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Suggested Citation:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. 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:"5 Direct Air Capture." National Academies of Sciences, Engineering, and Medicine. 2018. Negative Emissions Technologies and Reliable Sequestration: A Research Agenda. Washington, DC: The National Academies Press. doi: 10.17226/25259.
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5 Direct Air Capture INTRODUCTION Recent integrated assessment models (Fuss et al., 2013) have made clear the need to include negative emissions technologies as one component in a portfolio of solutions (e.g.; mitigation, energy efficiency, renewables, fuel-switching, etc.) to prevent 2°C warming by 2100. Among these negative emissions technologies is the direct removal of CO2 from the atmosphere, commonly referred to as direct air capture. For direct air capture systems to be considered a negative emissions technology, they should sequester the captured CO2 on a time scale that positively impacts climate change. Currently, the only reasonable approach for storing captured CO2 is geologic sequestration, which is covered in Chapter 7. Direct air capture has received significant attention in the public media because it provides a means to reverse CO2 emissions, can appear to be a relatively “easy fix” to climate change, and is a relatively new and high tech negative emissions technology (NET). In addition to negative emissions potential, direct air capture systems benefit from their inherent flexibility of placement, which can reduce the need for pipelines1 from the capture site to the sequestration reservoir. Furthermore, direct air capture systems have the flexibility to produce CO2 for the commodity market at a desired purity. However, thermodynamics sets a lower bound on the energy required to separate a mixture of gases. Dilute streams are more difficult to separate and require more energy than more concentrated mixtures. A discussion of the thermodynamic limitations appears in Appendix D. The approaches to direct air capture outlined in this chapter are technically feasible, but because CO2 in air is ~300-times more dilute than from a coal- fired power plant flue gas, the separation process for the same end CO2 purity will likely be costlier than capture from fossil fuel power plants. CO2 removal from gas streams is an important component of many industrial processes. The choice of removal technology is governed by the concentration and pressure of the gas stream. Physical solvents are used at high concentrations in natural gas processing and chemical production. Lower concentrations require use of chemical bases that react with CO2, a Lewis acid. Among the simplest of these are hydroxides and amines. These can be introduced either as components of a liquid (usually aqueous) solution, or as functional groups on the surface of a high surface area solid material. Thus, CO2 can be captured from dilute gas streams, including air at ~400 ppm CO2, by contact with basic liquids and solids. However, capture is only the first step. For manufactured2 direct air capture systems, it is essential that the capture agent, either liquid or solid, is capable of releasing CO2 at conditions of temperature and pressure that are accessible with low energy input, so that the capture agent can be used repetitively, and to prepare CO2 for some form of secure sequestration. Capture generally happens spontaneously using these chemical agents, and the more significant energy costs are incurred in the step that recovers and concentrates the captured CO2. Capture is generally an exothermic process and desorption for concentration is an endothermic process. In this chapter, two types of direct air capture CO2 separation processes are assessed: one employing 1) liquid solvents and one utilizing 2) solid sorbents. Material and energy balances are carried out and compared to quantify the net reduction of CO2 from the atmosphere depending upon the energy 1 Approximate cost of CO2 transportation via pipeline is $2.24/tonne CO2 per 100 km of dedicated pipeline (ref NETL) . 2 Manufactured DAC systems are those that utilize chemical or physical processes that are designed to capture CO2 from the ambient air. The adjective, “manufactured,” serves to distinguish such approaches from those that reply on natural phenomena such as CO2 uptake by plants or minerals in the natural environment. PREPUBLICATION COPY 131

132 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda sources assumed (e.g.; renewables, nuclear, natural gas, or coal). This analysis assists in identifying the technical challenges of each capture process to identify opportunities and needs for future research and development. Discussion and analysis of CO2 compression, transport, and subsequent geologic sequestration are covered separately in Chapter 7 on Geologic Sequestration and Appendix F. In addition to estimates of annual CO2 reduction potential, cost and capacity estimates associated with the various capture processes are also provided. BACKGROUND Economics (Literature Review) The cost of carbon capture for direct air capture systems has been a contentious issue. Literature estimates of direct air capture systems span roughly an order of magnitude, from 100 to 1000 $/t CO2 (Ishimoto et al., 2017). These estimates represent CO2 capture costs and not the cost of net CO2 removed from the atmosphere, with these costs tending to put direct air capture approaches among the most expensive atmospheric carbon dioxide removal options. One of the challenges in comparing estimates is that earlier reports often used different system boundaries; for example, not all steps needed for a complete cyclic process are accounted for in all studies. Some have utilized generic correlations for process operations; while others have carried out detailed optimizations of specific systems. As progress continues to be made on pilot and demonstration plants, more accurate measures of actual costs can be expected to become available. Estimates at the high end of the cost spectrum (1000 $/t CO2, House et al., 2011) did not assess a specific technology. Rather, they made estimates based on direct air capture energy requirements and assumed second-law efficiencies applied to the calculation of minimum separation energy based upon 75% air capture and 95% CO2 product. A range of energy resource costs from wind to natural gas were considered, leading to an approximate upper estimate of 1000 $/t CO2. Estimates of 641–819 $/t CO2 based on a benchmark liquid system were put forward in the first report to assess direct air capture, produced by the APS (Socolow et al., 2011). While comprehensive in its analysis, key limitations were introduced by the benchmark system chosen in the report. This system conceptually adapted the technology for CO2 capture from flue gas streams—countercurrent flow of gas and liquid caustic solutions in a packed column—to CO2 capture from air. Because of the much lower concentrations of CO2 in the latter case, the volume of gas flow per tonne of CO2 captured is much larger and the power requirements to overcome the pressure drop in the vertical packed tower configuration contribute to both significant capital and operating costs. While optimization of the operating conditions of this process design can reduce costs somewhat, estimated as 528–579 $/t CO2 (Mazzotti et al., 2013) and 309–580 $/t CO2(Zeman, 2014), the basic geometry and gas-liquid contact scheme remained the same; such designs are now recognized as not broadly applicable to direct air capture systems. As has been pointed out by several studies, altering the flow configuration to reduce pressure drop can dramatically reduce capture costs compared to the APS report benchmark system, which is based upon a more conventional approach mimicking postcombustion capture absorber technology. Holmes and Keith introduced a cross-flow scheme for the gas relative to the falling liquid combined with a novel scheme involving the co-capture of CO2 from air combined with an oxy-fired natural gas regeneration in a carbonate-based capture system, leading to reported costs between 336–389 $/t CO2 (Holmes and Keith, 2012b) and 93-220 $/t CO2 (Keith et al., 2018). For solid adsorbents, low pressure- drop configurations analogous to the “honeycomb” structure of monoliths for automobile catalytic converters and other ultra-low pressure drop configurations are preferred motifs (Realff and Eisenberger, 2012). These novel configurations will require further testing and demonstration in order for the lower- price points to be realized. Laboratory studies of processes based on both solid and liquid sorbents have tended to estimate lower operating costs. Examples include amine-functionalized sorbent processes estimated at 82–155 $/t PREPUBLICATION COPY

Direct Air Capture 133 CO2 (Kulkarni and Sholl, 2012), though this study only considered operating costs. Earlier cost estimates based on aqueous chemical capture designs were similar, at 60–145 $/t CO2 (Stolaroff et al., 2008) and 165 $/t CO2 for a complete system excluding sequestration (Keith et al., 2006). However, caution should be taken when making comparisons from study to study, as the completeness of the system considered, and the purity of the CO2 stream produced, vary among them. Commercial Status There are several companies aiming to commercialize direct air capture systems today. A list of these companies is provided in Table 5.1, who have focused mostly on units that operate on the scale of 1 Mt/y CO2 capture from the air. These companies are primarily privately funded. Many manufactured 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), and purity of the output CO2 stream. While pure (> 99%) CO2 is desired for geological storage or sequestration, more dilute streams containing 3–5% CO2 can still be useful for supply to enclosed greenhouses and algae farms (Wilcox et al., 2017). While commercial entities need to monetize CO2 to offset R&D costs and grow their business, it is important to note that if CO2 is separated from air to be utilized, the CO2 is required to be sequestered on a time scale that positively impacts climate to be considered a negative emissions technology. Of the companies listed in Table 5.1, all but Carbon Engineering utilize capture by amine (or ammonium) based solid sorbents, although some are considering other kinds of structured solid sorbents in their continued development; Carbon Engineering’s process involves aqueous hydroxide solutions that react with CO2 to precipitate a carbonate salt. Most approaches rely on heating or a combination of heat and vacuum to release captured CO2 from its bound state on the solid sorbent or, in the case of the precipitate in the Carbon Engineering process, to thermally decompose the carbonate. The resulting alkaline oxide, or hydroxide in the latter case, is then re-dissolved in the aqueous solution, thereby restoring its CO2 uptake capacity. Alternative methods of regenerating solid sorbents have been advanced by Wang et al., 2013, as well as companies like Infinitree (humidity swing). In the latter case, after capture under relatively dry conditions, exposure of the CO2-saturated sorbent to humid air under mild vacuum causes release of CO2. TABLE 5.1. Companies working to commercialize direct air capture systems. Company System Type Technology Regeneration Purity/ Scale Application Carbon liquid solvent potassium hydroxide temperature 99% pilot Engineering solution/ calcium 1 t/d carbonation Climeworks solid sorbent amine-functionalized temperature or 99% w/dilution demonstration filter vacuum depending on 900 t/y application Global solid sorbent amine-modified temperature 99% 1000 t/y Thermostat monolith and/or vacuum Infinitree solid sorbent ion-exchange humidity 3-5% algae Laboratory sorbent Skytree solid sorbent porous plastic beads temperature air purification, Appliance functionalized with greenhouses benzylamines (Alesi and Kitchin, 2012) PREPUBLICATION COPY

134 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda At the time of writing, all the companies have technologies that are either in the laboratory stage or have advanced to one-off pilot or demonstration plants. Climeworks has advanced the farthest, operating a 900 t/y demonstration plant in Switzerland where CO2 is used for various applications, rather than storing in geologic reservoirs. ANALYSIS: ENERGETICS, CARBON FOOTPRINTS, AND COSTS In this section, we present analyses of the energetics, carbon footprints, and economics of direct air capture systems based on 1) liquid solvents and 2) solid adsorbents. For both analyses, the following baseline assumptions were made: ● Plant capture rate from air = 1 Mt/y CO2 ● Concentration in air = 400 ppmv CO2 ● Volumetric flow rate ≥ 58,000 m3/s air ● Capture fraction from air ≥ 60+ CO2 ● Concentration of product ≥ 98% CO2 • Emission factors3 o Heat from natural gas = 227 g CO2/kWh o Heat from coal = 334 g CO2/kWh o Heat from nuclear = 4 gCO2/kWh o Heat from solar = 8.3 gCO2/kWh o Electricity from grid (U.S. average) = 743 gCO2/kWh o Electricity from natural gas = 450 gCO2/kWh o Electricity from coal = 950 gCO2/kWh o Electricity from nuclear = 12 gCO2/kWh o Electricity from solar = 25 gCO2/kWh o Electricity from wind = 11 gCO2/kWh • Plant life = 10 years4 When considering the design of a plant capable of capturing 1 MtCO2/yr from the air, one has to carefully consider the energy resources that are used to power the plant to determine the net removal of CO2 from the air. For instance, if fossil fuels are used to supply the energy in the absence of “conventional” carbon capture and storage (CCS), the net removal of CO2 from the air may be significantly reduced. When comparing costs of direct air capture across varying boundary conditions, one method of aligning the estimates for direct comparison is through the use of a cost factor, represented by: Cost Factor = 1/1-x (5.1) such that x is the CO2 emitted per CO2 captured. As x approaches 1, or for every tonne of CO2 captured, a tonne is released, the factor approaches infinity, as does the cost. On the other hand, as x approaches 0, the cost of net CO2 removed becomes closer to the cost of capture. Example technologies that would lead to x approaching zero may include the use of 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 instance, in the case of the liquid solvent approach, one of two considered in this study, temperatures of up to 900°C are required for regeneration. Technologies that may be able to achieve this may include concentrated solar power towers (DOE, 2013), combustion of low-carbon hydrogen, PV or wind-sourced electric 3 Emission factors for all electricity sources is from https://www.nrel.gov/analysis/life-cycle-assessment.html. 4 Same as used by U.S. Department of Energy for baseline power plant studies, such as in Draucker et al., 2010. PREPUBLICATION COPY

