Negative Emissions Technologies and Reliable Sequestration A Research Agenda (2019) / Chapter Skim
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5 Direct Air Capture
5 Direct Air Capture
Pages 189-246
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From page 189... ...
The direct air capture approaches described in this chapter are technically feasible, but because CO2 in air is ~300 times more dilute than in flue gas from a coal-fired power plant, the separation process for the same end CO2 purity will likely be more expensive than capture from fossil fuel power plants. CO2 removal from gas streams is an important component of many industrial processes.
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From page 190... ...
Capture is generally an exothermic process, and desorption for concentration is an endothermic process. This chapter evaluates two types of direct air capture CO2 separation processes: one employing liquid solvents and one utilizing solid sorbents.
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From page 191... ...
Such designs are now recognized as not broadly applicable to direct air capture systems. As highlighted by several studies, altering the flow configuration to reduce pressure drop can dramatically reduce capture costs compared to the APS benchmark system, which is based on a more conventional approach that mimics post-combustion capture absorber technology.
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From page 192... ...
Many direct air capture systems have been proposed. These can be distinguished by characteristics including the choice of liquid solvent or solid sorbent, method for CO2 release/capture (regeneration)
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From page 193... ...
Climeworks has advanced the farthest, operating a 900 t/y demonstration plant in Switzerland where CO2 is used for various applications, rather than stored in geologic reservoirs. ANALYSIS: ENERGETICS, CARBON FOOTPRINTS, AND COSTS This section presents the Committee's analyses of the energetics, carbon footprints, and economics of direct air capture systems based on liquid solvents and solid adsorbents.
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From page 194... ...
Examples of technologies that may lead to an x near zero are those that use low-carbon energy resources to supply the required heat and power to operate the system, which may be unique for a given direct air capture approach. For example, the liquid solvent approach requires temperatures of up to 900°C for regeneration.
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From page 195... ...
The committee followed different approaches to analyze the liquid solvent and solid sorbent direct air capture systems described below. The liquid solvent systems analysis is derived from a conceptual process design published by Carbon Engineering (Holmes and Keith, 2012; Keith et al., 2018)
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From page 196... ...
Therefore, rather than analyzing a specific process, a generic sorbent-based process is considered, and key process parameters varied to provide a range of energetic and process costs. Liquid Solvent Systems Process Description The two major components of a liquid solvent direct air capture process are the air contactor and regeneration facility (Figure 5.1)
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From page 197... ...
Moreover, the packing volume for their system is estimated at 20,000 m3, compared to a large cooling tower volume of 10,000 m3 and a conventional packed tower of about 285 m3. These considerations highlight that an optimized direct air capture contactor design 197
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From page 198... ...
. FIGURE 5.2 Conceptual drawing of the air contactor for a liquid solvent direct air capture system.
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From page 199... ...
Thus, a liquid solvent direct air capture system with a 1 Mt/y CO2 capture rate will require the addition of about 8.2 Mt/y water to make up for water loss. Solvent Pump To calculate the work required to pump KOH for even distribution across the packing material, the pressure drop, volumetric flow rate, and liquid density are required.
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From page 200... ...
2 to make CaCO3 and regenerate KOH for reuse in the air contactor: H2O + K2CO3 + Ca(OH) 2 → 2 KOH + CaCO3 Typical causticization efficiencies for sodium hydroxide (KOH efficiencies are lacking in the literature)
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From page 201... ...
Because of this large thermal requirement, CO2 emissions associated with traditional calcination processes are significant, ranging from 0.38 to 0.57 and 0.56 to 0.84 Mt/y CO2 for natural gas and coal firing, respectively. To minimize CO2 generated in the direct air capture process, any thermally generated CO2 could, in theory, become co-captured with that from ambient air.
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From page 202... ...
, leading to a footprint of 0.068 and 0.041 Mt/y CO2 using electricity-derived from coal and natural gas firing, respectively.4 Chemical Make-up Reagent loss may occur at several points in a liquid solvent direct air capture process. Because of the nature of direct air capture, foreign contaminants may enter the absorber (e.g., insects, birds, particulate matter, sulphur oxides [SOx]
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From page 203... ...
