I have been wanting to build a retort charcoal kiln, to process some of the wood I have that is too large for my chipper into charcoal. I have been on the lookout for some inexpensive tanks or firebricks to build one out of, when I happened across a used “smoker” for sale inexpensively within driving distance and drug it home today. It is made out of 3/16 steel, with a upper and lower chamber. I will have to reconfigure it a bit to act as a retort kiln. Attached are some pictures of the patient.
The upper chamber is 28" in diameter and 48" long, for about 17 cubic feet or 127 gallons of capacity. Lower chamber is 34" in diameter, same length.
Basic tasks for conversion are to:
• block off and seal off the upper (charcoal) chamber
• making a better a sealing upper door
• vent the wood gasses from the upper to the lower with some SCH40 pipe with drilled holes
• add a chimney to the top of the back box for exhaust
• insulate the upper chamber to hold the heat in
• and probably a few other tweaks and adjustments
I should have enough scrap steel to make all the adjustments, may need to purchase some high temperature insulation, weatherstripping, and perhaps a sealing "fireplace" door. I can move it around with my tractor, but I have also considered mounting the unit on a small trailer to make it more portable.
Anyway I am open to any thoughts and ideas on how to proceed.
Only had a few hours to work on the Retort today. I was able to round up enough scrap pipe and fittings to build a removable threaded chimney out of 3" sch 40 pipe. I cut out a vent hole and welded a coupling to the back box. Also built a rain deflecting hood for the upper exhaust out of a 3" adapter fitting and a scrap piece of channel. I also was able to build a blank off plate and get it welded in the back of the upper chamber, blocking off the old exhaust path when this unit was a smoker. Note there is nothing fun about trying to prep and weld the back of a 4' deep chamber, but I got it passable.
I am contemplating whether to build a door for the upper chamber from scratch, or to buy a "fireplace style" door (think salvaging a wood stove) and make it fit.
I'm not an expert nor even a welder but thoughts ...
- I worry that the chimney path from the burn chamber is too short, or rather that the hot gasses will exit the retort too quickly. This is just a a question of overall efficiency.
- Insulating the top will certainly help with the efficiency.
-3" diameter pipe seems far too small for a chimney. My BBQ has a bigger exit than that, 4" rocket stoves are generally considered the minimum... I can imagine that when opening the bottom chamber to check or refuel that the flames may decide they'd rather exit the front.
- Um, that schedule 40 steel pipe is galvanized? Bad things (toxic gas) happen when galvanized pipe is heated past 392F, so remember to be careful
- I'd think its easier to weld a good door on than to create one from scratch. The retort portion especially depends on a good seal, so I'd think you've got a higher rate of success on that path even if it requires extra work to frame out a structure.
Thank you for your feedback. In retrospect, yes the 3” diameter exhaust does look rather small, I was trying to use up scrap pipe I had on hand and the 3” looked way bigger than the existing 2” exhaust being used in the previous (smoker) configuration. After reviewing several retort builds with attention on the exhaust sizes, I concur the 3” is probably too small to work correctly.
Yes, I am aware of the issues with using galvanized where it will be heated, and plan to take precautions when it is used.
The current “exhaust” route runs out the back of the lower chamber, through the 30” tall 12x12 box, and through the 28” tall 3” exhaust pipe, exiting the “rain cap” at 7’-5” off the ground. If I doubled the exhaust length to about 60”, that would have it exiting around 10’ off the ground, which while not the end of the world would make the unit more difficult to store.
Looking around my scrap pile for larger pieces that would work, I have several tube and pipe pieces in the 3” to 4” range, and some box tubing that is 3”x5” square. For several reasons I would prefer to use what I have here versus purchasing other materials if possible.
Right now I am thinking of:
1: adding a 4” exhaust to supplement the 3” exhaust, (5 inch equivalent)
2: removing the 3” exhaust and running two 4” exhausts, (5.7 inch equivalent) or
3: removing the 3” exhaust and running two 3x5 box tube exhausts (6.2 inch equivalent)
I can run any of these options out at basically any height from 7'-5" up to 10’ tall (5’ chimney) or more.
