[Stoves] flames touching pot

[ajheggie at gmail.com]

On Wednesday 14 September 2011 01:01:23 Paul Olivier wrote:

>
> *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:

Yes I've seen these before and replied to you in a biochar exchange with 
Ronal. These are probably still a diffusion flame, just that as they are 
small the volume to surface area is still small enough for sufficient 
oxygen to diffuse into the offgas and burn completely without the glowing 
carbon stage.

There's nothing inherently wrong with a diffuse flame, as Alex points out 
a premixed blue flame can still have poor CO2:CO ratio.

>
> 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.

Yes these flames look long, are not premixed and if you increase fuel you 
say smoke is produced, which again make me wonder about the difference in 
composition, yet you say these are coffee husks? I'm afraid I still 
haven't seen a rice husk or a coffee husk, I should get out and about 
more! A pre mixed flame should not produce smoke as it is turned up as 
the air: fuel ratio should be constant. As you say the air only enters 
from the side and offgas only rises through the holes then it is not 
premixed.
> But I can turn the flame height down to less than 10 mm, without
> extinguishing the flame.

Which is good, the higher the turn down ratio the better for cooking.

> *This is not really a premix flame.*

Yes I see that now
> *But secondary air* is so dispersed along the periphery of the burner
> *that it looks like a premix flame.

As I said it is a diffuse flame with a surface area : volume ratio that 
allows complete combustion in one hit, like the blue at the bottom of a 
candle flame.
> I did not get a good premix look until I added the burner housing and
> the horizontal flange.*

I suggest this just added a bit of velocity to the air and this in turn 
caused enough turbulence to force air into the flame.
>
>
> *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?

I don't for sure but dull red is around 600-700C for a black body, I'd 
guess stainless would be around 0.9 emissivity , so a bit hotter, of 
course the outside is losing heat by radiation, conduction and convection 
so the other side of the steel can be hotter.
> 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*.
> *

Not really because there's a fuzzy line between the two. A retort is a 
pyrolyser and a kiln is one too but the kiln allows a bit of air in to 
create heat. Similarly there are few gasifiers that produce no char in 
their ash because to burn completely to ash would mean letting in too 
much oxygen to have a good cold gas efficiency of conversion.

> 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.

I cannot speculate any further on this, we need a gas analysis and some 
experts to enlighten us.
> *
> *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.

Yes, I was aware of the aspect ratio and the high silica content but not 
the rest. You have good results with coffee husks too and I have no idea 
why these results look better than wood.

AJH