[Stoves] Chinese stove photo sequence

Sun Jan 29 04:57:43 CST 2017

Greetings from Jakarta

This is the land of Wet Wood.

If you want to burn wet wood (assuming you can get it started with dry‎ wood) you have to preheat the primary air.

Because the primary air is normally fed into the bottom of the fuel, it will tend to dry the bottom first, loading the middle with even moisture. There two solutions to this.

First, use a central tube with multiple holes as per Dr Nurhuda's (wet) candle nut shell-burning gasifier‎.

Second, run it on its side as per the Kyrgyzstan Model 4 so that there is a portion of the fuel being dried all the time, not all of the fuel being dried some of the time.

This can also be done using the chain grate that Alex English has at work. Preheating the primary and and speeding up the chain grate turns out the type of char you want. High or low volatiles at the touch of a button.

Regards
Crispin with Iwan and Arya ‎heading to the new Jakarta Stove Lab - about to make its appearance

On 28 Jan 2017 at 10:14, Andrew
Heggie wrote:

> 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.
>

Thank you so much for this post
Andrew, it helps make a lot of sense of a
lot of things for me.  I appreciate this is
all probably old hat for most of you.  If
these findings about moisture contents
(and more detailed combustion
principles of TLUDs) have been written
up anywhere it would greatly help me
play catch up, but just this much
information helps enormously and your
explanation as to what is going on in my
specific experiments makes very good
sense, and helps me understand the
processes better.

> 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.
>

LOL, I seem to have confirmed that with
my 33%mc wood!  This all began when
my presumably approaching 25%mc
outdoor uncovered wood was
undoubtedly getting wetter as winter
progressed when it reached a point of
failing as I had been using it.  Guessing
mc is obviously a longer term acquired
skill if aiming to load a stove for a
predictable burn.  At present for me it is
easier to mix dry and wet
proportionately than to guess the mc of
a uniform batch for achieving similar
results, and obtaining and keeping
different mcs is also problematic.  The
real skill is to go camping, load a stove
with found wood in very varied
conditions and be able to control its
heat output, as opposed to successfully
coping with not quite the burn you
want.  Only long experience seems
likely to improve such outcomes, but
already this has become a great way to
cook.

> 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.
>

Yes, and this is also what civilises and
controls the fire so well of course when
well harnessed.  Even my daughter
posted me pictures of her 'upside down
fire' in her home living room fireplace
after I sent her a link, and she was very
pleased with it.

> 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 .
>

Yes, this makes sense to me.  Ironically,
in practice, because I have so greatly
extended the burn time, the whole
process is still going while I eat
breakfast, and then return to a very
robust combustion as opposed to just a
pile of ash, and so I actually get more
not less recoverable char.  So what you
say is counterintuitive to me at first, but
does make sense and I accept your
reasoning for it.  If I simply quenched
the stove at end of pyrolysis I'm sure my
char residues would be as you predict,
and I am consciously using the char to
extend the burn, both with the bottom
wet layer and if I refuel at the end as
well.  The concept of the char being
protected from being consumed by the
wood gas is good to have made explicit
what is going on at that stage, and how
that then changes once that 'protection'
ceases.

> 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.
>

Brilliant, that is such a great
visualisation, thank you.  It supports my
own observations, including the spiky
little very red flames that poke up above
the char like plant shoots breaking
through the soil in spring, and so
obviously emanate from the wet wood.
You describe a different mode of
combustion and that is exactly what it
looks like.

> 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.
>

Yes, and hence the most dangerous
stage for the integrity of the burn
chamber.  Makes great toast though :-)

Thanks again Andrew - much
appreciated.

Neil Taylor

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