Design of a Large-Scale Electrochemical CO2 Reduction Process

Abstract

The rise of CO2 concentration in the atmosphere is a leading cause of global warming. Utilizing CO2 ob- tained from point sources, such as chemical industries, as a feedstock to produce high energy density fuels and chemicals could mitigate the emission of CO2 as well as provide various economic bene- fits. One promising technology is the electrochemical reduction of CO2, however, the presence of contaminants in the industry-supplied feedstock and the separation of products downstream would be challenging in a continuously operated large-scale plant. To address these challenges and indentify the bottlenecks involved, it is important to study which pre-treatment and post-treatment steps are required and how to integrate these in a large-scale electrochemical CO2 reduction process.

The gas and liquid feed streams to the CO2 electrolyzers are first cleaned to the desired levels. The liquid feed stream is water from the river Rhine that is purified so the specifications of the water meet the requirements for type 1 water (ultrapure water). The gas feed stream is the flue gas stream of an average steel-producing plant in Europe and is cleaned to remove sulfur and nitrogen compounds. A two-step electrolysis process is used where CO2 is first reduced to CO, followed by the reduction of CO to C2+ products. The electrolyzer for the first step is a membrane electrode assembly-based flow cell with a current density of 300 𝑚𝐴 𝑐𝑚2− and a faradaic efficiency (FE) of 96% to CO. In the second step, a gas diffusion electrode-based flow cell with a current density of 300 𝑚𝐴 𝑐𝑚2− and a faradaic efficiency of 9.35%, 15.49%, 45.57%, and 16.38% towards acetic acid, ethanol, ethylene, and propanol, respectively, is used. The anolyte and catholyte in the reactors are recycled 3,500 and 2,000 times, respectively, to reduce the size of purification of the liquid feed stream section and increase the liquid product concentration. In both steps, unreacted CO2 and CO are recycled. The gaseous and liquid products are separated and purified to meet the industry standards using established separation techniques.

The total capital investment for a process with an industrial gas feed of 381.678 tons per hour is 4,053.5 million dollars with a daily operating cost of 7.403 million dollars. The daily income from selling the products is 2.363 million dollars, but this could increase if the FE towards acetic acid is increased since this product has the highest income per electron consumed. The net present value (NPV) for the base case, assuming current technological and market conditions, is -19.4 billion dollars after 15 years. To analyze which parameters have the most influence on the NPV, a sensitivity analysis is also per- formed with a better and optimistic scenario. It was found that the economic feasibility of the currently designed process is not limited by the technological progress, but mainly by the market conditions.

The target of this process is to reduce the emission of CO2, however, the operation of the plant itself contributes to some CO2 emissions. Therefore the process should consume more CO2 than it emits. The units that consume most energy and emit the most CO2 are the CO to C2+ products electrolyzer and the first distillation column in the liquid product separation section to remove acetic acid. It was found that the process is only carbon negative when the consumed energy is generated by nuclear, wind, or solar energy. The net CO2 emission is the lowest when nuclear or wind is used as an energy source. Generating all the required energy from these sustainable sources brings another challenge since the total installed capacity of these sources are currently not sufficient to cater to the needs of such a large-scale continuously operating CO2 electrolysis plant.

Keywords: Electrochemical CO2 reduction, large-scale, pre-treatment, post-treatment, technoeconomical analysis, energy analysis