Mulch grinders are large outdoor machines designed to take logs or waste wood from a logging mill and grind the wood to a specified desired mulch size.  An ARCCA expert was called to investigate a fire involving a mulch grinder.  During a drought, the owner/operator of a mulch production operation had used a mulch grinder to grind a batch of mulch.  Several hours after operation, and sometime during the night, witnesses saw a fire at the mulch production facility.  Subsequently the grinder was found burned.

Steps Taken:

  • ARCCA’s expert inspected the site and the grinder still in place.

  • During the site inspection, several large mounds of mulch were seen un-watered.

  • During the evidence examination, residual mulch on the grinder experienced auto-ignition, and the grinder had to be sprayed down to stop the fire.

  • During the site inspection, whenever mulch was being removed via bulldozer, smoke and smoldering could be seen inside the mulch.

Final Findings:

Based on the site and evidence inspection and the timing of the fire, the cause of the fire was not the mulch grinder.  Due to the drought and heat, conditions existed such that self-heating within the mulch piles was sufficient to cause auto-ignition.  The initial area of origin happened to be adjacent to the grinder.  The initial fires were relatively small and contained to individual piles of mulch.  However, once the fire spread to the grinder, it grew rapidly.

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