Bridging - Downdraft Gasifier

I’m planning my new build and I want to ensure I cover all bases before I start!

My current build works great but I always planned on re-building a more long term and solid unit. Most of the issues I’ve had have been slowly worked out and now the last one….bridging within the Gasifier, I’m guessing a lot of people come across this.

My design is loosely based off the Peterson and has several sections within (slowly decreasing in size), I’m having bridging issues appropriately once every 30 minutes, it’s not too much of a problem at the moment as I have a sort of plunger/poker mechanism built in to the lid but ideally I’d like to get rid of this.

Basically I’d just like some info on bridging, problems others have had and the best resolutions? Also is it feed stock or build issue or both?!!

Thanks in advance,



On the Peterson gasifier, his setup is meant to have the grate shook every so often. In the book it goes over that.

Mother Earth News with their gasifier had another way to get around bridging by alternating their nozzles, some pointing directly up and some pointing across. I’m not sure if they ship their plans overseas but they still sell them on their website.

Yevgen Kolyvan aka Joni, points all of his nozzles upwards at a 20 degree angle. But he also has his gasifier mounted to his rear bumper of a car and allows it to rattle and agitate.

While you mentioned it, what is your feedstock? Woodchips like to bridge really bad.


You might have a look at Tone’s thread Tractor with gas? He seems to be able to use fuel that would make almost any other gasifier bridge and become restricted. And his grate doesn’t move at all.


The only things i can think of is like a grate shaker as the wayne Keith design has available on a button switch, And i have heard that fuel sizes too big can be a problem, the smaller burn tubes may need to chunk fuel smaller. Im not an expert ,or one with a lot of experiance at this time though And too small of wood like larger wood chips, can cause bridging as well.stationary gasifiers not in a car or truck may need gasifier shakers and or hopper agitaters too match wood size or bridging fixes.


Yes, and mount it on the same frame as the generator.


Thanks Guys,

The file in using is soft wood, mainly pine, 2/3cm blacks(15%-20% moisture) I have a grate shocker fitted at the moment although this isn’t automated, it doesn’t really seem to help with the bridging.

I’ll have a look at the angle of the jets on the rebuild. Unfortunately the generator is going in to a soundproof structure so I won’t be able to use the vibration to help.

Thanks again.


Ashley it will definitely help if you look into utilizing the engine exhaust heat to dry fuel. Steve Unruh has preached that until his face turns purple.

Maybe use a steel drum with the exhaust piped into it, and use wire baskets to hold the wood.

Tar sweating out of the wood in the hopper can make it stick to itself, I’ve observed this when making incomplete batches of charcoal in a retort.


Brace the gasification unit to the generator.
Let the engine vibration shake you fuel and this will reduce bridging.
Feedstock, use wood cut into blocks.


I would consider using a right angle drill attachment, mounted inside the hopper, with a auger attached to it. I’d run it to an intermittent windshield wiper motor mounted to the outside of the hopper and run it at it’s longest interval. Sorry about the long links. Amazon.


Hello Ashley, yes upper system bridging is especially a problem with stationary small engine systems.

My own system is small VictoryGasWorks shop 2009-2012 test mule prototype all SS unit.
Was developed around a 6500 watt air-cooled 3600 RPM electrical generator. Up to small four cylinder inline engines. Mine does not have a moisture reducing hopper. Single walled SS. And we did use a lot of conifer woods.

There is the heart of the problems. Too small of engine to make the heats for the tar-ring/tar- cap rising heats melting that the larger engines driving systems can use.
And using a really tarry wood in conifer woods. Even kiln cook/dry confer woods have the wood-sugars still in them that will convert and cook out.

Putting the input wood into the hopper truly forced dry and warmed helps a lot. That means truly dry at 10% or less moisture. Warmed means just out of an engine heats forced air drying. Not bagged wood stored winter outside even under cover that will air re-hydrate back up to 20-25% moisture. Maritine climates do this. The U.K. is certainly maritime climate.

Engine rod shaking works aright. But hard to turn on, and then turn off when not needed . . .then causing more problems in compacting than bridging solving.
My hopper still had an electric motor eccentric weight mounting bracket. Works for command and control. But another electrical system to set up and maintain. And these shaking all eventually metals work harden and crack something. Loosen flange bolts. Vibration work flange seals to leaking. Adaped over motors eat their bearings. Purpose commercial vibrators are expensive.

What also helps is to limit the pyrolysis area volume exposed to core rising heat.
Almost all systems work quite well with small 1/2-3/4 of an hour batch loadings. Even with wettish woods and conifer woods too.
So why the evolved from the side feeders like the GEK systems, the Drizzler systems.
Jonas Hedenberg’s 7 years evolved side auger fed in chipped fuel systems is this too.
Look him up on YouTube by his name and woodgas.
Because he has evolved a neat pyrolysis zone windshield wiper gearbox and motor tampering down systems.
His evolved stationary CHP system was highlight video interviewed by the Omstalliningaresan woodgas promoter fellow here:

Even though not CC enable to be able to translate; you can see Jonas hands describing his tampering system at 7:28 → 8:05.
Jonas’s own videos show this system in action. The Swedish translator guys say he is not timing actuating but needs sensing somehow.
Just be aware Jonas is woodgas fueling four cylinder water cooled Volvo engine as his smallest.
4X the gasifier core heats to work with the versus you and I with our much smaller single cylinder engines.

And this is why BenP’s book system is putting produced gas heat right back into the pyrolysis zone where his experiences have shown you best benefit to prevent tars made gluing bridging.

So lots of approaches possible.
Heat management. Encompassing fuel wood just-in-time dying and warming.

