A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration (2022) / Chapter Skim
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8 Electrochemical Engineering Approaches
8 Electrochemical Engineering Approaches
Pages 209-238
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. Basic approaches exploit high-pH conditions created around the cathode to shift the equilibrium of the carbonate system toward a greater concentration of bicarbonate and/or carbonate ions (La Plante et al., 2021)
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Positively charged cations in the solution migrate to the cathode (negatively charged electrode) , which promotes reduction reactions, and the negatively charged anions migrate to the anode (positively charged electrode)
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Design Choices in Electrochemical Engineering A range of design choices may be available in the construction and the operation of electrochemical reaction systems. These choices are made so that overpotentials at the electrodes or within the cell are minimized, while maximizing current and yield efficiency either when using electricity as an input, or in the case of multiple energy inputs such as electrons and photons/phonons (Nørskov et al., 2004)
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Design Choices in Electrochemical Engineering A range of design choices may be available in the construction and the operation of electrochemical reaction systems. These choices are made so that overpotentials at the electrodes or within the cell are minimized, while maximizing current and yield efficiency either when using electricity as an input, or in the case of multiple energy inputs such as electrons and photons/phonons (Nørskov et al., 2004)
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have been developed and used to improve the performance and to reduce the cost of electrolysis cells. The lowest exchange current densities and the highest catalytic properties for the lowest material cost are desirable.
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investigated the thermal decomposition of desalination rejected brine. Similar to seawater electrolysis, the products are solid magnesium hydroxide (Mg[OH]
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(log10 Conc. (log10 Ionic Strength Conductivity (µS/ Electrolyte Ohmic Current Density -- Current Density -- mineral phase)
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Hybrid approaches A combination of electrochemical approaches that results in an increase in ocean alkalinity and the removal of CO2 from seawater; e.g., as a gas, or in mineral carbonates. Base Process In this approach the base liquid produced by electrochemistry is mixed with seawater/brine to force the precipitation of carbonate minerals.
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ELECTROCHEMICAL ENGINEERING APPROACHES 217 FIGURE 8.3 Simplified schematic of the acid process to remove CO2 from seawater. Systems for precipitating sodium hydroxide (NaOH)
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shows how much of the original dissolved inorganic carbon and calcium in the solution are removed through precipitation, and (d) shows the impact on seawater pH for both cases.
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FIGURE FIGURE FIGURE FIGURE FIGURE FIGURE 8.7 8.7 8.78.7 8.7 8.7 Simplified Simplified Simplified Simplified Simplified Simplified schematic schematic schematic schematic schematic schematic of ofof of of of water water water water water water electrolysis electrolysis electrolysis electrolysis electrolysis electrolysis for for forfor for for ocean ocean ocean ocean oceanocean alkalinity alkalinity alkalinity alkalinity alkalinity alkalinity enhancement. enhancement.
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In this context, electrochemical methods have a particular advantage because they may be capable of producing acidity/basicity, in situ, without a need for ancillary additives, synthetic electrolytes, etc.; in effect, the additive demand can be fulfilled by, ideally, zero-carbon electrons. Second, it is critical that the materials of construction, particularly the electrode materials and reactors are abundant.
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. Additionality and Downstream Effects Despite their enormous potential, electrochemical processes that couple with the world's oceans may exert unintended consequences.
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species, for example, when alkalinity enhancement of the aqueous phase allows for the additional drawdown of atmospheric CO2; and/ or (3) immobilize CO2 in mineral carbonates.
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The ocean contains approximately 2 mmol/kg (88 g CO2/m3) of dissolved inorganic carbon; aqueous CO2 constitutes only a small proportion of this (~10 µmol/kg at atmospheric pressure)
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to power operations when curtailment is not in effect or cheap renewable electricity is not available. Geospatial and Adjacent Infrastructure Considerations Unquestionably the need for seawater in electrochemical ocean CDR requires the siting of such processes and plants at sites that offer ready access to seawater.
