Climate Intervention Carbon Dioxide Removal and Reliable Sequestration (2015) / Chapter Skim
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3 Assessment of Possible Carbon Dioxide Removal and Long-Term Sequestration Systems
3 Assessment of Possible Carbon Dioxide Removal and Long-Term Sequestration Systems
Pages 39-96
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From page 39... ...
Land use emissions since 1750 total about 660 GtCO2, which suggests an upper limit to the physical potential of reforestation and afforestation to remove carbon dioxide from the atmosphere. In reality, the number would be much lower because society needs to manage previously forested land to meet the need for food and fiber, and these managed systems typically have lower average carbon stocks than they did prior to conversion.
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From page 40... ...
. The IPCC Fifth Assessment reports potential carbon sequestration rates of up to 1.5, 9.5, and 14 GtCO2/yr in 2030 for global afforestation and reforestation activities, depending on the mitigation scenario (IPCC, 2014b, Table 11.8)
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From page 41... ...
. Excluding deforestation, terrestrial ecosystems currently sequester carbon on a global scale, largely as a result of forest regrowth on lands previously cleared for agricultural use in the Northern Hemisphere and enhanced productivity in response to increasing carbon dioxide concentrations.
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From page 42... ...
. On average, the amount of organic carbon in intensively cultivated soils is much lower than the potential carbon sequestration capacity below ground.
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From page 43... ...
. Soil carbon can be increased by growing cover crops,2 leaving crop residues to decay in the field, applying manure or compost, using low- or no-till systems, and employing other land management techniques that increase soil structure and organic matter inputs.
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From page 44... ...
. Programs that set aside agricultural land can increase net carbon sequestration and provide wetland, stream, river, and lake protection, although indirect land use impacts (i.e., the creation of farmland in other regions or countries to offset the land set aside)
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From page 45... ...
If fossil fuel use has been eliminated in the area where the biomass is produced and energy needs are not being fully met, then combusting waste material to produce bioenergy would produce lower net greenhouse gas emissions than would production of biochar. If additional energy is not needed to meet human needs, then biochar production will reduce net greenhouse gas emissions relative to allowing that waste to decompose.
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From page 46... ...
Though these techniques are clearly not a solution by themselves, they can be valuable elements of a climate change mitigation portfolio. ACCELERATED WEATHERING METHODS AND MINERAL CARBONATION The long-term fate for most CO2 released to the atmosphere is first to become bicarbonate ions dissolved in the ocean and later to become carbonate sediments on the sea floor (Berner et al., 1983)
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From page 47... ...
Silicate weathering reactions can also affect marine chemistry in a way similar to dissolution of carbonate minerals. However, because silicate minerals do not in general 3 Inthe discussion here, for simplicity, the committee discusses calcium with the understanding that other divalent cations, such as magnesium, are also possible.
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From page 48... ...
. The long-term fate for most CO2 released into the atmosphere is to become carbonate sediments in the ocean, where the cations in the carbonate minerals are derived from silicate-mineral weathering reactions.
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From page 49... ...
Thus, these approaches favor coastally located facilities where there is ready access to seawater. Another approach is to encourage carbonate or silicate mineral weathering reactions to occur on land (Köhler et al., 2010; Schuiling and Krijgsman, 2006)
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From page 50... ...
. Scaling and Environmental Issues Carbonate minerals, silicate minerals, and seawater are all abundant and so there are no obvious fundamental physical constraints that limit the application of these approaches at the global scale.
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From page 51... ...
To have substantial effects on ocean carbonate chemistry at a global scale would involve mining and crushing hundreds of cubic kilometers of carbonate and/or silicate minerals. For comparison, in 2011, worldwide coal production was equivalent to about 9 km3 (USGS, 2013a)
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From page 52... ...
It is likely that the CO2 would have to be concentrated to some extent to improve mineral carbonation conversion on timescales of interest. Taking into account the total energy (4.65 GJ/tCO2)
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From page 53... ...
. Ol, olivine; Se, serpentine; CKD, cement kiln dust; FA, fly ash; SS, steel slag.
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From page 54... ...
Because these accelerated chemical weathering approaches are relatively low-tech in their fundamental concept, it should be possible to get improved cost estimates for accelerated chemical weathering facilities and operations. These cost estimates would need to take into account geographically specific conditions; the costs of mined minerals and their transportation are likely to comprise a substantial fraction of overall cost (Figure 3.1)
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From page 55... ...
, especially relative to downstream environ mental benefits and relative to the impacts of other CDR methods; • Testing and modeling various approaches at meaningful scales to better de termine the life-cycle economics, net cost/benefit, optimum siting, and global capacities and markets of accelerated mineral weathering in the context of CDR. In summary, only laboratory-scale experiments of ocean-based accelerated weathering have been carried out thus far.
