earth with a high proportion of organic material.