Biochar/stove/hot tub design

Hey, complete biochar/permaculture noob here, with strong aspirations to become less nooby. I’m hoping to build a biochar kiln in my garden that’s also an oven and also a hot tub. Initially it was just going to be a pizza oven, then things happened - much of the blame can be attributed to various threads on this forum.

Can you help me not blow myself up?! At this stage I’m just trying to get the design solid and make sure the physics is sound. We have a wood pellet stove in the house, which has previously exploded when smouldering pellets caused flue gas to build up and it suddenly ignited - blew the stove door glass out. We now have two young children so doing my best to avoid these kinds of situations.

So here’s the basic idea…

- inner chambers made of dense heat conductive material, perforations in the top for gas to escape. Intended for biochar production but might also be useful to cook food in one.
- outer chamber made of insulating material
- oven at the top to catch additional heat
- heat travels from the furnace through a gap that runs under the inner chamber, along the left end, over the top and down the right end. No gap along the long sides. Eventually biomass gasses ignite, furnace is no longer fed and gases burn until there’s none left to burn.
- flue runs under hot tub
- when you first light the stove, you could put a stopper in slot B and not in A, to direct heat up the flue and warm it up (or to just fire the hot tub). When hot, take it out and put in slot a to direct around the oven/retort.
- air comes in through a door at the furnace entrance and also through the stopper slot, whichever one doesn’t have a stopper in it
- idea is to cast it in a home brewed concrete, if I can make a mix that works - dense and heavy on the inside, insulating on outside. Going down the ‘geopolymer’ route as have saved up lots of coal ash, wood ash and clay. Will resort to buying it if this doesn’t work!

I’m most definitely trying to run before I walk here, but if you have any input on the basic design and can see any potential issues that might crop up I’d love to hear from you. Also keen not to make a smoke bomb as we have neighbours either side. Thanks in advance!

- Dave