[Terrapreta] Trench Method: First Attempt

[William Carr]

Hi.     I wrote about wanting to try making bio-char with a "trench  
method" a while back.

It has been too cold to plant tomatoes here up to now, so I had a  
little time on my hands.


I marked a patch, dug a trench and lined it with some branches and  
newspaper.

Taking a tip from TV chef Alton Brown, I then sprayed the newspaper  
with used cooking oil, using my garden sprayer.


The oil slows down the paper's tendency to flash up and burn down to  
ash.   It also adds heat to the fire.

I then piled on a lot of sticks, dead branches, green branches, etc.,  
and lit the fire.

Once it was going really well, I tried to smother the fire with damp  
straw and dirt.

********


I'm going to write this one off as a learning experience.


I'm posting these results to help anyone else attempting this method.




I DID make some bio-char.


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.


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.

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.

2) It would be great to have a source of green wood chips for this.    
I did put my name on the "free wood chips" list with the tree service  
Asplundh, but no answer yet.

3) I need to find a better way to control the air intake to the trench.


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.

That way I could get the fire going, and then just keep shoveling on  
green wood chips, then top with dirt.

A trickle of air from the pipe would keep the fire going, while the  
mass of green chips and dirt above would hold in the heat.


The chips would torrefy and then char.

The only reason I didn't do that this time (other than: that air  
compressor is HEAVY...)  is that I started to wonder how much air the  
fire would really need.

Any guesses?   Formulae?    I THINK a 3 horsepower air compressor  
could supply enough air to fire a twenty foot trench, but I'm not sure  
how to even begin looking that up.


**********



My secondary plan is to build a char-making stove based on the MIDGE  
woodgas design, writ large.    Sheet metal is expensive though, so I  
plan to salvage some from a dead washing machine.

The interesting thing about the MIDGE is that it gasifies stick wood  
very well, but the moment the volatiles are burned off, the stove goes  
out leaving perfect charcoal sticks behind.

William Carr