[Stoves] Chinese stove photo sequence

[Andrew Heggie]

On 27 January 2017 at 18:57,  <neiltm at uwclub.net> wrote:

> I would be interested to hear if anyone
> has successfully burned wood with such
> a high moisture content in a TLUD, or
> what anyone has designed a TLUD to
> tolerate?

Neil with all the strategic and political posts on [stoves] at the
moment your experiments, which the stoves list was  in the past good
at, may get swamped.

IIRC Ronal and Tom originally said that with "Denver dry" ( probably
10% moisture content on a wet basis) the burn was clean and
sustainable and left a char residue of 25% of the initial dry mass of
wood. As moisture content increased the initial change was that the
yield of residual char decreased. At the time it was suggested that
the downward pyrolysis front  would not establish with wood above
about 25% mc wwb.

I surmise the reason this might be is that the energy required for
pyrolysing a chunk of wood is considerably less than that needed to
evapourate it's water content. Because much of the energy from the
pyrolysis front is carried away upward by the convection of offgas
only the weaker radiative and conduction effects are available to
start pyrolysing the layer below.

If the wood has higher than ideal moisture content then the available
heat must evapourate moisture at 100C and then raise the part of the
wood to ~270C before  the nascent char can itself be formed. The first
thing the primary air then encounters is this fresh char which then
provides heat for the ongoing downward migration of the front but the
front has slowed down because of the time taken to evapourate water so
more char is consumed before it can be shielded  by the new offgas .

In your case  the 33% mc wwb, part seasoned or re wetted,hazel
modifies the front because the fuel no longer has a homogeneous
moisture content. Not having seen other than your photos I suggest on
encountering the wetter piece of wood the downward migration  at the
lump of wood ceases and this piece of wood may be burning in updraught
mode whilst the pyrolysis front continues down around it in the dryer
wood.

We know if left to its own devices as the pyrolysis front reaches the
bottom all the air available starts burning the char at a much higher
temperature than was present during the pyrolysis.

Andrew