Stone biochar kiln

Hello,
I wish to make a masonry version of the metal barrel type biochar kilns that have a rocket stove burn chamber and riser running thorough the middle of a metal barrel to heat up the wood/ biochar contained  in the barrel. Instead of a metal barel, i want to make a bigger one out of rocks ( porous basalt). because I have tons of it. Its light because of many  holes so its a bit insulating.    I would need some kind of refractory coating or mortar to fill in the inside cracks since it must be mostly air tight. I thought about mixing refractory cement or mortar with perlite for more insulation. I also have waterglass but never used it before.
Im not sure what to use. I want it to be durable, not just temporary.  Any help with construction methods would be most appreciated. I  was thinking about making it round

Also, I want to try to make the riser more than twice the normal length but S- shaped. ....so there is more surface area to heat the inside of the stone retort instead of just going straight up.
Most of the riser has to be made from metal duct work to transfer the heat to the retort.

Is that kind of riser going to work though? I put a pic here. I think it should still pull the draft ok.

Hey Peter,
I think you could build a fairly air proof, fairly stable round kiln using cob or clay mortar (clay rich soil, sand, maybe fiber).

As to the idea of a S shaped riser, it stops being a riser as soon as it goes horizontal. And usually you try to avoid going up and down. The top parts become a heat trap and then impede movement of the gases.

That said, I don’t completely understand what you’re trying to build here. Could you try to explain why a hot pipe should provide the energy instead of the gases themselves?
Could you link one of the metal kilns you’re mentioning?
Could you run the whole flue gas into the kiln without the piping?
Could you use the TLUD design instead?

Thanks Benjamin,
This link has pics of examples : https://ibb.co/album/D4xPb4 .  The Hookway retort is basically what I'm talking about. The Hornito retort is pretty much the same thing  as far as I can tell.  
The wood to be biochar-ed is in a sealed drum ( in my case I want it to be made from stone). The rocket stove ventilation is separate . I guess its possible to run the rocket exhaust through the biochar chamber if there was zero oxygen left in it, but these are tried proven designs meant to be sealed except for the wood gas that exits a small tube. Depending on the design, the tube can go down and just hook into the burn chamber.Then after the moisture is vented and good wood gas comes out, the gas fuels the who!e shebang and you don't have to add any more wood. There's another pic attached that's basically the same thing made a smaller size on Instructibles .

I was thinking about using a j- tube with the feed tube outside, and the burn tunnel and riser inside the retort. Then the metal riser exits through a sealed hole in the top. The bare metal of the riser heats the kiln from inside.
I want to make the j-tube mostly from stone and refractory or maybe cob but where I live in Hawaii there's no dirt here really...just a lot of compost. Some of it is really old and black muck in rare places, but it not clay or like minerals or sand. I kind of dont think its right for cob.
But we have tons of volcanic basalt of  a!l size. Its relatively  light with air holes.
I want the bottom part of the riser to be masonry,but a straight metal one needs to be connected to the riser hole in the bottom and go out the top of the retort.  That will need to be replaced over time, but  its not that long.

Since I want to make a bigger retort than 55 gals, I wanted to increase the surface area of hot duct so it can heat up the bigger space. If an S - shaped riser won't work, I could make it so there 2 smaller risers ( one feed tube and one burn chamber but 2 risers).  Either that or put a larger diameter duct inside the retort with reducers at the inlet and outlet.

I don't really know much or understand the tlud ones. Can it burn up waste plastic with no smoke?
Because that's my one main goal, not biochar so much ....and also I want to store some of the gas for fuel for cooking, etc.
Also carbon is left over still and  that can be used as fuel, sequestered, etc

I mainly need help with figuring out how to make a relatively airtight retort with rocks or masonry. I was thinking of a honeycomb design. I have basalt rock of all sizes.  The inside needs to be lined to fill cracks between rocks and make it mostly airtight.. I have waterglass and perlite ( insulating), so I thought about smearing that mix on the inside and filling all the cracks with it. I never used waterglass so I dont know if it will just end up cracking off? Or should I use metal lathe or chicken wire first on the inside and then cover it with refractory ?( I read water glass can protect metal from heat and oxidation. )
(I would also need a lid of some kind. I can make it heavy so its held down tight by gravity and maybe add a rope gasket)