Direct Air Capture 135 heating, and alternative designs of nuclear including high-temperature gas-cooled reactors (Harvey, 2017). In terms of solid-sorbent-based approaches to direct air capture, the second technology of focus in the current study, the temperature requirements for regeneration are significantly less (i.e., < 150°C), and hence, the options for providing the low-carbon energy differ, some of which may include, solar thermal, geothermal and light water nuclear reactors. It is also crucial to consider the embodied emissions of the materials required to build a plant capable of operating 1 MtCO2 per year. Although the embodied emissions are not included in the current analyses, the amount of steel and cement for these plants may be non-negligible. The current analysis makes clear that direct air capture, if fueled by low-carbon energy pathways, will have the greatest impact compared to using high-carbon energy pathways. However, fueling direct air capture plants with low-carbon energy resources in place of using those resources to directly replace fossil-fueled based point-source emitters requires careful consideration. The design of both direct air capture approaches outlined below include an air contactor and regeneration facility. In general, a practical process requires five key attributes: 1) Low-cost air contactor—so the contactor area can be large to minimize pressure drop, as the low concentration of CO2 in air requires huge gas volumes to pass through the contactor. 2) Optimal CO2-sorption thermodynamics—relates to having a sorption isotherm with suitably high CO2 uptake at CO2 partial pressures below 500 ppmv to minimize sorbent inventory and overall size of the process. The need for high CO2 uptake at low partial pressures suggests the sorbents need strong, chemical interactions with CO2, in contrast to separation processes that operate at higher CO2 partial pressures, where sorbents employing weaker, physical interactions may be used. 3) Rapid sorption/desorption kinetics—results in fast sorption and desorption, faster cycling, and therefore less sorbent needed for the same output. 4) Low sorbent regeneration energy—the CO2 binding energy must be high enough to achieve a good uptake capacity, but not so high that endothermic sorbent regeneration energy requires unacceptably high regenerator costs. Furthermore, effective process designs are needed to minimize the thermal mass of equipment that is repeatedly thermally cycled between sorption and desorption; that is, the sensible heat of the process should be minimized. 5) Low capital costs—applies to virtually any process but is particularly key for direct air capture systems, with the lifetime of sorbent media posing a potentially important capital cost for some designs. Different approaches were taken in assessing liquid solvent and solid sorbent direct air capture systems below. For liquid solvent systems, the analysis is built from a conceptual process design published by Carbon Engineering (Holmes and Keith, 2012a; Keith et al., 2018). It is important to note that the analysis carried out in this study took place prior to the recent publication of Keith et al., 2018, but after careful examination of their recent work, the committee determined that the current analysis aligns closest to the design “C” configuration, which omits the onsite power island and instead uses grid electricity to supply all electrical work, suitable in regions with available low-carbon electricity. However, this design configuration also assumes compression of CO2 to 15 MPa, while compression is not included in the current analysis. Compression results in an energy of 0.48 GJ/tCO2 and cost of $8/tCO2, assuming an electricity cost of $60/MWh - leading to additional emissions of 0.1 MtCO2 for every MtCO2 captured, assuming an average grid emissions factor of 744 kgCO2 / MWh. In particular, for the regeneration of their capture material, the heat is sourced through burning natural gas in an oxygen-fired kiln in which CO2 is produced through both the combustion of natural gas and the calcination of CaCO3, the material that is in part responsible for removing CO2 from the air. The analysis of this process includes both an assessment of the overall energetics and economics of the combined process, as well as that obtained by direct air capture only, excluding the CO2 produced from fossil fuel combustion. For solid sorbents systems, there are several companies (e.g., Climeworks, Global Thermostat, and Skytree, among others) pursuing this pathway, each developing their own unique proprietary process with different design features. Therefore, rather than analyzing a specific process, a generic sorbent-based process is considered, and key process parameters varied to give a range of energetic and process costs. PREPUBLICATION COPY

136 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 5.1. Simplified process flow diagram of a generic liquid solvent-based direct air capture system. 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). In this process, an aqueous potassium hydroxide solution (KOH) reacts with the CO2 from the air to form water and potassium carbonate (K2CO3) in an air contactor. The potassium carbonate aqueous solution is then fed to a causticizer, where it is reacted with calcium hydroxide (Ca(OH)2) to form calcium carbonate (CaCO3) precipitate. The CaCO3 slurry is then fed to clarificatory and filter press to remove water, before it is fed to a calciner where the CaCO3 precipitate is heated with natural gas in an oxy-fired kiln to about 900°C thereby producing solid calcium oxide (CaO) and high-purity CO2 gas that can be compressed and transported for long-term sequestration. Unit Operations Air Contactor The air contactor is used to contact the air with a KOH aqueous solution such that CO2 reacts to produce potassium carbonate (K2CO3): 2KOH + CO2 → H2O + K2CO3 The ambient air enters the contactor at 400 ppm and exits with 75% CO2 captured in the solvent as K2CO3. Due to the high stability of this product species, a caustization step is required to react K2CO3 with calcium hydroxide (Ca(OH)2) to form calcium carbonate (CaCO3), regenerating the KOH solution for reuse in the contactor. Contactor Sizing: In the air contactor, air is blown using fans over PVC-based packing material like that used in industrial cooling towers, as depicted in Figure 5.2. The solvent is a 1 M KOH aqueous solution that is sprayed uniformly over the packing material. The packing material assumed is Brentwood PREPUBLICATION COPY

Direct Air Capture 137 FIGURE 5.2. Conceptual drawing of the air contactor for a liquid solvent direct air capture system. SOURCE: Holmes and Keith, 2012b. XF12560. Holmes and Keith (2012) determined that a capture fraction of 0.75 CO2 in air was optimal based upon their solvent-based separation process. With an air velocity of 1.5 m/s and 75% CO2 capture from air, the contactor area needed to separate 1 Mt/y CO2 is 38,000 m2. The largest commercial packed towers have areas on the order of 100 m2, which would indicate the need to construct hundreds of towers to achieve 1 Mt/y CO2. Due to this challenge, Holmes and Keith have proposed adopting technology used in large-scale cooling towers and waste treatment plants. Their optimal air contactor design is approximately 20 m × 8 m × 2 00m and ten contactors would be needed to capture 1 Mt/y CO2, a considerable improvement over a conventional packed tower. 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 will be significantly different from that of a conventional coal or natural gas post-combustion carbon capture plant. Pressure Drop: When calculating the pressure drop, one has to consider the packing material composition (e.g., metal, plastic, ceramic) in addition to the nature of the air flow through the wetted packing material. For post-combustion applications, the flow is often modeled as counter-current (APS 2011; Mazzotti, 2013), while in the work of Keith et al. (2012, 2018), it has been modeled in a cross-flow configuration. The literature provides a number of pressure-drop correlations for conventional metal packing material with counter-flow configurations, but to the authors’ knowledge there does not exist in the open literature any pressure-drop correlations for the PVC packing material with the cross-flow configuration as described in Keith et al. (2012, 2018). For this reason, in the current work, a range in fan power energy consumption is established by considering separately the pressure drop associated with stainless steel and PVC packing materials. In the work of Keith et al. (2012, 2018), the pressure-drop across the packed tower is based on the following correlation developed specifically for the PVC packing material, Brentwood XF12560: . ∆ = 7.4 where ΔP is the pressure drop in (Pa), D is the column depth in (m), and v is the air velocity in (m/s). Based on Holmes and Keith’s design (v = 1.5 m/s, D =, 6–8 m), the resulting pressure-drop ΔP = 106–141 Pa (1.0–1.4 mbar). Mazzotti et al. (2013) showed that a novel, stainless steel packing material designed specifically for post- combustion capture may achieve a ΔP = 380 bar (v = 2.57 m/s, D= 3.6 m). It should be noted that this pressure drop was derived for a counter-current flow contactor where air velocity and capture fraction PREPUBLICATION COPY

138 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda were treated as optimizable variables. Implications behind the choice of packing material will be discussed in greater detail in the process economics section. Fan Work: From the pressure drop, the fan power (MW) required to drive 58,000 m3/s flowing air through the contactor can be calculated from: ∆ = where is the volumetric flow rate (m3/s) and is the fan electrical efficiency (60% assumed). This yields an air contactor fan work = 10–37 MW. Thus, for carbon capture rate of 1 Mt/y CO2, the fan energy required is 0.32–1.18 GJ/t CO2 (14.2–52.5 kJ/mol CO2) captured. This equates to 0.073–0.269 Mt/y CO2 emissions from coal-fired power and 0.044–0.160 Mt/y CO2 from natural gas-fired power, resulting in an average annual net CO2 capture of 0.83 Mt/y and 0.90 Mt/y, for coal and natural gas power to the fan, respectively. Water Loss: Depending on the molarity of the hydroxide solvent and the relative humidity, the water loss in the air contactor could be 1–30 mol/mol H2O per CO2 captured. Stolaroff et al., 2008 showed that increasing the concentration of hydroxide resulted in less water loss. Specifically, water loss was nearly eliminated for a ~ 7.2 M NaOH at 15°C and 65% relative humidity. However, Holmes and Keith, 2012b estimated the minimum hydroxide concentration to mitigate water loss was 2 M KOH. Stolaroff et al., 2008 showed that a water loss of 20 mol/mol H2O per CO2 captured is typical for low concentration hydroxide solutions (e.g., 1.3 M) at 65% relative humidity. 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 potassium hydroxide (KOH) for even distribution across the packing material, the pressure drop, volumetric flow rate, and liquid density are required. This information was not available for the system of Holmes and Keith, but they presented a rule of thumb in that the energy required for fluid pumping is approximately 15% of that required of the fan energy. This equates to between 0.048–0.065 GJ/t CO2 (2.13–2.84 kJ/mol CO2) captured. The additional CO2 generated by using coal or natural gas to generate electricity for solvent pumping, results in 0.013 and 0.0077 Mt/y CO2, respectively. Slaker In the slaker, CaO reacts with H2O exothermically to regenerate Ca(OH)2, which is reused in the causticizer: CaO + H2O → Ca(OH)2 Inert grit may be produced in the slaker and this impacts the efficiency of this step. Grit production depends on particle size, temperature, and the type of equipment used (Hassibi, 1999). Work for the slaking process has been estimated at 0.005 GJ/t CO2 (0.2 kJ/mol CO2) (Baciocchi et al., 2006), while efficiencies in the literature range between 0.95–0.99 (Emmett, 1986). Using an average efficiency, slaking work contributes additional emissions of 0.001 and 0.0007 Mt/y CO2 when the electricity is sourced from coal and natural gas, respectively. Despite the exothermic nature of the slaking reaction and heat exchange that occurs between the slaking and causticization steps, this low-grade heat may not be not easily integrated and becomes difficult to consider in the cumulative energy for regeneration. PREPUBLICATION COPY

Direct Air Capture 139 Causticizer In the causticizer, an K2CO3 aqueous solution is pumped from an exit stream in the air contactor and reacted with Ca(OH)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) range between 0.8–0.9, which means that there will be increased energy requirements to account for the additional processing needed to compensate for non-ideal conversions (Mahmoudkhani and Keith, 2009). However, the causticization step has negligible work requirements compared to other steps in the regeneration cycle (Baciocchi et al., 2006); thus, incremental changes in work due to causticization efficiency are manifested in downstream processes (e.g., clarification and filter press). Following the causticization reaction, the supernatant KOH (aq) solution is clarified, mixed with additional reclaimed solvent and pumped back to the absorber. The required work in the clarification step is estimated to be 0.109 GJ/t CO2 (4.8 kJ/mol CO2) assuming ideal conversion efficiencies in upstream processes (Baciocchi et al., 2006). Adjusting this work value for realized slaking and causticization efficiencies results in emissions of 0.025 and 0.015 Mt/y CO2 for coal and natural gas, respectively. Precipitated CaCO3 is filtered, thickened, and pressed in preparation for transport to the kiln for calcination. Heating and drying of the CaCO3 is necessary to remove as much water content as possible before passage to the energy-intensive calcination step. This preparation is also energy intensive, requiring an estimated 3.18 GJ/t CO2 (140 kJ/mol CO2), or the equivalent of 0.30 and 0.20 Mt/y additional CO2 emissions using heat derived from coal and natural gas, respectively. Calciner Following filtration, clarification, and drying CaCO3 must be heated to high temperatures (~ 900°C) in a calciner to form calcium oxide (quicklime) and highly concentrated CO2: CaCO3 → CaO + CO2 After calcination, the quicklime is returned to the slaker, where it reacts with water exothermically to regenerate Ca(OH)2 and heat the slaking solution to about 95°C (Baciocchi et al., 2006). Though it low-grade heat such this is often difficult to integrate, a separate CaCO3 steam drying process using heat recovered from lime hydration could offset thermal requirements for drying by 2.39 GJ/t CO2 (105 kJ/mol CO2) (Zeman, 2007). In addition, calcination efficiencies of over 0.9 have been reported in the literature (Stanmore and Gilot, 2005; Martinez et al., 2013). Heat requirements reported in the literature (Baciocchi et al., 2006; Zeman, 2007) for the calcining process range between 6–9 GJ/t CO2 (264–396 kJ/mol CO2). This includes an efficiency factor of 0.75 for the direct use of thermal energy. Due to this large thermal requirement, CO2 emissions associated with traditional calcination processes are significant, ranging between 0.38–0.57 and 0.56–0.84 Mt/y CO2 for natural gas and coal firing, respectively. To minimize CO2 generated in the manufactured direct air capture process, any thermally generated CO2 could, in theory, become co-captured with that from ambient air. However, the balance of post-kiln exhaust is largely nitrogen and if the end goal is to produce a near-pure (≥ 99%) CO2 stream, additional CO2 separation equipment is required. Oxygen-fired (oxy-fired) kilns can obviate the need for additional CO2 separation equipment, because they produce an exhaust that is composed of only CO2 and H2O; allowing the production of a near-pure CO2 stream after the water is condensed out. For heat recovery from the calciner, a heat-exchanger is used to cool the 900°C flue gas exiting to 200°C with the incoming gas. Then, the 200°C flue gas is passed through a condenser and further cooled to 30°C. Pure oxygen for the oxy-fired kiln is separated from air using an air separation unit (ASU), where its PREPUBLICATION COPY

