[Terrapreta] Charcoal properties

[Michael J Antal]

Hi Tom: charcoals with high VM content always have low surface areas and low pore volumes.  Activated carbons always have low VM content.  But the measurement of surface areas by gas sorption is very time consuming.  Note that SEM only reveals the largest macropores.  The micropores, that are far too small for SEM, impart surrface area to the char.  Regards, Michael.

----- Original Message -----
From: Tom Miles <tmiles at trmiles.com>
Date: Sunday, March 4, 2007 6:22 pm
Subject: RE: [Terrapreta] Charcoal properties
To: "'Michael J. Antal, Jr.'" <mantal at hawaii.edu>, terrapreta at bioenergylists.org

> Michael, Danny,
> 
> Is there a difference in the morphology of wood and agricultural 
> charcoalswith different volatile contents such as pore size or 
> porosity? Can it be
> seen in scanning electron microscopy (SEM) photos or measured with
> absorption techniques?
> 
> See images in J. Skjemstad, "Charcoal Carbon in US Agricultural Soils"
> linked at:
> http://terrapreta.bioenergylists.org/Skjemstadus
> 
> Thanks
> 
> Tom
> 
> 
> 
> 
> -----Original Message-----
> From: terrapreta-bounces at bioenergylists.org
> [mailto:terrapreta-bounces at bioenergylists.org] On Behalf Of 
> Michael J.
> Antal, Jr.
> Sent: Friday, March 02, 2007 12:29 PM
> To: terrapreta at bioenergylists.org
> Subject: [Terrapreta] Charcoal properties
> 
> Dear friends: terra preta is fascinating in part because it 
> involves so many
> disciplines.  My viewpoint is that of a fuel scientist/chemical 
> engineer.My laboratory produces well-characterized charcoals for a 
> wide variety of
> research endeavors, including carbon fuel cell studies, metallurgical
> charcoal applications, activated carbon production, and terra 
> preta research
> (with my colleagues Dr. Goro Uehara, Dr. Jonathan Deenik, and Tai 
> McClellanin the University of Hawaii’s College of Tropical 
> Agriculture and Human
> Resources).  With this message I wish to call your attention to the
> elementary properties of charcoal that I think about when I am 
> producing a
> charcoal for one of our research endeavors.
> 
> Both the feedstock and the process (i.e. pyrolysis) conditions 
> influence the
> properties of the charcoal product.  For example, oak wood has 
> little ash;
> consequently its charcoal also has little ash.  On the other hand, 
> ricehulls have much ash (nearly pure silica), and so does its 
> charcoal.Likewise corncobs produce a highly macroporous charcoal, 
> whereas sucrose
> charcoal lacks a macroporous structure.  But unfortunately, the 
> propertiesof the feedstock do not completely determine the 
> properties of the charcoal.
> For example, if pyrolysis is carried out at a high temperature, 
> some of the
> volatile ash components leave the charcoal.  In our work it is not 
> unusualto find that the charcoal contains as little as 20% of the 
> amount of ash
> that we expected on the basis of the feedstock ash content.  One 
> carboncompany produces an ash-free carbon for metallurgical 
> applications by simply
> heating a fossil carbon (usually coal) to such a high temperature that
> virtually all the minerals in the fossil carbon vaporize.
> 
> Likewise the pyrolysis temperature (usually called the “heat treatment
> temperature” or HTT) exerts a big influence on the properties of 
> the carbon.
> Fuel scientists employ proximate analysis to measure this 
> influence.  Let’s
> be clear: there is nothing approximate about proximate analysis!  
> Proximateanalysis determines the moisture content (mc), volatile 
> matter (VM) content,
> fixed carbon (fC) content, and ash content of a charcoal (or 
> fossil carbon).
> A good barbeque charcoal will have a VM content of 25 – 30%, 
> whereas a
> charcoal destined for metallurgical use often has VM content below 
> 10%.Increasing HTT lowers the VM content of the charcoal, but 
> there is not a
> simple relationship between the HTT and the charcoal’s VM content. 
> Why?
> The simplest explanation is that the thermocouple used to measure 
> the HTT
> measures the temperature of the pyrolysis environment: it does not 
> measurethe temperature of substrate during pyrolysis!  Pyrolytic 
> reactors designed
> to maximize “oil” (or gas) yields - and minimize the charcoal 
> yield - employ
> high heating rates.  Under these conditions the pyrolysis 
> reactions are
> endothermic; consequently there is a large temperature difference 
> betweenthe charcoal and its environment (i.e. the temperature of 
> the charcoal can
> be hundreds of °C lower than its environment).  On the other hand, a
> pyrolytic reactor that is designed to maximize the charcoal yield 
> will evoke
> exothermic pyrolysis reactions in the substrate, since the 
> reactions that
> form charcoal are exothermic.  In this case the temperature of the 
> charcoalcan be much higher than its HTT.  I can provide some 
> interesting papers on
> this subject for anyone who is interested.
> 
> In summary, both the feedstock and the pyrolysis process conditions
> influence the properties of the charcoal product, but they do not 
> determinethe properties (i.e. knowing the feedstock and process 
> conditions is not
> enough to predict the properties of the charoal).  The only way to 
> determinethe charcoal’s properties is to actually measure them.  
> We do proximate
> analyses of all our charcoals.  Often we do gas sorption 
> measurements to
> determine the carbon’s surface area and pore volume distribution.  
> Sometimeswe obtain an elemental analysis of the carbon, or an 
> analysis of its ash
> content.  For our carbon fuel cell work we measure the carbon’s 
> electricalconductivity, and with colleagues in the Hungarian 
> Academy of Sciences we do
> temperature-programmed desorption of biocarbons used in our fuel 
> cell.  We
> have done XRD, NMR, ESR, and MALDI-TOF MS analyses of some of our 
> charcoals.We have plans to expand our analysis capabilities into 
> other areas soon.
> 
> Tests by my colleague Professor Goro Uehara and his co-workers in 
> CTAHR have
> shown that the addition of some charcoals to the soil can be 
> harmful to
> plant growth.  Our analyses of the properties of this “harmful” 
> charcoalindicate that it would have been perfect for barbeque.  
> This illustrates the
> dangers of working with an uncharacterized charcoal purchased from 
> yourlocal grocery store.  Professor Uehara and his co-workers will 
> have more to
> say on this subject in the near future.  In the meantime I 
> emphasize that
> our understanding of charcoal’s beneficial and detrimental effects 
> on plant
> growth must rest (in part) upon measurements of the charcoal’s 
> properties.This is not an easy job and there are no short cuts 
> that I can find.
> 
> Like most of you, at present my terra preta research is not 
> funded, so the
> best I can do is provide well-characterized charcoals to my 
> colleagues here
> at UH.  In the future I may have the resources to provide well-
> characterizedcharcoals to other terra preta researchers as well.  
> I will let you know
> when this becomes possible.
> 
> Best wishes, Michael.
> 
> Michael J. Antal, Jr.
> Coral Industries Distinguished Professor of Renewable Energy Resources
> Hawaii Natural Energy Institute School of Ocean and Earth Science and
> Technology (SOEST) 1680 East-West Rd., POST 109 University of 
> Hawaii at
> Manoa Honolulu, HI 96822
> 
> Phone: 808/956-7267
> Fax: 808/956-2336
> http://www.hnei.hawaii.edu
> 
> 
> 
> 
> 
> 
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