Biochar retort kiln - best material to use around heat riser

Hello all,
I do appreciate the feedback in regards to my last post. I have decided I will probably scrap the cinderblock plan. Now I do have other questions. I have decided to go with a “hornito biochar kiln” type of design which is the combination of the rocket stove on the bottom (which I will probably go with Ianto Evan’s design) and the retort kiln on top, which will be used for pyrolysis. I am also leaning toward putting this kiln inside of my greenhouse as opposed to right outside (which I do have plenty of space for), this way instead of trying to capture the waste heat, the waste heat should just radiate off the kiln directly into the greenhouse. With this, should I go with a 55 gallon drum around the heat riser, or should I go with firebricks around the heat riser instead? I do have a budget that will allow me to go with either design. I hope my question makes sense, any input would be great. I am sure you all have seen these designs but I wanted to post a picture as well for further illustration.
Thanks!
-Mike

Interesting, i've made a fair bit of biochar but not used a system quite like that.

It looks like the pyrolysis gases as directed back through the burn chamber - as i understand it this is good for getting a clean burn, but not necessary to complete the charing process. At the point that the gases are flammable the pyrolysis is self sustaining in the retort anyway.

As far as building one of these inside a greenhouse goes, i'd be exceptionally wary. My experience of all biochar systems is that they get ferociously hot not something to be messing with in a confined space or near precious plants. I'd be leaning towards brick construction for the thermal mass and see if you can duct the exhaust gases through a bench in the greenhouse - gentle sustained heat seems to be the order of the day. If you have sufficient mass that your exhaust gases are cool you could even vent some directly into the green house to temporarily raise co2 levels ( there are serious safety implications here though as you don't want to breath that or lead to a carbon monoxide build up)

Mike

Mike,
Thank you for the input, and I would be interested in learning more about your work with biochar. Another idea we have been entertaining is the incorporation of an aquaponics system to possibly raise tilapia in the winter time. If I ducted the exhaust gasses through a (I assume cob) bench, do you think I could build this bench in a shape that will comfortably hold the tank? I have posted some photos below of the area I have to work with. The area is approximately 12’ (15 if I utilize the area I am currently growing a few things) by 5’.

For my own biochar making i messed around with various configurations, including a simple retort, and using an oil drum to make a Top Lit Up Draft (TLUD) burner.

My man conclusion was that making large amounts of biochar from our trimmings and scrap wood was very time consuming. Now i'm leaning more and more to a imple open fire and water quench. Simple, cost effective, time effective and relatively good yields. Biochar is now a useful bi-product of our pruning process but not an end in itself anymore.

Typically i would end up with a third of an oil drum of biochar from an afternoon of burning. An afternoon of my time is better spent on other jobs around the place.

From a big open fire i can get about five times the amount of char in the same time. Yes, i get less char for the amount of prunings burnt, bu t i am time limited not materials limited. I put quite a bit of effort into ensuring as clean a burn as possible (dry kindling, toplit etc...).

My personal preference now would be to encorporate biochar production into routine household systems - for example, a rocket based cook stove makes a small amount of biochar each time it is used. Over a reasonable period of time a decent quantity would build up.

I'm not an expert on cob bench designs so i'll leave someone else to comment on the designs of your greenhouse.

Mike

Mike,
I have never created biochar myself, but I had read that one of the main concerns is time consumption. This is a project more for personal education as I am an intern, so time is not much of an issue for me. We would like to capture the waste heat from this project, but our main objective is to create biochar and demonstrate to local farmers how they can do the same. There is a lot of waste biomass here due to beetle kill, so we think resources are plentiful. The farmers will also consider their time a valuable resource so we are entertaining different ideas as to quickly loading/unloading the kiln. Our main focus is to create biochar, so heating the greenhouse is sort of secondary. Instead of using the rocket stove to heat the cob structures, our design will have perlite right outside the heat riser for insulation, and to hold in the perlite we will use either a 55 gallon drum, or more firebricks on the outside. Right above the heat riser will sit the oxygen limited pyrolysis chamber for creating the biochar. We think this will work well, in reference to the first image I posted there is a great Youtube video on how the “hornito” design works, you can find that here: http://www.youtube.com/watch?v=hZD6hrVhZGc . I am also going to post a picture of our original design if you can make sense of it; this is a very basic sketch. I do have a CAD printout which is closer to scale, but I have not covered it with red ink, so it would be difficult to understand. We originally were going to go with concrete masonry units around the entire structure, so that is the material you see on the far ends. The two circles in the middle represent a 55 gallon drum in which the heat will travel through, and inside that drum sits the smaller circle, which represents the oxygen limited retort, which is a 30 gallon drum (in between the 55 gallon drum and the 30 gallon drum we planned on using spacers to allow heat to travel through. Although I think I am planning on altering this project now and starting fresh, it may still give you an idea of what I am trying to accomplish. Any input would be helpful.
Thanks,
Mike

I wanted to post a copy of the CAD drawing, this is all to scale based on standard sizes of: firebricks, CMU's, 55 gallon drums, and 30 gallon drums, the difference on this one is the steel lid. Ignore all the red ink, as we have scrapped some of the ideas

Ok, i think your scale is totally wrong - it sounds like you have acres of beetle killed trees to deal with. Cutting up a whole tree into chunks small enough to go through your small retort would be a monstrously slow job even before doing the burn itself. Our oil drum scale work just about copes with our woody hedge trimmings!

