Science of charcoal activation

This is a wery informative video on charcoal activation.

This information is wery important for charcoal gasification. Our gasifiers defenetly meet charcoal activation criteria in the reduction part of the glow lobe. What l am asking my self is does injecting water together with air in our gasifier allso aids to porosity of the char? In this case, this is a nother advantige to water vs EGR injection. Greater porosity equal greater surface area equals faster more complete reactions in the gasifier.


Hi Kristijan,

1: saddly i have to say that this video is far away from being accurate, i might elaborate on that later.
2: the carbon from the ashes in the gasifier ( grate slipping carbon) is amongst the highest activated ones, iodine nr more then 1000 mg/ gr carbon
3: the minimum nr to look for as being activated = 600mg or more, 1000 minimum if is high quality…
4: carbon made at high temperature in a retort system has the lowest iodine nr untill activated later on.
(retort has no oxygen to burn away the softest layers of carbon, opening more pores
5: the soot from the gasifiers have the highest iodine nr, hence the best for doing detox in my opinion :grin:
6: i have spent quite a large budget in getting things tested and certified in official labs during last 4 years… and many so called commercial activated carbons, not even come closed being accepted to be called activated at all…

7: i bet that, if any one put his slipped char ( from our gasifiers ) thru the same test as in the video, the found values would be a tenfold compared as showed in the video…


Thank you for weighing in on this issue. It is important to me because I am trying to find a simple way to make high adsorbing char for water filters in remote African villages. I was told by a university PHD charcoal researcher that my electrically conductive char from a high temperature (1600F) retort would have this high adsorbing quality. Your comment about “oxygen to burn away the softest layers of carbon” makes me wonder if charcoal made in a TLUD stove or kiln might be better. Can you recommend a simple test for adsorption that does not involve lab equipment?


Hi Bruce,

I have been testing , making, engineering charcoal since i am in Thailand… What i learned is in contradiction with this Phd researcher you mentioned.
Yes, retort charcoal is more pure and more conductive and No, the adsorption level decreases with higher temperatures and no oxygen. ( i have a lot of lab results for that )

Activated charcoal is charcoal made in a retort and subsequently “activated” by adding O2 from air, Co2 , H2o … removing the soft carbon from the hard carbon pores.
( tried to put all the knowhow short version it in a nutshell )

Depending the purpose and the quality of the water to treat, you construct a open barrel type, kiln, or similar i have seen already here on DOW, where as the glowing char falls down the bottom hole and is subsequently smothered with sand or water. ( kind of a gasifier :grin: )

If using open top barrel, the fire must be kept untill its smokeless, or after the initial covering from the drum, air must be added untill full glowing, hot as hell can be, but must be total smokeless, then cover and cool down.

If you test that conductivity, the best pieces are less then 3 Ohm

summerising: burning, glowing untill smokeless, cool down

The pure carbon will be alkaline if not washed, but sometimes that is preferable for drinking water with low PH

a simple test to check adsorption rate: how much alcohol can a piece of charcoal hold by weight, to test with methanol, alcohol, water and so on… where as i use methanol as “field test”

Observation, after washing the carbon with methanol, it becomes more hydrophilic hence adsorbs more water.

Hope this helps a bit



i would have thought it would make it even more hygroscopic

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Thank you Koen! Wery interasting.

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Thank you for this very helpful information. It sounds like the open kilns that we use for making field biochar will work fine for our water filter charcoal. We will simply switch from crop waste to wood to create the harder granules we want for our filters. The most important part of the filter is the biological layer that forms on top of the charcoal. It takes 11-14 days for this layer to form that traps and consumes the nasty micro-organisms. This approach is widely used with 1 meter tall sand filters. Charcoal’s properties allow us to make better filters in a standard 20 liter bucket.


Now back to engine fuel. Do you have observations or recommendations about the relative value of retort charcoal vs kiln charcoal for cooking fires and/or engine fuel?

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Hi Bruce,

Thats an easy one… at least IMHO, i only use retort fuel…
1: the yield rate is better
2: less attention to pay to obtain good quality
3: i have a electric retort at my disposal :grin:

joke aside, i have seen many good designs in open kiln, normal kiln, retorts…
It depends largely on the person/operator who’s making the charcoal…
Retort is more easy to teach and the operators can use other wast wood, scraps to cook the charcoal.
If well designed, then they can burn any domestic waste in a fair smokeless way together with the fuel cooking simultaneous water and food during the charcoaling process.
The only thing to watch/learn/observe: no more smoke coming out of the retort when burning hot outside ? then the charcoal isn done…

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Thank you Kristijan for starting this topic. Very timely for me.
BTW, I am a fan of Cory’s Lab.
Thank you Koen for weighing in to give us the benefit of your expertise on this topic.