Til recently, I’d only dealt with the very end of the wood chip supply chain, the part where the chips arrive in a lorry, ready to be tipped into the chip store and burned in the boiler. But in connection with a number of new projects, I’ll be more closely involved in the whole woodchip lifecycle: chipping, drying, storing, delivery etc. And while getting to grips with some practicalities a few interesting quirks have come to light. Maybe not earth-shattering, but interesting just the same so I thought I’d note a couple down here.
First, when you buy wood chip don’t forget you’re paying for water. So if you hold on to that chip for any length of time in the right conditions it will dry further, reducing your tonnage and so increasing your cost per tonne. For example, if you buy a tonne of fresh wood chip at 55% moisture content for £45 and then store it under cover and let it dry out, here’s how the total mass and £/tonne will change as the chip dries to around 25% moisture.
Fig 1 – Total mass and £/t for a tonne of wood chip as it dries
But while the cost per tonne goes up during drying, so does the useful energy content. Effectively you’re letting nature dry the wood rather than having to evaporate off all that water in the burn chamber of your boiler. And that makes a big difference – consider that it takes more than six times as much energy to evaporate water as it does to raise it from room temp to 100 degrees in the first place!
And perhaps surprisingly, as your chips dry your total mass is reduced but so too is your p/kWh (in other words the cost of your chips, or £45, divided by the net calorific value). Your energy is actually getting cheaper because you’ve got more of it as the wood dries. Shown below.
Fig 2 – Useful energy content and p/kWh for tonne of wood chip as it dries
The moral of the story? Don’t get hung up on price per tonne where wood chip is concerned. You need to know how much water you’re buying in order to estimate useful energy content. Also, look at seasoning of chip as an investment. Let nature do the hard work and spare your boiler.
Note that the figures above assume hard wood at a gross calorific value of 19.5 MJ/kg.
carbon limited 
Good stuff Casey, was thinking about this the other day (sadly). What about pellets? I assume their moisture content is fairly constant and therefore buying via the ton is not so problematic.
Yep, good pellets tend to be around 8% moisture so it’s easier to compare in terms of tonnes. However, the type of wood and amount of bark have a big effect on energy content (and ash), so there are still questions you have to ask in order to make in informed choice.
As does the supplier. Until the Technical standard is compulsory (it was due some time this year and may already have been implemented) all kinds of ‘impurities’ are finding their way in. This not only reduces calorific value but is detrimental to both local air quality and can seriously damage the boiler.
The domestic market is in trouble as well- several pellet suppliers are in real trouble or have already gone under. Unless you want to be a one man rescue operation (and if you have the extra room), woodchips seem a better investment right now
Casey,
How can your cost per tonne varaying, if you are not spending any penny on drying the wood chip? Could you please explain?
Hi Nabeel,
If you buy a tonne of moist wood chip and allow it to dry further, you’ll be left with less than a tonne of fuel (since a certain mass of water will have evaporated).
So to have a tonne of this new drier fuel, it will cost you more per tonne than you originally paid. This is because you’ll need to start with more than a tonne of the moist fuel to end up with a tonne of the drier fuel.
I hope that helps.