140 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda electric requirements are 0.30 GJ/t CO2 (13.2 kJ/mol CO2), leading to a footprint of 0.068 and 0.041 Mt/y CO2 using electricity-derived from coal and natural gas firing, respectively.5 Chemical Make-up Reagent loss may occur at several points in a liquid solvent direct air capture process. Due to the nature of direct air capture, foreign contaminants may enter the absorber (insects, birds, particulate matter, NOx and SOx) than can accumulate and combine with Ca-ions to form undesirable products. Further Ca- ion loss can occur during filtration and kiln firing. It is preferred to make-up Ca-ion loss with CaCO3 because of its relatively low cost (200 $/t delivered) and smaller carbon footprint than other lime products, such as quicklime (CaO) and slaked lime (Ca(OH)2)6. In addition to Ca-ion losses, KOH may be lost in the absorber through aerosol formation and spray drift (Keith and Holmes, 2012. Operational expenditures for chemical make-up have been estimated to be 0.90 $/t CO2 captured (Socolow et al., 2011). Assuming this cost is split into 0.20 $/t CO2 for KOH (aq) and 0.70 $/t CO2 for CaCO3(s), results in make-up requirements of 400 t KOH/y7 and 3,500 t CaCO3/y, respectively. Given that KOH is produced in the energy intensive chloralkali process (7 GJ/t KOH), make-up from KOH production yields a footprint of 590 t/y CO2. Emissions from CaCO3(s) make-up may be attributed to vehicle emissions from delivery (0.11 kg/t-mile CO2), accounting for the round-trip, and any additional disposal required from waste build-up in the loop.8 Mass and Energy Balance Estimated energy requirements for liquid solvent direct air capture systems are given in Table 5.2, depicted in Figure 5.3, and totals of 0.74-1.66 GJ/t CO2 and 9.18-12.18 GJ/t CO2, for electricity and thermal energy requirements, respectively. The fans, pumps, slaker, causticizer/clarifier, and air separation units are all assumed to run off of grid-sourced electricity, while the heater/dryer and calciner represent the thermal requirements of the overall system. As shown in Figure 5.3, collectively, the energy required from running the electric components of the system totals between 7 and 12% of the entire energy demand for the process. In particular, the dominant energy-intensive component of this process is the thermal regeneration of calcium oxide and subsequent production of high-purity CO2, followed by the step of heating and drying of CaCO3. These steps collectively reduce the net CO2 captured to between 0.11-0.42 Mt/y CO2 if natural gas is used as the thermal energy source and 0-0.11 Mt/y CO2 for coal. In other words, using coal as the thermal source results in nearly the equivalent emissions of CO2 as that captured. These estimates include a thermal credit of 1.5 GJ/t CO2 from the cooling of the calciner exhaust unit. Due to the uncertainty associated with a well-defined system that could recover the heat generated from the hydration reaction of the steam drying process, this credit was not included in these estimates. This is also a primary difference between the analysis of the current work and that of Keith et al. (Joule, 2018) since they assume significant heat integration resulting in an average thermal work requirement of 5.25 GJ/tCO2 compared to the lower bound of 8.4 GJ/tCO2 in the current work. Incorporating heat recovery methods as described in Zeman (2007), this may be reduced to 6 GJ/tCO2, but given the lack of clarity availabile in the open literature of the heat integration approaches, they were excluded from the current study. It is important also to note that since the calciner is oxy-fired, this approach in particular co-captures the CO2 from air in addition to that generated from burning natural gas 5 Based on 200 kWh per tonne O2 produced in the ASU, and 0.56 mol O2 per mol CaCO3 supplied to the calciner. 6 Emissions from CaCO3 mining range from 1.5 to 80 kWh/t, resulting in negligible CO2 emissions when compared to other steps outlined in this section. 7 Based on a bulk purchase price of $506.5 /t NaOH [Integrated Environmental Control Model (IECM)]. 8 This disposal may be considered analogous to reclaimer waste disposal in MEA regeneration ($260/t) [IECM]. PREPUBLICATION COPY

Direct Air Capture 141 TABLE 5.2. Liquid solvent direct air capture system unit operation energy requirements and CO2 generation. Energy Required CO2 Generated (Mt/y) Unit Operation (GJ/t CO2) Natural Gas Coal Contactor Fans 0.32–1.18 0.044–0.160 0.071–0.095 Solvent Pump 0.048–0.065 0.007–0.009 0.011–0.014 Slaker 0.005 0.0007 0.001 Causticizer/Clarifier 0.109 0.015 0.028 Air Separation Unit 0.30 0.041 0.028 Heater/Dryer 3.18 0.20 0.30 i Oxy-fired Calciner 6.0–9.0 0.38–0.57 0.57–0.85i Exhaust Gas Cooling -1.5 -0.11 -0.15 ii iii Additional heat Recovery -2.4 — — Total (w/o gas cooling credit) 9.9–14 0.69–1.00 1.00–1.31 Total (w/ gas cooling credit) 8.4–12.5 0.58–0.89 — i Emissions co-captured with that from ambient air. ii Heatrecovered from hydration of CaO for use in CaCO3 drying (Zeman, 2007). iiiNeglected in process total. 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). for the heat source. Taking this particular approach into consideration indicates that an additional 0.38- 0.57 Mt CO2/y can be produced at high-purity along with that captured directly from the air. Assuming an initial atmospheric concentration of CO2 at 400 ppm at 25°C, the minimum work of capturing 75% of the CO2 at a 98% purity is 0.45 GJ/t CO2 (20 kJ/mol CO2). Based upon the energy PREPUBLICATION COPY

142 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda requirements outlined for liquid solvent direct air capture systems, the “real” work is 8.4–13 GJ/t, leading to an exergy efficiency9 of 3.5–5.3%. Process Economics In assessing the costs and benefits of direct air capture, an area of contention has been the broad- range of costs reported in the literature. Cost comparisons made without first normalizing conditions and boundaries are misleading; thus, it is important to emphasize that the cost estimates presented here are for the separation and capture of CO2 from ambient air from modestly optimized, generic direct air capture systems operating at 75% capture with highly concentrated CO2 the product (~ 98% purity), which is needed to minimize compression costs and volume requirements for geological sequestration. These cost estimates reflect the total annual economic penalty incurred for removing 1 Mt CO2 from air on a per tonne CO2 captured basis. However, additional CO2 emissions can be generated in several of the steps required in manufactured direct air capture systems. It is important to account for these emissions directly in the avoided cost expression by assuming a penalty for any emissions generated. This cost of CO2 avoided is always higher than the cost of CO2 captured and approaches infinity as the amount of CO2 generated during capture approaches the amount captured. It is important to account for these emissions directly in the net cost expression by assuming a penalty for any emissions generated. This net cost of CO2 removed is always higher than the cost of CO2 captured and approaches infinity as the amount of CO2 generated during capture approaches the amount captured. The cost estimates presented here also vary by energy source and do not include costs for compression, transportation, injection, and sequestration.10 The estimated capital and operating costs for a 1 Mt/y CO2 liquid solvent direct air capture system is provided in Table 5.3. For this cost analysis, an optimistic scenario is presented based on optimal parameters (for instance, Keith et al., 2012 and Keith et al., 2018), where co-dependent parameters are jointly optimized to minimize system cost. Here, any literature values on installed equipment costs are taken directly, whereas direct equipment costs are multiplied by a 4.5 factor to convey total installed cost (Rudd and Watson, 1968). A realistic case is presented whereby parameters are set at their respective upper-bounds as indicated in Table 5.3. A realistic worst-case scenario still aims to minimize cost through single and joint-parameter optimization, but additional factors (e.g., higher cost of electricity, extent of heat integration, new technology multiplying factors, equipment quotes, etc.) elevate additional cost components leading to a higher total cost. These estimates yielded capture costs of 147- 264 $/t CO2 for natural gas-fueled systems and 140–254 $/t CO2 for coal-fueled systems (Table 5.4). The estimated net costs per CO2 removed were calculated to be 199-357 $/t CO2 for natural gas-fueled systems and approaches infinity for the case of coal-fueled systems, since more CO2 is generated than captured. Although the current work does not account for compression of CO2, this would add on the order of $8/tCO2 to the cost of net removal in order to directly compare to the cost numbers reported in the literature that do account for compression (APS, 2011; Mazzotti et al., 2013; Keith et al., 2018). The cost of net CO2 removed reported in the current work (199-357 $/tCO2 removed) may be compared to the avoided costs reported in the APS study (641–819 $/t CO2 avoided) and the related follow-up study of Mazzotti et al. (510 – 568 $/tCO2 avoided). Mazzotti et al. considered three Sulzer packing materials: Mellapak-250 Y (also used in APS 2011), Mellapak-500 Y, and Mellpak-CC, a novel stainless-steel packing material designed specifically for carbon capture. Optimization of the system around a specific packing material (Mellapak-250 Y) resulted in a 7% lowering of the avoided cost: $610/tCO2 (APS 2011) vs. $568/tCO2 (Mazzotti et al. 2013). Use of the advanced packing material (Mellapak-CC) resulted in an 9 Exergy efficiency is defined as the ratio of minimum work to real work: Wmin/Wreal. 10 Given that costs for compression, transportation, injection and storage for CO2 captured both through BECCS and direct air capture are assumed to be approximately the same, the report discusses them once in Chapter 7 on Geologic Sequestration and Appendix F. PREPUBLICATION COPY

Direct Air Capture 143 TABLE 5.3. Estimated capital (CAPEX) and operating (OPEX) 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 (2018), based on optimal percent capture of 75% and bed depth of 6-8 m and PVC packing material at ca. $250 /m3. Upper bound: projected cost of re-optimized Keith and Holmes configuration using stainless steel packing ($1500 / m3), shallow packing bed (3 m) and 1.5 × new technology cost factor. Slaker, Causticizer, 130–195 Lower bound: capital costs taken from Socolow et al., 2011 and Clarificator 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 manufactured direct air capture system. Air Separation Unit & 65-100 Lower bound: calculated from scaled CAPEX reported for ASU in Condenser the IECM (S.Rubin et al., 2007) IGCC process. Upper bound: 1.5× factor applied for integration with calcination in manufactured direct air capture system. Condenser cost scaled from IECM estimate and assumed negligible ($300,000) relative to ASU and other components Oxy-fired Calciner 270–540 Lower bound: price quote from industry source with 4.5× factor used for scaling ISBL equipment costs to full costs (Socolow et al, 2011). Upper bound: calciner price quoted in Socolow et al., 2011, with 4.5× factor applied. Note: one would need an oxy-fired natural gas and coal kilns for each case and commercial viability of these are unknown. CAPEX Subtotal ($M) 675-1255 CAPEX Annualized ($M/y) 81-151 Assumes a plant-life of 30 years and fixed charge factor of 12%. OPEX Cost ($M/y) Maintenance 18-33 Range calculated as 0.03 of total capital requirement. Labor 6-10 Range calculated as 0.30 of maintenance cost. Makeup & Waste Removal 5–7 Lower bound: assumes $500/ t KOH, $250/ t Ca(OH)2, $0.30/ t H2O, $260/ t waste disposal (Rubin et al., 2007). Upper bound: applies 1.5× factor to make-up OPEX. Natural Gas 25–35 Range calculated from low and high thermal requirements reported in Table 5.2, assuming NG 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. average bituminous coal, $48.40/ short ton, or $2.33 /GJ. Electricity 12–28 Range calculated from electrical requirements reported in Table 5.2, with electricity price of $60/MWh. OPEX Subtotal (NG) 66-113 OPEX Subtotal (coal) 59-103 even lower avoided cost of $510/tCO2. Both Mazzotti et al. and APS assume a counter-flow configuration in the development of the pressure drop relationship, which directly relates to the fan power required. This is different from that reported in Keith et al. (Joule, 2018), which is based on a novel PVC-based packing material with a pressure-drop correlation assuming a cross-flow configuration. This plastic packing is approximately 1/6 the cost of the metal packing assumed in APS and is expected to have a significantly lower pressure drop (ca. 10 pa/m) when compared to more commonly examined metallic packing materials (ca. 100 pa/m). If the plastic packing proves to be durable enough to withstand the PREPUBLICATION COPY

