[Stoves] 40% yield of charcoal

[AJH]

On Mon, 11 Jun 2007 08:27:53 +0200, Crispin Pemberton-Pigott wrote:

>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.

Yes because the atmospheric moisture is directly related to the
relative humidity and temperature.
 
>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. 

You must be aware first that it is not a straight line relationship,
the curve is "S" shaped (ogive I think) with the two lines roughly
parallel at any point depending on whether water is entering or
leaving the wood. Tom Miles recently posted a link to equilibrium
moisture contents but I haven't taken the time to refresh my memory
yet. Also it's reversible so energy will be given up when water
rebonds to the cell structure and taken in when it leaves.


> 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.

Yes


>  It means 
>that for some small increase or decrease in the atmospheric moisture, the 
>wood cell moisture content does not change at all.

No this isn't so, always the mc will change if the outside atmosphere
changes but this change will lag the same change in the opposite
direction, the only place the curves meet are when there is no water
left in the wood (over 105C I would guess) or when the cell water is
saturated.
>
>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.

I've no idea what range of wood moisture contents are possible but in
uk we see a range from 35% to 60% on a wet basis. Of course there is
no reason for heartwood to retain any moisture as it is no longer
actively involved in growing, it will be the leaves phloem and
meristematic material they will require water to facilitate exchange
of chemicals.

AJH