. Based on the energy requirements outlined for liquid solvent direct air capture systems, the "real" work is 8.2-11 GJ/t, leading to an exergy efficiency8 of 4.16.2 percent.
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From page 204... ...
FIGURE 5.3 Estimated energy requirements for a liquid solvent direct air capture system using a calcium carbonate cycle, where most of the energy is for CaCO3 preparation for calcination and CO2 liberation in the kiln (calculated at 900°C)
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From page 205... ...
The cost estimates presented here also vary by energy source and do not include costs for compression, transportation, injection, and sequestration.9 The estimated capital and operating costs for a 1 Mt/y CO2 liquid solvent direct air capture system are provided in Table 5.3. This cost analysis presents an optimistic scenario based on optimal parameters (for instance, Holmes and Keith [2012]
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From page 206... ...
Costs for a Generic Liquid Solvent Direct Air Capture System with a Capacity of 1 Mt/y CO2 Removal CAPEX Cost ($M) Comment Contactor array 210–420 Lower bound: reported cost of air contactor array from Holmes and Keith (2012)
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From page 207... ...
Natural gas 25-35 Range calculated from low and high thermal requirements reported in Table 5.2, assuming natural gas cost of $3.25/GJ. Coal 18-25 Range calculated from low and high thermal requirements reported in Table 5.2, assuming 2016 U.S.
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From page 208... ...
estimate would decrease by nearly $15/tCO2 before considering system optimization. Additionally, a two-thirds reduction in operational energy expenditures on fan power may be achieved via the reduced pressure drop, resulting in an additional cost savings of $7/tCO2 assuming electricity from natural gas at $60/MWh.
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From page 209... ...
Further, a water flow rate of 5.7 ×105 t/y H2O is needed to produce an average of 4.15 kmol/hr H2 to then produce the heat required for a direct air capture plant designed to remove 1 Mt/y CO2. The energy required for electrolysis dominates the energy operating costs as shown in Table 5.5, followed by the H2 compression energy required.
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From page 210... ...
Upper bound: 1.5 × factor to account for new technology. Though the Ca-recovery cycle is mature and well studied in the pulp and paper industry, learning costs may be associated with integration into a direct air capture system.
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From page 211... ...
230-365 Assumes a plant life of 30 years and fixed charge factor of 12%.
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. Solid Sorbent Systems Process Description Like liquid solvent systems, solid sorbent direct air capture systems have two main processes: adsorption and desorption that operate cyclically (Figure 5.4)
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From page 213... ...
and produce a concentrated CO2 stream. Regeneration is the most energy-intensive step for a solid sorbent direct air capture system and includes the thermal energy needed to induce CO2 desorption (ΔHads)
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From page 214... ...
An advantage of many recent solid sorbent–based direct air capture processes is that they do not require high temperature thermal energy. In an ideal scenario, the electrical energy needs should be met with renewable energy, and the thermal energy 12 For many sorbents, water uptake should be minimized to minimize the amount of water that must desorbed from the sorbent in each cycle, and its associated energy penalty.
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From page 215... ...
For each step in the solid sorbent direct air capture process, the CO2 emissions were evaluated under several scenarios, including providing the electrical energy from wind, solar thermal, nuclear, natural gas, or coal and thermal energy from solar thermal, nuclear, coal, or natural gas (Table 5.7)
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. While the use of coal to power a solid sorbent direct air capture system is not likely, it provides a useful worst-case emissions scenario, providing an upper bound to the problem.
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From page 217... ...
Because additional CO2 emissions can be generated in several of the steps required in direct air capture systems, the net costs of CO2 removed are also presented. The cost estimates presented vary by energy source and do not account for compression, transportation, injection, and sequestration (see Chapter 7 on geologic sequestration)
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From page 218... ...
