First Down Draft Charcoal Gasifier

Good idea Cody. Mine is only a 1/2 inch pipe but the same idea should work.

OK, we all knew I was going to do it eventually but how many thought I’d admit it?

I tried to start back up on gasoline after changing the oil and it backfired through the intake. I knew it was pulling easy but I thought I got oil in the cylinder (which it did) from tipping it up to add the oil.

Took the valve cover off and sure enough the intake valve was stuck open.

I cleaned up the carb and sprayed some cleaner in the hole behind the carb and got the valve unstuck. Put it all back together and run it on gasoline. It smoked a lot at first but cleared up.

I don’t think it was all caused from today’s run. It had been backfiring sometimes but all the moisture and a little tar finally built up enough to cause the valve to stick.

I have to go help a neighbor but I think I still need to adjust the valves. They seemed a little loose but I didn’t have my feeler gages right here.

At least small engines aren’t that hard to work on and nothing got bent or broke.


This was my idea for connecting a canning jar as the water trap. I got the 2" x 3" Fernco fitting but should have searched for a 1" x 3" one instead. The 3 inch part is closer to 3-1/2 inches before tightening the clamp and fits a wide mouth canning jar great. The 2 inch side is about 2-3/8 inches before clamping.

My pipe is only about 1-1/2" outside diameter so I’ll have to build an adapter to fit but this should seal a lot better than what I have now. Enough layers of duct tape on the smaller pipe would work but I need to make a new piece so I will probably use a bigger pipe. This should be under the boiling point so PVC pipe might even work.

I didn’t try adjusting the valves on the generator. If they have tar or carbon on them that would make them loose. I don’t have a new head gasket so don’t want to risk taking it apart to clean or lap them in. I will likely at least check them to see what they measure now.


That is a good observation. A Charcoal gasifier should not have to much water vapor. I would use less moisture in my fresh Charcoal.
How about the outside temperature where you are living.
Right now where I live it is normally a dry climate. But in the fall it becomes very wet and the moisture in the air increases. So at this time I can use drier Charcoal when running my gasifier, with no water added because the Charcoal is like a sponge it will soak up the moisture right out if the atmosphere.
This is why on my newest build it is going to have the hopper more like a wood gasifier to get the extra moisture out of the hopper. By using condensation tubes on the outside of the hopper wall.
I have found if I do not seal up my stored charcoal in a sealed containers it can not stay completely dry. So the humidity of the air around you does make a difference in how the charcoal works in the gasifiers and the performance of the gasifier and how the engine will perform.
Two of the things that will make leak gases, moisture and added air more then the 1 to 1.1 ratio for good combustion in the engine.
So you found a air leak down steam of the charbed that will weaken your rich gases. To much water down stream will cause the charbed to be to cold to work properly that a weak gas es.
You might be see this balancing act in the operating of a gasifier?
You found the one more problem and you are fixing it by removal of the extra rotors to much mass of steel in the wrong places. Now you will have more heat where it is needed to crack water vapor into Hydrogen.
You are fine tuning your gasifier. Good job. Reminder every gasifier is a is a little different in its operation because it is a custom built. You are the operator and you will make it work.


The temperature here is cool but the humidity is high.

I keep looking at the book and watching videos on the WK gasifier. I keep thinking about ways I could add the condensation tubes but was trying to keep this build from getting too complex. Wouldn’t really matter since it is going to be stationary so weight and size isn’t really an issue. The “hopper” and the “burn chamber” as well as the “ash pan” are all in the same 100 pound propane tank. I might eventually try adding extras on to this one trying to improve it or just get it tuned to run my generator as good as I can and save all the extras (hopper condensation tubes, heat exchangers, cyclone, etc.) for the next build. Each time I run it I try to get it to run just a little better and it is improving.

I did adjust the valves and pretty much got rid of the tapping sound. Wasn’t as bad as I thought but was a little loose.

Today I built a new pipe to hook up a better water trap jar.

I duct taped the old, leaking one to seal it up for this run because I was using it to run the generator to power the grinder and welder. It caught a little water but most of it ended up in the filter. Probably not enough slope to the water trap and it’s still drying out the new ashes and I guess my charcoal must be wetter than I thought even though most of it was freshly made.

