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Which is better value for money - Air dried @25% MC or Kiln dried Firewood @ 20%MC ?

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This is a question I have spent some time over trying to determine an accurate answer given the variables.

 

Yet again, never, ever have I seen a credible financial comparison between the 2 fuel types.

 

I have compared the 2 fuel types below to illustrate the main differences to facilitate calculating the cost differences:

 

Air dried Firewood @ 25% MC

 

Lower cost

Lower kWh / kg

Burns for longer

Less logs per kg - therefore as firewood bought by volume, less logs needed to be burnt to give the estimated  kWh / kg

 

Kiln dried Firewood @ 20% MC

 

Higher cost

Higher kWh / kg

Burns for less time

More logs per kg - therefore as firewood bought by volume, more logs needed to be burnt to give the estimated  kWh / kg

 

I have left out all my specific data for each of the above Firewood type characteristics, so not to muddy the water and allow other to give their own independent views.

 

Of course the values for the above fuel type differences will generally be assumed values for the purpose of this post.

Possibly some people will have had experience of Kiln dried @ below 20% MC, but in my experience by the the time it is burnt on average the majority of Kiln dried will be circa. 20%.

 

Openspaceman will surely be a main contender to provide eloquent conclusions, if he has the time to reply. 

 

Any replies most appreciated from those interested.

 

Edited by arboriculturist

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Suspect you have made an error in the kiln dried cost statement ?

 

Also with regard to weight - nothing can 'weigh less per kg'?!

Edited by waterbuoy

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I would be interested in reading from someone with more knowledge on the KW side but I'll give it a go.

 

Just to be picky, the comparison above should be weighs less per cube and in an ideal situation 20% moisture wood should last longer as it isn't using energy to dry itself. Admittedly in reality it can burn faster and hotter as people keep chucking wood on instead of closing down the fire.

 

I've seen a few studies of KW to moisture including Here but they always vary a bit, then I suppose you just have to work out the cost of drying another 5%.

 

Kiln drying costs us around £10 a cube to get to below 15% so say £6 to 20%

 

We get RHI so it's around break even for us but if we had to put £6 on top of our price (smallest worst value load) of £147 for 1.8cube it would go from £82 to £88 roughly so a 7% increase in price. I can't read the study because I'm on my phone for a conclusion on KW per £ unfortunately.

 

 

 

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5 hours ago, waterbuoy said:

Suspect you have made an error in the kiln dried cost statement ?

 

Also with regard to weight - nothing can 'weigh less per kg'?!

Apologies, a few typos and errors - corrected now (always get a 3rd party to proof read before posting ?  ) 

Edited by arboriculturist

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12 hours ago, kevinjohnsonmbe said:

Which is best VfM? 
 

Despite all the calc’s and variables, surely it’s a simple matter of cost. 

Yes, the thing is there is much wringing of hands and wailing about using wood to dry wood but the physics of it is all the same , energy is energy whether we use solar heat for the drying or intercept it first on its way to gaining entropy and store it in a tree.

 

The bigger argument is whether our value system, money and wealth, are good measures by which we make decisions. If not we need to come up with another one that functions as well as this.

 

@gdh has put some numbers on it  and it's simple to work out the difference in energy available in 25 or 20% mc wood but the rest is so many variables. One would need to know the parameters of the stove and the flue temperature to decide on which was better for which stove. From the supply side one would need to know the cost of covered space, seasonality of labour, cost of capital equipment etc. In practice a person makes simple decisions to run a business and one thing leads to another, if another business makes different decisions and as a result out competes the first then it is more successful, that's what drives capitalism, not basing decisions on what's good for the environment, that's what regulations do to keep everything acceptable to most of we also rans.

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One cube of firewood will create the same amount of heat when it's burned whether it has 20% or 25% moisture content.  The difference will be whether that extra water stops it burning properly by reducing the temperature or in other ways. 

 

Some heat will be lost turning the water into water vapour but the science of that is beyond me at the moment.  It's apparently not just as simple as each kg of water using 0.6 kWh of energy to turn into vapour, the actual value depend on the temperature at which it happens.  I am also not sure whether "bound" moisture when wood is below fibre saturation takes additional energy to separate it from the wood.  So although I intended to do a rough calculation, I've had to admit defeat.

 

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What about air dried at 20% ?

3 hours ago, aesmith said:

One cube of firewood will create the same amount of heat when it's burned whether it has 20% or 25% moisture content.  The difference will be whether that extra water stops it burning properly by reducing the temperature or in other ways. 

 

Some heat will be lost turning the water into water vapour but the science of that is beyond me at the moment.  It's apparently not just as simple as each kg of water using 0.6 kWh of energy to turn into vapour, the actual value depend on the temperature at which it happens.  I am also not sure whether "bound" moisture when wood is below fibre saturation takes additional energy to separate it from the wood.  So although I intended to do a rough calculation, I've had to admit defeat.

 

What about air dried at 20% ? ?

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Surely 20% is 20% whether air dried or kiln dried?  Or are you thinking that the kiln drying would have altered the properties of the wood as well as removing water?   I expect that's possible.  Some woodworkers prefer air dried, and I'll bet that kiln drying for firewood is a lot more brutal than for joinery timber.

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