(best-worst, 1-5) Desorption heat Solar 0.008-0.01 0.004-0.04 Nuclear 0.004-0.005 0.002-0.02 Natural gas 0.22-0.30 0.12-1.2 Coal 0.32-0.44 0.17-1.7 Air contactor fans Solar 0.0004-0.008 0.0005-0.026 Wind 0.002-0.003 0.0002-0.012 Nuclear 0.002-0.004 0.0002-0.013 Natural gas 0.07-0.14 0.01-0.47 Coal 0.15-0.3 0.019-1 Vacuum pump Solar (0.93-1.9)
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From page 219... ...
For a generic solid sorbent system, with all parameters varied within the ranges listed in Table 5.5, the committee calculated carbon capture costs ranging from $18/tCO2 to over $1,000/t CO2. The combination of every best-case parameter resulted in the lower bound (1-best)
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From page 220... ...
Summary of Analysis of Solvent and Solid Sorbent Direct Air Capture Systems Table 5.11 presents the estimated energy required for direct air capture, along with the CO2 footprint and net CO2 removal assuming a plant designed to capture 1 Mt/y CO2. Both liquid solvent and solid sorbent cases have been considered, with scenarios that vary to meet the electric and thermal needs of the direct air capture plant.
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From page 221... ...
Costs for a Generic Solid Sorbent Direct Air Capture System with a Capacity of 1 Mt/y CO2 Removal Parameters 1-Best 2-Low 3-Mid 4-High 5-Worst Adsorbent CAPEX 3.6 70 122 186 988 Adsorption OPEX 1.3 9 12 19 4.3 Blower CAPEX 3.6 2.1 3.7 6.7 13.7 Vacuum pump CAPEX 4.5 2.6 4.7 8.5 17.4 Steam OPEX 2.5 2.2 2.4 3 43 Condenser CAPEX 0.03 0.07 0.075 0.1 0.4 Contactor CAPEX 2.2 1.3 2.3 4.1 8.4 Vacuum pump OPEX 0.3 0.2 0.2 0.24 0.3 Energy Requirements The thermal component of the energy required to operate a direct air capture plant dominates the electric component because of the need for strong CO2-binding chemistry. The electricity required is used to operate fans and pumps and can be minimized through the design of a shallow contactor to minimize pressure drop through the system.
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From page 222... ...
System Cap- Net Re Electric Thermal Electric Thermal Electric Thermal (Mt/y CO2) tured moveda NG NG 0.74-1.7 7.7-10.7 0.11-0.23 0.47-0.66 0.11-0.42 147-264 199-357 coal NG 0.74-1.7 7.7-10.7 0.18-0.38 0.47-0.66 0-0.35 147-264 233-419 wind NG 0.74-1.7 7.7-10.7 0.004-0.009 0.47-0.66 0.34-0.53 141-265 156-293 Liquid Solvent solar NG 0.74-1.7 7.7-10.7 0.01-0.03 0.47-0.66 0.31-0.52 145-265 165-294 nuclear NG 0.74-1.7 7.7-10.7 0.01-0.02 0.47-0.66 0.32-0.52 154-279 173-310 solar H2b 11.6-19.8 7.7-10.7 0.01-0.03 0 0.99 317-501 320-506 solar 0.0004 solar 0.55-1.1 3.4-4.8 0.008-0.01 0.892-0.992 88-228 89-256 0.008 nuclear nuclear 0.55-1.1 3.4-4.8 0.002-0.004 0.004-0.005 0.91-0.994 88-228 89-250 0.0004- 113-326 Solid Sorbentc solar NG 0.55-1.1 3.4-4.8 0.22-0.30 0.70-0.78 88-228 0.008 wind NG 0.55-1.1 3.4-4.8 0.002-0.003 0.22-0.30 0.70-0.78 88-228 113-326 NG NG 0.55-1.1 3.4-4.8 0.07-0.14 0.22-0.30 0.56-0.71 88-228 124-407 coal coal 0.55-1.1 3.4-4.8 0.15-0.3 0.32-0.44 0.26-0.53 88-228 166-877 a Assuming the use of an oxy-fired kiln to provide heat from natural gas in the calcination process, leading to greater CO 2 production and hence lower cost of net CO2 removal, using a basis of 1.3 Mt CO2 for NG/NG, 1.2 Mt CO2 for coal/NG.
