[Stoves] [biochar-policy] Re: Equipment required for testingstoves
Tom Miles
tmiles at trmiles.com
Wed Dec 12 23:22:03 CST 2012
In my experience burning crop and woody residues in developing countries, including Brazil, and in the developed world, I have observed that much of the charcoal is created by most of the residues burning at higher temperatures in an oxidative, combustion, environment rather than through mostly pyrolysis. A pile of residue often burns in an “oven” of fuel with a high temperature core and a cool outside. The fuel is clearly pyrolyzed but the fire eventually breaks through leaving a flaming mess of charcoal and ash. Grasses heat and burn quickly due to thin stalks. The charcoal yield from a hot fast prairie fire is very low. Nonetheless they do leave charcoal. We even designed devices to break up the charcoal and quench smoldering thatch when we were open burning large grass fields. This is likely the miscellaneous mess that Amazonian farmers left in their garden plots around their houses which mixed with other wastes. The areas of terra preta and terra mulata surrounded the dwellings and were surprisingly smaller than we imagine when we hear of the vast areas they encompass. When we visited these sites I found that reality seemed to match what I had imagined from reading the anthropology that I have previously cited. After working a little with pyrolysis chars, gasification chars and combustion chars it is easy to see how they would blend well with nutrient courses to compost. So the Fischer-Glaser theory of biochar creation through composting charcoal makes a lot of sense to me.
Tom
From: Stoves [mailto:stoves-bounces at lists.bioenergylists.org] On Behalf Of Kevin
Sent: Wednesday, December 12, 2012 8:11 PM
To: Discussion of biomass cooking stoves; biochar-policy at yahoogroups.com; Ron Larson
Subject: Re: [Stoves] [biochar-policy] Re: Equipment required for testingstoves
Dear Tom
----- Original Message -----
From: Tom Miles <mailto:tmiles at trmiles.com>
To: biochar-policy at yahoogroups.com ; rongretlarson at comcast.net ; 'Discussion of biomass cooking stoves' <mailto:stoves at lists.bioenergylists.org>
Sent: Wednesday, December 12, 2012 2:18 AM
Subject: Re: [Stoves] [biochar-policy] Re: Equipment required for testingstoves
Charcoal burns in direct contact with air well above 600 C so any fire at the charcoal stage would be above 500-600C.
# There are two issues here:
1: The temperature at which charcoal burns
2: The average temperature experienced by the charcoal remaining after it was produced.
Certainly, charcoal can burn at temperatures well above 600C in air. Surface temperatures of burning char can be very different than the core temperature. Consider a flash fire, with high surface temperatures for a short time. Core temperatures of the wood or char remaining can be very much lower. This is a very complex heat transfer problem... unsteady state three dimensional heat transfer to bodies of irregular shape, with change in phase. What is important is the properties of the "unburned charcoal" remaining for potential use as biochar.
Higher temperature oxidized chars have great adsorption properties. They compost readily. Great way to make terra preta.
# Certainly, such "designer chars" could have very superior properties, but at greater cost. The "bottom line" for the Farmer or Grower is the "Benefit/Cost Ratio." Additionally, there may be special soil conditions that such "designer chars" can handle better than "regular biochar" that works adequately well for most common soil conditions.
Best wishes,
Kevin
Tom
From: biochar-policy at yahoogroups.com [mailto:biochar-policy at yahoogroups.com] On Behalf Of Kevin
Sent: Tuesday, December 11, 2012 8:18 PM
To: rongretlarson at comcast.net; Discussion of biomass cooking stoves; biochar-policy
Cc: Alex English; Tom Miles
Subject: [biochar-policy] Re: [Stoves] Equipment required for testing stoves
Dear Ron
----- Original Message -----
From: rongretlarson at comcast.net
To: Discussion of biomass cooking stoves <mailto:stoves at lists.bioenergylists.org> ; biochar-policy <mailto:biochar-policy at yahoogroups.com>
Cc: Alex English <mailto:english at kingston.net> ; Kevin Chisholm <mailto:kchisholm at ca.inter.net> ; Tom Miles <mailto:tmiles at trmiles.com>
Sent: Tuesday, December 11, 2012 2:09 AM
Subject: Re: [Stoves] Equipment required for testing stoves
Lists (adding biochar-policy also), Kevin, Alex, Tom (who I add, because he speaks Portuguese and might have caught an answer when we were in Manaus a few years ago)
See below
_____
From: "Kevin" <kchisholm at ca.inter.net>
To: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>, "Alex English" <english at kingston.net>
Cc: "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
Sent: Monday, December 10, 2012 8:24:34 PM
Subject: Re: [Stoves] Equipment required for testing stoves
Dear Ron
Would you agree that the Amazonians made Terra Preta with low temperature char?
