[Stoves] Chinese stove photo sequence

[neiltm at uwclub.net]

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