I am open for suggestions on how to proceed. Thanks!
Glad you know about the zinc ... always hard to broach the topic and not leave you feeling accused of being a moron!
On the exhaust route ... I'm not so concerned about the height of the chimney (arise up lthough its good to consider it), and more about the path of it. I don't know how you could improve it in this case, so its more of a warning against underperforming. Your (inherited) design has the flame under the retort and then the heat flow is just under the retort, to the back box and then up and away. In contrast my rocket pizza oven has the heat rise up and hit the bottom of the front half of the inner barrerl, rise up around the sides of that half, pass through a channel to the back half of the barrel, flow down around the back half of the barrel and then out the chimney. So the heat wraps around the inner barrel for as much heat exchange as possible. From what I've seen of simple retorts I'm not worried that you can't make charcoal, just noting that this is relatively inefficient.
For the chimney, a 2" make sense for a smoker, really limiting the draw. You've got the cross sections figured out, but just remember that surface friction still means that two small pipes don't have equal performance of a large pipe. That said, I think you might be onto something with two different chimneys because you might be able to close off one or the other to tune the draw to the phase of burn. A simple disc on top of the pipe allows you to damp the chimney draw, so toss the largest diameter tube you've got on there and then you can damp it down.
No worries, safety first. Yes, the inherited setup doesn’t have the exhaust wrapping around the chamber as much as most designs which will inhibit performance to some extent. I am hoping that some insulation will retain enough heat for it to work reasonably, as altering that would entail a major redesign.
Per your suggestions, I put a new exhaust on the retort today, instead of using two smaller tubes I recycled some old air compressor tanks into an exhaust, it is 9” then 6” with a damper in the 6” portion. That should work much better than the initial setup.
I had some time after work today so I got some 2" angle iron scrap and welded up a "door frame" to make a nice sturdy flange for the door to seal to. I then took the original "door" for the smoker and welded it in place to seal up the charcoal chamber front. I then cut out the door opening to match the door flange and welded the door frame to it.
I have a tube frame the same size as the door frame, I am going to weld a piece of plate to the frame and call that my "door." Probably use some nuts and bolts to make it shut. I will need to round up some fireplace gasket to get it sealed tightly. After I finish the door, I just need to plug the old exhaust and install the vent/woodgas piping and the retort should be ready for a test firing.
This is a very cool project. I gave up on retorts because the ones I saw were too small and I figured I'd never get enough volume of biochar. I'm glad you're finding a way to make it work. I predict others will see your project and find a way to make something similar work for them , too.
I have not had much time to work on this project recently, however I was able to cut out a piece of plate for the charcoal chamber door and start welding the tube frame reinforcing to it. My current plan is to drill through the door and the mating flange, and have the door be a bolted connection. I am thinking 6 bolts would be good.
I also grabbed a few "fireplace gasket" kits from the store, I plan to glue that onto the door part for a good seal.
Had a bit more time to work on the Retort Kiln, was able to do some finishing work on the upper chamber door and glue on the "fireplace gasket" to help it seal. I also cleaned up the mating surface on the other side and painted it with High Temperature Paint.
I drilled through holes through the door and mating flange, and welded some nuts to the backside of the flange. Thus I will only have to run in six bolts to shut the lid. Also I added the super important lower brackets to support the (heavy) door while I attach the bolts.
Next I started working on the piping to vent the steam and ultimately take the wood gas to the lower chamber to burn. I am recycling some existing pipe from the scrap pile. Note extra precautions are to me made because it is galvanized, aka don't try this at home.
My current plan for "insulation" is to build a "standoff" out of angle iron a couple inches from the main body and install some metal roofing to create an airspace. This will act as both to reduce heat loss through capturing an airspace, similar to a double pane window. This should also allow the convection heating of the upper part of the chamber from heat rising. If this amount of heat retention ends up insufficient, I will likely line the inside of the metal roofing with some high temperature insulation.