Here are some ideas for a more appropriate scale for your resources:

A large steel hoop charcoal kiln. These are used by foresters to make charcoal for bbq's but have many advantages for your application. They are mobile, so can be setup where your trees are. They make a valuable product - lumpwood charcoal - and the smaller particle stuff is your biochar. If you want to turn all of it to biochar you can crush it up by driving over it repeatedly with a tractor etc... On a hard surface. They take much larger pieces of fuel wood than your small retort, so need much less processing. Selling the charcoal from just pne or two burns would pay for the hoop.

If you have access to a digger you could try a big pit burn - dig a pit about 5ft deep and fill it with while trees etc to a height of around 3 ft above ground level. Light the top of it. A few hours later once it has burned down below the level of the pit cover it again with soil. Leave it for a few days to complete pyrolysis, checking occassionally to make sure the soil cover is intact. When you are confident it is cooled you uncover it again. I would thoroughly wet it, dig it out by machine and crush it ready for use. You will get a total mixture of partially burned and properly charred wood. All of it is fine to apply to the soil.

Mike

My biochar drum method

Half way down this page - no pictures i'm afraid, i'll get to it one day.

Mike,
That seems like a decent method in making biochar, but please don’t take offense, it does not seem necessarily clean. I do understand that to create biochar in a sustainable manner, it is important to use resources that are site specific. Where I am I have literately tons of wood chips I am able to use, my point in mentioning the beetle kill is because some of the farmers we demonstrate to may have trees close by. Where I am employed our mission is “Helping people by championing small-scale, local, and sustainable solutions to reduce poverty, promote healthy communities, and protect natural resources. With this being said we try to focus on the economic model that “small is beautiful” as opposed to “bigger is better”. We are okay with this kiln being small, we just want to make sure we can burn as cleanly as possible, and as efficient as possible. The rocket stove allows you to use significantly less firewood to reach extremely high temperatures due to the highly insulated burn chamber.
Thanks!
-Mike

Mike, no worries about me taking offence - I know it isn't the cleanest approach. Quick question in response though - do you ever have bonfires on site? If so you are already making that same pollution in the burn process, but without any of the mitigating benefits of producing biochar alongside it.

We probably have a bonfire half a dozen times per year on our 8 acres. Large diameter stuff goes for the wood stove, small diameter stuff goes for compost or gets put through the small chipper we have. Anything in the middle ground is likely to get burned, along with various perennial weeds (bindweed in particular) with cannot be disposed of reliably by composting. We try to make this as clean a burn as possible - material is left to dry for a reasonable period, the fire is constructed carefully and managed to flame rather than smoke. We burn a lot of material on one site, rather than have many small fires.

As a decent sized bonfire burns the embers fall and build up in a nice layer, under ashes. The ashes protect the embers from oxygen so only the gases burn. Once the flames have died down and all the woody material has broken down to embers it is straight forward to douse the whole fire with lots of water, turning those embers into lots of biochar.

We make lots of biochar, with a lot less labour input than previously

Our periodic bonfires now contribute in a positive way to our soil by providing biochar

Careful management has reduced the number of fires we have, and the air pollution due to an open burn.

I did this this week and, from a single bonfire, produced around 10 times as much biochar as I would have done from my old barrel methods. I would guestimate that I saved around 20 hours labour compared to preparing each batch of materials for the burn, monitoring 10 separate burns, quenching the resulting char etc...

Ultimately it is a trade off between quick and effective, vrs slow and clean... I suspect that low yields in small batches will result in your system being under used.

Mike, where are you in the world for this Project?
Do you have Access to already chipped Wood?

Michael, I liked your answers so that am releaved for the fires I made this late spring!!

I also did not know how to process all my trimmed branches so that they could fit into a TLUD!!
So I made an opened fire.
I put as much as posible all at once.
It was just imposible to burn all at one time because I do not have enough distance with trees for this.

All adequately enough burned pieces of Wood were dragged out of the fire and watered.
I had a 2 parts fire, 1 burning and 1 cooling.

I just understand now that you seem to say that the biochar should be reduced to small pieces (which size?) before they are good for incorporating into the soil...

Thanks for the learning

I started a thread on the Jolly Roger Build I'm about to embark on. Just looking for final thoughts on the various vents and airway dimensions. https://permies.com/t/28553/energy/Build-Jolly-Roger

Appreciate any thoughts!