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From page 56... ...
. The strength of the marine biological pump and resulting ocean carbon sequestration depends, among other factors, on the quantity of the phytoplanktonic nutrients nitrogen and phosphorus in the global ocean and the completeness with which the supply of these nutrients to the surface ocean are utilized by phytoplankton.
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From page 57... ...
Third, if the elemental ratio of carbon to nutrients in organic matter were to increase from the average value at present, then the net new flux of carbon to depth would also increase. Fourth, a reduction in the biological formation of particulate inorganic carbon in the surface ocean would increase surface alkalinity and enhance ocean carbon sequestration.
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From page 58... ...
Other related ocean biological CDR approaches have been proposed but have been studied in less detail than ocean iron fertilization (Williamson et al., 2012)
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From page 59... ...
. However, as this section describes, there are still unresolved questions with respect to the effectiveness and potential unintended consequences of large-scale ocean iron fertilization.
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From page 60... ...
. Because of the large natural background levels and variability of subsurface dissolved inorganic carbon, the direct measurement of small changes in ocean carbon sequestration at depth from ocean iron fertilization experiments is challenging.
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From page 61... ...
It is also likely that iron fertilization will have downstream effects on nutrient supply, and thus productivity and food web dynamics, in other ocean regions. An intended consequence of ocean iron fertilization involves shifting plankton community composition toward larger cells that will lead to enhanced downward-sinking flux; the long-term impact of this shift on higher trophic levels, including fish, seabirds, and marine mammals, is not well known but may be addressable in part by studying analogous regions with substantial natural iron fertilization.
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From page 62... ...
These are discussed further in Chapter 4. Looking forward, the committee highlights several important future research directions: • Understanding the effectiveness of iron inputs on stimulating biological or ganic carbon production and increasing carbon export; • Determining the fate of the sinking organic carbon and iron in the subsurface ocean as a result of deliberate ocean iron fertilization; • Assessing potential downstream effects that may limit biological productivity or change other aspects of biogeochemistry in other regions; • Detection and accounting of net changes in subsurface ocean carbon seques tration and the effective lifetime of the carbon sequestration; and • Understanding the ecological and biogeochemical consequences of extended and large-scale iron fertilization.
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From page 63... ...
The committee considers this an immature CDR technology with high technical and environmental risk. BIOENERGY WITH CARBON CAPTURE AND SEQUESTRATION AND DIRECT AIR CAPTURE AND SEQUESTRATION Bioenergy with Carbon Capture and Sequestration BECCS is a process in which biomass is converted to heat, electricity, or liquid or gas fuels, followed by CO2 capture and sequestration.
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From page 64... ...
Prior to that point, there is no difference in net carbon emissions to the atmosphere whether the CCS is tied to bioenergy or fossil fuel use. Large-scale expansion of biomass plantations may displace forests that have significant biodiversity that the new growth would lack.
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From page 65... ...
. Hence, adoption of bioenergy reliance at this scale will be constrained by available land and resources and the secondary impacts on greenhouse gas emissions (e.g., N2O)
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From page 66... ...
The high carbon-to-energy ratio of bioenergy feedstocks (roughly equal to that of coal and half that of natural gas for dry biomass) and the decrease in net energy resulting from the combustion of bioenergy feedstocks with a high moisture content mean that, in the most common situation, there is lower net reduction in GHG emissions relative to using the same CCS capacity with fossil fuel–generated energy, particularly natural gas–generated energy.
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From page 67... ...
.) However, that potential is likely to be significantly constrained for some time, if not indefinitely, by the need for most arable land to be used to meet global food demand and the competing demand to use global CCS capacity to sequester fossil fuel emissions.
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From page 68... ...
C L I M AT E I N T E R V E N T I O N : C a r b o n D i o x i d e R e m o v a l a n d R e l i a b l e S e q u e s t r a t i o n ogy will be the primary solution for the required scale of significant CO2 reductions due to its negative environmental impacts, water requirements, and moderately high cost. Solvent-based approaches to chemically scrubbing CO2 out of the atmosphere are considered here without focus on solid sorbents due to the infancy in adsorptionbased processes compared to solvent-based processes for CO2 separation.
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From page 69... ...
For instance, House et al.'s $1,000/ton estimate is based on the first and second laws of thermodynamics, assuming 90 percent capture and 95 percent purity combined with a Sherwood analysis based on the dilution of CO2 in the atmosphere. In addition, this cost assumes the energy source is CO2 free since using natural gas or coal would result in greater 69
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From page 70... ...