This is the new water trap all ready to try out. For the big pipe adapter I cut some slots and bent it to fit the smaller pipe. I haven’t tried it yet but looks like it should work.

My gasoline gage on the generator is empty but I did run it a little after the run on charcoal gas to try to flush out any of the moisture or tar it might have picked up from today’s run. When the valve got stuck I didn’t end the run on gasoline but that wouldn’t have helped with the tar built up on the intake side of the carb. I assume that was from mixing raw wood with the charcoal with the stack of brake rotors. Hopefully switching back to straight charcoal fixes that problem but there could still be some tar in the pipes and hoses that might need cleaned out yet.


Winter is going to be here soon and this gasifier is working. I think I’m basically down to the boring 75% left to do now.

I run it one more time today then took it apart, hosed out the filter and all the pipes and hoses.

Then I moved it under a roof and put it back together.

It still has hot charcoal in it from today’s run but it’s shut down and cooling. The bottom around the main burn area is still too hot to hold onto for very long but the rest is either cold or just a little warm.

Not sure if this will be run here or just stored. I’ll put some metal on the wall to shield it from the heat if I do run it and it might be moved around to fit the generator beside it if I do run it.

There’s still more to do like replace the nozzle and attach the top on with a different method but I didn’t want to have to dig it out of the snow so my daily updates will probably slow way down.

Thanks to everyone that encouraged me and helped me get this figured out and built. I’ll collect more charcoal over the winter from the furnace and I might still work on it or use it but, like I said, winter is coming soon.

I might rig up another filter for the smaller simple fire for more portable uses like running the generator when the power goes out or I want to run the log splitter. I haven’t tried this down draft gasifier with anything other than the generator but there’s no reason it wouldn’t run the log splitter if I move it close enough to run a hose to it.


Just make sure you keep that door open, carbon monoxide is a very real danger with gasifiers.

Looks like that building isn’t sealed up much so you should be fine.


That’s the end of my sawmill building. The two ends are open and there’s a big opening on the side where the logs are loaded. No problem with ventilation.


Brian, thanks for this topic that got me started on my own first down drafter. My 50 year old trash-find tractor took some TLC but she is now running strong. Also I have the basic form of the gasifier. I might not get back to this until Spring, so I will wait until then to start my own new topic. Here are a couple of teaser pics until then.


For anyone interested, here’s what my 1/2 inch pipe flute nozzle looks like after the last run.

I knew it was melting even before that run but it took some more off it turning it into mostly a straight nozzle. The back of the propane tank was getting hot so most of the hot charcoal was going horizontally before being drawn down and out through the grate.

My thin stainless steel fire tube melted a bit where the heat was going horizontally too.

It still isn’t too bad but obviously not a long term solution.

I emptied most of the charcoal out to get to the nozzle and to examine the tube. Probably won’t refill it since the cold weather is coming and it needs some more work.

My charcoal on the last run was too big but it still run the generator. Smaller pieces probably would have made better gas and maybe less heat escaping with the gas.

Through out this winter I hope to find more materials to build it better but I’m happy with the progress so far.


I’m going to try Cody’s idea to make a thicker nozzle.

This is a piece of my original 1/2" pipe nozzle for the threads that is pounded into a 3/4" pipe. Then a bigger pipe will be pounded over the 3/4" pipe to make it thicker.

This is still only about 1/4" thick walls but twice what I was using. I would have liked to have the 1/2" pipe the whole way but had to grind it to even get it forced into the 3/4" pipe.

Took a lot of welding wire and not the prettiest job but I welded the threaded section onto the pipe and the bigger pipe over the other one as well as closed the end. I used a washer to fill in most of the space and just blobbed on the weld to close it all up.

I still need to drill the flute holes but the pipe is hot and I haven’t been using the gasifier since I moved it so the holes will have to wait for another day. I’m not sure if I should drill it like it was or start small and work my way back up again. The intake side is only 1/2" just like I had been using and what my simple fire uses but then goes to 3/4" pipe where the heat will be.

I also got a couple bigger springs that I might use to hold down the lid but they might be too strong. I added another coat of silicone inside the lid around the stove rope to hopefully get it to seat better but I still want it to be able to pop up if it needs to.

The springs won’t fit the hold down that is on it now so I will probably cut them off and make a different type. I liked Matt’s hold down clamps but couldn’t locate that picture on this phone. Might try to figure out the top spring idea that I think Cody suggested. Again, no real rush.