144 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda TABLE 5.4. Summary of carbon capture costs for a liquid solvent direct air capture system powered by natural gas or coal. Cost ($/tCO2) Natural Gas Coal Capture Costi 147-264 140-254 Net-Removed Costii 199-357 ∞ Produced Cost, Oxy-Fired Calcineriii 113-203 ∞ i Basis = 1Mt net CO2 removed from air. ii Basis = per net unit of CO2 removed with an average of 0.3 MtCO2 for natural gas and zero for coal iii Basis = per net unit of CO2 produced including co-capture of CO2 from natural gas oxy-fired kiln with an average of 1.3 MtCO2. caustic solvent over the life of the plant, the APS CAPEX estimate would decrease by nearly $15/tCO2 before considering system optimization. Additionally, a 2/3 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. This emphasizes the need for demonstration- scale projects in this field so that novel materials for packing such as plastics coupled to unique configurations such as cross-flow, can be tested and verified. An additional difference in the APS system design is the vertical absorber approach with an array of 330 squat scrubber towers with a total cross-sectional area of 37,000 m2. Though quoted at 50% capture, the higher air velocity (2.0 m/s versus 1.5 m/s considered here) yields a cross-sectional area comparable to the system described in this report (38,000 m2). However, the design of 330 squat towers is shown to be capital intensive with a total installed cost of $1.3B—roughly 60% of the total system cost. Conversely, Keith and Holmes (2012) demonstrated a total installed cost of about $150M for an array of 10 air contactors with the design shown in Figure 5.2. Finally, the APS reported a calciner with an installed cost of $540M. However, industrial calciners with output compatible with 1Mt/y CO2 capture systems may be purchased for about $60M, leading to a total installed cost of $270M—50% less than reached by the APS study. As previously discussed, Keith et al. (2018), suggest that the cost differences are likely due to in part to the design configuration decisions, such as PVC packing coupled to a cross-flow configuration compared to metal packing coupled to a counter-flow configuration, with the former resulting in a lower pressure drop in addition to reduced capital expense in addition to the horizontal absorber design and extensive heat integration. It should be noted that although new materials and configurations may result in reduced costs, without the opportunity to test them under realistic conditions (e.g., real environment and extended time), it will be difficult for the lower bounds of these cost estimates to be realized. The current work accounts for these previous studies (APS, 2011; Mazzotti et al., 2013; Keith et al., 2018) and provides a broad range of energies and costs that encompasses all of the steps in the solvent-based separation process. The broad range of energies and costs confirms the need for research and development in this space so that a true baseline cost for DAC may be established. In addition to considering natural gas and coal resources for fueling the direct air capture plant, in an attempt to minimize “x” in the cost factor of Eqn 5.1, a low-carbon route based on solar PV and electrolytic H2 to meet the power and heat requirements, respectively, was also considered. An additional route based purely on solar PV with the assumption of using an electric-fired kiln for the calcination process was also investigated, with the cost details presented in Appendix D. Table 5.5 shows the cost breakdown of the capital and operating and maintenance based on this low-carbon scenario, which results in an average net removed CO2 cost range of $317-501/tCO2. The primary difference in terms of the capital expense of this pathway is the replacement of an oxy-fired kiln with the H2-fired kiln , the absence of an air separation unit, an electrolyzer for H2 production, a compressor and pressurized storage tank for on-site H2 storage, and the installation of PV modules, inverters, and battery storage for on-site electrical generation. The energy required for operating fans, solvent pumps, the slaker, causticizer/clarifier, and gas cooling unit as detailed in Table 5.2 are used as input parameters to determine the energy costs of PV solar including battery storage so that the system PREPUBLICATION COPY

Direct Air Capture 145 TABLE 5.5. Economic costs associated PV, Storage, and H2-Fired Calciner for Solvent-Based direct air capture CAPEX Cost ($M) Comment Contactor Array 210–420 Lower bound: reported cost of air contactor array from Holmes and Keith (2018), based on optimal percent capture of 75% and bed depth of 6-8m and PVC packing material at ca. $250 /m3. Upper bound: projected cost of re-optimized Keith and Holmes configuration using stainless steel packing ($1500 / m3), shallow packing bed (3 m) and 1.5× new technology cost factor. Slaker/Causticizer/ 130–195 Lower bound: capital costs taken from APS 2011 report Clarificator 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 manufactured direct air capture system. H2-Fired Calciner 360–720 Lower bound: price quote from industry source for oxy- fired kiln with 6× factor used for scaling ISBL equipment costs to full costs and to account for new technology. This may be too low due to the uncertainty of the commercial-availability of a H2-fired kiln. Efficiency of 95% assumed. Upper bound: calciner price quoted in APS 2011 report, with 6× factor applied to account for new technology. Condenser 0.3 Condenser cost scaled from IECM estimate and assumed negligible ($300,000) relative to other components. Water 1.1 Water Investment @ $2/tonne (@ 3.6×103 - 4.7×103 kmolH2/hr, 5.7×105 tonnes water required per year, assuming negligible losses). Electrolyzer 260–420 Alkaline (mature) $850-1500/kW; assuming HHV of 283.74 MJ/kmol H2 (IEA, 2015b) giving electrolyzer power requirement of 310-525 MW. PV+Battery 865-1465 Direct electricity needs, i.e., 33-73 kJ/molCO2 for direct air capture processing, 430-730 kJ/ molCO2 for electrolyzer, and 51-68 kJ/molCO2 for H2 compression. Assumes total installed cost of $2.2/WAC including PV modules and inverter, with battery storage adding an additional $15/MWh. (Fu et al., 2017). Compressor 22-37 88% efficiency compression to 18MPa, $70/kWH2 (IEA, 2015b; Ogden, 2004). Pressurized Tank 73-207 $236-394/kWH2ii CAPEX Subtotal 1921–3045 Annualized Capital Payment ($M/yr) 230–365 Assumes a plant-life of 30 years and fixed charge factor of 12%. OPEX Cost ($M/yr) Maintenance 58–91 Range calculated as 0.03 of total capital requirement. Labor 17–27 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. PREPUBLICATION COPY

146 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Table 5.5 Continued CAPEX Cost ($M) Comment PV+Battery 6.7–11.3 Assumed as $18/kWac (Fu et al., 2017). OPEX Subtotal 87–136 Cost = Avoided Cost ($/tCO2 yr-1)a PV+Storage+H2-Fired 317–501 a Basis = 1Mt CO2 is operated in a continuous fashion. Further, a water flow rate of 5.7 ×105 tH2O/yr is needed to produce an average of 4.15 kmol H2/hr to produce the heat required for a direct air capture plant designed to remove 1 MtCO2/yr. The energy required for electrolysis dominates the energy operating costs as shown in Table 5.5, followed by the H2 compression energy required. 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. In these systems, air is blown through a solid adsorbent contained within an air contactor, where the CO2 in the air is adsorbed onto the solid adsorbent. Next, the solid adsorbent with CO2 is exposed to heat and/or vacuum to liberate the CO2 from the solid adsorbent. Finally, the solid sorbent is cooled before it is ready to start the process again from the beginning. Owing to the suitability of temperature swing adsorption (TSA) for capturing ultra-dilute species (Lively and Realff, 2016), a generic adsorption process employing either just TSA or TSA in combination with vacuum swing adsorption (VSA) has been assessed to place probable bounds on energy consumption, CO2 emissions, and associated costs for solid sorbent systems.11 In this section, a generic, hypothetical process is described, alongside its estimated energy use and consequent CO2 emissions. FIGURE 5.4. Schematic illustration of two-step, sorbent-based direct air capture process: 1) air contacts the sorbent using a gas-solid contactor (left); and 2) a heating the system with possible application of vacuum is used to desorb the CO2 from the sorbent followed by cooling before returning to the initial state (right). 11 Such a process does not map to the humidity swing approach employed by Infinitree, as outlined in Table 5.1. PREPUBLICATION COPY

Direct Air Capture 147 Unit Operations Adsorption Air is blown through a solid structure (contactor) that contains a suitable CO2-adsorbing material and CO2-depleted air is emitted from the process. In the adsorber, the main contributor to energy use is the electrical energy required for fans to drive air through the contactor containing the solid sorbent. The primary driver for the energy consumption associated with this step is the pressure drop through contactor. This part of the process deviates substantially from the more often studied flue gas separations. Desorption After the solid sorbent has been saturated with CO2, it is moved to the desorber12 where heat (TSA) or heat and vacuum (TSA/VSA) systems are used to desorb CO2 (regeneration) 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) and heat-up the sorbent, contactor and other equipment (ΔHsens), as well as electrical energy needed for vacuum pumps (if employed). Energy consumption in the condenser is deemed negligible, though some heat could be recovered if integrated into steam generation. This was not considered here. Overall, the energetics for this energy-intensive step of the process are the same as for a similar regeneration step in a process targeting a more concentrated feed (e.g. capture from flue gas). Owing to the energy intensity of this desorption step, process design innovations in this step can have a large impact on the overall process efficiency. Designs that give rapid heat transfer, as well as minimize the CO2 partial pressure over the adsorption media, are advantageous, providing both concentration and thermal driving forces for CO2 desorption. Mass and Energy Balance In general, solid sorbent system designs aim to 1) minimize pressure drop for flow through the air-sorbent contactor; 2) minimize contactor mass while maximizing sorbent mass (thus minimizing the sensible heat energy losses); 3) maximize the CO2 uptake; and 4) advantageously manage the water uptake.13 For the generic process considered here, key process parameters were varied within a physically realistic range, Table 5.5. Adapting Realff and Kawajiri’s methodology (Sinha et al., 2017), individual contributors to the energy consumed in the process and the cost of CO2 capture were estimated. Estimated process energy intensities for the generic solid sorbent direct air capture system were obtained by varying each parameter within the range provided in Table 5.5. The calculated thermal and electrical energy requirements are reported in Table 5.6, with the associated CO2 emissions if the energy were provided by coal, natural gas, nuclear, wind, or solar given in Table 5.7 (NREL, 2013). The electrical energy consumption was costed at an average grid price ($0.06/kWh) and the thermal energy cost was derived by considering the extra steam that would have to be produced to replace the electrical energy delivered from the condensing turbine of a power plant (Sinha et al., 2017). The estimated energy consumption falls in a similar range reported for other processes in the literature (Figure 5.5, Broehm et al., 2015). 12 Or the adsorber is switched into desorption mode, if a single unit is deployed. 13 For many sorbents, one will want to minimize water uptake, to minimize the amount of water that must desorbed from the sorbent in each cycle, and its associated energy penalty. However, some adsorbents may benefit by co- adsorbing water, as CO2 uptake may increase, in which case water uptake must be managed advantageously. Water adsorption can also be managed to balance the production of fresh water as a coproduct. PREPUBLICATION COPY

148 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Because of the wide parameter variation that is possible, five scenarios were considered that represent different degrees of process optimization and performance (i.e., best case, low, mid, high, and worst case). The combination of every best-case parameter resulted in the lower bound (1-best case), a scenario that may be unachievable due to correlations amongst various parameters, where optimizing one may necessarily move a second parameter away from an optimum using currently known materials and approaches. Similarly, there are an infinite number of ways to design a poor process with a very high energy consumption, with this scenario (5-worst-case) presented here an example where all the most TABLE 5.5. Model parameters that affect estimated performance of a solid sorbent direct air capture process. Some parameters (inputs) were varied within a physically realistic range, based on literature reports and outputs were calculated from the model. Parameter Units Range Inputs Contactor to Adsorbent Ratio kg/kg 0.10–4.0 Adsorbent Purchase Cost $/kg 15–100 Adsorbent Lifetime y 0.25–5.0 Sorbent Total Capacity (at 400 ppm) mol/kg 0.5–1.5 i Desorption Swing Capacity mol/mol 0.75–0.90 Air Velocity m/s 1–5 Desorption Pressure (VSA) bar 0.2—1.0 Desorption Final Temperature (TSA) K 340–373 Heat of Adsorption (CO2) kJ/mol 40–90 Outputs Adsorption Time min 8–50 Desorption Time min 7–35 ii Mass Transfer Coefficient 1/s 0.01–0.1 Pressure Drop Pa 300–1400 i Fraction of CO2 adsorbed that is desorbed and recovered as product. iiLumped linear driving force coefficient accounting for all resistances, see Appendix D. TABLE 5.6. Estimated unit operation energy requirements for solid sorbent direct air capture systems. Energy Required (GJ/t CO2) Step Type Mid-Range Full-Range (best- (low-high, 2-4) worst, 1-5) Desorption Heat (100°C sat. steam) Thermal 3.4–4.8 1.85–19.3 Air Contactor Fans Electrical 0.55–1.12 0.08–3.79 Desorption Vacuum Pump Electrical (110–140) x 10-4 (4–910) x 10-4 Total 3.95-5.92 1.93-23.09 PREPUBLICATION COPY

Direct Air Capture 149 TABLE 5.7. Estimated CO2 emissions generated by a solid sorbent direct air capture system that removes 1 Mt/y CO2 depending on energy source. Carbon Emissions (Mt/y CO2) Step Energy Source Mid-Range Full-Range (low-high, 2-4) (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) x 10-6 (0.0015–2.8) x 10-5 Wind (0.47–0.7) x 10-6 (0.0059–13) x 10-6 Nuclear (0.47-0.93) x 10-6 (0.0059–14) x 10-6 Natural Gas (1.6–3.3) x 10-5 (0.029–50) x 10-5 Coal (0.35–0.7) x 10-4 (0.0056–10.8) x 10-4 Total Solar / Solar 0.0084–0.018 0.0045–0.066 Nuclear / Nuclear 0.006-0.009 0.0022-0.032 Solar / Natural Gas 0.22–0.30 0.12–1.2 Wind / Natural Gas 0.22–0.30 0.12–1.2 Natural Gas / Natural Gas 0.29–0.44 0.13–1.67 Coal / Coal 0.47–0.74 0.19–2.7 i Emission factors for different energy sources are referenced near the start of this chapter (NREL, 2013). PREPUBLICATION COPY

