Properties of a good wood gasifier

Wood gasifier
What are the properties of a good gasifier supposed to be?

  1. Produce quality gas with the same properties over a wide flow range
    - quality gas:
    high temperature in the reduction zone with the presence of charcoal through which pyrolysis gases must exit
    adequate humidity of gases traveling through the reduction zone, the possibility of condensation of steam in the gasifier
  • gas with the same properties, at different flows:
    isolated reduction zone so as to maintain a high temperature and temperature accumulation in this part
    good installation of air nozzles with the smallest possible cross-section, which still meets the needs of consumption, the highest possible speed of air beams for blowing (20 m / s)
    suitable cross-section and height of the combustion and reduction zone
  1. Good gasifier efficiency
    good gasifier insulation, hot exhaust gas preheats fresh air and wood inside
    cooling for condensation is performed only when necessary - water cooling

  2. Easy to start and maintain

Direct ignition opening in the heart of the gasifier
Ash cleaning door

I ask for critical comments regarding the above statements


Hi, Tone!
Most items are good…
I guess your air velocity is taken as cold air velocity…
In that case, there is not much “room” for doubling
the ambient temperature by preheating to 2 X Kelvin–“step”,
using the same nozzle diameters.

A little adding…
Plank silo drying
Silo mantel condensation
Thyroid burning
Radial reduction over an insulated heat flexing (twin)plate,
manually tilted at cleaning.
Vertical “hanging” angle-iron grate-ring.

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Max thank you very much, I will follow your instructions, I will draw a sketch shortly and you will find it easier to comment on the details, because as they say, a picture says more than a thousand words.

Different parts of the system will use/age/wear and need to be changed out.
Many, many build-up designs out there are all-welded up. Makes for quick easier first building. Much crying later after a hew hundred of hours of useage.
Reparability is different than a “modular” made swappable experimenters systems a few have offered up.
The WK and VesaM’s and some of the Finnish systems design in reparability.
And this requires a base in hard running and thousands of hours of actual worked gasifier using.
Steve unruh


Hi Steve!
We are not “Teslonians”!??

Max, I don’t expect this, just for any good advice, criticism, or experience I would be grateful for.

Making interchangable parts heart is a chalange. It means you need things to fit air tight, withstand the heat and come apart easy. Not something l am comfortable with.

Personaly l like to weld it all shut but with a about 300% tolerance. I use thick hevy steel for everything heat connected, and top up with thick strong wealds. Air mantel where ever l can!
I learned my lesson many times. Easyest way to be sure you can eliminate a leak if a problem occurs is not give it a chance to develop. Stove rope and RTV silicone are a strict no go for any hot part of the gasifier for me! Just wealds and threads.

Sharp corners shuld be avoided too. Only place l ever had a weld crack is under the restriction of my cube shaped gasifier. Not a critical weald but shows it shuld be avoided!


Hi, Tone!
This is a “classic” joke; for years we have had fun following a certain type of “building” style, and had a good smile.
Good advise from colleges is always welcome and jokes are jokes. They are kept apart.

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Hi Tone,
for dimensioning of the hearth, I found this website. It is in Swedish, but a translator might help. Or the Scandinavians in this forum. :wink:
The main dimensions are calculated by the gas consumption of the engine. It is Imbert-style.
Hope it is helpful for you.


Thanks, Til!
Google (the evil empire) will translate for you while viewing with Google Chrome.
Good information, plus many more html “chapters” in that “book”. :grin:


Ha! Ha! Good joke Max.
No. You and I are Practicalists’. (English allows you to make up your own words for new concepts)

Kristijan I do understand the hard hurting bite after a fellow has struggled with internal gasses bypassing leaks.
Regardless the restriction plate (the number one worst heats gasses flows erosion hammered part) has time and times again proven edge sealed with just sloped formed char-ash. You just have to have enough flat edges overlap and allow enough time for this char-ash to naturally developed. Then drop-in; pull-out changeable. The expansion floating outer edge of this means no welds cracking. No walls and plates temperatures differential buckling.
The Oxidization Zone hearth walls are another use damaged heat area.
My 1st generation Ben’sBuilt Victory hearth handles this quite well with removable stand out air nozzles; protected by ~13mm of internal spun alumina-silica insulative mat held in place with a drop in SS tube sleeve. The jets protruding through this sleeve mat retainer. This sleeve-retainer is drop-in, pull-out; bottom sealed down buried in the char-ash slope resting on the restrictor plate. Sleeve heats erosion damaged? Pull out. Replace. It is just a plain round tube from flat stock with air nozzle holes in it. His evolved system carried over into his Ben’s Book published system.
Better. IF my hearth outer tube was a larger inside diameter I would just ledge mount a row of vertical $3.00 insulative stove hearth bricks. These last 6000 to 20000 hours inside my hard ran wood stove. The more insulative softer grades of thermal bricks are easily hole sawn for the air nozzle openings. No need for these to be edge gasses sealed as other have tried. Let them loose float. Lower ledge supported edges buried in the upper char-ash slope. upper edges retained with an angle iron segmented ring. Also drop in replaceable.
Grate system IMHO should be lift out easily replaceable too.
The corrosion prone hopper NOT welded on: or worse; one piece with the hearth tube, but a separatable component. Ugg!! Not flat flanges with many bolts! That is high pressure capable industial work. No. Spend the time/effort to make a V-clamp attachment system. WK’s as an example of this.

