First Down Draft Charcoal Gasifier

I want to try making a down draft gasifier since my first simple fire gasifier is almost complete and working.

As I showed in my simple fire thread, I’m starting with a propane tank.

I estimated wrong and this is the same diameter as the 20# tank I used as a filter so this one might be a 30#.

I filled it with water 3 times and then cut the bottom off leaving the top ring as a possible stand.

Now I’m not sure how to continue.

I have a brake rotor that almost fits and there’s a ridge near the bottom that might hold it in place but not sure if I should have the dish up or down.

I could cut the center out completely or add a plate with more holes and call that my grate.

Another choice is a smaller brake rotor.

But there would be gaps around the outside of I put it at the ridge or very little space for the outlet pipe if I just set it on the bottom.

This might be what I go with unless I get some better ideas before I do it. Grind the outside to make a ring that will sit on the ridge on the big rotor using it as the grate and put the smaller one on top leaving a ring to hold ashes or charcoal as insulation.

Also, I’m not sure about the nozzle(s). If I remember right, there should be 4 to 6 inches between the nozzle and the grate.

This will be charcoal but I’d be lying if I said I don’t plan to at least try some “rocket fuel” or at least feed it some charcoal that didn’t fully convert. This will be for small engines possibly a lawn mower but the first test will likely be the generator that I currently run off the simple fire because it’s already set up.

I had planned to just take it apart and dump it to clean it but might try to put an ash clean out port at the bottom.

Being the first Down Draft gasifier I’ve ever tried to build I might be doing it wrong. I’m not afraid to fail but if anyone sees any problems with my plans I’m not afraid to change my plans.


Look up the Swedish Mako charcoal gasifier to give yourself some inspiration. I would try to make a grate out of rebar or some other bar material. Space the bars about 3/8" apart, the smaller char will hold itself together. Take a section of propane tank or a hoop that will fit inside the tank and weld the bars to that.

Tack on some angle iron as standoffs to the inside of the tank for the grate to rest on, and any vibration or shaking should agitate the grate. You want it to be able to move around a little. Since it’s just sitting on pegs it should come out easy enough if you tip over the gasifier for cleaning.

For nozzles I’d take some 3/8 couplers, cut them in half, and use those as a way to screw in pipe nipple nozzles.
I don’t know exactly what engine CC size you’re aiming for but a wild guess would say 3 of some 3/8" pipe nipples would be plenty.

If you’d like to easily shut it down but stay simple use whole couplers and use pipe plugs when shutting down as your caps. You could also build an air jacket around these holes if you want to put a valve in place for shutting off more easily. An air jacket would be safer for fire safety reasons.

Angle the nozzles slightly downwards so ash/slag will not plug them up.

For a hopper you could try welding or brazing on a 5 gallon steel bucket with a removable lid if you find the 30lb tank isn’t enough, but sometimes a smaller short run gasifier is more efficient. If I had one in front of me I’d tell you if they’re close in circumference. The steel is thin so take your time to prevent warping. Little tacks all around.

For the gas exit I’d try to use anything bigger than 1". I’d aim for 2" pipe. You can always reduce the pipe size as it’s going to the engine.

A gas cooler will absolutely be necessary and you’ll either want a metal flange, threaded, or hard welded joint for that. It’s normal for the exiting gas to come out hot in a downdraft just like with raw wood gasifiers, but many find it comes out fairly cool.


Thanks Cody. That gives me a lot to think about.

This is the specs for the generator. The lawn mower is probably similar if not a little smaller but I’m not sure this will be best for the lawn mower if it needs a cooler but I’ll have to think about it. The mako gasifier looks like the cooler is built around the hopper.

I take it the grate doesn’t need to be tight against the sides then. I was thinking I had to keep the flow only through the center. That should make things lighter than the way I was thinking.

I’ll have to look and see what scrap I have. I still have a little more left to do on the simple fire too but I’m curious to see the difference between the updraft and down draft even if it ends up being bigger than I wanted for a lawn mower.


Yeah, the Mako was usually mounted on the front of a passenger car.

If you want to make sure flow only goes to the center you could take one of those brake discs and cut out the center. Like an oversized restriction.

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This is the simplest downdraft l can think of. Suitible for rocket fuel too.

Its not much more complicated thain a Simple fire. Has no air manifold thugh, so the nozzles (preffer 5) are each sticking out radialy.

Ash is placed here (red) to form a cone.

Edit: hmm l draw the nozzles too long. They shuld protrude less in the hearth but you get the idea.

Also, char downdrafts dont require a restriction so you can save a lot of weight by not imusing any brake rotor at all. Also, a restriction will severely prolong startup, going without it l wuld be surpriced if the gasifier needs more thain 30-60s to bd up to operating temps.


brian, you should know roughly the diameter of the glowing ball, when you use rocket fuel, i think…the whole cross section should be glowing, so no tar can bypass…if you prefer protecting coal layers near the metal, i would say it is not tar safe, therefore only coal for fuel…
my understanding…


I still don’t have my plan figured out but did some more welding today powered by charcoal gas from the simple fire.

