Does a wood pellet or biomass store need CO detection and dust explosion protection?
Abstract
Yes for CO: stored pellets emit carbon monoxide and use up oxygen, so stores need ventilation, CO checks before entry and often a fixed CO monitor. Dust protection depends on size: industrial silos and conveyors need dust zoning and venting; small stores mainly limit fines.
Yes, in most cases, but for different reasons and to different degrees. Wood pellets give off carbon monoxide (CO) and consume oxygen as they age in storage, so every enclosed pellet store, domestic or industrial, needs ventilation, a safe entry procedure with CO measurement and, increasingly, a fixed CO monitor. Pellet dust and fines are combustible and can explode, so industrial silos, conveyors, elevators and filters need dust zoning and explosion protection; a small domestic store mainly needs measures that keep fines and dust clouds down.
Short answer
- CO hazard: wood pellets emit CO, carbon dioxide and aldehydes during storage, more so when warm. Measured levels range from about 100 ppm in domestic storage rooms to over 14,000 ppm, with oxygen as low as 0.8 %, in closed ship holds.
- CO measures: permanent ventilation of the store to outdoors, ventilating before entry, measuring CO (and oxygen) before entering, never entering alone, warning signs, and a fixed CO monitor where people may enter or where the store connects to living space.
- Dust hazard: intact pellets are too coarse to form an explosive cloud, but the fines and dust produced by handling are combustible wood dust.
- Industrial plants: silos, bucket elevators, conveyors, hammer mills and dust filters need area classification for dust (Zones 20, 21, 22), Ex equipment and explosion protection such as venting, suppression and isolation.
- Self-heating: large silos can self-heat and smoulder; rising CO and carbon dioxide in the headspace are a warning sign, although laboratory work shows fine particle emissions can rise earlier.
- Standards: ISO 20023:2018 covers small-scale stores (up to about 100 t); ISO 20024:2020 covers commercial and industrial handling.
Why pellet stores produce carbon monoxide
Compounds in the wood keep oxidising slowly after the pellets are made, even at room temperature. The reaction uses oxygen and releases CO, carbon dioxide and volatile organic compounds such as the aldehyde hexanal. Emission rates depend on pellet type, age, moisture and temperature. In a closed space the CO builds up and the oxygen falls.
Swedish research published in the Annals of Occupational Hygiene measured up to 123 mg/m³ of CO (roughly 100 ppm) inside domestic storage rooms, where emissions peaked after about two months of storage in the warm season, and 56 mg/m³ in a pellet warehouse. A later study by the same group, prompted by a fatal accident during unloading of a ship in Sweden, found 1,460 to 14,650 ppm of CO and only 0.8 to 16.9 % oxygen in the closed holds of five pellet ships, with almost the same levels in the adjacent staircases. A dangerous atmosphere developed within a week of loading.
For comparison, the EU indicative occupational exposure limit values for CO (Directive (EU) 2017/164) are 20 ppm as an 8-hour average and 100 ppm for 15 minutes. CO is colourless and odourless, so nobody notices it without a detector.
What a safe pellet store looks like
ISO 20023:2018 sets requirements for small-scale and residential stores, including ventilation of the store (with an informative calculation annex), entry procedures that differ for stores up to 15 t and for larger or underground stores, stationary CO monitors and protection against burn-back and back-gassing from the boiler. In practice:
| Measure | Purpose |
|---|---|
| Permanent ventilation openings or ducts to outdoors | dilute CO continuously |
| Fixed CO monitor at the store entrance or in adjacent rooms | warn before and during entry, and of leakage into living space |
| Ventilate and measure CO and oxygen before entry; stand-by person | prevent poisoning and asphyxia |
| Boiler and store protected against burn-back and back-gassing; filling checklist followed | keep flue gas and fire out of the store |
| Warning sign at the door | inform others and the fire service |
For ships, silos and large stores the ship study recommends measuring CO and oxygen before entry, forced ventilation of access spaces, labelling and training; confined-space rules apply.
Dust explosion protection
Wood dust is explosible: HSE guidance gives wood flour Kst values of about 83 to 192 bar·m/s, and OSHA lists wood flour among its St 2 examples. Pellets are typically several millimetres in diameter and do not form explosive clouds themselves, but conveying, dropping and screening abrade them into fines and fine dust. That dust is the explosion hazard.
- Industrial and commercial plants must assess dust explosion risks like any other combustible dust plant: inside silos, elevators, conveyors, hammer mills and filters an explosive cloud occurs during filling and handling, typically giving Zone 20 or Zone 21 inside, and Zone 22 where dust escapes around transfer points. Explosion venting, suppression and isolation protect the equipment, and housekeeping prevents layers that could feed a secondary explosion. Large silos can also self-heat; CO and carbon dioxide rise as smouldering develops, though a 2020 laboratory study found that fine particle emissions rose before any significant increase in CO. ISO 20024:2020 gives a risk-based approach for these sites.
- Small domestic stores fill pneumatically from a delivery truck. The aim is to limit fines and dust movement (pellets with a low fines content, careful filling, a store kept closed to living space) and to keep ignition sources such as unprotected lamps and switches out of the store. ISO 20023 includes explosion prevention and measures against dust accumulation among its topics.
Common mistakes
- Entering a store "just for a minute". CO and oxygen depletion can incapacitate quickly; measure first.
- Sealing the store airtight to keep pellets dry. Ventilation to outdoors is part of the design.
- Relying on smell. CO is odourless; the aldehyde smell of pellets says nothing about CO.
- Treating pellets as non-dusty. The dust and fines from handling are what explode.
Related on excodex
- What makes a dust explosive? Particle size, concentration and the dust explosion pentagon
- Dust explosion protection: venting, suppression, isolation and containment
- Electrochemical toxic gas sensors: how they work and their limitations
- Is wood dust from a woodworking shop an explosion hazard?
- Is the inside of a silo always classified as Zone 20?
- Tools: carbon monoxide in the gas library, IEC 60079-10-2
Standards and further reading
- ISO 20023:2018, Solid biofuels, safety of solid biofuel pellets, safe handling and storage of wood pellets in residential and other small-scale applications: ansi.org preview
- ISO 20024:2020, Solid biofuels, safe handling and storage of solid biofuel pellets in commercial and industrial applications (under revision): iso.org
- IEC 60079-10-2, Classification of areas, explosive dust atmospheres; HSE HSG103, Safe handling of combustible dusts
- Svedberg et al., Emission of hexanal and carbon monoxide from storage of wood pellets, Annals of Occupational Hygiene 48 (2004): chalmers.se
- Svedberg, Samuelsson, Melin, Hazardous off-gassing of carbon monoxide and oxygen depletion during ocean transportation of wood pellets, Annals of Occupational Hygiene 52 (2008): chalmers.se
- Soto-Garcia et al., VOC emissions from multiple wood pellet types and concentrations in indoor air, Energy and Fuels (2015): cdc.gov
- Bluvshtein et al., Early detection of smoldering in silos: organic material emissions as precursors, Fire Safety Journal 114 (2020): hvlopen.brage.unit.no
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.