Time to go from updraft to downdraft charcoal gasifier?

Thanks Giorgio, that document looks great. I just translated it with Google translate for documents (the one good thing Google does). I had the same idea about the location of the gas outlet. The barrel of the gasifier has served as a test bed and has various openings in different locations already. I am going to plumb the original air input a few inches above the bottom to be the outlet. Hopefully I will have that done in a few days. (It’s one of the few days without rain this week and the wife needs some landscaping work done.)


One of the Russian members of this forum saw decent success with a hotter gas exit, close to the Reduction Zone. It is already prudent to have a gas cooler even with updraft for the eventual temp increase and prevent damage to filter media or engine.


He has a small hearth with a big hopper, comparatively to the burn tube area. He draws the gas off while it’s still hot.

He essentially started with a Koen style gasifier and modified it to his needs. He ultimately didn’t want the gasifier anymore because he found solar was sufficient to his expectations of powering a fridge on a boat.

I like these tighter zoned updrafts, I wonder how well they feed with an expanded hopper like that though. You’d want some thick material to hold up to the heat.


Well, I got the outlet plumbed today after all. Could only cut up two large trees before running out of bar oil.

I did a test run and noticed two things. My gas is still a little poor. It flares well, but not sustainably. I think I have to unload the gasifier and sift the charcoal. I let the gasifier bake for a while with the loading door open the last time to drive off water. But I think it may have burned up too much of the charcoal and now I may have ash mixed in. There doesn’t appear to be any water in the gas thankfully.

The other thing is that the outlet is very cool. I had not expected that. The outside of the barrel is cool up to the top of the charcoal, and then warm above that. So before with the outlet at the top, it was always warm if not hot.

All in all, I think this is progress. If reloading the gasifier yields good gas, I will be very happy. Unfortunately, the gasifier in the document that Giorgio provided has tweaked my interest. I may have to build it to see if there is merit in that design.


marty, when you will build the downdraft gasifier from the document, with your water cooled nozzle from top will be perfect…this document was my basic design for my first gasifier, but without heat resistant mass around the hearth…the nozzle melts away quickly in the position how showed at the document because of too much heat exposition…therefore once found the right nozzle hight, i made the nozzle coming in from the side, but , how said, with your water cooled nozzle should be no problem…in stationary units weight is no problem, but on my mowers the unit must be very light, so all protection from the metal only by coal around the heat area, but there is always the danger that some umidity can bypass…but a advantage is , if somewhere is clogged by ash, the gas can bypass through the coal…during operation i need no grate shaking, only when cleaning out after work i use the shaker


for a downdraft build also this design could be conveniant, instead of firebricks, what restrickts eventually too much and leds to hearth plugging, a robust ring, for example old stove plate rings can be used …with more or less rings different inner diameter…


Nice ideas Giorgio.

Next test will be with a fresh load of charcoal. If that goes well, I think I will create a side mounted nozzle. I want a simple, removable, spring loaded barrel top to eliminate any air leakage from the loading door I have. While easy to operate, I think there are too many possible air leakage points. Moving the nozzle to the side wall will allow this to happen. I plan to have it angled downwards at about a 45 degree angle. Hopefully, it will remain impervious to the heat as it is now vertically.

Thanks for the help, I think I’m inching closer to a successful design.


Martin, I think as long as it’s water cooled it should be alright. However I’ve melted nozzles in a sort of diagonal draft setup. It’s hot enough to turn the fancy silicon carbide tubes I bought to glass, normally those never melt.


Wow Cody. I’m pretty good at melting things too. Sometimes I think that all I’ve made is a forge instead of a gasifier. :wink:

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marty, your watercooled nozzle works with water under pressure or thermosyphon system?
in the inner pipe cold water down, in the outer pipe hot water up ?
thanks giorgio


Hi Giorgio, right now I’m just using water from a faucet. We get our water from a mountain stream, so I have plenty to waste! Ultimately, I am going to add a heat exchanger and make a closed loop system, probably needing a pump.

I was running the water down through the inner shell and then up through the outer shell. But I’m going to switch that so the warm water flows up around the air tube and warms the air some. This will also cut down on condensation in the air inlet tube. My original thought was to keep the end of the tube as cool as possible, but really if the water is 10 deg C or 90 deg C, compared to 1500 deg, it won’t make much of a difference.


I just completed an extended run test with the nozzle. After about 1 1/2 hours, the nozzle had a medium thick coating of fine ash around the air outlet hole that was easily brushed off. The hole was not obstructed and the steel was undamaged. This was with a slow flow of water and no water drip. I didn’t measure it, but the flow was probably less than a liter per minute. The exiting water temperature was about that of hot tap water. I think the heat exchanger I have in mind will easily cool this flow. I don’t think it will thermal syphon, but I’ll see.

