Re: flash carbonization of biomass

["Ron Larson" <ronallarson@qwest.net>]

Andrew:  Sorry for the long delay in responding to your very thoughtful
response of Dec. 29 (to mine of 12/25) on charcoal-making (including CM
stoves).


1.  You said (after quoting me on Antal's new discovery):
    ".... I thought the major achievement of his process was the
high yield of carbon as opposed to high volatiles charcoal.  This
should have good implications for low ash charcoal for use in high
grade metal refining......"
    (RWL1):  I need help on this from both yourself and Mike.   Are you
suggesting 1) that Mike does not have high volatiles and 2) that high carbon
means low ash, and/or 3) that high grade metal refining requires low ash.
My confusion comes from not knowing enough about different grades of
charcoal and their uses - which I am sure both you and Mike do.
    I once gave some stove-produced charcoal to a woman using it for burning
incense - she didn't like it as it (I think) had too much "high volatiles".
On the other hand, I have heard others on this list say that high volatiles
was an advantage in lighting charcoal for cooking.   Could you give us a
tutorial on charcoal - especially as desired in rural areas for cooking?

2.  After another quote from me asking Mike about small scale pressurization
for cooking, you said:  "
    "High pressure is very limiting as the vessel size increases, which has
implications for mechanised loading. What a lot of people seem to fail
to appreciate is that charring even at low pressures can be a quick
process."

    (RWL2):  Your response is (naturally) dictated by the very impressive
large scale (one person operation but megascale compared to a cook stove)
work that I had the privilege to see in operation in October - including
mechanised loading.  But my view of a "pressurized cook stove" is one that
is of course hand-loaded and is smaller than the usual pressure cookers I
see.  While agreeing with you completely, I'd like your opinion on whether
there is some fundamental reason to forget working on pressurized
small-scale cook stoves.
    On your second point, I guess I agree - but need clarification.  First,
we are generally not interested in speed of charcoal-making with a CM stove.
My interest in pressurization is more in being able to get a superior and
higher quantity of charcoal.   Maybe to also use the pressure productively
(perhaps).

3a.  The third quote of mine you used was directed to "waste" heat
utilization.  You referred to our "cocoa tin" discussion  and said:
    "The drawback of this retort heated from outside approach is that the
heat
transfer can only take place through the walls of the vessel and
thence from char particle to unreacted biomass, but it does make use
of the offgas being flared in a supporting fire. There is likely to be
gradation of the extent of charring from outside to middle."
    (RWL3a)  This also was the case with the similar CM stove developed by
Professor Grover in India.  It was not rapidly controllable either.  The
retort's principal advantage is that it effectively prevents air entry.
(but not so easy to obtain complete closure.)

3b.  You continued:  " Traditionally char was made in kilns, basically an
air starved fire in
a container, I believe the idd stove is a kiln in this respect. The
kiln has a heat transfer advantage in that the offgas and combustion
products circulate around as yet unreacted biomass. The disadvantage
is that valuable char is consumed preferentially to offgas to achieve
this."
    (RWL3b):  Again, a need for further discussion. 1.) I don't see the
"heat transfer advantage".  The CM UD (or should I say UD CM?  I choose the
former only because it is eawier to say) stove puts about the same about of
hot gas past about the same amount of unreacted wood, it would seem (albeit
a much longer path in a large kiln.   In the CM UD stove, there is a
continued similar parallel gas flow path  whereas in most kilns they seem to
be mostly bottom lit (my reading of the charcoal kiln literature) - and the
air flow is dictated either by moving a number of chimneys or by air holes
that appear as the dirt-covered pit kiln collapses.
    [Oops - after rereading, I realized your comparison was not between a
kiln and a CM UD, but rather between a retort and a kiln.  But I decided to
leave my comment, since it might help someone.)

