[Terrapreta] Let's talk retort materials for a little bit

[MFH]

In blue..

 

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From: Greg and April [mailto:gregandapril at earthlink.net] 
Sent: Friday, 16 May 2008 12:18 AM
To: MFH; 'Terra Preta'
Subject: Re: [Terrapreta] Let's talk retort materials for a little bit

 

Interspaced between the ************ .



----- Original Message ----- 
From: MFH 
To: 'Greg and April' ; 'Terra Preta' 
Sent: Wednesday, May 14, 2008 15:10
Subject: RE: [Terrapreta] Let's talk retort materials for a little bit


If by "retort" you mean the drum used to contain the material to be
pyrolised, then the decision about material is between cost and longevity.
 
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That is exactly what I'm talking about.


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Normal steel will last for years and won't heat-distort if the retort is
heated and cooled reasonably evenly.
 
A retort made from "boiler plate" will last longer and be more resistant to
distortion.

 

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LOL - I tried looking up the definention of "Boilerplate", and very little
of it had to do with metal. - Google for "boilerplate steel" otherwise
you'll get all sorts of cut and paste nonsense

 

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Cast iron is better still. I've been hunting for scrap cast iron pipes.
 
My experience in this area is from using various materials in various shapes
as fireboxes under drying beds for produce like cocoa beans or copra. We
once fluked some cast iron pipes left behind by the Japs from WWII, which
proved to have very long lives when used as fireboxes. Also had some made
using normal steel plate and also boiler plate, with the latter having a
longer life.

 

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How does one make a firebox from pipe? Cut to desired length, weld a cover
on one end or stand it on one end in a pad of wet concrete, cut a hole near
the base of one end, add firewood and light. Use the heat out of the top for
whatever.

 

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Shape was important. Flat surfaces as in a box-shaped firebox distorted in a
matter of weeks. Cylindrical shapes were best, particularly if the sides
were supported by concrete walls thus preventing a gradual change from
cylindrical to oval. Best of all was to line the inside of the cylinder with
2" of cast refractory cement, giving a life despite 24/365 use of many
years. This is not suitable for char production as the cement has a
considerable insulating effect.

 

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I was thinking of something a long the lines of having an octagional cross
section.    

 

Now before you start wearing groves in your head, scratching it, trying to
figure that out, let me explain.

 

It's easier to obtain flat plates at lower cost and weld them into shape,
than to try and get a single piece of round stock of the same thickness and
volume.    As for the possability of warping, a number of brackets can be
installed that will resist warping, again these brackets can be made from
easy to obtain flat stock.   To be airtight your welds are going to have to
be continuous at every join - that's a lot of welding. Heaps cheaper and
simpler is a tubular shape. Steel fabricating workshops normally have
"rollers" that can handle 3mm plate. Even if you just get the plate
pre-rolled then weld at home, there's only one weld.

 

I also decieded that I wanted to be able to unload the retort with a shovel
and a flat bottom makes it easy for a square point shovel can slide along
it.  Yes, but if the retort is raised above the ground it is easy to make a
shaped scraper to get the last char out after taking most with a shovel.

 

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316 or other stainless steels may be okay but at a serious cost escalation.
I'd suggest that its more cost-effective to use the cheapest steel and toss
it out in a couple of years.
 
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That is some thing to consider, althow there is also the cost of rebuilding
it to consider as well. 
 

Greg H.

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