C L I M AT E I N T E R V E N T I O N : C a r b o n D i o x i d e R e m o v a l a n d R e l i a b l e S e q u e s t r a t i o n Figure 3-5a Bitmapped FIGURE 3.5 Carbon Engineering's slab air-contactor design is shown as an example of the design of a DAC plant. The surface area is optimized to achieve maximum air contact for reasonable CO2 capture, and the width of the column is shallow to minimize pressure drop and subsequent energy requirements. Comparing with Figure 3.6, it is clear that the design and footprint of a separation system is dependent on the starting CO2 concentration.
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From page 71... ...
This image provides an example for dimensional comparison to the DAC plant in Figure 3.5, not a comparison of scale as the annual removal rate by the National Carbon Capture Center is small since it is for demonstration purposes only. SOURCE: Courtesy of Frank Morton, Business Development Manager of the National Carbon Capture Center, Southern Company.
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From page 72... ...
C L I M AT E I N T E R V E N T I O N : C a r b o n D i o x i d e R e m o v a l a n d R e l i a b l e S e q u e s t r a t i o n TABLE 3.1 Comparison of Assumptions and Costs of DAC in the Literature Cost [$/tCO2 captured] Capture Regeneration Total Assumptions Reference Yes Yes 1,000 Calculation based on minimum House et al., 2011 work.
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From page 73... ...
. Also, alternative uses of the concentrated CO2 need to be considered, for example, its conversion via accelerated mineral weathering to solid carbonate or dissolved bicarbonate for stable ocean sequestration (see Accelerated Weathering Methods and Mineral Carbonation)
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From page 74... ...
or by chemical or geochemical reactions (see Accelerated Mineral Weathering with Land-Ocean Sequestration) causes the partial pressure of CO2 (pCO2)
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From page 75... ...
Assuming that solar energy is used to fuel the DAC process and that ~100,000,000 acres of Bureau of Land Management (BLM) land are available in the southwestern United States, this could lead to a removal of ~13 GtCO2/yr and a cumulative removal of ~1,100 GtCO2 up to 2100 (see Table 2.2 as well)
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From page 76... ...
C L I M AT E I N T E R V E N T I O N : C a r b o n D i o x i d e R e m o v a l a n d R e l i a b l e S e q u e s t r a t i o n FIGURE 3.7 U.S. CO2 sequestration capacity estimates for various geological reservoirs.
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From page 77... ...
reports that in order for CCS to make up 17 percent of the CO2 mitigation portfolio through 2050, the scale of CCS needs to increase from the order of millions of tons of CO2 per year to ~7 GtCO2/yr (Global CCS Institute, 2013; IEA, 2013b)
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From page 78... ...
SOURCE: GEA, 2012. 15 with minerals in the rock, leading to the formation of carbonate minerals in which the CO2 is chemically transformed and, hence, more permanently trapped.
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From page 79... ...
Assessment of Possible Carbon Dioxide Removal and Long-Term Sequestration Systems FIGURE 3.9 Comparison between "allowable" and expected emissions (left) and percent stored CO2 remaining (right)
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From page 80... ...
C L I M AT E I N T E R V E N T I O N : C a r b o n D i o x i d e R e m o v a l a n d R e l i a b l e S e q u e s t r a t i o n oil produced will reduce the cost of CO2 disposal, but in these cases the majority of the CO2 is recovered and reused. The purchase price of CO2 is about $40/tCO2 to $50/tCO2 for EOR operations (Benson et al., 2012)
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From page 81... ...
Economic considerations indicate that application of these approaches, if they can be cost competitive, would largely be limited to coastal environments with co-located availability of concentrated CO2 streams, carbonate or silicate minerals, and ocean water (Rau and Caldeira, 1999)
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From page 82... ...
. In the section Accelerated Weathering and Mineral Carbonation, the transformation of CO2 with alkalinity to form stable or dissolved carbonates was reviewed.
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From page 83... ...
A study by Sridhar and Hill (2011) estimated that replacing 10 percent of building materials with carbonate minerals has the potential to reduce CO2 emissions by 1.6 Gt/yr.
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From page 84... ...
In addition, these carbonate minerals could potentially be left in dissolved form where they could be released into the ocean, thereby countering acidification caused by passive uptake of excess CO2 from the atmosphere.
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From page 85... ...
Although capture from point-source emitters coupled to sequestration (i.e., CCS) is not considered a CDR technology, it is included in Table 3.2 for comparison with the CDR technologies considered in this report.
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From page 86... ...
86 CDR Summary Table 3.2 CO2 Capture Approaches Committee Confidence: High Medium Low Point- Accelerated Accelerated Source Direct Air Biological Biological Weathering Weathering Capture Capture Land Based Ocean Based Land Based Ocean Based Afforestation, Fuel/fuel soil, land Ocean iron NOTES: gas management fertilization Technological readiness, speed to deployment, technical risk Mature technology (ready to deploy quickly, low technical risk) : technology exists at scale Intermediate maturity technology: prototypes exist, not to scale Immature technology (not ready to deploy quickly, high technical risk)
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From page 87... ...