Remember. The topic OP asked for Properties of a GOOD system. Not just a get-you-going system.

Steve unruh


Yes you are right, l forgot to mention the restriction is the only thing l dont weld. It just sits in a gasifier and can be pulled out, like a WK choke. Insulates and seals its self with ash.

Well the way l see it you have 2 options. Make a gasifier out of materials that can withstand the heat, take all the different expansion spots in to account… do not weld! let the heat hang in there and hope for the best. Example is old time ww2 gasifiers, cast iron hour glass of cheramic hearths. Good for the flywheel effect.
Or, make the heat conduct out of the critical spots and distribute elsewere. In the intake air per instance. The importance here is to have enough mass on certain places that can conduct heat fast enough. WK is one clear example of that, or my flute nozzle for a char gasifier.

Nozzles… l used thick stainless bolts drilled trugh. After many hours and many miles they still had the burr from drilling. Charcoal is a different story, using it pure did erode my inch thick ss rod with a 9mm hole…

Me and Tone talked on the phone the other day, discussing possibilitys for a good gasifier. He laughed when l sayd for me, the making of the gasifier hearth is the simplest part and l stand fully behind that statement. Its things later on that are a pain to make right, cleanout ports, filters, mixers…

This is a example of how l imagine a simplest, bullet proof raw wood gasifier hearth anyone can make in a hour or two

Thick firetube, thick nozzles, welded strongly for a good thermal contact. Thick restriction sits on a shelf. A DOMED air mantel around, air intake at the bottom in a tangent like a cyclone for good turbolence/heat pickup by the air.

Grate not shown here!

I use this design now, but on stereoides :smile: reason being l wanted to make a hybrid gasifier that can run anything from raw wood to pure charcoal, but prefferably a mix of both.

9mm thick firetube, double level of nozzles, super thick restriction. In contrast to the original concept, in spirit of making the gasifier withstand pure charcoal l allso added finns on the firetube and welded 6mm acid proof stainless sacrificial plates inside the firetube. Probably overkill but l wanted to see how they perform.
In a way it uses some concepts of a WK…

I will probably make a cheramic wool insulated reduction zone like this later when l get to it.

This gasifier run my Mercedes but just a few hoppers worth. Sof far so good but the real test will come now when l put it on my Ĺ koda


Hi Kristijan,
You picture a horizontal V element grate. An All-Must-Flows-Trough design.
MaxG now proposes a vertical can/cage V element grating system.
I have come to insist on a By-Pass flows capable (up and over the edge, long path trough a retained char stack); command movement active-possible, grating systems.
Gives a wider range of gas-loadings and fuel types use flexibility. Think the WK, Victory hearths, some early thru GEK III grateing systems. The FEMA chain hanger type. All By-pass flow types.

How to make V clamp attached hoppers?
Collect up a variety of steel wheels. Especially the unusual sized compact spare steel wheels.
This will give you a concave cupped inverted ring cutting off the edge bead seat to weld onto your round hopper vessel. Use an overlap weld joint.
Then using a 13mm/1/2" smooth steel rod Heat/Beat/Repeat to form a matching round seat.
Weld this onto your hearth upper tube.
Source different small, medium, large barrel V clamping rings. You can cut these down to the proper diameter. Again heating and reforming.
Use a length of the woven flat fiberglass stove tape glued onto the concave seat as your seal.

Works. All just hand Fab work. Still a lot les work that making up two flat flanges. Kept flat /made flat again after welding on. With then many bolts. Ugg.


First property, is it must flow consistently through all stages and process. With out consistent flow you will not establish the processes at any stage. Generally this requires automation at the smaller scale and for stationary machines with out an operator.