Edit: took a while to find it but here’s the picture of Matt’s lid clamp I might copy.

I don’t remember where I found that picture. I think it was one of his older builds. Looked fairly easy to build and the spring tension could be adjusted.

I don’t know where I found this drawing and it isn’t exactly how my lid is made but probably close to the idea I think Cody suggested earlier.


Researching for the lid/poof designs I found this:

In there Koen says:

You want 2 functions, filling lid and puffing lid…
1: make one lid that can be closed tight, the easy way, no springs…
2: on that lid, or another place, make an overpressure valve that never will be opened unless you have a “puff”
On stationary designs they use aluminium foil rupture disks, but i am sure a larger tennis ball will work to
A larger , heavy rubber ball, 4" size, on top of a 4" pipe as a blow off valve maybe ?

That might be an option if I can’t get the lid to seal good enough while still being loose enough to POOF.


Drilled the holes in the new flute nozzle.

As far as I could tell /remember, the last one had 3/8" holes as the biggest so this is a little bit bigger except for the furthest one.

I estimated a little off to center the holes in the burn tube better but should work. The nozzle will be turned so the holes face down.

I’m not ready to run it again but have close to 3 buckets of charcoal ready to dump in and half of my little barrel that I have been collecting from the house furnace that will need ground and screened. Should be many hours worth of fuel waiting to be used and I collect more each day.

Also got a couple grease barrels that I cleaned out by making some charcoal in. The tilted barrel method worked pretty good but, with the house needing some heat now, grabbing some coals from the furnace is working good and doesn’t waste the heat although it is slower but I haven’t been running the gasifier lately so I don’t need to make large amounts at a time.


Seeing Bruce working on his downdraft gasifier I thought I should try mine again in its new home.

The problem is that there is a lot of wood in the way and some of it is still too good to call firewood. I did chop up a wheelbarrow full of wood from inside and finally cleared out enough space to bring the miter saw inside. I also hooked up a light at the end of the sawmill track so I can see the height settings just in case I ever run it when it is getting too dark to see the numbers.

This picture was before clearing out some space but I got the generator moved.

It took way longer than usual to get it running on charcoal this time. I had completely emptied it after the move so I could get to the nozzle. It was refilled with mostly the same charcoal I took out and that had sit in open buckets under cover but it has been raining a lot recently.

Once I did get it running it just didn’t have the power I was hoping for.

I got about 3 ounces of water out of the cooler and at least that much more out of the filter. The charcoal must have absorbed a lot of moisture and the cooler isn’t condensing all of it.

After shutting it down smoke was coming out of the lid. Opening it up it didn’t use a lot of charcoal but poking around at it I noticed some glowing coals near the top. Obviously the lid isn’t sealing while running like I had hoped. This could explain the lack of power and possibly the amount of water making it through. I knew my springs that are on it aren’t strong enough but hoped the vacuum would draw it down while running but it must not. I have bigger springs just haven’t rebuilt the hold downs yet. Pressing down on the lid I could stop the smoke so stronger springs should fix it but I might need more than two clamping locations to pull it down evenly.

I was able to cut up close to another wheelbarrow load of wet slabs using the miter saw on this run but it really loaded down the generator which tells me I don’t have the power it should.

Hopefully fixing the lid solves the problems.

I never did figure out how big this barrel is. I’m guessing 15 gallons but might be more. This charcoal was all collected from the house furnace and it hasn’t been real cold here yet. I should be able to get pretty much if I keep collecting it over the winter.


I tried the gasifier again last night and it worked a little better and got less collected water. I had added a stack of washers to tighten the lid down which also eliminated any chance of the lid acting as a “puffing lid”. The lid not sealing good enough MIGHT have been part of the problem but didn’t really fix the low power or the amount of water.

I’m pretty sure I know what the problem is why the gasifier is lacking power compared to the Simple Fire or even what this one had before.

This is a view inside the gasifier. There is a mixture of sizes and shapes of charcoal with a lot of it bigger than it should be. I did sort out some brands that were not only bigger but not fully converted to charcoal.

In the screen (~1/8 inch mesh) is some of the charcoal right out of the gasifier and in the bucket is after running it through the grinder and screening off the fine dust.

