[Stoves] Practical Stoves- Introducing The Versatile Stove

[ajheggie at gmail.com]

[Default] On Thu, 3 Nov 2016 01:48:20 -0700,kgharris
<kgharris at sonic.net> wrote:

> I will have to think about the wet wood.  It might be using more energy to evaporate the water then there is energy in the wood.  The Prime stove has a nice design which holds wet wood to the outside of the stove so that waste heat will dry it.  Making provisions to dry it before burning it might be better then burning it wet. 

Hi Kirk

I agree that we should  quantify the cost of burning wet wood but I
think the problem is more to do with the way it affects combustion
than the energy loss, though the effects are intimately interrelated.

I generally use a figure of 2.7MJ per litre or kg of water evaporated
and exhausted with the other flue gases so it's energy "cost" in
burning a 1kg log at 50% moisture content is 2/7*.5 =1.35MJ. The log
contains about 18.6MJ/kg so burned perfectly it would release 9.3 MJ
so the cost in burning the log is about 15% of the gross heat from the
log.

Consider the three Ts advocated for good combustion, Time  for the
gases to burn out, Temperature  for them not to be  quenched before
they burn out and Turbulence to ensure good mixing increases the
ability for a fuel gas to meet an oxygen molecule.

The more significant effect to my mind is that this heat loss both
lowers the Temperature, increases the mass flow and  dilutes the
woodgas given off such that it is unlikely to complete combustion, so
it is vented as un burnt gases and vapours as smoke, robbing the fire
of some of the otherwise available chemical energy.

Worse still is the problem that the wet log  has to give up it's water
vapour before it can itself pyrolyse and burn, so it is still
smouldering long after its heat was needed.

AJH