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A life cycle assessment case study of coal-fired electricity generation with humidity swing direct air capture of CO2 versus MEA-based post-combustion capture Coen Van der Giesen, Christoph Johannes Meinrenken, Rene Kleijn, Benjamin Sprecher, Klaus S. Lackner, and Gert Jan Kramer Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05028 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016
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Environmental Science & Technology
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A life cycle assessment case study of coal-fired electricity generation with
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humidity swing direct air capture of CO2 versus MEA-based post-combustion
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capture
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Coen van der Giesen†,§,*, Christoph J. Meinrenken‡,§,*, René Kleijn†, Benjamin Sprecher†, Klaus S.
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Lackner¥, Gert Jan Kramer†,″
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†
Institute of Environmental Science, Leiden University, P.O. Box 9518, 2300 RA Leiden, The
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Netherlands
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‡
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¥
Earth Institute, Columbia University, 500 W. 120th St., 918 Mudd, New York, NY 10027, USA Center for Negative Carbon Emissions, Arizona State University, PO Box 873005, Tempe, AZ 85287,
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USA
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″
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Utrecht, The Netherlands
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§
Copernicus Institute of Sustainable Development, Utrecht University, Heidelberglaan 2, 3584 CS
These authors contributed equally to the work
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Most carbon capture and storage (CCS) envisions capturing CO2 from flue gas. Direct air capture
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(DAC) of CO2 has hitherto been deemed unviable because of the higher energy associated with
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capture at low atmospheric concentrations. We present a Life Cycle Assessment of coal-fired
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electricity generation that compares monoethanolamine (MEA) based post-combustion-capture
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(PCC) of CO2 with distributed, humidity-swing-based DAC (HS-DAC). Given suitable temperature,
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humidity, wind, and water availability, HS-DAC can be largely passive. Comparing energy
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requirements of HS-DAC and MEA-PCC, we find the parasitic load of HS-DAC is less than twice that
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of MEA-PCC (60-72 kJ/mol versus 33-46 kJ/mol, respectively). We also compare other
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environmental impacts as a function of net greenhouse gas (GHG) mitigation: To achieve the same
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73% mitigation as MEA-PCC, HS-DAC would increase 9 other environmental impacts by on average
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38%, whereas MEA-PCC would increase them by 31%. Powering distributed HS-DAC with
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photovoltaics (instead of coal) while including re-capture of all background GHG, reduces this
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increase to 18%, hypothetically enabling coal-based electricity with net-zero life-cycle GHG. We 1 ACS Paragon Plus Environment
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conclude that, in suitable geographies, HS-DAC can complement MEA-PCC to enable CO2 capture
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independent of time and location of emissions and re-capture background GHG from fossil-based
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electricity beyond flue stack emissions.
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INTRODUCTION
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To keep global warming below 1.5-2°C targets, a balanced mix of greenhouse gas (GHG) mitigation
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measures is needed. Among these, carbon capture and storage (CCS) is projected to play an
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important role, as are increasing energy efficiency and deployment of renewable and nuclear
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power.1–3 Besides traditional carbon capture from point sources, additional research into a wider
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portfolio of carbon dioxide removal (CDR) technologies, including distributed solutions, is seen as
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more and more important.4–6 We henceforth refer to all such capture and storage options as CCS.
