Gas puts out silo fires

All fuels oxidise slowly, and the right combination of heat and moisture can result in a pyrotechnical process in which the pellets self-combust. If this results in a fire, poisonous gases can develop as well as the risk of explosion.

“Silo fires are very difficult,” says Lasse Larsson, risk manager at Lantmännen. “They involve big volumes and a fire centre that is almost impossible to access.”

As luck would have it, these fires seldom occur. However, because the frequency is so low the fire service has limited experience in putting them out. This was the reason that two years ago the Räddningsverket, Swedish Rescue Services Agency, the Statens Provningsanstalt SP (Government Testing Institute), Lantmännen and AGA began a project aimed at producing a strategy for extinguishing these fires.

“Research into self-combustion was already available,” says Larsson. “Looking at it, we concluded that the best way to put out these fires was to use nitrogen.”

Nitrogen is an inert gas similar to carbon dioxide that forces out oxygen, starving the fire and also reducing the risk of explosion.

At the fire in Kristinehamn, both nitrogen and carbon dioxide were used. The carbon dioxide was added at the top of the silo, to act as a lid. Meanwhile, a hole approximately 50 millimetres in diameter was made at the bottom to feed in a nitrogen pipe close to the centre of the fire. It took five days to extinguish the fire and empty the silo, as well as some 14 tonnes of nitrogen and 35 tonnes of carbon dioxide

“You could say that the fire in Kristinehamn was something of an unplanned large-scale test, and it has given us a lot more knowledge,” says Larsson.

Carbon dioxide and nitrogen were delivered to the site via two AGA tankers on the same day the fire was discovered. At the same time two AGA service technicians were called to the fire site to organise the necessary equipment and prepare the intake, which meant no valuable time was wasted.

One conclusion of the project was that it would be good for all silos to have bottom intakes to make it possible to feed in nitrogen, and this was confirmed in the Kristinehamn fire. This would avoid to the necessity of drilling a hole at the time of the fire. Fixed measurement points on all silos is also high on the list of recommendations.

“But there are so many old silos in the countryside, we are talking about a huge number,” says Larsson. “Rebuilding all of them will have to be done gradually.”