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Wet Firewood

I have accidentally entered the energy business.  About a third of the logs left after Lincoln Electric maintained the power lines were cut to non-marketable lengths, so the alternative was turning them into firewood.  Earlier, I wrote of the differences between species as firewood – this time I’m looking at the cost of water in firewood.

In general, a cord of green wood contains about a ton of water.  The problem isn’t the action of heating the water – raising the temperature of water is an easy calculation.  A British Thermal Unit (BTU) is the amount of heat required to raise one pound of water by one degree Fahrenheit.    The problem is converting ice to water, and water to steam.  It takes twice as much heat to raise a pound of ice one degree as it does a pound of water.  There’s 144 BTU difference changing from ice to water.  Then, once the water is heated to boiling, we need another 970 BTU to change a pound of water into steam.

15% moisture, in Douglas Fir that’s been drying a year. It would be about 30% moisture freshly cut.

The references tell me that firewood is “dry” when it is still holding 20% moisture.  The dry larch on my porch tests from 11% to 15%, so I’m feeling good.  Let’s guess that there’s a pound of water in an 8 pound chunk of stovewood, and I’m bringing it in at zero degrees – no temperature at all.  Makes the calculations easier.  This calculation is for 12.5% water – fairly dry wood.  Double the numbers for 25%, and remember 20% moisture is considered dry. 

Changing one pound of ice at zero degrees to steam that can go up the chimney takes:
 64 BTU to bring the ice from 0 degrees to 32 degrees
144 BTU to change ice to water
180 BTU to bring the water from 32 degrees to 212 degrees
970 BTU to change water to steam and send it out the stack

1358 BTU total, just to get rid of one pound of water, from about 69,000 BTU in my 8 pound piece of dry stovewood – a loss of roughly 2 percent.  You won’t notice it on my 15% larch.  On the other hand, a cord of lodgepole has 23% less BTU than larch, but the calculations for water stay the same.

And that assumes 100 percent efficiency – which I don’t have.  The moisture meter cost less than $20 – and I think it’s worth having on hand. 

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