[Stoves] More on flaming pyrolysis

[AJH]

On Sat, 2 Jun 2007 11:22:43 -0600, Ron Larson wrote:
>  Fortunately, I don't believe the answer 
>should impact us very much in the design process, but I believe the dialog 
>is important as we can better understand what is going on.

Agreed
>
>    Tom Reed wished for a gas analyzer.  Maybe 8-10 years ago I reported on 
>this
>list (I think) on tests I had made with some borrowed (from the University
>of Denver) expensive hydrocarbon gas monitoring apparatus.  The (well
>described by others) 1) very low carbon dioxide, 2) lots of CO and 3) zero
>oxygen above the pyrolysis front (but still in the charcoal and below the
>secondary air port) were easy to see.

Of course in my scenario I would expect to see low CO2 for very dry
wood and this then increasing for more moist wood.

>    a.  I found a big improvement in reproduceability and ease of operation
>when I arranged the fuel supply like hexagonally close-packed pencils on
>end.  If too tight, but especially if too loose, the operation could go 
>awry.  I learned
>from experience.  When the fuel is chip- or seed-like, the resistance to air
>flow goes way up - requiring a fan or a very shallow fuel bed. 

When I first learned of the technique from you I was very excited as I
wished to emulate and old woodland management practice and make use of
"bundles" of twigs. Whilst with care it did work and had all the
benefits (and I note the comment about resistance in a woodchip bed)
you explain, it easily did go wrong. I think the biggest issue was a
hot carbonised  small twig breaking off and falling down into the
lower regions, this does not happen with chips.

> With a fan,
>there is a much smaller chance of getting char (and most people are not now
>caring about char - but for "terra preta" purposes this is crucial  (and I
>believe there are annual income aspects that should be determinative).  But
>in terms of the energy required to heat up the fuel region just below the
>"flaming pyrolysis" level, the hexagonal packing allows a nice clear shot
>for the optical energy from the flame to pass unhindered to lower levels. 

As you say the downward passing of heat can be radiative or
conductive, in this case convection actually works against the
downward movement of heat and forced air will exacerbate this.

> I
>was always amazed that the front almost always (after learning how to pack
>well) moved uniformly downwards (proportional in speed to the amount of
>primary air).  I wish I knew of a reason (a physical law) that this should
>have been so.

Me too, the best I can put it down to is the radiation constant from
the fixed bed and the specific heat and conductance of the wood being
constants. The experiment needs replicating for constant air mass
flows and differing sizes of wood and moisture contents.

>  It was only when there was a big (unintended) void in the
>fuel supply that a single preferred path would develop (quickly leading to
>combustion along that path).

Yes once the pyrolysis front distorts from a horizontal plane the
uniform downward movement breaks up, the first indication is usually a
light blue haze.


>  So to Andrew, I ask for more information on 
>his feeling that there was (or should be?) a size limit to TLUD.  I got up 
>to 30 cm diameter

I gave up at 28cm, the diameter of one of my oil drums, I brazed
several together.

>I think - but I can see why Andrew might think/know there is an upper size
>limit. 

There may be an upper size limit for any one pyrolysis front but there
would be ways of paralleling stacks up within one unit. The reason I
moved away from favouring tlud as a means of making charcoal was the
low yield and too high a temperature to maximise fixed carbon for a
given mass of dry wood.

> But for "normal" (ie less than 10-15 kW) stoves, I don't think 
>container size need be a problem.  Andrew has been developing a much larger 
>scale pyrolysis
>unit - not a stove (last I heard).

I agree

I have been playing with a number of concepts for much larger
carbonisers. The high pressure one just got too expensive. You'd laugh
at the "Heath Robinson" small scale version of an open field burner I
have set up in my garden at the moment. The terra preta discussion
prompted me to revive my "under-a-cover" burner for habitat management
by burning. This is similar in concept to Tom Miles "phoenix" stubble
burner but for burning higher moisture content material in situ. We've
had mixed success with some fancy aspirators that just didn't form
enough depression or they fouled the secondary air ratio. So I've
resorted to changing down from a 1/2 dimension device to my oil drum
sizes on grounds of development cost. My thoughts are the radiative
feedback idea I prototyped did actually enable pyrolysis on the fly
(albeit very slowly, our target was 1/2 hectare/day and I don't think
we got very close). So it could adapt to arable slash and char *if*
the char was incorporated under the surface to quench complete
combustion.
>
>    b.  Within the piece (my pencil-like twig/branch) being pyrolyzed, the
>resistance to gas flow is much lower (factor of ten or more?) longitudinall


Yes you also see this with wet branches in a fire, the moisture mostly
bubbles out of the ends.

>this supports Andrew's position on char combustion providing much/most of
>the radiant energy.  With chips or small pieces lying horizontally, this
>preferred longitudinal direction aspect disappears - and I have no insights
>on what is happening then.

I cannot add to any other of your conjectures but agree if this had
been of mainstream interest it would have been thoroughly researched,
possibly the dominance of other energy sources since Victorian times
is the reason we still reference such old texts when discussing
pyrolysis of wood, the great thinkers went where the money was?

AJH