Spontaneous combustion, or self-ignition, has been a topic that followed me over the years. The basic concept is that an exothermic decomposition or oxydation process creates more heat than the heat losses in the surrounding area.
Linseed oil, paint filters and a deer target
My first encounter with the topic was as an undergraduate student at the University of Maryland when Prof. Jose Torero asked me to prepare a self-ignition demonstration for a fire investigation course for the Bureau of Alcohol, Tobacco, and Firearms (ATF). Similarly to what is shown in the photos below, I used cotton rags and linseed oil for the demonstration. With some practice, supported by careful measurements and observations, we created a box that burst into flames in the parking lot while Jose was explaining some details. Success!
The interested reader is encouraged to look up the fire at One Meridian Plaza in Philadelphia, which was caused by cotton rags soaked in linseed oil and resulted in three firefighter fatalities.
Note that the iodine number is key for the self-ignition cases involving oils – the higher the iodine number, the more prone the oil is to self-ignition. The figure below shows that linseed oil has the highest value.
After my studies at UMD, I worked for one year at Combustion Science and Engineering (CSE) in Columbia, Maryland. There, I worked on three cases of self-ignition – paint-loaded filters, deer target for hunting and more cotton stained with linseed oil. As these were actual engineering cases that needed data and reports, I had to study the theory a bit closer before commencing. Several theories exist, with each being most suited in different domains of the Biot-number (ratio of the conductive heat transfer to the thermal conductivity). I ended up using the one by Frank-Kamenetskii, with experiments carried out with cubes. The basic concept is to place cubes of different sizes in an oven that is heated to a steady temperature. The critical temperature is the one that leads to self-ignition, i.e., ignition in the center of the tested material, rather than on the surface (which would indicate that the oven temperature is too high for self-ignition, as regular ignition occurs).
Semenov, Thomas and Frank-Kamenetskii illustrations [made by Laura Schmidt based on Fire Dynamics by Dougal Drysdale].
The first study was on waste filters from the ventilation system used during spray painting of objects. Several fires had been experienced in Iowa, and the task was to figure out why these fires happened and how they could be prevented.
As the paint was quite volatile, it was particularly essential not to set the oven temperature too high. There were also several other factors in play, such as the volatility of the paint used. The eventual breakthrough came when I noticed the importance of the packing density. With that parameter in place, we solved the problem and could deliver a report with a quite simple solution – empty the waste containers more often! The problem had been that when the containers got too packed, the exothermic oxydation process of the paint had a faster heat release than the losses in the densely packed filters.
The second self-ignition case was a bow and arrow target (not the one displayed) that had a history of catching fire on the way from the production site to the stores where it was sold. In particular, the rear part of the unassembled target was indicated to be the problem. Again, our task was to figure out why it happened and how it could be solved. The same method was applied – cubes in the oven. As this material was more homogeneous, the task was simpler, and the critical size-temperature relationship for self-ignition was established. The solution – the back part of the deer had to be reduced in dimension unless the expanding foam formulation was to be changed. The former was easier, and these targets are now leaner deer, or the back part comes as two pieces! This story was later developed by Combustion Science and Engineering to be a student exercise - see link (The Bowes Decorating Company).
Here is an overview of some self-ignition cases and how they were solved.
Case Study | Product | Solution |
|---|---|---|
Production | Deer target | Redesign |
Renovation work | Cotton rags with linseed oil | Risk assessment and implemented risk management strategy |
Storage | Milk powder | Control temperature and size of containers |
Farming | Biomass piles | Reduce size and apply chemicals (to stop the biological process that initiates the process) |
Waste management | Paint loaded filters | Process change |
As you can see, there are many hidden dangers in products in our everyday life, so pay attention to warning labels and how you store and dispose of products. From the point of view of a forensic fire engineer, self-ignition must be considered as a scenario. If in doubt, consult with a fire laboratory that can carry out the type of testing described here.
That’s all for today.
Grunde
References:
Jomaas, G., Richard Roby, P. E., Carpenter, D., & Schemel, M. C., 2002. Spontaneous Combustion of Paint Loaded Filters.
Beever, P., 1984. Spontaneous ignition of milk powders in a spray-drying plant. Journal of the Society of Dairy Technology, Vol. 37, No. 2.
Drysdale, D., 2011. An Introduction to Fire Dynamics: Third Edition, Wiley Blackwell.
Duane, T.C. & Synnott, E.C., 1992. Ignition characteristics of spray-dried milk product powders in oven tests. Journal of Food Engineering, 17(3), pp.163–176.
Gray, B., 2016. Chapter 20: Spontaneous Combustion and Self-Heating. In: SFPE Handbook of Fire Protection Engineering, 5th edition, New York: Springer-Verlag.
Do you have any burning matters to discuss with Grunde? Reach out through the Burning Matters feedback form.
Interested in finding out more about the author? Check out this article 🙌🏻
