[Stoves] Airflow For Biomass Fired Appliances- Natural Draft Stoves

[Alex English]

Yes, in the stoves world TLUDs with fans are comparatively clean 
burning. I'm just saying it doesn't necessarily follow that they are at 
their best when their flame is shortest.

Any burner will have a range of emissions including a sweet spot where 
it performs best. There are lots of variables to consider. Some require 
better instrumentation.
For a TLUD with a fan, or a boiler with a fan, you can shorten the flame 
with extra secondary air. As you know, if it isn't needed for combustion 
then is robs heat, that may be needed for optimum combustion. Yes 
long/tall flames can have sooty tips. So there are potential trade offs. 
When I can see the flame and my combustion analyser at the same time I 
have often seen that a long flame has lower CO/CO2, less excess air and 
higher heat transfer efficiency.  Not enough air and it will be even 
longer with poorer emissions. Flame colour is a clue, its the numbers 
that inform. Better mixing from higher pressure blowers/fans can shift 
the range of flame lengths shorter. Stated another way, the optimum 
flame length is unlikely the shortest.

I'm being warmed by my ND pellet stove right now. After the secondary 
air ports the flame travels horizontally through a 2.5 inch tube, 12" 
long. For this firing rate the sweet spot is when the  flame fingers are 
shooting out six inches past the tube end. More secondary air shortens 
it back inside the tube. Less secondary lengthens it and turns it more 
orange and larger. CO/CO2 increases in both cases. There will be no 
visual emissions from the chimney for any of these scenarios. Real time 
PM and NOx numbers might enlighten this tale some.

  I've seen a large chip boiler cut its CO in half improve 4% points of 
thermal efficiency just by closing some secondary air ports. The flame 
lengthened by roughly 25%.

Years ago when I was testing my Reed style fan TLUD on low power. It had 
the smallest of flames but the flame didn't fill the chamber cross 
section of the chamber below the pot. Some of the pyrolysis products 
were sneeken past the flame and condensing brown (not soot) on the pot. 
Less secondary air, a larger flame, and perhaps a different geometry 
could have helped. The problem went away at higher firing rates with a 
bigger and somewhat taller flame.....
....but I burnt the food and went hungry:(

Alex




On 14/04/2013 5:18 PM, Lanny Henson wrote:
> A response from Alex English! made my day.
> Fan powered TLUDS have a nice short flame height, are they not clean 
> burning?
> Lanny
>
>     ----- Original Message -----
>     *From:* Alex English <mailto:english at kingston.net>
>     *To:* Discussion of biomass cooking stoves
>     <mailto:stoves at lists.bioenergylists.org>
>     *Sent:* Sunday, April 14, 2013 2:25 PM
>     *Subject:* Re: [Stoves] Airflow For Biomass Fired Appliances-
>     Natural Draft Stoves
>
>     Lanny,
>     I understand the comment below but I think flame height can be
>     shortened with excessive excess air. The lowest emissions CO/CO2
>     and highest temperatures, at some power levels, in appliances that
>     I have tested has often been when there is a significantly taller
>     tail of flame.
>
>     However, don't believe  all tall tails :)
>     Alex
>

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