Had some time this evening to keep working on this, I was able to use a hole saw to cut the holes to allow the upper chamber to vent to the lower chamber, or to atmosphere via the gate valve. Assembled the pipes and got it welded to the body of the retort. Drilled the lower pipe with holes (3/16) to act as gas jet burners once the steam is off and the woodgas starts coming out. I trimmed the upper pipe, only goes into the upper chamber a few inches. I need to get a cap for the lower pipe end.
I then used one of the circular drops from the hole saw and welded up the old chimney hole from where the "smoker" used to vent out the front top. Because the lower door hit the new upper door frame, I trimmed the tops of the lower door where the wood is fed, and welded them in place.
Lastly I got some old angle I had lying around and started building the standoffs for the "insulation" standoff. Made three identical frames, welded two to the front and back of the main body edges, and one with an extra crosspiece back behind the chimney. This will be the surface I mount the metal roofing on. For anyone who has not figured it out, this is not a "precision" type project.
Still plodding away on this. I was able to find adapters in my pipe pile to put a plug on the end of the "burner" pipe in the firebox. I then began putting some scrap metal roofing on the standoff frame. This is serving to trap heat near the retort body, to keep the temperature in the charcoal chamber elevated. Got the back, sides, and top basically done.
I am planning on making a removable "door" of cladding for the upper part of the front to cover over the actual charcoal chamber door to keep heat in there. I am not planning on cladding the underside or the lower part of the front where the burn chamber is. My gut feel is that the current amount of heat trapping will allow the charcoal to process without requiring excessive firewood to keep the process going. Hopefully the firewood is only until the wood gas begins, and the wood gas finishes the process through. If it doesn't work out like that, I will go back and add actual high temperature insulation below the cladding.
I like reading about and seeing your project. I don't have anywhere the skills, the space, nor the materials to do something like that. Other people on the list do, however, and will probably want to use some of your ideas to make their project work.
Thank you for the encouragement. No real skills needed to do a project like this, just a few tools and some persistence.
Thank you as well, hopefully the report on its function can be positive! If not, then expect updates on redesign.
It was a rainy day, but I was able to get the "door cover" built for the upper part of the front this evening. The unit is now 95% done, which is done enough to run a proof of concept load through it. I will try to do that as soon as I get some free time to monitor it, at least for the first batch.
If you are looking for something to hold heat, I can recommend looking at coating your contraption with Satanite. Its a foundry cement used inside metal forges so it will take high heat for long periods of time without breaking down. Its about 5# for 20 USD plus shipping. For your purposes, since your heat is not so high, you might be able to cut it with locally acquired sand. Its pretty insulating by itself.
Its meant to be used over a refractory blanket inside the forge. For your purposes I don't know if the mortar alone would be sufficient. The blanket tends to be more expensive at about 100 USD for 50 feet x 2 feet roll. The upside is that you can try the cement alone and if that's not enough, you can wrap it with the blanket and then apply another layer of cement to seal out any dirt or moisture from getting into the blanket.
Rock wool as a material would work but the binders that hold it into bats disintegrates if it gets too hot,
Just my 2 cents...
Money may not make people happy but it will get you all the warm fuzzy puppies you can cuddle and that makes most people happy.
Thank you for the suggestions on high temperature insulation. I am still hoping not to need additional insulation, but we will see after the first batch. I like the suggestion of coating the blanket, I was hesitant to use a refractory type blanket out of concern about the insulation getting wet/dirty.
I actually intended to run a batch this evening after work, however after I loaded the charcoal chamber and lower firebox, I noticed that I had forgotten to weld up a drain pipe left over from its previous life as a smoker. I blame that the charcoal door setup hid it from plain view... so I got out my flux core mig and assorted tools and welded it shut in the field as I had already moved it far from the garage and loaded it with wood. That ate up too much time this evening, so I will have to run the load another day.
So as an update, the unit was backdrafting smoke badly out the firebox door, so I ended up cutting the smaller 6" upper chimney section off while it was running, leaving only the lower larger part. That solved most of the backdraft issues, the shorter chimney still seemed to draft well. After looking at the smoke path in the firebox the transition from the firebox to the rear stack needs smoothed out. I will also probably buy a new upper chimney section.