The grinder might still be leaving too big of pieces through. I had modified it to make less dust but it looks like I might have gone too far. My best option might be to rebuild the grinder again (third time’s a charm) and make it more like Gary’s where it can just be filled and left to work and end up with engine grade charcoal without the extra dusty hand screening.

Having too big of charcoal would probably explain the lack of power and maybe even the amount of moisture getting into the gas. I hoped I could get away without so much dusty grinding and screening but I suspect this size of charcoal is leaving the gas pass through without fully converting to carbon MONoxide or Hydrogen. Being less restrictive could also explain why the heat traveled upwards and ignited some of the charcoal closer to the top although that could have been from a leaky lid or from starting it by blowing air into the nozzle.

Unrelated but I run out of propane for my torch so I lit it with an old gasoline blowtorch.

One other thing I noticed on my test run was that the generator would speed up if I run the miter saw continuously instead of turning it on and off. I’m guessing the increased load was drawing more air and getting the fire hotter making better gas. POSSIBLY increasing the size of the nozzle holes would help. Before I replaced the nozzle it was running with a burnt out end so basically an open 1/2 inch pipe. Considering the original plans for the Simple Fire called for a 1 inch pipe nozzle, my flute nozzle could just be restricting the air flow or creating higer pressure jets of air that forces its way through the bigger pieces of charcoal too fast.

Oh well, this is all part of the remaining 75%.

Instead of making another post, I’ll just add that I dug down to the nozzle and the charcoal looked closer to the right size. Maybe running it a couple times burnt down the bigger pieces or maybe the big stuff was just my last unprocessed charcoal. Anyway, I drilled the 3 closest holes out to 1/2" and left the last one at 3/8". Won’t know if that makes a difference until I try it again.


Charcoal making:
I switched from screens to winnowing with a fan. The screens are so slow and fussy. Dropping the ground char in front of a fan is dead simple and much faster for me. An additional bonus is the dust being directed away…
Getting your grinder/ processing dialed in is one of those foundational pieces that makes gasification really doable…looking forward to watching your build!


Run again and a little better with a lot less collected water but still lacking power.

The back side of the tank (opposite the nozzle) was really hot so my welded plug on the end of that nozzle must be leaking. I just need to let it go out, empty the charcoal back out, and grind/reweld the end.

Got more wet firewood cut up using the gasifier as power but it still doesn’t have the power it did with the last nozzle.

Some soapy water and compressed air showed that I was right.

Took a few tries to get it sealed up and it might not be perfect but better than it was. I ended up using arc welding rods to fill in the end. Still has the slag problem but was faster and saved my flux core wire. Of course, that job had to be done using gasoline.

I put it back together and added the charcoal back again. Took a while to get it running but I think part of that is not having the propane torch to test the gas.

Eventually I got it running and it did a lot better. Probably close to what I had before. I still had the arc welder close so tried burning up the ends of my rods left over and it did it. I assume it would run the wire feed welder like it had been but didn’t try it.

I shut it off and carefully checked inside. It smoked when I took the lid off and then ignited. The charcoal level was getting very close to the nozzle. I put the lid back on and a few pulls on the rope and it was running again.

When I shut it off again, I restarted on gasoline to make sure I cleaned out any moisture or other bad stuff from the engine and tried the miter saw. It run better on gasoline but still loaded the generator down. Maybe that saw uses more power than I thought. I couldn’t find an Amp rating on it.

I need to prepare a couple more buckets of charcoal and should fix the lid better but I think I got it back to how it was running before I moved it and changed the nozzle.


Hi Brian, have you screened your Charcoal down to just 1/8" to 1/4" with 1/16" fines in your mix of engine grade Charcoal. Screen out all ash or smaller Charcoal fines in the mix? With no more then 10% moisture in the Charcoal mix. Maybe using 5% moisture first if you are finding moisture in the hopper.
Why I am asking this, is if I use to large of Charcoal in my Charcoal down draft double flute gasifier, I will have weaker gases and my generator will not perform at it best that it can on Charcoal gases.
Kristijan L and others have said It over and over with the Charcoal bed below the nozzles it can not have a lot of voids between the Charcoal for air to pass by when is burning. The idea is to get as much surface place for the air to come into contact with the white hot active charcoal to make good quality gases and lots of it.
Your gasifier looks great it should produce good gases.


