[Terrapreta] Trench Method: First Attempt

[AJH]

On Sun, 25 May 2008 22:10:07 -0400, William Carr wrote:

>Not half what I should have gotten:   I conclude I didn't use enough  
>earth to seal the trench.
>
>I spent five minutes sealing the trench and fifteen watching for flare- 
>ups.   That wasn't long enough.

I've made char using this method and found over a number of days the
char continued to burn, my conclusion was to make sure the material
had charred thoroughly and then quench it. Therapeutic doesn't mind
wet char, barbecues do! 

>
>
>Also, some of the branches I used were way too big.   By trying to  
>keep the fire hot enough to keep those logs burning, I let in enough  
>oxygen to finish charring the small diameter branches.

From what I have read when this method was used in UK it was separate
from clamp burning of the cordwood, the raw material was the smaller
stuff tied into faggots or bavins, so yes classifying the material
before the burn makes sense.
>
>So, this is what I would do differently:
>
>1) No large branches.    Cut them up with the chainsaw and/or run them  
>through the chipper first.

I'm not keen on this chipper idea as it reduces gas circulation but
dry chips sprinkled on top would be OK, in fact as the heat builds up
you can probably get away with green chip, because their dimensions
are so small the water can be quickly driven out. Remember drying is
the time limiting process in pyrolysis, because it is so endothermic
not the pyrolysis itself which becomes exothermic, and self
sustaining, in the range 300-450C.
>
>2) It would be great to have a source of green wood chips for this. 


I entirely disagree on this, fast char making like this depends on the
wood being as dry as possible, slow char making as practiced in Japan
for producing perfect charred logs is a different matter and the burn
lasts for days, I'd guess terrapreta has no need for perfect char.   
>
>3) I need to find a better way to control the air intake to the trench.

I disagree again, the point of the trench is to prevent air getting to
the bottom or sides of the char, the char is formed by adding dry
brushwood to the fire in the pit such that you can see no grey ash
forming on the char bed but the char bed remains hot enough to
pyrolyse the disintegrating stick. The fresh wood quickly evolves a
yellow-white smoke which immediately burns in a yellow flame above the
pit, if you only see white-blue smoke and little flame you are burning
char rather than offgas.
>
>
>And I think I have an idea on that.     I have some steel pipe lying  
>around.    I could perforate it,  bury it in the trench and hook it to  
>a compressed air supply.

This will only serve to burn the char you already have at the bottom
of the pit.

What I found was that hardwood timber harvested in the winter, in UK,
and the cordwood cut out for sale of firewood, left the lop and top
over the site in twigs of less than 7.5 cms and down and between 1
metre and 6 metre length. By May they were dried out well enough to
pyrolyse. To scale up from the little pit I would have needed to dig a
longer trench in the weald clay, because of site restrictions this was
not allowed so I used a standard two tier ring kiln. Rather than tie
faggots I used a grapple loader to crush grabfuls of lop and top. with
a fire lit in the bottom  I continued loading it throughout the
morning and we estimate that 10m^3 (bulk) of char formed in the kiln
when I sealed it after 2 hours. I think I can find pictures but it was
in the days before digital cameras and they will be 35mm prints.

AJH