[Stoves] is this new?

Sat Jan 19 23:46:44 CST 2013

Ron –

Can you guesstimate the degree of improvement more quantitatively? Crispin
places (correctly) a lot of attention on EA (Excess Air). Can your meter
measure this simultaneously with the CO readout? Or do you need a hood,
etc? What are typical O2 readings where you are measuring CO? Was the flame
fully complete where you measured? (little additional secondary air coming
in?)

I used a combustion analyzer that reads simultaneously CO and O2. Excess
air is calculated from O2 and fuel composition (which I had on hand from
the Seigert analysis I posted a while back)
I stuck the probe horizontal just above the outlet of my inner cylinder.

I would be very happy to hear that I did it completely wrong!

Anyway, these tests weren't intended to demonstrate improvement, just to
explore the concept. I found it totally unintuitive and surprising that the
draft from a short section could counteract the flow of producer gas being
forced by a fan.

I called it a "heat pump" to emphasize that the inner cylinder was actively
pulling secondary air rather than the producer gas "bumping into" ambient
air at the outlet of the burner. I suppose I was also thinking of a way to
distinguish that it wasn't a momentum effect (like a jet pump entraining
secondary air) but a buoyancy effect from the difference in density between
the cool ambient air and hot flue gas.
But "heat pump" is not a good choice because it's already used in HVAC.

Always looking for better terminology, what do others think?

Marc Paré
B.S. Mechanical Engineering
Georgia Institute of Technology | Université de Technologie de Compiègne

my cv, etc. | http://notwandering.com

On Sun, Jan 20, 2013 at 11:49 AM, <rongretlarson at comcast.net> wrote:

