[Stoves] Is there a role for combining torrefaction and char-making stoves?

[ajheggie at gmail.com]

On Thu, 01 Mar 2012 10:54:25 +0100, Ronald Hongsermeier wrote:

>isn't that a part of the energy equation that takes more energy to make 
>it react than it gives off when it reacts? 

Yes it means that stage of the reaction needs heat energy to be put
in. It says nothing about the chemicals that are being driven off.

So drying wood is exothermic, you need to supply sufficient heat to
turn the liquid water in the cells into a vapour that is carried away.
Given that you need a difference in temperature for heat to flow from
one substance to another you need a higher temperature than that
needed to boil water to drive this reaction. So typically we oven dry
wood at 105C  for a number of hours. This removes all the free cell
water by soon after 100C but leaves some 20% water that is weakly
bound to the cell walls. This weak bond needs a bit more energy than
just the latent heat of evaporation of water to drive the last 20%
away. Even then I suspect not all the water will be evolved.

Once you get above 150C Volatile Aromatic Compounds are emitted, some
of these are known carcinogens but they characterise the smell of hot
wood.

All these preceding reactions are endothermic but the endothermy is
all to do with the heating up and changing state of water and VOCs to
vapours, its all increased enthalpy not a change in chemistry.

It's  wrong to be too prescriptive here as one stage has a variable
transition to the next, because wood is a mixture of chemicals.

The next stage is what interests torrefiers and It probably occurs
between 170 and 280C and I don't have a clue what is happening or what
is being driven off but the consensus seems to be that hemicellulose
and possibly cellulose start to depolymerise and where the breaks
occur OH groups are lost. This makes the remaining strands of
aliphatic (chainlike) sugar multiples no longer attracting to water,
when cooled this is what I think of as torrefied ( never having
apparatus to hold wood steadily at these temperatures. The endothermy
here is that energy necessary to drive off the OH groups.

After this the hemicellulose, cellulose and lignin start to break down
into the tars, gases ( like CO CO2 CH3 H2) and vapours which we expect
from full blown pyrolysis.

I wish I had paid more attention in the school lab 45 years ago.
Fractionating columns, glass retorts and logwood chips were all there
to play with...

AJH