[RWL1: I think others may have an answer - maybe based on spectroscopy. I will start looking but don't know that field well enough to know what others may have concluded about these ancient soils. I fear that 500 years (minimum) in soil may hide the initial character that we can readily see in a University setting. I know from being in several Amazonian biochar "pits" that it is pretty hard to find a piece big enough to test. I think it entirely possible that char left over from simple three-stone fires could have been made at 500-600 C (or higher). Would you call those temperatures high or low?]
# KC1: Obviously, I am speculating, but I would speculate that Terra Preta was made with charcoal from a number of sources:
1: Char residue remaining after charring or burning of "wood waste from initial jungle clearing.
2: Char residue remaining from charring or burning of agricultural waste and weeds
3: Char and ash residue from cooking fires and possibly smudge pots
4: Organic fertilizer supplements, from humanure, composted food scraps, and probably dredgings from oxbow lakes.
I would consider 500-600 C to be a "low temperature char." I haven't seen any references to the existence of bellows technology in ancient Amazonia, that would be necessary to produce significantly higher temperatures.
It is likely that the Amazonian Terrapretians would have quickly noticed if such low temperature char additions to their agricultural practises were causing poor results. If that was the case, they likely would have taken steps to avoid application of char to fields. They would be looking for short term benefits or harm. Given that they used char on a widespread basis, and that it was basically low temperature char, it would thus seem that Terra Preta worked with low temperature char, and it worked relatively quickly, not requiring an aging period of several years.
If so, are there any test results to show that an "intermediate temperature char" would give better results than the "low temperature char?"
[RWL2: I think that people like Dr. Johannes Lehmann and Evelyn Krull may be getting to an answer for some specific soil and species. I keep looking for it.
My note below to Alex was to make it easier for users to know what is being used Some of my favorite biochar scientists like Drs. Julie Major and Christoph Steiner were forced to use char bought off the side of a remote Amazonian road.
I haven't seen any data emphasizing tests with a range of temperatures. Drs. Stephen Joseph (low) and Hugh McLaughlin (high) recommend different temperature regimes.]
#KC2: Is it perhaps likely that "basic low temperature biochar" is good for general agricultural applications, but that intermediate and high temperature chars may be better for addressing special agricultural problems.
I seem to recall that "high temperature char" and/or "activated char" gives inferior results in a biochar application.
[RWL3: How about giving a cite for that?]
#KC3: Unfortunately, I cannot point to a specific cite.
Does this impression make sense to you?
[RWL4: No - certainly not as a universal truth/]
#KC4: Universal truths are scarce and hard to find. :-) If you put yourself in the circumstances of an Amazonian Terrapretian of 3,000 years ago, what would you do differently?
If so, is there a "preferred char making temperature range"?
[RWL5: I am sure that it depends a lot on the intended recipient soil - and probably on the plant species. #KC5:1 Certainly! Jungle woods can vary in density from balsa at about 10 lb/cubic foot, to Lignum Vitae, at about 68 pounds per cubic foot.
My focus in this exchange below with Alex is to give soil researchers and stove users a better idea of even getting close to knowing what char-T they are using.
#KC5:2 Char making temperature is easy to determine. It should thus be very easy to determine the importance, or lack thereof, of char making temperature. It would be very good to know this, to reduce variables in a test analysis.
There is a good bit of information out there relating pH to production temperature - but pH also depends on fuel size and ash content and pH changes over time. Who knows what else leads to a "preference"?
And we also hear from Dr. Spokas that what happens after char production is maybe as much or more important. I think it is absolutely amazing that we hear so many good reports (and few bad reports) when we know so little even about the char-production temperature - and even the wood species, etc, etc, etc. Ron]
#KC: With that many "good reports" and so few "bad reports", and with so many char variables, this would tend to downplay the importance of char variables. The "bad reports", presuming that they were competently done, could provide very important insights into what works, and what doesn't.
Best wishes,
Kevin
Best wishes,
Kevin
----- Original Message -----
From: rongretlarson at comcast.net
To: Alex English <mailto:english at kingston.net>
Cc: Discussion of biomass cooking stoves <mailto:stoves at lists.bioenergylists.org>
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" <english at kingston.net>
To: rongretlarson at comcast.net, "Discussion of biomass cooking stoves" <stoves at lists.bioenergylists.org>
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, rongretlarson at comcast.net 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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