Dear Jock
I presume that you do not have a combustion analyser. It is rare. So, based on looking only, the gap for the secondary air entrance looks really large. This remark is based on the available draft (high) and the physical size (large). I expect that the excess air level is pretty high (2-300%) and that you will get better performance by making it smaller. So I have a practical suggestion. Make a fixed collar with holes maybe 13mm in diameter and cover the slot completely. Make the holes 14mm apart centre to centre (i.e. 1mm larger distance than the diameter). Then make a matching sliding collar that you can shift around from side to side. The details I leave to you as to how to do that. There are lots of examples around. What will happen is that the incoming air will probably not be a heck of a lot less total than the open slot, and it will enter with a great deal more velocity through the holes. This will send a blast of air from each jet opening which, if you are lucky, will just meet in the centre. That will guarantee that there is secondary air mixed through the whole vertical column of gas. Because there are gaps in the incoming air stream (where there is no hole) it will make a great deal of turbulence on each side because the gas is rising, and the flame that surrounds each incoming air jet will further disturb the smooth flow. The result you are looking for is a very much shortened flame (total height above the gap) and hotter gas temperature after the flame has finished burning (plus about 2 inches). If you get the air ratio right, the gas temperature will go up, and the burn rate will remain the same. Note (important note actually): the draft is split between the primary and secondary air entrances. That means if you close the secondary gap partly with a perforated collar, it will increase the draft on the primary side for the same hole sizes. You may find the burn rate increases after making the change. That may not be what you want so reduced the primary air hole (total area) until you get you burn rate back. The ideal is to be able to tune the secondary air while having the primary air give you the burn rate you want. Each time you change the primary or secondary air flow, you affect the other flow because they are splitting the available draft and it is a zero sum equation. Also, if you get the flames to come to the centre and stay away from the walls the tins will last a heck of a lot longer. And once you get that right, you can put another metal sleeve around the upper shell (which gets the hottest) with a gap of at least 13mm radially away from the one we see now, and close the bottom back to the body connecting just below the present top of the lower can. This will cause hot secondary air to enter the holes in the sliding collar instead of cold air. This will reduce the CO and PM, while limiting the EA at the same time. The air flow in this condition is quite sensitive to the negative draft of having to pull the secondary air downwards as it heats. Small changes will make a big difference to the EA. By sliding that preheater up and down the lower you can find the best position but that will require access to a combustion analyser. Have fun! Regards Crispin From: Stoves [mailto:[email protected]] On Behalf Of Jock Gill Sent: Saturday, August 31, 2013 2:46 PM To: Discussion of biomass cooking stoves Subject: [Stoves] A practical application for pyrolysis: grilling Friends, Erin has kindly posted some notes I sent her on a practical application of pyrolysis using a modified Weber grill. This is slow cooking the carbon negative way. The notes also over making grass tablet biochar. http://stoves.bioenergylists.org/content/grass-tablet-biochar Questions? Suggestions? I look forward to hearing from you. Regards, Jock Jock Gill P.O. Box 3 Peacham, VT 05862 Cell: (617) 449-8111 :> Extract CO2 from the atmosphere! <: Sent from my iPad
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