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Heat riser width to length question

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New member, big fan, first post!

The question:
Is 10 inches wide and 3.5 feet long going to function properly as a heat riser or does it need to be longer for that width to really draw the air and get to the high heat?

The reason I ask:
I am building a rocket stove biochar retort kiln. I'm planning on putting the heat riser in the center of a barrel with external insulation and small holes at the bottom of the barrel near the heat riser to feed wood gas as the fuel once I reach that point in the burn.

I know metal isn't the ideal material for the heat riser due to the carbon burning out and breaking down after repeated use, but I went crawling through a junk yard and found some sort of welded steel tank that is about 3.5 feet long and measures 10 inches wide. The steel is about 5/8 or maybe even 3/4 an inch thick (I'll measure it once I cut the ends open with a torch).

Being so thick, I think it would last a very long time in a rocket stove. I also found some unusual barrels that are the same height as the heat riser and much wider than a normal 55 gallon barrel, I think the pair will work great as long as I weld another thick piece of steel to the bottom to handle the high heat.

Eventually I'd like to utilize the heat in a RMH design while making biochar since I am going to be making LOTS of biochar for my huge garden over the next few years. I am considering pumping the heat through a pottery kiln made of fire bricks, and/or from there into a large thermal mass in the barn to give the animals some extra warmth in the harsh winter months, and maybe into a green house.

If you have any feedback about the original question or design ideas I'd love to hear it. Thank you for reading!
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Location: Okanogan Highlands, Washington
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Metal in rocket stoves: Problem is not just the carbon burning out, but at that size with proper insulation you may easily reach temperatures that warp or melt the iron/steel itself.
Strongly recommend using refractory materials (fire brick, kiln brick, or at least something with a fire clay base component) for a project like that.
There's a lot of discussion (some with more grammar) if you search these forums.

Biochar in rocket stoves: Completely different project goals. The right height for a heat riser in a rocket (cooking or heating) is the height that results in burning all the smoke. The right height for a biochar retort is the height that lets you burn a really smoky fire, then do secondary combustion with the smoke (or store it, if you are really ambitious and like black goo). Completely different goals, different designs. There was a design for tall biochar retorts made for Chinese open-air markets that might be a good starting place if you want to use that metal cylinder for a low-temperature biochar retort.

Just FYI: this comes with personal bias, as I'm deeply skeptical of biochar generally. Fire is compelling, I'm the first to admit; and I think that fascination and emotional reward sometimes contributes to people wanting to continue a fire-related practice given the slightest excuse. Thus, the rationale behind a particular practice (it sequesters carbon! It's a Good Thing for Soils!) doesn't get examined very closely.

Soil benefits: The clay-heavy, slow-draining, nutrient-poor, organic-matter-eating tropical rainforest soils that were the original example of 'terra preta' are not common everywhere; dirt can be made of just about anything, and different soils have different needs. Only one of the dozens of biochar-users that I know has tested the benefits of biochar for his particular soils, found it good on a small scale, before expanding. He's in a heavy-rainfall area with clay-rich soils. Are the people on alkaline, silty, loamy, or sandy soils doing similar tests before getting excited about biochar?

Carbon sequestration: To me, it seems like burning dirty fires to produce biochar will release a lot of carbon, sterilize and destroy fertility potential in the organic matter used. The likelihood of damaging carbon sinks by collecting organic matter for biochar production, or of biochar being used to 'greenwash' unsustainable forestry and farming practices, seems higher than the likelihood of users/producers making good choices at all stages (appropriate wastes as fuel not deforestation; appropriate use of the primary combustion gases for necessary energy; appropriate use of the charcoal end product by testing soils for application rates and organic fertilizers needed).

I've seen 3 examples that I thought had enough good points to outweigh the waste. Never seen a good long-term study comparing the biochar to compost from a similar original feedstock.
Dozens of examples where someone was in the "I made some biochar! now what?" stage; that's part of why I think this is a fascination issue, a solution in search of a problem.
And dozens of examples where I feel strongly that the 'problems' solved by biochar could be better solved by other means (composting, breaking up the battery chicken farm into reasonable units with other types of farms so their manure doesn't need as much processing to be useful, improving efficiency and insulation to reduce fuel needs rather than trying to manage a two-stage burn or gasifier plant, reducing combustion engine use with lifestyle changes rather than trying to bend over backwards to find alternative fuels so we can continue riding around on flaming explosion-based technology instead of things with legs.)

If we all switched to wood for our current energy usage, the world's forests would be gone in less than a year.
The biochar process is hot enough to deal with noxious weeds, but not medical waste or other noxious matter; and it's too inefficient for me to endorse it for other purposes. I'm in the field of using renewable fuels for clean-burning heat, not biochar, for the above reasons.

That's my two cents.

Hope the practical tips are some use, anyway.
If you feel you've addressed the above issues and your situation calls for biochar in a deeply appropriate, high-integrity way, feel free to post some more info and pictures and become the 4th good example in my biochar books.

Jeremy Mecham
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Thank you Erica, that was a fantastic and well thought out response. A couple things in regard to my design, fuel, and intentions.

I recently bought 40 acres of ranch property in AZ. I will be getting a well drilled in the near future and I am moving there to start a homestead in about 2.5 years. The temperatures are not insane like a lot of AZ, average summer days are mid 80's, sometimes peaking closer to 100. I collected soil samples that I dissolved in a glass jar and it looks like I'm at about 95% sand with some silt and organic matter as well. So I started looking into ways to improve soil nutrients, bring in more organic matter, retain more water, all that kind of stuff. Well a week ago I learned about biochar and have done a lot of reading and watching videos. I have to say that the responsible creation of it, as well as the explanation of how it works makes sense. I'm completely sold on the idea to be honest. I believe it could do wonders for my sandy soils when mixed with compost. I think worst case I would end up retaining more water for my plants and orchard.

So I have 2.5 years to get my soil in shape before I move there and start planting. My plan is actually to make the bone char class of biochar due to the high phosphorous levels. I have spoke with a local wild game processor and worked out a deal to take as much deer and elk bone as I would like.

As for my rocket stove design, it looks like my guesstimate was off on the tube width, my internal diameter is 7.5 inches which should work fine for the 36 inch height. I may run into some issues with warping but now that I already have it I will give it a try. The design should burn every bit as clean as any rocket stove though. I will start a small fire in the J tube section, once it gets enough heat to the heat riser I should start to burn efficiently and clean. Then once I start getting gas from the bone, the small holes in the bottom of the burn chamber will feed the fire and keep it burning with minimal additional wood being fed.

Again, this is all making sense in my head, but in reality who knows. I'm guessing there will be some experimentation in both making and using biochar. One thing I will stick to though is being responsible with it. I will only burn clean, I will utilize as much of the heat as possible in as many ways as possible, and if I don't see positive results using biochar, I will stop using it.

I have started making a video of the creation of the retort, when it's all said and done I will put it on YouTube and show the design and how well it works. Then down the road, I'll also show the results of using the bone char.

Thanks again for your feedback.
Acetylsalicylic acid is aspirin. This could be handy too:
Native Bee Guide by Crown Bees
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