150 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 5.5. Literature reported energy requirements for solid sorbent direct air capture systems and values calculated in this study (adapted from Broehm et al., 2015). pessimistic values from were used. These two cases are shown for completeness, though the committee does not expect practical operation at either extreme. More realistically, three estimates using parameters in the middle of the range, are given (2-low, 3-mid, 4-high), where the descriptive words refer to anticipated carbon emissions and energy consumption. An overview of the approach used for the calculations is provided in Appendix D, along with specific parameters used for each case. An advantage of many recent solid sorbent based manufactured 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 used should be acquired from low temperature waste heat when such heat sources are suitable and available. Doing so helps to maximize net CO2 removal. Furthermore, use of waste heat could provide important stepping stones for early installations to operate with more privileged economics, potentially off-setting the disadvantage of being early on the technology learning curve. Nonetheless, because deployment needed to impact negative emissions on a global scale will require on purpose heat and power, in all scenarios considered in this chapter, the energy used is sourced exclusively for the direct air capture process, and no assumption of waste heat use is made. 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. The calculated energy requirements suggest that the worst-case scenario (5-worst) would be unable to provide negative emissions under any scenario where fossil energy was used, even those that use renewable energy for electricity, due to extensive thermal energy requirements provided by fossil energy. However, even the worst case scenario provided negative emissions if solar thermal or nuclear energy provided the energy for operation. By contrast, most other scenarios are substantially carbon negative, with the more realistic estimates (2-low to 4-high) being negative even when coal was used to provide all the energy (0.47–0.74 Mt CO2 emitted per Mt CO2 captured). 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. In the near-term one could envision rapid deployment using natural gas to provide PREPUBLICATION COPY

Direct Air Capture 151 thermal energy. Such a scenario yields a process with acceptable negative emissions (0.29–0.44 Mt CO2 emitted per Mt CO2 captured). Negative emissions drop further when renewable electricity is used (Table 5.7) and further still when thermal energy is generated from renewable sources. Nuclear energy provides another low emissions option. Like liquid solvent systems, solid sorbent direct air capture systems have a minimum work of 0.45 GJ/t CO2 required for capturing 60-75% CO2 from air to a 99% pure CO2 stream. Based upon the energy requirements outlined for solid sorbent direct air capture systems, the “real” work is 1.9–23.1 GJ/t CO2, leading to an exergy efficiency range of 2–24%, with the middle range scenarios (2-low to 4-high) being 7.6-11.4%. Process Economics As noted above, the cost estimates presented here are for the separation and capture of CO2 from ambient air from modestly optimized, generic direct air capture systems operating at 65-75% capture with highly concentrated CO2 the product (~99% purity). These cost estimates reflect the total annual economic penalty incurred for removing 1 Mt CO2 from air on a per tonne CO2 captured basis. Because additional CO2 emissions can be generated in several of the steps required in manufactured direct air capture systems, the net costs of CO2 removed are also presented. The cost estimates presented vary by energy source and do not include costs for compression, transportation, injection, and sequestration (see Chapter 7 on Geologic Sequestration). In this section, estimated costs of CO2 capture for the range of scenarios considered are given in Table 5.8 and Table 5.9. The two main phases of the cyclic adsorption process are shown above in Figure 5.4. In the adsorption phase, air is contacted with a solid structure that contains a suitable CO2-adsorbing material, with air depleted in CO2 being the exit stream from the process. In this step, key contributors to the process cost include 1) the energy required to pass the air over or through the adsorbing material, 2) the cost of the adsorbent, and 3) the cost of the contactor and other equipment such as the fans that provide airflow. For routine equipment, like the blowers and vacuum pumps, a factor of 4 was applied to the purchase cost to represent the total installed cost. For more innovative components, such as the gas-solid contactor, a factor of 6 was applied. It is instructive to compare the capital cost for the air-sorbent contactor between the solvent and solid sorbent case. The total capital cost for the contactor in the solvent case ranged from $210-420M. In contrast, for the solid sorbent case, the cost ranges from $13-84M. Considering that the solid sorbent case has a 10 fold higher surface area per volume, the order of magnitude of these costs are similar. In the desorption phase of the process, heat (TSA) or heat and vacuum (TSA/VSA) are applied to the system to induce CO2 desorption and recover a concentrated product. This second step incurs substantially more operating costs, including costs associated with the energy needed to induce desorption (ΔHads), the energy required to heat the sorbent, contactor and other equipment (ΔHsens), and energy necessary to operate the vacuum pump, if such a pump is employed. Amongst capital costs assigned to this step, the cost of the pump and condenser are included, whereas other costs are accounted for in the first step. Adapting the methodology described by Realff and Kawajiri (Sinha et al., 2017), the individual contributors to the cost of CO2 capture were estimated for different parameter sets. Carbon capture costs from 18 $/t CO2 and to over 1000 $/t CO2 are calculated for a generic solid sorbent system by varying all parameters within the ranges given in Table 5.5. The combination of every best-case parameter resulted in the lower bound (1-best), a scenario that is likely unachievable due to correlations amongst various parameters, where optimizing one may necessarily move a second parameter away from an optimum using currently known materials and approaches. Unsurprisingly, there are an infinite number of ways to design a poor process that yield costs over 1000 $/t CO2, with the calculated upper bound in this parameter space represented by the worst case (5-worst). As noted above, three other, PREPUBLICATION COPY

152 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda TABLE 5.8. Input parameters used for cost estimates for the generic solid sorbent direct air capture system, with selected outputs. Parameters 1-Best 2-Low 3-Mid 4-High 5-Worst Adsorbent Purchase Cost ($/kg) 15 50 50 50 100 Adsorbent Life (y) 5 0.5 0.5 0.5 0.25 Sorbent Total Capacity (mol/kg) 1.5 1.0 1.0 1.0 0.5 Desorption Swing Capacity (mol/mol) 0.90 0.8 0.8 0.8 0.75 Contactor to Adsorbent Ratio (kg/kg) 0.1 0.1 0.2 1.0 4.0 Desorption Pressure (bar) 0.2 0.5 0.5 0.5 1.0 Outputs Final Desorption Temperature (K) 340 360 360 360 373 Cycle Time (min) 39 16 28 42 26 TABLE 5.9. Estimated annualized capital (CAPEX) and operating (OPEX) 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 Total Cost 18 88 148 228 1080 more realistic scenarios were considered as well (2-low, 3-mid, 4-high). A detailed overview of the approach used for the calculations is provided in Appendix D, and the specific parameter used in each case are given in Table 5.8. A sensitivity analysis of the impact of the various parameters is shown in Table 5.9. All five scenarios are shown, where it becomes clear that the adsorbent CAPEX dominates the overall cost, and by comparison, other capital and operating costs are not substantial cost drivers. This demonstrates the importance of adsorbent cost and lifetime and shows how further adsorbent material innovations can potentially bring down costs. As noted above, the cost estimates span a wide range. Disregarding the lower bound as perhaps not realistically achievable and the upper bound as prohibitively expensive, the middle range of scenarios is perhaps most instructive. These estimates yielded capture costs of 88–228 $/t CO2 for a generic solid sorbent direct air capture system. It is plausible that these costs could be reached within the next decade, noting that Climeworks has reported a cost of capture of about $600/t CO2 for their first generation commercial plant (Daniel Egger, pers. comm., October 11, 2018). From this point, costs can be expected to be reduced as the process design and process operation improve, falling into the range calculated above. Through this analysis of solid sorbent direct air capture systems, the following summary observations can be made. First, processes that are not specifically optimized for direct air capture will result in costs that fall within the range estimated by House et al. (≥ $1000/t CO2). Second, direct air capture processes that employ physically realistic process parameters designed for direct air capture systems in mind can offer costs in the range of 100–600 $/t CO2. Lastly, large-scale processes (over 1 PREPUBLICATION COPY

Direct Air Capture 153 Mt/y CO2) employing known materials and gas-solid contactors in the most promising scenarios, could offer costs that approach 100 $/tCO2, though no such large-scale, continuously operating installation is known at this time. Summary of Analysis of Solvent and Solid Sorbent Direct Air Capture Systems The estimated energy required for direct air capture, along with the CO2 footprint of the process and net CO2 removal assuming a plant designed to capture 1 MtCO2 per year as shown in Table 5.10. Both liquid solvent and solid sorbent cases have been considered with varying scenarios associated with meeting the electric and thermal needs of the direct air capture plant. Energy Requirements The thermal component of the energy required to operate an direct air capture plant is dominant over the electric component due to the need to have 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. The strong-binding chemistry is necessary to produce high-purity CO2 from dilute CO2 in the air, i.e., approximately 400 ppm. The thermal requirement for regeneration of the material used for capture may be satisfied by burning natural gas directly, with the heat generated used for regeneration directly or indirectly through the production of steam. Another option for meeting the thermal requirement is H2 combustion, which results in zero CO2 emissions. It is clear from Table 5.10 that the thermal requirement for the liquid solvent system14 is significantly larger than that of the solid sorbent-based approach. This is because the liquid solvent approach involves heating CaCO3 up to 900°C to produce the high-purity CO2, while the temperature required for the solid sorbent regeneration is much lower at approximately 100°C. A range of energy estimates are presented for the solid sorbent-based approach since a number of scenarios have been considered varying a number of the adsorption optimization parameters. It is also important to note that the electric requirement is similar, regardless of the approach. For the solvent-based approach to direct air capture, H2 combustion was also considered, with H2 produced through electrolysis. If using the grid mix of electricity, this approach increases that component significantly; however, one could also source the electricity from carbon-free nuclear, wind or solar, which would maximize the impact of this pathway to direct air capture. Carbon Footprint If the electricity or thermal energy requirements are met using fossil fuels, then this will result in significant CO2 emissions, thereby reducing the effect of direct air capture plant in terms of CO2 removal from the air. A grid mix has been assumed along with scenarios that include carbon-free paths such as nuclear, wind and solar thermal, in addition to fossil-intensive paths such as coal and natural gas. The CO2 generated from meeting the energy requirements increases as one would expect from carbon-free sources such as nuclear, wind or solar thermal, to natural gas, to coal being the energy resource with the greatest CO2 emissions. It is also important to note that since the solvent-based approach regenerates 14 For the particular solvent system studied, deploying high temperature heat. Solvent systems are not inherently disadvantaged compared to solid sorbent systems, and both can operate in high or low temperature regimes if the sorption/desorption chemistry is designed to do so. PREPUBLICATION COPY

154 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda TABLE 5.10. Summary of estimated energy requirements, CO2 footprint, and carbon capture for 1 Mt/y CO2 liquid solvent and solid sorbent direct air capture systems. Energy Required CO2 Generated Net CO2 Capture Cost Direct Air Energy Source (GJ/t CO2) (Mt/y CO2) Removed ($/t CO2) Capture System Therm Net Electric Thermal Electric Thermal Electric (Mt/y CO2) Captured al Removed 0.47- NG NG 0.74-1.7 7.7-10.7 0.11-0.23 0.11-0.42 147-264 199-357 0.66 0.47- coal NG 0.74-1.7 7.7-10.7 0.18-0.38 0-0.35 147-264 233-419 0.66 0.47- Liquid wind NG 0.74-1.7 7.7-10.7 0.004-0.009 0.34-0.53 141-265 156-293 0.66 Solvent 0.47- solar NG 0.74-1.7 7.7-10.7 0.01-0.03 0.31-0.52 145-265 165-294 0.66 0.47- nuclear NG 0.74-1.7 7.7-10.7 0.01-0.02 0.32-0.52 154-279 173-310 0.66 2 solar H2 11.6-19.8 7.7-10.7 0.01-0.03 0 0.99 317-501 320-506 0.008- 88-228 solar solar 0.55-1.1 3.4-4.8 0.0004-0.008 0.892-0.992 89-256 0.01 0.004- 88-228 nuclear nuclear 0.55-1.1 3.4-4.8 0.002-0.004 0.91-0.994 89-250 0.005 0.22- solar NG 0.55-1.1 3.4-4.8 0.0004-0.008 0.70-0.78 88-228 113-326 Solid 0.30 Sorbent3 0.22- 88-228 wind NG 0.55-1.1 3.4-4.8 0.002-0.003 0.70-0.78 113-326 0.30 0.22- 88-228 NG NG 0.55-1.1 3.4-4.8 0.07-0.14 0.56-0.71 124-407 0.30 0.32- 88-228 coal coal 0.55-1.1 3.4-4.8 0.15-0.3 0.26-0.53 166-877 0.44 1assuming the use of an oxy-fired kiln to provide heat from natural gas in the calcination process leading to greater CO2 production and hence, smaller cost of net CO2 removal, using a basis of 1.3 Mt CO2 for NG/NG, 1.2 MtCO2 for coal/NG. 2assuming all hydrogen is produced via electrolysis using near zero-carbon power; 3range spanning from 2-low to 4-high scenarios. CaCO3 in an oxy-fired kiln, it is easily able to capture the CO2 generated from burning natural gas to meet the thermal requirements in addition to maximizing the removal of CO2 from the air (Keith et al., 2018). In fact, on average, an additional 0.5 MtCO2 per year is produced and captured along with the CO2 from air by condensing the exhaust mixture of CO2 and water vapor. It should be noted that any process, in principle, can employ fossil energy with carbon capture from the energy emissions to reduce its carbon footprint at the expense of added capital and operating costs. Such a scenario was considered here for the solvent case simply because it is an inherent part of the Carbon Engineering design. Carbon Removal Cost If fossil-based energy resources are used to provide the energy requirements of a direct air capture system, then to accurately determine the cost of removing CO2 from the air, one needs to consider the net CO2 removed since burning fossil fuels produces CO2. On average, the costs for net CO2 removed range between $89 up to $877/tCO2, depending on which adsorption scenario is considered for the solid sorbent-based approach, while the costs range between $156 to $506/tCO2 for the solvent-based approach, depending on the use of natural gas or renewable H2 for the thermal source. IMPACT POTENTIAL Direct air capture flux and capacity potential has no fundamental physical limit, making its primary limitation financial. The impact potential limit is the investment required to scale direct air PREPUBLICATION COPY

