When pellets are not actually pellets

Cheap ‘Pellets’ – do they actually provide a saving, even in the short term?

When the re:heat engineers were asked if they could come and help with a re-commission of a boiler operating on a cheap batch of ‘wood pellets’ we responded with a cautious ‘well, we’ll come and have a look and see what we can do’.  As a customer focussed business, keen as ever to keep our clients happy, we thought we’d call in and take a look……… take a look at what we found:

Dusty wood pellets

Whilst we fully appreciate that client has saved themselves a few quid by taking the rejects from a wood pellet mill, we don’t think they took into consideration the possible effects of using this fuel on their boiler.  It may not actually be dust either – it can often be classified as wood flour which has different combustion properties to wood pellets or sawdust.  When wood dust becomes airborne and is introduced to an ignition source, it can lead to an explosion.  Biomass boilers designed to run on wood dust have a very different design – often a similar design to a power station – where wood dust is injected into a flow of air via a high-pressure fan and introduced into a controlled combustion chamber to release energy in the form of an explosion.

Burning wood dust or flour in a standard biomass boiler is certainly not recommended.  In a standard hearth, the grate is designed to allow air to flow over pellets, creating an even burn, efficiently extracting the available energy from the pellets.  The energy released isn’t as consistent and cannot be controlled as easily.

With dust on the hearth, the combustion air cannot circulate around the fuel, leading to ‘hot-spots’ and in turn reducing the life expectancy of the grate components and the boiler itself.

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The Health and Safety Executive also has a lot to say on this subject:

http://www.hse.gov.uk/woodworking/fire.htm

And you may even find some real life case-studies of explosions if you look carefully.