Downdraft gasifier for 5000 watt generator

Just wanted to post a picture and description of what I have been working on. This gasifier is a downdraft with a nozzle above the restriction zone and a stack at the top that also lets in air. Both have valves to adjust the air. The gasifier also has a water scrubber filter, followed by a hay filter. It then has an aluminum cooling tube and another cyclone expansion tank. The blower and engine mixing valves follow this. This gasifier has a valve between the main unit and water scrubber to allow to be closed off to operate it in updraft mode to warm up the unit. It could also be used as an updraft charcoal gasifier if I wanted it too. The main unit also has a homemade refractory lining made of a mixture of clay,perlite, and portland. I’ve fired it a few times before I siliconed the lid on to bake in the refractory. What do you guys think? I’m going to hook the bilge blower up to it tomorrow and give it a shot!

Looks neat Jonathan,

Good luck on the firing and keep us posted.

Hey JohnathanT
I like that you have incorporated a pressurized paint-pot into your system.
These are an overlooked, underutilized great piece of obtainium with open access easy top sealing.

Tips on your firing up is to always have fully made wood char around and just above that internal air nozzle.
Have to maintain replenishing this consumed up char with internal dropping down rising heat cooked out wood particles.
And given enough fuel wood/char consumed you will have the remaining wood cell wall mineral ash building up.
Be min 1% to as high as 10% ash of the original wood fuel weight/volume building up in the hearth bottom depending on your base fuel wood source you are using.
So for trialing initially gasses exchanges flow restricting ash build up will not give you a problem. With fuel wood/char consumption this flow restrictions problem will sneak up on you and become a real head scratcher.

You make no mention of a char to ash separating lower grate.
And I see no lower hearth system port to remove out settled separated out ash build up.

Load and trial fire it up anyway to cook your refractory and get some Woo-Rah flaring and even some engine running time. Then once you see this ash building up, you will know just how much you will have to modify for your own actual unique local fuelwood and usage.
Actual burning/wood consuming is ALWAYS the best way to learn.
Really. Wording-it is just a bunch of compressed heated air. Loaded engines will not run on just air. They need on-hands fuels experiences.

Good job of DOing so far man

Portland is not your friend. It expands abruptly at 500c.

Thankyou guys for the comments. Steve U, good to see you on here again. Yes that is a stainless steel paint pot that I bought for 50 bucks. It acts as a water bong scrubber. Gas goes down to the bottom of it and gets filtered through the water. The grate I am using is a pump strainer basket. It has a thread for a 2 inch pipe nipple which is my restriction size. The sides of the basket have 3/8 holes drilled it from the factory. I cut 1/4 inch slots across the bottom of it. The bottom propane tank is where the gas outlet and ash cleanout is. I tried firing the thing up today and couldn’t get a flare to burn. I think I need more cooling for the gas. I am going to use air preheat to do some of this. It was also windy today so that might be part of the reason. I hope I don’t have airleaks. I have siliconed EVERY metal to metal joint. When I shut the unit down, I had a puffback that went out of the top stack/air inlet at the top of the unit. The tractor exhaust flapper that I welded to the top of the stack fuctioned properly. Just a few things to figure out and I am going to make it happen. And terry, I know portland isn’t the best thing to use. On the next build I’m going to use some heatstop refractory or fireclay. I would rather use heatstop because its better. A word to those who thing about using anything with sodium silicate in it. It sucks bigtime! I hope I can get this thing working!

Hello Jonathan ,

Where on the system were you trying to flare? Over in the PVC section or closer to the gasifier? What is in the the big white tank and about how many gallons or cubic feet of space is involved ?


Looks like you have aluminum dryer vent tubing. That stuff leaks like crazy. I almost wore out the starter on my lawnmower with that. If you replace that your chances of a flare are much greater.

Wayne, the blower is hooked up where the rubber coupling is on the pvc. The plastic barrel is 30 gallons. The aluminum dryer vent hooks to a pvc pipe that goes down to the bottom of the barrel. The pvc fittings thread into the 2 inch bung on the top of the barrel. Terry, did you have trouble with the semi-rigid aluminum tubing? I know they make a much cheaper grade of dryer vent that is basically aluminum foil. I hope this isn’t the culprit because I would be hardpressed finding something else to make the connection. Maybe pvc pipe? I am going to close one of the air inlets and use a shopvac to pressurize the entire system. I will spray every joint with soapy water and see if I have a leak. With the blower hooked up it pulled pretty hard on the air inlets. Very easy to light the gasifier. I really think I need more cooling for the gas. The pipe going into the water scrubber got pretty hot. Another thought is that the water in the scrubber started steaming and the gas is just too hot and wet to light. I will try all these suggestions and hopefully get to the bottom off it soon! Thankyou guys for the advice.

