Time to go from updraft to downdraft charcoal gasifier?

I have been debugging my updraft gasifier the past few months. I am wondering if it is time to go downdraft. Here’s the chronology of what leads me to this.

A few months ago during the southern hemisphere summer, the gasifier was putting out good gas. For an air nozzle, I was using a horizontal steel pipe with a cap that had several 1/4” holes drilled in it.

On one run, I got the generator running well and kept an eye on it for the first 10 or 15 minutes. Stupidly, I walked away for about an hour and came back to this melted mess.

I then devised a ceramic nozzle. I welded a holder for a brick that had holes drilled through it. The holder was at the bottom of the gasifier.

I got good gas again, but slag was building up on the brick.

I then cut a brick in half to make a slit opening. It was easier than drilling holes. I added a water drip in the form of an ultrasonic mister. I figured it was an easy way to control the amount of water and could be powered by the generator. That way, when the generator shut down, so would the water input. I did a run for about an hour. I didn’t have a buck step up converter to control the mister yet, so I ran it at full voltage I had some water condensing out in the heat exchanger, so I was surprised that when I removed the nozzle for inspection, it had thick layers of slag.

Fast forward a couple of months. At this point I came to the conclusion that the only way to avoid slag on the nozzle was to have it pointing downwards. So I built one that would fit in the bung hole of the gasifier barrel and protected it from heat with a water jacket. The nozzle had a single 7/8” hole – the diameter of the inner air tube.

I ran the new nozzle for an hour with water mist. The nozzle had no damage, but the quality of gas was poor. There was a bunch of water condensing out at the heat exchanger and final filter. At least the nozzle had no heat damage at all.

I then narrowed the opening to a single 7/16” hole. I still got poor quality gas – there is way too much water in it. I think I flooded the charcoal load when I ran with the mister. After a run I left the loading door open to let the heat bake off water in the charcoal. Then I did another run and the gas was good. Good flare and good generator running. However, after adding in more charcoal, I’m back to condensing water out at the heat exchanger and the gas quality is down.

I think the charcoal I made several months ago has absorbed moisture from the atmosphere. Summer here is extremely dry, but fall and winter is extremely wet and humid. Unless I get a number of barrels I can hermetically seal, the updraft style of gasifier won’t work.

So I am looking for a good downdraft design where the water might be converted to H2. Also, I always liked the ability to clean the gasifier without completely emptying it like the updraft.

I am thinking that a hearth made with ceramic bricks and a water cooled nozzle pointing downwards like what I currently have may be a viable design.

I wonder what size to make the hearth. When I run my 3.5kw generator, I develop what appears to be a basketball sized mass of burning charcoal. This surprises me, as I expected something more akin to the size of a hearth in a downdraft.

So, can anyone suggest a design they can point me towards? Thanks!


Hi Marty , if your looking to try a downdraft your best to go with Matt’s design , have a look on his thrive off grid postings he goes through it all in detail .
If you want to keep the updraft system you have now i would start by getting a Hexoloy nozzle and have that in the bottom of your drum vertically , you will still get slag build up but its not a big problem i ran mine for maybe 2 or 3 months before cleaning out some of the slag volcano build ups around the nozzle , ,by the way my nozzle has been in the same gasifier since 2018 and run many many hundreds of hours running a 7.5kw generator flat out , never once used water to boost the gas , just be mindful that before each start up after a week or two to push a soft wood rod up from underneath to clear any slag build up that may be starting to close off the nozzle .
Orrrr if you need to use a water drip then i would go with Kristijans Flute style nozzle mounted horizontally , that works great with dripping water turning to steam instantly have a look on the Nozzle for charcoal posting they should all be on there .
All the best Dave

Cheers dave


If you wanna try a flute updraft, I would suggest trying this idea out that Bill went with.

Slag buildup is inevitable with updraft. It’s just part of the maintenance schedule. I had to scrape mine every once in a while but normally got away with just poking out the holes and clearing them.

Benefits of a vertical nozzle or Flute is the water has time to turn to steam and convert better to hydrogen, and the nozzle(s) are below the heat.


we switched to a down draft on the boat. seems to handle poor-er charcoal well… and LOVES water.


marty, thank you for your nice documentation and observation, very interesting…for clean out the gasifier with the bottom -updraft nozzle is not needed to shovel out all the hopper, make a cleaning port on the sidewall and clean out with a hooke all the ash and slags and stop when clean coal comes down …(Transmission belt made from tires on woodgas small engine - #85 by giorgio)


marty, another thought because umidity in the coal…we have the same situation with winter umidity (Charcoal from twigs...simple, quick and effective without smoke - #5 by giorgio)
this containers (white plastic) can help for hermetic storage, if available…
this is the design how i like to build my updraft gasifier for the chain tractor…instead making the gas outlet on top i have the possibility with this design to bring the gas outlet with different pipe lenght near possible to the reduction area…so i guess not much umidity goes in the system but collects under the hopper lid, instead when gas outlet is on top of the hopper, all vapour is sucked in the system…
only the gas comes out more hot and needs probably some cooler…
your cruncher is very smart…has it got some modifications yet?


