Dear Crispin,
I think the nozzle of an oxygen-acetylene cutting torch is identical to
a propane-oxygen ditto. The ring of small premixed flames serves to
preheat the workpiece (steel) to a temperature where the central jet of
oxygen can begin to burn, heat, melt and disperse the molten steel.
Best regards,
Peter Verhaart
On 16/01/2012 04:50, Crispin Pemberton-Pigott wrote:
Dear Alex
Right on the money re retention time, methinks. The theoretical temperature is
often far above the actual achievement is because of the long flames /
residence time. If the rapid mixing at the flame site shortens the flame, stage
1 of high temp is achieved. The next is to put in no more air than necessary.
And to be as hot as possible.
Looking at the narrow aspect of high flame temp only, and given Andrew's
excellent question about a flame-within-flame perhaps if the cooler flame was
used to heat an O2-free carbon containing gas from the char gasifier, you get
the most heat in the smallest space. On that basis it is not necessary to run
the flames together, it is just one option.
Suppose the air for the 'hot flame' was preheated to 1000, then calculate what
the heat yield is. Perhaps the temperature of the reaction is limited and the
preheating does not add anything to the upper temperature limit. But it does
allow the limit to be reached, perhaps.
Perhaps the limit would be reached more reliably. For a greater total mass of
gases. For a longer physical distance for better heat transfer.
So I am offering an alternative to the flame-in-flame: sequential burning with
heat retention.
On the face of it the double flame sounds more likely to work. If it was
constructed like an oxy-acetylene torch flame (cutting torch) there is a
multi-jet flame ring around a central port with the high oxygen flame. The
limiting problem I see is that if the gas-air is premixed, it _has_ to be cool
enough not to light spontaneously. That inclines me to think the preheating
approach could be incorporated with beneficial results. Say, very hot air
entering the centre right at the point of hot-gas emergence (the CO) from its
own jet.
The mixing of the interior flame is going to mess up the smooth flow of the
cooler 'supporting flame' around it.
Maybe very neat multiple air jets could prevent it.
Have you seen a propane + oxygen cutting torch nozzle? It is not like the
acetylene one. It has a greater number of smaller air jets. The purpose is to
shorten the flame and increase the actual available maximum temperature (as
opposed to the theoretical maximum).
Lots of food for thought.
Regards
Crispin
-----Original Message-----
From: Alex English<[email protected]>
Sender: [email protected]
Date: Sun, 15 Jan 2012 13:09:00
To: Discussion of biomass cooking stoves<[email protected]>
Reply-To: Discussion of biomass cooking stoves
<[email protected]>
Subject: Re: [Stoves] Inverted top lit updraught
Andrew,
Can we melt steel with torrified wood?
I'm trying to follow your thinking on the propane flame, inner/outer
business. Are your suggesting that the pyrolysis gasses be kept
separated from the CO from charcoal gasification in one device like the
Dasifier and then burn the latter inside the former.
The other Das trick to elevating the useful temperature is the intense
mixing he generates with his compressor induced flows, thereby
maximizing the heat flux in a small volume.
Is the necessary retention time for complete combustion inversely
proportional to the turbulent mixing rate of the fuel gas and air?
Alex
An 1 Further musings: [1] it should be just about possible to melt steel
with a wood flame if the wood is perfectly dry, in practice it doesn't
happen and iron was not produced until charcoal was used, the reason is
simple; whilst the charcoal only has<50% of the heat of the raw wood it
was produced from, it is dry nearly pure carbon with about 30MJ per kg
available to be released on full oxidation. C+O2+4N2=>4N2+ CO2+30MJ heat
per kg of C is a low massflow reaction and heats of over 2000C are
achievable. Consider a propane torch flame, when premixed with the
correct amount of air the flame is two blue flames, a bright blue inner
flame surrounded by a paler blue enveloping flame, the hottest part is
at the tip of the inner cone where nearly all of the premixed oxygen is
consumed burning hydrogen and some carbon to H2O and CO/CO2, remaining
CO is then burned in the outer enveloping flame by oxygen that has
survive the first flame and oxygen that diffuses in from the surrounding
air. Can we consider doing something similar with the dasifier, using
the high mass flow ~1600C pyrolysis offgas flame to form a hot envelope
around the lower heat capacity but higher temperature CO+N2 flame?
Without destroying the burner pipe from the heat? AJH
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