However, we do not know how long biochar lasts when used as a soil amendment [eg. 1]. Also, it's only useful in this way when loaded and applied to poor soil in the tropics [2].
For long term storage of CO2, it might be better to just bury the main products of pyrolysis (biochar and pyrolysis oil) where hard coal and the oil once used to be - deep in the earth. Burying both products should also make it cheaper (from a carbon mitigation point of view), since it's hard to make useful products from the pyrolysis oil. The biochar also doesn't need to be as clean as when used for soil. One can even pyrolyse old tires (and possibly plastic).
The problem is, of course, that there are no long-term studies about the stability of biochar (and the bio-oil) in deep layers.
The reference you provide says 556 years. It is actually lasts far longer the deeper you put the biochar in the soil since we do know that breakdown requires oxygen and/or UV light. In some soils the average age of the carbon is 5000 years.
The best way to implement this is by slash-and-char of the tropical forests. I have done the calculations and using less than 30% of te tropical forests we can pull out all the CO2 being addded from human activity. It would also create a viable industry in some of the most poor regions in the world.
Why not produce electricity, hot water and hot air by gasifying the wood, and then burying the biochar that is left over after the gasification process?
All Power Labs (allpowerlabs.com) produces small scale reactors that can do just that.
The good char systems do this. Some of the really clever systems can be used to fix nitrogen and turn the biochar into fertiliser. the whole thing can be combined with improving local agriculture slash-and-burn regions.
Would it be feasible to do something similar with ocean water? I’m thinking Northern California or even south - somewhere trees aren’t prevelant already and without using scarce water.
However, we do not know how long biochar lasts when used as a soil amendment [eg. 1]. Also, it's only useful in this way when loaded and applied to poor soil in the tropics [2].
For long term storage of CO2, it might be better to just bury the main products of pyrolysis (biochar and pyrolysis oil) where hard coal and the oil once used to be - deep in the earth. Burying both products should also make it cheaper (from a carbon mitigation point of view), since it's hard to make useful products from the pyrolysis oil. The biochar also doesn't need to be as clean as when used for soil. One can even pyrolyse old tires (and possibly plastic).
The problem is, of course, that there are no long-term studies about the stability of biochar (and the bio-oil) in deep layers.
[1] https://onlinelibrary.wiley.com/doi/abs/10.1111/gcbb.12266 [2] http://iopscience.iop.org/article/10.1088/1748-9326/aa67bd/m...