[Stoves] World stove, fantasy draft, flame caps, TLOD, TFOD, DS

[Kelpie Wilson]

Hi friends,
sorry but I get the email digest and cannot figure out how to reply to a
subject thread, so I'll just start a new one.

This is so interesting! I just got back from a workshop tour of Nebraska
and Kansas, teaching flame cap pyrolysis for making biochar from invasive
eastern red cedar. This use of waste biomass for biochar has been my focus
in recent years, rather than stoves or TLUDs.

So many times, I have seen flames from these kilns flowing upward and then
turning back down. The flame tips have a downward hook at the top as
something sucks the the air back down. These are Down Suckers, thank you
Paul for that term!

You can see a nice example of this pictured in my slides here:
https://www.slideshare.net/kelpiew/flame-carbonizers-for-biochar-in-practice-and-theory

Of course these flame cap kilns have no bottom air, but in the beginning
stages of a burn, the air does flow down the sides and up the center of the
fuel stack so they do operate in an updraft mode of sorts. So some of it is
the air flow down the sides that is sucking the air from above and causing
the flame tips to hook downward. However, in the later stages of the burn,
we have completely filled in the kiln with hot glowing coals and air still
comes from the top because the combustion must create a negative pressure
to pull air in from the top. Which of these down-sucking affects might be
taking place in a small stove?

I also have to agree with Gordon, and with Bill Knauss, that there must be
positive pressure from the wood gas as heat forces the volatiles out of the
wood. That would be how the Everything Nice/World Stove works. Heat from
the flame on top volatilizes gas down inside the stove. Pressurized gas
flows up to the top of the fuel bed, feeding the flame, but some flows out
through the side holes, gaining speed as it passes through the venturis,
creating more negative pressure to draw more gas. In the annular space it
pre-mixes with secondary air for better combustion. This pre-mixed gas then
merges with the flame on top.

It is not all about draft and buoyancy. Gas flows also occur from
volatilization and oxidation.

So the World Stove is definitely not a down-draft gasifier, but it is more
than a TLUD. In fact, I have felt for awhile that a flame cap kiln is more
like a retort than anything, since it is closed on the bottom. The flame
itself closes the retort and acts as heat exchanger at the same time.

The World Stove just acts like a self-heating retort with a flame cap, as
pressurized syngas leaves and recirculates to the flame.

I agree that I would really like to see someone evaluate Nat's claims about
N-enriched char from his stove. That would be proof that down-sucking gas
actually moves from the top through the fuel like a sweep gas. Not too sure
that could actually happen. The venturi effect would have to be very strong.

Kelpie

-- 

*Ms.Kelpie WilsonWilson Biochar Associates <http://www.wilsonbiochar.com>*
Email: kelpiew at gmail.com
Oregon home office: 541-592-3083
Mobile: 541-218-9890
Skype: kelpie.wilson
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