New TLUD burning slow and smokey

I've been using the Toucan TLUD (1 gallon) design for awhile now and wanted something bigger, so I started from scratch using a 20 gallon galvanized trash can as my burner. For the "crown" section I used an inverted 3 gallon galvanized livestock water dish with a 3" hole in the center and an 8"x2' chimney. There are about 50 or so half-inch holes in the bottom of the garbage can, and a dozen or so in the crown.

I did two burns today and the first burned out after the top 6" charred, and the second one burned REAL SLOW, but eventually did get to the bottom, although it wasn't an even burn and I still had some raw wood left. I'm chalking that up to poor choice in fuel. Not all of it was the same diameter...it ranged from chunks of bark to 4" or so diameter by 2' length logs. In general, I got little ash and more char from the second burn than I've gotten from all of my runs with the 1 gallon TLUD combined, but it was a SLOW burn and it was VERY smokey. Several times I thought it was going to go out. I never did get the "rocket effect" of flames coming out of the chimney like I did with the small version.

I know smoke can indicate too much air or not enough air, and I'm inclined to believe that this is not enough air. With the lack of large flame, slow burn time, and smoke, does that sound right? If so, would it make more sense to put more ventilation holes in the bottom of the trash can, in the crown area, or both?

Jeffrey Roberts wrote: Not all of it was the same diameter...it ranged from chunks of bark to 4" or so diameter by 2' length logs.

That might be your problem, I've never tried to burn such big chunks in my TLUD. I load it from a pile of wood chips and there is only an occasional big piece of wood or some hefty branches.

I would suggest loading the small material in the bottom of the TLUD and leaving big chunks for the top. It will take them longer to release all of their combustible gases, which they can do as they slowly travel down the barrel with the flame front.

I'll try that - I actually had it backwards from the way you suggested, but that makes sense about the gases. I was able to do all sorts of different sizes in my smaller TLUD (relatively speaking, of course, since it was only a gallon) at the same time without an issue though.

Here's something else I'm wondering about but I don't want to jump on any hunches yet because there's no going back once I drill more holes. If there's more ventilation in the crown or chimney, it will ignite the gases (although any ignition should light up all of them, right?). More burning of gases = more heat = more draw. Does that sound right? Or if this problem persists, should I go with more ventilation in the bottom? It's propped up on bricks, so plenty of ways for air to get in...

It sounds to me like it draws air just fine, no more holes in the bottom needed. I only rarely get the gases to ignite in the chimney, usually at the beginning of the burn. That's when the whole mass of material is heating up and there will be a lot of off-gassing and there will be something in the gas still left to burn as it exits the chimney. Then it settles down and pour out copious amounts of white smoke -- a little bit of particulate, but most of the "smoke" is just the water vapor produced by combustion condensing. I judge when the biochar burn it done by when there is no more water vapor condensing and just wisps of white particulate smoke left. That tells me all the water has been driven off and what is remaining in the barrel is char.

What size of container are you burning in? My only previous experience was with the 1 gallon one and it smoked at the beginning, burned totally clear, and then my indicator that it was done was when it started smoking again towards the end. It's interesting to see different perspectives on this because there's a lot of variation!

Okay - second run of the stove, and the results were worse with smaller fuel. I filled it with finger-diameter sticks from top to bottom, lit it up, and it burned great on one side (maybe too fast, even) but didn't burn evenly so the whole other side was basically untouched. The whole thing was REALLY smokey this time, but I have an observation:

It burns great without the crown part or chimney (so it's essentially a campfire in a garbage can), but as soon as I put the crown on (with or without the chimney) it would totally kill it. If I took the chimney off and lifted the crown part up an inch or so, it burst back into flames with no problem. To me, this indicates not enough air flow.... should I enlarge the hole in the top of the crown area? The whole top of the crown is about 15" wide, and the hole is approximately 3" in diameter. The other holes in the sides of the crown (10 of them) are 1/2" in diameter. Maybe more of them? Looking at the Toucan TLUD design, the crown is very open for its size and mine is not.

Thoughts?

I use a 55 gallon drum, and my purpose is to make biochar. I agree totally with the observation that the chimney cuts down on the combustion. I have a 6" diameter by 14" tall chimney that sits on top of the lid, and if I remove the entire lid+chimney in the middle of a burn, it will start drawing more air and soon burst into flame. Then, when the lid+chimney goes back on, it quenches the flames and it goes back to the smouldering type burn that you want when you are making charcoal.

You didn't say in your original post what your purpose was with this TLUD. If this is a cook stove of some type, then you DO want flames, so I would say dispense with the chimney altogether.

My purpose is biochar also. Looking at plans for the 55 gallon drum stoves as you're talking about, as well as the 1 gallon stoves, the hole in the chimney is 50% of the total diameter of the burn chamber. On mine it's less than half of that. I think tomorrow I'm going to widen it and try another burn. I know for biochar production it should be a slower burn than if I were using it for cooking, but most of what I'm reading says there should be little to no smoke so I think mine's burning way too slowly.....

Have you seen this video?



After several lousy attempts to make biochar, I decided to do just like this video, and it really works well. It takes anywhere from 4-6 hours to do a complete burn where all the material in the barrel is completely charred, but that is not a problem as it doesn't need to be constantly tended. The only difference I have is that my chimney is not a 30 gal. barrel, but is just two no.10 food cans fitted together.

And don't believe these people that say there should be no smoke. When water vapor released during combustion hits the ambient air outside, it condenses and the steam cloud continues until you run out of cellulose to burn. When it's all charcoal in the barrel, and that begins to burn, that's when the exhaust is pure CO2 and you will have no smoke.

I have not, but I will check it out now. I'm surprised that your chimney is so small compared to the size of the drum. Does it get good enough draft to burn clean or is it smokey?

EDIT: Now that i'm watching this, I realize I have seen it but didn't put too much stock into it because I watched it when I was still building my 1 gallon TLUD and didn't think much of it applied. I will watch it again though since this is larger

It pours out copious amounts of white water vapor (good thing my neighbors don't complain). When that tails off, I know the burn is coming to an end.

When doing a biochar burn, there should be no smoke other than usually at thebeginning or end. Steam is not smoke, it if water burning off. One thing to consider is whether your wood is dry or wet. If it is wet then it probably wont burn well, or burn without smoke. I just finshed building a J-Ro TLUD built under the watchful eye of Art Donnely of Seachar.org. I was hoping to burn this weekend, but we had torrential rain the last few days, and everything is wet. I hope to get a burn in soon.

Best of luck with all your biochar efforts.