Grate and Reduction Design Ideas and Discussions

Hi, Jan-Ola
16. of November 2016

Your “mapping” shows clearly where the low-end borders of your gasifier are. And generally.

Increasing the reduction volume and decreasing the bit-size leads to constipation, as ash-blowing does not work well enough in the current flow area.

Another (mechanical) way would be to introduce light sieving
at (every) some bunkering stop.
Meaning, to help the inadequate ash-blowing by letting the “small stuff” flow down between “just-right” angle bars, from one side to the other, alternating.
This could help keeping a bigger reduction volume chemically active by avoiding it to “drown” in ash.

Another idea is to make a grate with a slightly finer “mesh” than the current one…
The current one seems to favor “big or nothing”.

Meaning the ratio between angleirons and the “valley” width.

Narrower “valley” to gap!

Now, a big bit can “live” on the “valley”, as all smaller ones are consumed by the gas-stream over the gap and dropped through.

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Hi, Max

I must admit I’m a bit confused by your statements.
First of all:

Nothing wrong with that statement, BUT that was NOT the state of the gasifier I was describing above. Rather the opposite - needing smaller fuel to cool things down. AND I have never increased my reduction volume.

OR are you saying 4:1 - 5:1 is unnecessary high vacuum to keep gastemps down? Could gastemps be lowered with lower vacuum only by keeping char surfaces clean and active by increasing the velocities?

Another question: Grate suggestion have conflicting statements.

Does that mean a tighter grate for more velocity? OR a more open grate for more slipping?

Hi, Jan-Ola!

Ok! Sorry to have caused a confusion!

I did willingly not start with your “state of art”, as it is already well known to everybody following your thread.

“And generally”. Was meant to focusing onto something else than the conclusion of your “mapping”. (kartläggning)

Focusing = if…

  1. I was first repeating the way it cannot be developed
    towards, and reasons for it.

  2. Then, a (mechanical) way to still be able to go that way.

  3. Still another item: Increase the restriction diameter slightly (1/4"–1/2") in stages.

Now, decreasing the output gas temperature is obviously a question of fulfilling the oxidation and the reduction process.

The grate architecture is important for the mechanically “surviving” char bits; make the valleys slightly narrower than the gaps, and the “oversizing” of surviving bits will decline.

In the same reduction volume you want the bit mean-size to decrease by not “preserving” big bits on too broad valleys.

Going in to mechanical means later.


Hi, Bob!
16. of November 2016

Next “round” I expect to be on Jan-Ola’s Thread…

Hopefully Chris is “mopping up” our …

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It’s hard to talk to everyone on a topic that is brought up in discussions and you are not on everyone’s else’s thread.

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OK, I’ve moved some of the posts. It’s hard to untangle a discussion like this and not leave some holes, so I err on the side of caution. Also note that moved posts can appear out of chronological order.

A tool you may not be aware of, is the “reply as topic”. I’d like everyone to make an effort to use this when appropriate.

  • Click the post timestamp, in the top right corner of any post (might say 1d, for 1 day ago, or maybe 1h, or 24m, etc).

  • In the popup box, click “+ New Topic”.

This way your response will be linked to the old discussion, the post you’re replying to will be quoted in your own post, and your post will be linked under the post you respond to. But it’s a new topic, and the discussion doesn’t disturb the existing conversation.

If you want to reference a post in an existing thread, just link to it, like this,

or like this (leave a space in front of link):

Wide Band Electronic Mixer Control Tutorial

or like this (no space in front of link):

Posts with links to them show a link to posts which are referring to it. For example, if you look at Matt’s post, you’ll see a link back to my post here.

Help me help you… it’s a lot easier if you self-organize the posts than to have me try and clean it up later.



After reading it over and over again I see what you are talking about, my brain has the light bulb turned on, but not on bright yet. Lol.

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Thanks Chris for the instructions.

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I like threads like this one, to have all the focus in one topic.

I was jusr about to do my ashpit cleanout so l decided to allso take a look in the charbed and show you guys the insides.

This was a experimental design, but it prooved well so l stuck with it. The restriction plate is already a bit deformed and cracked (cast iron) and the grate has a few scales on but we will see how long it lasts. Both can easyaly be changed.
The grate is 2.5mm mild steel.


