What causes a batch box to smoke?

Sometimes i've seen my 4" batch box burn clean, and other times black smoke has poured out of the chimney
At times i can rearrage the logs inside the fire chamber to increase the flame and suddenly the smoke disappears.
I'm getting operational temperatures of 500-800 deg C inside the burn chamber.
Thanks,

It was probably never going to work but I stuck a camp TLUD stove on the chimney exit to see if it would burn the smoke.  It worked quite well but when I increased the smoke it just smothered the gasifying stove.

From my understanding, batch rockets are quite fickle and need to be built to the correct specifications, theses are all based as a % of the riser diameter.
I have read numerous times on this forum that you need to stick with the recommended dimension as any deviation from  those figures will result in possible malfunction.
Then you have to bare in mind air leaks from a dry stacking could easily effect the burn too.
Perhaps you could try a 6” J tube version as there is far more leeway with these  than a batch box.

Paul had a recent podcast talking about batch box versus J-tube stoves.  The consensus was that J-tubes are the way to go until you've built multiple (Paul said 10) successful rocket stoves or heaters.

Batch box heaters are cool, and they resemble a traditional metal box stove, but it's harder to get them to work right, when working right involves burning all the smoke and not making creosote.

I feel the batch boxes are superior to the traditional j tube, if much more finicky to build. As long as you stick to Peter's dimensions all should work well, but I do remember them saying that the smaller stoves are much more problematic with clean burning and a consistent draw, perhaps the small 4" design  is the issue more than anything else. Oversizing really isn't an issue with the batch so upgrading to a 6" wouldn't be a terrible idea

I'm building a mini pizza oven so I suspect 6" will over fuel the oven.  So, my next option might have to be to build a larger oven if that is the case suited for a 6" oven.

Regarding building 10 J tubes, that implies there's no one to help you.  But we do have experienced BBR builders here who can advise.

I'm thinking that if I have smoke for too long, then there's something wrong with my fire triangle.  If I see a flaming double vortex entering the riser, then I know mixing should be adequate, and that the temperature of the gases is sufficient as they pass through the port.  I added a secondary air channel and still I saw smoke.  So, I'm now deducing that I'm losing the heat too rapidly from the riser .. I've got too many fire bricks forming the riser.  I don't believe I'm over fueling the fire.

Take a look at the rocket oven design which recently finished a kickstarter that Paul set up ( https://permies.com/t/87936/rocket-ovens-kickstarter ), it is a 6" system and it's not too big for baking. I got to start it up for the pizza party and it started easily with zero smoke and you can adjust the temperature. That's part system size and part how much you feed it- you can partially cover the feed with brick and burn one stick at a time, or leave it open and burn 8 sticks at once.

Batch box isn't at all superior/better than a J tube, it is different though as it is meeting a different need at a cost. You can (properly) load a batch box with more wood and close the door and leave it be for longer than you can a J tube. But it will measurably burn more wood than the J tube for the same resulting heat according to Donkey, and usually will cost more to build (properly) due to the door construction. Of course you can spend more money on any design based on the parts you use, but the batch box simply has a few more parts. I think Donkey said the batch box will burn about 20% more wood than the J tube as far as efficiency (meaning more wood for the same heat), but it might have been someone else in the room at the time.

Not only does the batch box need to be properly sized, but loading the wood also requires precision to avoid air flow issues. If you stack too tight against the port you will have problems, and if you light the tinder at the door end you will likely get smoke back. J tube designs can be built far more bulletproof (or Gilligan proof to borrow Paul's terminology) but can also smoke back if a person tries hard enough to do everything wrong or it hasn't been cleaned lately.

If you are getting smoke out the exhaust then the burn chamber temperature is low, from reading about burn tunnel temps in https://permies.com/t/93105/Cooking-inch-batch-rocket-stove that would be the likely issue. Without proper sizing/spacing, sealing of joints, and insulation of burn tunnel/riser the temp will drop and/or the draw will suffer. The temps you are reporting are less than half of what you should get if built properly.

When visiting Paul and Jocelyn's place I saw a couple batch box feeds and a few J tube feeds all in service and they all work great if used properly, as they were well designed and built. I've previously seen rocket mass heaters that were in poorer shape at another site (needing cleaning or with less than ideal exhaust setups) which were more temperamental or smoked at times.

The podcasts on batch box heaters has some good wisdom regarding "improvements" as well, lots of people try to improve a design without understanding that the original design took years of testing to evolve to what it is. Batch box designs are still being tweaked to try and match J tube efficiency. Under perfect conditions (like Peter being the person starting/feeding his custom designs) batch box systems are probably right there though. But if all the corners are cut to make something from the few supplies on hand, there really isn't an answer beyond "rebuild it properly". I hope that doesn't come across as mean, but I do think this is the issue.

I watched this video

and even with a ceramic fiber riser Erica's face grimaces at 9:30 as she comments on smoke inside her rocket oven.  I presume it's a black oven (haven't seen the plans) on this account, and I just don't know if the health risks of cooking with ceramic fiber in your face has been properly documented. There doesn't appear to be an increased risk of cancer, but who knows about other fiber related lung diseases? At least I hope they used rigidizer.

I think she's talking about 500 deg F in her oven, and I get those temperatures as well (eventually), and I'll get there faster once I fill in the air gaps.

It's a white oven and I think they got it up to 800 a few times inside the oven itself, but in the burn tunnel and heat riser it's usually at 1500+ degrees. The insulated fiber materials usually used for RMHs are rated for 2300+ degrees so there shouldn't be any off gassing involved there. This oven just uses insulated firebrick if I recall, there's no inner riser wrapped in ceramic fiber.

Perhaps there was some hole or leak into the oven chamber allowing smoke in, or some pizza bits fell off onto the metal surface and charred, to cause smoke inside, but it wouldn't be from the burning wood if operating normally.

I'm also seeing temperatures of 800+ degrees Celsius or 1472 deg F in my burn chamber so I don't think my temperatures are that off.

It's not off gassing that's the concern. It's flaking off of microscopic filamentous particles that get carried in the combustion stream and pollute the air that you might breathe.

I see Peter has just said in another thread that 4" BBR stoves need to be carefully tended continuously otherwise they smoke.
So, I guess I have to upsize it.