[Stoves] Wattle cuttings

[Andrew Heggie]

Ronal you seem to be responding to original post from elsewhere? I
don't see a copy on stoves.

David also has an article on the stoves website

http://stoves.bioenergylists.org/content/elsa-stove

I've looked at some of the videos and there seems to be different
iterations of the stove dating from 2014.

Just two comments at this stage:

On 7 May 2017 at 02:28, Ronal W. Larson <rongretlarson at comcast.net> wrote:

> It's weight is about 1kg and at about 15 MJ for air dried wood should
> produce about 4kw of heating power. (Of which we use 70 % in the gasifier).
>
> [RWL2:  I see numbers above 15 MJ/kg for wood (and you use 18 MJ/kg for
> pellets).

I believe softwood pellets can exceed 20MJ/kg because of their higher
lignin content.


>
> [RWL5:   Dave is here saying he is recommending a larger diameter for the
> less dense wood stick fuel.

This makes logical sense in that the power varies with the amount of
offgas being produced, as pellets are denser for a given amount of
primary air the same offgas will be produced from the smaller area
with the denser fuel. One of the reasons a pellet stove burns well is
that the combustion takes place in a more compact volume.

Crispin made a comment in the "List of woods for TLUDs?" thread
regarding denser fuels needing to be more finely  comminuted in order
that more surface area is initially presented to the descending
pyrolysis front.

Actually this bears a bit more thought in itself. The less dense fuels
are so because there is more air between the fibres and air is an
insulator. So that the surface of a  "light" wood like willow or
poplar will reach pyrolysis temperature faster than a dense wood like
beech, which conducts heat into it's interior and this heat loss means
the surface stays cooler for longer. Way back Tom Reed introduced us
to the concept of Biot number  which is a measure of the ratio between
heat received at the surface to the heat conducted to the interior of
a particle.

So for a given air flow and with woods of the same moisture content
and size does the pyrolysis front migrate migrate downward faster?
Because more heat should initially be available to radiate and conduct
downward.

Does the net effect mean the pyrolysis zone is deeper with one or
other densities? At end of burn the result is much the same as all
particles are charred.

Andrew