[Stoves] flames touching pot

Tue Sep 13 19:01:23 CDT 2011

Andrew,

I really appreciate the response you have below.
I note in particular what you write:

*The difficulty with achieving the same with wood gas is that the woodgas
is inherently hot as it meets the air. It would be interesting to cool
the gas from a tlud ( which would mean losing the fuel value of all the
tars present as they would condense) and then using a premixed burner on
the remaining cooled gases.

*I inherited so much from Alexis Belonio and his work in rice hull
gasification.
I like the idea of the two rings of holes in the Belonio burner design.
But Belonio did not direct secondary air onto these two rings of holes,
and this resulted in a long diffusion flame.
When I put a housing around the burner burner
with a horizontal flange to direct air onto the burner holes,
the long diffusion flame shortened into* little blue soldiers*, as you see
in:
http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0444.JPG
http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0445.JPG
When enough secondary air is allowed to enter,
the flame is totally blue right from the very beginning of the burn.
But it can easily happen that there is too much secondary air, as you see in
this photo:
http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0634.JPG
I will make an adjustable air disk at the bottom of the burner housing
to control the amount of secondary air that enters.

However when I switch from rice hulls to coffee husks,
what was an excess of secondary air on rice hulls becomes close to ideal.
This is the bluest flame I have ever obtained on coffee husks:
http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0646.JPG
http://dl.dropbox.com/u/22013094/150%20Burner/IMG_0652.JPG
If I turn the flame any higher than what you see in these two pictures,
smoke is produced.
But I can turn the flame height down to less than 10 mm, without
extinguishing the flame.
*
*You write:*
One obvious exception here is those gasifier stoves, as promoted by Alex
Belonio and REAP, that seem to be simple yet have a premixed blue flame.

*This is not really a premix flame.*
*But secondary air* is so dispersed along the periphery of the burner
*that it looks like a premix flame.
I did not get a good premix look until I added the burner housing and the
horizontal flange.*

*You write:*
A "proper" gasifier operates at 1100C and the producer gas exits at over
850C,

*I think that the Belonio gasifier is a true gasifier.
Alexis said that his unit operates in the range of 1000C.
I think that he is right, because when lighting is subdued,
the outside of the reactor glows red hot at that point where gasification is
taking place.
Do you know at what temperature stainless steel turns red?
This claim that I make of true gasification puts me in gentle conflict with
Ron Larson.
Ron says that I should not use the word *gasifier *but *pyrolizer*.
*
so all the tarry compounds are reacted out to the simple gases, CO
and H2 which then can be burned in a blue  premixed or diffuse flame but
simple stoves don't reach these temperatures, so there must be something
special about rice husks that allows the offgas to burn without glowing
sooty particles.
*
Note that at the beginning of the burn on rice hulls in a Belonio gasifier,
the flame is not fully blue.
It only turns fully blue about half-way down the reactor.
But if the secondary air is correctly supplied to the burner,
the gas burns blue right from the very beginning.
*
*You write*:
All that I can think is that the husk pyrolyses so
rapidly that there is little opportunity for secondary compounds to form
and the offgas consists of simple compounds and true gases. My other
thought is that the high silica content in the husks similarly acts to
produce simple pyrolysis compounds and gases.*

Air rises up through the rice hulls in the reactor in a uniform manner.
Rice hulls are uniform in size, shape and moisture content.
They are thin and have a large surface area.
In the rice hull, lignin and silica are bonded in a special way,
and this supposedly makes them difficult to combust.
Also, 2.1% to 6.0% of the *rice hull* consists of a waxy biopolyester called
cutin,
which might affect the speed at which the hull gasifies.

