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A dust explosion in the fuel station

A dust explosion in the fuel station of a power plant created a serious event. Unfortunately, people suffered burns, but fatalities were avoided.
A power plant, built next to an industrial plant supplies heat and steam to the establishment and to the local district heating network. The power plant’s maximum electricity output is 85 MW. The fuel used is mainly bio-fuel, such as cull lumber and sawing waste. The plant has a fluidised-bed boiler which makes it possible to use a variety of fuel types.

Since wood fuel contains branches and stumps, the fuel to be used is crushed to an adequate consistency before transporting it to the boilers. Along with stumps, big stones may end up in the crushers, which can generate sparks.

Conveyor system on fire
One evening there was a dust explosion in the fuel crusher. The explosion propagated along the partitioned conveyor tunnels and caused a fire in the conveyor system while setting off secondary explosions, which propagated a fire in the receiving hoppers and in a parallel fuel network.

At the time of these events, an employee of the transportation company was cleaning the fuel unloading station area according to received instructions. Gases from the dust explosion were released in the unloading station and caused burns to the driver. Car drivers working at the parallel fuel station also suffered moderate burns.

Rapid action saves thousands
The power plant personnel immediately began initial extinguishing action and extinguishing vapour was discharged through the fuel conveyor system. The fire department arrived in 15 minutes, began extensive extinguishing action and emptied the fuel conveyors.

The fire did not reach the fuel silos. Thanks to rapid extinguishing action the property damage did not reach EUR 100,000. The greatest damage was caused by a pressure wave which tore off walls, doors and fuel conveyors.

Actions to prevent similar accidents
After the event, it appeared that the spark detectors in the crusher had functioned properly. However, these detectors cannot prevent dust explosions which occur immediately when a certain dust mixture obtains sufficient ignition energy.

To identify the cause, the fuel side was closely examined. It was soon discovered that, during the day, some non-compliant loads of too-dry bio-fuel had arrived at the plant, which might have caused the formation of potentially explosive dust. The following actions were taken to prevent similar accidents:

1) The crushers were provided with explosion vents and channels.
2) The reception of fuel at a low humidity level was no longer accepted.
3) The use of extinguishing vapour was increased with extremely dry fuel and during maintenance and repair work.
4) In the future, the entire workforce will receive training on the functioning and location of extinguishing vapour valves.

Rapid fire protection may prevent the very worst consequences, but it is always possible to learn from an accident and try to anticipate dangerous situations.



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