[Stoves] 40% yield of charcoal

[Crispin Pemberton-Pigott]

Dear Andrew

You have confirmed some thing I suspected by didn't have an explanation for.

>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.

Just in case this is not clear to some, the hysteresis curve is produced by 
plotting the atmospheric moisture against the moisture content of the wood. 
Even if the external conditions change extremely slowly, the moisture 
content in the wood never keeps pace with the atmosphere, both on the way up 
or on the way down because some extra energy is required to put the water in 
or take it out.  This is not caused by the time it takes to get the moisture 
deep inside a think piece of wood. That is something else.  Even with a thin 
flake of wood, there is a lag because of the energy requirement.  It means 
that for some small increase or decrease in the atmospheric moisture, the 
wood cell moisture content does not change at all.

Something else worth mentioning is the large percentage of wood mass that is 
water bound in this manner.  It is quite a lot of the total.  We have Acacia 
species in Swaziland that are so dry they can be cut and used in a fire 
immediately.  That is their normal condition.  It will be interesting to 
find out if they have a 25% (or so) moisture content which would indicate to 
me that there is some minimum level of cell wall moisture required for a 
tree to grow.  Perhaps there are a whole group of trees that have well below 
average moisture contents producing interesting wood properties.

The preferred species for cooking in the lowveldt region from here north 
through eastern Zimbabwe turn out to be slow growing trees containing high 
salt crystal levels.  These salts burn/explode on a micro scale and snuff 
the flames on a continuous basis because of their fire-limiting properties. 
The result is a fire that glows beautifully, rather than burning with red 
and yellow flames.  Such wood is great for roasting meat and simmering pots. 
The salt crystals are visible in micrographs of wood cells.  I would not be 
surprised to find there is a link between trees with low cell wall moisture 
and the accumulation of such salts.

Regards
Crispin