Definition :

The solid storage option is possible with Metal-carbonate CO2 cracking, bio-methane thermochemical cycle cracking or bio-methane plasma cracking. All the cracking options generates solid carbon as carbon black (equivalent to coal dust), carbon nanotubes (CNT) or graphene.

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The structure of carbon black, CNT or graphene are really stable and allow long term storage but a specific risk concerns the small size of the particles and the impact on health if not handled properly. Due to its stable form, with appropriate hermetic storage, it can be transported easily and stored underground, for example in unused coal mines. The graphene can be compacted (optional) and stored in hangars since it is a very valuable material that can be sold for additional revenue generation if it is allowed by regulation (principle of additionality of carbon credits (apparently possible based on information obtained from consultancy Vertis).

It is difficult to find information about solid carbon storage cost, since today it is used by industrial processes and not as carbon storage option. However, the cost of storage should be similar to biomass burying so around 7$ / tCO2 and 28$ / tCO2.

Using molar masses we know that in order to obtain 1 kg of carbon we need 3,67 kg of CO2. So store 1 kg of carbon is equivalent to store 1,56 kg of CO2.

Datas : Technical data was found in ^1.

^1: Fasihi, M., Efimova, O. and Breyer, C., 2019. Techno-economic assessment of CO2 direct air capture plants. Journal of cleaner production, 224, pp.957-980.