[Terrapreta] Economics of biochar

[AJH]

On Tue, 8 Jan 2008 16:14:45 -0600, Sean K. Barry wrote:


>What happens if the number for the yield of charcoal to dry weight of the feedstock is raised to ~30-40%, approaching stoichiometric?  I've read that the energy content of the charcoal goes to maybe ~60%?

I'm not sure what you mean as the terminology looks a bit strange but
yes you can control the composition of the charcoal by changing the
temperature and if you made a low temperature char with a high yield,
say from a softwood stem, then the yield may be 45% and the energy
retained in the char may be ~24MJ/kg so that would be in the 60% ball
park. My experiment was a simple carbonisation experiment with no
controls on temperatures.
>
>In your pyrolysis reaction (carbonization) 2.97 MJ / 18.8 MJ = ~16% of the total energy is in the 11% yield of charcoal ?
>
>Or, is it 2.97 MJ / 15.4 MJ = ~19% of the total energy is in the 11% yield of charcoal ?

I calculated 19% but it was based on guestimates for the initial and
final calorific contents.

>The speed of the pyrolysis reaction is driven by the particle size and the ratio of feedstock to oxidant (lambda).

To my mind that is more a stage of combustion rather than
carbonisation.
>
>Most carbohydrates in biomass have a C:H:O ratio of 1:2:1, like sugar C6H12O6. 

not really because you are dealing with mixtures, there's been a
recent posting on [stoves] suggesting C3H8O3 is a better model for the
global equation.

>I am looking at a design for a down-draft gasifier.  The feedstock and the air-oxidant are both entered through the top of the reactor.  The pyrolysis reaction proceeds below inside a charcoal bed.  It lies in a reaction zone near the top, but below fresh feedstock and above a cooler, moving (downward) charcoal bed.  The charcoal and "producer" gas both exit out the bottom of the reactor.

Isn't this the classic stratified downdraught design that needs
tweaking with tuyeres to work properly? Sure it will make a high char
ash but the whole thing is driven by the consumption of char, that's
what provides the temperature to crack the pyrolysis products. We
don't need to do this as we're not interested in offgas quality, we
want a high char yield. I think...

AJH