[Stoves] Biomass stoves v. PV-induction cooking (re: Frank)

Norman Baker ntbakerphd at gmail.com
Wed Feb 1 12:10:17 CST 2017


Andrew and guys;

I could be wrong and am willing to be in the face of new evidence. But I do
not think it is a simple convection effect at the surface. Obviously it is
a convection effect but not due, I think, to being at a cooler surface.
First, we used a heat shield (closed at the top) and the the temps inside
the heat shield were about 200 degrees C above ambient. Core was typically
at 450 -500C. External surface at the pyrolysis front was typically at 350
C without the heat shield. Second, we observed the pyrogas moving in any
direction you could imagine. That was a real surprise. In concert with this
unexpected gas movement, we also observed (quite consistently I have to
add) smaller pieces of pyrolyzing wood chips lose their mass and volume and
then simply cascade down into between the larger wood chips for an average
distance of about 20 cm below the pyrolysis front. Often, they would induce
pyrolysis along their path and sometimes would fall fast enough to simple
induce an area of pyrolysis at a greater depth. That phenomenon was
amazingly consistent. The new area or path pf pyrolysis would create a
small channel that would offer a more open less pressurized path for
primary air to flow. Along with it, the pyrogas would be sucked along (in
almost any direction) and create what we saw with the gas going in any
direction as well as up as expected. I'm interpreting here because we did
not actually see it internally but did see it at the periphery of the
barrel.

That said, we early on found that any long or unusually large wood chips
would create bridging and air channels the pyrolysis would follow downward.
We also soon figured out the woodchips needed to be "settled in" as we
filled the barrel. Consequently we would put in a scoop of chips and shake
the bejusus out of it to make them "settle in" for more consistency of
burn. This is just another way of saying more consistency of porosity for
airflow. We also did a few burns where we would put a handful of coarse
sawdust in a spot to see how it inhibited the migration of the pyrolysis
front. Of course, it would inhibit the pyrolysis front since primary air
had trouble getting through the sawdust.

Consistency of particle size is important and that is a parameter we could
ask landscape bark suppliers to work on. If you have ever been to a lumber
mill that has chipping operations, size consistency is simply a matter of
putting in the appropriate screens to separate particle sizes. If however,
we use woodchips where there is some inconsistency to particle sizes, and
pyrolysis proceeds as we alluded to, we simply need to understand that and
accommodate that in our analyses.

That phase of our research is almost a side issue that was easy to do for a
quick and dirty experiment to understand what was happening. It could and
should be repeated just to document what was happening. At the time, both
Paul and I were on our knees with our faces close to the tempered glass in
the heat shield. It was fascinating to watch something we did not expect.
At one point Paul looked at me and said "Blimey!" I looked at him and said
"What the hell!".

I too would expect this to happen in larger diameter TLUD's. But I think it
is mostly a function of inconsistent particle sizes.

Unfortunately, I was not able to snip the videos I had on hand in to 20
second quickies to show people what was going on at ethos. If I have the
opportunity, this next year I think I will take some videos of the entire
process. This will include falling of smaller particle sizes, development
of the small channels of pyrolysis, the evolution of tars and water vapors
inside the glass, the passage of the main pyrolysis front, and the
combustion of the tars from the glass as the front proceeds past the Pyrex
dish. Two things we found interesting were unusually large particle sizes
created channels that were often visible in the finished biochar after a
burn cooled. The small channels created by the cascading small woodchips
were never visible after the burn cooled. It is tempting to create a TLUD
and do a burn that would extend the main pyrolysis front to below the Pyrex
glass, and then flood the TLUD from the bottom with water to capture the
small channels that we saw. This would extinguish pyrolysis and show us if
the small channels are a surface effect or extend throughout the feedstock.
I suspect, at this point, the small channels are found throughout the
diameter of the feedstock.

By the way, the temperature tolerances of the Pyrex dish are greatly
exceeded when used in a TLUD. Simply avoid drastic temperature changes and
let the temperatures build slowly and then cool slowly. Once, without
thinking about it, we dropped the wet carpeting in on the biochar to stop
pyrolysis. The wet carpeting hit the Pyrex glass and it shattered
instantaneously.

Norm

On Wed, Feb 1, 2017 at 7:13 AM, Ronal W. Larson <rongretlarson at comcast.net>
wrote:

> Andrew:  cc list and Norm
>
>         As Norm and I talked, we came to the same conclusion.  But it is
> and was a remarkable video to see ANY downward flowing gases.  I’d like to
> know if anyone with computational fluid dynamic simulation capability can
> report on this being observed as well - and whether they can report any
> negative implications, etc.
>
>         Presumably this could suggest that larger is either better or
> worse.
>
>         Thanks for the feedback.   Your own char-making was with very
> large diameters.  Might you have seen anything similar?
>
> Ron
>
>
> > On Feb 1, 2017, at 2:22 AM, Andrew Heggie <aj.heggie at gmail.com> wrote:
> >
> > On 31 January 2017 at 17:04, Ronal W. Larson <rongretlarson at comcast.net>
> wrote:
> >
> >> 2.   As an example of an unexpected unknown that I saw for the first
> time
> >> yesterday:  Norm Baker is doing some extremely strong work on very large
> >> TLUDs, and he gave an outstanding talk on his 16th model (called “Ring
> of
> >> Fire”).  He later showed me parts of a 1.5 hour run with a pyrex window
> >> inserted into the side of his barrel.  Neither of us know how to
> explain the
> >> clear picture of pyrolysis gases (smoke) traveling DOWN near this window
> >> (mostly - not periodically).  Anyone else seen this?  (pretty hard
> without a
> >> window). Clearly the overall gas flow is upward - as he produces plenty
> >> (35%) of high quality char.
> >
> > Is this not a simple convection effect, the  outside of the  barrel
> > being cooled by ambient air whilst the core is at ~600C?
> >
> > I would expect this effect to be more pronounced the  larger the
> diameter.
> >
> > I doubt it affects the burn much unless it causes a "fold" in the
> burning layer.
> >
> > Andrew
> >
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