In this chapter: why stored wood pellets, sawdust and animal feed can heat up on their own until they catch fire, how to detect it in time, what to do (and what not to do) when there is a fire inside a silo, where fires start in the process, and what the plant's fire plan must include.
A fire that starts without a spark
Almost every fire needs an ignition source. The material stored in a silo can catch fire without one. This is called self-heating: the material itself generates heat as it reacts with air or through the activity of microorganisms. If that heat is produced faster than it dissipates, the temperature rises until the material starts to smolder, from the inside.
The Swedish research institute RISE, author of a reference handbook on silo fires, sums it up this way: in many cases, a silo fire starts from self-heating of the stored material; oxidation and biological activity can, under unfavorable conditions, end in slow combustion deep inside the material, which is why it is very hard to detect1. Days can go by with no visible signs on the outside. Dust explosion, a different but related hazard, is covered in Chapter 3.
Why wood pellets heat up on their own
Wood contains fats and fatty acids. When it is ground, dried and pelletized, those substances are exposed to air and oxidize. Svedberg et al., in a 2004 study on pellet storage, attributed the formation of carbon monoxide to this oxidation of fats and fatty acids2. The same reaction that releases gases also releases heat5. That is why, in a pellet silo, self-heating and CO emission go together: CO is an early warning that the material is reacting (for the danger CO poses to people, see Chapter 7).
What makes pellets react more:
- Freshly made pellets. The UK HSE states that gas emission is highest in the first six weeks after manufacture3.
- Temperature. The higher the storage temperature, the stronger the reaction3.
- Type of wood. According to the HSE, pine emits more than spruce3.
- Moisture. Researchers at the University of British Columbia, in a paper published by the Wood Pellet Association of Canada (WPAC), point out that self-heating incidents probably start with a temperature rise caused by moisture absorption and condensation, which then speeds up the oxidation of the wood4.
- Fines and broken pellets. Dust and broken pellets increase the surface area exposed to air45.
- Very wide silos. Slender structures dissipate heat faster than wide ones4.
What pellet researchers recommend
The same University of British Columbia paper proposes, among other measures4:
- keep pellets dry but not too dry, around 7 to 8% moisture;
- do not store hot pellets at temperatures above 35 °C;
- reduce the amount of broken pellets and dust entering the silo;
- prefer silos 10 m in diameter or smaller;
- ventilate to cool, but limit the intake of humid air, and shut off the air supply immediately if self-heating is already underway, because air feeds the reaction.
There is also a standardized method for measuring how much heat a pellet generates: ISO 20049-1:2020, by isothermal calorimetry, with a classification test for wood pellets at 60 °C. The result applies to that sample, with its quality and age, and does not identify the cause of the heating6. It is useful for comparing raw materials or batches in a laboratory.
Animal feed and meals, too
Animal feed, meals and by-products such as oilseed meals do not behave the same way as wood pellets, but they can also heat up, especially when water gets in. RISE lists biological activity among the causes of self-heating in silos1.
A case documented in the accident database of the French Ministry of the Environment (ARIA) shows this clearly. On October 30, 2017, at a feed mill in Lapeyrouse, France, the supervisor noticed that no material was coming out of a bin and, when he opened the inspection hatch under the hopper, saw smoke. About 100 tonnes of sunflower meal had heated up inside a 232 m³ bin, with a hot spot of 100 °C. Firefighters had to cut a panel to empty it, and the smoke came back the next day. The probable cause was water that got in through a faulty seal. The loss was about 41,000 euros, with no injuries7.
The lesson is simple: a leak or a joint that lets water into a meal or feed silo is not a minor maintenance issue. It can be the start of a fire.
How to detect self-heating in time
Since the fire starts on the inside, looking is not enough. The signs to watch for are15:
- Temperature at several points in the material, with temperature probes or cables inside the silo. WPAC identifies this as the most important prevention measure5.
- Carbon monoxide at the top of the silo. A sustained increase is one of the first signs5.
- A strong or burnt odor.
- Heavy condensation on the silo roof.
- Caked, darkened or hot material at discharge.
RISE stresses that what matters is the trend: recording measurements and watching how they change lets you see dangerous conditions before smoke appears1.
