[Stoves] 40% yield of charcoal

[AJH]

On Sun, 10 Jun 2007 13:46:29 +0200, Crispin Pemberton-Pigott wrote:

>
>Cornelio sent us http://www.fao.org/docrep/X5328e/x5328e05.htm which says:
>
>"Air dry or "seasoned" wood still contains 12-18% of adsorbed (sic) water."


I think they did mean adsorbed but I'm not sure they are technically
correct, activated carbon adsorbs organic compounds onto its surface
by attracting them with weak (possibly van der waals?? forces).
cellulose does something similar with this water.

I look on it like this:

The wood is a porous structure which contains free space in and
between the cell walls. This space contains basically water and
atmospheric gases. This part of the moisture content comes out fairly
easily, it can be wrung out or pushed out. Removing it causes no
changes to the dimensions of the piece of wood.

The remaining water is about 25% by weight of the dry mass of the
wood. This moisture is "associated" with the cell structure, it's not
a strict chemical bond but it is part of the structure. As you remove
this water it causes the remaining wood to change shape, quite a lot
tangential to the rings, slightly less radial to the rings and very
little along its length. The stresses this causes show up as the
characteristic fractures in the end grain of wood, and timber
seasoning aims to minimise this.

As this wood is weakly bound to the cell structure it needs addition
of a small amount of energy in addition to the latent heat of
vapourisation to free it. The availability of heat to do this is
linked to the ambient temperature and the propensity for the water
than to leave as vapour is links to the water vapour pressure in the
atmosphere, i.e. the outside humidity. So the wood settles at a
moisture content that is related to temperature and humidity. This
then is the equilibrium moisture content. We can see that this bonding
contains energy because the equilibrium moisture content varies
differently depending whether the wood is drying, as outside
conditions move the equilibrium to accept more water, or re wetting,
as water is deposited from the atmosphere back into the wood. The
curve showing this difference is a typical broad "S" hysteresis curve.

AJH