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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 472
Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 473
Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Page 474
Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Suggested Citation:"Appendix D: CO2 Flux Calculation." National Academies of Sciences, Engineering, and Medicine. 2019. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

APPENDIX D CO2 Flux Calculation Appendix D The flux of carbon dioxide (CO Flux Calculation parameter in de- CO2 ) across an interface is an important 2 termining the amount of material required for a given amount of separation in addi- The flux of size of the contact area required. The flux of CO across parameter in determining the tion to the carbon dioxide (CO2) across an interface is an important an interface may be 2 𝑝𝑝"#$ amount of material required for a given amount of separation in addition to the size of the contact area represented by: 𝐽𝐽"#$ = 𝐶𝐶' 𝑘𝑘) 𝐸𝐸 = 𝑘𝑘) 𝐸𝐸 required. The flux of CO2 across an interface may be represented by: 𝐻𝐻 Such that, JCO2 = flux of CO2 across the interface from gas to liquid in the case of a solvent or from gas to pore in Such that, the case of a solid sorbent JCO2 = flux of CO across the interface case of a solvent, in the case of a solvent or Ci = concentration of CO22at the interface. In thefrom gas to liquid this is the Henry’s law solubility. In the case fromsolid to pore in could be theaequilibrium capacity of the material based upon a Henry’s law of a gas sorbent it the case of solid sorbent estimation approach. Ci = concentration of CO at the interface. In the case of a solvent, this is the Henry’s H = Henry’s law constant in units2of atm·cm3/mol pCO2 = partial pressureIn the case of a solid sorbent it could be the equilibrium capacity of the law solubility. of CO 2 kl = mass-transferbased upon InHenry’s lawa solvent, this is strictly the liquid-phase mass-transfer material coefficient. a the case of estimation approach. coefficient. In the case ofconstant in units of atm∙cm3/mol H = Henry’s law a solid sorbent or mineral, this could be an effective mass-transfer coefficient, based upon all of the dominating diffusion resistances present in the system (see Ruthven, 1984 for details), pCO2 = water on the pellet, CO2 that is, partial pressure of macropore diffusion, etc. E = enhancement factor, which is only present when a chemical reaction takes place kl = mass-transfer to exhibit the the case of a solvent, this is of a CO separation process for an For simplicity, and coefficient. In unique requirements requiredstrictly the liquid-phase 2 extremely dilute system (air) compared tocase of a soliddilute system (coal exhaust), only solvent-based mass-transfer coefficient. In the a moderately sorbent or mineral, this could be an separation will be mass-transfer the remainder of this discussion. The enhancement factor,resis- be effective considered in coefficient, based upon all of the dominating diffusion E, can calculated for a present in the system (see Ruthven, 1984 for details), that is, water on the pel- the rate tances chemically-reacting solvent based upon the concentration of reactant present and of chemical macropore diffusion, etc. reaction may take place instantaneously, which means CO2 reacts let, reaction. For instance, the at the interface, indicating that the base reacting with CO2 is much greater in concentration than CO2 𝐷𝐷0 𝑐𝑐0 𝐸𝐸' = 1 + E the interface. In factor, which is such that present at = enhancementthis case, E = Ei, only present when a chemical reaction takes place 𝑧𝑧𝑧𝑧𝑐𝑐' For simplicity, and to exhibit the unique requirements required of a CO2 separation process for an extremely dilute system (air) compared to a moderately dilute system where DB equals the diffusivity of the base, cB is the concentration of the base, z is the stoichiometric (coal exhaust), only solvent-based separation will be considered in the remainder of coefficient of the base in the chemical reaction, D is the CO2 diffusivity, and ci is the interfacial concentration of CO2. The enhancement factor, E, can be calculated for a chemically-reacting this discussion. Most solvents to based uponreactconcentration of reactant present and thefast pseudo-first-order solvent date do not the with CO2 instantaneously, but rather via a rate of chemical = 𝐷𝐷𝐷𝐷𝑐𝑐0 reacts at the interface, indicating that𝐸𝐸the base reacting with CO2 is much greater in reaction,reaction. For instance, the reaction may take place instantaneously, which means CO2 in which case, 𝑝𝑝"#$ 𝐽𝐽"#$ = 𝑘𝑘) 𝐷𝐷𝐷𝐷𝑐𝑐0 k is the rate of the CO2 present at the this case, Equation 1 E = Ei, rewritten such thatconcentration thanchemical reaction. Ininterface. In this case, can be such that as: 𝐻𝐻 which would represent the CO2 flux across the gas-liquid interface of the majority of solvents known Figure D.1 shows the interfacial concentration as a function of 𝑝𝑝"#$ and H or air capture (top) and today. 469 the flue gas of natural gas and coal-fired power plants (bottom). To illustrate the impact that dilution has on the CO2 flux, taking amine-based solvents as an example, for air capture, ci is ~10, while for coal-fired