If there is a fire inside a silo
A silo fire is not like an ordinary fire, and firefighters have little experience with them. In 2023, RISE published a guideline with 13 measures and 4 warnings for these cases, based on its Silo Fires handbook1. The basic method it recommends is to inert the silo with nitrogen injected from the bottom to displace oxygen and slow the combustion. Only once the fire is under control should you empty it in a controlled way, putting out the hot material as it comes out. This work can take days.
All staff should know the four warnings in that guideline1:
- Do not enter a building without a personal gas monitor or self-contained breathing apparatus. There may be very high levels of CO and carbon dioxide, with little oxygen.
- Do not use water inside the silo, especially if it contains pellets. Pellets swell, can form bridges and, in the worst case, cause the structure to collapse. Water can also generate CO and hydrogen.
- Do not open the silo. The incoming air feeds the fire, can trigger gas or dust explosions, and can spread the fire through connected conveyors.
- Be careful with liquid nitrogen: at −196 °C, the liquid or the very cold gas causes injuries.
The same guideline recommends that the extinguishing plan for a silo fire be prepared in advance, between the silo owner and the fire department, because every installation and every fire is different1. That includes knowing where to get nitrogen in your area and where it can be injected into each silo.
Fires that start in the process
Not all fires start in storage. In Chapter 3, we looked at the cases of Waycross (2011), New Hampshire (2011), Inferno Wood Pellet (2014) and Horizon Biofuels (2025). Two places come up again and again:
- The cooler, where hot pellets pass through and where an ember can form and travel.
- Cyclones, filters and dust collection ducts, where fine dust concentrates in an enclosed space.
In two of those cases, New Hampshire and Inferno Wood Pellet, investigators noted that the ducts or the dust storage had no spark detection. These systems detect sparks or embers traveling inside a duct and put them out before they reach a filter, a cyclone or a silo. They are a tool of the third line of defense from Chapter 3 (leaving nothing that can ignite the dust). They complement housekeeping, venting, suppression and isolation; they do not replace them. Where to install them, and what type, is determined by each plant's risk assessment.
Control of the other ignition sources in the process (hot surfaces, tramp metal, welding, electrical equipment) is covered in Chapters 3 and 5.
The fire plan
What the regulations require
In the United States, OSHA requires two plans, in writing when the company has more than ten employees:
- Fire prevention plan (29 CFR 1910.39): a list of the major fire hazards and ignition sources, with their controls; the protective equipment for each hazard; procedures to control accumulations of combustible waste; maintenance of safeguards on heat-producing equipment; and who is responsible for each item8.
- Emergency action plan (29 CFR 1910.38): how the alarm is raised; how and by which routes people evacuate; what those who must shut down critical equipment before leaving do; how all staff are accounted for after evacuation; who performs rescue or medical duties; and whom to contact. It also requires an alarm system with a distinct signal for each purpose9.
In Argentina, Chapter 18 of Decreto 351/79 (articles 160 to 187) and its Annex VII regulate fire protection. The fire load of each area is used to classify its risk and to determine, among other things, construction requirements and the number and rating of extinguishers10.
Extinguishers
OSHA's standard on portable fire extinguishers (29 CFR 1910.157) is a good example of the minimum11:
- mounted, located and identified so that they are readily accessible;
- for fires in ordinary solid materials (Class A, such as wood and meals), no more than 22.9 m of travel distance to the nearest extinguisher;
- monthly visual inspection and recorded annual maintenance;
- training on their use at hiring and at least once a year, if staff are expected to use them.
An extinguisher is for an incipient fire in the building or on open equipment. It is useless for a fire inside a silo, a filter or a closed duct: there, what was said above applies, and opening the equipment can make everything worse.
The fire brigade
A plant can decide that all staff evacuate when the alarm sounds and that the fire department fights the fire, or it can set up its own fire brigade. If it sets up a brigade, the OSHA standard (29 CFR 1910.156) requires a written policy stating how it is organized, what functions it performs and what training it receives; training at least annually for all members and quarterly for those who fight interior structural fires; and that those people be physically fit for the job12.
Whichever option you choose, the fire department in your area should get to know the plant before an emergency: where the silos are, what material they hold, where the power is shut off and where nitrogen can be injected.
Chapter checklist
- Do we measure the material temperature in pellet and feed silos, at several points, and record the trend?
- Do we measure CO at the top of the pellet silos?
- Do we inspect silo roofs, joints and openings to keep water out?
