[Stoves] Rapid mixing

Ronal W. Larson rongretlarson at comcast.net
Thu Sep 29 20:04:04 CDT 2016 

Alex, cc list adding Paul Olivier (not usually on this list)

	1.  Nice new test.  I am unaware of anyone else who has demonstrated premixing (except see below on one).  Really good to see any work along these lines.

	2.  Some comments:
		a.  It seems the flame diameter never exceeded about 3 inches.  I think the flame doesn’t expand out to fill the full 6” max diameter because the pressure inside the flame is lower than outside.  Lower because the number of particles goes down with both CO and H2.  Ex:  2 H2 + O2 .>> 2 H2O  (3 molecules goes to 2);  same for the CO equation 2 CO + O2 >> 2 CO2.
		b.  It would be nice to see more in the vertical direction.  Perhaps doing this with a glass cone or a set of small observational holes might give some more insights.  I can conceive that your flame gets smaller in diameter as it rises (because all flames seem to do so).
		c.   There should be some data around on flame speed for TLUD-type gases.  This 3” flame diameter number could be a way of learning more on the gas constituency.  That is - why not a 2” or 4” flame when viewed from above?
		d.   There might be less than full mixing in your geometry;  putting in some barriers ( a few inches of mineral wool?) to vary the time (or geometry) for mixing distances might be interesting.  I remember reading once of variations in the length of Bunsen burners (which is different because   CH4  + 2 O2 >> CO2 + 2 H2O give equality of 3 on each side.  
	As an aside:  with the LPG gases, propane (C3H8) and butane (C4H10), you have more particles after combustion.  It looks like you could repeat this pyrolysis gas experiment with these three other gases - and we might learn something.  (Remembering there would be temperature differences that would make it difficult to draw conclusions.)
		e.  Can you say a bit on the secondary air inlet.  What total area?  Any chance there is, or could be, a swirl?  (to give more time for pre-mixing).  Similar would be seeing the effect of changing the length of the riser, before getting to the flame.
		f.  If you had a 3” (or more) riser pipe diameter instead of 2”, would the flame have appeared at the secondary air holes?  
		g.   Not sure what you would learn from this, but what if the 2” riser suddenly expanded to 4” (no cone).   My guess is that the short video would look about the same - about a 3” diameter flame as viewed from above.  Another way of saying this is that the upward flowing pyrolysis gases flare out, and happen to be the right velocity when they are about a 3” diameter.  (but you are avoiding eddies with your geometry.)
		h.  This last rambling thought makes me ask whether you can now (or soon) change both the primary and secondary air quantities?  I think there is a possibility of some interesting changes.  For instance, if you don’t have enough secondary, then the needed combustion air will have to flow downward in the cone.  If you have a lot more secondary than you need (or could change its temperature), then maybe the 3” flame diameter would presumably change a little.
		i.  Same for primary air variations.  As you point out there should be a change of gas composition that might tell us something new.  At the present time, you have a fuel bed of 12” diameter.  What is the fuel-bed height, and can you say more on the pellet (if pellet) size?  How long to go through a run?  If you are saving the char - how much?

	3. What led you to try this experiment?  (Were you expecting the very nice looking flame - and why?)

	4.  Pre-mixing could be advantageous for cooking (being such a nice looking flame) for health reasons (assuming you are dropping the count of 2.5 micron particulates).  Have you tried to see what sort of “plaque” might be left on a cold surface above the flame?
	Another advantage for cook stoves is that you could get multiple burners going.  (And it should be easy with smaller pipets each burner)

	5.  Paul Olivier and I talked over some TLUD experiments with pre-mixing several years ago  (very different geometry).  He didn’t like the results - so I am adding him to this message for comments.
	

Thanks for the sharing - and (in advance) for any answer to my too-many questions above.  Questions on your few sentences below are all covered above.

Ron




> On Sep 29, 2016, at 3:37 PM, alex english <aenglish444 at gmail.com> wrote:
> 
> Crispin,
> I don't know about the chemistry in the reaction zone, except that I would expect more hydrocarbon "tars" to remain with the char at lower temperatures.
> 
> I am not currently tooled for combustion gas analysis. When I was, this flame would test at <100ppm CO and 8~10% CO2.
> 
> Secondary air was not preheated beyond incidental ambient heat from the stove. it would be easy to get significant controllable preheat off the hot edge of the funnel.
> 
> Alex
> 
> 
> On Sep 29, 2016 1:01 PM, "Crispin Pemberton-Pigott" <crispinpigott at outlook.com <mailto:crispinpigott at outlook.com>> wrote:
> Thanks alex
> 
>  
> 
> If you run a higher superficial velocity I would expect to see the flame turn more to the blue, but it is already blue. Is it possible the H and all what we would normally see in the ‘flaming pyrolysis’ is taking place inside the bed? That would be pretty hot, no so?
> 
>  
> 
> Very interesting. Great video. What’s the CO/CO2 ratio in that condition?
> 
>  
> 
> When the premixed air is added, what is the temperature going in? Harold thinks we should investigate that. I agree. I’ll ask Riaz at CAU.
> 
>  
> 
> Crispin
> 
>  
> 
>  
> 
>  
> 
> No, but it is produced with a pyrolysis front at a temperature (500c ish) at the low end of normal for a TLUDs filled with wood pellets. There are some unanswered ( or demonstrated) questions here. To match burn rate with a higher py-fr-tp I need to use a smaller fuel chamber and increase the superficial velocity. This will perhaps produce a different gas composition. We'll see...
> 
> Alex
> 
>  
> 
> On Sep 29, 2016 9:00 AM, "Crispin Pemberton-Pigott" <crispinpigott at outlook.com <mailto:crispinpigott at outlook.com>> wrote:
> 
> Dear Alex
> 
>  
> 
> is that flame produced late in the pyrolysis? ‎By that I means in a carbon 'evaporating' stage.
> 
>  
> 
> Thanks
> 
> Crispin 
> 
> 
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