[Terrapreta] Retort Gasification Question

[William Carr]

Okay, this is sort of a gasification question founded on my Terra  
Preta experiments.



Hopefully there are some Gasification experts here...


********Charcoal Making Background -- Skip if you know this stuff  
already**************



There are basically two ways I know of to make charcoal.   Both  
require some form of containment vessel, even if it's only a pit or  
trench.


You can either burn some of the wood IN the vessel to generate the  
heat, and then suffocate the fire at the right point to capture the  
charcoal;

Or you can supply all the heat externally, OUTSIDE the vessel.

Build a big fire, put the wood in a sealed vessel (called a retort)  
and tap off the flammable exhaust fumes to help fuel the fire.



In theory you could make charcoal this way in your BBQ grill -- by the  
time I'm done cooking burgers there's usually plenty of heat left in  
the grill, it's a shame to waste it.

Why not make more charcoal with that heat?

******************************************


What  I've been trying lately are different mixes of biomass waste to  
make charcoal for Terra Preta.


I'd like to know what's going on, chemically speaking, during the  
process.   Also, what might happen differently with different mixes.




For example, I have a Corn stove.   Unless I pay more for cleaned  
corn, every fifty pound bag has about two pounds of corn dust in it.

This stuff is somewhat useful as fertilizer in my garden, and can act  
as a weed suppressant.


It also makes fairly good charcoal for Terra Preta.



I save tin cans from canned peas, etc, and cut the tops off with a can  
opener that leaves a "safe" edge on the can lid.   That makes it easy  
to put the lid back on.

I make a little hole in the lid to vent exhaust gas.

I fill the can with the corn dust, cap it with the lid, and set it  
inside the corn stove.

Sometimes I put a wider can upside down over the first can to hold in  
exhaust gasses and hold out oxygen.


The heat inside the corn stove bakes the loose corn dust.



And then the magic happens.



When I take the can out and let it cool, the corn dust is baked into a  
cylindrical column of charcoal, just a little bit shrunken, with the  
groove marks of the can embedded in the surface.



This charcoal is baked HARD, like a brick.   It's a lucky thing the  
char contracts or it would be very difficult to extract from the can.


To break the charcoal up, I use a hammer.     And no, it doesn't  
crumble easily.

It takes several swings of a ball-peen hammer or  hard jabs with a  
5/8ths inch steel rod to break the charcoal in half.


I suppose a sledgehammer would be faster, but I don't want bits of  
charcoal flying everywhere.


 From corn chaff comes solid lump charcoal.

And it burns fairly well, although with more flames than bagged  
charcoal.

I broke off a lump and threw it into the corn stove.   The edges  
started to glow after a minute or so, and it gradually burned up over  
the course of six or seven minutes.


My future experiments will include trying :

A) loose sawdust,

B) sawdust mixed with corn dust,

C) corn dust mixed with sand,

D) Either sawdust or corn dust with clay powder added, and

E) corn dust plus sawdust and used cooking oil.



It's that last mixture that has me most curious.


I would like to know, what will happen in a semi-sealed can containing  
corn dust and sawdust, mixed thoroughly with used fryer oil.

The oil will vaporize first.   If there's a vent in the can, the  
flammable vapor will leak out and burn as soon as it reaches hot oxygen.

That will force the temperature up faster and cause the fuel inside  
the can to carbonize quickly.


*******Afterthoughts***********


I realized after thinking of this oil/corn-dust/sawdust mixture that I  
don't REALLY know what's going on in the retort.

The classic example of making wood alcohol is easy to follow.

As the wood bakes at above 500 degrees Fahrenheit, it emits water  
vapor, acetic acid, methanol, and burnable gasses like Carbon Monoxide.


What is going on when the corn dust bakes is probably similar.   But  
just HOW similar?



In my reading, I found a reference that says if you cook sawdust to  
300 degrees you get soft black powder, and at 700 degrees hard, black  
solids.

That latter appears to be what's happening here.

I'd like to know what role the water vapor plays in the process, and  
what exactly binds the char together so strongly.



I have sketched out a charcoal kiln using a 5 gallon steel bucket, a  
stainless steel trash can, and a piece of five foot long secondhand  
chimney pipe.

The goal is to build a MIDGE stove, load the kiln made from the  
chimney pipe with the soup cans, and set it on top of the MIDGE.

I should get organized, I suppose, and weigh the cans before and after  
carbonization to get an idea of the efficiency of the process.

A nice goal would be to up-size the process to larger batches,  
assuming I could find a free source of biomass.

There's a whole cottage industry doing this in India, but I don't have  
free access to sugar cane leaves.


Thanks for reading,


William Carr