A few things come to mind…
I’m right there with Terry… Ditch the tubing, even it’s tight now… It will leak. Especially after a backfire.
Do you have good “starting” char filled to above the nozzles?
If not, that could be part the excessive heat.
PVC will work if it’s not too hot
You can turn the blower around for your leak test, it’ll make plenty of pressure for that

Pictures and video are very helpful… What was coming out of the fan and what color was it?
Keep your head up, you are very close.

Yes, it’s the semi ridged stuff I’m taking about.

Do you have a safety valve installed on the plastic drum? Wouldn’t want all that gas to go “pop”!

If you have too much pressure for testing, it may blow the soapy water clear off the leak and you won’t detect it.

No safety valve. Any ideas for one? The exhaust flapper on the main unit worked but it would probably leak on the barrel. I could replace the tubing with pvc. No chance of it leaking then. The unit was designed to work in updraft mode for making char and warming up. I put charcoal from a bonfire in it at the bottom, but I don’t know if it was above the nozzle. I will fire it in updraft mode to char the wood completely. Both air valves are adjustable so less air at the restriction might help with things. The gas coming out of the blower was cool for about 10 minutes and then everything started heating up. I think hot, wet gas is a culprit as well. Thankyou guys for the suggestions!

Good Morning Jonathan,

Agree with the above comments from both Terry’s .

Just a few comments to think about ,

When the gasifier is being pulled or drawn by an internal combustion engine the air / fuel ratio mixture has to be in the ball park for the motor to run . The motor has to be running to draw the gasifier. If the gas mixture is to rich the motor will quit or if too much air is getting in the system the mixture becomes too lean and the adjustment have to be turned down to compensate and if they are closed and the motor still running it means oxygen is leaking into the system . With enough leaks the mixture will become to lean to run the motor ( vacuum pump ) and quit . By the motor quieting it is a safety warning that something is not right and air maybe leaking in .

If we are using an electric vacuum blower it does’t care what the air mixtures are, it just keeps on pulling. Any points between the vacuum motor and post gasification where the temps are under the flash point ( past the grate enough that the temps have dropped below self ignition ) that air accidentally leak in can cause the air /fuel mix to become explosive . ( The only time we want this mixture to become explosive is the last few inches in the system at the mixing bowl just before it inters the intake manifold ) If trying to flare and air has leaked into the system it is possible to have an explosive mix and the flame will run back into you piping and continue to the source of the air leaks . If the flame is running back through the piping it is no big deal but if it gets to a storage of properly mixed gas it can be a big bang ( this is the reason I ask about the capacity of your white drum an also Mike B post about me running like hell when I saw a flare sucked back in the pipe at Argos )

With a new gasifier and before the char bed has been fully established air can make it past the restriction and grate and set up conditions very similar to air leaking in post gasification with the exception of the exit gasses being very hot. The gasifier kida goes in the heater mode or if not enough heat there yet it could run an explosive mix through the system .

One small example of a gasifier going into the heater mode being pulled by a vacuum blower is Bill S unit at Argos … The unit had been flaring gas for a long period of time and doing real good. I felt of the fuel hopped and it was getting hot I mention maybe we should add some fuel but the flame was to pretty to be messed with . In about a min the flame sucked back in the piping and there was a small bang and it quit ( I ran as soon as I saw the flame run back in the pipe . Very little gas storage on the system so no danger ) I think as it was running out of fuel oxygen made it past the grate and on into the system . Had the unit been powering a motor the motor would begin to lose power an quit prior to the above event . The electric vacuum motor doesn’t care if the mix becomes too lean and just keeps pulling ( pop )

On your system I think I would arrange the blowers so the polarity could be switch so you could run the gasifier in up draft with the hopper lid open and reversing back and forth to build a deep bed of char. Once a good bed of char is established mount the blower so it will pressurize the nozzles pushing the system vs pulling the system . This will make any possible air leaks post gasification null and void and if there are any they will show with smoke or haze escaping .

I think all of the pictures of truck flares at Argos were bilge blowers putting gentle pressure to the gasifier nozzles ( gasifier fresh air intake )


Jonathan ,

One other thought . Until you learn your gasifier you may want to flare before your water filter just to eliminate the possibility of it wetting the gas .

Great explanation above Keith. I have had the most explosive mixtures at shutdown/idle when there is heat-soak. I didn’t have a problem with lighting Larry’s system since it had way too much air in it and a sprung top.

Here we go re-arranging your system.
I would remove your plastic expansion tank. For a relief valve you would need the whole top to vent.