Mechanical. In that is wood bit sizing. Dutch John found that roughly your fuel bits sizing had to be ~1/7 of your downdraft internal diameter. Sheee. Small engine. Small gasifier. Small wood bits making. Most guys refuse this part of it.

Another way will be said is to just go with a divorced process and just go with a wood charcoal system. Actually, valid with a few IF’s.
IF you have wood that makes a good sound handle-able charcoal that can be handled, sieved, crunch sized, sieved, then bagged and stored. Conifers are very poor at this.
And IF your local lets you make the wood charcoal.

Steve unruh


On a small unit enough process heat is an issue.
So consider an external heat input from the close coupled engine exhaust to add heat to the fuel in the distillation zone.

Scaling becomes important.
Insulation and air preheat.
Too much heat can cause the different zone not to work properly.
Depth of char bed can change.
Increasing diameter and slowing down the gas flow as temperatures increase will crack the tars cooked out sooner and higher up because of the change in distillation zone temperatures.

Shape of fuel bunker might work better tall in skinny and a coal unit but that might not be the optimum for a wood chip unit.

Lots of reasons and ways to do things
Ah spoilers…


A downdraft gasifier already has a good ability to crack tars and volatiles. Taking your fuel all the way to charcoal isn’t necessary. You might heat your wood fuel to a lower temperature, IE Torrefaction, and cook out enough tars to avoid bridging.

Try using engine exhaust to “dry” and heat treat a charge of wood. See if the charge sticks together while hot (and cool) after treatment. You only need worry about the surface of the wood so that might make it easier.

If the treated charge doesn’t stick together in the test then it shouldn’t stick in your hopper. If it does… well running engine exhaust through a basket of wood shouldn’t be too much trouble so you aren’t out much?

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A couple of things here, not to be argumentive.
I strongly believe torrification browning the outside of the wood is pre-creating a can be later active charcoal outer surface. Then when that piece is combustion heated the inner outgassing volatiles have to pass through this Hot active outer char layer. I have verified this time and again in my bulk wood stove. 40% wet wood put directly onto a glowing bed of charcoal does this first flame surface converting. Then a better than should be energy release. Works best with HOT, low flow.

The other thing is I only use engine exhaust heat indirectly to dry and warm wood. I’ve seen others do and claim engine exhaust burbled up through wood chunks. Dan Nichols; some of the IISC GPCL system pictures. As a former Auto Emissions Tech I know a well set up IC engine will exhaust emit more weight of combustion moisture that the liquid fuel put into it. I think direct engine exhaust dying will be limited down to making a hot still above 20% wet wood.


so say a box with exhaust piped in and out (more or less a muffler?) heating top surface with wood chunks above? Then enclose somewhat to retain the heat energy and dry the wood with someway for wood moisture to escape, in my head I see a big hot plate heated by engine exhaust?


You wouldn’t want the exhaust to condense - I’m in full agreement there.


Now what fun is that…

For the sake of argument lets say I do not proof read enough.
And to clarify what meant…
If you have a small gasifier and its not able to generate enough heat as suggested using engine exhaust to heat the outside of the unit in a jacket like on an imbert above the tuyers.
Now you have some process heat to help with a monrator and to heat the fuel before it goes into the pyrolysis zone

I agree pushing exhaust through the wood chips would be counter productive.

Pre-drying of wood chips with exhaust gases …


This is where you start chasing your tail around and around. The first thing any gasser needs to understand; is there are multiple processes. Each process effects the process down stream and each process down stream effects the process above.

If your grate is not flowing at the right rate, then this has slowed your over all flows. Fuel in the oxidation zone will have too long of a dwell time to oxidize, reducing your fuel too far. The fuel above this in the drying zone; the flow will slow giving more time for hopper gases to fuse the fuel together. The oxidation zone now reduces the fuel too far leaving a void. This situation is not just local to the hopper and all processes are effected.

I would recommend a both a hopper and grate agitator. You can control them with readily available cycle timers on ebay or amazon. You then simply need to dial them in and remember that they effect each other.

Process and flow are the first things you must get working. You must establish consistent process first, then you can adjust jetting, fuel size, restriction and adjust other parameters.

These timer boards are already pre programmed. They have something like 20 different programs you can choose and the timer on / off range is plenty for this. They are very easy to use and wire.


Well Ashley you did ask and now i think you have many many options from the experts to try out and see what works best for your machine .
I’m no expert at all when it comes to gasifiying wood , but i am playing with downdraft charcoal machines at the moment and find a strange situation where for no reason my fuel stops flowing down , how is that possible ? its not anti gravity charcoal i have made is it , nope its down to heat rising i think and causing the char just above to form a sort of mid air cave , that will collapse if shook, poked or vibrated over the years a few ways to over come this effect has been posted ,sadly now lost on the Yahoo wood and charcoal groups , but a couple of ways i can remember was to have a internal spring loaded side wall or top that pushed down when material collapsed below it , and just recently Matt Ryder on his Thrive off grid ( small engine ) post 1534 has or is testing a device to help move material away from a backwall to push it towards the hot zone .
Another way if it was me would to use something along the lines of the pyro touch that was used by the Drizzler inventers , once it can be installed through a air tight seal that would for sure move your material i think even if it was held up with hot tar .


The small mass of the chips is the problem. It’s own weight in the hopper will compact it and even damp sand when tamped can be made to hold itself up when material is removed to form an arch. Even slightly damp chips would tend to react the same way with the nozzles burning away an inverted bowl in the chips. Without some kind of mechanical movement to break it loose I can’t see it collapsing.