[Stoves] Reducing smoke with steam

[andrew]

On Wednesday 02 January 2008 11:59, IPC wrote:

> At atmospheric pressure, pyrolysis of virtually all hydrocarbons
> is essentially complete at 450 deg C, so 700 deg C is way too
> high.

I wasn't referring to pyrolysis but to cracking, by heat and partial 
oxidation, producing sooty particulates. Even simple hydrocarbons 
like CH4 will produce soot if air starved. I used to pass a factory 
that made carbon black for tyres, carbon brushes etc, from natural 
gas and this was done by partial oxidation. The flare stack where 
they burned off the steam and CO after the soot had been removed was 
a beautiful blue flame.

Even so we know that some of the tars produced during wood pyrolysis 
are not volatilised till quite high temperatures.
>
> Yes, there is an endothermic reaction
> C + H2O ---> CO + H2 delH = 7.3kJ/g water
> but it must be offset against the exotherm
> CO + H2O ---> CO2 + H2 delH = -2.3kJ/g water
> (and similar for unburned hydrocarbons)
> So the endotherm would play only a small role, and you would get
> that energy back with interest when the CO and H2 combusted once
> there was enough air.

Yes we know that the effect is something within the flame and that 
overall as long as the flame is clean the energy balance isn't 
changed apart from the mass flow of the introduced steam.

Even with the equations above it shows that the steam does take 
energy from the flame but we can only know how it will affect the 
energy balance in the flame if we know the cv of the gases being 
flared and the total massflow.

Now we know that some fuels have a high propensity to form sooty 
particles, Tom Reed has, in the past, referred to a lamp flame 
length test that is used to classify these (liquid) fuels (and that 
this was a measure of the ability to resist detonation in an engine, 
which we all probably know is also an area where steam is added when 
an engine is working at its limit). This seemed to fit in with the 
idea that the diffuse flame simply did not have enough surface area 
to volume ratio to allow oxygen to diffuse and completely oxidise 
all the gaseous products in the flame, hence hydrogen was stripped 
and oxidised to steam whilst the remaining carbon clumped and 
carried on out of the flame, resisting oxidation. 

I haven't tried, or have the easy means to try, to add steam to a 
sooty flame, so I haven't even seen it demonstrated but Peter's idea 
that the steam had a physical effect by moderating the flame 
temperature didn't sit quite right as any inert gas should have a 
similar effect, which is why I suggested the water gas reaction 
could both moderate the temperature in the middle of the flame and 
subdue high temperature cracking by partial oxidation as well as 
producing a small amount of H2 and CO.
>
> At flame temperatures, the pyrolysis products react very rapidly
> with steam - faster than they do with oxygen - which is why steam
> injection is effective.
>
> Smoke is combustible, and therefore represents an energy loss.  If
> it takes a bit of steam to stop the smoke, you win air-pollution
> wise at minimal cost.

Yes, I'm not knocking it if it works, just wondering what is 
happening.

Being able to increase the length of a clean diffuse flame by adding 
steam may be interesting to those using oil lamps.

AJH