Risk of carbon monoxide release during the storage of wood pellets

Wood pellets for boilers are normally stored in a large sealed hopper/tank or a storage room that has a screw feeder (auger) connected to the boiler. Alternatively, the hopper/tank can be mounted over the boiler for gravity feeding. Due to the enclosed nature of these hoppers/tanks/rooms, the atmosphere inside can become oxygen depleted and a toxic atmosphere containing carbon monoxide can accumulate. The chemical reactions responsible for carbon monoxide production from wood pellets are assumed to be an auto-oxidation process, especially oxidation of the fatty acids to be found in wood⁴.

Experimentation has shown³ that small quantities of wood pellets can produce life-threatening quantities of carbon monoxide in a confined space and that there are various factors that will affect the amount of carbon monoxide produced:

• Age - pellets will produce more carbon monoxide within the first six weeks of being manufactured.
• Temperature - more carbon monoxide is produced at higher temperatures.
• Wood type - pellets made from pine contain more unsaturated fatty acids than spruce so produce more carbon monoxide.
• Other factors - carbon monoxide levels will also increase with the amount of available oxygen present, exposed pellet surface area and amount of mechanical abrasion of the pellets that has taken place.

Note: In addition to the risk of carbon monoxide from wood pellets there is also a possibility of carbon monoxide being present because of a back-flow of flue gases via the fuel supply mechanism from the boiler. Causes for this include inadequate equipment being installed or a poorly designed flue.