I assume your gas coming off the gasifier is too hot to go directly to the hay filter (the reason for the water scrubber). I think the hay filter could pull enough moisture from the system to get a flare.

I, unlike the rest of the folks here, have a Chinese gasifier that uses a “water scrubber”. Personally, I think it does a pretty good job of capturing tar and soot. HOWEVER----- water doesn’t necessarily stay where you want it to stay. Whether you are pushing on the water with a blower or pulling on it with engine, the height of the water is going to change with the relative pressure/suction applied. Just ask folks on this site how many times they have sucked their U-tube manometers dry (creating and automatic air leak). Your water scrubber tank is subject to the same thing. If you had, for instance, 4 inches of water in your “tank”, and had an input tube immersed 3" deep into the water, it wouldn’t take much suction to uncover that tube rendering the scrubber totally useless. Besides, when that does occur, the water starts to bounce up a down, creating all sorts of issues.
The only reason I don’t say the same thing about a blower right now is that I assume you have very little back pressure downstream from your blower to the outside world,

But, if you are going to run an engine, I can tell you that it’s pretty easy for me to get close to 40"wc suction with my generator making about 3000 watts of electricity.
If there’s a moral to this story, here it is:
Just like the old saying: “parents, do you know where your child is?”, water scrubber gasser: “Do you know where your water level is?”

With my system, I have to blow and suck fairly carefully to keep things working correctly.

Pete Stanaitis

PeteS yours is an excellent post.
I personally have used only once and come to hate water gasses scrubbing.
Hated the then contaminated water disposal problems I created.
Means I have learned little about the practicalities of actually using them.
Your voice of experience shows well shows the wisdom that actual hands-on expereinces will always beat out speculating every time.
Others like P.C.; J.B.; B.P.; L.J. are using pumped water wet gasses cooling&washing systems and happy with it.
You the one talking about the practical details of it! Good on you.

Ha! And WayneK I’d though you said you knew little about small stationary systems. Knew enough to quit hold’en 'em, drop 'em, and run away!
One of the best detailed posts on following the process with visual and hand felt clues following the heat and oxygen internal “what’s happening” that I’ve ever read.

JohnathanT my input would be since your stated intent is to engine-generator run with this produced woodgas then as Wayne pointed out start engine running with it.
Only the engine will be the true judge of good woodgas IF your intent is too use it as an IC engine fuel.
And show the gases usable value quicker than any other way.

Blower/sucking flare changes the whole system dynamics far from what an actual engine charateristics on the system will be as you can read others saying too.
Yep, blowing needed in the very beginning, and if you intend to in-system char bed developing.

Flare judging for engine fuel is one of the biggest jokes on the NET about woodgas.

I have many times now engine ran with produced gas that would not flare at all.
Flaring has it’s own set of tricks involving gas speed, and air mixing.
And that picture “perfect” blue gas is a horrible grade of IC engine gas needing too much ignition timing advance and an odd fuelgas/air mixture. HAS LOW ENGINE POWER.
Give me “dirty” translucent yellow flare gas any day and I’ll make better engine power with far less tweeking.

If you use a single cylinder air cooled push rod activiated overhead valve air cooled engine to “goat” with; all woodgas conditions, including actual smoke, are easily recoverable from except for two.

You can valve cover off; carburator drop; and spark plug out; solvent wash away any tars or soots or goo’ed stuck open valve problems.
One/two bolts more, be push rods pulled out and can straighted any bent. Pushrod tube bent too then just the cylinder head off with four more bolts. Same for any actual bent valve fixin’.
If you’ve ever watched the VictoryGasWorks videos that poor little red Brigg& Stratton gnerator had all these done to it many times and was still capable of near 90% new power.
This after many hunderds of hours, with many different woodgas systems experimental developments, on lots of weird bio-mass fuels, and then switched back to gasoline for baseline power comparing.

The two bad gas condition NOT IC engine recoverable from are carried through abrasive ash damaging the piston rings and cylinder wall.
Not going to happen with your current downstream set-up.
2nd one is direct running of too hot of too uncooled produced gas (above ~250) as it will engine oil burn off on these same areas.
Give the same problem with premature metals surface wearing away and loss of compression problem.

Don’t want to risk/repair your gen-set, then use an old rotary lawn mower as your development goat.
Leave the blade on and close off around the lower housing to the ground with sand bags to trap in the blade air churned to load the engine better for a stonger fuel gas suction demand.
Ha! Sand bag the deck top down too so it does not hovercraft float.
Works much better than a just an unloaded, board mounted engine.

