Wood can burn. Wood pellets and chips belong to the important secondary fuels category in modern power plants, furnaces or heat generators, so this is quite obvious.

However, although this feature of wood is known in general, it is still not common knowledge that this combustibility makes wood handling processes some of the most hazardous in industry from an explosion standpoint; explosion incident statistics are evidence enough.

But what makes wood material and wood handling processes so special in terms of explosion hazards?

This question can be answered quite simply. Considering the requirements of the dust explosion phenomenon – combustible dust, air/oxygen and effective ignition sources – nearly all wood handling installations have the ‘perfect mixture’ for dust explosions to occur.

Using the example of a fibreboard plant, where all typical wood industry-related equipment, such as silos, conveyors, screens, mills, dryers, cyclones and dust collectors are found, an explosion hazard is even more likely, as all the above-mentioned equipment creates wood dust, causing explosive atmospheres. Additionally, given mechanical moving parts, as well as drying processes, ignition sources are easily generated.

In Autumn 2012, this deadly combination led to one of the most severe wood dust explosions in history in South America, where five people lost their lives and the entire plant was shut down for more than five months.

Avoid and protect

The explosion safety concept for such plants is typically made up of a combination of explosion prevention measures and explosion protection measures (reducing explosion effects to an acceptable level).

Explosion prevention means taking measures to prevent the formation of explosive dust clouds, as well as avoiding ignition sources by de-dusting, housekeeping, grounding, proper maintenance and/or spark extinguishers.

We know that even if all preventive measures are applied (especially with regard to the latter), this approach might lead to misapplication of spark extinguishers that might not work if particles are large.

It also cannot suppress an explosion and only addresses the ignition risk arising from small, hot particles. Nor does it prevent ignition sources from tramp metal or hot surfaces.

That is why protective measures also have to be applied in most wood handling installations. They typically apply one of these approaches:

  • Explosion resistant design (make equipment so sturdy it will withstand explosion overpressures of up to 10 bar)
  • Explosion pressure venting (allowing escape of pressure and flame via a predetermined breaking point on the installation)
  • Explosion suppression (incorporating a rapid fire extinguisher to stop the flame)
  • Explosion isolation (prevent flame and/or pressure going down or upstream )

Due to minimal maintenance requirements and low investment costs, passive explosion protection approaches, such as explosion pressure venting, are most commonly used in wood handling facilities.

Such blast panels can be combined with flame-trapping mesh materials allowing various applications to be protected by socalled flameless vents.

As with any comprehensive safety concept, even a fully protected plant can only be secure when all human actions, situations and conditions are taken into account.

In practice, this means plant management has to be aware of the explosion risk in general, implement available explosion safety measures and educate personnel.

Awareness of the need for combustible dust explosion safety also has to be raised so catastrophic events are not likely to endanger health, lives, or the business.

Therefore, risk analysis should be carried out to identify hazards and to allow the implementation of appropriate safety measures. BeWAre unexpecTed fire Bugs When conducting a risk analysis, all circumstances have to be considered. A last illustration impressively shows that even small ‘bugs’ can affect explosion risk.

In this case, several sawmill operators carried out a risk analysis and decided not to protect their installations handling wood chips with, normally, high humidity content. These conditions changed following a pine beetle infestation that led to many dead/dried trees.

After a long period of quarantine, these trees were purchased at a low price and brought to the sawmill. Due to the pine beetle infestation, the resulting wood chips were drier than usual and many explosions occurred in the sawmill plants, leading to long down times and several injured people.