After three and a half hours around 9:00 est, the unit was still venting lots of steam. I chalk this to loading the large charcoal chamber with a large amount of big wet wood more than some inherent design flaw, but time will tell. I decided to leave the vent open and just let the fire die out in the unit as it vents steam. I will try lighting the unit again tomorrow, hopefully will get it to the point of gasification then.
I think this has shown me that from a practical standpoint I need to incorporate a stack and dry period into my process, otherwise I will have to use more lumber than necessary in the firebox just to drive out the excessive moisture as steam.
The temperature gun shows the unit is getting plenty hot, but without running a full cycle I can not yet draw any conclusions as to whether additional insulation is needed. My current thought is that if the unit can sustain gasification to finish the pyrolysis process without having to add additional wood for heat after gasification starts, then I will consider the current insulation successful as is.
So, I ran the unit today for about 6 hours, it continued to vent steam and wood vinegar, but did not get to the point of gasification. I believe this indicates that either filling the charcoal chamber with 10+ inch diameter wet wood was a mistake or the unit is horribly inefficient. Or both.
I am letting the fire die down and the unit cool down overnight.
I think I will try to light as hot a fire in it as I can tomorrow and see if it will start to gasify.
I began stacking some wood under cover to begin to dry out for future batches.
John - you're miles ahead of me here ... but I gotta say the 10" diameter wet log seems the problem!
Everything I've ever seen about making charcoal seems to use a) dry wood and b) the smallest scraps possible. Smaller pieces pack more densely so the process is more efficient, and because we're trying to make char and not charcoal, the size of the output really doesn't matter (and, grains are probably better than chunks for spreading, etc).
I just did a cursory search on kiln drying times ... without speaking authoritatively it looks like a 2x4 can spend as long as a week in a kiln. I know that a solar lumber dryer can take about a month. So ... there's a lot of steam to remove. You might let time and sunshine do most of the work...
Thanks for the feedback. I knew the large wet wood was a stretch when I did it. I was hoping it would work without drying to save extra time and handling, but I will start drying out any further wood before processing. I have a carport that I can start drying the lumber under.
I still hold out some hope to process at least medium sized pieces, maybe I could split larger ones like firewood to increase surface area to volume.
So, long story short the other day I decided to open the chamber to check the progress rather than continue to fire the retort. I found the unit had turned some of the wood that was touching the bottom of the chamber into charcoal, however it was quite obvious that the majority of the lumber had not begun to char significantly. There was also a "pool" of condensate water at the bottom, as the unit cools down it seems moisture condenses on the upper surfaces of the chamber. I dumped some of the loose charcoal into my compost pile.
Since it is now obvious that my lumber needs to dry some before processing, I have some time to redesign as my lumber begins to dry. Although the unit may have "worked" with dry wood, I am seeing enough signs that it is not retaining enough heat to my liking.
Based on my observations, I believe the unit needs to either conduct more combustion heat into the charcoal chamber and/or more effectively retain the heat in the charcoal chamber. The process wood needs to be reduced in size to more rapidly dry and then char. Lastly the lower burn chamber still needs improvement in drafting or a second chimney stack added. I believe these are the major issues to be solved going forward, but please chime in if I am missing something.
The inherited design has a not-sloped 48" deep burn chamber, which causes smoke to inevitably come out the front of the unit, even when the chimney is drawing quite well. Rather than continue to fight this, I am considering adding auxiliary chimney stacks to the front of the unit. This would be easy to do by just having two pipes come out the two sides, more difficult would be attempting to "wrap" the exhaust around the upper chamber, also requiring a cladding redesign. I could enlarge the rear chimney and hope the increased draw eliminates this problem, but with a 48" chamber I am not sure that would work. Additionally the unit seems less hot at the front, drawing exhaust out the front may assist that.
The unit needs insulated better. I am figuring either some type of high temperature batting covered in either metal foil / sealing compound or to refine the cladding to be more tightly closed and fill the space between with perlite or vermiculite.
