Scientist can identify pyrolytic carbons but I don’t recall what evidence has
been put forth for TP soils. Christoph Steiner and others have described a
smoldering process of burning crop residues in the Amazons. Various
presentations have shown that some Amazonian cultures had a keen understanding
of the use of the char and ash from one crop and how it should be applied to
improve the productivity of another crop. (See Lehman et. al. eds., Amazonian
Dark Earths, Woods et. al. eds., Amazonian Dark Earths: Wm Sombrek’s Vision).
The anthropological presentations at the 2010 IBI conference in Brazil did not
point to pyrolysis (i.e. low temperature) as a principal tool. It was likely
high temperature combustion, or if smoldering, poor combustion rather than
pyrolysis that created the bulk of the char which, when composted with other
organic matter and blended in the soil became terra preta (Fischer and Glaser
2012, Synergisms between Compost and Biochar for Sustainable Soil
Amelioration).
The value of high temperature versus low temperature char and even pyrolyzed,
gasified or combusted char, gets blurred when you leave it to process in the
physical, chemical and biological environment of the soil. We have yet to see
complete analyses of the impact of the microbiology on the biochar, mineral and
organic mix influenced by the plants. Depending on the metric used we have seen
better results from composted high temperature gasification chars than from
blended low temperature pyrolysis chars. We have even seen negative effects of
some low temperature chars on biological activity. Those results may seem
counter to our understanding of earlier research (e.g. Lehman et. al.) You also
have the variable of woody feedstock vs grasses. These have very different
cellulosic, lignin and ash compositions. There have been unexpected results
with water holding capacity chars from grasses vs. woody chars.
It’s just not that simple. Take any char and a smart gardener and they will
learn to use it appropriately by mixing, or promoting an environment for,
nutrients, other organics, organisms and soils. (See for example, Biochar in
Horticulture, 2012, Horticulture Australia)
Let’s learn how to use all the chars productively and then let the scientists
analyze the successful recipes to come up with theories about temperatures,
organisms and chemistry. (One of my favorite references for learning about the
chemistry of why we cook the way we do is Harold McGee’s On Food and Cooking.)
Tom
From: Stoves [mailto:[email protected]] On Behalf Of Kevin
Sent: Monday, December 10, 2012 7:25 PM
To: Discussion of biomass cooking stoves; Alex English
Cc: Discussion of biomass cooking stoves
Subject: Re: [Stoves] Equipment required for testing stoves
Dear Ron
Would you agree that the Amazonians made Terra Preta with low temperature char?
If so, are there any test results to show that an "intermediate temperature
char" would give better results than the "low temperature char?"
I seem to recall that "high temperature char" and/or "activated char" gives
inferior results in a biochar application. Does this impression make sense to
you? If so, is there a "preferred char making temperature range"?
Best wishes,
Kevin
----- Original Message -----
From: [email protected]
To: Alex English <mailto:[email protected]>
Cc: Discussion of biomass cooking stoves
<mailto:[email protected]>
Sent: Monday, December 10, 2012 9:46 PM
Subject: Re: [Stoves] Equipment required for testing stoves
Alex:
Thanks:
I see only a few remaining questions related to the thermocouples. My
interest is only in being able to report to the soil scientists the temperature
at which the char was produced.
Q1. I think we should be able to say that a time average of a central
thermocouple measurement showing a slight drop over time of the highest numbers
is a pretty good estimate - that could be reproduced for "any" similar "flaming
pyrolysis" approacd. The properties (pH, surface areas, labile component, etc)
of such char should be compared (a Master's thesis?) with char produced via
other means. I think Nat Mulcahy's non-flaming pyrolysis approach can produce
varying temperature char. An all-electric heating approach in any oxygen-free
environment , operated at different temperature should also be used to compare
the char properties with those from stoves. Maybe that data is already out
there??
Q2. I think there could be some influence of the initial fuel moisture
content. Do you (anyone) have an opinion on that? I am trying to avoid
having to always measure temperatures, but still be able to give an indication
of the "likely" char temperature, by knowing how long a specific volume or
weight of fuel lasted.
Q3. I wonder if the char temperature as measured by a thermocouple system
like yours would also be a function of the fuel itself (species,
characteristic size, shape, etc.)
Q4. I am pretty sure that the top and bottom char will be significantly
different in a typical cooking cycle, where a very high flame temperature is
desired at first (affecting only the top part of the fuel load), and then a
much lower temperature desired later (affecting only the lowest portion of the
fuel load). My question, for anyone, is whether an average temperature is at
all valuable, if the average (obtained from the total duration of the
pyrolysis) covered a wide range of production temperatures. Actually I have
heard so many different opinions on the best production temperature - maybe a
mixture of char temperatures might be an advantage. Thoughts?
Ron
_____
From: "Alex English" <[email protected]>
To: [email protected], "Discussion of biomass cooking stoves"
<[email protected]>
Sent: Monday, December 10, 2012 4:38:56 AM
Subject: Re: [Stoves] Equipment required for testing stoves
Ron,
On 09/12/2012 8:33 PM, [email protected] wrote:
Alex etal
Thanks for the cite. I think I understand most of the plot - which was of
amazing duration!. I am especially amazed at how uniform (and high) the flame
temperature was in the late time plot, even as the other plots were dropping.
It is a very steady gas producer. Conditions are constant except for the
distance and path composition between the pyrolysis front and the burner. If it
can be done over 100cm then why not 200 or 300.
a. Since you have this one from 2000, you probably have quite a few more
- from which I/we might extract a good bit more information/ Any other similar
plots around that you can post?
No I don't.
b. I am surprised that the "pyrolysis gas temperature" was so much lower
than the temperature of the char. Where was the probe for this measurement -
and had there been some mixing of secondary air at this point?
No mixing of secondary air at that point. That occurs in and above in a 5cm
burner mixing pipe. The tmperature difference is largely due to the nature of
unshielded thermocouples in gas.For the most part thermocouples radiate away
heat according to the temperatures of the surfaces that make up the sphere
around them. A thermocouple buried in the pellets that are all carbonizing at
700C will give a fairly accurate measurement. A thermocouple in the gas above
the top of the pellet bed will radiate to the pellet bed and, in this case the
uninsulated container walls. The more that pellet bed shrinks the larger the
portion of the radiant sphere that is the cool container walls. The larger the
thermocouple, the greater the radiant cooling , the lower the measurement. The
higher the temperature the greater the radiant loss, to the forth power. All
the gas is also radiating and convecting heat to the container walls. So there
are two reasons for a slow drop in gas temperature, and one reason for not
trusting either. The same holds true for the absolute value of post combustion
measurement.
There are gas-aspirated pyrometers which shield a thermocouple with ceramic
layers that approach gas temperatures and give better numbers. We will soon be
using an 10 footer to probe the chain grate stoker gasses in carbonizer-
pyrolysis-gasifier mode.
Grate fun.
c. What is the present disposition of this equipment?
Its in the recoverable bone yard. I should have shown it to Crispin when he was
here.....or perhaps not:)
Alex
Nice work
Ron
_____
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