[Terrapreta] micro-porosity: char vs. charcoal

[Greg and April]

I think that some of this has to do with what was covered back in Dec under the heading of "torrefaction vs. Carbonization "    ( starting on 12/11/07 ).

The amount of time a piece of char is held at a given temperature appears to be as critical to the amount of micro-porosity as the % of VM left in the char.

At higher reaction temperatures and smaller the piece of starting material, the inner core temperature reaches the critical time/temp faster, thus developing the porosity in a shorter amount of time.

This is not to say that this can not be done with lower temps or longer charring times - quite the opposite in fact.

Take a look at this web page:

http://www.twinoaksforge.com/BLADSMITHING/MAKING%20CHARCOAL.htm

and in particular the pic below of a piece of char that on an entirely different web page ( that I can't find right now ) the maker states that he believes it to be ' almost over done ':

http://www.twinoaksforge.com/images/charclose.JPG 

That looks to me, to be very porous in nature, and it was made with a kiln made from steel 55 gal drums, cinder blocks, and scrap sheet rock.

Greg H.



  ----- Original Message ----- 
  From: Gerald Van Koeverden 
  To: Terra Preta 
  Sent: Thursday, January 03, 2008 12:30
  Subject: [Terrapreta] micro-porosity: char vs. charcoal


  Micro-porosity seems to take extended periods of time to develop properly...so that charcoal as produced in reactors will probably be very porous.  But what about char?  It only spends a second or two in the furnace...?

  Gerrit

  At 250ºC for heating times up to 144 hours, and at 300ºC for heating times up to 24 hours, the micropore volume remained below the detection level. At 300ºC for heating times of 48 and 72 hours, the micropore volumes were 0.043 and 0.072 cm3/g, respectively. At 350ºC, the micropore volumes were <0.001, 0.070, and 0.222 cm3/g when heated for 1, 8, and 24 hours, respectively. At 400ºC, the micropore volumes were <0.001, 0.201, and 0.306 cm3/g when heated for 1, 8, and 24 hours, respectively. At 500ºC, the micropore volumes were 0.079, and 0.209 cm3/g when heated for 1, and 8 hours, respectively."

  from: Changes in Composition and Porosity Occurring During the Thermal Degradation of Wood and Wood Components

  By David W. Rutherford, Robert L. Wershaw, and Larry G. Cox

     http://pubs.usgs.gov/sir/2004/5292/#N100B0



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