> Marc cc list  & Crispin
>
>     I have searched around a bit unsuccessfully for more on the Vesto -
> which I have never seen.  So this in part is to ask Crispin if
> cross-sectional drawings exist - since it seems to have numerous nice
> features.  But the VESTO seems to be quite different from your test, so I
> only add comments on Crispin's remarks that related to your experiment -
> and then jump down to yours (Marc's).
>
>
> See below
>
>
> ------------------------------
> *From: *"Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
> *To: *"Discussion of biomass cooking stoves" <
> stoves at lists.bioenergylists.org>
> *Sent: *Saturday, January 19, 2013 3:38:52 PM
> *Subject: *Re: [Stoves] is this new?
>
> Dear Marc
>
>
>
> I think this is properly called Counter-flow secondary air. I have used it
> in the Vesto with the addition of a second concentric ‘air tube’ between
> the loose one you are using and the combusting gas. That innermost tube is
> the combustion chamber and the air tube is the secondary air preheater. The
> loose one is akin to the stove body which is used to create a negative
> pressure in the sense that the air is drawn into the stove heating *
> downwards* instead of upwards.   *[RWL:   I confess I am not seeing this
> geometry.}*
>
>
>
> There is wisdom in this which is that the negative draft on the downward
> flowing air is counter-balanced by the hotter gases rising in the chimney
> with the hotter gasses ‘winning’ the draft contest.
>
>
>
> If you get the downward path (and its temperature) right balancing
> (almost) an upward hotter flow in the centre, you can get a low EA value
> (with corresponding low CO and high heat transfer efficiency) at different
> power levels – something notably missing from the cheap can-stoves.
>
>
>
> One of the drawbacks of nearly all the current crop of gasifiers and batch
> loaded stoves is they are not very controllable for power, and when they
> are, there is little to no control over the secondary air volume unless
> there is a fan involved.   *[RWL:   Can't quite agree.  It doesn't make
> any sense to me to design a TLUD without primary air control.  Agree that
> secondary air is rarely controlled.*
>
>
>
> By using the layout you have described, or a triple version as per a
> Vesto, you can have self-regulating (or close to it) secondary air supply
> without having to operate a second air controller. The variation in draft
> does it automatically.
>
>
>
> The position of the external air entry holes on the Vesto and the lower
> chamber below the controller are at the height they are to create a
> reasonable balance on the draft in the centre of the system that pulls in
> primary and secondary air. The smaller holes through the air tube at the
> level of the secondary entrance are to allow in additional secondary air if
> the primary air is shut down rapidly (which would otherwise cause a very
> low EA condition and smoke – which you seem to have experienced, although
> for a different reason).  *[RWL:  I need a cross-sectional drawing to
> understand this last.    Not sure that Marc experienced smoke??]]*
>
>
>
> You can get the more common secondary air preheating by running the air up
> the outside of the pyrolysing chamber with air entry at the bottom (see the
> $1 Grasifier) but it is ‘unregulated’ by the draft inside – it operates
> based on the heating that comes through the exterior wall. Stoves like this
> include the original 1984 Tsotso Stove by David Hancock (the famous), the
> Peko Pe and Paul’s gasifiers, the POCA charcoal stove and the metal+clay
> Anglo SupraNova (though I plan to edit that last stove in a couple of
> months to be more advanced).
>
>
>      *[RWL:   I don't have enough familiarity with the above named stoves
> to comment.]*
>
>
>
> Something you might try is to place the loose pipe on a ring that pretty
> much = the inside diameter of the chamber, but loose enough to fall with
> its own weight. The drill a bunch of holes at the bottom to allow in the
> secondary air through the cylinder. I suggest 600mm2 per kW.  Ignite the
> rice hull then place the pipe+ring on top with the ring on the bottom. As
> the fuel drops in volume, the chimney will sink, always sitting on top of
> the fuel and letting in the secondary air immediately above the fuel
> level.   *[RWL:   I agree with the idea of an added washer shape, but I
> believe the needed flame holding (and minimum char burn)  can follow with a
> fixed "washer" and cylinder height.   I think a "floating" tube will create
> problems in the resulting increasing space between the chimney top and the
> cook pot.   That distance is also very important - in achieving high
> efficiency.*
>
> *     Crispin's dropping ring+chimney might work, but I hope you or
> someone can try the same but fixed.  I think the ring will be as hot either
> way -as the flame, not the hot char, should establish that ring
> temperature.   Varying the ID of this disk  (or cone?) could provide some
> interesting data as well.   Maybe the pyrolysis gases should exit through a
> ring rather than a hole (the inner solid circular part supported by at
> least three "thin" strips.]*
>
>
>
> The advantage of this is that it will definitely keep the flame going and
> keep the top of the fuel bed really hot, hopefully burning some of the char
> at all times, this preserving the ignition of the gases. As the gas is
> already ‘gas’ by the time if emerges from the fuel, the secondary air holes
> can be at or near the bottom – a few rows perhaps.  *[RWL:   This not
> clear.  Are we talking the bottom of the chimney  region or the fuel
> region?  I don't see any advantage to placing secondary air holes within
> the fuel region  (which is at the "bottom")* The incoming gas *must*reach the centre point (look inside to see the flames)
> *[RWL:   Agree on "must";  the last clause is not clear - The flame
> height and shape will depend on turbulence and the diffusion of pyrolysis
> gases into the secondary air stream(s).]*. A too-large diameter tube is a
> common mistake in the design of these. A central air pipe is often added to
> overcome a problem that should not have been there in the first place.
> *[RWL:   This may be, but I have seen a few designs with excellent
> turbulent mixing due to the interior secondary air pipe.  I think an
> interior secondary air pipe may be a generally useful design feature - and
> maybe in the Bellonio (Olivier?) design you are working with.     Perhaps
> Crispin could give us a cite on who has been using an interior secondary
> air pipe.   I'd like to hear their thoughts.*
>
>
> The inward distance travelled by the secondary air varies with the draft
> applied and the hole diameter.   *[RWL:     Sort of agree.  But if there
> is symmetry, the lowest flamelets will turn up at the center and all the
