[Gasification] heat required for drying

[AJH]

On Sat, 16 Dec 2006 05:05:34 -0600, Paul Francesco wrote:

>Granted extra attention may have to be paid to later removal of 
>moisture.

I think this is the problem, even though the water would be entering
the gasification reaction as steam, heat would need to be added to it
to raise it to the temperature of the gasifier's oxidation zone,
somewhere greater than 1100C, this heat would be robbed from the
gasifier. Even if the steam did not enter any reactions (which it
would highly endothermically) it would still dilute the offgas
sufficiently to reduce its calorific value.

So the perceived wisdom is that the wood should enter the gasifier as
dry as possible to reduce the amount of gasification air needed and
hence cut massflow, this leads to the highest cv offgas and the least
amount of heat lost as sensible heat in the offgas.

As has been pointed out getting the last amounts of water out of the
wood involves higher temperatures as this water is weakly bound to the
cell material, so you need to look at equilibrium moisture contents to
see what temperature and rh the latter part of the drying cycle needs
to work at. Also you need to be aware that dry wood will pick up
moisture from the atmosphere as the rh increases, so there are limits
to dryness that is worth drying to if the wood is to be stored prior
to use.

From looking at equilibrium moisture contents you will also see that
there is a trade off between the amount of heat you add to air to
reduce its rh, and hence increase its drying ability, and the amount
of air that needs to be pumped through the heap of wood, at a cost in
fan energy.

So if we decide to use low grade heat, from a condenser or a cooling
jacket then we have to pump a lot of air, compromise the heat exchange
or build bigger heat exchangers.



AJH