Giorgio, I’m thinking of adding a grate like in your drawing. I am thinking of constructing it with 3/4" angle iron and I am wondering what size openings to make. Any suggestions? How did you construct your grate?

Thanks, Marty


thanks for nozzle reporting…what inner diameter you have in the air feed pipe? how much ccm has your motor?
my gasifiers on the mowers must be very light, so i have around 2mm metal and less everywhere , also the grate of the last gasifier, actually in work, has 8mm splits and 2mm thickness, but it is downdraft… i try to avoid metall contact to the very hot zone, except nozzle of course, so there is everywhere coal around for protection. my motors has only 250 ccm, only the motorcultivator with 550 ccm runs recently for a test with the red bear mower gasifier , with no damage though bigger engine.
on my stationary unit where weight is no problem, i have a cast steel self made grate with drilled holes about 12mm diameter from a electric cooking plate…

for your modification i would take a ash grate from stove scrap for gas exit…they are cast iron and should keep up…

the rectangular stove ash grate (for hot gas exit) has 5 mm splits, i think with this distance not much coal goes sucked through…
the round piece was the grate from the stationary unit before it got the wider hearth plate grate, all both cast iron…
on my chain tractor i will try updraft with the lower gas exit how in the design, the main reason is the nozzle in the bottom, how eddy ramos uses…this position is less as possible heat exposed…but this project maybee in summer…


Giorgio, much thanks for the pictures and descriptions.

The air pipe has an inner diameter of 7/8" (22 mm) with a restriction plate on the end that has a 7/16" hole (11 mm). I have a generator with a 338 cm3 motor.

I wish I could find stove scrap like you have. It is difficult in Chile to find such treasures! I think I will have to construct the grate with the materials I have on hand, angle iron in this case.

An open question to anyone who cares to comment: What orientation makes the most sense? The vertex of the angle upwards or downwards? I am leaning toward downwards because I figure ash/charcoal will file the valley and provide some insulation to the steel.



Use the angles so they collect ash. I’d go so far as to pre fill them with ashes.


Ash on top protects; steel on bottom radiates.


Been away for a while, just found this. Wow, great topic and thread. Yes, Maxgasman taught us (or me, at least) about upward facing angles for grates, about a dozen years ago.
After debating back and forth on my F150 charcoal gasifier, I’m going to stick with the updraft. Sure, with the updraft you wind up with an initial bed of dampish charcoal (after a long shut down), and that moisture doesn’t go through the reduction zone, but that can be vented/flared/dried upon startup. Otherwise, I like the predictable “partial combustion”/reduction zones of the updraft, as well as the inherent cooling and filtering. Cody, I think, asked for some nozzle size calculation comparisons, and I’ve about gotten that finished, will post soon.


Well, I just finished ver 3.0 of the gasifier.

I only ran it a short time, about 10 minutes, but it performed well. Good gas, low outlet temps and no water condensing in the radiator.

It is a crossflow with the outlet above the grate. The outlet is shown in the upper right of the barrel. I figured the path through the charcoal would help cool the gases and perhaps keep some heat away from the grate.

I shortened the water cooled nozzle and introduced it through the side of the barrel at a downward angle. It can be easily removed by screwing it out of the holder mounted on the side of the drum. This allows the entire top of the drum to be removed for easy loading and cleaning when needed. I also modified the ash door. It is spring loaded now with the handle pushing on the center of the door for better distribution of pressure. It is easier to open and close too.

I did a post mortem of the last gasifier and I determined why it wasn’t performing well. In a nutshell - air leaks. I had used an old barrel because I was only experimenting with charcoal instead of wood. It had some fluid in it, so I figured it was sound. Well maybe in the beginning but not anymore!

Lots of holes in the bottom. I think I also had air leaks at the old ash door. I’m not sure what came first, the holes in the drum or the air leak at the door. There was a lot of heat near the door and it warped. Could have happened after I sprayed water on it to cool it. Anyway, it looks like the heat migrated to the exhaust and carried over water evaporated, but not reacted, into the exhaust. This steam heated the piping to the radiator and condensed out into the water trap. The same moist charcoal isn’t producing any water at the trap now and the exhaust piping is much cooler.

Here is the eye from hell:

The water cooled nozzle seems to be working well. I tried to run the generator today, but my starter isn’t working and the rope pull retractor failed. :rage: I will do a longer run with the generator once I fix the starter or at least the rope pull. I am anxious to see how the nozzle and the grate fare.

If everything looks good, then the last thing I’m going to do is construct a heat exchanger for the water from the nozzle.


Wow, you do nice work! We like videos too.


As Don said that’s a really nice build , don’t worry about a starter rope just grab a cordless drill with a short extension bar and socket and spin it up that way , also helps if say your nut is metric on the generator then use a imperial socket its a little looser and so spins off no problem , cant wait too hear how it performs on a longer run .