Direct Air Capture 155 capture as well as the availability of geologic storage to sequester the captured CO2. Available pore space must be shared with the CO2 produced from conventional carbon capture efforts in addition to BECCS. It has been stated often in mainstream literature that an advantage of a direct air capture plant is that “it can be placed anywhere.” Although there may be opportunities where direct air capture can be deployed in locations where BECCS cannot, since arable land is not required for direct air capture, thereby indicating that direct air capture may have a competitive advantage in accessing remote pore volumes, one should approach this assessment with caution. This is because for direct air capture to take place on any significant scale (i.e., thousands of tons CO2 removed per year), as discussed in this report, it requires significant infrastructure, energy, and land. At 1 Gt/y CO2 removal and 100 $/t CO2 for combined separation, transport and reliable sequestration, the total investment would be about $100 billion per year or 0.5% of U.S. GDP. At a global scale of 5 Gt/y CO2 removal and 100 $/t CO2, the total investment would be about $500 billion or 0.6% of global GDP. Achieving this rate and scale of CO2 removal will require substantial investments in fundamental research, demonstration, and deployment. To maximize the net emissions removed from the air and the ultimate impact that direct air capture and sequestration could have, the use of renewable energy resources should be maximized where possible. The integration of renewable energy along with base load natural gas, or use of combined heat and power units, could be a cost-effective approach to carrying DACS out at scale. SECONDARY IMPACTS Land Direct air capture systems have much fewer land requirements than afforestation/reforestation and bioenergy with carbon capture approaches and do not require the use of arable land. Therefore, direct air capture impacts on biodiversity would also be much smaller. Consider the Amazon rainforest as an example. The net primary production of the Amazon is approximately 270 km2 per MtCO2/year. With a land area of 5.5 million km2, this equates to an annual CO2 removal of about 20 GtCO2. As will be discussed in this section, the land area requirement for the equivalent CO2 removal using direct air capture is roughly 40x smaller at 7 km2 per MtCO2 if powered by natural gas. If you consider a temperate deciduous forest with a net primary production of 390 km2 per MtCO2/year and an average tree density of 200 per acre, a single tree acts to remove (net), on average, 50 kgCO2/year; in this sense, a 1MtCO2 direct air capture system does the work of 20 million tree equivalents, or a forest spanning 100,000 acres. In general, the land that is required for direct air capture is impacted by the size of the contactor and the spacing requirements of multiple contactors and contactor configuration. The land area estimates discussed in this section are those required to capture 1 Mt/y CO2 at 65-75% capture. Liquid Solvent Systems: In the contactor design of Keith and Holmes, the cross-sectional inlet area is oriented normal to the land surface. This use of vertical space minimizes direct land use per contactor structure. For example, a 4000 m2 inlet area is achieved through a structure containing packing dimensions of 20 m high × 200 m long × 8 m wide. These packing dimensions are a result of a full structural engineering analysis and cost optimization that examined sensitivity to height and width (Holmes and Keith, 2012b). If the packing material is housed in a shell structure that is 110% of the packing dimensions, this leads to a direct land use of roughly 2000 m2 per contactor, or half the inlet cross-sectional area. At 400 ppm CO2 in air and 100% capture efficiency, the capture of 1 t/y CO2 from air corresponds to an air volumetric flow rate = 4.09 × 10-2 m3/s. Assuming an air inlet velocity = 1.5 m/s and CO2 capture efficiency = 75%, an air contactor cross-sectional area = 38,000 m2 is obtained from: = ∙ PREPUBLICATION COPY

156 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 5.6. Direct and indirect land use for a direct air capture system air contactor configuration. When positioning adjacent contactors, care should be taken to allow for the outgas of one contactor to fully equilibrate to ambient conditions before entering the next contactor. At the optimized conditions considered in this study (75% capture and V = 1.5 ms-1), a cross- sectional area of roughly 38,000 m2 is required to capture 1 Mt/y CO2 or the equivalent of 10 air contactor units with packing dimension of 20 m × 200 m × 8 m per unit. The array of multiple contactors needs to be arranged around a centralized regeneration facility and should be positioned to minimize piping and other associated infrastructure costs. An important consideration in contactor arrangement involves the region where CO2-depleted air exits the contactor. To maximize separation efficiency, this region should not feed into the intake of an adjacent contactor. Rather, appropriate spacing is required for proper tropospheric mixing to occur such that air entering an adjacent contactor has fully equilibrated to ambient conditions (400 ppm CO2) (Figure 5.6). A centralized regeneration facility including the causticizer, slaker, calciner, air separation unit, and other auxiliary equipment is expected to have a direct land impact of approximately 20% that of the air contactor array (Keith et al., 2018). When multiple contractors are positioned to minimize piping and other infrastructure costs, the direct land required is approximately 24,000 m2 (~6 acres) including the regeneration facility (Keith et al., 2018). Indirect land use accounts for the spacing between contactors if multiple direct air capture plants are to be constructed in a single area. In a single-plant design, though there would be no adjacent plant, the region of tropospheric mixing may pose risks not yet well understood due to the lower local concentration of CO2. For example, plants of the C3 photosynthetic genotype (80-95% of all species) grown under glacial conditions (CO2 < 200 ppm) 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). To avoid unwanted consequences or potential trophic cascades related to this CO2 depleted region, this land area is assigned as indirect land use regardless of a single or multiple plant design. When indirect land use is considered, the total land requirement jumps by about 300 times to 7 km2 (~1730 acres). PREPUBLICATION COPY

Direct Air Capture 157 The land area discussed excludes consideration of an onsite power island. The average size of a natural gas plant in the United States is 30 acres, or 1400 m2 per MW (Stevens et al., 2017). Indirect land use associated with resource production, but excluding transmission and transportation, increases the land requirement to approximately 8100 m2 per MW, or 2.4 square kilometers for a power requirement of 300 MW. Additional land may be required if on-site renewable energy (e.g., PV solar panels or concentrated solar thermal) is used to offset any portion of the process electric and thermal requirements. The National Renewable Energy Laboratory reports a generation-weighted15 total land use of 3.0 acres GWh-1 yr-1 for concentrated solar thermal, and 5.5 acres GWh-1 yr-1 for small 2-axis flat panel PV power plants (Ong et al., 2013). If solar is used to offset 25% of the electric and thermal requirements, an additional 3600 acres of total land area is required. In the theoretical limit where solar power and CSP is used to offset all electric and thermal requirements, total land use escalates to 14500 acres, or roughly 58.6 square kilometers. 100 such facilities (representing 100 MtCO2 removal per year) would require a land area roughly the size of Delaware. The National Renewable Energy Laboratory also reports land use data for wind generation. Here, due to the wide range of wind configurations and absence of a universally accepted metric for land use in wind plants, the average value for total land area is 40 ±25 acres GWh-1 yr-1. Though this total land area requirement is larger than solar, a key advantage of wind power rests in the ability to use land in between turbines, considering that the turbine footprint is less than 10% of the directly impacted land area. Direct land 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 on-site electrolysis of H2 using solar power. The electric demand here ranges from 400 to 500+ MW for a 1 MtCO2 removal plant. Using the generation-weighted average land intensity for solar-PV power production quoted above, the land footprint for this configuration is 19,250 – 25,500 acres, or roughly 80 to 100 km2. An important consideration in these land area calculations pertains to the inter-contactor spacing depicted in Figure 5.6. The direct land use of a configuration, including the contactor array and regeneration equipment, but excluding land for power, is 0.3% of the total land footprint; thus, a direct air capture operator might choose to use the indirect land space to house—in part—on-site power infrastructure, assuming the presence of such equipment had a negligible impact on the equilibration of contactor outgas to ambient levels (400 ppm). For example, this space may be suitable for the installation of low-lying solar panels, whereas wind turbines could potentially reduce wind speed which would impact both the rate of tropospheric air mixing, and—potentially—the velocity of air entering an adjacent contactor. For optimal land use, is important to understand the impact of different land uses on overall direct air capture plant performance. Solid Sorbent Systems: Similar contactor spacing constraints exist as discussed above for the solvent case. Today, companies developing commercial direct air capture technologies are targeting designs that remove CO2 from the air at areal intensities of ~2–200 kt/y-acre CO2 depending on the technology. This land footprint accounts for the land area for process equipment, areal mixing, and safety margins, with the air/solid contacting equipment covering only a small fraction of the total plant area, typically <5%. For the hypothetical adsorption direct air capture process considered above, a single direct air capture plant would capture ~200-1370 kg CO2/m2-y.16 To capture 1 Mt/y CO2, given the capture rates mentioned above, numerous such units would typically be deployed (scaling out, rather than scaling up). Given the span of capture rates above, a land area requirement of 200-1250 acres of land is estimated for 1 Mt/y CO2, with the middle three scenarios (2-low-4-high) requiring 300-425 acres. These areal requirements 15 Total land area requirements are subject to variability due to location, array configuration, derate factor, and tracking technology, and range from 2–7 acres/GWh/y for small 2-axis PV, and 2–8 acres/GWh/y for concentrated solar power. 16 This is the range spanned by the five scenarios, from 1--best to 5-best. PREPUBLICATION COPY

158 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda consider only the direct air capture plant. To additionally account for local power generation, the land requirements increase to 550–800 acres for natural gas based thermal and electrical energy, or 1355-2450 acres for natural gas based thermal energy and solar based electrical energy, assuming no space in the direct air capture plant footprint can accommodate power generation equipment. Water Water loss in direct air capture processes primarily come from the sorbent-air contacting process. In both the proposed solid and solution-based direct air capture processes, most water use in contained in closed-loop systems, whereby water is continuously recycled. Nonetheless, nearly all processes offer potential for water loss and therefore water use, and this parameter should be carefully considered when any new process is developed. Liquid Solvent Systems: Water loss in the contactor is mainly due to evaporation, with a trivial contribution from drift loss. As outlined in the Section on Energetics and Carbon Footprint, 8.2 Mt of makeup water should be supplemented to offset this loss annually. This value is calculated at 65% humidity, 16 ℃, 2M KOH(aq) solution and may increase if the manufactured direct air capture plant is placed in more arid conditions. Stolaroff et al. 2008 show that if the relative humidity is reduced to 50%, the evaporative water loss increases from 20 mol H2O / mol CO2 to 80 mol H2O/ mol CO2, a 4-fold increase. Higher molarity solutions have a lower vapor pressure and experience lower evaporative losses. The direct air capture operator may choose to mitigate water loss through the adjustment of solvent molarity based on ambient conditions. “Drift” loss is a phenomenon described by the cooling industry as the escape of droplets from the contactor as a result of the cross-flow configuration. Measurements were carried out as described by Keith et al. (2018) indicating that airborne KOH concentrations less than 0.6 mg/m3 air flow out of the contactor, which is below the National Institute for Occupational Safety and Health upper exposure limit of 2.0 mg/m3. Demonstration-scale projects should include further measurements of this kind to minimize the risk associated with the configuration choice. Cooling water is required to condense out water vapor from the calcination flue gas. At a flue gas output of roughly 640 ft3/min, roughly 1300L / min of water is required. This water is largely re- circulated and does not contribute significantly to the overall water consumption. In the oxy-firing process, combustion water and CO2 are produced through natural gas combustion in an oxygen-enriched environment. According to Keith et al., 2018, before compressing the CO2, there is a water “knock-out” stage. This water is combined with a make-up feed of 531 t/h in a settling tank, to be subsequently mixed with CaO to produced calcium hydroxide for the contactor. The majority of process waste involves Ca-based solids that precipitate out of the sorbent cycle due to contaminants that enter via the contactor. The generic solvent process does not produce a significant amount of wastewater and on-site wastewater treatment is not anticipated. Solid Sorbent Systems: The various direct air capture companies currently employ processes that vary widely in regard to fresh water usage. 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 consider employing 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. In an alternate approach, sorbent regeneration can be accomplished by indirect heat transfer such that steam is contained in a fully closed system, allowing for negligible water losses in some cases. It has been reported that under some operating conditions, solid sorbent based processes produce water fresh, which is harvested from the air concurrently with the CO2 captured. For a typical configuration of the hypothetical adsorption-based direct air capture process considered in this chapter, water loss would amount to ~1.6 MtH2O/y for CO2 capture of 1 Mt/y CO2 (about 4 mole of PREPUBLICATION COPY

