Gas filtration with Charcoal?

What would happen if you filtered (cooled) wood gas through a column of fine charcoal medium? Would it effectively remove remaining tar and allow the CO and H2 to pass through?

Thank you (I am a newbie)



(When it plugged up you could use it as fuel and replace it with new?)

1 Like

Just found this article too.

1 Like

Hi Andrew, welcome to the DOW site. Yes you can filter with charcoal. But I think it would be better to use hay or sawdust chips and just throw it into the compost pile when done. Reusing charcoal will be a little tricky. First you will have to wash it clean and dry to 10% moisture content. If it is fine smaller charcoal it would need to be mixed with wood if using it in a wood gasifier. I would not use it in a charcoal gasifier with because it could contain a lot of tar. Unlike regular charcoal that does not contain tar in it.
Tar is the enemy to engines.
A up draft charcoal gasifier does do some filtering when the gases move throw the charcoal cooling it. But it is also burning the charcoal up in a short time too.


THank you !!

I have also wondered about hot pumice stone as a filtration media. I have seen blacksmiths use it with their forge and it significantly decreases smoking at least…

Thanks again, lots to learn !!


Yes Andrew some have used lava rocks, puffed clay, or pumice for a hot filtration media. It doesn’t substitute a hay or cloth filter. The rocks are placed usually before the gas cooler so the gas is very hot.


Yes pumice, and other kinds of materials even cinder red rocks and ceramic balls are used with oil baths filters. They are very messy to clean out. The simplest is grass hay you can wash it down to clean it or sawdust use it and throw in into your compost pile when done. Easy and cheap to come by too. Some one on this site used all three, larger charcoal on the bottom above the charcoal was grass hay and sawdust chips on top with more hay. This could be washed down over and over for cleaning.
I use just hay and it works fine and wash it down every few hundred miles or so. Only fine soot makes it through the hay filter it will not hurt the engine. It will actually burn and make power in the engine.


Charcoal needs to be red hot to crack tar, that’s just the chemistry of it. At best your fine charcoal would act as a mechanical filter but tar would more or less pass through.

Saw dust is a popular filtration medium but again… that’s just a mechanical filter. If present in the gas, some tar will precipitate out on the substrate but not enough to solve a tar problem.

The lived experience of gasifier experimenters is that you either need to not make tar in the first place or crack it in the hot zone of the gasifier. Filtering tar out has proved very difficult.

All that said - welcome! If you want to learn about gasification you’ve found the right place.

I sat on a draft for too long - others chimed in already. I’ll add a link to a research paper on this issue:


Generaly, the saying is that the best way to get rid of tar is not making it at all.

Untill now, l have only seen sucessfull tar removal once, with a huge and soffisticated electrofilter. With any more classical method, geting all the tar out is quite impossible.


On AnthonyB’s linked up paper the key to begin is to go to page #11, graph illustration Figure 1.2
THEN the papers limited studies scope will make sense; “Tar Removal from low-temperatures gasifiers”
Their high point on the studied all-biomass’s systems only going up to 800C.
So . . . . wanna’ make the least streamed out tars using good woods as you fuels (not all-biomass)??
Never let your core temperatures fall below 850C in the lower hearth reduction zone for fuel gas to be sent to an IC engine.
As a practical matter you will have to operate at 900-1000C to not fall below that 850C threshold.
And this is metals destroying temperatures.
Why we have as much as possible evolved to ash slope shielding of the base structure’s metals. DO use large root contact air nozzles bases for heat out transferring.
Do use drop-in/pull out reduction throat (choker) plate and sleeves. Make grates replaceable too. Other heat transferring elaborations as in the WK’s.

Not explained in this papers chart is that the change in the tars made percentages is the vaporization and converting to fuel gasses, first of the lightest Primary tars group as the processing temperature goes up. Up a bit more than the less volatile Secondary tars groups get converted too.
Finally leaving just the Tertiary thick-as-asphalt tars.
You can see these in the tar dumps the hard driving vehicle guys have to do.
NOT filtered out but condensed out and collected before sending downstream to the IC engine.

Most of the made woodtars as just more engine fuel molecules to be utilize.
Get it core HOT. Keep it core HOT.
Nothing you do downstream can make-up for poor hearth temperatures management.
Steve Unruh


Other than breaking down steel components is there any advantage or disavantage to running at temps above 1000C using a refractory hearth for instance?


Depends. Above about 1000c things begin to behave differently. The glowzone becomes what l call “supernova state”. It starts throwing out vast amounts of gas while not expanding the glowzone. A tiny hearth can then propell a big engine down the highway, but its also ash melting temps…


Thank you ! Appreciate the input


Good point. THank you


Thank you for your input. Hopefully formed ceramic parts become cheaper. I have seen them in Italy in gasifying stoves with ridiculously high temp. They are kinda brittle but seem to ignore heat well

Thanks again


Tom, on my Gasifier I like running the grate temps. at 1300 °f up to 1730 °f so the inside firetube core is 2000 °f and higher no tars are going to be making it through unconverted to good gases. The most dangerous time for making tar is at start ups. Also when you are just starting the new gasifier up. It is best to load the ashes in first filling all void and sealing it up. What falls on down leave it in the bottom ash area. Clean off the grate and fill charcoal up to above the nozzles with good popcorn size and larger charcoal to avoid making tar and getting it down pass the grate and into the rest of the system. Start up blowers get the charcoal good and hot then add dry wood on top of the hot bed. If you have charcoal brands put them in first before raw dry wood. I know you and steve know these things but some of the newer members do not know this, and how important it is not to get tar into the gasifier system in the first place. Like others have said you can not filter out all the tars.
The Hot Heat in the charcoal lobe zone and time to convert is the only good answer to converting the tars to good gases. Even dripping the gooy black tars into this 2000 °f to 3000 °f lobe zone well convert all the to good gases.
I like it Hot, Hot. 2000 °f on up in the fire tube.


Ceramic isn’t the end all be all, thermal mass of the steel and having shelves for ash to form up is the simplest way to protect the gasifier from the heat. And like SteveU said, exchanging the heat out of the steel into the oncoming air of the nozzles. Preheating the air is known to increase methane and reduce inert nitrogen thus giving more potent gas.

Cold air is known to warp nozzles that are buried in the glow zone’s heat, so preheating has many benefits. The gasifier can use its own heat to preheat the incoming air with no necessary outside input, save for maybe a heat exchanger around an engine muffler but that would be considered using the system’s heat in my opinion.

There are some coatings you could put over the steel, infrared reflective ones but that would add one more thing to the maintenance schedule.


Its not just the heat. Problem is the ash, and cheramics collect lots of molten ash.


What I would favor AndrewT,
is lined with loose fit refractory bricks.
They come in deferent grades of harnesses and temperature resistance grades.
I often get mine up to glowing in my woodstoves. And Kristijan is correct that is when the wood ash will melt and stick. As a not-expensive formed casting then it is just a matter of steel edge scraping the ash crust off. Sometimes the brick will break. Sometimes they will even break across from a cold wood introduction thermal shock. Slip out and replace.
Again. No big deal. $3.00-5.50 USD each.
My own gasifier was never designed to accommodate retrofitting these inside.

Someday a new build up to prove my premise.
Steve unruh


I am continually learning just the same as the new people Bob. I know less about the theory and practice of gasification than any of the devoted folks here. I do updraft charcoal to run a generator. Just about as basic as it gets. I read and listen but seldom understand what you other guys are talking about. If I could purge my head of a lifetime of useless information there might be room to learn something new. You may have noticed that my knowledge of cars and engines plateaued somewhere around 1975