Christa (cc list) Thanks very much for the lead. It helped. But like Marc, I still don't have a full knowledge of all the air paths for the VESTO. In particular, I think the VESTO was designed to eventually combust all of the char - the opposite of what I have been promoting. But of this I am still not sure.
Thanks also for your 2011 report/manual. It is almost certainly the best handbook around for both gasifying and pyrolyzing stoves . I found this for "foodandfuel": http://www.pciaonline.org/node/625 But could not find a web site. Is there one? Should see you at ETHOS in a bit more than a week. Ron (responding to Marc's message of almost the same time next) ----- Original Message ----- From: "CHRISTA ROTH" <[email protected]> To: "Discussion of biomass cooking stoves" <[email protected]> Sent: Sunday, January 20, 2013 2:13:08 AM Subject: Re: [Stoves] is this new? 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 ----- Original Message ----- 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 600mm 2 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/ _______________________________________________ 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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