KANBrief 4/13
Wood pellets are now firmly established on the European market for heating fuel. Around 2,300,000 tons of pellets were produced in 2013 in Germany alone, serving as a natural source of heat in 320,000 boilers and stoves (DEPI; German Pellet Institute). In storage however, pellets emit carbon monoxide gas. This has already caused several fatalities. KAN sees a need for action in this area. Its response has included initiating the production of a DGUV informative publication.
Pellets consist of wood chips that are compressed into a cylindrical form under high pressure without the addition of chemicals. They are typically produced in large saw and planing mills, where the required raw material arises as a by-product. The pellets are transported to consumers down a long logistics chain which may include ocean-going shipping, dock storage, and various other forms of stores at producers' and dealers' premises. Finally, the pellets are generally transported by road and blown into fuel stores, which are often privately owned.
Small, but not without their dangers
It was not widely appreciated until recently that life-threatening concentrations of carbon monoxide (CO) may arise in pellet storage areas. In addition to the possibility of CO being emitted from boilers that are not functioning correctly, a risk particularly exists in closed storage areas of harmful or even fatal concentrations of CO being produced by autoxidation of the unsaturated fatty acids contained within the wood. The scale of these emissions varies according to factors including the type of wood used, the age of the pellets (being particularly high during the first six weeks following production), the duration of storage, the temperature, and the availability of oxygen in the store. Evidence suggests that the mechanical stress upon the pellets during the charging process is also relevant (Emhofer & Pointer. Lagertechnik und Sicherheit bei der Pelletlagerung. Bioenergy2020+, Graz, 2009; Staatsministerium für Wirtschaft, Arbeit und Verkehr Sachsen. Abschlussbericht zur Sonderaktion „Lagerung von Holzpellets“, 2013, article (pdf)).
Between 2002 and 2011 alone, 13 fatal accidents caused by carbon monoxide poisoning were reported in Europe, ten of these in ships' holds, bulk stores and silos. Three occurred in 2010 or later in stores in private households (cf. Gauthier et.al. Lethal Carbon Monoxide Poisoning in Wood Pellet Storerooms. Düsseldorf/Zürich, 2012). Further accidents in pellet stores in residential property that were however not fatal also emerged later. Both private consumers and maintenance and repair personnel are affected.
Need for action
It is evident that the existing provisions in various areas of the legislation – such as that governing chemicals, the environment, or construction have not been effective in reducing the risk of accidents. It is therefore reasonable to conclude that additional provisions, more closely adapted to the conditions encountered in practice, are needed governing preventive technical measures and user behaviour. It must also be ensured that this information reaches the operator reliably.
In the light of this, KAN has proposed that the Expert Committee Trade and Logistics draw up a DGUV informative publication ("DGUV-Information") on the storage of wood pellets. Measurements are first to be performed in existing pellet stores, giving consideration to the different process scenarios and capacity utilization levels. The findings are then to be used to produce a guidance document with the support of the relevant technical circles. In order for those affected to be provided with preliminary information as soon as possible, the DGUV's Expert Committee Trade and Logistics will publish a preliminary informative document as early as the beginning of 2014.
Since relevant factors may include not only storage of the pellets but also their production and transport and possibly even installation design, a viable prevention solution must be comprehensive in its approach. KAN therefore lobbies for:
• Standards governing pellet-fired boilers to be amended with the inclusion of safety requirements (such as for air vents in the stores), and for user instructions to make comprehensive reference to the risks to commercial and private users
• Review of whether the CO emissions from pellets can be reduced by technical modifications and whether the user information required in the standards can be improved
• The current draft of VDI Technical Rule 3464, "Emission control – Storage of wood pellets at the end user […]", to give adequate consideration to existing findings, for example concerning minimum ventilation times
Finally, it should also be considered how the German construction legislation could contribute more explicitly than is currently the case to the safe design of pellet stores, for example via the combustion ordinances issued by the German regional governments.
Michael Robert
robert@kan.de