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Relative (dis-) advantages of different CCS technologies
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CCS is not tied to a specific technology. For coal-fired electricity generation, arguably the most
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mature technology is post combustion capture (PCC) using monethanolamine (MEA), which scrubs
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CO2 directly from the flue gas of fossil fuel power plants.7–9 It is often argued that there are no
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technological obstacles to large-scale implementation of PCC; rather that its main hurdle is a lack of
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policy or market mechanisms to incentivize adopting PCC amidst several challenges:1,9 The power
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plant must be located in sufficient vicinity of a suitable storage site, or else CO2 transport from the
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plant to such a site will become prohibitively expensive. PCC results in reducing operating efficiency
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compared to a conventional power plant. It can best be implemented in modern coal-fired power
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plants, i.e. fewer than 29% of currently installed coal-fired power plants globally.1,9 Further, as an
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optimum design choice, MEA-PCC typically captures 85 - 90% of the flue stack CO2 emissions10 but
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none of the so-called background emissions (e.g., from mining and transporting coal, or building and
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decommissioning the plant itself). MEA-PCC thus typically eliminates only ~70% of total life cycle
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greenhouse gas (GHG) emissions of coal-based electricity.11,12 Finally, many power plants are located
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near population centers, a fact that can be problematic for transport and storage of CO2. The IEA 2 ACS Paragon Plus Environment
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notes that “examples in Germany and The Netherlands illustrate that under-appreciation of public
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concerns over CO2 storage can easily be fatal for CCS”.1
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But CO2 does not necessarily have to be captured at the point of combustion. A CCS alternative to
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PCC is Direct Air Capture (DAC), a form of CDR that captures CO2 directly from the atmosphere.6,13,14
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Relative dis-advantages and advantages of DAC versus PCC have been previously discussed.10,15 One
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possible advantage of DAC is that its location and schedule of operation is not necessarily tied to the
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emitting power plant itself. This flexibility imbues the technology with benefits15–17 that may partially
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alleviate above limitations of PCC-based CCS: With regard to location, DAC plants can be built at or
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near the final CO2 storage site, so that CO2 transport infrastructure can be largely avoided. This could
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alleviate social opposition arising from CO2 transport through populated areas.18 And because its unit
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size and operating schedule is not tied to the power plant itself, DAC could be scaled to capture not
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90% but 100% of CO2 emissions emitted from a plant and in addition compensate for background
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GHG emissions, thus allowing for true, net-zero GHG electricity from coal. Similarly, DAC could
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compensate for emissions from non-stationary GHG sources in sectors where costs or technological
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feasibility may prohibit phasing out fossil fuels (e.g., aviation). The Royal Society notes that ‘there is
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surprising value in the economic freedom to build a capture plant where it is cheapest to do so and
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near the best sequestration site’.18
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Above theoretical advantages notwithstanding, capturing low concentrations of CO2 from air simply
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is thermodynamically more challenging than from concentrated point sources.15 In a recent,
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comprehensive review of DAC and other CDR, DAC is expected to consume up to 45GJ per ton carbon
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equivalent (540kJ/mol CO2) – levels that would arguably render DAC economically unviable.19
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Whether as part of a portfolio of climate responses4 or as a last-resort backstop technology to reduce
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atmospheric CO2 levels via negative emissions,20 improved understanding of DAC's energy
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requirement and environmental impacts is therefore crucial.
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Humidity-swing DAC (HS-DAC)
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Previous research on DAC has focused on gas scrubbing technologies using sorbents such as CaOH2,
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KOH and NaOH.15,21,22 The binding potential of these is relatively high, and therefore large amounts of
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high-exergy energy (electricity and/or high grade heat) are required to release the CO2 during
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sorbent regeneration.21,23 In contrast, HS-DAC utilizes ambient-temperature heat for the sorbent
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regeneration: It is realized through water drying off the sorbent, i.e. spontaneously evaporating at
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ambient conditions (driven by the surrounding air and, if available, sun shine).24 This is akin to other
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DAC technologies, such as temperature-swing DAC, which can use low grade heat for cycling the
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sorbent material and whose energy balance analysis thus also shows limited requirement for
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mechanical (or electrical) energy.25,26 However, for HS-DAC, the relatively low energy consumption of
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the overall capture process is in part achieved through reliance on ambient wind conditions to drive
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air through filters and later dry them for regeneration. This causes one of HS-DAC’s main
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disadvantages: Its performance becomes weather/geography dependent.27 In this study, we quantify
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this dependence via sensitivity analyses.
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Previously, relative (dis-)advantages of HS-DAC have been analyzed,6 current understanding of its
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molecular process28 and thermodynamics24 described, and its energy consumption has been
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estimated at 50kJ/mol.29 Briefly, a non-toxic anionic exchange resin such as the commercially
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available MarathonA adsorbs CO2 when dry and releases it when exposed to 100% humidity. Driven
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by ~2m/sec or higher ambient wind, air flows through filters in which the resin adsorbs some CO2,
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reducing the air’s CO2 concentration from 400ppm to ~360 ppm (design-dependent). Using an
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electrically powered conveyor system, the filters enter a desorption chamber. After evacuating to
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0.01 bar the sorbent is sprayed with water, causing the sorbent to release CO2. The resulting low-
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pressure gas mix in the desorption chamber (CO2 and H2O,