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From page 223... ...
Carbon Removal Cost If fossil-based energy resources are used to provide the energy requirements of a direct air capture system, then an accurate estimate of the cost to removing CO2 from the air requires consideration of the net CO2 removed because burning fossil fuels produces CO2. On average, the costs for net CO2 removed for the solid sorbent–based approach range from $89 to $877/tCO2, depending on the adsorption scenario, while the costs range for the solvent-based approach range from $156 to $506/tCO2, depending on the use of natural gas or renewable H2 for the thermal source.
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From page 224... ...
To maximize the net emissions removed from the air and the ultimate impact of direct air capture and sequestration, the use of renewable energy resources should be maximized where possible. The integration of renewable energy with base load natural gas, or use of combined heat and power units, could be a cost-effective approach to scaling up direct air capture and sequestration.
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From page 225... ...
Liquid Solvent Systems: In the contactor design of Keith and Holmes, the cross In general,inletland that is required for direct air capture is impacted by the size of the contactor sectional the area is oriented normal to the land surface. This use of vertical space and the spacing requirements ofuse per contactor structure.
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From page 226... ...
are known to suffer from compromised survival and limited reproduction. Further, these conditions may affect plant tolerance to drought, heat, and other stressors, an important consideration if direct air capture siting includes arable land (Sage and Cowling, 1999; Ward, 2005)
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From page 227... ...
use may be avoided altogether through contracts with off-shore wind farms, which typically experience higher capacity factors than their land-based counterparts. An alternative configuration for sorbent-based direct air capture involves onsite electrolysis of H2 using solar power.
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From page 228... ...
In both the proposed solid and solution-based direct air capture processes, most water use is contained in closed-loop systems, whereby water is continuously recycled. Nonetheless, nearly all processes have the potential for water loss, and this parameter should be carefully considered when any new process is developed.
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From page 229... ...
The hypothetical adsorption-based direct air capture process analyzed here, which relies on T/VSA using saturated steam condensation on the adsorbent and contactor as the mode of heat transfer, can result in water loss to the environment. For those who employ this approach, such as Global Thermostat, the potential water loss is usually accepted as a consequence of the improved heat transfer and overall process performance offered by this mode of heat transfer.
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From page 230... ...
Future R&D efforts should carefully consider water production/use. Environmental One potential environmental impact of direct air capture processes is the depletion of CO2 from the air exiting the contactor.
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From page 231... ...
This in turn limits the ability of policymakers to understand the costs to deploy direct air capture to achieve the scale of negative emissions needed to comply with the Paris agreement. As such, the most significant research will bolster public support for an array of pilot-scale studies of integrated direct air capture processes that can be operated for extended time periods to assess process performance and reliability, and will provide the data necessary to improve and refine process techno-economic models.
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From page 232... ...
Examples of basic science innovations that could significantly advance direct air capture technologies include: (i) Low-cost solid sorbents, ideally costing <$50/kg, that are designed in conjunc tion with a suitable gas/solid contactor capable of deployment at scale.
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From page 233... ...
Justification • Simulate, synthesize, test new 20-30 10 Project Cost: ~ $1M Basic Science and Applied Research materials (solvent/sorbents) Project Duration: ~3 y • Design, model, test novel equipment Project Number: 20-30/y concepts Project Staff: ~ 1 FTE • Design and model novel system concepts, some specifically targeting renewable integration Establish independent evaluation for 3-5 10 Contracts: 2 • materials performance testing, Contract Staff: 3-5 FTE characterization, validation • public materials database creation and management • Scale materials synthesis to > 100 kg 10-15 10 Project Cost: ~ $5M scale Project Duration: 3 y • Design and test novel equipment for Project Number: 2-3/y pilot scale Project Staff: ~ 3 FTE • Test system innovations on integrated lab-scale direct air capture system (> 100 kg/d CO2)
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From page 234... ...