details), that is, water on the pellet, macropore diffusion, etc. l H = Henry’s the case of a solid of atm·cm3 coefficient. Inlaw constant in unitsonly present/mol a chemical reaction takes place E = enhancement factor, which issorbent or mineral, this could be an effective mass-transfer coefficient, when pCO = partial pressure of CO2 based2upon all of the dominating diffusion resistances present in the system CO2 Ruthven, 1984 for for an For simplicity, and to exhibit the unique requirements required of a (see separation process kl = mass-transfer coefficient. In the case of a diffusion, etc. details), that is, water on(air) pellet, macropore solvent, this is strictly the liquid-phaseonly solvent-based mass-transfer extremely dilute system the compared to a moderately dilute system (coal exhaust), coefficient. P E the case Dwhich is sorbent or mineral, a chemical be an effective place In N D I X of a solid only present when this could reaction takes mass-transfer coefficient, E = enhancement factor, separation P be considered in the remainder of this discussion. The enhancement factor, E, can be A will based upon simplicity, and to exhibit the unique requirements required of a CO2 separation 1984 forfor an For all of the dominating diffusion resistances present in the system reactant present and the rate (see Ruthven, process calculated for a chemically-reacting solvent based upon the concentration of details), that is, water on the compared to a moderately dilute system (coal exhaust), only solvent-based extremely dilute system (air) pellet, macropore diffusion, etc. of chemical reaction. For instance, the reaction may take place instantaneously, which means CO2 reacts E = enhancement factor, separation will indicatingwhich is base reactingwhen discussion. reaction takes place only present this a chemical The enhancement factor, at the interface,be considered in the remainder ofwith CO2 is much greater in concentration E, can be2 that the than CO of chemicaldilute system (air) compared to a moderately𝐷𝐷dilute instantaneously, whichonly solvent-based 0 𝑐𝑐0 calculated for a chemically-reacting solvent basedrequirements required of a CO2 separation process for an For simplicity, and to exhibit the unique upon 𝐸𝐸 with separation will be considered the base reacting = 1 +discussion. greater in concentration E, can be present at the interface. In this case, E = Ei, such that the concentration of reactant present and the rate calculated for a chemically-reactingE = Ei, such that 𝑧𝑧𝑧𝑧𝑐𝑐' concentration of reactant present and the rate extremely reaction. For instance, the reaction may take place system (coal exhaust), means CO2 reacts at the interface, indicating that in the remainder' of thisCO2 is muchThe enhancement factor, than CO2 coefficient of theindicating that the base reacting= is the 𝐷𝐷0 themuch greater incconcentration is theCO where base in the chemical reaction, DwithcB is 𝑐𝑐0 concentration i the interfacial solvent based upon the 𝐸𝐸 at the interface,DB equals the diffusivity of the' base,+CO2 is diffusivity, and ofis thebase, z than 1 presentDB equals the diffusivitycase, base, cB is the concentration of the base, z is the stoichiometric where at thereaction. For instance, the reaction may take place instantaneously, which means CO reacts interface. In this of the the 𝑧𝑧𝑧𝑧𝑐𝑐' of chemical 2 CO2 and ci is the do not react the base, B is CO . 𝐷𝐷0 𝑐𝑐 2 concentration of CO2. coefficient of the base inthat chemical reaction, D is the CO2 diffusivity, stoichiometric In this 𝐸𝐸 is the coefficient of the base in the chemical reaction, 'D= 1 2 CO20diffusivity, and ci is the interfacial + present atequals the diffusivity case, E = Ei,csuchthe concentration of the base, z is the stoichiometric the interface. 𝑧𝑧𝑧𝑧𝑐𝑐 where DB Most solvents to date interfacialof with CO2 instantaneously, but rather via a fast pseudo-first-order concentration of concentration of CO2. to date do not react with CO2 𝐷𝐷𝐷𝐷𝑐𝑐 ' 𝐸𝐸 = concentration of the base, z is via a fast reaction, in which case, 0 Most solvents instantaneously, but rather 𝑝𝑝 where DB equals the diffusivity ofwith base, instantaneously, the CO2 cB is Most thatpseudo-first-order chemical reaction. In the case, Equation 1 can a fast pseudo-first-order the stoichiometric such solventsthe date ofinnot reaction, in which case, the CO2but rather via be ci is the interfacial k is to rate dothe react 𝐽𝐽"#$ =𝐸𝐸 = $ 𝐷𝐷𝐷𝐷𝑐𝑐 𝐷𝐷𝐷𝐷𝑐𝑐0 𝑘𝑘) reaction, in which base the chemical reaction, D "# this rewritten as: 𝐻𝐻 coefficient of the case, is diffusivity, and 0 concentration of CO2. 