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UBC Theses and Dissertations

Development of a continuous reactor for the electro-chemical reduction of carbon dioxide Li, Hui

Abstract

A laboratory bench-scale continuous reactor has been developed for the electro-reduction of CO₂ to formate salts in aqueous solution. This novel reactor operated in the "trickle-bed" mode with co-current flow of reactant gas and catholyte liquid through a flow-by 3-D cathode, to overcome the twin problems of reactant supply and mass transfer limitation encountered in previous work on the electro-reduction of CO₂. Two single-cell electro-chemical reactors with cation membrane separators, Reactor A (150 mm long by 30 mm wide) and Reactor B (680 mm long by 50 mm wide), were constructed and used in experiments to study the catalyst (cathode) materials, the effects of process variables on reactor performance and the reactor scale up with respect to a speculative industrial process for reduction of CO₂. In Reactor A, a variety of tin and lead catalysts (cathodes) were investigated, including tin deposited graphite felt, copper mesh, tin coated copper mesh, lead shot and granules, tin shot and tin granules. Among these catalysts tin coated copper mesh and tin granules were the most intensively studied. The results here showed the primary and secondary reactions were respectively the reduction of CO₂ to formate (HCOO⁻) and of water to hydrogen, while up to 5% of the current went to production of CO, CH₄ and C₂H₄. Factorial, fractional factorial and parametric experimental designs were employed with both tinned copper mesh and tin granule cathodes to study the separate and combined effects of the following ten process variables on reactor performance: catalyst life (operating time), cathode specific surface area, cathode thickness, catholyte flow rate, current density, CO₂ concentration in the feed gas, electrolyte conductivity, electrolyte species, gas flow rate and temperature. For these experiments on Reactor A the current ranged from 1A to 14A with corresponding superficial current density from 0.22 to 3.11 kA m⁻². The formate current efficiency and specific energy consumption ranged respectively from 10 to 96% and 130 to 1300 kWh kmol⁻¹ formate, while the product concentration of KHCO₂ was up to 0.08 M in single pass flow. In addition, the combinations of two cations (K⁺ and Na⁺) and four anions (HCO₃⁻, CO₃²⁻, Cl⁻ and HCOO⁻) at various concentrations were studied as the catholyte, the best of which was found to be a solution of 0.5 M KHCO₃ with up to 2 M KCl as the supporting electrolyte (when necessary). The anolyte used in the present work was 1 to 2 M KOH. In Reactor B tin granules were used as the main cathode material, due to the relatively long lifetime (300 mins vs 30 mins) and high selectivity for formate of tin granules compared with other cathodes in the present work. The effects on formate current efficiency of current density, fluid flow rates, back-pressure, formate concentration in the catholyte feed and cathode pretreatment techniques have been studied. The current in Reactor B ranged from 20 to 101 A with corresponding superficial current density from 0.62 to 3.14 kAm⁻². The results obtained are encouraging, e.g. the formate current efficiency reached 63% at current density of 3.14 kA/m², a reactor voltage of 3.9 V, specific energy 332 kWh kmol⁻¹ formate and product concentration of up to 1.03 M KHCO₂ in single pass flow. However the problems of cathode deterioration over time (3-7 hours) and formate cross-over must be solved before this process could be commercially viable. A crude reactor model has been established as a guide to the development and scaleup of the process of electro-chemical reduction of CO₂. In addition, a conceptual process design and subsequent economic projections were made for the on-site production of sodium formate by reduction of CO₂ from an 80 MW fossil fuel based power generation plant (600 tonne/day CO₂ emissions). For such a CO₂ converting plant to be commercially viable the electro-chemical reactors should operate at superficial current densities above about 1 kA m⁻² and preferably in the range 2 to 3 kA m⁻². With a current density of 2.2 kA m⁻² the total installed capital cost is estimated at 5.5 x 10⁸ $US (2005) +/- 20% and the return on investment is about 24%/year, based on a carbon credit of 25$US per tonne CO₂ and assuming all products (NaHCO₂, NaHCO₃, H₂, O₂) are sold at prevailing bulk chemical prices. Concerning the energy requirement, electro-chemical reduction would only be feasible as a means to control CO₂ emissions in regions where non-fossil based energy, such as solar, wind or nuclear, is abundantly available. The present work has broken new ground with respect to the design of electrochemical reactors for CO₂ reduction and demonstrated the feasibility of carrying out this process on an industrial scale--with the caveat that the effective cathode life must be extended from three to several thousand hours and an improved cation membrane separator is needed to prevent formate cross-over.

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