[Stoves] venturi system -ratios of air and gas?

[Paul Medwell]

This discussion seems to have gotten off-topic somewhat, nevertheless...

I thoroughly agree that nonpremixed flames emit harmful pollutants. The levels of which are non-negligible and indeed can be downright worrying. However, the previous inference that "nonpremixed flames are terrible and should never be used" may give some readers the false impression that nonpremixed flames should be avoided at all cost. I don't believe this is true, and given the enormous range of practical nonpremixed combustors currently in-use (such as engines and furnaces) that successfully meet emissions regulations, I stand by this claim. Certainly premixed flames offer some advantages in the reduction of some emissions (such as soot), but at the cost of increased design complexity, increased operational complexity, increased safety risks, and can increase some other emissions.

I also thoroughly agree that the use of a spoon is not a good or reliable approach to measure soot volume fraction -- it was simply intended to be an indicator of the relative levels of soot within a flame and what is emitted from a flame. Soot within a flame is highly desirable, and a great amount of research effort goes into making as much soot as possible within a flame for many applications (such as furnaces). Of course, soot formation in other applications should be ameliorated (such as engines). In almost all cases the goal is to completely oxidise the soot prior to its release (although sometimes it is a desired product). Just because a flame is yellow does not imply that it emits a lot of soot -- it is certainly likely to emit some (potentially a lot), but don't forget that most of it can get oxidised within the flame (but only if properly designed).

Since this is digressing from the objectives of this mailing list I shan't continue this discussion. Nevertheless, I welcome continued debate on this, and other topics. Coincidentally, if anyone has a strong interest in soot outside of cookstoves, I draw your attention to the International Sooting Flames workshop series:
http://www.adelaide.edu.au/cet/isfworkshop



________________________________
From: Stoves [stoves-bounces at lists.bioenergylists.org] on behalf of Philip Lloyd [plloyd at mweb.co.za]
Sent: Sunday, 10 January 2016 3:59 PM
To: 'Discussion of biomass cooking stoves'
Subject: Re: [Stoves] venturi system -ratios of air and gas?

Dear Paul

It is entirely fair to call an unmixed flame dirty.  It emits fine black carbon, much of it in the PM2,5 range and therefore breathable. There is still some hydrogen in the soot, so you essentially have a polyaromatic hydrocarbon (PAH).  Those are the chemicals we now know gave the little chimney sweeps scrotal cancer – the first identified link between cancer and industrial malpractice.  And rather than using a spoon to detect the carbon, use a white plate at an angle above the candle flame. The spoon heats rapidly, and you lose the thermal diffusion capture mechanism; the plate stays cooler longer and thermal diffusion plays an ongoing role.

Thermal diffusion is also called thermophoresis – there is a slightly useful Wikipedia article https://en.wikipedia.org/wiki/Thermophoresis. If you establish a thermal gradient in a fluid, the heavier species in the fluid will acquire more kinetic energy than the lighter ones and will move down the gradient towards the cooler surface.  The effect was used early in the Manhattan project to enrich uranium isotopes.

Prof Philip Lloyd
Energy Institute
Cape Peninsula University of Technology
PO Box 1906
Bellville 7535
Tel: 021 959 4323
Fax: 086 778 0257
Cell: 083 441 5247
PA: Nadia 021 959 4330

From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Paul Medwell
Sent: 10 January 2016 02:53
To: Discussion of biomass cooking stoves
Subject: Re: [Stoves] venturi system -ratios of air and gas?

Sorry to protract this discussion, but...

It is not entirely fair to call a nonpremixed flame "dirty". A nonpremixed Bunsen burner flame is typically yellow, caused by the presence of in-situ soot. However, very little of that soot is released from the flame. There are enormous differences between soot in a flame and soot emitted from a flame.

The candle is a classic example. Its primary purpose is to generate soot -- without soot in a candle flame it would emit very little light. Comparatively little of the soot that is generated is actually emitted from the flame. Next time you are at a fancy dinner, place you spoon above the flame and see how much soot is released: there will be some, but surprisingly little. Put your spoon in the flame for the same amount of time and you'll soon see how much soot is within the flame.
Soot within flames is often highly desirable: it is one of the primary mechanisms for heat release. A typical furnace runs nonpremixed flames for the sole purpose of generating as much in-situ soot as possible to radiatively heat the furnace.


On 9/1/16 5:18 PM, Philip Lloyd wrote:
There is quite a good pic of flames at various mixed levels in https://en.wikipedia.org/wiki/Bunsen_burner. When there is no premixing, the flame is yellow and dirty (not all carbon is combusted, as in a candle flame). When you get to a stoichiometric flame, all the combustion takes place at the blue cone – the almost invisible “flame” above it is mainly hot gas radiating; minimal combustion takes place there.

Prof Philip Lloyd
Energy Institute
Cape Peninsula University of Technology
PO Box 1906
Bellville 7535
Tel: 021 959 4323
Fax: 086 778 0257
Cell: 083 441 5247
PA: Nadia 021 959 4330

This email has been protected by YAC (Yet Another Cleaner) www.yac.mx<http://www.yac.mx?source=email>
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