Ron, find some info on the VeSto in the micro-gasfication manual page 39 http://www.giz.de/Themen/en/dokumente/giz2011-en-micro-gasification.pdf see you later this week christa
Am 20.01.2013 um 05:49 schrieb [email protected]: > 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" <[email protected]> > To: "Discussion of biomass cooking stoves" <[email protected]> > 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 > > I slid a metal cylinder into the opening of the top of the reactor, leaving a > gap along the sides. Here's a picture: > <image001.png> > 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 > [email protected] > > 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/ > > _______________________________________________ > Stoves mailing list > > to Send a Message to the list, use the email address > [email protected] > > 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/ >
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