- Do we check that pellets do not go into storage hot, damp or with a lot of fines?
- Do staff know that water is not used inside a pellet silo and that a silo on fire must not be opened?
- Is there an extinguishing plan for silo fires prepared with the fire department, including the nitrogen supply?
- Do the ducts, cyclones and filters have the spark detection called for by the risk assessment?
- Do we have written prevention and emergency plans, with people responsible for them?
- Are extinguishers accessible, inspected every month and maintained every year?
- Have staff received training in extinguisher use and evacuation this year?
What to leave to a professional
The fire load calculation, the number and location of extinguishers, the temperature and gas detection system in silos, spark detection and the emergency plan must be defined by a licensed fire protection professional, together with your insurer and the local fire department. The extinguishing plan for a silo fire is prepared with the fire department, before you need it.
At Meelko, we take fire risk into account from the line design stage: pellet cooling before storage, dust collection, the location of cyclones and filters, and the size of the silos are defined together with the plant's capacity. If you want to review your project or your plant, tell us about your project.
Chapter sources
- Persson H. and Fjellgaard Mikalsen R. (eds.). Silo fire guideline, 13 measures and 4 warnings. RISE Research Institutes of Sweden, RISE Report 2023:68, Trondheim, 2023. Based on the handbook Silo Fires (RISE/MSB, 2012). https://www.dsb.no/siteassets/rapporter-og-publikasjoner/rise/rise-report-2023-68-silo-fire-guideline-final.pdf ↑
- Svedberg U. R. A., Högberg H.-E., Högberg J. and Galle B. "Emission of Hexanal and Carbon Monoxide from Storage of Wood Pellets, a Potential Occupational and Domestic Health Hazard". Annals of Occupational Hygiene 48(4):339-349, 2004. doi: 10.1093/annhyg/meh015 ↑
- Health and Safety Executive (HSE, United Kingdom). Safety alert OPSTD 3-2012: Risk of carbon monoxide release during the storage of wood pellets, November 5, 2012. https://www.hse.gov.uk/safetybulletins/co-wood-pellets.htm ↑
- Sokhansanj S., Yazdan Panah F. and Lee J. S. (University of British Columbia). "Managing Silo Size and Humidity Key to Controlling Self-heating in Pellets". Wood Pellet Association of Canada, December 21, 2023. https://pellet.org/news/managing-silo-size-and-humidity-key-to-controlling-self-heating-in-pellets/ ↑
- Yazdan Panah F. (Wood Pellet Association of Canada). Safe Wood Pellet Storage: Preventing, Detecting, and Managing Self-Heating Incidents. Technical deep dive: causes, contributing factors, and detection methods for self-heating (presentation, 2026). The presentation itself states that its recommendations are for guidance only and are not legal requirements. https://pellet.org/wp-content/uploads/2026/03/technical-deep-dive-wpac-presentation.pdf ↑
- ISO 20049-1:2020, Solid biofuels — Determination of self-heating of pelletized biofuels — Part 1: Isothermal calorimetry. Confirmed in 2025. https://www.iso.org/standard/66904.html ↑
- ARIA database (BARPI, French Ministry of Ecological Transition). Accident No. 50891, October 30, 2017, Lapeyrouse (63). https://www.aria.developpement-durable.gouv.fr/accident/50891_en/?lang=en ↑
- OSHA. 29 CFR 1910.39, Fire prevention plans. https://www.law.cornell.edu/cfr/text/29/1910.39 ↑
- OSHA. 29 CFR 1910.38, Emergency action plans. https://www.law.cornell.edu/cfr/text/29/1910.38 ↑
- Argentina. Decreto 351/79, Annex I, Chapter 18, Protección contra incendios (articles 160 to 187), https://www.argentina.gob.ar/normativa/recurso/32030/dto351-1979-anexo1/htm, and Annex VII (fire load, risks 1 to 7, siting, construction and extinguishing conditions, and extinguisher rating tables), https://www.argentina.gob.ar/normativa/recurso/32030/dto351-1979-anexo7/htm ↑
- OSHA. 29 CFR 1910.157, Portable fire extinguishers. https://www.law.cornell.edu/cfr/text/29/1910.157 ↑
- OSHA. 29 CFR 1910.156, Fire brigades. https://www.law.cornell.edu/cfr/text/29/1910.156 ↑
