Abstract
Carbon Dioxide removal from air (CDR) combined with permanent solid storage can be accomplished via carbon mineralization in ultramafic rocks in at least four ways:
1. Surficial CDR: CO2-bearing air and surface waters are reacted with crushed and or ground mine tailings, alkaline industrial wastes, or sedimentary formations rich in reactive rock fragments, all with a high proportion of reactive surface area. This can be implemented at a low cost, but most proposed methods have a very large area footprint at the gigatonne scale. The area requirement can be greatly reduced by calcining (heating to produce pure CO2 for permanent storage or use) followed by recycling of MgO, CaO, Na2O, … Such looping methods have predicted costs that are as low or lower than for direct air capture with synthetic sorbents or solvents (DACSS), and a similar area footprint.
2. In situ CDR: CO2-bearing surface waters are circulated through rock formations at depth. These methods potentially have a cost similar to that of surficial carbon mineralization, and a giant storage capacity with reduced surface area requirements, but they involve uncertain feedbacks between permeability, reactive surface area, and reaction rate, providing a fascinating topic for fundamental research. Furthermore, the size, injectivity, permeability, geomechanics, and microstructure of key subsurface reservoirs for in situ CDR remain almost entirely unexplored.
3&4. Combined partial enrichment of CO2 using direct air capture with synthetic sorbents (DACSS) plus surficial carbon mineralization (3) or in situ carbon mineralization (4). Energy requirements and total costs for partial enrichment of CO2 are substantially lower than for enrichment to high purity. CO2 enriched air can be sparged through mine tailings at the surface, and/or through water to increase dissolved carbon concentrations prior to circulation through rock reactants. Such combined or hybrid approaches have not been investigated thoroughly, and offer many avenues for optimization.
This paper reviews previously proposed methods, and describes some possible new methods, for carbon mineralization to achieve Gt-scale removal of CO2 from air. Two figures from the paper are provided here. The first is for a proposed system of MgO-MgCO3 looping, in which weathering of MgO removes CO2 from air, and heating (calcining) removes CO2 from magnesium carbonate minerals, for storage or use. The second illustrates estimated costs and rates of CO2 uptake per borehole, in a system combining partial CO2 capture from air using direct air capture technology with synthetic sorbents, combined with carbon mineralization via reaction of CO2-enriched water with subsurface peridotites. [Display omitted]
•Combined CO2 removal from air (CDR) + solid storage via weathering of existing ultramafic tailings is relatively inexpensive•Mining rocks for CDR and solid storage may be cost-competitive with direct air capture using synthetic sorbents (DACSS)•Cost and area for surficial CDR can be greatly reduced by calcining, storing or selling CO2, and re-weathering MgO, CaO•In situ carbon mineralization, reacting CO2-bearing fluid with subsurface rocks, may be cost competitive with DACSS•Hybrid methods, e.g., DACSS to enrich CO2 to a few % + carbon mineralization, may reduce cost and area requirements