7 billion tonnes of alkaline materials are produced globally each year as a product or by-product of industrial activity. The aqueous dissolution of these materials creates high pH solutions that ...dissolves CO
to store carbon in the form of solid carbonate minerals or dissolved bicarbonate ions. Here we show that these materials have a carbon dioxide storage potential of 2.9-8.5 billion tonnes per year by 2100, and may contribute a substantial proportion of the negative emissions required to limit global temperature change to <2 °C.
Abstract
To avoid dangerous climate change, new technologies must remove billions of tonnes of CO
2
from the atmosphere every year by mid-century. Here we detail a land-based enhanced weathering ...cycle utilizing magnesite (MgCO
3
) feedstock to repeatedly capture CO
2
from the atmosphere. In this process, MgCO
3
is calcined, producing caustic magnesia (MgO) and high-purity CO
2
. This MgO is spread over land to carbonate for a year by reacting with atmospheric CO
2
. The carbonate minerals are then recollected and re-calcined. The reproduced MgO is spread over land to carbonate again. We show this process could cost approximately $46–159 tCO
2
−1
net removed from the atmosphere, considering grid and solar electricity without post-processing costs. This technology may achieve lower costs than projections for more extensively engineered Direct Air Capture methods. It has the scalable potential to remove at least 2–3 GtCO
2
year
−1
, and may make a meaningful contribution to mitigating climate change.
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
Terrestrial enhanced weathering, the spreading of ultramafic silicate rock flour to enhance natural weathering rates, has been suggested as part of a strategy to reduce global atmospheric CO2 levels. ...We budget potential CO2 sequestration against associated CO2 emissions to assess the net CO2 removal of terrestrial enhanced weathering. We combine global spatial data sets of potential source rocks, transport networks, and application areas with associated CO2 emissions in optimistic and pessimistic scenarios. The results show that the choice of source rocks and material comminution technique dominate the CO2 efficiency of enhanced weathering. CO2 emissions from transport amount to on average 0.5-3% of potentially sequestered CO2. The emissions of material mining and application are negligible. After accounting for all emissions, 0.5-1.0 t CO2 can be sequestered on average per tonne of rock, translating into a unit cost from 1.6 to 9.9 GJ per tonne CO2 sequestered by enhanced weathering. However, to control or reduce atmospheric CO2 concentrations substantially with enhanced weathering would require very large amounts of rock. Before enhanced weathering could be applied on large scales, more research is needed to assess weathering rates, potential side effects, social acceptability, and mechanisms of governance.
Abstract
The cement industry, an industry characterised by low margins, is responsible for approximately 7% of anthropogenic CO
2
equivalent (CO
2e
) emissions and holds the highest carbon intensity ...of any industry per unit of revenue. To encourage complete decarbonisation of the cement industry, strategies must be found in which CO
2e
emission reductions are incentivised. Here we show through integrated techno-economic modelling that CO
2
mineralisation of silicate minerals, aiming to store CO
2
in solid form, results in CO
2e
emission reductions of 8–33% while generating additional profit of up to €32 per tonne of cement. To create positive CO
2
mineralisation business cases two conditions are paramount: the resulting products must be used as a supplementary material in cement blends in the construction industry (e.g., for bridges or buildings) and the storage of CO
2
in minerals must be eligible for emission certificates or similar. Additionally, mineral transport and composition of the product are decisive.
Over the coming century humanity may need to find reservoirs to store several trillions of tons of carbon dioxide (CO2) emitted from fossil fuel combustion, which would otherwise cause dangerous ...climate change if it were left in the atmosphere. Carbon storage in the ocean as bicarbonate ions (by increasing ocean alkalinity) has received very little attention. Yet recent work suggests sufficient capacity to sequester copious quantities of CO2. It may be possible to sequester hundreds of billions to trillions of tons of C without surpassing postindustrial average carbonate saturation states in the surface ocean. When globally distributed, the impact of elevated alkalinity is potentially small and may help ameliorate the effects of ocean acidification. However, the local impact around addition sites may be more acute but is specific to the mineral and technology. The alkalinity of the ocean increases naturally because of rock weathering in which >1.5 mol of carbon are removed from the atmosphere for every mole of magnesium or calcium dissolved from silicate minerals (e.g., wollastonite, olivine, and anorthite) and 0.5 mol for carbonate minerals (e.g., calcite and dolomite). These processes are responsible for naturally sequestering 0.5 billion tons of CO2 per year. Alkalinity is reduced in the ocean through carbonate mineral precipitation, which is almost exclusively formed from biological activity. Most of the previous work on the biological response to changes in carbonate chemistry have focused on acidifying conditions. More research is required to understand carbonate precipitation at elevated alkalinity to constrain the longevity of carbon storage. A range of technologies have been proposed to increase ocean alkalinity (accelerated weathering of limestone, enhanced weathering, electrochemical promoted weathering, and ocean liming), the cost of which may be comparable to alternative carbon sequestration proposals (e.g., $20–100 tCO2−1). There are still many unanswered technical, environmental, social, and ethical questions, but the scale of the carbon sequestration challenge warrants research to address these.
Key Points
The ocean naturally stores a very large quantity of carbon as dissolved carbonate and bicarbonate ions
It may be possible to store additional carbon in this sink to mitigate climate change at costs that are comparable to conventional mitigation
Research is needed to understand the impacts and the feasibility of this approach
Chemical weathering is an integral part of both the rock and carbon cycles and is being affected by changes in land use, particularly as a result of agricultural practices such as tilling, mineral ...fertilization, or liming to adjust soil pH. These human activities have already altered the terrestrial chemical cycles and land‐ocean flux of major elements, although the extent remains difficult to quantify. When deployed on a grand scale, Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be used to remove CO2 from the atmosphere. The release of cations during the dissolution of such silicate minerals would convert dissolved CO2 to bicarbonate, increasing the alkalinity and pH of natural waters. Some products of mineral dissolution would precipitate in soils or be taken up by ecosystems, but a significant portion would be transported to the coastal zone and the open ocean, where the increase in alkalinity would partially counteract “ocean acidification” associated with the current marked increase in atmospheric CO2. Other elements released during this mineral dissolution, like Si, P, or K, could stimulate biological productivity, further helping to remove CO2 from the atmosphere. On land, the terrestrial carbon pool would likely increase in response to Enhanced Weathering in areas where ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution. In the ocean, the biological carbon pumps (which export organic matter and CaCO3 to the deep ocean) may be altered by the resulting influx of nutrients and alkalinity to the ocean. This review merges current interdisciplinary knowledge about Enhanced Weathering, the processes involved, and the applicability as well as some of the consequences and risks of applying the method.
Key Points
Enhanced Weathering impacts the C‐cycle, NPP and ocean acidity
Enhanced Weathering alters besides the C‐cycle further nutrient cycles
The rock for crop concept is already applied as Enhanced Weathering
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