As a self-ignition is the spontaneous ignition of combustible materials. Ignition always takes place as soon as the self-ignition temperature is reached. Catalysts or surface effects can lower this “threshold temperature”.
Self-ignition of organic material
Self-ignition can occur if sufficiently large amounts of highly flammable organic material ( e.g. hay , coal , flour , oil-soaked rags or wood chips) are stored with sufficient, but not too strong, ventilation. Various processes can lead to spontaneous combustion in these materials.
On the one hand, in carbohydrate-rich materials, biological oxidation ( putrefaction ) can lead to a sufficient temperature increase to trigger chemical processes that lead to inflammation if sufficient heat dissipation is not guaranteed. This is explained in more detail under Self-ignition of damp hay .
However, the same mechanism applies not just to hay, but to any large, sufficiently moist accumulation of carbohydrate-rich material. In certain circumstances, compost heaps can also ignite. Also household waste from landfill sites can be prone to spontaneous combustion, which is why large areas here with thermal imaging cameras are monitored.
On the other hand, unsaturated compounds such as vegetable oils , if they are distributed over a large surface, can generate enough heat through direct oxidation with atmospheric oxygen to trigger a smoldering fire and ultimately an open fire under the appropriate conditions (large amounts of loosely stored bulk material ) . Oil-soaked rags must therefore be stored in tightly closing metal containers after use or taken directly to a controlled incineration (e.g. in a workshop oven). This requirement applies in particular to the processing of hardening oils such as linseed oil varnish or, to a greater extent, to modern hard oils. A loosely crumpled soaked rag offers an almost ideal combination of reaction surface and heat build-up, which can very likely lead to critical areas.
In the case of improperly cleaned laundry soiled with such vegetable oils, self-ignition is promoted by the poor heat dissipation during storage in larger piles or piles. The use of tumble dryers or ironers increases this risk further, provided the laundry is not cooled down at the end of the process.
Are also threatened by spontaneous combustion coal stockpiles and underground coal deposits (see coal fire and burning heaps ), which lie close to the surface or were exposed (for example, in China). Coal heaps are therefore often cooled with water or relocated.
Spontaneous chemical self-ignition
Some substances react with the oxygen in the air even at room temperature with the appearance of a flame. So cesium , rubidium , white phosphorus , lithium , aerosols of metal powders such as z. B. iron, aluminum, many substituted phosphines or silanes spontaneously at room temperature on contact with atmospheric oxygen; they are called pyrophoric .
An enlarged surface strongly increases the tendency towards inflammation. Freshly made magnesium powder heats up in the air until it spontaneously ignites. Larger amounts of metal dust (e.g. waste mixtures) can become very hot inside due to their own insulating effect, which means that even less reactive metals such as aluminum can tend to self-ignite. In the home or in the hobby workshop there is an increased risk of spontaneous combustion due to residues of liquid coating materials such as B. natural resins or grease or oil soaked rags or cleaning wool.
Particular care must be taken when handling pure oxygen, especially in a liquid state, as many substances ignite particularly easily in the presence of pure oxygen. For this reason, the fittings on oxygen cylinders must be free of fats and oils in order to prevent self-ignition.
- Keyword spontaneous ignition. In: Meyer's large pocket dictionary. ISBN 3-411-11044-9 .
- Karlsruhe Institute of Technology (KIT) - Research Center for Fire Protection Technology: Investigation of the self-ignition of self-igniting substances. Research Reports No. 8 (online) (PDF; 2.7 MB), No. 13 (online) (PDF; 1.7 MB), No. 24 (online) (PDF; 1.9 MB)
- Deutsche Montan Technologie & Federal Institute for Materials Research and Testing: Understanding self-ignition of coal ( Memento from December 28, 2013 in the Internet Archive ) (PDF; 642 kB)
- G. Krause: New ways of determining auto-ignition temperatures . In: bulk goods . tape 12 , no. 6 , 2006.
- Charles A. Browne : The Spontaneous Heating and Ignition of Hay and Other Agricultural Products . In: Science . tape 77 , no. 1992 , 1933, pp. 223-229 , doi : 10.1126 / science.77.1992.223 , PMID 17773057 .
- Franz-Josef Sehr : The fire extinguishing system in Obertiefenbach from earlier times . In: Yearbook for the Limburg-Weilburg district 1994 . The district committee of the Limburg-Weilburg district, Limburg-Weilburg 1993, p. 151-153 .
- Self-ignition of laundry - fats and oils are to blame , Vorarlberg Fire Prevention Office, accessed on August 8, 2018.
- Axel Bojanowski : Fire under the earth. In: süddeutsche.de. Süddeutsche Zeitung Digitale Medien GmbH / Süddeutsche Zeitung GmbH, May 17, 2010, accessed on December 2, 2017 : “In China, huge coal seams are burning and threatening large cities. The fires are considered one of the greatest ecological disasters in the world. German researchers should now help with the extinguishing. "
- polishing likes: Spontaneous ignition of waste that arises when polishing aluminum . Retrieved on December 19, 2017 (German).
- Dipl.-Biol. Bettina Huck: Self-igniting substances. In: Lexicon contribution from Arbeitsschutz Office. Haufe Group , accessed October 15, 2019 .