I cut and welded in a pipe for the gas outlet and the ring around the top.

My weld connecting the pipe to the tank is still pretty lumpy and I burnt through the thin pipe a few times. The top ring turned out better because I got the simple fire gasifier running better.

I used up the remains of my flux core wire and had to put another 2 pound spool on the welder.

I had some chunks of rebar welded inside to hold a grate which I didn’t decide on yet but that was when the simple fire gasifier wasn’t working right and those broke back off.

I removed the filter pads in the simple fire filter and replaced them with pieces of felt covering the planer shaving and stopped using the water drip. I had too much water dripping in the first run and got moisture in that wasn’t getting converted to Hydrogen. Combined with the plugged up filter I just wasn’t getting enough power.

I refilled the simple fire a few times today and it needs refilled again. It didn’t run until it shut off but was getting pretty hot.

Hopefully this gasifier works at least as good as the simple fire. That one got a good workout today and had to save me a lot of gasoline. 3 runs today and, except for the first run, it did really good working hard most of the time. I was able to turn the welder on high so got better penetration but had to do hundreds of small tacks to keep from burning through the thinner metal.


For water drip, you want about 1 drop per second. In your case maybe 1 every 2 seconds.


I tried an experiment to see if I understand how the charcoal down draft should work.

Not as small of holes as it should be but thought I’d try it.

I drilled 5 holes around 5" up from the grate but didn’t have any 3/8" pipe to make nozzles.

I had to use bigger size charcoal because of the size of holes in the grate. The nozzles are just holes. The lid fits really tight but wasn’t sealed.

It was easy to light but no luck at getting a flare at all.

I dumped it out and set my brake rotor on the grate and tried more charcoal. The bottom still had to be bigger sized but then put smaller charcoal up over the nozzle holes. Again, it was easy to light and made smoke that went away if the torch was held in front of it but no flames.

I skipped a lot of the steps and didn’t use all engine grade size charcoal but thought I could get an indication that I was even on the right track.

I should have put grease or something around the lid to seal it temporarily. I suppose if air was leaking in there (which it surely was) it would pass right through the charcoal unchanged.

I didn’t make an ash cone either. With the large holes it would have just gone right through. A lot of the charcoal fell through even with the larger size on the bottom.

Am I even on the right track?


I am no expert on downdraft but can you get some nozzle material sorted for your holes that will at least keep the heat away from the outside of the container and more towards the inside , also the grate material will do for a quick test but i would imagine is not going to last very long .
How high above the gas out pipe are your nozzle holes if its too close it may not have enough dwell time .
Oh and look for air leaks if no flare its 9 out of 10 times


Okay start checking that all of your nozzles were fired up you should see at yellow glow in each hole of each of the nozzles. Yes you can use larger charcoal in the bottom on the grate, but you need to use finer charcoal up by the nozzles and above that point in the hopper to make good quality gases to flare with. And the charcoal needs to be dry no more that 10% moisture in the charcoal above the nozzles if you are not using a drip system on the nozzles.
Very Important that there are no leak in your hopper lid for sealing it up. Yes ,You should have a ash cone below the nozzles. A 3" to 4" ash ring plate on top of the grate to help seal the inside edge up in the firetube. Put ashes forming a shallow cone shape before lighting the nozzles up. This ash will continue to be forming when the unit is running. Even if one of your nozzle hole are not fired up this will let air in and make weak gases until it is on fire too. The larger the diameter, the nozzles are from each other can make it difficult to get started. Light up each of the nozzles. Any air leaks down stream of the grate area can weaken you gases. Only add air after the cooling tubes and filter just before going into your engine or flaring. If just flaring no filter is required. It works best if you are sucking on the gasifier with a vacuum pump or blower. Air leaks are the majority of the problems for weak gases.


BrianM., hatchet or machete down size your charcoal. Be some too small and dusts made but it’s the quickest, easiest. Do not use that pictured hammer. It will pulverize too much.

For just testing concepts make a tin sniped sheet metal ring to lay over the metal grid for an ash ledge. (Hide the theft of a baking tin from your s.w.m.b.o.)
Might as well put a cross turned second layer under that drop-in ring of that expanded metal grid to tighten up your openings.

You just have too many factors now too far off.
Steve Unruh


You need to seal around the brake rotor so that the gas must penetrate into the heart of the gasifier and follow to the center of the brake rotor. All you are doing here is the air is entering and making C02 < (lots of smoke and no burny) then going straight down and bias around the brake rotor. You also need the jets to be high enough so that the CO2 can convert to CO before it leaves the charcoal and exits the grate.

Chargas also can be very hard to light and get to sustain until you add a water drip or mist. If you have someone that can help have them spray water into the nozzle with a spray bottle as you try to light. Note there is some lag time.


