[Stoves] High temperature combustion in a ND TLUD

[Jock Gill]

Julien,

A few observations from today:

1. The larger the diameter of a TLUD, the larger the volume required for the combustion chamber;

To this point, I reduced the size of the central column to a one inch pipe that I extended to 5 inches in height - up from three.  This makes the FXJ burner significantly deeper and taller.

2. The larger the diameter of a TLUD, the larger the secondary air ports have to be;

To this end, I increase the size of my ports from 3/8s to 7/16ths of an inch.  One half inch diameter ports would be better.  Hard to drill in a tin can.  Larger airports also reduce the draft on the primary air, which helps control the rate of pyrolysis in operation mode.  Extremely little O2 is required in operation mode, esp. with any pre-heating of the primary air supply.

3. Design a TLUD's primary air ports for the oxygen requirements for the middle of the burn, NOT the startup phase.

To this end, I now start the TLUD over a grate with unrestricted access to primary air.  This supports the higher oxygen requirements during the startup phase.  Additional starter may have to be used at about the 5 - 7 minute mark to complete startup and transition to operation mode.

Once startup is accomplished, place the unit in its outer can.  In this position, the primary air is restricted by the narrow gap between the bottom of the TLUD and the outer can.  See the photo below:



Startup mode is in process.  No smoke.  Placed like this, the reactor has maximum access to primary air as is required at startup.  Note the use of washers:

1. Larger washers on the bottom of the draft can center, and stabilize, it on top of the reactor.

2. The smaller washers at the bottom of the reactor establish a very small gap between the reactor's bottom and the outer can.  This provides very restricted access to primary air.  Clearly, a TLUD requires one level of O2 at start up but a much small amount of O2 in operation mode.  Asking one set of primary air holes to be "right" for both situations while the reactor placed in a single fixed position is not optimal.

As I am not going to be able to enlarge the secondary air ports in this reactor to ½ inch, I will add some additional ports above the bottom row of ports.



This is the bottom of the reactor can after a extremely hot and dirty run with 3/8th inch secondary airports.  Increasing the secondary air ports cools off the system a bit both by introducing more air as well as by reducing the draft pressure on the primary air.  As you know, the PA control the rate of pyrolysis and thus the rate of gas production.

A subsequent run with 7/16ths secondary air ports resulted in much less thermal damage to the reactor bottom.

I might consider reducing the number of primary air holes to just 30.  Altho the 37 I am now using are at the limit of what is required to establish self sustaining pyrolysis.  So I probably won't.  The larger number of small holes gives a good distribution of primary air.



This is the one inch x five inch copper pipe I am  now using as a central column in my burner after a run with the 7/16ths secondary air holes.  The temperature in the FXJ burner chamber is simply burning up the pipe.  I'll have to find a 5 inch length of heat resistant stainless steel pipe.

In the end, I was able to get much closer to a no soot run.  There was a several minute peak of sub-optimal and sooty combustion -- too rich a fuel/air mix.  The majority of the 43 minute run on 500 grams of wood pellets was reasonably to very clean.  I note that there were zero creosote deposits as the burner temperature as high enough to  "crack and combust" all of the volatiles.  Or perhaps the black flakes in the photo above are all that is left of the volatiles?  They certainly did not appear to escape the system.



Vigorous and well formed, sharply defined, flamelet jets are my design goal.  The dark areas are the centers of the secondary air jets.  The bright lines are where the burning gases at the edges of each jet combine.   Lazy flames, as seen in the lower left, are not good.  This unit was operating outdoors in a 10 gallon can.  The breezes would of course create disturbances in the air flows,  leading to such things as the lazy flame mentioned above.  Once the reactor was in the ten gallon can in full operation mode, it was in little danger of being blown out by a breeze.  In startup mode, fully exposed to the breezes, the initial phase of the pyrolysis is susceptible to being blown out.

When I get a chance to use a furnace thermometer, I will be able to tell more about the temperatures produced by this system.

Comments, suggestions?

More as it is.

Jock

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

Cell: (617) 449-8111

google.com/+JockGill

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