Feb. 12, 2018 - On July 5, 2010 a wood pellet silo in Norway exploded when firefighters released inert carbon dioxide into the headspace to lower the oxygen content and suppress a smouldering fire. The lesson from this incident is that the use of carbon dioxide to suppress silo fires is unsafe.
Smoldering fires produce flammable pyrolysis gasses. The gasses can travel and accumulate, for example in the headspace of the silo. The release of carbon dioxide from high-pressure cylinders can generate static electricity with sufficient energy to ignite the pyrolysis gases.
Smouldering fires in wood pellets storages can occur for a number of reasons. There are plenty of examples in industry where pellets self-heat deep inside an undisturbed pile. Another known cause is mechanical friction heat in, for example, a roller bearing, which can ignite dust particles. Embers can be difficult to detect and they can travel in conveyor systems and start fires in storage areas.
Water is often an unsuccessful method of fighting smouldering fires in bulk storage silos. Water from sprinkler or deluge systems will only cause damage to the silo and is ineffective in suppressing deep seated fires as the water will generally tunnel down through the outside of the material instead of wetting it through.
Suppress fires with inert gases
Alternative firefighting strategies have been devised which use the injection of inert gases to suppress combustion. Inert gases can deplete the oxygen available for combustion and quench the pyrolysis. The most commonly available inert gases in large quantities are nitrogen and carbon dioxide.
Oxygen-deficient smouldering fires produce pyrolysis gases. A typical pyrolysis gas is carbon monoxide, which is poisonous and flammable. The presence of unburned pyrolysis gases is a known hazard to firefighters. If a compartment fire has little or no ventilation, leading to an oxygen-deficient environment, large amounts of unburned gases will accumulate. The gases may remain at a temperature hotter than the auto-ignition temperature. The sudden access to air by breaking a window or opening a door may result in large flames rapidly expanding towards the source of oxygen; this is known as a backdraft.
Mixtures of fuel and air will burn only if the concentration of fuel is within certain limits, the so-called flammability limits. The limits for methane, for example, are 5-15 per cent volume. Carbon monoxide has a much wider flammability interval, the lower and upper flammability limits are 12.5 - 74 volume per cent. Mixtures of pyrolysis gases and air, at temperatures below the auto-ignition temperature are therefore likely to be in the ignitable range and able to cause an explosion if they meet an ignition source. Carbon dioxide may provide that source of ignition.
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