[Terrapreta] Long term durability of Low Temp Chars

[AJH]

On Thu, 17 May 2007 16:36:26 -0500, Sean K. Barry wrote:

>Hi AJH,
>
>As I understood Dr. Antal's work (I may be wrong and you can sure check with him), but the charcoal formed during the pyrolysis which he did, was 93-95% fixed carbon (pure carbon, not carbon in volatile matter).

Dr.Antal's process takes place under pressure and high temperature, in
these conditions a chemical equilibrium favours formation of solid
carbon, so soot from thermal decomposition products of the pyrolysis
offgas are deposited back on the char matrix, giving a high fixed
carbon yield. Something similar is claimed for a liquid phase
"pressure cooking" of biomass with a catalyst. 

For most of our purposes these high tech solutions will not be
affordable whereas modified kiln/retort technology giving a high
(possibly 45% of dry matter) solid biochar yield may be affordable but
we still need to establish what portion of the total char in addition
to the fixed carbon we can successfully claim as being sequestered.

>  So, I assume that were that charcoal incorporated into soil, 93-95% of it's weight could be considered as sequestered carbon.

Yes if the biochar is 93-95% fixed carbon but most low temperature
carbonising will be yielding <80% and that is for woody wastes with a
decent proportion of lignin, non woody agri waste with low lignin will
do worse.


>  I hope I did get your point, that not all of the entire weight of charcoal put into soil could be considered as tonnage of carbon, which would then be considered as permanently sequestered.  I'm suggesting that it's 93-95% of the weight of the charcoal (the 5-7% would be part of the molecules of decomposable volatile matter).

Agreed but see above, with simple carbonising techniques as currently
practiced we will not get better than 80% fixed carbon without
sacrificing a lot of yield.

Andrew Heggie