Small rocket core for heating biochar retort kiln...questions

I have a kiln design that I am working on. The outer wall will be several panels of stainless steel that are bolted together. There will be ceramic fiber blanket on the inner wall. There will be a 55 gallon drum inside of the SS cylinder that will hold the wood to be pyrolized.

I've seen similar designs with the kiln walls made out of masonry of differing kinds where they have the barrel on bricks and then build a fire through the walls of the kiln (and under the barrel). I built something similar to this last year but this is a new design for me and I really want to utilize a rocket core for more efficient heating fuel.

**I'm not asking for any design help on the kiln build...just wondering if I can build a rocket core as described below and have it function properly.**

I want to bury firebrick (I have access to both full and split sized) in the ground to make a "J tube" of sorts. I want the burn tunnel to be buried so that the top of the burn tunnel is level with the ground and the fuel feed sticks up just a touch (or however much it needs to). However, in order to have the kiln walls on the level ground as well, there would not be a heat riser. I am wondering if the SS kiln wall in and of itself could effectively act as the heat riser. This would mean that the rocket core would have fire brick for the burn tunnel and the fuel feed and then just have an opening at the top of the burn tunnel on the kiln side to release heat into the kiln. Would something like this work? Thoughts? See my basic Sketchup images for an idea of what I am envisioning. Let me know if my description is not clear enough and I'll try to better explain.

I'm looking at doing something almost exactly like this. I need to make some biochar for clients out of materials like nut shells and chipped coconut husks (coir), and these don't really play very nicely in a flame cap kiln. I'm looking at mild steel plate, angle fittings for bolting together, and ceramic fibre blanket held in place with stainless steel mesh.

Your drawings look very similar to what Kobus Venter designed here. His kiln can be used three different ways: as an open flame-cap vessel sort of like a Ring of Fire, with an inner retort surrounded by sacrificial feedstock as a TLUD, or as you're planning with a rocket combustion core exiting underneath the inner drum.

According to Kobus this works quite well. It's not a full-on rocket by permies standards without the matching cross section insulated heat riser, but because the goal is to transfer the heat into the retort to initiate pyrolysis it meets the functional requirements and I would expect it to use a lot less fuel than the TLUD for better results thanks to the high temperature generated. The chamber will act as a heat riser thanks to the insulation. The bit I'm wondering about is where the extra air will come from to fully combust all the syngas coming out of the retort...will it draw through the burn tunnel, or should we add a secondary air port at the bottom of the chamber?

Experimentation awaits. We should compare notes as we build these things.

Thanks for the response.

I didn't intend to get into the weeds with the kiln specific stuff because of the forum topic area not fitting. But yes, it is fun to play around with different designs. I have been working with biochar a lot the past year. I've tried many small scale kiln styles/designs. I've also chatted with Kobus about the Black Ripple Kiln and about my previous kiln design. The previous design was two carbon steel sheets with ceramic fiber blanket insulation in-between as the kiln walls. The barrel would go inside and I used wood chips to stuff between the walls and the barrel and then be top lit to heat the barrel and start pyrolysis. It worked really well and I could make biochar out of 8" diameter logs, all the way down to twigs. I have ideas for burning wood chips, nut shells, etc as well but haven't tried in this retort. The last kiln design burned out the inner metal wall rather quickly so now I'm moving on to something similar but with SS outer wall, ceramic fiber inner wall (not sandwiched between another sheet of metal), and (ideally) rocket core to heat. I'd love to have a discussion about design privately or in a different forum thread! I don't get to chat about this much with others...haha.

Anyway, back on topic, I was thinking last night that I'm pretty sure that my idea will work for two reasons.

1) I at first built a small portable core and covered it in aluminum sheet. What I'm thinking of now is similar...between what Kobus suggests and what I built before. The core "worked" but I didn't get enough heat to start pyrolysis after an hour (I think my core was too small). I got the burn going by other means and the heat/flames (I let the gases come out of the bottom of the barrel for a more complete burn) melted the aluminum covering on the kiln half...dumb of me to have tried the core that way with those intense temps. I'll get around to fixing the portable core and use it for a rocket stove.

2) Later, with the old design I had a 6" diameter black stove pipe, 2' in length with a 90 degrees elbow on either end, buried under the kiln in a similar fashion as the portable rocket core. This was for increased airflow at the bottom of the kiln. Once things got warm the draw from the pipe was awesome. Eventually, I would have to cap the outside elbow because the burns were so intense that they would start pushing gasses out of the pipe. But overall it worked great and I assume that a buried firebrick core would work similarly.

Here is what I'm going to try for the buried firebrick core. The top of the vertical sections from the feed area and the tiny heat riser will be level with the ground.