Thanks.
Paul Olivier

On Wed, Sep 14, 2011 at 4:37 AM, <ajheggie at gmail.com> wrote:

> On Sunday 11 September 2011 18:09:56 John Davies wrote:
>
> >
> > I was using one of these today, and observed how the flame spread out
> > across the bottom of the pot. The flames were touching the pot, but no
> > soot. The difference I see is that it is a blue flame, while our stoves
> > tend to produce a yellow flame.
> >
> >
> >
> > The question is why the yellow flame produces soot , but not the blue
> > one ?
>
> As has been said the yellow is carbon particles glowing in the heat of the
> flame, being black they have a high emissivity and the yellow colour is
> characteristic of the temperature, incidentally this radiation of a hot
> body is a reason that a standard thermocouple under reads a flame
> temperature because it is heating up in the flame but then radiating heat
> away so it never reaches flame temperature but only an equilibrium
> between gaining heat from the flame and re radiating heat from the tip.
>
> So how do these glowing sooty particles get into the flame and what
> happens to them? My take is that for a number of reasons there is a
> complex mixture of fuel chemicals in a wood flame that are the results
> both of combustion, pyrolysis and then secondary combining of primary
> pyrolysis species. Much of this recombining of pyrolysis products takes
> place within the wood particle. If the wood particles are very small and
> heated very quickly there is little opportunity for the initial pyrolysis
> species to recombine and hence we have less char and tar and more vapours
> and true gases arriving in the flame as fuel.
>
> Mostly our wood gases rise from the primary combustion or pyrolysis in an
> oxygen starved state so they meet the oxygen in the air at the flame
> boundary ( a flame is simply an area of combining gases) and this oxygen
> is initially dissociated on at the flame boundary. If you observe a
> candle flame you can see a dark area in the middle of the flame by the
> wick but below that a bluish base to the flame and the yellow area is
> atop and around both of these. In a wood gas flame this blue area is non
> existant or hard to see but what is happening is the dark area is an area
> of volatilised fuel and the blue area is a part of the flame where there
> is complete combustion. The yellow area is where just sufficient oxygen
> is diffusing into the flame from the outside to strip hydrogen away and
> oxidise it but insufficient to completely mix and burn out the fuel
> gases. So there is incomplete combustion in the flame and the sooty
> particles live in the flame until they meet sufficient oxygen near the
> flame surface to burn out completely. If the fuel production is too high
> (in the candle if the wick is too long) then the volume to surface area
> of the flame is never sufficient allow enough oxygen to the carbon
> bearing particles and they swiftly cool off and the tip of the flame
> tails off into sooty smoke. The same happens if a cool surface is poked
> into the flame, it quenches the reaction and prevents the soot burning.
> So better mixing and turbulence shorten the flame which needs to complete
> combustion before it touches the pot.
>
> OTOH Methane and propane are simple gases that mix completely with oxygen
> molecules when cold and have no interaction until they meet a flame, at
> which stage, as they are in the right proportion, they immediately burn
> out completely without any glowing products of incomplete combustion.
> Take a bunsen burner and block the air hole and you turn the flame from
> two short blue cones to a lazy yellow sooty flame.
>
> The difficulty with achieving the same with wood gas is that the woodgas
> is inherently hot as it meets the air. It would be interesting to cool
> the gas from a tlud ( which would mean losing the fuel value of all the
> tars present as they would condense) and then using a premixed burner on
> the remaining cooled gases.
>
> One obvious exception here is those gasifier stoves, as promoted by Alex
> Belonio and REAP, that seem to be simple yet have a premixed blue flame.
> A "proper" gasifier operates at 1100C and the producer gas exits at over
> 850C, so all the tarry compounds are reacted out to the simple gases, CO
> and H2 which then can be burned in a blue  premixed or diffuse flame but
> simple stoves don't reach these temperatures, so there must be something
> special about rice husks that allows the offgas to burn without glowing
> sooty particles. All that I can think is that the husk pyrolyses so
> rapidly that there is little opportunity for secondary compounds to form
> and the offgas consists of simple compounds and true gases. My other
> thought is that the high silica content in the husks similarly acts to
> produce simple pyrolysis compounds and gases.
>
> AJH
>
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-- 
Paul A. Olivier PhD
27C Pham Hong Thai Street
Dalat
Vietnam

Louisiana telephone: 1-337-447-4124 (rings Vietnam)
Mobile: 090-694-1573 (in Vietnam)
Skype address: Xpolivier
http://www.esrla.com/
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