Charcoal downdraft vs wood downdraft

I’m looking to build a new gasifier. I was thinking of staying with charcoal and just making a larger updraft style because I know that it should work for what I need and it’s nice and simple. My desire is to have a longer runtime for my Predator 8750 Inverter generator (420cc 12HP). My small gasifier only gives me about 20-30 minutes before it gets too hot. It really has been just an experimental unit up to this point.

However, I keep thinking that maybe I’ll branch (no pun intended) off into a downdraft or cross-draft charcoal gasifier design so I can have the capability of adding other non-charcoal feed stocks if I want to but still use charcoal as the primary fuel. I have a couple water heater tanks, a few propane tanks, and some old fire extinguishers in my materials pile, so I have a a decent amount of materials for whatever gasifier design I end up going with.

So, I have seen a few downdraft charcoal gasifier designs but I am wondering what the main differences are between a charcoal downdraft and a wood downdraft gasifier? Are charcoal downdrafts not as complicated or have less strict measurements for things like restriction/nozzle number/nozzle size? I know there are people here who have made both, so I am sure someone will be able to let me know.

Thanks in advance.



Charcoal downdrafts are more simple, and ones like the Mako design with a ring of nozzles you can blend in some wood, but you have to account for the heat. If you don’t have a water drip or some moisture in the fuel it’ll get hot very quickly. They almost always used special nozzle tips made from Kanthal/Ferrochrome. My Diagonal Draft is based on a design from the Pegasus book which is titled as “Driving On Wood” in the Library. I wouldn’t go with a regular crossdraft, but instead raise the nozzle by a few inches and have it point down, up to 45 degrees downwards.
Something about changing from a horizontal flow to a diagonal downwards flow helps you if your fuel isn’t perfect. You can also just make a regular grate for a Diagonal Draft, just make sure it takes up the entire barrel if you go for a really simple design like I did. The plain grate also solves an issue of Crossdrafts, the ash issue of vertical grates getting plugged with slag.

The Gengas book also has a lot of drawings of charcoal gasifiers from WW2.
It shows the Mako design, it’s really a downdraft gasifier without a restriction when you get down to it.

If I were to build another charcoal gasifier it would be a Mako and use the Silicon Carbide nozzles that are about 1/2" inner diameter, pressed into pipe nipples that screw into couplers welded to a pipe ring or just an air jacket. Depending on engine size dictates how many nozzles and how big of a diameter nozzle tip you need.


Hi Bew , if you are happy with your setup apart from the run time , that 100lbs propane bottle you have there would give you a run time of around 3 hours of full on power draw as long as the nozzle is roughly 3 inches from the bottom and the gas exit is as high as possible , i cut a 5 inch hole on top side of my tank and made a plate to cover it for fill ups and a trap door about 8 inches above the Hight of the nozzle to allow me to swap out nozzles and clean clinkers ect .
I replaced my gas bottle gasifier about 5 years ago and use a 44 gal drum with a removable top for easy reloading .


Benefit of a down draft or cross draft is you can mix in raw fuel in. I think a down draft will process water more effectively and they run cooler. If your gasifier is getting too hot you dont have enough reduction, the gas should be cool and should never get hot enough where it is too hot to touch with your hand.

Ive now put some hours with fuel mixtures and will now be recommending a 50% mixture as the maximum. With a moisture range from 0 to 10% and correlating to the amount you mix in. 50% will require zero moisture, where a 20% mix the wood can be as high as 10%.


Before your work with downdraft charcoal gasifiers, I understood that the reason for the updraft not needing a cooler was because the gas was going through a mass of charcoal above the reaction which cooled it down before it exited the gasifier. With the downdraft gasifier, the gas leaves the gasifier immediately. I would think it would retain more heat. So, why does the gas come out cool in the downdraft gasifier?


Nope not the case the process is endothermic as long as the charbed is large enough to absorb the heat in process the result is the same. I now have over 80 machines proving this :slight_smile:

The DFX S2 and 3 both when running the 8750’s and the 9500 the gas out hose is generally just warmer than ambient temperature.

Another thing the gas leaving immediately is actually cooling the gasifier. This heat is not left to be absorbed back into the gasifier. Air goes in, then reaction process, then product gas out. left over moisture is also taken out rather than getting absorbed into fuel above changing the MC over the time and consumption of the hopper load.


I have wondered about this for a while. Thanks, Matt, for your explanation–amazing. Now for a downdraft as simple as the SimpleFire. I think Cody is working on that.


Well, if we can figure out a sizing chart for them. The one I built can’t get the Sierra to run on charcoal alone and I don’t want to leave my injectors running and risk burning them up.

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I had a downdraft at one stage that would only make very weak gas , i guessed at the time it was down to distance from nozzle to gas exit .I have been wanting to carry on testing by trying Matts proven design i even have the ammo box but then i never seem to have enough time to hand or a decent enough workshop at home , be a lot easier for me if i could buy a unit from a pro builder like Matt but due to distance shipping costs that aint never gonna happen unfortunately .


