The tragic fatal explosion that claimed a Firefighter’s life last week has a lot of folks claiming this was a COM DUST event. But after close examination of scene photos, I am confident this event was a syn-gas explosion with possibly some “fines” igniting during the event. These events are not all that rare and almost always involve syn-gas. In 2001, a local business has a rotary kiln dryer for wood chips. The business is one of the worlds largest pet bedding manufacturers. The kiln began smoking so the FD was called. After some time, the FD decided they needed to open the kiln and as soon as they did, a backdraft occurred burning several FF’s. Now we do not yet know if this silo was opened and this led to this event; but we do know the wood chip silo held something highly energetic, such as syn-gas. Here is how this highly flammable gas is produced inside a wood chip silo…
Here is the mechanical breakdown of how this systemic failure occurs.
The root cause usually begins weeks before the explosion with moisture. When damp wood chips are piled deep inside a confined silo, microbial and fungal activity begins breaking down the biomass. This biological decay is exothermic. Because the chips in the center of the silo act as an excellent insulator, the heat cannot dissipate. Once the internal temperature reaches roughly 170°F to 200°F (75°C to 93°C), the biological action dies off, but chemical oxidation takes over. The pile enters a thermal runaway, eventually reaching the auto-ignition temperature of the wood, creating a deep-seated, smoldering fire.
Because the smoldering fire is buried deep under tons of chips inside a relatively sealed silo, it is starved of oxygen. It cannot burn cleanly. Instead of full combustion, the wood undergoes pyrolysis1. The extreme heat chemically decomposes the surrounding wood chips, off-gassing massive volumes of highly flammable gas. This “wood gas” is primarily composed of:
- Carbon Monoxide (CO) (LEL: 12.5%)
- Methane (CH4) (LEL: 5.0%)
- Hydrogen (H2) (LEL: 4.0%)
This flammable gas mixture slowly percolates upward, filling the voids between the chips and the headspace of the silo. The silo is now acting as a massive, unvented gas generator.
The silo is now packed with an explosive atmosphere, with the smoldering fire at the bottom serving as a constant ignition source. All it lacks is oxygen to bring the gas-air mixture into the flammable range, or a pathway for the gas to hit an external spark.
An operator notices smoke, opens a bottom access door, or opens the top roof hatch to inspect the level. A rush of fresh oxygen enters the silo, mixes with the hot syngas, and hits the smoldering embers. The resulting gas deflagration blows the roof off the silo.
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As the smoldering fire consumes material at the bottom, a bridge or “rat hole” forms in the chips. When that bridge collapses, it suddenly shifts the hot gases, mixing them with incoming air and the glowing embers.
Even if the facility strictly handles “wood chips,” it is physically impossible to transport them without generating fines.
- As the chips are blown, augered, or dropped into the silo, mechanical attrition (friction) breaks off tiny particles.
- These combustible dust fines settle on the silo walls, ledges, and structural supports.
- When the primary gas explosion pops, the shockwave physically shakes the silo, dislodging this accumulated dust and suspending it in the air. The fireball from the gas explosion instantly ignites this newly formed dust cloud, triggering a secondary, often much more destructive, dust explosion.
When evaluating this hazard under standards like NFPA 664, Prevention of Fires and Explosions in Wood Processing, the mitigation strategy cannot just rely on explosion venting. It requires active CO monitoring in the silo headspace to detect the pyrolysis off-gassing long before the LEL is reached.
- Pyrolysis converts organic materials like biomass, plastic waste, or municipal refuse into syngas (synthesis gas) by heating them to high temperatures (typically 500°C to 900°C) in a strictly oxygen-free environment. This thermal cracking breaks long-chain polymers into a mixture primarily composed of hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), and methane (CH4). https://www.ars.usda.gov/northeast-area/wyndmoor-pa/eastern-regional-research-center/docs/biomass-pyrolysis-research-1/what-is-pyrolysis/ ↩︎