Research into the cause of fires involving toluene has always unearthed one certain fact, which is that devastating results are inevitable if there is an electrostatic charge. But why does the chemical ignite so quickly?
In the huge blaze at a refinery in Köln-Godorf on January 9th, 2014, a tank filled with toluene burnt out after an explosion. No details were given but electrostatics was blamed as an ignition source.
In the last years we investigated several toluene fires and so the question arose, why toluene in confirmed electrostatic ignitions of flammable liquids was affected in an above-average degree.
Toluene is assigned to explosion group IIA and is about as sensitive to ignitions as, e.g., benzene, methanol, and acetone. But in which way does toluene differ from these liquids?
How Electrostatic Charges Can Occur
First of all there is the electrical conductivity which is the lower the higher the tendency for electrostatic charging is. In the range of liquids, toluene shows the lowest value with its conductivity of 0.1 – 10 Picosiemens/m. It follows, when toluene is flowing through supply lines, it will be electrostatically charged very quickly, e.g., negatively, when flowing through a metal pipe.
Charges created in this way only show ignition energies of about 0.5 Millijoule. This is sufficient for igniting many flammable liquids whose minimum ignition energy (MIE) lies in between 0.2 und 0.5 Millijoule but not for igniting combustible dusts.
The "Explosion Range" – In the Danger Zone
It is well known that gases and vapours are only ignitable within their explosion range, that means, between the lower explosion limit (flashpoint) and the upper one. Below the mixtures are »too lean«, above they are »too rich«. But within the explosion range the need of ignition energy is in no way constant, it reaches a little above the stoichiometric mixture, well known as λ=1 -Lambda sensor in car engines- its lowest value, the MIE.
Towards the borders of the explosion range, the need of ignition energy rises for more than an order of magnitude. It follows, that common electrostatic discharge appearances (brush discharges from charged isolating materials, spark discharges from charged persons) can only ignite stoichiometric fuel/gas- mixtures.
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