𝑝𝑝"# Most solvents to date do not react with CO2 instantaneously, but rather via a fast pseudo-first-order 𝐽𝐽"#$ = 𝐸𝐸 = $ 𝐷𝐷𝐷𝐷𝑐𝑐0𝐷𝐷𝐷𝐷𝑐𝑐0 𝑘𝑘 which would represent the CO2 flux across the In this case, Equation 1 can be rewritten as: known such that k is the rate of the chemical reaction. gas-liquid interface of the majority of solvents Figure D.1 shows as: interfacial concentration as) a function of 𝑝𝑝"#$ and H or air capture (top) and 𝐻𝐻 reaction, in which case, today. such that k is the rate of the chemical reaction. In this case, the equation on page 469 today. CO2 flux, taking amine-based solvents as an𝑝𝑝example, 𝐷𝐷𝐷𝐷𝑐𝑐air capture, ci is ~10, while for coal-fired can be rewritten the 𝐽𝐽"#$ = 𝑘𝑘) for 0 "#$ whichthat k of the rate gasthe chemical reaction.gas-liquid interface of the majority of solvents known has the flue gas isrepresentof and 2 flux across the plants (bottom). To illustrate the impact that dilution such would natural the CO coal-fired power In this case, Equation 1 can be rewritten as: 𝐻𝐻 D.1 shows the interfacial concentration as function of 𝑝𝑝"#$ and H on the flue gas,Figurerepresent the CO flux across the gas-liquidainterface of mol/cm3. Basedair capture (top) and it is approximately 250 times greater at approximately 2,500 the majority or solvents known which would of upon the flux Figure D.1 shows the interfacial concentration as a function of 𝑝𝑝"#$ and H or upon the flux and 2 equation,whichnatural gasto enhance the flux of plantsthe gas-liquidillustrate the the majority of of air the flue gas ofmeans that and coal-fired power CO2 across the gas-sorbent interface for thatcase today. this would represent the CO flux across (bottom). To interface of impact the dilution has on the CO2 product kLE must be 250. The mass an example, for air capture, ci at most a factor coal-fired capture, theflux, taking amine-based solvents as transfer coefficient changes byis ~10, while for of 10. The 2 flue gas, it is approximately ,250 times greaterthe approximately 2,500 mol/cm3. Based air capture (top) solvents known today. mass-transfer coefficient, kL is dependent on at process parameters used in contacting the air and equation,Figure natural gas the interfacial powerCO2 across the gas-sorbent CO the impactthecap- of airhas the flue gas of this means that toand coal-fired concentration as a function of p interface for air case D.1 shows enhance the flux of plants (bottom). To illustrate and H or that dilution flux, taking must be 250. The mass transfer coefficient capture, 2c is most a factor coal-fired on the CO2 product kLE amine-based solvents as an example, for airchanges byi at ~10, while forof 10. The capture, the (top) and the flue gas of natural gas and coal-fired power plants (bottom). To flue gas,ture approximately 250 times greater at approximately 2,500 mol/cm3. Based the air and mass-transfer coefficient, kL, is dependent on the process parameters used in contacting upon the flux it is equation, this means impact that dilution has CO21across the taking amine-based solvents as illustrate thethat to enhance the flux ofon the CO2 flux,gas-sorbent interface for the case of air capture,an example,kfor air capture, ciThe mass transfercoal-fired flue gas, itby approximately of 10. The the product LE must be 250. is ~10, while for coefficient changes is at most a factor mass-transfer coefficient, kat is dependent on2,500 mol/cm3. Based upon the flux equation, this 250 times greater L, approximately the process parameters used in contacting the air and 1 means that to enhance the flux of CO2 across the gas-sorbent interface for the case of air capture, the product kLE must be 250. The mass transfer coefficient changes by at most a factor of 10. The mass-transfer coefficient, kL, is dependent on the process 1 parameters used in contacting the air and sorbent. For instance, the air can interact with the sorbent via coated packing or bubbles. In addition, the way in which the air flows over the solvent impacts this parameter, that is, cross-flow vs counter-flow. The enhancement factor depends primarily on the rate constant of the chemical reaction between CO2 and the chemical binding to it. Hence, a chemical with a rate constant 25 times greater, combined with a process that allows for the mass-transfer coefficient to be maximized, has the potential to create a flux for air capture equivalent to the more concentrated scenario of coal-fired flue gas exhaust. It is not necessary for the air cap- ture flux to match that of flue gas for its success, but improving materials in this way could results in more competitive costing because the capital expense would surely decrease with fewer required units for separation. Hence, the development of new chemistries that could lead to enhancing the rate of chemical reaction with CO2 could 470

