Making Large Rocket Incinerator...could use some he!p

Hello,
This is my first post here. I live off grid in Hawaii and I am currently very motivated to make a large rocket incinerator . I attached a rough basic drawing. I am partially inspired by the one at the
SECMOL campus. ( https://www.linkedin.com/pulse/low-cost-high-temperature-waste-disposal-incinerator-prototype-rohit )

Mine is a bit different. I couldn't find the size of one at SECMOL but I think it seems a bit smaller than my planned one.

I have an idea they may simp!ify things . It does not have a closed trash opening with a separate air opening. The trash and air enter from the same vertical end . I have a stainless drum from a washing machine with extra holes in the end and I want to use that as a"fire grate" and to put the trash in. Those drums already make cleaner fires on their own because so much air gets in.

So I was thinking the feed chamber could be a few inches larger in diameter than the stainless drum. That would allow a huge air space around the drum with no trash clogging the airway. If done right, It seems to me this would eliminate the need for secondary air pipes like the SECMOL one needed to burn cleanly. It would also not need a cover over the burn chamber( except maybe while starting it or when it cools.)

I have many porous volcanic rock here to build it with of all sizes. Im not sure yet the best way to make it. I was thinking about making forms with rolls of chicken wire to help lay the stone around to make if easier to make uniform. I read commercial refractory mortar is probably my best bet for mortar, but I wanted ask people here since I do not have much experience making these. Then to make it insulated and reflect heat back, I am planning on putting on a coating of waterglass mixed with some perlite. Or is there better ways?

I just am not sure about the best dimensions. I read about the 1:2:3 general rule, but I  cannot seem to find info on the comparative diameter of the feed,  burn chamber, and riser. I have been assuming they are best being the same diameter, but is that rigbt?
I have people say " its not rocket science" but when I'm making some huge labor intensive incinerator, I dont want to guess at things. There must be an ideal size of the burn chamber and riser for this application. An incinerator does not need to conserve fuel, but just the opposite. And it needs to burn hot and clean. The rise is 14 feet tall, so that's the biggest challenge. I was thinking about just using a few of stove pipes side by side.
Should their square inch area be the same as the area of the horizontal burn chamber?
I don't understand what happens if its smaller ( like if I used just one or two stovepipes). Is that really bad? Or could it be better if it slows things down so the exhaust gases get to sit there a bit longer and get hotter and burn cleaner ( as long as it does not restrict oxygen too much)?

My stainless steel washing drum is 22" in diameter and 14" tall. So I was thinking the feed tube would be about 28" inner diameter. That would allow a 3 " wide gap all around the drum for airflow. That's just a guess though. I know there is a " best" air to fuel area ratio for the inlet. I dont think extra air injector pipes are needed if enough air gets in. I know the Demontfort incinerators have a secondary burn chamber that gets all the 02 it needs just from the intake, so I know it is possible.

If anyone could help me with the size if the riser and horizontal burn chamber, it would be much appreciated.

I would really like to know the opinion of people more experienced in this area even if they think its not a good idea.

Aloha
Pete

Hi Pete,
Welcome to Permies!

That is one huge J-Tube design you want to create.
I can tell you the dimensions for an 8" J-Tube, and you can enlarge them to suit your needs.
The feed tube is 8" x 8" and traditionally is 16" deep
The Burn tunnel is 8" x 8" and has a roof length of 12" or less.
The riser is 8" and up to 48" tall.

The feed tube and burn tunnel should have the same diameter; the riser can be larger if necessary.

That is a gigantic J-tube! The dimensions generally scale proportionally, though at such sizes, I don't know if anybody really knows how it will perform. As there is a lower limit around 4" where the combustion to surface area ratio causes excessive losses and reduced temperatures, there may also be an effect at much larger sizes where temperatures get so high that you approach blast furnace conditions, with temperatures far above 2000 F. I would absolutely expect even stainless steel to be degraded quickly, and perhaps even melt.

You will need to use only refractory materials in your core. I don't know how your lava rock behaves, though I suspect it will start melting by 2000 F. Also, temperatures around 2500 F or so cause nitrogen to burn, creating NO2 and similar compounds which are harmful. You really don't want temperatures much over 2000 F. The highest grade firebrick I know of is rated for 2600 F.

