[Stoves] Model R TLUD update

Wed Sep 27 14:30:19 CDT 2017

Jock,

You are following a very good pathway.  I followed a very different path (turn-down) but came to similar ideas.  I think one of the canopy devices from my stove will work for you.  It operates differently from your disk.  Instead of forcing the gas out to the edge, it pushes some gasses out and brings some air in.  Your design will give about 57 cm of mixing surface, a canopy device will give you about 200 cm of mixing surface.  Better mixing means faster burning, which as Crispin says is a good thing.  Send me a shipping address and I will send you a canopy device to try.  I will be leaving on a two week trip on Monday, Oct. 2, so make it quick if you want to try it.  I still have time to make and ship you a canopy before leaving, but just barely.  

Did you see the document I sent the list about a month ago.  If not I will send it again, with its updates.  Read the parts about the canopy, they apply to what you are doing.  Ignore the parts about injection, they may be over-weighted.  I underestimated the aggressiveness of diffusion at the higher temperatures in the stove.

Concerning the ignition of the gasses above the disk.  For the Wonderwerk 316 stove we have the same problem.  We installed holders that hold the combustor section with the canopy up about an inch above the reactor, allowing air to enter to feed the starter flame and get the pyrolysis front going.  Then the combustor can be turned 45 degrees to set slowly down onto the reactor section.   This allows the gasses above the canopy to be ignited by the starter flame.  Setting it down to quickly puts the secondary flame out.  It takes about 30 seconds before the pyrolysis front is ready.  During that 30 seconds the starter flame is pre-heating the combustor.

Your fellow stover,

Kirk H.

Sent from Mail for Windows 10

From: Jock Gill
Sent: Wednesday, September 27, 2017 10:02 AM
To: Discussion of biomass cooking stoves
Cc: Kelpie Wilson; Hugh McLaughlin; Erich Knight
Subject: [Stoves] Model R TLUD update

Stovers,

As Crispin suggested, I tested my model R TLUD with a 12 CM diameter disk suspended about 1 cm below the entry of the secondary air.  This disk covers about 64% of the pyrolysis surface (15 cm in diameter).  An interesting result is that this clearly creates two very distinct chambers in the system.   One below the disk and a second between the disk the cap. The bottom chamber is the pyrolysis chamber. The upper chamber is functionally a combustion chamber.  This is made clear by the fact that the pyrolysis in the lower chamber will NOT ignite the gases in the combustion chamber.  These gases must be lit by a second ignition process.  The pyrolytic gases will “smoke” until the fuel air mix is rich enough to sustain combustion in the upper chamber.

I am still working on an ignition system for the top chamber.  The pyrolysis in the lower chamber is easily started with wood pellets soaked for 5 minutes in 90% rubbing alcohol.  The cross over to combustion of the gases in the upper chamber happens around 10  minutes into the process.  At this point, I carefully squirt some gelled alcohol on top of the cap.  It immediately starts to vaporize. This creates an explosive  mix in the draft can which is very easily lit — but must be done from the edges to avoid a flash burn when the alcohol gases ignite explosively.  I usually have to repeat this process two or three times until the fuel/secondary air mix is rich enough to sustain combustion.  Then it is off to the races.

Latest run was just over 38 minutes on 450 grams of wood pellets (my cost is about 12.5 cents US).  Temperatures exceeding 1,200 degrees F were observed in the exit gases.   The conversion of the wood pellets to charcoal was complete and the charcoal passed all of the usual tests easily.

This is a 18 cm diameter stove pipe cap.  It works as well or better as when I test with the larger 21 cm diameter cap.

This shows the 12 cm diameter disk suspended from the smaller stove pipe cap.  It hangs about 4 CM down from the top of the cap and rests about 1 cm below the entry point of the secondary air.

Primary air is pretty aggressive.  It had to be somewhat increased to adjust for the reduction in draft caused by the 12 cm disk. I expect a can with somewhat smaller primary air holes would also work.  A key concern is to get the pyrolysis hot enough to completely convert all of the biomass to good quality carbon.

This is the top of the reactor can.  Notice the lip. It limits the diameter of the disk.  It also prevents the gases from raising straight up and deflects them a bit towards the center.  Note: the reactor can has an interior diameter of 15 cm.  The disk has a diameter of 12 CM, thus the pyrolytic gases are forced way towards the sides of the reactor and must rise up thru a narrow gap, ~ 1.5 cm.  This effectively creates two very separate chambers, one below the disk and one above.  The lower chamber will get very little effect from secondary air.  The stove pipe cap rests on the three angle bracket arms.  

It is easy to see that the pyrolysis creates the gas unevenly across the surface.  As the point of greatest gas production moves around, it causes turbulence in the flow of gas up into the combustion chamber.  It is quite interesting to watch — if you do not burn off your eyebrows. I use a step ladder to get safer viewing position.

The assembled model R TLUD with the smaller 18 CM diameter cap.  The draft can on top is critical.  If it is too short, the run the will go long and the conversion of the wood to charcoal will be incomplete.

This is the larger 21 cm diameter stove pipe cap.  It also works.  The pattern and size of the exhaust holes doesn’t seem to matter too much.  Visual evidence suggests more blue in the flame when  using the smaller cap. As the smaller cap provides a narrower gap for the secondary air flow, this may allow for hotter temperatures the combustion chamber?

Comments?  Suggestions?  I hope others will build on these ideas and take them further.

Regards,

Jock

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

Regenerate the Commons

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