Appendix D FIGURE D.1  Interfacial concentration of CO2 as a function of CO2 partial pressure between 0.0004-0.001 atm (top) and 0.04-0.14 atm (bottom) and Henry’s law constant between 10,000-70 000 atm-cm3/mol, spanning that of ionic liquids to potassium carbonate. SOURCE: Wilcox et al., 2014. 471

APPENDIX D be of interest in terms of future research. These new chemistries could also play a role in enhancing the flux of CO2 for more concentrated CO2 cases as well. SOLID SORBENT SYSTEM To assess the bounds of a generic, hypothetical air capture process employing adsorp- tion as the separation technology, the methodology outlined in the work of Sinha et al. (2017) was employed. This approach, applying mass and energy balances to each individual step in the overall process, calculating energy requirements, and then assessing the costs of the necessary capital equipment, has been applied to various scenarios to arrive at overall costs for the separations process. In the analysis employed, an array of critical parameters was defined within a range of values deemed physically realistic, and from these parameters other key parameters are calculated (Figure D.2). A stepwise approach to calculating process energetics and costs was then applied. In the first step, the contactor:adsorbent ratio, adsorbent purchase cost, and adsorbent lifetime are defined. The contactor is the structure that FIGURE D.2  Approach to calculating energy costs and process economics for a generic adsorption- based air capture process. 472

Appendix D provides for high surface area gas-solid contacting, whereas the adsorbent is the chemical agent that binds the CO2. From these values, the adsorbent and contactor costs are determined. In the limiting case where the adsorbent and contactor are the same, the contactor cost goes to zero. This case was not specifically considered here. Next, the total CO2 capacity was defined, the ratio of the CO2 and H2O capacities, and the fraction of captured CO2 desorbed and collected as product (CO2 swing capacity) were defined. These define the total moles of CO2 captured, which is a key parameter in the denominator of the cost and energy. The heat of adsorption of CO2 is also de- fined, constrained within a range (ΔHCO2). The desorption time is also defined based on a transient energy balance calculation using 100°C saturated steam as the heat transfer media, which transfers heat through the heat of condensation. In these cal- culations, the model envisaged that the steam is directly contacted with the sorbent, providing both a concentration and thermal driving force for desorption. In this ap- proach, the sorbent quickly attains a pseudo-steady-state capacity of adsorbed water, becoming hydrated after steaming, with some water desorption occurring via evapo- rative cooling upon exposure to fans blowing air through the material in the next adsorption step. Based on the water transfer rate, the amount of water lost by evapo- ration was calculated such that the heat transfer was sufficient to reduce the sorbent/ contactor combination to the initial conditions based on the sensible heat require- ment of the adsorbent/contactor system (Jeong et al., 2010). This is essential to re- initialize the system for the next adsorption step. Given the high content of adsorbed water on the sorbent surface owing to the direct steaming heat transfer approach, the amount of water adsorbed from the air in each adsorption cycle is estimated to be minimal in this gas-solid contacting strategy. In contrast, alternate strategies are being employed in practice, whereby heat is transferred indirectly and the steam does not directly contact the sorbent. This alternate approach might offer less efficient heat transfer but better protects the sorbent from possible degradation by direct steam contact. This approach also would result in more water being extracted from humidity in the air. No attempt was made to model every known process, but rather to use the representative, generic process for energy and cost estimations. In the next phase, the pressure drop is derived from a calculation that considers the velocity, contactor length, and radius of a contactor channel. The mass-transfer coeffi- cient is derived from the velocity calculation. The mass-transfer coefficient is a lumped parameter that accounts for all potential resistances to mass transfer, including film resistance, macropore resistances, as well as micropore resistances. The adsorption time depends on the mass transfer coefficient, pressure drop, and velocity. From these parameters, blower or fan operating costs were determined. 473

