[Stoves] Yellow and blue flames explained and: Why flames should not touch the pot

Paul S. Anderson psanders at ilstu.edu
Wed Sep 14 09:09:52 CDT 2011


Dear stovers

What Andrew Heggie has written below is the best explanation of yellow  
and blue flames that I have read in my 10 years working on stoves!!!   
His content mainly is not new, but how it is stated so clearly is what  
I commend.  I will put it on the website that I am developing for TLUDs.

Paul
-- 
Paul S. Anderson, PhD
Known to some as:  Dr. TLUD    Doc    Professor
Phone (USA): 309-452-7072   SKYPE: paultlud   Email: psanders at ilstu.edu
www.gtz.de/de/dokumente/giz2011-en-micro-gasification.pdf   (Best ref.)


Quoting ajheggie at gmail.com:

> 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 pyrolyzes 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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