Direct Air Capture 159 water lost per mole of CO2 captured). This value could vary significantly depending on the ambient humidity at the capture site, and as noted above, in some scenarios fresh water can actually be produced. In drier climates, water loss would be expected to be larger, whereas it would be smaller in more humid regions. Additionally, water will be needed in the synthesis of the solid sorbents, and in the limit of short sorbent lifetimes, water consumption for this purpose could be substantial. At least one company, Infinitree, is developing an alternate solid sorbent-based technology that deploys an entirely different capture approach. Rather than using a T/VSA process and solid amine-based adsorbents, Infinitree is developing a technology that deploys quaternary ammonium-based sorbents that operate using a swing in humidity to induce CO2 adsorption and desorption. In this approach, which differs substantially from the hypothetical process outlined in this chapter, CO2 is captured under dry conditions, and then desorbed and concentrated under humid conditions. In the academic literature, water loss from hypothetical direct air capture processes has thus far only received marginal attention relative to the focus placed on other factors, such as energy use. Future research and development efforts need to carefully consider water production/use. Environmental One potential environmental impact of manufactured direct air capture processes is the depletion of CO2 from the air exiting the contactor. Though many studies continue to examine the environmental impact of elevated atmospheric CO2, fewer have examined the impact of lowered CO2 levels. De Marchin et al. demonstrated that reduced CO2 leads to lower PSII photochemical efficiency in algae cultures (de Marchin et al., 2015). This region of local CO2 depletion could have adverse effects on crop efficiency and the overall health of local habitats. Thus, direct air capture siting should take into consideration the nature and role of regions directly “downwind” of large CO2-scrubbing contactors. Liquid Solvent Systems: The generic solvent manufactured direct air capture process involves two chemical-intensive processes- (i) contact of ambient CO2 in a caustic KOH(aq) solution, and (ii) regeneration of KOH through a Ca-based causticization and chemical swing cycle. Both of these processes are mature, well-studied and long-employed in industry: KOH(aq) is used to scrub CO2 as a pre-stage in cryogenic air separation (Holmes and Keith, 2012b), and the Ca-based recovery cycle is based on the Kraft process employed in the pulp and paper industry (Baciocchi et al., 2006). Wastewater is not generated in significant amounts in this process, and solid waste build-up in the recovery cycle should have similar environmental implications and disposal guidelines as the reclaimer waste in a traditional MEA-scrubbing operation. Solid Sorbent Systems: For the generic adsorption-based direct air capture process, the primary means of chemical release is from the active CO2 adsorbing materials, which are intermittently exposed to the ambient air. Most existing companies employ amine-based solid adsorbents, and these species are not indefinitely stable under aerobic conditions. Studies of VOC emissions from conventional carbon capture plants employing liquid amine solutions as capture agents suggest that amine-based sorbents can break down over time into species, such as ammonia, nitrosamines and other nitrogen containing compounds that can cause potential damage to organisms or the environment (Azzi et al., 2014; de Koeijer et al., 2013; Karl et al., 2011; Karl et al., 2014; Ravnum et al., 2014; Zhang et al., 2014). Little is known about emissions from solid amine-based adsorbents, and this is an area where research is needed to clarify the potential emissions. PREPUBLICATION COPY

160 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda RESEARCH AGENDA At this time, it is not possible to select either solid sorbent or liquid solvent as a leading technology. Concerted R&D on both approaches is needed, with the understanding that scalability, cost and suitability for different locations will vary. However, limitations in basic science and engineering knowledge to do not appear to limit the deployment of manufactured direct air capture processes today. Rather, the absence of a natural economic driver, such as a cost on carbon, limits the rapid testing and deployment of direct air capture. Consequently, slow deployment limits the amount of publicly available data for techno-economic analyses of various known approaches to direct air capture. This in turn limits policymakers from understanding the costs of deploying manufactured direct air capture to achieve the scale of negative emissions needed to meet the targets of the Paris Agreement. As such, the most significant research need is public support for an array of pilot scale studies of integrated direct air capture processes that can be operated for extended periods of time to assess process performance and reliability, providing the data necessary to improve and refine process techno-economic models. It should be noted, direct air capture solves only part of the problem since it only captures CO2; it does not on its own sequester the capture gas. Nonetheless, advances in basic science and engineering can continue to reduce the costs of manufactured direct air capture. In the section below, the committee lays out a research agenda for direct air capture to contribute to the removal of carbon dioxide from the atmosphere, including the estimated cost for the research and potential options for implementation (Table 5.11). Basic Research While basic science innovations are not the primary barrier to initial deployment of manufactured direct air capture technologies, advances are needed to expand the scope of approaches for direct air capture, providing new opportunities for technology breakthroughs that will drive down costs. For example, for liquid-solvent direct air capture, advanced process designs, (e.g., shallow contactor to minimize pressure drop, improved packing material properties and contactor designs) may be developed. In addition, improvements in the material properties of solvents and sorbents also has the potential to drive down costs. For example, the two key parameters that impact the capital design of the separation process for solvent-based direct air capture are kinetics of reaction and solvent capacity. The overall kinetics of CO2 capture are also impacted by the diffusion kinetics, i.e., the time in which it takes CO2 to diffuse from the air to the chemical binding agent. In a similar manner, the diffusion of CO2 out of the material is important in terms of producing high-purity CO2 as a final product. A slower reaction, slower diffusion, and a lower capacity all lead to the need for more material to capture a given amount of CO2. This naturally leads to an increase in the number of units required for capture, directly leading to an increase in capital cost of the overall system. Hence, increasing kinetics and capacity (e.g., through catalysis using novel solvents) can lead to a smaller solvent requirement and potentially lower capital expense. The Committee recommends a program that includes several areas of basic research on the order of $30M per year over the course of 10 years. This would include approximately 30 programs per year over several areas each, with estimated budgets of ~$1M over 3 years of the project, covering basic research projects as well as early phase technology development. PREPUBLICATION COPY

Direct Air Capture 161 TABLE 5.11. Recommended direct air capture research agenda: tasks, budget, duration, and justification. Annual Phase Tasks Duration Justification Budget Basic Science & ● Simulate, synthesize, test new materials $20M-$30M 10 y Project Cost: ~ $1M Applied (solvent/sorbents) Project Duration: ~3 y Research ● 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 $3M-$5M 10 y Contracts: 2 ● materials performance testing, Contractor Staff: 3-5 FTE characterization, validation ● public materials database creation and management Development ● Scale materials synthesis to > 100 kg scale $10M-$15M 10 y Project Cost: ~ $5M ● Design and test novel equipment for pilot Project Duration: 3 y scale Project Number: 2-3/y ● Test system innovations on integrated lab- Project Staff: ~ 3 FTE scale direct air capture system (> 100 kg/d CO2) Establish third-party evaluation for $3M-$10M 10 y Contracts: 2-5 ● material synthesis economic analyses Contract Staff: 3-5 ● bulk materials performance testing, Fully Loaded FTE: $500k characterization, and validation ● equipment testing, characterization and validation ● basic engineering design package ● creation and management of public database of materials and equipment ● stage-gate for demonstration-scale pilot plants ● build a "pilot-scale" plant of "10-100 t/y" to provide input data for "demonstration- scale" at 1000 t/y Demonstration ● Design, build, and test pilot-scale direct air $20M-$40M 10 y Project Cost: ~ $20M capture systems (> 1000 t/y CO2) Project Duration: 3 y 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, and 3-5 projects in years 8-10. Establish national direct air capture test $10M-$20M 10 y Contracts:: 1 center to Contract Staff: 20-30 FTE ● support pilot plant demonstration projects Fully Loaded FTE: $500k ● develop third-party front-end engineering design and economic analysis ● maintain public record of pilot plant performance Deployment ● Scale up/out siting factors to optimize $100M 10 y Project Cost: $100M direct air capture performance (>10,000 Project Duration: 3-5 y t/y) Project Number: 1/y Project Staff: 60-70 FTE Project Number: 1 every 2 years PREPUBLICATION COPY

162 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda Annual Phase Tasks Duration Justification Budget Project Staff: 60-70 FTE First projects after 3-year pilot projects (1000 t/y CO2) depending on success of technologies from funding above, if success above justified such large investment. Engage national direct air capture test center $15M-$20M 10 y Contract: 1 to Contract Staff: 30-40 FTE ● Support full-scale plant demonstration Fully Loaded FTE: $500k projects ● Maintain public record of full-scale plant performance and economics Examples of basic science innovations that could allow for significant advances in manufactured direct air capture technologies include: (i) Low-cost solid sorbents, ideally costing <<$50/kg, that are designed in conjunction with a suitable gas/solid contactor capable of deployment at scale. Solid sorbents are typically developed in physically unrealistic (for direct air capture) contactors, such as fixed beds, and often the sorbents are made from prohibitively expensive materials. Low cost, scalable sorbents developed in conjunction with appropriate contactors, enabled by scientists and process engineers working together at the earliest stages of development, will facilitate more rapid development of practical, scalable direct air capture processes. (ii) Development of strategies that minimize the large thermal requirements of direct air capture processes will be crucial to reducing the operating costs of these systems. Examples may include but are not limited to lean solvents (with or without catalysts) as well as gas/solid contactors that limit the mass of material not directly involved in binding CO2, thereby minimizing the sensible heat load. Also, CO2-selective yet less strongly binding materials for CO2 capture that lead to reduced regeneration energy are sought. (iii) Design and synthesis of new materials with enhanced CO2 sorption capacity and reaction and diffusion kinetics are needed, specifically new materials that bind CO2 sufficiently strongly to remove it from air under ambient conditions; materials that use new binding pathways or mechanisms are especially sought. Simulation and modeling could be coupled to experiments to assist in the optimal design of new materials. Such materials should be characterized not only in regard to their uptake capacity, but also their uptake kinetics and cyclic stability under varied humidity conditions in a practical gas/solid contactor. Materials should give swing capacities and reaction kinetics that are competitive with or exceed the state-of-the-art. (iv) Advances in solvent, solid sorbent or contactor design that lead to an increase in the mass-transfer coefficient or assist in driving down the capital costs of the system a) liquid solvents: advanced packing materials (plastics vs. metals) and optimization of solvent properties (density, surface tension, and viscosity) to maximize packing coating b) solid sorbents: advantageously controlling water sorption, optimally designing the sorbent pore size distribution, maximizing swing capacity, increasing sorbent durability and lifetime. (v) Identification of potential degradation products released into the environment by solvents and solid sorbents, especially from solid amine-based adsorbents that are being widely considered for deployment. PREPUBLICATION COPY

Direct Air Capture 163 (vi) New processes that are tailored to the unique constraints of direct air capture, e.g., high gas throughput and low pressure drop. (vii) Life-cycle analyses of known and new direct air capture processes are needed, specifically with regard to CO2 emissions from sorbent production and use (given the sensitivity of the solid sorbent-based processes to sorbent lifetime) as well as water use. As noted above, improvements in CO2 solvents and sorbents are needed to reduce direct air capture system costs. However, accurate and repeatable sorption measurements under controlled conditions can be difficult (simultaneous CO2 and H2O uptake measurements are sought), sometimes leading to conflicting results in the literature. Thus, the Committee recommends that independent material performance characterization (e.g. gas/vapor sorption, Gibbs energy of formation, heat capacity, thermal conductivity, thermal expansion, thermochemical stability, etc.) using standardized testing methodologies be an integral part of the research agenda. In addition, preliminary material synthesis cost estimates should also be independently evaluated. Lastly, these materials performance data should be regularly compiled and made publicly available. Suitable institutions for independent evaluations could be a U.S. Department of Energy National Laboratory, the Department of Commerce National Institute of Standards and Technology (NIST), a non-profit research organization, or even a manufacturer of sorption equipment. Preferably, more than one vendor would be used to ensure quality and reproducibility of measurements. An exemplary institution that performs an analogous service for solar cell research is the National Center for Photovoltaics (NCPV) 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. Development Materials New materials synthesis can be expensive at the early stages, with many novel materials produced at the gram-scale. Scale-up to even kilogram-scale can often be cost prohibitive without some innovations in materials synthesis. Therefore, the Committee recommends that some research funding be provided for materials synthesis scale-up, where research should aim to develop cost effective methods for synthesizing more than 100 kg of material. To support these efforts, the Committee also recommends that funding be set-aside to engage third-party vendors in carrying-out detailed cost analyses for synthesizing new materials of interest. Furthermore, to support materials development, the Committee recommends establishing a national center for testing bulk materials using standardized hardware to compare performance on even basis (see above section). This center should also maintain a database of materials tested and their performance results. Unlike the independent testing of new materials under the basic research section, this center should focus on bulk materials testing (> 100 kg) that includes real- world challenges (contaminants, attrition, cycle life, etc.). Components System components and equipment designs (e.g.; heat exchangers, contactors, regenerators, monoliths, compressors, pumps) with novel aspects that achieve more effective mass and thermal transport and/or integrated unit operations (process intensification) are of interest to reduce overall direct air capture system costs. Development funding to fabricate and test component hardware at pilot-scale (> 1,000 t/y CO2) is needed. To supplement this work, a vendor should be identified to carry-out PREPUBLICATION COPY

