Calorific value of pellets – it doesn’t make a lot of difference does it?

The short answer is yes, but that wouldn’t be much help without an explanation and evidence. So here goes, what is the calorific value, why is it important and what can affect it?

The calorific value (CV) is the amount of energy released as heat when a compound undergoes complete combustion with oxygen. Usually given in GJ per tonne, each pellet manufacturer will show a different calorific value.

So the higher the value the more energy per/unit of fuel you will receive, therefore the more energy output per tonne, the less fuel you need to burn for the same energy output and the more RHI/ tonne you can potentially earn.

However, as with everything in life, it’s not quite as simple as looking for the highest value; you need to be sure that you are comparing like with like.  Whenever you compare the calorific value of pellets, it is a must to check you are comparing the same measurement, this  is important because calorific values can be measured as gross calorific value or net calorific value.  The difference is that in gross calorific value the energy content is measured as though the energy in the water vapour were recovered whereas in the net calorific value it is assumed that the energy in the water vapour is lost. Industry has settled on using net calorific value as the appropriate unit as this represents real outputs rather than theoretical outputs.

What affects the CV of pellets?

Moisture content can have the greatest affect on the CV of wood pellets and on the efficiency of your boiler. The rule here is the drier the better, but in a range of say 6-8% moisture content – any lower or higher will impact on the pellet quality.

Different timber species have differing energy output on a volume basis, but should be similar on tonnage basis. We predominately use spruce and larch blends – the dominant species in our local forests, both of which have high energy content.

Pellet quality is an important factor in energy content. We see density and fines as two of the more significant variables. With durability the combustion period is optimised when the pellets are neither too dense nor too soft. Having high fines content again affects combustion and reduces the burn time meaning the energy available from the fines is effectively lost.

A quick internet search shows that calorific values are available but again to make comparison hard some are in Mj/Kg and others are in kWh/tonne, how do you work out a comparison? In a word, Cheat – there are loads of websites to help with conversions.

Surely it doesn’t make that much difference? Well here is an example of a performance comparison carried out by one of our customers.

The comparison was carried out by a client who runs a poultry farm in Northern Ireland. He had taken the decision to try a low cost pellet because small changes to input costs have a significant effect on profitability within the agricultural sector, including heat generation.  He ran a trial to test the relative cost per tonne in energy terms. The evidence was stark, measured at the heat meter after the boiler efficiency is factored in, the energy output from the ‘lower cost’ pellet was 3,500 GJ/tonne against 4,500 GJ for Land Energy pellets.

This simple check demonstrated a 23% reduction in energy output from the ‘low cost’ pellet. Measured in £/GJ the ‘low cost’ pellet actually cost 13% more due to lower output per tonne. This is just focusing on the energy content – the losses are even greater when you look at the increased wear rates to the boiler and increased de-ashing required for ‘low cost’ pellets.

So once you have checked that the fuel is on the BSL list, that it carries a quality mark such as EN Plus, then check out the net CV value. If it isn’t obvious, ask, any self-respecting wood pellet supplier will be happy to answer your questions – backed up with a bang-up-to-date laboratory report – and you will be able to make the decision about your most cost effective source of fuel.

Woodlets

By Hugh Montgomery 24th February 2016

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