> other higher flamelets will not make it to the center line.    If the
> secondary air can be "canted", then a beneficial swirl can be achieved.*
>
>
>
> Obviously another concentric pipe fixed above the loose one can be the pot
> support.  *[RWL:   If  one "pipe" could slide inside the other, this
> would overcome my objection to a variable gap near the pot. But I
> think/hope the added worries with a slip fit are probably not needed.  ]*
>
>
>
> What this whole apparatus does is recreate the combustion conditions that
> are afforded by a downdraft combustor, without the downdraft combustor’s
> ability to be refuelled while running. If an updraft batch process is OK
> for the application, it is easier to apply the heat to a single pot
> directly above.    *[RWL:   Have to question some of this.  A charmaking
> downdraft also has to be bottom lit - with the pyrolysis front moving
> upwards.   I certainly agree on the difficulties of working with
> downdraft.]  *  To vary the power of the stove, control the primary air.
> *[RWL:  Definitely agree - I see some misunderstanding on this - but I
> know Marc does understand it.  The relation is linear.*
>
> *
> *
>
> *    More below in responding to Marc.]*
>
>
>
> Regards
>
> Crispin
>
>
>
> So, I was playing around with burners on a Belonio rice husk gasifier last
> night.
>
> If you're not familiar, there are a bunch of photos of the basic design on
> google image: batch stove images<https://www.google.com/search?q=belonio+batch+stove&sugexp=chrome,mod%3D16&um=1&ie=UTF-8&hl=en&tbm=isch&source=og&sa=N&tab=wi&ei=kh76UMnkBOiriAforYHQCw&biw=1282&bih=717&sei=lB76UMTnOaSjigfp3YGgDw>
>
>
>
> I slid a metal cylinder into the opening of the top of the reactor,
> leaving a gap along the sides. Here's a picture:
>
> [image: Inline image 1]
>
> Now, normally when you take the burner top off of these stoves, there's no
> combustion inside because there is no secondary air available.
>
> Well, I saw a roaring flame inside after sliding in the metal cylinder
> (option #2 in the diagram)
>
>       *[RWL:   In your #2 drawing, you show some flames in the outer
> annulus.  Did you observe that always, some of the time, or never?   Such
> outer flames look like a problem - not an asset.   And controllable or
> minimized with an interior blocking ring.]*
>
>
>
> As far as I can tell, the cylinder acts like a chimney, causing a pressure
> drop which sucks producer gas from the bed, not allowing it to escape
> through the gap on the sides.
>
> As a result, secondary air sinks through the gap and you get combustion at
> the bottom of the cylinder.   *[RWL:  Absolutely.  Crispin has said it
> correctly.   Maybe "sink" is OK - but there is a decided pressure
> difference caused by the interior combustion and hot rising gases.]*
>
>
>
> Has anyone seen something like this before? I can't think of any examples.
> I called it a "heat pump" in my field notes.
>
>     *[RWL:   Most of the early two-can versions around 1996 found it
> necessary to shield the secondary air holes from the wind and so there was
> often an outer cylinder - and some preheating.  The air could generally
> enter from either the top or bottom.  I do not recall the inner cylinder
> geometry you have just tested, which should provide much greater
> pre-heating.    I can't see a reason to encourage "heat pump" terminology.
> *]
>
>
>
> With the right dimensions is might be a good auto-regulating burner: more
> producer gas producers more heat, pulling in more secondary air.   *{RWL:
> You or someone needs to see how self regulating this can be.  I agree that
> the tendencies are in the right direction.     But I think an alternative
> would be finding a way to independently modify this secondary airflow
> Maybe two concentric cylinders whose relative angular rotation could vary
> the secondary air flow.   This could give some quicker results also -
> rather than changing the air flow pattern through new holes or slits for
> each inner cylinder.   I can conceive that the right EA might be determined
> by judging the vigor of boiling.
> *
>
> *     You are describing a geometry where you might be able to get a
> swirl easily - slits at the bottom that are bent to give angular velocity
> to the incoming secondary air could be a big help in achieving more
> complete combustion  - and not possible with the (more expensive)
> Bellonio-Oliver burner design.*
>
>
> I think it could be useful for charcoal stoves as well as TLUDs.
>
>      *[RWL:  There are already "cylindrical" products on the market to
> start charcoal barbecues more quickly.  But I hope we can forget about
> charcoal-using stoves - as being inferior in health,  BC (black carbon),
> efficiency, and other ways.*   *Users seem to prefer putting pots
> directly on the char - and so the cylinder (and the advantages of
> preheating) are apt to not be used much.
> *
>
>
> I measured lower CO than usual with Belonio burners. Similar excess air
> levels (though I only tested two sizings of the metal cylinder).
>
>
>    *[RWL:   *A few hours after this message, you wrote to Tom Miles:
>      *"Tom, I measured CO with a probe at the top of the cylinder using
> my UEi combustion analyzer.*
> *I've got the bottom of the line model that only measures CO and O2. It
> has trouble with CO higher than 1300 ppm, but I wasn't getting any higher
> than 400 ppm during these tests."*
> * *
>
>     *[RWL:   I think it very exciting that you achieved this level of
> CO.  Can you extrapolate and say that your combustion efficiency was way
> over 99% ?  Can you guesstimate the degree of improvement more
> quantitatively?
>      Crispin places (correctly) a lot of attention on EA  (Excess Air).
> Can your meter measure this simultaneously with the CO readout?  Or do you
> need a hood, etc?  What are typical O2 readings where you are measuring
> CO?   Was the flame fully complete where you measured?  (little additional
> secondary air coming in?)*
> *
>      Thanks for giving this report.*      Ron]
>
>
>
>
> Marc Paré
> B.S. Mechanical Engineering
> Georgia Institute of Technology | Université de Technologie de Compiègne
>
>
> _______________________________________________
> Stoves mailing list
>
> to Send a Message to the list, use the email address
> stoves at lists.bioenergylists.org
>
> to UNSUBSCRIBE or Change your List Settings use the web page
>
> http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
>
> for more Biomass Cooking Stoves,  News and Information see our web site:
> http://www.bioenergylists.org/
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130120/bd2b72ee/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.png
Type: image/png
Size: 40570 bytes
Desc: not available
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20130120/bd2b72ee/attachment.png>