Generally, the feed tube is the lowest temperature zone, with the later burn tunnel and lower riser the hottest. I am not sure how air velocity through the core will scale; I don't think it will be fully linear, as in three times faster than in an 8" J-tube. Thus, there may not be an advantage in making a 14' tall riser aside from the chimney draft effect. I don't think the horizontal burn tunnel needs to be fully twice as long as the feed depth. 28" diameter as you show is so huge that I think you would need industrial engineering and materials to construct and operate it safely. Even 12 to 16" square would have a very large capacity and high temperatures for incinerating.

What kinds of material do you want to incinerate? What size is the largest thing you would need to put in? What volume per day would you be burning?

For the riser, it needs to be one single cross section, not split up, as that would introduce more friction and slow the system.

Rather than trying to hold the trash in a drum, I think it would work better to make a firebrick "grate" of closely spaced fins standing up some from the floor, with the slots following the airflow streamlines so they get swept out by the draft. What would be the minimum common size of items in the load? Spacing to hold most of those up would allow good flow around the load. Fins/slots running down the sides of the feed might work well to get airflow around the load without dead spots.

On studying the SECMOL incinerator article, I note that is only around maybe 10" diameter (hard to tell for sure). It achieved up to 2000 F (1100 C), with a rather crude construction. Note that the riser is entirely ceramic fiber, and the base is clay bricks which will get fired nicely at the temperatures they are achieving.

Do you have a good quantity of wood or other easily combustible material to sustain the incineration of more problematic or erratic materials? You would want a good clean hot burn going before introducing plastics.

I'm wondering if some controlled water injection would be a viable way of keeping the combustion temps in the "safe zone" on a monster like this. There are gasifiers and pyrolysis technologies that require the feedstock to have a high moisture content and they burn efficiently and super clean. The one that I'm most familiar with has stack gas temps peaking at about 1000 C.

Thank you Glen, Phil, and Thomas for the  very helpful replies!!
I meant to reply sooner but was unable to until now.

After reading the replies, I think my plans are much too large for my needs and will be too hot anyway.
I compost, so most of my garbage that I want to burn is plastic.I live alone so I do not create that much trash, but I cannot drive due to an illness and I have about 30 big trash bags of mainly plastic  piled up from the past 4 years as I was recovering and moving to my land ( moving seems to generate a lot extra trash for me).

So normally I would not need a huge incinerator but I wanted to make one big enough to handle a full 30 gal trash bag at least .

Maybe its dillusional, but I envision throwing filled 30 gal trash bags in without opening them and watching them easily disappear with little smoke!  I know with the right design, that is possible though.

Also I really wanted to use my stainless cloth dryer drum because they are great fire " pits" with not much smoke all by themselves (using wood though) , so I thought coupled with a rocket stove, it would be extra efficient, p!us it's big enough to hold a filled trash bag. Plus using a drum like that would make possible a true downdraft design.

I understand how it might melt though... but my design is a downdraft one ,so my thinking was that  the stainless drum would be in the coolest spot of the rocket stove right below the air intake. Like Glen said,  the hottest part would be the later part of the burn tunnel and lower riser. So I was thinking that stainless drum would be cooled enough by the incoming downdraft air that it would not get super heated like below it. I like Glen's grate idea but it seems like it might only work at first but then get clogged with smaller particles and cinders after not too long. I cannot really understand it without a pic though. I'm not sure if it would allow for a downdraft design either.

What would happen if I made the burn chamber large enough to hold a filled trash bag ( with or without using the stainless dryer drum), but made the burn chamber and riser a lot smaller in diameter than my plan? That would be much easier to build and also might prevent it from becoming ridicouslouly hot ...but would it still be able to burn up all the smoke up so its smokeless?

I would really like to make something that I can  Just through whole trash bag without opening  them up and dealing with a big mess. Also , since I live in hawaii, most of the trash is not very dry. I could possibly make a plastic hoop house to use as a solar dryer if absolutely necessary, but I'd really rather not have to dump all my trash out and deal with that mess. Thank you Phil for that info about certain furnaces actually needing wet feedstock to prevent overheating ...I did not know that. So I guess it's possible to make one that handles wet trash.

