[Stoves] Comments about T-LUDs

[AJH]

On Sat, 07 Oct 2006 22:48:08 -0500, Paul S. Anderson wrote:

>This is outside of my field, but of considerable interest.  For 
>example, is some
>specific biomass exceptionally high in it yield of char?  Or, as Jeff asks,
>could his pond scum be exceptionally low in its yield of char?  

The simple answer is I don't know but I've been told that the lignin
fraction contributes the major part of the char when charcoaling wood,
this lignin is the glue that stiffens up wood compared with annual
plants so it suggests that plants without lignin would yield less
char. I've certainly found char from some plants have little ability
to support combustion, banana skins was one.

What strikes a discord is that lignin is one step away from
carbohydrate toward a hydrocarbon. Hydrocarbon plastics do not seem to
have any char residue when volatilised.


>We know from
>Alexis Belonio and observation of T-LUD gasification of rice husks that the
>small, thin husks yield only even smaller and thinner flecks of char.  And
>algae strands are certainly smaller and thinner than husks.

We also know that speed of reaction is important, the faster flash
pyrolysis methods yield far less char and higher gases and condensable
liquids, small size and large surface area will increase the speed and
so this might favour lower char yield?

Also temperature of reaction is very important. Charcoal is still a
mixture of the carbon matrix and associated tars that have not been
volatilised. We know that as temperatures approach 1000C that char
yield from wood drops to 15% with nearly 100% of this fixed carbon yet
at 400C you can retain 45% of the dry weight of the wood as char but
the fixed carbon is ~70%, this suggests there is still 30% of the
weight in oxygenated tarry compounds in the char. I think that there
must be two things competing here one the basically exothermic
splitting off of carbon from the carbohydrate and two an endothermic
production of volatiles from the remainder.

SO if I am near right we know that thermal treatment of wood
(pyrolysis) always drives off volatile oxygenated organic compounds
which contain a increasing proportion of the available carbon as the
temperature goes up, these compounds contain a lot of the wood's
energy. At the same time we have seen on the [gasification] list a
claim for another treatment that enables the lysis of the biomass to
yield just carbon and water without any external thermal input other
than that necessary to raise the temperature to 180C in a sealed
container for a few hours. Now from what I replied to Frank it seems
my model of Cx(H2O)y for wood is not valid, as the process must derive
energy from somewhere. In fact we know the lignin in wood is a
polyphenol compound which seems to be one step nearer a hydrocarbon by
the plant having eliminated an oxygen atom (presumably at some energy
cost to plant) so there is an imbalance from the simple model. Not
being a chemist I'm guessing that the oxydation of this small
imbalance of hydrogen plus the breaking of carbon bonds must be
driving the reaction.  Un fortunately no figures on heat losses from
the process were given.

Given time I think I could model the energies but a chemist may have
them to hand already??

AJH