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</o:shapelayout></xml><![endif]--></head><body lang=EN-CA link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dear Marc<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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 <i>downwards</i> instead of upwards.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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).<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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).<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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<sup>2</sup> 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.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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. The incoming gas <i>must</i> reach the centre point (look inside to see the flames). 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. The inward distance travelled by the secondary air varies with the draft applied and the hole diameter.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Obviously another concentric pipe fixed above the loose one can be the pot support. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>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. To vary the power of the stove, control the primary air.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Regards<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>So, I was playing around with burners on a Belonio rice husk gasifier last night. <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>If you're not familiar, there are a bunch of photos of the basic design on google image: <a href="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">batch stove images</a><o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><o:p> </o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>I slid a metal cylinder into the opening of the top of the reactor, leaving a gap along the sides. Here's a picture:<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><img border=0 width=504 height=556 id="_x0000_i1025" src="cid:image001.png@01CDF669.C0E2F230" alt="Inline image 1"><o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Now, normally when you take the burner top off of these stoves, there's no combustion inside because there is no secondary air available.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Well, I saw a roaring flame inside after sliding in the metal cylinder (option #2 in the diagram)<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><o:p> </o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>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. <o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>As a result, secondary air sinks through the gap and you get combustion at the bottom of the cylinder.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><o:p> </o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Has anyone seen something like this before? I can't think of any examples. I called it a "heat pump" in my field notes.<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><o:p> </o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>With the right dimensions is might be a good auto-regulating burner: more producer gas producers more heat, pulling in more secondary air. <o:p></o:p></p></div><div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>I think it could be useful for charcoal stoves as well as TLUDs. <o:p></o:p></p></div></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>I measured lower CO than usual with Belonio burners. Similar excess air levels (though I only tested two sizings of the metal cylinder).<o:p></o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><o:p> </o:p></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'>Marc Paré<br>B.S. Mechanical Engineering<br>Georgia Institute of Technology | Université de Technologie de Compiègne<br><br><o:p></o:p></p></div></div></body></html>