Glen said "What would be the minimum common size of items in the load? Spacing to hold most of those up would allow good flow around the load"

Most of the stuff is not large ( like plastic meat trays , wrappers, etc. But like I said I do not want to have dump everything out. Does it really need airflow around everything all at once? If I put a whole 30 gal filled trash bag in, the outside parts would get the air first and burn, but the inside would just get hot and smoke and make flammable producer gas which I'm hoping would all just burn up in the burn tunnel. I don't care if it all burns up really fast. I just want it to burn cleanly in the end with little smoke. I don't care if it takes all day to burn up 1 filled trash bag  ( though I know it probably won't take that long).

I also have plenty of solar electric power here so if the design I make has insufficient airflow, I could put in an adjustable blower to make an ideal airflow. I thought that also might allow me to use a smaller size burn tunnel and riser. And if my trash is especially wet, I could turn the blower up higher ( and lower if dry).

"Do you have a good quantity of wood or other easily combustible material to sustain the incineration of more problematic or erratic materials? You would want a good clean hot burn going before introducing plastics"

Thank you, yes I have plenty of good coppice wood ( strawberry guava) so that shouldn't be a problem.

Btw, the rock I have here to work with is volcanic porous basalt. Its melts at 1200 c I believe. Its also a bit insulating I assume just because of the many small holes. I'm planning on using store bought refractory mortar but then I want to line the inside with a refractory also ....sodium silicate ( waterglass) made from kitty litter and lye .

I attached another diagram with a modified design like I just talked about. I don't know if it will work good or not. Its got a removable lid with a hole the same size as the new smaller size burn tunnel .And also a blower if necessary. To sum my goa!s again : throw in full 30 gal bag if mainly plastic trash not fully dry and make it disappear with little smoke (I also have plenty of wood to help)

Thanks again for the helpful input!
Pete

I would suggest a bit of inverse design thinking based on a prototype that I'm working on. Make a normal, tried-and-true J-tube and put a retort above the heat riser so that the hot exhaust gases are hitting it. The retort needs to be sealed except for a pipe that leads back to the intake of the burn tunnel. Put your plastics and questionable stuff inside the retort and feed the rocket with normal firewood.

This design takes all of the evolved gas that comes from the thermal decomposition of the plastics and puts it into the zone where it can be fully and cleanly combusted. The high temperatures, airflow, and turbulence in the burn tunnel will make this a great way to completely destroy the waste material, and as a bonus you'll get a little biochar out of each batch from any organic material that was mixed in.

Oh, and it won't need to be anywhere this huge. You could probably do what you need with an 8" J-tube and use a steel drum as your retort, although I'm planning to scale up to a 12" system for the biochar retort that I'm working on. The retorts in my scenario will be metal skip bins that have reached the end of their service life but don't have any holes yet, and the whole shebang would be housed in a 20-foot container.

Thank you Phil! That is an awesome idea!

Great minds think alike I was recently thinking about a similar idea . But instead of burning all the gasses up inside the riser area, the tube could go to the outside and bubble up through some water to cool/ clean it and collect in an upturned tank .....like a methane digester setup. And then it could be used as fuel for cooking stoves, etc.

I was thinking of using a pressure pot (normally used for spraying) - attached pic).. I got that idea from a guy on YouTube who made a wood gasifier out of such a pressure pot. He replaced the rubber basket with a wood stove rope gasket.

There could another tube that goes to the riser ( like you said) with a valve(s) so if the fuel collection tank is fuel, the extra gasses could be burned in the riser like in your design.

Those pressure pots are made from thick steel or stainless  steel but I'm still not sure how long they will last in the high heat. It might be better to use recycled drums like you mentioned and replace when necessary. Those pressure pots are not cheap, but if they last long,  it would be worth it to me. They make them in all sizes. A 10 gallon one would fit a lot of plastic trash and be very convenient . Even a 5 gallon size would fit quite a bit if stuffed.

But if they do not last long, it might be better to use recycled drums like you mentioned and replace them when needed.

In your design, does the top of the skip bin protrube up outside of the riser very far, or just a little ? I'm thinking most if it is inside the riser? I'm guessing you have to weld a top on it?

I was born in 1974 long before the internet and had to use libraries for info and ideas, so I really appreciate the power of the internet. It dramatically speeds up the evolution of ideas ( when people are not looking a cats

Thank you again for sharing!!

Pete