[Stoves] Inverted top lit updraught

Peter Verhaart pietverhaart at bigpond.com
Wed Jan 18 00:58:45 CST 2012


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<english at kingston.net>
> Sender: stoves-bounces at lists.bioenergylists.org
> Date: Sun, 15 Jan 2012 13:09:00
> To: Discussion of biomass cooking stoves<stoves at lists.bioenergylists.org>
> Reply-To: Discussion of biomass cooking stoves
> 	<stoves at lists.bioenergylists.org>
> 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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