No. I still have larger pieces up to around 3/4" (some might even be bigger) with the finest pieces about 1/8".

I will need to modify or remake my grinder to get smaller pieces and saving the finer than 1/8" pieces would be nice. I tried that with the Simple Fire but ended up with a lot of dust clogging my filter so I quit saving the pieces that went through the 1/8" screen but didn’t pass through a window screen. Now that I think about it, Gary Gilmore uses a plate over the top of the charcoal to stop charcoal from being sucked out in his updraft Simple Fire. I couldn’t find a picture of it but in his plans it does say:

You may want to put a screen on top of the charcoal if you are running an engine larger than 8 HP. With out the screen, some charcoal chunks may get sucked over into the filter.

My problem with the dust was caused by BLOWING air into the nozzle to get it burning or to flare it which would have blown the fine particles out of the outlet and into my filter. I’m not sure just the engine suction would have done that.

This is very likely the problem with the lower power and probably why the water is still making it through. The last run had very little water caught but that same charcoal had been in the gasifier so most of the moisture probably already got caught or used. I haven’t emptied the burn chamber or the charcoal below the nozzle so it was probably getting smaller and packed in better which could explain why the last run was the best one I’ve had recently but it still isn’t as good as I have seen it run.

I seem to get different opinions on the best size of charcoal to use so I’m guessing every gasifier is a little bit different. Unless I misunderstood different comments, some people call anything smaller than 1/4" biochar and other people use a window screen to get just the dust out. I think Cody was one of the ones that used to use fine flakes made with a modified chipper. Could get more weight per volume so longer run times.

This gasifier is working and I keep getting it running better. There’s still more I plan to do to it and I need to get the charcoal grinding and processing dialed in like Chuckw said.


I think the size of the charcoal will depend on your build to some degree , if it was a updraft simple fire type then anywhere between 1/4 up too 3/4 works great as its small enough to help stop rat holes getting up towards the top and not fine enough to block the gasses .
On the few downdrafts i have made i use the same size feed stock as that’s what my crusher makes ,i imagine that the smaller the pieces the more reactive area you have to make the gas and the time/distance is shorter so that must help .
That’s how my brain see’s it anyway , maybe i am wrong and that’s why i have not had consistent run times without intervention like i do with my updraft gasifiers .
When i am feeling better i intend to get stuck into solving my issues with run times over this summer as long as we don’t get too hot of a summer .


I haven’t run this one long enough to know what works and what doesn’t. I could get 2 hours runtime with my Simple Fire and I’m hoping this one can at least get double that.

I used up some of the propane tank as the ash pan and have part of the inside (where the burn chamber is) shrunk down and have ashes packed in around it so that lost more volume but I know it still holds at least 2 buckets of charcoal and probably 3 buckets. When I run it before, if I remember right, I run it twice about 2 hour each time without refilling it. I’m not sure I could get 6 hours out of it without refilling. I did need to poke it down between the 2 hour runs but smaller or at least more consistant charcoal sizes should get it to flow better. The last few runs it didn’t get the pocket that needed poked down so I’m hoping that problem was just from having too big of charcoal pieces.

I switched to the flute nozzle and am up to 3 holes @ 1/2" (~12mm) and 1 hole @ 3/8" (~10mm) but I had run the simple fire and this one with just a 1/2" open pipe but it did melt. It was running my 212cc engine on the generator pretty good with that size nozzle.

I don’t remember exactly what the distance from my nozzle to my grate is but I measured the simple fire where the charcoal level would stop running and made it at least that far. I’m thinking it ended up around 10" (~250mm) but I could be wrong.

I don’t regret building this down draft gasifier but it would have been a lot easier to just make it a bigger updraft one. The grate and clean out port is something I wanted to make cleaning it out easier. The bigger opening at the top for easier filling and accessing the nozzle was another thing I wanted. The biggest reason I wanted to try the down draft was to be able to run damp charcoal and not need a water drip to add hydrogen and to not need to be as picky about the fuel size or quality. So far those benefits haven’t really worked out. It does run my generator but I’m condensing water from the gas. I have run it with various sizes of charcoal and even threw some raw wood in on some of my earlier tests but ended up with a stuck intake valve from the tar.