Spontaneous inflammation
To Heuselbstentzündung it can for the storage of hay , for example in Heustöcken come, when the water content is too high. In order to prevent this, the respective operators of a hay store are obliged to monitor it closely; in an emergency, disregard can be punished as negligent arson .
mechanism
If fresh hay is brought in and still has a level of moisture that, in equilibrium with the air in the haystack, leads to a relative humidity of more than 85%, there is biological activity of fungi and, with even higher moisture contents, also of bacteria and depending on the age of the Heus breathing of living grass cells instead. The respiratory warmth formed by residual respiration from not yet completely dried leaves in hay is considered to be one of the first contributions to hay warming.
During these processes, energy is released in the form of heat , which increases the temperature inside the haystack. At 40–50 ° C, the previously dominant mesophilic microflora dies and thermophilic bacteria and fungi begin to multiply. Now - provided there is sufficient water and air supply - very high metabolic rates can be achieved, similar to thermophilic composting , which drives the temperature to 60–70 ° C. There are different options:
- If the humidity of the hay is rather low and the ventilation is good, the heated areas dry out due to the high temperature and the biological activity comes to a standstill.
- With a very high moisture content and adequate ventilation, sufficient heat is dissipated through evaporation and the temperature is stabilized at around 70 ° C, so that the reaction proceeds to a humus-like end product as the temperature slowly falls, as in composting . With poor ventilation, on the other hand - which is more likely in a haystack because of the rapid structural loss under these conditions - rotting soon occurs with rapidly falling temperatures and digested sludge.
- Temperatures higher than 80 ° C in compost, piles of wood chips or hay stores can only be attributed to purely abiotic-chemical processes. The breakdown of pectins , protein and other compounds can cause the temperature to rise further. In the absence of air, pyrophoric gases and vapors of fermentation alcohols develop . Gases are considered pyrophoric if their ignition temperature is below 100 ° C. Even if the enthalpy of evaporation of the water has a cooling effect at first, the material can self-ignite in dried-out areas . From 180 ° C also begin Verkohlungsprozesse . During the molecular disintegration, the temperature can jump by 150 ° C within two to three days to around 250 ° C and autonomous self-ignition becomes possible. The material begins to smolder , smoke develops, the released heat of combustion and the resulting updraft , which sucks in fresh air, and the high oxygen content of the wood constituents (e.g. lignin ) lead to the rapid expansion of the fire source .
- In a very narrow humidity range, which corresponds to an equilibrium air humidity of about 96.5% relative humidity, hay heats up further from about 70 ° C due to the low evaporative cooling and the poor thermal conductivity of the hay due to direct oxidation of cellulose and other ingredients due to atmospheric oxygen independent of biological activity . However, this reaction initially depends on the presence of water and is quite slow. This reaction accelerates with increasing temperature, takes place more and more independently of the presence of water and turns into a smoldering fire . If there is sufficient air supply and otherwise suitable conditions, this can ignite and turn into an open fire.
Prevention and control
To prevent spontaneous ignition in haystacks, ensure that the hay is sufficiently dry before storage. Mechanical hay ventilation systems can also help prevent fires when the hay is not sufficiently dry.
If there is a suspicion that a haystack self-ignition could occur, so-called hay probes are used, which measure the temperature inside the haystack. This should be below 50 ° C. If this limit is exceeded, an even stricter control must be carried out. The fire brigade must be alerted at the latest at a temperature of 70 ° C.
Since hay quickly begins to char at high temperatures, channels easily form, which can often be recognized by depressions and depressions on the surface. In this state, haysticks are extremely unstable and should not be entered at all or only with adequate security.
The fire brigade determines the temperatures of the hay at various points and tries to localize heat sources and to investigate the heat distribution in the haystack. If the temperature is too high, the haystack will be removed. This is done with extreme caution, as contact of the heat source with atmospheric oxygen can lead to immediate ignition. If necessary, the hay can be cooled with water, but this should only be used in a spray jet to avoid unnecessary air movement.
A modern alternative to this method is the use of a hay weir blower cooling device , which makes the removal of the haystack superfluous, but is very time-consuming.
If a haystick self-ignites, the fire usually spreads so quickly that often only damage limitation can be carried out. It is therefore necessary to call in the fire brigade at an early stage.
See also
literature
- Walter Hamilton, Ulrich Kortt (arr.): Handbook for the firefighter . Boorberg Verlag, Stuttgart, Munich, Hanover, Berlin, Weimar, Dresden 2004; ISBN 3-415-03176-4
- Dieter Karlsch, Walter Jonas: Die Roten Hefte, Issue 47 - Fire protection in agriculture . 3. Edition. Kohlhammer, Stuttgart 1993, ISBN 3-17-012104-9 , pp. 32-36 .
- Henry Peter Rothbaum: Spontaneous Combustion of Hay . In: Journal of Applied Chemistry , 13, 291-302. Society of Chemical Industry, London 1963.
- Charles A. Browne : The spontaneous heating and ignition of hay and other agricultural products. In: Science 77, 1933, pp. 223-229.
Individual evidence
- ↑ Josef Eckl: Preventing Self-Inflammation , Weihenstephan University of Applied Sciences, PDF file
- ↑ a b c Frank Stolt: Self-ignition of crops. In: FEUERWEHR magazine . 9/11, kripo.at (PDF).
- ↑ Technical rules for hazardous substances - Working with gases - Risk assessment. TRGS 407, June 2013
- ↑ § 16 BayVBB. January 1, 2013, accessed June 23, 2018 .