[Stoves] pre-mix of wood-smoke with air? and how?

[Paul S. Anderson]

Dear Martin,

Your message was long, yes.  But with some relevant points.

Quoting "Boll, Martin Dr." <boll.bn at t-online.de>:
>
> In fact I saw simple woodgas, (without air) streaming out of a hole of a can
> approx 2 or 3mm diameter, that I could not ignite, because of blowing out
> the igniting flame. Therefore I think there had to be used a "Prallplatte"
> (=baffle?) to have somewhere inbetween the nozzle and the baffle the right
> speed to form a stable flame or/and to re-ignite. (

I suspect that this was with a forced-air TLUD.  (Natural draft does not get
such movement, at least not for very long or unless there is some 
extra-special
chimney).

You are raising a good point.

I learned from Tom Reed (and others) about the need to constrict the 
flow of the
gases so that there was sufficient speed and therefore more turbulance.  But I
only did it with a natural draft TLUD, the Champion Stove that has a
"concentrator disk" with a 3-inch (7.5 cm) hole in the middle.  BUT this is at
or above the entry of the secondary air that comes in under that disk, so in
effect that stove has some "early mixing".  I believe the flame is actually
below the hole when the gas volume is low, but could be slightly above 
the hole
when the gas volume is high.  I never tried to put a short tube/cylinder
vertically on top of the hole.  Such a cylinder might confine the mix of air
and gases enough to get some of the mixing that you are discussing, that is,
before the actual location of the flame.  With hindsight now, a 2 or 3 
cm mixer
(or "flame-delay") might have a good impact.

> Isn't the premixing by butane-burners the reason for clean burning?
(Snip/edit)
> ...propane/butane ... has to be (??) pre-mixed to burn
> without soot by burning in a low pressure-jet/nozzles?

I have no experience nor studies about this, but I became aware of it a 
week ago
when I cannibalized a discarded natural-gas stove for pot supports, etc.  I
noticed the pre-mixer and I am thinking about it concerning my Lily alcohol
burner.

> As far as I have seen Alexis Belonios pictures, the burning-gasses are moved
> with much more pressure. I think/guess/feel the pressure made by a fan
> produces a comparable pressure that makes the jets of a propane/butane
> camping-stove-burner. But take this with the famous corn of salt: If the
> flame-speed is far less, the jet has to be slower. Possibly is the lower
> caloric-worth of woodgas helpful to burn the jet with lower pressure/mix
> completely.

I believe that Belonio does NOT have much pressure, certainly not anything
similar to the pressure of propane/butane tanks.  Those gases are under much
higher pressure (I think) and the gases are released via very small openings
and can cause entrainment of the air to enter and be mixed.  That is not the
case with the pyro-gases.

However, you have observed "woodgas, (without air) streaming out of a 
hole of a
can > approx 2 or 3mm diameter, that I could not ignite, because of 
blowing out
> the igniting flame.
THAT is good.  But I feel sure that forced primary air is needed.  That stream
of gases should be your starting point.

First, simply place a metal mesh (perhaps 1 mm openings) over the streaming
gases and then try to ignite.  The mesh causes turbulance in the flow AND can
be a "flame holder" if hot enough.

Second.  How much can you entrain (confine) to the mixing tube WITH some
secondary air, and still have NO flame in the tube?  But you also want to be
able to ignite (with control) that mix when it leaves the mixing tube.  
IF that
gives you a better (cleaner, more controllable) secondary combustion, then you
have a very important result.  We can solve the easier problems later about
keeping that mixing tube hot enough to prevent condensation.

Sounds interesting.  You have caused me to re-think my earlier comments.

Said slightly differently, those of us working on the gasifier stoves are now
able to make the gases reasonably well.  Quite possibly the future progress
could be more focused on how to combust those gases better than we do at
present.

Paul


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