Justification • Design, build, and test pilot-scale 20-40 10 Project Cost: ~ $20M direct air capture systems (> 1000 t/y Project Duration: 3 y CO2) Project Number: 1-2/y average Project Staff: 10-15 FTE Nominally, 3-5 projects in years 1-3, 5-10 projects in years 4-6, Demonstration and 3-5 projects in years 8-10 Establish national direct air capture test 10-20 10 Contracts: 1 center to Contract Staff: 20-30 FTE • support pilot plant demonstration Fully Loaded FTE: $500K projects • develop third-party front-end engineering design and economic analysis • maintain public record of pilot plant performance 234
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From page 235... ...
Deployment depending on success of technologies from funding above, if success above justified such large investment Engage National Direct Air Capture Test 15-20 10 Contract: 1 Center to Contract Staff: 30-40 FTE • support full-scale plant Fully Loaded FTE: $500k demonstration projects • maintain public record of full-scale plant performance and economics 235
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From page 236... ...
(vii) Life-cycle analyses of known and new direct air capture processes, specifically with regard to CO2 emissions from sorbent production and use (given the sensitivity of the solid sorbent-based processes to sorbent lifetime)
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From page 237... ...
at the National Renewable Energy Laboratory (NREL) , where the standards for solar cell efficiency measurements have been established, providing annual publication of the best research-cell efficiencies.
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From page 238... ...
An additional driver to improving system design is the need for a low pressure drop in the contactor to minimize the electricity requirement. In the case of the solvent-based approach, 20 percent of the capital costs and 30 percent of the operating and maintenance costs of the entire direct air capture plant are associated with the air contactor.
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From page 239... ...
To compensate for the shallow bed depth, the direct air capture contactor must have a large surface area to be able to capture the equivalent CO2. Figure 5.7 shows that the total cost of the direct air capture air contactor decreases with increasing packing depth to a critical depth of about 8 m, after which the costs begin to increase as the fan power plays a more dominant role in the total cost.
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From page 240... ...
Demonstration The most significant barrier to the assessment and deployment of direct air capture processes is the absence of process-scale operational data to perform accurate techno-economic analyses. Currently there is no incentive for privately funded demonstration projects to provide such data.
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From page 241... ...
Although these opportunities are good for the budding array of commercial entities that are developing direct air capture technologies, they offer limited growth opportunities for the industry overall. An array of publicly funded demonstration-scale studies of integrated direct air capture processes that can be operated for extended time periods is needed to generate data on process performance and reliability and to improve and refine process techno-economic models.
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From page 242... ...
To this end, publicly funded pilot-scale studies that provide long-term data from field operations are needed to more accurately model sorbent durability and lifetime, which significantly affect the overall process cost of solid sorbent–based direct air capture processes. Pilot-scale studies could also assess optimal site locations and plant configurations.
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From page 243... ...
has the appropriate infrastructure to manage direct air capture research, development, and demonstration projects through a typical grant process that distributes funds to projects at universities, nonprofit research organizations, start-up companies, and large companies. Contractors that provide independent materials testing, component testing, techno-economic analysis, and professional engineering design can also be managed through the U.S.
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From page 244... ...
Notably, before funds are committed for d emonstration-scale projects, detailed third-party engineering and economic assessments must demonstrate the potential for achieving CO2 removal at a cost of < $300/t. Data Management Data collection, organization, and public release is a critical component of a modern direct air capture research agenda.
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Direct Air Capture FIGURE 5.9 National Carbon Capture Center, Wilsonville, Alabama. NOTES: Funded by the Office of Fossil Energy, U.S.
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From page 246... ...
N E G AT I V E E M I S S I O N S T E C H N O LO G I E S A N D R E L I A B L E S E Q U E S T R AT I O N FIGURE 5.10 Recommended annual U.S. federal funding allocations for basic and applied research, d emonstration, development, and deployment of direct air capture technology.
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