Thanks Kristijan for the look inside your gasifier. I see your clean out is directly under the grate.
If you would just grind a little off the edge of the plate on each side it will come out easier. That is what I had to do on my choke/reduction plate the ash fills in on the flat spots that are ground off when placed back in. Love the design work you have done. You keep giving me more ideas to do on the mini WK Gasifier I want to built. A lot of the Members ideas are going into that next build that I want to do. But first I need to finish “The Gasifier That Wants To Be”, charcoal gasifier build and get it up and going.

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Yes this will have to be done eventualy.

On my next restriction model l will try to make eaven less char/metal contact, and go stainless. The fact that the restriction plate with about half a inch thickness of iron got so hot it got deformed tells mee this is a huge heatsink (expecialy at startups). I might eaven consider a non metalic restriction, l had some great resaults with superhot enviroments and white foamy insulation bricks. We will see.


If it had a little lip or ring on top to fill up with ash it would protect it more. This how Wayne cut the rotor out on mine. It left a ridge for ash to collect on, and help form a ash cone inside the fire tube. Lots of little things that add up in a gasifier design to help protect it from the high heat.

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I can’t take credit for my reduction tube setup. That was Arvid’s and Max’s idea. But I’ve given it some thought and I think the way my restriction sits on top of the tube takes advantage of the heat in the restriction area. Heat conducts down the tube into the reduction area where it’s needed the most and help keep things going during short low loads - WK’s flywheel effect in miniature.
I don’t know if the outer edge of your grate is tight against the firetube. If it is, a reduction tube could actually sit on the grate and the void towards the firetube filled with ash all the way up into the oxidation ash cone.


Hmm interasting thinking. Just the opposite of myne.
You are saying have more metal char contact so the metal heats the char in low loads?

The grate is tight. In a way l do use that effect. When l lift the restriction l see the center (reduction char) and only ash on the rim all to the top of the restriction.

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Yes. A little bit, for a little while. That’s the flywheel effect. Works the same as preheat. Both good for ideling when already up to temp.

The other route is saving heat. Insulation and no metal contact. Good for fast startups but less accumulated heat to use at redlights.

What way to go depends on what you’re looking for. Most of the time you really don’t need any of them. That’s the route they went during WW2. Less complexity, but will still take you from A to B.


Hmm makes sence. I am on the “fast startup bad idle type” with my setup.

The real question is am l happy with that? If one realy has to choose between fast startup and good long idle, l wuld vote for fast lightup l think.

As for insulating, everything is ash insulated plus the entire firetube is insulated with thick rockwoll.


Kristijan, if you added preheated air off your exhaust system, then you could have both. With extra warm preheated air going into your gasifier intake it would help at idling. I notice this if I start my truck up and warm the engine/exhaust system first, to just starting the gasifier up on the suction vacuum. And then start up the truck, idling is better once the truck exhaust heat exchanger inside warms up the incoming air. After the whole gasifier system is up to operating temperature at the grate around 1100*f and hotter, the flywheel effect kicks in. I think at that point I could just run ambient air into the gasifier by, bypassing the exhaust preheater and it would not matter. Colder air would give me better performance in certain conditions. Lots of different ways to do this wood gasification and make the vehicle go down the road with a SWEM.


Hi, Kristijan!

The differencies are not so dramatic, if you keep the “blow through” area on the grate near matching the restriction area and the reduction volume less than the oxidation volume.
Using one dimension of tube from the nozzle-roots down to the ash-pan.

This type of flow will be rather swift whole the way trough.
It usually blows it free of ash after a while on idle when giving full throttle.

In GMR Imberts the grate area is as wide as the outer shell. It has very slow gas streaming through it; part of the gas never goes through it at all.

In this case shaking or sieving has its clear place. Without it, burn through from the restriction downwards or sideways is common. Too much shaking causes side-blowing as well…


just going to poke this back up top, and see if anyone who played with some of these designs has any more to report on how well it worked, or things they changed to make it better


Thought I’d snag Dustin’s picture of his V grate. Looks solid built I’ll have to take a page out of this book.

He says he went pinky width maybe 1/2" I’m thinking 3/8" gap distance.