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to convey the CO2 to sequestration sites, or to develop solid waste handling and valorization facilities (e.g., for the potential use of mineral carbonates and hydroxides as construction materials)
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The environmental impact associated with rock extraction is discussed in Chapter 7. Gaseous Chlorine Production In seawater or brine electrochemical reactions (equation 8.5)
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, with membrane-based processes performing favorably. Reduced Dissolved Inorganic Carbon in Effluent Waters Processes that remove CO2 from seawater will, by intention, result in very low dissolved inorganic carbon (DIC)
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suggests an energy requirement of 3.1 MWh for the acid process and 4.4 MWh for the base process per ton of CO2 removed.9 There is no comprehensive technoeconomic assessment for processes that effect CDR and use electrochemistry to increase ocean alkalinity. House et al.
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suggest that using a chlor-alkali system to produce NaOH for ocean alkalinity enhancement may cost on the order of $533–$668/t CO2, without including costs associated with acid disposal. If additional revenue was generated from the sale of Cl2 and H2 gas, then the costs for FIGURE 8.12 Bare module costs for an aqueous electrolyzer for a given power capacity.
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. In the absence of cost improvements on conventional electrochemical systems, the cost of ocean alkalinity enhancement through electrochemical methods may be between $150 and $700/t CO2 removed.
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ELECTROCHEMICAL ENGINEERING APPROACHES 231 TABLE 8.4 A Simplified Techno-Economic Overview of Diaphragm and Membrane Chlor-alkali Systems, and Then the Application of the Produced NaOH for CDR by Ocean Alkalinity Enhancement Diaphragm Membrane System size (kt/yr) Chlorine production 179 Sodium hydroxide production 197 Hydrogen production 5 Maximum CO2 removal 286 Power (MW)
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and/or marine "dumping" under the London Convention and Protocol. As discussed in Chapter 2, the parties to the London Protocol have agreed to an amendment, dealing specifically with the discharge of materials into ocean waters for the purposes of "marine geoengineering."12 The amendment, which has not yet entered into force, establishes a framework under which parties to the London Convention may approve certain marine geoengineering activities involving ocean discharges.13 While electrochemical engineering approaches could be considered a form of "marine geoengineering," at the time of writing, the framework only applied to activities relating to ocean fertilization.
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. 8.6 SUMMARY OF CARBON DIOXIDE REMOVAL POTENTIAL The criteria for assessing the potential of electrochemical processes as a feasible approach to ocean CDR, described in Sections 8.2–8.5, is summarized in Table 8.5.
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Provision of sufficient electrical power will likely have remote impacts. Social considerations Similar to OAE and to any industrial site.
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Coupling to Whole-Rock Dissolution Systems Optimum system design will likely require careful coupling between the electrochemical component and that which facilitates rock dissolution. Furthermore, large-scale electrochemical approaches would use a range of local rock resources, and research to assess the impact of realworld rock chemistries on system performance and/or effluent quality is needed, including the potential of using waste rock (e.g., mine tailings, quarry fines, etc.)
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These should be broadened to also consider novel designs of electrolyzers and electrochemical reactors, electrode materials that can minimize the production of unwanted by-products (e.g., Cl2 gas) , electrochemical reactor architectures, and hybrid approaches that both remove CO2 and increase ocean alkalinity.
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8.3 Assessment of environmental impact and What is the life-cycle environmental impact of Low 7.5 10 acid management strategies approaches? 8.4 Coupling whole rock dissolution to Is it possible to scale electrochemical approaches Low 7.5 10 electrochemical reactors and systems while managing waste acid production?
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As such, the research agenda should also include provision for resource mapping, stakeholder engagement, and wider economic impact from an ocean-based electrochemical CDR industry operating at a climate-relevant scale. The outcomes of the research should be transparently disseminated such that independent assessment of cost and environmental impact can be made.
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