As for it being modular, yes this is the way to go especially if you are just beginning. This way you can revise and you are going to revise it many times. As for sealing this is what ceramic seal is for. Its what it is used for. never use silicone it wont work at these temps. You do not need perfect air tight seals on a gasifier, if you are sucking air past these seals, you dont have a seal issue, you have a build issue. (See First Property ^^) Ive been using the ceramic seals from the very beginning of our modular design units and have never had an issue. Your flanges must be 12 Ga Steel or heavier, never use Stainless Steel unless you want to warp that will cause a leak then failure. You really need to understand your architecture and stress growth of SS. Generally need computer software for this. SS is terrible material for this and offers absolutely zero advantages. In the gasifier reactor it will stress harden and fail. and then in your filter system you want it to cool your gas. SS retains heat this the opposite of what you are trying to do. There are coatings you can get that will outlast the machine. You can even use aluminum if you wanted.

When I seal the systems its not just the flange clamping that is doing the sealing. It is the joint corner of the vessel and the flange that is really doing the sealing. I use high tolerance washers on all bolts, much like your brake lines on your car have copper washers for the banjo line fitting. Steel can do this just as well.

IMG_1233 IMG_1234 IMG_1235 IMG_1236 IMG_1238 IMG_1239 IMG_1241


Next is your filter system. A good system will have very good cooling. The cooler your gas the more energy potential pr volume it will have along with better moisture dropping. This is not a steam injection system and steam injection does not work by simply introducing moisture into the intake air steam. Wet fuel equals wet fouled spark plug, resulting in weak engine performance followed by the entire system crashing and angry operator :frowning_face:
Next is gas scrubbing if the gas is not clean enough add more filters. Easy enough.

Run time, a good system will meet the desired run time simply put. Pellet fuels are the best option to meet long run times. They are three as energy dense pr volume as most other fuels and you dont need an auger feed. They can simply be gravity fed via a huge hopper and are the only fuel capable of flowing through a reducer to the hearth. Someone mentioned this fuel is energy intensive to produce. In the bigger picture it is no more energy intensive than any other fuel and it is far more viable. First thing I want to address is auger systems, to bulk feed chips; you need a minimum 3 HP motor and it will still jam I guarantee it. Every time this auger runs its 2.2 kW of energy input to bulk feed chip fuels. This a big loss in energy some forget to factor. Chipping fuel with the smaller chippers will generate waste. 25 to 50 % of what you chip will not be usable. The energy you used to chip that fuel is wasted, the energy potential of that fuel is wasted and your time is wasted. Pellet fuels only require a cheap wood chipper / mulcher and you only need to process the amount you need;; all of it will be used. Chip fuel sorting is not fast or fun, it takes time and your energy. You will have less time into feeding a hammer mill that can process 100 lbs of fuel in minutes. Then a 3 kW pellet mill can produce 150 lbs of fuel in an hour. This can run directly from the gasifier so basically its fueling itself as you can get them electrically driven. As for the other equipment you can run that on charpellets using a small charcoal unit. The charpellets from the wood gasifier are perfect fuel for this, with very little screening effort. So zero gasoline input here. Keep in mind I am talking about bulk feed systems here. If an hour run time is satisfactory then chunk fuel is probably the best way to go. But for me; my clients want these machines to run 8 to 16 hours a day with zero intervention. Turn it on and forget about it for the next 8 to 16 hours. I have achieved this with these machines they will run until they are out of fuel and its easy to do, as you do forget they are even running. Those that have experience know what runtime anxiety is; where you are constantly listening to your machine so that you can intervene to keep it running. A good wood Gasifier does not have this issue.

Only issue with pellets, is they will not work in a traditional down draft gasifier. You need to control combustion moisture so it can not migrate into the hopper and foul the fuel. Otherwise it will soak it up and turn to mush!. The Drizzler or my system are the only systems I know that will work. I will open source my tech but you will need to sign an NDA.


Matt, l have run pellets on a more or less traditional downdraft. The trick is simple. Keep the heat in the hopper above boiling point of water. My insulated hopper on my Chevy got up to 250 c, estimate, enough that the wood on top of the hopper started turning brown. No chance of moisture here HOWEVER l only filled in pellets for long highway drives. The goal was l burn a whole hopper of chunks after the pellets were gone, so that the gasifier cools down on chunks.


Ok can you shut down on a full hopper? This more of what Im talking about.

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No, like l sayd, that wuld be bad.

Well, except in case the pellet level was low enough that the pellets were allredy partialy torrefied by heat. But it was a risky buisness, stoped doing that after a few times l had sawdust to clear out…


Oh, by the way, since we are at it… there is allso a fuel that flows good, doesent turn to mush when moist, requires litle to no energy imput to make (even emits it!), doesent contain tar… can anyone guess what that is? Hint: its black, begins on C… :wink:


Here I am attaching a sketch of a gasifier that might meet quite a few requirements of a good gasifier.