Will charcoal (carbon) oxidize in the air?

[Blokle]

TL;DR Summary

Will charcoal (carbon) oxidize in the air?

Wikipedia — Carbon sequestration claims that burying charcoal into the soil offsets CO2 and thus reduces the greenhouse effect:

In the soil, the biochar carbon is unavailable for oxidation to CO2 and consequential atmospheric release.

Is it a must to cover charcoal/biochar with the soil to prevent the air and rain water exposure to avoid oxidation, i.e. offset carbon dioxide? Would it otherwise oxidize to CO2 under normal conditions (no fire) if left above the ground?

 

[Borek]

Mostly matter of kinetics - CO₂ is stable, mixture of C and O₂ is not. No idea about exact numbers, but I expect them to slowly react.

 

[Vanadium 50]

TL;DR Summary: Will charcoal (carbon) oxidize in the air?

Of course. See https://en.wikipedia.org/wiki/Barbecue

But if you bury it in the ground its no longer in the air, is it?

 

[Lord Jestocost]

TL;DR Summary: Will charcoal (carbon) oxidize in the air?

Wikipedia — Carbon sequestration claims that burying charcoal into the soil offsets CO2 and thus reduces the greenhouse effect:
Is it a must to cover charcoal/biochar with the soil to prevent the air and rain water exposure to avoid oxidation, i.e. offset carbon dioxide? Would it otherwise oxidize to CO2 under normal conditions (no fire) if left above the ground?

A "thermogravimetric analysis of charcoal" in air can give some indications of the relevant temperatures.

 

[Blokle]

Of course. See https://en.wikipedia.org/wiki/Barbecue

But if you bury it in the ground its no longer in the air, is it?

I have specifically emphasized that no fire is involved. Only air and rain water. One of the answers I got elsewhere:

"The spontaneous oxidation of charcoal by air at room temperature happens spontaneously. But it is so slow that it can be forgotten. The half-life of the reaction iis much much longer than the duration of the universe."

 

[Rive]

Is it a must to cover charcoal/biochar with the soil to prevent the air and rain water exposure to avoid oxidation, i.e. offset carbon dioxide?

Yes.
Coal piles (for long term storage) got compacted to prevent/reduce air present in the pile: though in this case preventing spontaneous combustion is also a thing.

 

[Vanadium 50]

A) In real life fire is involved. Even in forests (forest fires happen at decade-level time scales and sequestration needs to work on multi-century time scales)
B. In real life, biology is involved. Charcoal doesn't last forever, and returns to the ecosystem.
C. Understanding sequestration by making unrealistic simplifications is unlikely to work.

 

[TeethWhitener]

"The spontaneous oxidation of charcoal by air at room temperature happens spontaneously. But it is so slow that it can be forgotten. The half-life of the reaction iis much much longer than the duration of the universe."

The activation energy of the carbon-oxygen reaction will depend on the exact form of the carbon, but a rough estimate is 200 kJ/mol or so. Putting this into the Arrhenius equation gives $k\approx A\times10^{-37}$ at T=300K. I don't know the pre-exponential (A) factor, but it is definitely not going to be on the order of $10^{37}$. The half-life for the reaction will therefore be incredibly long. At least at room temperature, for all practical purposes, a diamond is forever.

As others have mentioned, however, if it's on the surface of the earth, it's not guaranteed to stay at room temperature.

 

[Blokle]

The activation energy of the carbon-oxygen reaction will depend on the exact form of the carbon, but a rough estimate is 200 kJ/mol or so. Putting this into the Arrhenius equation gives $k\approx A\times10^{-37}$ at T=300K. I don't know the pre-exponential (A) factor, but it is definitely not going to be on the order of $10^{37}$. The half-life for the reaction will therefore be incredibly long. At least at room temperature, for all practical purposes, a diamond is forever.

As others have mentioned, however, if it's on the surface of the earth, it's not guaranteed to stay at room temperature.

Thank you!

1. Let's assume we are in a hot climate. 45 degrees Celsius 24/7... Can we still assume that there will be no oxidation of charcoal for all practical purposes?

2. Is humidity a factor here? Or even more - if we put the charcoal regularly into direct contact with water - will this accelerate oxidation?

 

[TeethWhitener]

1. Let's assume we are in a hot climate. 45 degrees Celsius 24/7... Can we still assume that there will be no oxidation of charcoal for all practical purposes?

What I meant by "not guaranteed to stay at room temperature" is less "hot climate" and more "struck by lightning." Nonetheless the Arrhenius equation is:
$$k(t)=Ae^{-\frac{E_a}{RT}}$$
and changing T from 300K to 318 K isn't going to make much of a difference.

2. Is humidity a factor here? Or even more - if we put the charcoal regularly into direct contact with water - will this accelerate oxidation?

Not really. But again, as many people have tried to stress in this thread, keep in mind that the timescale for carbon sequestration as a geoengineering solution to climate change is on the order of 1,000-10,000 years. If you have millions of tons of charcoal just sitting on the surface of the earth for that long, it's going to have to deal with fire, volcanism, lightning, heck even just a piece of flint falling on a piece of metal by sheer accident and causing a spark.

 

[Blokle]

A) In real life fire is involved. Even in forests (forest fires happen at decade-level time scales and sequestration needs to work on multi-century time scales)
B. In real life, biology is involved. Charcoal doesn't last forever, and returns to the ecosystem.
C. Understanding sequestration by making unrealistic simplifications is unlikely to work.

A) You are right that in the long term there will be fires if one just dumps a lot of charcoal above the ground. But I was thinking about possible useful ways of utilization of charcoal instead of just burying it. E.g. this patent suggests to use carbon as hydroponic growing medium. This way you can keep charcoals in small portions (wet, since regularly irrigated) so the danger of fire is not that big. But I wanted to make sure that the oxidation doesn't reverses the effect and the CO2 doesn't return to the atmosphere. So far it seems to be a feasible and useful alternative to just bury charcoal to offset CO2. Or do I miss something?

B) What do you mean by "biology is involved"? Do some living organisms eat the carbon and turn it to CO2?

 

[Blokle]

Yes.
Coal piles (for long term storage) got compacted to prevent/reduce air present in the pile: though in this case preventing spontaneous combustion is also a thing.

Interesting article. Thank you!

However it speaks about coal which, contrary to charcoal, is not a pure carbon. I might be wrong, but these sentences of the article seemingly suggest that the oxidation process doesn't involve C but rather other substances:

"What was immediately apparent from the coal proximate and ultimate analyses was that the volatile matter and hydrogen content had decreased disproportionately, and the fixed carbon, elemental carbon, and oxygen content all increased disproportionately—clearly indicating oxidation losses."

I'm not a chemists but my understanding of the above was - if C were to turn into CO2 gas during oxidation its portion within the solid mixture (=coals) should have decreased and not increased... Is this correct?