Avoiding toxic gases from the pellet bunker
Wood pellets for heating systems and heaters are regarded as sustainable carbon dioxide-neutral fuel. However, the wood shavings pressed into oblong little sticks have potentially hazardous properties when stored in large quantities: the manufacturing process involving cutting up, heating and drying of the wood and the shavings gives rise to auto-oxidation processes in certain wood components. For example, gases are formed from unsaturated fatty acids, such as the harmful carbon monoxide (CO) and various aldehydes. Even months after production, the pellets can still emit gases. In storage rooms, the concentration can be so high that it can lead to severe poisoning in persons entering the rooms. “Many house owners and operators of heating systems fuelled with pellets are unaware of this hazard potential”, says Professor Dr. Dr. Andreas Hensel, President of the Federal Institute for Risk Assessment (BfR). “According to experimental studies, concentrations of up to several thousand ppm carbon monoxide can be reached inside pellet storage rooms which is sufficient for lethal poisoning.” How high the concentration is in a given storage room depends on the quantity stored, the temperature, ventilation and the age of the pellets.
Several human cases of carbon monoxide poisoning including lethal ones have been reported to the BfR. The persons had spent time in wood pellet storage rooms, so-called pellet bunkers, or they wanted to enter such rooms to carry out maintenance works. These storage areas were airtight, meaning that extremely high concentrations of carbon monoxide had accumulated in the rooms without being noticed by the casualties.
Carbon monoxide is colourless and odourless. In consequence, even high concentrations of the gas are not perceived by humans. Already at a concentration of 200 ppm or more in the breathing air, prolonged stays in such environments can lead to headaches. At 800 ppm, light-headedness and nausea are experienced after a short time. Still higher concentrations result in a loss of consciousness after as little as a few minutes.
The accidents reported to the BfR show that precautionary measures such as brief airing of pellet storage areas prior to access are not sufficient to lower the gas concentrations to a harmless level. Even mere warnings drawing attention to the poisoning hazard do not offer adequate protection. An investigation conducted by the state authority found that within the area covered by the study, the majority of storage rooms had inadequate warnings. This applied to one-family homes, larger residential complexes and the premises of operators of large storage rooms.
Due to their inherent hazard potential, wood pellet storage areas should, through suitable measures, be equipped in such a way that no gases from them can escape into residential, working, and other rooms such as normal cellar rooms that are accessed by humans. Such measures are, for example, continuous aeration and ventilation of the storage rooms through contact with the open air and installation of CO detectors. On the other hand, it should be ensured that the toxic gases are diverted as far as possible from rooms used by humans. Additional safety measures which warn and protect against CO should be taken. The same applies to the storage of woodchips.
Information on the problem of safe management of pellets can be found in the following publications:
http://www.arbeitsschutz.sachsen.de/download/Abschlussbericht_Pellets.pdf
http://www.fnr-server.de/ftp/pdf/berichte/22031508.pdf
http://www.fnr-server.de/ftp/pdf/berichte/22021708.pdf
About the BfR
The Federal Institute for Risk Assessment (BfR) is a scientific institution within the portfolio of the Federal Ministry of Food and Agriculture (BMEL). It advises the Federal Government and Federal Laender on questions of food, chemical and product safety. The BfR conducts its own research on topics that are closely linked to its assessment tasks.