APPENDIX D TABLE D.1  Parameters Used to Calculate Lower-Bound and Upper-Bound Costs for Direct Air Capture with Solid Sorbents Process Parameters Lower/Upper Bounds Adsorbent purchase cost ($/kg) 15/100 Adsorbent lifetime (yrs) 5/0.25 Sorbent total capacity (mol/kg) 1.5/0.5 Desorption swing capacity 0.9/0.75 SCmax CO2:water ratio 1:2/1:40 Desorption pressure (bar) 0.2/1 Final desorption temperature (K) 340/373 Thermal Energy Outputs MJ/mole 0.08/0.85 GJ/tCO2 1.85/19.3 kW-hr/tCO2 514/5367 Electrical Energy Outputs MJ/mole 0.003/0.167 GJ/tCO2 0.08/3.79 kW-hr/tCO2 20/1055 Cost $/tCO2 14/1065 NOTE: Energy expenditure associated with upper- and lower bound-process configurations. 474

Appendix D In the last step, the desorption pressure was varied within a range and the final de- sorption temperature of the contactor/adsorbent material was similarly defined. By conducting a transient heat transfer calculation using 100°C saturated steam as the heat transfer media, as noted above, with a final desorption temperature defined, the desorption time was calculated. Other outputs from this step include the vacuum operating costs as well as the desorption energy costs (steam costs). The calculation procedure and outputs from each step is schematically shown in Fig- ure D.2. The range of parameter variation is given in Table D.1. The conditions leading to the lower- and upper-bound costs are outlined here. As noted above, the combination of all the most favorable parameters, using realistic physical parameters, suggests that a hypothetical cost as low as $18/tCO2 can be esti- mated. The upper-bound cost ($1,060/tCO2) is not a true upper bound, because there are an infinite number of ways to deploy direct air capture with high costs. The term upper bound solely signifies the upper end of the range considered in the calcula- tions. The committee does not feel that the lower bound cost is practically achievable, though no physical bounds prevent the cost of direct air capture from dropping below $100/tCO2. RENEWABLE ROUTES WITH SOLVENT-BASED SYSTEMS Costing Air Capture from Renewables—achieving 1Mt CO2 avoided per year Two routes to costing air capture using 100 percent renewables were considered: (1) electricity sourced from combined photovoltaics (PV) and battery storage for direct use and (2) electricity sourced from PV for electrolysis (Figure D.3; Table D.2), followed by H2 storage as described in detail in Chapter 5. In these low-carbon scenarios, solar thermal and geothermal were not considered because of their temperature limita- tions and inability to reach the required calcination temperature of 900°C. It should be noted however that technologies are emerging, such as concentrated power towers (DOE, 2013) and alternative nuclear designs (Harvey, 2017), that involve high-­ temperature gas-cooled reactors that may be suitable low-carbon routes for integra- tion into air capture approaches requiring high temperatures for regeneration. SECOND-LAW EFFICIENCY CALCULATION The second law efficiency calculation for the separation of CO2 from the atmosphere for a process employing a solid sorbent, as described above, is given here. 475