I think we should be using the same volume of charbed as a typical down draft imbert as a rule of thumb. It will at least get you in the ball park. Better to have more than not enough for what ever application.

My char bed I think is plenty, about a 19" diameter by probably 10" tall since I went to the 45 degree downwards direction.

But for that 4.3L my nozzle was only 1" ID. I think it will run the Mazda’s 2L engine though, because the Sierra ran with just a dribble of gasoline using the injectors with the pump turned off when I tested it today.

Gas is very humid I should definitely add a condensate tank and probably surround the bag filter with hay to coalesce the moisture out.

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Im finding that the nozzles do not need to be very large. The 3/4 pipe is good for 1000 cc I have not tested any larger than this so it may be possible to run even larger displacement with the 3/4 pipe size. However I tried a 1/2" pipe nozzle for the 420 cc range generator and the gas seemed weaker and the machine ran inconsistent.

Yeah when you start getting over 2 liter engine displacement this is where I think running two units will work better. Two units with 1 inch nozzles should run the 4.3 and even larger V8’s just fine.

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Downdraft charcoal offers limited tar cracking capacity and a lower pressure drop across a consistent fuel bed.

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Why would it matter? If the char bed is large enough; what is limiting tar cracking? If anything the down draft will have better tar cracking as the charbed is static. In the case of the downdraft if tar is getting past the charbed then that just means the charbed is too small. An updraft if there is tar in the fuel above it has no chance to properly process as it is post reaction zone. I dont see how it would matter as far as presure goes up or down seems an up draft would be more restrictive and not as consistent as a down draft.

A downdraft all reactions and flows are fixed and better controlled.


Choices of wording.
Bed of fuel in an up draft will effect pressure drops as the fuel is consumed.
Low and consistent is better than variable with an up draft ( I know that not what I wrote but it is what I meant )

Limited tar cracking.
A gasifier that up drafts is not real good at cracking any tars that may be present in low grade charcoal.
A side draft or my preference down ( even that does not quite cover all variations in what a creative mind can build ) is less likely to allow tar to get past the grate.

SO I am not disagreeing Matt.
We are on the same page, its just not that clear from my post.


Right on just making sure im not missing something. lol

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Im just not making sense.


Velocity needs to be high enough to generate enough heat to drive the process.
Nothing different here than any other gasifier type from that perspective.

Conservation of heat energy for the gas prosecution process is just as important as tuyer size and gas flow speed through the super heat char bed…

I have no new ideas to offer on this.
I have expended a lot of time and effort on findings ways to get high velocity without highflow restriction, long residence time in the super heated combustion and reduction zone the right amount of hydro cracking.

Humid gas though thats a sign of failure to crack.
Too much water maybe?
Not enough heat or time in the reduction zone…
Maybe the fuel is the problem not enough hot reactive surfaces to catch the water and crack it

Just random thoughts.
The gasifier of any type should be trapping heat inside, recycling heat inside to drive the process.
Adding water to cool the reaction should only be as much as needed to reach the optimum temperatures of operation in the largest achievable reduction zone.

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Wallace it seems to be standard fare for a downdraft charcoal gasifier to have humid gas. Kristijan is of the thought that it’s beneficial especially for the cooling rails to prevent soot buildup.

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Does excess help?
Too much sugar does not make a better pie.

If there is too much water its just a drag on the system.
If you need water too cool the components then its a problem with the components and not a need for more cooling.

Point here is there is an optimum amount of water required for a char unit, beyond that more does not make better gas.
if excess water is passing through a unit then the question is why?
Something in the design is not cracking the water, or something is not designed correctly so that it requires extra cooling…

I come to the conclusion that you need an excess of water by about 5-10% to make sure you have enough for the water gas shift .
This was because of inefficiencies/bad designs in my reactors or operating outside of the envelope…
Back off the water gas quality drops temperatures rise.

This isn’t optimal but its reality

In an ideal world when you burn a fuel you would make a mixture that us exactly the correct amount.
in a burner system for dry solids that something that needs an excess of air for complete combustion.
In an engine you might want a richer mixture for the right combination of flame speed and ignition.
Again not optimal but because of something that has skewed requirements.

Seeing a wet filter in a gas cleaning unit tell me I have done something wrong.
Because you see something and others see something as a common feature of how the systems operate this does not mean its ideal…

Like I said I don;t have answers but observations.
Well I do have some answers and some ideas and some results too.
But nothing thats easy and practical.

I don;t know Cody.
All I know is my gut says maybe…
I follow that till I hits a problem.
Crumple the paper cut up the steel try something different.

Ideal temperature control and moister control in a fluid bed with fuel introduced at the same time as air and its lead me to look a things a lot different.
It has not solved any problems…yet.

What i am trying to say is a gasifier is a Chemical reactor

like any reactor in a chemical plant you want to design it to do something as efficiently as possible to make your product from your feed stock.
Everything about the design the qualities of the feed, operating temperatures and pressure mixing all of this is about making the product and avoiding un reacted chemical or unwanted by products