Assessment of potential environmental impacts of an Enhanced Rock Weathering process for carbon dioxide removal in the form of bicarbonates by means of Life Cycle Assessment

In addition to rapid emissions reduction, different Carbon Dioxide Removal (CDR) processes are required to be deployed in the order of many Gt y^-1 for limiting the global temperature increase to the ambitious target set by the Paris Agreement.

Enhanced Rock Weathering (ERW) and River Alkalinity Enhancement (RAE) are CDR approaches that mimic and accelerate the natural process of rock weathering. These processes remove CO₂ from the atmosphere and store it permanently in the sea in the form of bicarbonates, thanks to the spread of grinded alkaline materials (e.g., limestone, slaked lime or dolomite) in different environments, e.g., on croplands or in rivers.

The use of limestone or dolomite involves a long time for their dissolution, which can be accelerated by reducing the size of the particles in the order of micrometres. Alternatively, using slaked lime (SL) decreases the energy requirement for grinding because SL dissolution is faster at larger particle size than limestone. On the other hand, the production of SL causes unavoidable process CO₂ emission and energy consumption during the calcination (i.e., the thermal decomposition of limestone into lime).

Here, a process that produces decarbonized SL for ERW or RAE is analyzed. It consists of the use of renewable electric energy for the calcination of limestone and of the storage of CO₂. Two alternative CO₂ storage systems are considered: geological storage and marine storage in the form of bicarbonates. The former is more studied and currently deployed with an annual global capacity of about 50 MtCO₂ per year. However, geological storage has some drawbacks, such as the long time required for the identification of a formation suitable for storage, and a high financial risk because of the money loss in case the formation will result unsuitable. Furthermore, suitable geological formations are unevenly geographically distributed in the world and the long-term sustainability of the injection rate is uncertain. The latter storage approach, still in the first phases of the development, consists of the formation of bicarbonates by reacting CO₂ from the calcination with seawater. Then, part of the decarbonized SL is used for balancing the pH, so a carbon-enriched marine solution with the same pH of the seawater is released. Unlike geological storage, this storage methodology is modular with certain and constant injection rate and can be deployed in every site near the coast.

The potential environmental impacts of the process with the two different CO₂ storage technologies are analyzed through the Life Cycle Assessment (LCA) methodology. In addition to climate change, 15 impact categories are assessed according to the Environmental Footprint method implemented in Simapro software. The impacts are calculated on the basis of the mass and energy balance of the processes.

The limitations of the LCA methodology for assessing the overall environmental impacts of these processes will also be investigated. In particular, the lack of an impact category able to assesses the potential river environment remediation or the contrast to ocean acidification when the added alkalinity reaches the sea.