emerging, such as concentrated power towers (DOE, 2013) and alternative nuclear designs (Harvey 2017), that involve high-temperature gas-cooled reactors that may be suitable low-carbon routes for integration into air capture approaches requiring high temperatures for regeneration. <<TABLE D.2 NEAR HERE>> APPENDIX D SECOND-LAW EFFICIENCY CALCULATION The second law efficiency calculation for the separation of CO2 from the atmosphere for a process employing a solid sorbent, as described above, is given here. This estimate demonstrates that an air capture process using solid sorbents and oper- ating near ambient conditions, using low temperature thermal energy, can be surpris- ingly efficient. Despite the high dilution of the source gas, a process suitably designed TABLE D.2  Economic Costs Associated PV + Storage for Solvent-Based Air Capture CAPEX Cost ($M) Comment Contactor array 150-250 Lower bound: anticipated cost of 10 air contactor array from Holmes and Keith (2012), based on optimal percent capture of 75% and bed depth of 6-8 m. Upper bound: scaled cost for system at high percentage capture (90%+) and deeper packing bed, with 1.5× learning cost factor. Slaker/causticizer/ 130-195 Lower bound: capital costs taken from Socolow et al. clarificator (2011) report and adjusted to 2016 USD. 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. Electric-fired calciner 270 Lower bound: price quote from industry source for oxy-fired kiln with 4.5× factor used for scaling Inside Battery Limits (ISBL) equipment costs to full costs. Assuming the lower-bound estimate because this may be lower than an oxycombustion or H2- fired kiln since electric-fired kiln are commercially available. Efficiency of 80% assumed. 476

Appendix D TABLE D.2 Continued CAPEX Cost ($M) Comment CAPEX Subtotal 550-715 Annualized Capital Payment ($M/y) 62-80 Assumes a plant life of 30 years and fixed charge factor of 0.11278 (Rubin et al., 2007). OPEX Cost Comment ($M/y) Maintenance 23-40 Range calculated as 0.03 of total capital requirement. Labor 7-12 Range calculated as 0.30 of maintenance cost. Makeup (H2O, KOH, Ca(OH)2) and 5-7 Lower bound: assumes $500/t KOH, $250/t Ca(OH)2, waste removal $0.30/ t H2O, $260/t waste disposal (Rubin et al., 2007). Upper bound: applies 1.5 factor to make-up OPEX. PV+battery 294-389 Levelized cost of energy “PV Plus Storage” $92/ MWh (Lazard, 2016). Assumes total capital costs of ~ $3,900/kW including PV and battery energy storage. Assuming direct electricity needs of 21-27 kJ/ molCO2. Assuming electric kiln and electric heater have efficiencies of 80%, requires an electricity demand to meet thermal energy demands of 485- 643 kJ/molCO2. OPEX Subtotal 329-448 Cost = Net Removed CO2 Cost ($/tCO2 yr-1)a PV+Battery 391-528 a Basis = 1Mt CO2 477

APPENDIX D and optimized to target ambient CO2 can achieve the separation with unexpected efficiency. See Chapter 5 for calculated efficiencies. FIGURE D.3  Consideration of two routes to meeting the thermal needs of air capture, (1) electrolysis of H2, which is stored and used for H2-fired kiln (top) or (2) battery storage of renewable electrons for direct electric heating of an electric-fired kiln. 478

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To achieve goals for climate and economic growth, “negative emissions technologies” (NETs) that remove and sequester carbon dioxide from the air will need to play a significant role in mitigating climate change. Unlike carbon capture and storage technologies that remove carbon dioxide emissions directly from large point sources such as coal power plants, NETs remove carbon dioxide directly from the atmosphere or enhance natural carbon sinks. Storing the carbon dioxide from NETs has the same impact on the atmosphere and climate as simultaneously preventing an equal amount of carbon dioxide from being emitted. Recent analyses found that deploying NETs may be less expensive and less disruptive than reducing some emissions, such as a substantial portion of agricultural and land-use emissions and some transportation emissions.

In 2015, the National Academies published Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration, which described and initially assessed NETs and sequestration technologies. This report acknowledged the relative paucity of research on NETs and recommended development of a research agenda that covers all aspects of NETs from fundamental science to full-scale deployment. To address this need, Negative Emissions Technologies and Reliable Sequestration: A Research Agenda assesses the benefits, risks, and “sustainable scale potential” for NETs and sequestration. This report also defines the essential components of a research and development program, including its estimated costs and potential impact.

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