Effect per unit cost for pilot scale with currently available technology High: dollars per ton CO2 (i.e., <$10/tCO2) Medium: tens of dollars per ton CO2 (i.e., $10/tCO2 < x < $100/tCO2)
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From page 88... ...
88 Table 3.2 Continued Point- Accelerated Accelerated Source Direct Air Biological Biological Weathering Weathering Capture Capture Land Based Ocean Based Land Based Ocean Based Afforestation, Fuel/fuel soil, land Ocean iron NOTES: gas management fertilization Negative environmental consequences Minor: mostly local impacts; can be mitigated consistent with current national environmental protection standards Medium: potentially serious impacts that may be difficult to mitigate to current environmental protection standards Major: severe national or global impacts incompatible with current environmental protection standards; impacts may exceed environmental benefits of climate change mitigation
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From page 89... ...
Environmental co-benefits High: numerous and/or very likely co benefits, such as protection of watersheds from erosion, wildlife habitat and diversity, recreational opportunities, or reduction in ocean acidification Medium: modest or uncertain co-benefits Low: very few or no co-benefits Sociopolitical risks (include national security) Minor: limited and mostly local economic and social impacts Medium: potential for serious national or regional economic, social, political, or security impacts that may be difficult for governments to manage Major: potential for severe national and regional economic hardship, social dislocation, political instability, and civil or military conflict continued 89
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From page 90... ...
90 Table 3.2 Continued Point- Accelerated Accelerated Source Direct Air Biological Biological Weathering Weathering Capture Capture Land Based Ocean Based Land Based Ocean Based Afforestation, Fuel/fuel soil, land Ocean iron NOTES: gas management fertilization Governance challenges for deployment at scale No novel governance challenges Governance challenges likely to be primarily territorial, but with some legitimate interest by other states Potential for substantial adverse effects across international borders or to an international commons Risk of detrimental deployment from unilateral and uncoordinated actors Low risks: few actors (individuals, groups, nations) have large enough resources to deploy technique and motivation to do so Medium risks High risks: many actors with resources and motivation
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From page 91... ...
(geologic) Ocean Form Ocean For example, as part Accelerated Accelerated of CCS, BECCS, or Ocean iron weathering weathering in or NOTES: Land management DACS fertilization on land near ocean Technological readiness, speed to deployment, and technical risk Mature technology (ready to deploy quickly, low technical risk)
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From page 92... ...
(geologic) Ocean Form Ocean For example, as part Accelerated Accelerated of CCS, BECCS, or Ocean iron weathering weathering in or NOTES: Land management DACS fertilization on land near ocean Persistence (sequestration lifetime)
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From page 93... ...
Verifiability: Ability to detect and quantify the rate at which CO2 was captured and added to the sequestration reservoir Easily verifiable: existing and planned observation systems can verify without retasking Moderately easy to verify: existing observation systems would need retasking or known technology would need to be deployed Difficult to verify: new technology or methods would need to be developed or deployed Verifiability: Ability to detect and quantify the rate at which CO2 is leaking out of the reservoir Easily verifiable: existing and planned observation systems can verify without retasking Moderately easy to verify: existing observation systems would need retasking or known technology would need to be deployed Difficult to verify: new technology or methods would need to be developed or deployed continued 93
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From page 94... ...
(geologic) Ocean Form Ocean For example, as part Accelerated Accelerated of CCS, BECCS, or Ocean iron weathering weathering in or NOTES: Land management DACS fertilization on land near ocean Verifiability: Ability to quantify increase in carbon stocks of the sequestration reservoir (i.e., verification of change in carbon mass stored)
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From page 95... ...
Medium: potentially serious impacts that may be difficult to mitigate to current environmental protection standards Major: severe national or global impacts incompatible with current environmental protection standards; impacts may exceed environmental benefits of climate change mitigation Sociopolitical risks (include national security) Minor: limited and mostly local economic and social impacts Medium: potential for serious national or regional economic, social, political, or security impacts that may be difficult for governments to manage Major: potential for severe national and regional economic hardship, social dislocation, political instability, and civil or military conflict continued 95
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From page 96... ...
(geologic) Ocean Form Ocean For example, as part Accelerated Accelerated of CCS, BECCS, or Ocean iron weathering weathering in or NOTES: Land management DACS fertilization on land near ocean Governance challenges for deployment at scale No novel governance challenges Governance challenges likely to be primarily territorial, but with some legitimate interest by other states Potential for substantial adverse effects across international borders or to an international commons
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