Longer term perspectives mineral sequestration and CO recycling

Mineral sequestration of CO2 consists of transforming it into a stable substance through a carbonation reaction with a rock presenting a basic activity.

The main difficulty stems from the fact that such reactions are comparatively slow. Two channels are currently being explored:

- Ex situ mineral sequestration is operated above ground in an industrial installation by reacting CO2 with ground rocks or with solid waste. The main drawback is the need to manipulate, grind and store considerable volumes of solid materials. Different solid phases have been investigated for use in such a process.

Typically, CO2 is reacted with crushed olivine and serpentine (magnesium-rich silicate rocks) from a mine. Industrial wastes, such as blast furnace slag, composed of iron and calcium silicates, can also be used.

- In situ mineral sequestration is operated by injecting CO2 in a natural setting where basic rocks of magmatic origin, such as basalt, are present. Such an operation is performed in a very similar way to that used in the case of geological storage. The carbonation reaction between CO2 and the surrounding rock occurs very slowly and results in the formation of a stable compound. The porosity of magmatic rocks such as basalts is generally low and it is therefore difficult to inject large amounts of CO2.

The porosity and injectivity of such rocks can be increased through fracturation operations, but such operations are costly and it appears difficult to use them on a large scale in the near future.

Carbon dioxide recycling includes all possible uses in industrial or biological processes. Unfortunately the quantities of CO2 which can be recycled in that way are limited. Currently, the main application in the chemical industry is the production of urea, which requires around 80 million tons of CO2 per year [92]. A significant outlet is provided by the agro-food industry which, in Europe, consumes around 2.7 million tons of CO2. A CO2 rich atmosphere is used for accelerating the growth of plants cultivated in greenhouses. This outlet is also limited, but it is possible to widen the applications of the biomass produced in a CO2 enriched atmosphere by investigating new applications in the agro-food, chemical and energy industries.

The production of algae in bioreactors is presently considered for sequestering CO2 produced by a fossil fuel power plant. Such algae can then be converted into biofuels (diesel from the fatty fractions and/or ethanol from sugars derived from the biomass). Despite the fact that this area is very active, it remains difficult to ascertain the economic competitiveness of these developments.

It is necessary also to assess the overall carbon balance: the combustion of such biofuels cannot be considered as carbon neutral, because the carbon used for growing the biomass is provided by a fossil fuel and not recovered from the atmosphere.

EOR is currently the main large scale application of CO2 injection and it should continue to grow in the future.

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