164 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda standardized, third-party equipment testing and validation. Private sector vendors that have industrial gas handling experience would be preferable to research institutions. Systems Development (design, construction, testing) of integrated bench-scale systems (> 100 kg/d) that assimilate low-carbon energy sources in new and cost-effective ways should be supported. Because of the large thermal requirements for regeneration of solvents and solid sorbents by temperature swing, developing strategies that minimize the emissions from fossil energy use for heating are needed. These include potential for integration to utilize waste heat from other processes, utilization of electricity that would otherwise be shed by generators and grid operators during periods of low demand, as well as low-carbon energy sources. This is important because most manufactured direct air capture processes considered to date are estimated to have significant carbon footprints if powered by unabated natural gas. The strategies noted above to reduce carbon footprint are not mutually exclusive. For example, since different direct air capture processes require different quality thermal energy, including heat sources ranging from low temperature, low value heat (70-130°C) to high value, high temperature heat (700-900°C), strategies to provide thermal energy for direct air capture processes from low-carbon sources (e.g., concentrated solar, geothermal, bio-energy, nuclear, etc.) over a wide operating range are needed, in addition to utilization of waste heat. Similarly, all manufactured direct air capture processes considered to date require electricity. Therefore, strategies to power direct air capture processes with low-carbon electricity, potentially coupled with electrical storage enabling use of off-peak or curtailed electric power, are needed. An additional driver to improving system design is the need to have a low pressure drop in the contactor to minimize the electricity requirement. In the case of the solvent-based approach, 20% of the capital costs and 30% of the operating and maintenance costs of the entire direct air capture plant are associated with the air contactor. Figure 5.7 shows the relationship between packing depth and total estimated cost of the air contactor of a solvent-based direct air capture plant. The total estimated cost is the sum of operating and capital costs associated with the contactor. The operating costs are based upon the costs of electricity for fans or blowers, while the capital costs are directly related to the material for both the infrastructure and the solvent or sorbent materials. As described previously, the typical characteristic design of the air contactor is based upon the need for large surface area to maximize the air contacted and subsequent CO2 captured, while still being relatively shallow to minimize pressure drop and the subsequent expense of fan power to process the large amounts of air required. For instance, typical contactor depths are between 6 to 8 m for direct air capture, whereas in an example for post-combustion capture, the Petra Nova absorption contactor (“tower”) is nearly 15x deeper at 115 m. The deeper the packing depth, however, the greater the amount of CO2 captured. Hence, there is an optimization in terms of maximizing contactor depth, while minimizing fan power. To compensate for the shallow bed depth, the direct air capture contactor must have a large surface area to be capable of capturing the equivalent CO2. From Figure 5.7 it can be seen that the total cost of the direct air capture air contactor decreases with increasing packing depth to a critical depth around 8m, after which the costs begin to increase as the fan power begins to play a more dominant role in the total cost. As the cost of electricity decreases, as expected, this relationship also decreases. This highlights the opportunity that may be gained by coupling direct air capture with low-carbon energy. If for instance, one had available wind or solar at the optimal price point, it could allow for a design with a deeper contactor and subsequent smaller surface area, which could lead to a reduction in the capital investment required in the direct air capture plant. Figure 5.8 shows the relationship between the electricity cost and annualized capital (M$/yr) and packing depth. As can be seen, after a depth of approximately 8 m, the electricity cost begins to dominate PREPUBLICATION COPY

Direct Air Capture 165 FIGURE 5.7. Relationship between packing depth and total cost of a direct air capture plant assuming costs of electricity of $30, $50, and $100/MWh. FIGURE 5.8. Relationship between the electricity, capital and total cost and the air contactor packing depth. PREPUBLICATION COPY

166 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda the total cost of the air contactor. Therefore, to minimize the cost of the air contactor, it must have a shallow design, thereby leading to a significantly large surface area to capture a sufficient amount of CO2. To support the development of these systems, professional engineering design firms should be engaged to work with researchers to develop basic engineering design packages for novel systems, including: mass and energy balances, process flowsheets, preliminary piping and instrument diagrams, main equipment definitions and sizing, preliminary bill of materials, risk assessment, and process economics analysis. These engineering assessments will serve as a stage-gate, before which any demonstration-scale pilot projects are funded. Only projects with the potential to sequester CO2 at a cost of < 300 $/t should be considered for pilot demonstrations. Additional systems-level areas of research may include the use of modeling and simulation to create better integrated designs, assessing opportunities to use existing hardware and infrastructure (e.g., HVAC systems in buildings or combined heat and power systems) for direct air capture optimization and cost reduction, integration with industrial systems (e.g., steel and cement making) which have large quantities of low-high quality waste heat available, opportunities to fabricate contactors out of ultra-low capital materials (e.g., plastics), and system modeling to assess viability of using direct air capture as a dedicated load option to solve congestion or curtailment issues in local or regional grids as previously suggested. Demonstration The most significant barrier limiting the assessment and deployment of manufactured direct air capture processes is the absence of process-scale operational data that are needed for accurate techno- economic analyses. Currently there is no incentive for privately funded demonstration projects to provide data needed for accurate techno-economic analyses. Today, essentially all process-scale operations have been conducted in private companies, who may not have an interest in disclosing operational data. Furthermore, in the absence of a globally accepted cost of carbon, there is little demand for processes designed for direct air capture and sequestration. To this end, the limited commercial entities that exist are generally not practicing carbon removal, because the CO2 obtained by direct air capture is sold for productive use, for example in greenhouses, in the food and beverage industry, or in other areas. While these opportunities are good for the budding array of commercial entities that are developing manufactured direct air capture technologies, they offer a rather limited set of opportunities for this industry to grow. Parallel to the basic research and technology development programs above, an array of publicly- funded demonstration-scale studies of integrated direct air capture processes that can be operated for extended periods of time is needed to provide the data needed to assess process performance and reliability and improve and refine process techno-economic models. The Committee recommends a program that supports on the order of 3 pilot-scale projects per at ~$20M each. Such projects should capture on the order of 1,000 t/y CO2 per project. Such pilot-scale studies could assess the performance of solid sorbent-based processes, for example. As noted previously, the overall process cost is most sensitive to the capital cost of the solid sorbent. The capital cost of the sorbent is largely dictated by the purchase cost of the sorbent, along with its operational lifetime. To date, chiefly academic studies exploring sorbent stability have been published, with essentially no data available from commercial entities. Furthermore, most academic studies test under controlled, idealized laboratory conditions for limited periods of time, and such results often cannot be easily extrapolated to larger scale operation. To this end, publicly funded pilot scale studies that provide the long-term data from field operation are needed to more accurately model this variable, sorbent durability and lifetime, which most significantly affects the overall process cost of solid sorbent based manufactured direct air capture processes. Pilot-scale studies could also assess optimal site locations and plant configurations. It is anticipated that different direct air capture processes will be suited for specific deployment locales PREPUBLICATION COPY

Direct Air Capture 167 depending on the specific process characteristics involved (e.g., varied climates such as humid/arid, warm/cool, etc.). Furthermore, deployment location will impact operating and capital costs, which can be optimized based upon the available energy resources locally. For instance, if low-cost “stranded” carbon- free electricity (e.g., wind in Texas or Oklahoma) is available, then a plant might be designed in such a way to have a deeper bed, which would lead to enhanced capture, but greater electrical operating costs in the form of fan power to overcome the pressure drop. In these cases, the overall capital investment may be lower since the contactor surface area requirement may be smaller to capture the equivalent amount of CO2. Public funding for manufactured direct air capture demonstration projects are also needed to accelerate the momentum down the operational learning curves for the existing companies, and to provide incentive for new companies to enter the field. Furthermore, public funding will ensure that operational data are made available for the non-commercial research community to carry out independent techno- economic analyses, to guide policymakers and to make process innovations that will lead to improved direct air capture technologies. Availability of data for the energetics and costing of the processes will benefit the optimization of future designs to minimize these parameters, thereby leading to more rapid deployment of efficient processes. A single agency should be designated to set guidelines for data disclosure and sharing such that data are reported in a useful, uniform way, and that sufficient data are made available for accurate analysis of process costs and energetics. Deployment Manufactured direct air capture processes are expected to be initially deployed near locations where suitable geological sequestration is available, limiting or removing the need for development of long distance CO2 pipeline networks. Broad deployment of CCS from point sources will require creation of a more extensive CO2 pipeline network, which can be leveraged by direct air capture processes if appropriately sited. Other siting factors, such as (i) the impact of the potential for locally depleting the ambient CO2 concentration and its impact on agriculture or indigenous plant life, (ii) siting near appropriate sources of water when needed, and (iii) proximity to renewable energy or thermal opportunities should be considered to optimize the performance of the direct air capture plant and reduce impacts on the local communities. Direct air capture processes may be scaled up (deployment of larger units) or scaled out (deployment of a large number of small units) to achieve necessary carbon removal targets, and it is not currently clear that one deployment mode is better than the other. Demonstration-scale plants on the order of 10,000 tCO2/y are of a relevant size that will provide a better understanding of optimal siting for the various approaches. Given the scale and financial commitment (~$100M per project) required for commercial-scale direct air capture systems, public investments should only be made after detailed engineering and economics analyses (stage-gate) have been performed that demonstrate a path to commercially viable processes. Public investments that subsidize initial non-reoccurring engineering (NRE) costs for promising direct air capture technologies could be beneficial in accelerating direct air capture technology deployment. Implementation of the Research Agenda Institutions The U.S. Department of Energy’s Office of Fossil Energy and National Energy Technology Laboratory (NETL) has the appropriate infrastructure to manage direct air capture research, development, PREPUBLICATION COPY

168 Negative Emissions Technologies and Reliable Sequestration: A Research Agenda FIGURE 5.9. National Carbon Capture Center, Wilsonville, Alabama. Funded by the Office of Fossil Energy, U.S. Department of Energy and operated by the Southern Company Services. Award No: DE-NT0000749, period: Oct 2008–Sep 2014, funding: $251M, federal: $200M; and Award No: DE-FE0022596, period: Jun 2014–May 2019, funding: $187M, federal: $37M. (Source: NCCC, 2017) and demonstration projects through a typical proposal and grant funding process that distributes funds to projects at universities, non-profit 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. Department of Energy’s existing infrastructure. For development and demonstration testing of direct air capture components and systems, a centralized facility/national testbed akin to the NETL’s National Carbon Capture Center operated by the Southern Company (Figure 5.9) is recommended. Funding Scale of Funding Over the last few decades, federal R&D funding as percentage of total funding has consistently shifted from demonstration and development to basic and applied research. To develop commercially- viable direct air capture systems, funding levels will need to shift toward development and demonstration. The justification for this is as follows: It is generally accepted that the cost of scaling-up a process PREPUBLICATION COPY

Direct Air Capture 169 follows the "2/3-law", where the capital cost of plant kn at unit capacity cn, the cost of scaling goes kn = k0(cn/c0)2/3, where k0 and c0 are the unit cost and capacity of the reference plant. Assuming that a bench- scale process is 0.1% the capacity of a full-scale plant, pilot-scale is 1% of full-scale and, demonstration- scale is 10% of full-scale; then for every dollar spent on bench-scale development, roughly 5 dollars should be spent on pilot-scale, and 20 dollars on demonstration-scale. Timing of Funding Today, direct air capture technological maturity spans the spectrum from basic materials research to pre-commercial system development and demonstration. To build a pipeline of technology from basic and applied research through deployment of systems, the Committee recommends funding be staggered, with an early focus on research and develop that later feeds into demonstration and deployment projects over a period of 15 years, as depicted in Figure 5.10. Each phase of funding should have stage gates with technical and economics metrics. Notably, before moving to demonstration scale, detailed third-party engineering and economic assessments that demonstrate the potential for achieving CO2 removal at a cost of < 300 $/t should be considered for demonstration-scale funding. Data Management Data collecting, organizing, and establishment of public accessibility is a critical function of a modern direct air capture research agenda. Direct air capture specifically needs a central repository for the materials data as well as the results of engineering analyses and testing of DAC systems. Along with this, standard engineering assessment methodologies need to be created, analogous to Matuszewski, 2014. FIGURE 5.10. Recommend annual U.S. federal funding allocations for basic and applied research, demonstration, development, and deployment of direct air capture technology. PREPUBLICATION COPY

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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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