The wood to be processed is basically lumber to large to be chipped, but too small/damaged/curved to be processed for lumber. Most pieces will be 6"+ in diameter. I now expect that the pieces will need to be split like firewood prior to processing.
As an update the project is not dead, I just shifted gears to getting my process wood ready. Since the feed stock needed to be well dried before processing I have been cutting and stacking wood, as a byproduct of some left over pieces and new clearing for a small pecan orchard I am putting in. I now have about 8 ricks of wood drying under cover, and I have more to go. I plan to split the wood before processing, to reduce the size of the pieces.
I have had some time to reflect on my design, a welcome change from “rushing”. To improve the efficiency of the unit, I now plan to wrap the exhaust around the upper part of the retort charcoal chamber, instead of only heating from the bottom. I still plan to add insulation to the unit, but without the additional heat transfer of a wrapped exhaust I fear the retort would not be able to self-fire once gasification starts.
This will set me back a bit on the project, as I will have to redo the cladding work, but such is life. I moved the unit back close to my garage today to begin working on the exhaust changes.
I think you will find you have much, much better results with dry wood. I make a lot of charcoal and any time I tried to use wood that wasn't really dry, my results were not very good. Now I don't even attempt to make charcoal with wood that isn't fully dry.
As an aside, I love your project and can't wait to see more results.
I am glad to hear that your experience shows better results with dry wood, I am hopeful that my wood air drying will produce similar results. I have even thought about using an old window to make a small “solar kiln” for final heat assisted drying right before processing.
So after getting plenty of wood squirreled away to begin to dry out, I began to make the modification to my retort kiln. I decided to use an old air compressor tank to extend the exhaust across the top of the unit to increase the heat transfer from the exhaust gasses. A few calculations show that the cross-sectional area of a 17” diameter semicircle atop a 29” diameter tank should be about 83 inches square, which should be sufficient for the main exhaust (similar to a 10” diameter circle). I got the tank cut apart, need to make the modifications and weld it up. The picture shows the basic idea, the items in the way will be trimmed back so it can sit flat. I will also add a new exhaust stack to the front.
After I redo the exhaust, I will redo the metal roof heat shielding above the new exhaust and add a floor to the bottom. I will also cover and seal up all the openings so that it closes up tighter as I have ordered some perlite which I plan to use as high temperature insulation to fill the cavity between the retort kiln unit and the cladding.
I am still plugging away at redoing the biochar kiln to be more efficient while my wood is drying out. I am working on this outside so weather (and time) are seriously limiting factors. I need to get this completed however if I hope to have any biochar ready in time for a garden this spring.
I was about to get the old chimney cut away for the new exhaust crossover over the top of the unit. I will need to weld on some new sides to transition from the rear exhaust stack to the new crossover. The metal chimney on top is just for example, I actually have another old air tank I will repurpose for the new upper exhaust.
I also was able to open up the upper portion of the firebox to rear exhaust stack so that should flow better. I also removed several pieces of steel that were in the way in the firebox and exhaust area that were not really needed anymore. Climbing in the firebox to use tools to clean up that portion was less than fun.
My bags of perlite were delivered, once I get all the modification made, I will redo the cladding to be perlite-tight and will pour it in between the unit and the cladding as a loose fill insulation. A bit of a hassle to get the cladding tight, but not impossible.
Thanks, I am hoping it will be ready for a second test in the next few weeks.
I got most of the fabricating done on the new wraparound exhaust path for the retort kiln today. Made it out of two recycled air tanks and some random scrap pieces lying around. Don’t hold my welding against me, it was done in a hurry outside with occasional wind and less than ideal fit-ups.
I made new upper supports for a new sloped top roof, I still need to make a frame for the bottom and start closing off gaps so that the cladding can be filled with perlite.
I had a little bit of time to work on the kiln today, I did some pondering and measuring on how to best close and seal up the cladding. I also got the old door gasketing removed, cleaned up, and was able to weld a few strips of steel to better hold the gasket in place with the adhesive. The initial try of just having it on a flat plate did not fare so well, it seems to work better when pinned in place by some sides.
