[Stoves] Oversized stoves / thermal insulation

[Crispin Pemberton-Pigott]

Dear Andrew

>For a given mass flow and similar products of combustion the flame length
is directly related to 
>combustion time, the shorter the flame the shorter the time for completion
of any reactions. 

Agreed. I have not been able to get a high volatiles lignite with what I
think it a high CxHy level in the form of oils (bitumen?) to mix well with
air hot enough to get a short flame. It might be possible with a fan or
maybe a taller chimney but it is not looking very good at the moment. I have
been restricting myself to 3 metre chimneys because people will fit that
onto whatever is made, but 4 or 5 is a possibility.

What I observe is that the time (temperature and turbulence having been
taken care of) is dominated by the need to break down the C+ radicals that
are probably forming in the flame as the Hydrogen breaks away.

>My supposition is that the bonding in the hydrocarbon vapours is such that
it favours splitting a hydrogen 
>atom away and oxidising this leaving a higher carbon compound behind and it
is this that glows in the 
>diffuse flame until it is oxidised within the flame,

Exactly.

I tried funnelling the flame into a 100mm diameter pipe. Works fine. CO(EF)
bottomed below 20. [CO ppm multiplied by (excess air+1). It is a form of
expressing the combustion efficiency in a manner that uses only two direct
measurements: CO and O2.]

Then I tried 80mm. Good as well. Then 65. Great. Again CO(EF) below 20. 

So I went to 32mm. No luck at all - pretty much ruled it out in December
last year. It simple cools the gas stream so much that it won't let the
chimney work properly. It is a starting issue but translates into running
issue because it can't start properly.  Then came 40mm. Very difficult, not
clean - too much cooling by the walls and channelling the flame into a
smaller hole means extending the physical length of the burn because the
velocity has increased. No go. 

Back to 65mm - very good. Cheap and nasty construction and still had a
CO(EF) below 200 with ease. CO/CO2 reached 0.07% and sat there. That was in
March '10. The length of the burn at say 9 kW was about 500mm or so. As that
is not containable in a standard stove, it pretty much demands a new layout.
In SA we got a shorter version working very well with a 50mm diameter but
for 5.5 kW (meaning about 1/2 the gas volume). More about that in a few
weeks.

>The flame is radiating heat away from itself constantly so it is losing
temperature and high 
>temperature is one thing that leads to good combustion. 

The pipe (made of ceramic or metal) keeps the heat in place. That is the
main gain from the layout. The short open combustion space need to be
treated more like the end-lit cross-draft we are experimenting with in UB at
the moment. The GTZ-7.2 is a version of that with a hopper/bunker on the
back. There we got a CO(EF) below 10 for an hour at a time; long periods
below 1000 (which is already a pretty clean burn).

>If the combustion chamber is better insulated and can re radiate heat back
to the flame then I 
>would expect  the flame length to increase as these PICs contribute their
heat and continue burning.

Mmmm....well if the heat is kept in the flame is shortened - all the
required air is present in the mix for the whole time. It just seems the
practical issues result in a need not to cool the thing too much and to not
to slow it down (by making it big) or the mixing breaks down (fails) and CO
is left over at the end.

>I'm guessing there's a bit of a corollary with the lamp test that Tom Reed
told us about for 
>establishing the detonation point of a hydrocarbon by the maximum flame
length 

What does he mean by the detonation point?

Thanks
Crispin