I had to shake my head and smile while reading your report Marty. I have had all the same experiences including making a nozzle out of a fire brick and melting it. Dave said that would happen but I did it anyway. I have had a flare that spit droplets of water out with the smoke. Flared fine but would not run an engine. I use the hexaloy Dave mentioned and it is very durable but probably not easy for you to get. I also have a pipe cap nozzle like you have shown sitting on a horizontal intake pipe with an elbow so the cap ends up vertical but have not run it long enough to say if it will hold up or not. I also agree with Dave about using Matt’s design. He has already done all the experimentation for us. If I get enough time I want to convert an old WK design I have with a 8 inch fire tube to down draft charcoal. I built it to run a 390 cc generator but never used it because at the time I didn’t really know how to run it. Maybe a little smarter now. Anyway thanks for the report. It should save a lot of new comers a lot of the hassles both of us experienced.


Thanks all for the comments and advice. I like the idea of storing charcoal in one of those plastic containers, but I’ll have to see if they’re available here. I’m sure if they are, they will be much more expensive then anywhere else in the world. :slightly_frowning_face: (From experience). If I go that way, at least I wouldn’t have to change anything on the gasifier.

Giorgio, I haven’t worked on the charcoal crusher for months. It was working pretty well with the wooden crush plates. One thing I have to do though is change the belt drive. It is prone to slipping, so I need either to get a double pulley drive or maybe a timing belt drive for it. A flywheel would also be beneficial. Beyond that, a major rebuild would steepen and lengthen the screen underneath to let the charcoal exit better. At that point I would probably go to steel plates. A good winter project.


Dave, I started going through Matt’s thread and noticed that he has several designs. Which one are you referencing? Thanks.


Most likely his latest design which is essentially an Imbert gasifier that he can switch between wood chips or charcoal by adding a restriction.

Edit: Matt still uses water drip in this gasifier. The design is very clever how he set it up.


Cody, is that the cylindrical weld up kit or the rectangular shaped gasifier? Thanks.


The cylindrical one. He has YouTube videos showing how it’s made up.

He’s gone through a few revisions already. He’s doing R&D live on it basically.


marty, with your excellent water cooled nozzle from top you can easy modyfie your gasifier on downdraft system…gas exit where you have the door down, old stove grate in front, and firebricks around to make smaller the diameter, so no umidity can bypass the glow zone…


Hello Martin, I don’t have much experience with charcoal gasifiers, because I think that gasification of wood produces much better gas, because the conversion of tar gases makes it possible. Otherwise, I try to achieve a wider layer of glowing coal in my gasifiers, which is lifted and cleaned by the generated gases from the fine coal and the ash is removed from this area. I also pay a lot of attention to preheating the air to a high temperature countercurrently with hot gases and preparing dry wood. Recently, I have been using torofied wood, which gives a really strong gas, because this way the gasifier does not lose energy by expelling excess water.


A general question - in a downdraft, does the hearth size vary between wood and charcoal? When I ran a wood downdraft, the 2" reduction zone tube would be filled with unburned charcoal. When running the updraft charcoal, I observed a basket ball sized mass of burning charcoal around the nozzle. So I wonder if the reduction zone size changes with fuel? I am worried that the charcoal will burn down to the grate if too small.


You dont want a restriction in a charcoal down draft. You now just need volume from the grate to the Nozzle to accomodate the " Basket Ball " reduction zone. You will want to be able to service this area to remove the inevetable clinkers.


Hi Marty i re read your post and have to say your weather conditions almost sounds the same as mine , where about’s are you i noticed the Ferns in the back ground and i wondered if you might be living somewhere in a high altitude , all the years i have made and stored charcoal though i have always kept in steel drums and i live up in the hills in Melbourne and i don’t need rain here as we are so high up when the clouds roll by they cover everything in moisture there is always a mist of water on cloudy days even when there is no rain we get wet .
But only once have i ever heard my updraft pop and crackle as i first lit it up and after 5 mins lights up and runs well , and i hate to say this but once lit i walk away , i have even left it running and gone down the hill to the station to collect the wife after work many many times :melting_face:


Hi Dave,
We are near Caburgua, not too high (600m), but in the foot hills of the Andes about 40km from Argentina. It is a lush and verdant forest here, but just over the border is a dry desert. All the weather coming off of the Pacific eastward is drained of moisture here.

My mistake was storing the charcoal in mesh feedbags. I’ll have to get some steel drums. The local place I got them before closed down last year, so I’ll have to make a long trip to a city to get more. Which means mucho gasoline, tolls and having to bring a trailer into a city because I will have to buy a bunch instead of one at a time. :rage: I suppose this problem is small potatoes next to what is going on in the world and the reason I am here in the first place.

I was considering going to downdraft also because of cleaning issues. Whenever I open the cleanout door, a ton of unburned fuel spills out. Giorgio had a good suggestion that the door should be at the end of a short tunnel. Then you can dig out the burning area without too much of the fuel above spilling out.

So I will probably give the updraft another try. I guess it would avoid issues of needing a grate shaker. :slightly_smiling_face:


Updrafts do have their shortcomings, but I am happy with the three I built, which run a lawn mower, generator, grist mill, and car.


marty, in a thread about chargas use in columbia -german language-
HV_Druck holzgas columbien pdf.pdf (3.1 MB)
they have had the same problem with moisture in the coal…they ventilated for 15 minutes with open hopper lid to bring out the umidity, than it works well…
others in this document have had a drying barrel where the motor exhaust goes theough for drying…in this way you need only one thight barrel for stock dry coal for the next run…
also here in italy al scrap is immediately very expensive, if a interested person shows up…
i think it could also helpful if the gas exit is not on top, but nearly close as possible to the reduction area, protected by a old stove grate for not get to much coal pieces sucked in the gas stream…