Heat output vs moisture content

[aesmith]

Hi,

 

I understand it's generally recommended to get firewood down below 20%, but wondered if there were any figures floating around relating useable heat output at different moisture contents. It's just for general interest, but also because I've noticed adverts now and again for kiln dried firewood which seems pretty far fetched.

 

I thought about having a go at calculating loss due to latent heat, but when I think about it I'm not sure that would be accurate. As I understand it the water is bound to the wood structure in some way, so maybe takes more energy to extract the water in the first place.

 

Anyway, as I say I would be interested if there were any figures floating around.

 

Tony S

[Tom D]

There are figures out there for wood chip boilers, its surprising how much it affects the output. Logs will be similar although I would imagine chips will dry out faster in the fire making the effect of water in logs even worse. A 1kg log at 30% moisture contains 300ml of water, thats a can of coke! in every log! Imagine chucking that on the fire each time....

[Stubby]

And its not just the heat loss converting that water to steam its the gumming up of the flue as well ....

[difflock]

I did somewhere see that one should deduct 1% heat energy for each extra % moisture content.

So if timber at 20% moisture gives a nominal 4kw*hr heat energy

then a log at 10% moisture gives 4.4kw*hr

or 30% moisture gives 3.6kw*hr

or 50% moisture gives 2kw*hr

m

[Lucan]

See page 2

 

http://www.forestry.gov.uk/pdf/eng-woodfuel-woodasfuelguide.pdf/$FILE/eng-woodfuel-woodasfuelguide.pdf

[gensetsteve]

I also think the wetter the log the more you need to open the vents until eventually most of the heat goes up the chimney. My logs for the house sit in a clear poly tunnel for 2 years on average. The idea that dry logs burn too quick is rubbish in a woodburner you can clamp it down till they burn just right with max heat.

[openspaceman]

And its not just the heat loss converting that water to steam its the gumming up of the flue as well ....

 

They tend to be inter related, moisture needs a lot of heat to turn it to steam, 1 litre of water needs 4.186 kJoule for each degree you heat it up so you need 4.186*90=94.186kJ to get it from 10C to boiling but then have to add 2.300kJ to turn it to steam.

 

It's robbing this heat from the fire that quenches the flames and prevents combustion from completing and it's the products of incomplete combustion that then condense out with the steam to foul the chimney.

[openspaceman]

I did somewhere see that one should deduct 1% heat energy for each extra % moisture content.

So if timber at 20% moisture gives a nominal 4kw*hr heat energy

then a log at 10% moisture gives 4.4kw*hr

or 30% moisture gives 3.6kw*hr

or 50% moisture gives 2kw*hr

m

 

It can be worse than a straight line correlation as the higher the moisture content the more primary air you need and this adds massflow which means the fire may never reach a good working temperature. Also the dew point of the flue gases is lower so condensation occurs at a higher temperature which is why it is necessary for the flue gas to be at above 100C as it leaves the stack, given that a condensing gas boiler will vent its flue at 50C with the water as visible droplets and you can see a woodfire will always be wasting more flue heat than a condensing boiler.

 

Having said that a general ruel I use is to take the oven dry weight of logs I have and multiply by 18.6MJ per kg od (for hardwood) then subtract 2.7MJ for every kg of water (neglecting water formed in combustion).

 

So a 2kg log at 15% mc wwb will have 1.7 kg od and .3kg moisture and burned cleanly the most heat you will get out will be 1.7*18.6-.3*2.7=30.81MJ in practice you cannot get this heat into the room because you have to raise the temperature of the combustion air to the flue temperature and dump it up the stack, whilst a gas fire may only need 10% more air than the stoichiometric amount (this is the amount of air needed for there to be the exact amount of oxygen to combine with the fuel) the woodfire will typically need 100% more air and this excess air gets a free ride from the room and up the chimney carrying heat with it.

[SamWhiting12]

I find the Biomass centre very useful for comparing energy values between different values.

 

Fuel costs per kWh

[aesmith]

Thanks, there's some good figures there. I suppose manufactured pellets would be about 10 or 15% M/C, would that be correct? Or are they lower and kept low by keeping them in sealed packaging.