Design and Performance of an Integrated System of Reactor Modules for Solar Energy Conversion into Chemicals

Kurzfassung

The European chemical industry is one of the largest manufacturing sectors in Europe providing vital material inputs to many more industries and thus is a critically important economic factor. At the same time, it is highly reliant on fossil fuels resulting in the release of 155 million metric tons of CO2 equivalent in 2021 (EU-27) accounting for 5% of total net GHG emissions. This motivates the development of innovative technologies and processes enabling more sustainable ways of chemicals production. Against this background, in this contribution a solar-powered, integrated system of flow reactor modules for the production of chemicals from CO2 and H2O is presented that utilizes direct solar irradiation and solar derived electricity as energy inputs. The flow reactor modules are characterized as photo-electrochemical (PEC), photo-thermal (PT) and electrochemical (EC). They are arranged in series to produce ethylene (C2H4) as target product as shown in the schematic flow diagram of the integrated system together with main process streams and species. The directly irradiated PEC module produces H2 via water splitting and feeds the likewise irradiated PT module. CO2 is fed along the produced H2 to the PT to form CO via the rWGS reaction where CH4 is formed as side product in the Sabatier reaction. After the removal of unreacted CO2, the product gas mixture enters the EC module which is an electrolyzer for the CO reduction reaction to form the target product C2H4 with other carbon containing value products emerging as side products. System Operation and Experimental Results A demonstrator system including supporting optical and balance-of-plant components was assembled and tested under concentrated, simulated solar light. Over the course of the experimental testing the PEC, PT and EC reactor modules were operated for a total duration of 24.3, 21.8 and 13.7 hours, respectively, where differences in duration follow from the sequential start-up procedure. A peak production rate of 56.7 mmol/h of C2H4 was obtained under mixed feed conditions (i.e. CH4 and H2 were present besides CO) in the EC module corresponding to a Faradaic Efficiency of C2H4 of 27.0%. The PT module produced CO at a rate of 356 mmol/h with a CO2 conversion of 43% and selectivity towards CO of 58.8% based on CO2 while H2 was generated in the PEC module at a rate of 6.89 mol/h at a solar-to-hydrogen efficiency (STH) of 14.5% based on ΔG_(R,0) of which 3.7 mol/h were utilized in the PT. While operational challenges were observed and performance matching of the individual reactors could be improved, the physically integrated system was capable of producing relevant quantities of ethylene, as well as valuable by-products demonstrating the general feasibility of this innovative approach towards sustainable chemicals production.