[Stoves] flames touching pot

[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 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