Wood-pellet and wood-chip boilers are increasingly being used in homes, businesses and community buildings as an alternative, renewable energy source to oil- or gas-fired boilers. They are also being considered more often for use in large-scale power generation.
There have been concerns, though, for some time that the risks associated with the storage of wood pellets – particularly the release of carbon monoxide (CO) and absorption of oxygen – are not well understood.
An HSE Safety Notice was issued in 2012, warning manufacturers, suppliers and installers to control the potential risks. However, there are fears that the hazards associated with wood-chip stores are still inadequately understood.
After a number of fatalities in Europe related to wood stores, the Health and Safety Executive commissioned a study of the potentially dangerous atmospheres that may be generated in wood-pellet and wood-chip stores. It aimed to obtain information on how wood and chips are stored before use, how the buildup of CO is controlled in fuel stores, and what health and safety information suppliers and users have on the storage and use of wood pellets and chips.
The researchers also set out to measure the levels of CO and other relevant gases in wood stores, and to compile an initial view on best practices and procedures for the industry, to help control and manage the risk of dangerous atmospheres.
Staff must be trained in safe working practices and emergency procedures
They visited six sites with small-scale wood pellet- or chip-fuelled systems, and one with a large-scale pellet store to study procedures, risk-management systems and controls, and to measure ventilation rates, levels of gases and vapours in the stores, and the microbiological content of the fuel.
Laboratory research was also carried out into carbon monoxide and carbon dioxide emissions – and oxygen depletion – from wood pellets and chips.
The site visits and laboratory tests found that:
- Potentially dangerous atmospheres may be generated in wood-pellet and wood-chip stores
- Wood pellets produce CO and carbon dioxide (CO2), and deplete oxygen in the store
- Wood chips produce CO2 and some CO, and deplete oxygen
- Dangerous atmospheres in fuel stores may also arise because of poor exhaust ventilation of boiler combustion gases
- There was limited knowledge of the hazards associated with wood pellets and chips at sites with small boiler systems, and a greater level of knowledge at the large pellet store
- There was limited communication – of the health and safety issues relating to wood-fuel storage – between the companies supplying and maintaining the boilers, those manufacturing and supplying the fuel, and the users of the systems.
The 2012 HSE Safety Notice raised general awareness of the problems, but not all of its recommendations had been implemented at individual sites.
There are several simple measures that can be implemented to improve awareness of the hazards associated with the storage of wood chips and pellets in confined stores. Journal readers should be aware of these and ensure that any installation with which they are associated incorporates them.
On any site that falls within the scope of the Construction Design and Management Regulations, there is an obligation on the principal designer to address these issues. The Confined Space Regulations 1997 may also apply.
The potential risk of exposure to hazardous atmospheres should be controlled by restricting access to the stores and displaying clear warning signs. Carbon monoxide alarms will provide warning of dangerous levels of this gas. Where chips or pellets are stored in a confined space, staff who are authorised to work in the area must be adequately trained in the appropriate safe working practices, and made fully aware of the emergency procedures to be followed in the event of any incident in or around the fuel store.
While unplanned ventilation within store rooms may reduce the buildup of dangerous toxic atmospheres, the industry is wary of employing increased ventilation because of the potential for ingress of moisture and deterioration of the fuel.
However, appropriate planned ventilation will help to control the buildup of hazardous gases. This should also take account of ambient temperature and humidity, as these affect the rate of off-gassing (the buildup of toxic gases).
The HSE recommends that minimum ventilation standards should be determined to prevent significant off-gassing during fuel storage. It also says that wood pellet and wood chip fuel suppliers and distributors should produce and distribute comprehensive and consistent guidance about storage for end users;
this guidance which should include material-safety
Given the ongoing interest in biomass installations, safe design of fuel storage is a key element of such systems, and readers should be aware of the latest HSE findings – whether in the UK or elsewhere – as the science and health impacts are the same around the world.
- Safe storage of wood-pellet and wood-chip fuel, a report by the Health and Safety Executive
- HSE (2012) Safety Notice OPSTD 3-2012 Risk of carbon monoxide release during the storage of wood pellets
- HSE (1997) Confined Space Regulations 1997 Approved Code of Practice, Regulations and guidance. HSE Books, 2nd edition (2009) ISBN 9780717662333
Dr Hywel Davies is technical director at CIBSE