Today I finished the last of the welding that I need to do in preparation for installing the cladding. Basically I have welded up "floors" to close off the bottom of the cladding cavity. This seal is about halfway up the firebox, based on where it would be easy for me to accomplish the seal, and will fully encapsulate the charcoal chamber and the wrap around part of the exhaust. In a perfect world I would have sealed under the entire bottom, but it would have been much much more work for limited thermal gains.
Next I will install metal roofing on the sides similar to how I had it before, except with more fasteners to hold it tighter. Then I will pour the perlite into the body to cladding cavity before I install the metal on the roof.
I also reinstalled the fireplace gasket onto the door, with the main seal installed between the two metal stops.
I am wanting to get this project done very soon, as I am only a few weeks away from wanting to start my annual vegetable garden and I would like to incorporate some biochar into the soil at that time. Thus I need some time to run some batches and inoculate before then. Thus I need to finish the cladding work and run a test batch very soon.
So over the past few days I have been able to get the unit mostly (95%-ish) completed. I got the cladding installed and filled with perlite. I tapped on all the panels to get the perlite to settle, then got the roof installed pouring in additional perlite as I went. I used 7.5 bags, or 15 cubic feet.
I still would like to place additional flashing around the chimney-roof intersection to make that more water resistant, put an a layer of insulation on the retort chamber door, and would like to add a few additional pieces of roofing trim on the unit. But the unit is probably done enough at this point to run a test batch through.
I cut and split some of my air drying wood into some smaller pieces. I loaded the charcoal chamber full of dry-ish white oak, most of which was 1-3” thick, and roughly 6-8” wide by 12-18” long. These were drops from table slab milling, and was some of the driest scrap lumber I had lying around.
After closing the chamber, I lit the fire. The unit ran kind of smokey for the first bit until the unit got warmed up, after that the unit flowed very well with minimal backdrafting despite the longer exhaust route. I lit the unit in the afternoon and it produced steam and wood vinegar in good volume for many hours. The unit is not taking nearly as much fire wood to keep the retort hot and venting as it did previously, which I attribute to the insulation. Five hours into the burn the unit was still venting steam/wood vinegar, but as it was later in the day I decided to let the fire die down and the unit keep venting as the fire died out and the unit cools off overnight. I am hopeful that most of the moisture was baked off today, and sometime soon into the next firing can begin the flow of woodgas.
I know the amount of time venting steam is a function of the moisture in the process wood when I start. I am interested in rigging up a solar wood dryer, if I could make something that held about the same volume as the retort I could rotate the process lumber into the dryer the for a day or two before the retort to extract a bit more moisture using solar energy rather than firewood. I hope to rig up something to this effect out of some scrap/junk I have lying about, mainly to get rid of some of this scrap/junk towards a useful cause.
I loaded the unit with more wood in the burn chamber early this morning and lit it. By lunch it was exhausting white gas like crazy out of the charcoal chamber, so I partially closed the valve to shoot the gas into the burn chamber and this time it ignited. I don’t think I have enough holes in the pipe in the burn chamber, so instead of shutting the valve completely I lit the gas stream coming off of the valve. This kept excess pressure from building up against the door gasket. It sounded exactly like a handheld blowtorch when it was burning. The gas was also shooting out of the burner into the burn chamber and was burning very very hot, discolored the cladding closest to that spot. Within a few hours the gas stream had tapered off, so I shut the valve all the way. By this evening the gas had stopped flowing, but the unit was still plenty hot. I decided not to try to open the charcoal chamber until tomorrow. I am excited to open the unit up tomorrow and see what I have.
A couple of fixes I see the unit needs, I would like to drill more holes in the burner pipe so there would be less backpressure so that I could close the valve if I wanted to. Secondly, in the future I only plan to light the unit first thing in the morning, so that it doesn’t go through an overnight cool down period. It takes a lot of heat to get up to temperature and the unit smokes like crazy as it first warms up. Lastly I want to have something in the unit for the wood gas flame front to hit other than the side of the unit, perhaps some firebrick stacked on edge along the right side would reduce further cladding warpage/discoloration.
