[Stoves] Stove dynamics with the exotherm during pyrolysis

[ajheggie at gmail.com]

I find it a bit awkward knowing where to comment on this conversation
from another group but here goes:

I watched the video which was interesting with some confusingly laid
out graphs. It was fairly heavily into a sales pitch for making an
engineered  char via a method that could be patented and sold in a
proprietary way, as such it was steering potential buyers in the
required direction. To my mind often we find people revert to "dog and
stick" methods when the high tech approach gets too expensive, after
all a cheaply made char  may not do the job as well but you can make
lots of it.

I tend to glaze over with anything that relies on pellets as being
inappropriate for stoves discussions.


One thing did strike me was that the general requirement to make char
at 600C did seem to favour TLUD because I believe this is the sort of
temperature we see in a burn with decent control of primary air.

Some insertions below

On Sun, 9 Aug 2020 at 07:51, Anderson, Paul <psanders at ilstu.edu> wrote:

>
>
> IMBO   (In My Biased Opinion).   I would says “Yes, forget retorts.”   Now in need to explain my thinking.

Too sweeping a statement IMO but probably agree in this context.
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>
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> 1.  A retort does not provide a uniform supply of heat which cooks desire.   Retorts do not give combustible gases from the very early minutes.   And, as shown in Hugh’s research, when the heat does come, it will be much in a relatively short time (again, not good for cooking).

Agreed I don't see retorts being of much use for cooking because of
the reasons you cite for lack of control, in fact all biomass stoves
are lacking compared with gas or electicity when it comes to turn down
ratio.
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>
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> 2.  From  the little that I saw of Kevin’s simple (very low cost) TLUD, there was minimal or no attempt to restrict the entry of the primary air.  The result has two disadvantages:
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>
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> A.  There was a faster than desired release of the gases (and the resultant cooking flame).
>
> B.  The created char was subjected to too much O2 (in the surplus air) and subsequently burned (char-gasified) to the point that you are lamenting the low yield of char.

Also I suspect the temperature gets a bit higher and moves the char to
a more heavily graphitised product, I believe that if you heat any
char or hydrocarbon to 3000C it loses any semblance of being amorphous
and becomes graphite, layers of graphene with no interstitial spaces.
My brother used to talk of making lightweight aero engine pistons by
heating preformed pitch by electrical resistance to 3000C.
>
>
>
> 3.  Based on 2 above, the there is some solution to both problems when there is actual control of the entry of primary air.   Such control come with an extra cost.   IMO, that cost (for a mostly sealed container and control of air supply) is what makes the TLUDs stoves successful.  Success with  TLUD technology is closely tied with air control.

Yes
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> 4.  My experience with dry maize cobs as fuel is quite successful.   Cobs are less dense than woody stuff,  so the TLUD fuel chamber needs to be larger.   That, unfortunately, also adds to the cost.

I've never tried it but I believe is what is called corn stover in
America ( apparently nothing to do with stoves but from estover, the
ancient right in britain to gather fuel.
>
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> 6.  Tying this back to Hugh’s work with exothermic boost of heat, that boost does take place in TLUD stoves, but it is occurring at a consistent rate in each millimeter of the descending migratory pyrolytic front. (MPF).
>


Very much so, the control of the downward migrating pyrolysis front
means there is only a thin layer of new material being heated,
pyrolysed and then residing as char in the oxygen free zone at any one
time that prevents a thermal runaway.

I see thermal runaway in situations other than retorts where the mass
of wood gradually gets heated to  pyrolysis temperature and then the
exothermy cracks in above 330C and pyrolysis accelerates as a chain
reaction in the wood, with the only heat losses from the system being
the hot offgas.  In a retort the heat input is limited by heat
transfer through the retort wall but once the contents get hot enough
the evolution of offgas takes off. Yury Yudevich got around this in
his huge carbonisers by batch loading retorts in sequence to even the
heating out.

Something similar happens if you load bone dry wood into a wood stove,
often the evolution of offgas becomes too great for the secondary air
to oxidise it and black smoke ensues.

My guess is also that as a TLUD favours using wood of a small cross
section there is less opportunity for secondary reactions of pyrolysis
compounds within the wood being further reacted to soot so less carbon
is deposited in the  char matrix.


We expected a higher yield of char from our barbecue charcoal making
kiln  because it was pressurised and we used chunkier bits of wood,
this meant not only was the residence time of the pyrolytic compounds
increased within the wood but also the equilibrium favoured more
internal deposition, unfortunately we didn't run it long enough to get
usable figures.

Andrew