“Anyone can see a forest fire. Skill lies in sniffing the first smoke.” — Robert A. Heinlein
One of my children has a house in The Ville, the neighborhood in north St. Louis that a tornado ripped through in May 2025. Since that tornado, I’ve become adept with a chainsaw, cutting into firewood the felled oak trees that narrowly missed crushing the house. These are just four of the thousands of trees that the EF3 twister destroyed. Most are destined to be chipped. Some will end up in wood pellets to be used as fuel, primarily in Europe, where they are serious about reducing their dependence on fossil fuels.
There are almost 2,000 wood pellet mills in the world. About 10% of them are in North America, and one of them, Horizon Biofuels, is in Fremont, Nebraska. On Tuesday morning, July 29, 2025, an explosion at the Horizon Biofuels pellet mill killed three people: employee Dylan Danielson and his two daughters, Hayven, 12 years old, and Fayeah, 8 years old, who were at the plant waiting for their dad to finish his shift so he could take them to a doctor’s appointment.
What happened? Why? How do we prevent it from happening again?
Press Accounts
Two days after the explosion, the media reported that the Nebraska State Fire Marshal’s office had determined that 1) the incident was an accident and 2) that the cause of the explosion was a dust fire. In the absence of other information, the media also made much of an OSHA citation in 2012 with five serious violations at Horizon Biofuels involving Lockout-Tagout, Forklift training, HazComm, and one for maintaining cleanliness around woodworking machinery. The last violation specifically regarded wood dust accumulation at two milling machines.
More relevant was the report that came out a week after the incident. A nearby plant complained in January to the Nebraska Department of Environment and Energy about sawdust coating vehicles and clogging storm drainage ditches. Inspectors verified the problem and directed the pellet mill to clean out the storm drains. Then, in February, the plant manager admitted to state officials that the dust collection system was not working correctly.
Interesting, but not an explanation.
The Wood Pelleting Process
To understand what might have happened, it is helpful to understand the wood pelleting process.
The process starts with wood waste: saw dust, wood chips, wood shavings, small tree limbs and branches, and scraps from furniture manufacturers.
The facility then grinds it to pieces, which is an opportunity to create dust. At this point, the wood contains 40~50% moisture.
The next step is to dry the wood pieces to 10~15% moisture.
The facility then mills the dried wood pieces again, into fiber. This milling creates more dust.
The fiber then goes to pellet presses which compress the fiber at high pressure and elevated temperature. Not only does this compress the fiber into pellets, but it activates lignin and hemicellulose that are naturally in the wood to act as binders. Some facilities also use synthetic binders, glues, or starch to bind the pellets. Horizon Biofuels may be using synthetic binders, which might explain why early reports mentioned “alcohol-based materials at the site.”
From the presses, the pellets go to air coolers, then to screeners that separate fines from the finished product. The finished product is either packaged or put into bulk-storage silos.
Flammable Hazards Beyond Combustible Dust
While the pellets are in storage, they oxidize. Oxidation, an exothermic process, leads to the formation of carbon monoxide, which is not only toxic but a flammable gas. Its flammable limits are 12% to 75% by volume. The best housekeeping will not remove the carbon monoxide. Moisture and high temperatures promote the auto-oxidation of unsaturated fatty acids and terpenes in the wood. The auto-oxidation, an exothermic reaction, further increases the temperature, increasing the rate of carbon monoxide generation. While the pellets do not become hot enough to reach the autoignition temperature of carbon monoxide – 609 C (1,128°F) – they can become hot enough to ignite wood, which ignites in the range of 200 C to 500 C (390°F to 930°F), depending on a number of factors. This in turn can ignite the carbon monoxide.
Another flammable gas generated by wood pellets is hexanal, also called hexanaldehyde or caproaldehyde. Similarly formed by auto-oxidation, hexanal is specifically the product of the oxidation of linoleic acid. All tree species contain linoleic acid, but the amount is much higher in softwoods, like pine and spruce, than in hardwoods, like oak, maple and hickory. The flash point of hexanal is 32 C (90°F) and its autoignition temperature is 220 C (428°F).
The Dogwood Alliance, which wants to shut down the wood pellet industry, reports that 51 incidents of fires or explosions occurred at U.S. wood pellet plants between 2001 and 2022.
A Global Concern
Wood pellet production is not just a U.S. phenomenon. Or even a North American phenomenon. Biomass Magazine recently listed 116 active pellet mills in the U.S. and another 44 active pellet mills in Canada, with another 14 mills proposed or under construction. These account for about 10% of the pellet mills in the world; according to Timber-online, there were 847 pellet mills in the EU in 2023, 263 pellet mills in Asia, and 1,623 pellet mills globally.
A quick search revealed over a dozen pellet mill fires and explosions in Europe and Asia since 2010, in Germany, Belgium, Netherlands, Denmark, the United Kingdom, and Japan.
Controlling Combustible Dust Is Necessary But Not Sufficient
Controlling dust at wood pellet mills is necessary for preventing catastrophic fire and explosions, but it is not sufficient. We do not know yet what happened at Horizon Biofuels. The Nebraska State Fire Marshal’s initial finding that the explosion was caused by “an accidental dust fire” is not going to be sufficient to prevent recurrences of this incident. Fortunately, the U.S. Chemical Safety Board has deployed to Fremont to investigate. I hope they get a chance to complete their work, work we should all support, before the current administration closes their doors. The world over, fathers and daughters—and mothers and sons—deserve that much.