So I was able to crack open the retort around lunch today, and was pleased to see fully carbonized biochar. It “clinks” when cracks, and there are no signs of remaining tars/VOCs of any kind. As blazing hot as the unit got during the gasification stage I was not surprised, but it is always good so see something actually work.
When emptied, the charcoal filled three old garbage cans, so about 100 gallons of char total. Note I did not crush/pack it down, but the big pieces broke apart when put in the can. I threw the next load of hardwood in the burn chamber, but did not yet light the unit as I want to make a few tweaks in the firebox first.
Now I need to decide how to start inoculating the char. It seems reasonable that I could create about a thousand gallons of char in the near future, so I am tempted to try several different strategies for inoculation and I will have multiple batches to try things out on. I plan to mix as much char as I can make into my annual garden bed this year, 1000 gallons is about 133 cubic feet, which is about 1.33 inches thick across a 1200 square foot garden, or about 10% by volume if incorporated 16” deep.
We grilled out today, so I stole a few handfuls of the char to grill with. It turned out great, a little bit of a learning curve vs brickettes but not too hard. I prefer the flavor to propane, plus I was able to get the grill much hotter for a good sear.
Off topic I know, but I will set aside a few gallons for non agricultural purposes as it is handy to have some real charcoal around.
Had time to mess with the charcoal retort again. We have nicknamed it the flaming hippo. I started by drilling some more burner holes for the wood gas in the burn chamber as well as drilling them out slightly larger. This should allow more gas into the burn chamber without causing excess backpressure in the charcoal chamber. I also lined the opposite side with some firebrick, so that the flame front isn’t directly hitting the side of the steel firebox.
I loaded the firebox with some wood and lit it off. After about 2.5 hours I partially shut the valve to pipe some of the gas into the burn chamber and lit the remaining gas coming out of the valve. The unit got very hot, I noted the steel grate on the burn chamber floor was glowing red. I cracked the valve open a bit more so that more gas was vented out and burned versus burned internally. I left it running with about half of the gas in and half vented out until the flame front started to die down, when I then shut the valve all the way. Within six hours of lighting the gassing had greatly reduced, the fire was allowed to go out, and the unit began to cool down before nightfall.
Overall I am happy with the unit, at this point I don’t really think I have any more tweaks planned, just need to run batches through the unit.
To date I have run four batches through the flaming hippo. The second batch of hardwood processed nicely, but the “firewood” type pieces I used for the latter batches don’t pack as tightly so it only makes about 1.5-2 trash cans, so between 50-64 gallons a batch. The third batch I tried pine, it made a mess with creosote dripping and it wasn’t as dry as I thought it was so it took forever for it to start gasifying, which eats into the efficiency by requiring more wood to keep it burning. Didn’t finish in one day, the cooldown overnight makes additional wood-tar mess everywhere. Not planning on doing that again, so the softwoods are destined for the burn chamber not the process chamber, and all process wood must be dry. The fourth batch was hardwood again, reasonably dry so it processed in about 6-7 hours total from lighting to the end of gasification.
The unit takes some time to come up to temp, I start a moderate fire and let it slowly burn and vent fully open. Then once it has cooked a lot of water off to get it to start gasifying I add wood to build up the fire in the burn chamber to get the temperature up in the unit. Once it starts gasifying I partially shut the valve, with the additional burner holes the unit can quickly get very hot, so I basically keep about half of the gas vented as the unit does not need the additional heat. I use the amount of the burn chamber floor that is glowing orange as my thermostat, if it looks excessive I open the vent more. If smoke starts coming out the front that is also a clue it has too much wood gas in the burn chamber. Once the raging inferno starts to die down I completely shut the vent and let the wood gas continue to burn itself until it is out. I let the unit cool overnight before opening.
The cladding has sustained some warping from the extreme temperatures and the limited ability for it to flex given how perlite tight I was trying to make it. It is working fine, so I am not worried about it.
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