Practice starting up in either case with bottle fed in propane vapor into your “T” or “Wye” mixer.
Much easer to get a hand cranker running on propane bled-in then switch over to a drawn in woodgas than straight woodgas starting in my experiences. Even save you a lot of electric starter wear minutes.

When you are gasifier heating up blowing/sucking bypass flare/vent off all of your downstream to save the downstream from as much hearth system still too cold dirty tars and smoke building up as possible.
You’ll be thankful of the reduced downstream maintenance cleaning up and flow clogging problems there.

Real world woodgas using is not a steady state, idealized “Lab” thing.
In the real world a good operator “warms up” stabilizes before loaded working. Post heavy loaded working “winds down” “cooling off” gradually. An then “wipes down” before walking away.
Horses, frying pans, hard worked engines: YOU out sweat working - it’s all the same principle.
Unless . . . you LIKE scraping the dried on crud off that frying pan left unattended overnight the next morning??


Thankyou guys for the suggestions. Wayne, I can blow into the system with a shopvac and close off the filter train so it can build a char bed. I bet this is the reason I haven’t got a flare yet. Maybe I will replace the barrel with an oil bath filter that I was going to use but didn’t. The thought of a barrel of wood gas blowing up is starting to scare me. This would be the thing that people would remember about woodgas, that it blew up. I don’t want to mess it up for me or anybody else. Got an obligation to build something safe that works good! I will drain the water out the scrubber untill I get things working reliably. I think welding the stack on top of the gasifier was a good idea. It should make warming the unit up easier and less smokier. It also doubles as an air inlet that is fully adjustible. All the open top gasifiers I built were not good and gave you a major smokebath. I don’t see how the drizzler folks aren’t complaining of smoke going everywhere! I will build up a bigger charbed and pressure feed the system to get it to work. Then I will replace that barrel. I keep you guys posted.

Good news everybody. Got flammable gas this morning! Steve Unruh was right. I didn’t have a proper charcoal bed built up. I turned the blower on and lit the wood in the reactor from the top. I could not get flammable gas until all of it was glowing charcoal. Then the gas lit out of the blower every time I put a lighter too it. It can’t be stated enough to beginners that you have to have enough charcoal to start with or it won’t work. I need to build a proper burner coming from the blower. I need to put some kind of puff back protection on the buffer barrel. I need to install air preheating. Things to do, but the good news is that the unit works and I don’t have air leaks. I will replace the aluminum tubing when I get some money to do so. Thankyou guys for the advice and suggestions. I will keep you posted. The stack on the top works great. Lets in air, works as a chimney when needed, has a puffback lid, will do the same thing as an opentop gasifier without the smokebath, and acts as a vent when you shut the unit down. I think other gasifier builders should look into this concept.

Thanks for the good news !


Good Deal JohnathanT
Ach, but you give me too much credit.
WayneK and TerryK said the same in the middle of their posts about the char loading.
In fact, everyone responding back gave you very good experienced info.
Review these often later for then understood and appreciated advices.

Aluminum gas stream tubing will become a self correcting problem. Won’t even have to wait for an overpressure JiffyPop event like TerryG had. That corrugated aluminum is already white corroding/pin-holing as we speak now having been exposed to hot, always humid and alkaline woodgas.
You’ll run into this a lot with woodgas where a “was running just fine!” system when sits unused, has corrosion perforating, and corrosions/tars/sooted frozen valves movements, and clogged drains problems.
Nature of woodgas systems. Nature of all refining/distilling/cooking operations. Workers/operators in these indusrties know all of this well. When the LabGeeks and Chef’s have to clean and restore to useage their own used equipment a lot more practical gets done versus the esoteric.
Whole different evolution figuring operator shutdown, materials/design changes and pre-starting up check lists to make systems real-world reasonable and daily useable.

Able to flare now . . . once you ARE up to engine running your next face slap coming will be when you realize that quite often an IC engine will refuse to start-up and run on different kind of flared gas. Wasted a lot of time in my first woodgas year learning that one. A Mike LaRosa lesson for me.

If you are wise, somewhere along the line you will join the crowd of now experenced loaded engine running woodgassers reserving flares just for crowds demoing and nights photo’ing.
Cooled, filtered gas appearnce once experienced enough is a better judge for actual engine usability.
Hint: will be a translucent blueish/grey haze. Avoid breathing it as it should be a min 4% to 16% CO.
Sensitive local areas it really should be flare burnt off.
But BEST if IC engine purified into converted CO2 and H2O water vapors, useable Work and useable Heat.
Nature hates wasting of her bounty gifts. She is always the ultimate judge of the ethics of our endeavors.

Steve Unruh