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Any new small charcoal retorts out there?

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I've not disappeared.... I've been taking it all in & trying to get my head around what biochar is. Thought I'd gotten my head around it; that was until I read Openspacemans post. Actually, if I admit it.... I'm very confused!!

 

 

 

Feel free to ask, I'm no expert but have been following developments since around 2000.

 

Do a search on terra preta des indios because it was work on the fertility of these dark soils that triggered the biochar business.

 

I am not a fan of using stainless in reducing atmospheres.

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I am not a fan of using stainless in reducing atmospheres.

 

But that's based on corrosion properties and the stripping of the Cr oxide?

 

The rate of loss of section in retorts does not appear to be particularly high, and in a reducing atmosphere this would be expected. The limiting factor appears to be high temperature strength, manifested in creep resistance. Figure 1 in the link below indicates the relative strength vs. temperature of low carbon steels (e.g. mild steel) vs. the ferritic and austenitic grades:

 

SSINA: Stainless Steel: Composition/Properties

 

You can see that at around 650degC the ferritic stainless grades have twice the strength of mild steel. Austenitic grades such as A2 (304) and A4 (316) have far higher strength but they also have much lower thermal conductivity and high coefficents of thermal expansion (CTE). High CTE is an issue if heating is not uniform and/or the shape is constrained at points as some parts will expand more than others and the differential stresses may result in distortion.

 

It's these bulk properties which make stainless attractive. If you look at the power industry where the standard grade for steam pipes is P91 this also uses chromium and molybdenum to achieve its high temperature strength - it is in effect very close to the ferritic stainless grades. In theory, grades such as ASTM A515 would be a better choice as it has excellent high temperature strength (typical uses include fireboxes in boilers) but they are pretty much unobtainable in the right thickness so stainless is probably the best practical option. Based on the relative properties of the ferritic and austenitic grades, I think it would be tricky to design a horizontal retort with even enough thermal stresses to make it in an austentic grade so would go for a ferritic. For a vertical retort, 304 would probably be my first choice based on availability, price and properties.

 

Alec

Edited by agg221

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way over the top of my head on this topic but would boiler plate be any good ?

 

The materials part of this thread has got rather 'in depth'... :blushing:

 

Essentially yes, boiler plate would be a good starting point. Boilers are made of quite a few different grades of steel (or indeed cast iron) and all are generally the right sort of thing. The steel grade ASTM A515 I mentioned above is a form of boiler plate. There are other boiler plate grades which won't survive so well so it would be useful to know which one it was, but all will be better than mild steel.

 

The main difficulty with boiler plate is that it is usually rather thick. The aim is to make a small retort, which generally means you want thinner, lighter walls but if you weren't looking to make it portable and had a good source of boiler plate it would be an excellent option.

 

Alec

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But that's based on corrosion properties and the stripping of the Cr oxide?

 

The rate of loss of section in retorts does not appear to be particularly high, and in a reducing atmosphere this would be expected. The limiting factor appears to be high temperature strength, manifested in creep resistance. Figure 1 in the link below indicates the relative strength vs. temperature of low carbon steels (e.g. mild steel) vs. the ferritic and austenitic grades:

 

SSINA: Stainless Steel: Composition/Properties

 

You can see that at around 650degC the ferritic stainless grades have twice the strength of mild steel. Austenitic grades such as A2 (304) and A4 (316) have far higher strength but they also have much lower thermal conductivity and high coefficents of thermal expansion (CTE). High CTE is an issue if heating is not uniform and/or the shape is constrained at points as some parts will expand more than others and the differential stresses may result in distortion.

 

It's these bulk properties which make stainless attractive. If you look at the power industry where the standard grade for steam pipes is P91 this also uses chromium and molybdenum to achieve its high temperature strength - it is in effect very close to the ferritic stainless grades. In theory, grades such as ASTM A515 would be a better choice as it has excellent high temperature strength (typical uses include fireboxes in boilers) but they are pretty much unobtainable in the right thickness so stainless is probably the best practical option. Based on the relative properties of the ferritic and austenitic grades, I think it would be tricky to design a horizontal retort with even enough thermal stresses to make it in an austentic grade so would go for a ferritic. For a vertical retort, 304 would probably be my first choice based on availability, price and properties.

 

Alec

 

Thanks Alec. I've tried finding ferretic stainless steel barrels on the internet but not had any luck. I have seen the 316 ones mentioned though.

cheers, Steve

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But that's based on corrosion properties and the stripping of the Cr oxide?

 

Yes plus the expense

 

 

You can see that at around 650degC the ferritic stainless grades have twice the strength of mild steel. Austenitic grades such as A2 (304) and A4 (316) have far higher strength but they also have much lower thermal conductivity and high coefficents of thermal expansion (CTE). High CTE is an issue if heating is not uniform and/or the shape is constrained at points as some parts will expand more than others and the differential stresses may result in distortion.

 

I'm not sure of the significance of the conductivity but it is a limiting factor on retorts where the walls are the only means of heat exchange, this is why I believe drying should take place before pyrolysis. From what I have seen it's the very fact that the fire outside the retort that causes the problems because it is an uncontrolled temperature, with around 450C inside the retort and 800+C out there is a lot of scope for damage. Which is why I think the temperature outside needs to be controllable, even though I would probably not use this sort of retort.

 

I bow to your superior knowledge of steel grades as I am not sure what SS was used in the retort I saw but the life was not sufficiently longer to make it worthwhile over 45 gallon drums which lasted for 10 or so burns,.

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Was taking my life in my hands googling flange fittings :lol:

 

My mate gave up on page 438 of google when he was looking to buy some grease nipples

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I looked up boiler plate but it does look a tad on the thick side

 

Alec I asked our stockholder about ferratic grades and they looked at me blankly and said never heard of it. Is there a trade name for this? All they have in stock is 304 and 316. This is for the liner and not under any load other than heat stress during the burn

Thick-Plates-in-Stock-400-x-600.jpg.df5ad152e47ba253ac7a27688fb1b616.jpg

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Alec I asked our stockholder about ferratic grades and they looked at me blankly and said never heard of it. Is there a trade name for this? All they have in stock is 304 and 316. This is for the liner and not under any load other than heat stress during the burn

 

Scrub that. Looking further down the tables in your link I think I get it now. The 400 grades may have lower working temperature but will suffer less due to expansion and shrinkage?

Please keep it simple as this is way over my head :001_smile:

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