A question regarding ashes and clinkers

In considering the design of a charcoal gasifier, how much stock is placed in dealing with the ash and clinker build up within, and it’s movement inside the unit?

Some designs use a grate, and while I see how that may help with ashes, I can’t see how clinkers would pass through.

I guess the final question for me is: When designing a charcoal gasifier, should I put a bunch of effort into ensuring (if even possible) that ashes and clinkers travel towards, and through the grate? Or should I just concede that in real world useage, I will have to periodically empty out the unit and clean out ash and clinkers as they build up, and that this is just the nature of the beast with charcoal gasification?

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Reaction temperature affects the formation of clinkers. You can use exhaust gas recirculation to help keep the reaction temperature below 1100C where ashes begin to vitrify.

Also see Don’s grate and clean-out solution: Don's Geo Tracker Project - 2- - Charcoal - #131 by don_mannes

If that link doesn’t work go to Don’s Geo Tracker Project - 2-- Charcoal, entry 131


Hi Will, having charcoal space below the nozzle for ash and clunkers to go is the best thing I have seen. And controlling it with exhaust is the most important thing to do, like Bruce has said.
I like the idea of a box below the nozzle , (Simple Fire) design and mine works in the down draft mode. I will have a slide gate at the top of the box that can be closed to stop the flow of charcoal when opening the box for clean out of ashes and or clinkers. This is my current design of my Simple Fire. I am not finished with the build yet, will show pictures when completed.


Well, there is about 3% ash in your charcoal so you can roughly calculate your periodical emptying rate.
I think the best way is

  • lowering the temperarure under slaging temps
  • have space under the nozzle(s) dor broken off clinkers to sccumulate
  • have a way of brakeing the slag from the nozzle
  • perionicaly clean, and meanwhile ibspect the sistem!

Thanks guys!

I’ve been through Don’s thread many times and plan to emulate (steal) several of his design details😁. I figured I wasn’t going to get much positive flow towards the grate as I plan on an updraft with a nozzle dead center on the bottom like Don has, but with a non rotating puck. Thanks for the suggestion to keep the lobe below slagging temps with exhaust or water, I hadn’t considered that. I am concerned mostly with clinkers as my initial fuel will be pyrolysed straw pellets, and straw has double the ash compared to wood.

Beyond these, it looks like designing in a convenient, easy access plan to do periodic clean outs is in order!

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Please tell us more about your straw pellets. Are you making them?


Mine is an up draft with shakeable grate. I’ll say I can just shake the grates for three firings and on the forth fire up I have a poker (fence wire about 8" long) that frees any slag or what ever. I think a volcano forms up and around the grated round hole at the bottom. I leave it be because I think it helps protect the inside. I normally would never dump the charcoal out of this design, just a few shakes and a poke from time to time. If it is slow to start it needs a poke. If the charcoal doesn’t seem to be nicely laying on the grates, seems to be up in there, it needs a poke.


Yes I will be making them. I am building a flat die mill this winter, and will be making dies for wood and straw. I bought an old hammer mill last winter and rebuilt it. I will be either buying or building a wood chipper as well. I hope to alleviate my fuel availability issues by utilizing both wood and straw for motor fuel. Hammer milled straw, and chipped then milled wood.

I work at a Fab/Machine shop and one of the perks is the owner lets us work on our own projects in the shop, which is awesome for guys like us that tinker😊.

I will be making a thread detailing the mill when it’s done for anyone wanting to duplicate my efforts.


Do you have any pictures of the bottom of your unit showing the grate? Are you able to get the clinkers to come out of the gasifier through the grate? Any issues with melting the grate?

Toward the end of the topic:

Only when a screen like grate was deeper into the gasifier.

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Thanks, how’s that refractory cement holding up?

Hi Will, One of the things that has surprised me regarding the charcoal gasifier is that the ash does not clinker. It can accumulate around the nozzle but it easily crumbles when poked or picked out. I also have a forge and get clinkers when using charcoal so know they can form, but they don’t show up in the gasifier. Not sure why though. You will need some method to remove the ash. Whether inverting the gasifier and dumping the load or an opening that you can shovel ash from.
As you note, straw has much more ash than wood so you may get clinker formation there. You will just have to give it a try and see. I’m curious.
Gary in PA


Thanks Gary, I recall Don saying something about clinkers forming around the nozzle too, also Kristijan with the clinker “nests” around the air holes on the heavy pipe which supplied the air into the unit. In all cases it seems ash and clinker formation is pretty minimal. I’m sure straw will be a little more problematic!

Must be OK, but to be honest I never bother to check it. Any more I just use this stuff. It runs at least as good as gasoline and I would say a clone runs better on gas than gasoline because a stock clone carb tends to be a bit problematic.

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Because I have no experience with charcoal gasifiers. Can someone tell me what clinkers are? Are they spent charcoal and therefor contain no energy? Are they contaminants and need to be disposed of? Or do they have enough carbon worthy of saving and maybe used in a grill for cooking?

When you burn wood or coal, you get ashes, right? In a fireplace, or at the campground. The ashes consist of the non-combustible minerals that were part of the fuel.
If the fire burned out on its own, then pretty much all of the carbon has gotten burned out too.
If you poke around in the ashes, they are usually light and fluffy.
That’s what happens in an open fire, for the most part, because there is no forced air added.
But, if you add forced air, the fire gets hotter. This higher temperature tends to melt the ashes into a glassy sort of material that we call “clinker”. Still NO heat value.
We I was a kid, everybody in the neighborhood heated with coal. we had lots of clinker to contend with. The only use we had for it was to throw onto the driveway in the winter to get the car unstuck.

I, too, do a fair amount of forging. We actaully have clinker contests, to see who can get the biggest solid “donut” out of the forge after a day’s work.
Our success varies a lot with the coal itself. Some of it has a lot of ash (AKA clinker, in a forge) and some not so much.
Moral of this tale (from my perspective):
If your grate is getting very hot, let’s say over 1800 degrees F. or so, you will get clinker.
If your grate only runs at 1000 to 1200 or less, you won’t get much clinker.
These temps are only approximate, just to make the point that clinker = hot, ashes = not so hot.

Pete Stanaitis


So are the clinkers gray or black?
The ‘ash’ I get from my gasifier is black. From my understanding, that would indicate carbon. I also understood if I emptied out gray dust, that would indicate an air leak. Is that not correct?
My thought is the clinkers would be black because of an oxygen starved environment? That’s why I thought they may contain more energy.


Bill, back in the day of wood and other raw biomass gasification I would some times get these black glass like rocks. No fuel in those. With charcoal if I find any they can be just small crusty stuff that can be easy to crush with my fingers.

Mainly it is minerals but some times there could be flux like stuff mixed in that lower the melting point of the minerals and that mixture can turn to glass like substance. Some of those non fuel things get burned out when the charcoal is produced. Kiln and retort produced charcoal may not be the same. Most of the time I shake the grates and white powder ash falls out but most of my wood has some bark on it along with who knows what like mud and what kind of mud? Last weekend I gave it a poke with my length of fence wire and a nail fell out. One other time a big staple fell out.


Thank you Jeff.
That gave me a much better understanding.


I never had clinker when running char… i did when I was doing biomass… but never char. I saw some dirt clod-like ash formation near the nozzle, but I could squeeze them with my hand and they would crumble… Never like slag.