[Stoves] Adding value to charcoal from TLUD stoves

[Julien Winter]

Dear Stovers;

We have been discussing whether or not the biochar made in a TLUD can be
improved by allowing the TLUD to continue operating for a few minutes in
the char-gasification phase after the flaming pyrolysis phase has
completed.  In the discussion, some research findings of Dr. Peterson were
discussed.

I wrote to Dr. Peterson, to let him know that is work was being discussed
on the "Stoves" mailing list, and to get his input.  He wrote me back, and
agreed to have his comments forwarded.

Our communications were as follows:




DR. PETERSON'S REPLY

Dear Dr. Winter,

Thank you for the comments and alerting me to the discussion happening on
the stoves at lists.bioenergylists.org. I will try to answer your questions as
best as I can. Please see my responses in red text below:

I am open to discussion on this topic as it is very interesting yet
complex. Thanks for your interest and I hope my responses can add to the
discussion.

Regards,

Steve

DR. PETERSON INSERTED HIS COMMENTS INTO MY (Julien) EMAIL TO HIM.  MY EMAL
AND HIS RESPONSES ARE AS FOLLOWS:

JULIEN'S EMAIL:

Dear Dr. Peterson;

Re: Peterson, SC; Jackson, MA.  2014.  Simplifying pyrolysis: Using
gasification to produce corn stover and wheat straw biochar for sorptive
and horticultural media.   Industrial Crops and Products: 53, 228-235.

I read your above paper with a lot of interest.  It is very good and very
timely, because as you are obviously aware, there is a big effort to
promote gasifier cookstoves around the world.  Biochar produced from these
stoves is a critical asset, yet we know virtually nothing about its quality.

Your work has come up for discussion on  stoves at lists.bioenergylists.org.

First, Rajan Philip contributed the observation that volatiles accumulating
in the char above the flaming pyrolysis front could be revolatilized if the
TLUD was allowed to progress for a few minutes in char-gasification mode,
after the flaming-mode had finished.

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-March/008494.html


DR. PETERSON'S REPLY:

Let me start by saying that during a gasification run using the TLUD,
ideally we want to stop the process right after the last amount of
feedstock near the bottom of the inner cylinder has been converted into
char, because after the flame goes out at the top of the TLUD, you no
longer have a controlled area of combustion. So after the top flame goes
out, and oxygen is available to the entire column of char (which is still
typically 200-600 deg C), the char will begin to combust and turn to ash.
We have to deal with this to some extent even after dumping our biochar
into the stainless steel trash can after the process is finished because
there is air in the trash can when we dump the char in. We quickly seal it
with the lid, but we will have some small amount of biochar turn to ash
using this process. Our goal during this procedure is to try and minimize
ash.

I really don't see how letting the TLUD sit there after the top flame goes
out would help matters, because the reason the top flame goes out is
because the combustible gases present in the feedstock have been exhausted
once the pyrolysis front has moved completely through it. After that point
the char in the inner cylinder would cool. The combustion flame at the top
of the TLUD helps to burn volatiles off, so I would hypothesize that
letting the TLUD sit there after the flaming mode was finished would
increase volatiles as the biochar cooled, since they are no longer being
combusted from the top of the stove.


JULIEN'S EMAIL:

In the back of my mind, I remembered having seen your recent work, so I
contributed the following:

http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/2014-April/008498.html

In your work, you show how the temperature declines in the char above the
flaming pyrolysis front, which could (?) make the condensation of tars
possible.

>From your paper, it seems to me that you stopped reaction as soon as the
flaming pyrolysis front hit the grate.


DR. PETERSON'S REPLY:

This is correct. I admit I am not an expert on tars and our focus was on
the biochar and not tar when we wrote the Industrial Crops & Products
paper. Can tars condense at 300 deg C? Because the temperature at all
points along the vertical length of the TLUD, during the entire run, never
usually got much lower than that (shown in Fig. 2 of that paper).


JULIEN'S EMAIL:

When you analyzed the biochar, you found lower surface area in gasifier
biochar than retort biochar.

To my mind, your work helps to support the hypothesis that tars can
condense in gasifier biochar, and occlude porespace.


DR. PETERSON'S REPLY:

It very well may. The other phenomena that I think may be occurring as a
function of temperature is that structural integrity of the biochar
decreases as pore space increases and thus occlusions may be due to the
biochar structure itself fracturing and 'caving in' to occlude surface
area. It's a very complex problem, and may be feedstock dependent.

JULIEN'S EMAIL:

However, I wonder if you would like to comment on Rajan Philip's hypothesis
that a short period of char gasifation can revolatilize the tars?

DR. PETERSON'S REPLY:

See first response above.


END OF COMMUNICATION



-- 
Julien Winter
Cobourg, ON, CANADA
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