You are on the right track for sure.

I always put a layr of biger charcoal on the grate too, the size you have, but on top only engine grade. Size realy does matter when it comes to gasifiers…

Speaking of wich, your gasifier now, with no nozzles, is big enaugh to run a car. As is you will not get optimal performance on a smaller engine. Nozzles will shrink the glowzone considerably.

The role of nozzles is to direct the heat inward. You loose a lot of energy to the walls of the gasifier now, energy that wuld otherwise be used to make gas.

Its wery important for your lid to seal well. If there is air siping in from the top, the flame front moves towards it and you end up with a mess. Specialy if you use damp charcoal, more on that later.

Your blower… since you use it to suck rather thain push, those blowers are usualy a problem because they are designed to pull some air also trugh the motor for cooling, this can drasticaly dilute your gas. You can find the slots where it sucks air in and tape them over for a flare, but not for long to not overheat the motor.

The brake rotor. You put it on the grate directly right? Thats fine with this style gasifier, but like Matt sayd, it must seal. Best sealant in a gasifier is ash. Just pack ash around the outer edge.

The fuel. Like l sayd, engine grade size, but when you get other things right you will be able to use damp charcoal. You will have to experiment what your gasifier likes but to start with, about 15% per weight of charcoal is good. Nowdays l dont enen weight, l pour water over my dry charcoal and mix and when it stops dusting its just about perfectly moistened.

Moist fuel will be easyer to handle, it will eliminate the need for a water drip, it will be self regulatory (more load, more fuel burns, more water evaporates), it will protect your nozzles from melting, and it will give you a “empty hopper alarm”. When the fuel level gets low, the moisture is gone and gas gets weak, you will hear it in the engine or it will even dye if you dont tighten the air mixer, a nice feature


Wow, that’s a lot of good suggestions. Nice to see I’m kind of on the right track.

I assume this is close to what I need.

I think I might really need some kind of access port or I will need to rebuild the ash cone every time I clean it out.

The ring from my brake drum is a little under 5 inches diameter and about 2 inches high. From the grate to the nozzles is 5 inches. The higher I make the nozzles, the less usable fuel I will have but I could seal those holes and move them if needed. I think I read 4 to 6 inches is needed so went in the middle.

Not much left on this scrap pipe but I could probably cut a ring off to add in to shrink the hole more.

Not much reason to test it again until I find nozzles and seal the lid.


You can check our DXF here. Note the distance between the air nozzles and the grate. There is 14 inches of space between them. The grate is an active grate. It just sits on a spindle shaft from the bottom and there is a handle you can simply actuate back fourth to move it radially. This eliminates the need to clean it out unless you have clinker build up. In that case the grate spindle is held up by a lock collar that you can release to drop it down. We have bad backs here so we make this equipment as easy to service as possible. Besides we are lazy too.


I couldn’t find small 3/8" pipe but wanted to try the asconebqh3rq1ryuilqreey

SIDE NOTE: In a former life, I worked really hard to make grams of CONDUCTIVE charcoal trying to make batteries or super capacitors. Now I got it laying all over. Sure makes typing on a touch screen tough.

What I was trying to type was I wanted to try that ash cone and see if my nozzle height could work.

Matt, I think this gasifier is more like your ammo box version. I only have about 12" from my grate to the top of the container so my height has to be a lot lower to have any reserve fuel above the nozzles.

F.Y.I - US military surplus 60mm ammunition cans measure approximately 13" x 11" x 5.5"

One of the comments at the link above says:

You want 4 to 6 inches between your grate and the jet on this size gasifier. Larger you would increase this distance. You only need a few inches under the grate just enough to allow the gas through un restricted.

I put some pieces of 1/2" hardware cloth on the grate to help keep the charcoal from just dropping though. Then I put a layer of bigger pieces and then engine grade charcoal over the nozzles. Then, just for fun, I put some broken up, dry branches and some more bigger charcoal.

I played around for a while then taped up all but one nozzle and put a small stainless steel tube through that hole and blew air into it.

I leaned a steel can over the outlet and got the invisible (at least in daylight) flare which indicates that this idea SHOULD work.

My lid has a layer of silicone and grease on it but just held down with gravity. I didn’t want to bolt this lid down. Got a POOF during these tests and it lifted the lid. Nothing very impressive but probably a good idea NOT to bolt it solid.

I’ll have to get some thin stove rope and more high temp silicone for the lid and find some springs or something to hold it down. I feel better about putting more time and effort into this since I at least got a flare out of it.


You have to have that distance for it to work. You cant make this work without the depth for the reaction process to take place. It will just make C02 because there is not enough to for it to convert to CO.

Its like you are trying to fly an air plane but it wont fly, because you are trying to keep the wings small. You are going to need to make the wings bigger .


That ammo box gasifier also has 14 inches of reduction. That is a cross daft unit, your building a down draft.


I think it’s work smarter not harder.