    2)  There is a charcoal loss in both kilns and the CM UD by combustion -
but I don't believe this is intentional.  Rather, I believe that a
well-managed kiln (and CM UD) continues in operation by virtue of the
pyrolysis reaction being exothermal.  My "proof" of lack of combustion is
that the charcoal from a CM UD is rarely white (except where near the
secondary air source) - no ash.

3c.  You said:  "So one needs to combine the attributes of a kiln's good
heat transfer,
with those of a retort's high char yield. This is the approach Lurgi
used in coal retorts and I believe the Simcoa plant. Mike appears to
be taking the same route and it is also my preferred approach.
....."
    (RWL3c):  I think we probably have a lot to learn from the world of coke
making.  Can you provide some references to the Lurgi/Simcoa processes?  As
I interpret this, you are saying that you prefer the retort to the kiln -
and probably even at the small CM UD stove level.  My reaction is that I
don't know enough about the retort approach to make a choice at the large
scale level (except that most charcoal kilns are venting only - no flaring -
horrible!).  I am bothered by Mkie Antal's response that his approach does
not release a combustible gas - presumably too moist, maybe because he (like
many charcoal makers, including yourself) wants to utilize material of any
moisture content.   I am not willing to accept that the retort approach is
better for cookstove operations - it has a very slow response time.  Do you
agree?  Should I be taking most of your comments in this message as not
applying to cookstoves?

3d.   You said next:  "This is why I am not keen on promoting IDD as a means
of making
charcoal, though I am happy to accept its by product is charcoal that
is produced with little pollution and as such is better than much
charcoal making. I think Tom Reed agrees with me the IDD pyrolysis
front is powered by charcoal burning, plainly this detracts from
charcoal yield. With the advanced clean charcoal making I, and Mike
Antal, are playing with any heat necessary for maintaining pyrolysis
comes from burning some of the offgas and recirculating this hot gas
stream through the char. There are a number of ways of configuring
this, none patentable IMO but nonetheless the actual designs will be
proprietary and subject to confidentiality."
    (RWL3d):  I like your first sentence.  I also believe that rapid
controllability of power output is of high value - as is having a saleable
co-product.
    I invite Tom and others  to comment on the second, which I don't believe
is true - but
I am of course willing to give up some charcoal, even if true.  The CM UD
stove charcoal yield (25%?) is about equal to that from the best of (heavily
polluting) rural charcoal makers.
    There is no single optimization demand on processes with co-products.
So far, the only big drawback I hear is that education is required.  You,
Mike, and I (but not everyone) agree that we should be trying to get rid of
the traditional kiln approach.  Which approach will end up being successful
will depend on faactors we can now probably not even guess.  We have to
fight thousands of years of traditional charcoal making  (and my concerns
are not only that the gases are un-healthy, but also global warming
concerns, and desertifications and deforestation from something that is so
wasteful.
    I understand your and Mike's concerns about confidentiality - and
appreciate all that you do help us with good ideas.

(3e)  The implementation I am looking at uses quite basic technology and
should be possible to deploy in the sort of situations envisaged by
the Karves' project. A benefit to my mind is that this method can
control the temperature and cook time of the biomass to vary the
resulting char from post torrefied wood through Tom's seasweep and up
to metallurgical grades. My interest is in high volatiles charcoal and
I have made material which is friable like charcoal but retains 45% of
the mass of the original dry matter.
    (RWL3e):  Having seen your work up close, I can attest that your steam
drying technology is quite exciting to me - and I understand you  know how
to control temperature - which I don't think we will get to for the rural
stove market.  I would like to hear more about your reason for interest in
the high volatiles market - and whether there is a lesson there for the
rural stove area - both for those producing charcoal briquettes and for
those of us interested in CM stoves (either UD or DD).

Summary -  Andrew.  Thanks for these valuable inputs on charcoal. Always fun
to correspond with you.

     I would still like to hear more from anyone on the subject of higher
pressure as applied to small (order of 1 kg per hour?) CM operations.  I am
leaning against pressurization and retorts, but for reasons of poor
controllability.  Can a retort approach be done at very small scale with
rapid control response and low cost?

Ron







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