[Stoves] SX iCan simplified and improved

[Jock Gill]

Fellow stovers,

Starting a cold stove with tightly controlled system air, primary air, and drafts is an interesting problem.

If you use an outer sleeve with no holes for system air at its base, for down drafting air into the system, there is no heat for pre-heating or to create the draft.  A fan assist would be nice.  I should point out that a closed outer sleeve also risks creating reinforcing feedback loops that can create essentially a run away pyrolysis.

If the outer sleeve provides too little system air via ports on its bottom, the pyrolysis will also fail due to insufficient primary air.

So I am working toward the minimum limit of air for the system via ports on the outer sleeve [can.]  6 holes made with a can opener is way more than needed.  It defeats the advantages of pre-heating.  3 holes made with a can opener is slightly too little.  So now I have added three ¼ inch holes and things are about right.

The important point appears to be that you have to control BOTH air entering the total system AND the primary air entering the reactor can.  They are, I now realize,  two different, but tightly coupled, problems.

The pyrolysis can still be hard to establish and even fail due to lack of draft in the system.  So I replace the draft can with the three deflectors with a draft can that had nothing in the path of the exhaust gases: a straight pipe.  More on this latter. I also reduced the secondary air gap 50% to ¼ inch.  This improves the draft on BOTH the primary air controlling the pyrolysis as well as the secondary air controlling the turbulence and combustion qualities.

Currently, this configuration, with a secondary air gap of ¼ inch, shows some down drafting of the make up air at the top of the gap between the outer can and the inner draft can: ie some preheating is going on.  At the bottom of the outer can [sleeve], match flames are sucked into the system.  This tends to cool the primary air helping to prevent run way pyrolysis.

The above configuration also starts very easily with gelled alcohol drizzled just around outer edge of the TLUD.

Run times on 150 grams of year old wood pellet feedstock are about 50 minutes.  Charcoal yield is about 19%.  The charcoal passes all of the usual simple tests.

In general, the tricks appears to be:

1. Enough system air for primary and secondary air, combined with well tuned draft, to sustain the startup phase, but not cool the system unnecessarily.  Find the minimum limit that sustains the pyrolysis but achieves a well behaved combustion of the gases.  Do not use more than you need.  The more you use, the more out of control the system will be during the period of maximum gas production as the pyrolysis zone approaches the primary air source. Run times will be shorter and charcoal harvest reduced.  A key aspect of this is to control the draft pressure in the system as it has critical effect on both the primary and secondary air.

Note:  As the pyrolysis zone approaches the primary air source, the ratio of O to CO changes to favor oxygen.  This greatly reduces the blue in the flames -- or so I think.  At the last, when there is no more uncharred biomass, some carbon is burnt which restores a favorable CO balance and the flames turn blue again.  At start up, blue flames can be sustained for 15 or so minutes if oxygen is kept to the absolute minimum in the pyrolysis zone: No oxygen coming in from the top and the minimum coming up with the primary air.  Using a disk helps with keeping oxygen from coming in from the top.

2. Enough secondary air to keep combustion clean, but not so much as to cool things off or reduce the draft below the minimum required.  Find the secondary air minimum limit.

3. Make best use of the draft can by keeping it free of objects which will reduce its effect.  If you do use deflectors in the draft can, they appear to reduce the efficient combustion of the gases leading to more red-orange flames escaping from the top of the draft can.  In my first comparison test, the deflectors did not have much influence on run time or charcoal harvested.

4. It turns out that a central disk below the secondary air source is all that is required for better results.  If this disk is 2/3s of the diameter of the TLUD, it will cover 44% of the surface area of the TLUD.  This is is probably the minimum disk size.  It will be worth exploring disks that cover a larger percent of the TLUD surface area.  What is the minimum gap between the disk edge and the wall of the reactor can that is required for good results.

5. Tis it better to try to bring the secondary air into the very center of pyrolytic gases or is it better to force the pyrolytic gases to the secondary air?  Which choice allows a greater TLUD diameter?

A few photos:



My test set up in my wood stove.  The stove reduces as many variables as possible and provides ventilation for the combustion gases.The outer sleeve can shown has no bottom holes for supplying air to the system.  I eventually cut three equally spaced holes with a can opener and then drill three ¼ inch holes centered between the cut holes.  Knowing what I know now, I might try with only the three cut holes.  



This is a "Julia Childs" shot into a mirror looking down into the SX iCan.  This was about 15 minutes into a test run.  Note:  The deflector washers have been removed. The distortions are caused by the fact that I managed to melt the plastic mirror the other day.  



This is a view of around minute 35.  It looks like oxygen is now overwhelming the CO and thus creating yellow-red flames.  Importantly, the combustion appears to be complete enough that there is not a lot of unburnt material that draws flames up and out of the top of the draft can.  There is some, but relatively little compared to many other stoves I have built and tested.  The glow in the middle is from the combustion of the pyrolytic gases at the edge of the disk as they meet the inrushing secondary air.

Please remember that the work I do is aimed at creating the most affordable tools possible for a teacher to use to introduce carbon negative energy via pyrolysis, as well as the biochar co-product.  The principal barrier to this being taught in public schools in the US is that none of the key words are in any of the tests the teachers have to teach to.  This is a scandal as it prevents young people from learning about real options to current dependencies on fossil carbon.  It is a bureaucratic  barrier to innovation and thinking outside the box. 

Comments?  Suggestions?
Jock

Jock Gill
P.O. Box 3
Peacham,  VT 05862

google.com/+JockGill

:> Extract CO2 from the atmosphere! <:

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