[Stoves] Inverted top lit updraught

Sun Jan 15 12:50:01 CST 2012

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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