Remediation of Black Sea ecosystem and pure H2 generation via H2S-H2O co-electrolysis in a proton-conducting membrane cell stack reactor: A feasibility study of the integrated and autonomous approach

The present work explores the feasibility of an integrated and autonomous scaled up process towards the remediation of the Black Sea ecosystem with simultaneous H2 generation through the co-electrolysis of rich H2S/H2O seawater mixtures. The core unit of the proposed process is a proton-conducting membrane cell stack reactor (electrolyzer), where H2S in excess H2O mixtures are fed at the anode and co-electrolyzed to protons (H+), which are transferred through the electrolyte to the inert exposed cathode towards H2 generation. The proposed scaled-up process aims towards a Black Sea water intake of up to 2000 tn/hr and involves four distinct operating steps, i.e.: i) pumping Black Sea water from 1 km depth (H2S∼14 ppm) and H2S concentration enrichment up to 1 v/v% H2S-H2O, ii) Η2 production through H2S-H2O co-electrolysis at 850 °C and 2 bar, iii) purification and separation of the proton-conducting electrochemical membrane reactor effluent (H2 and SO2) and iv) H2SO4 production from off-gases. Overall heat management is accomplished through a natural gas high pressure burner along with flue gas power recovery (combined cycle) and the process system is assessed in terms of operating flexibility, electrical/heat requirements and economic perspectives. As was revealed, the decreased concentration of H2S/H2O mixtures (from 1 to 0.1 v/v%) results in a higher H2 and H2SO4 generation at the expense of higher heating/electrical demands, whereas the variation on the Black Sea water intake (from 650 to 1950 tn/hr) can be appropriately adjusted to regulate the corresponding operating costs. Based on a parametric sensitivity analysis, it was revealed that a H2S concentration of 1 v/v% and a water intake flow corresponding to a hydrogen production of >40 kg/h can promise favorable financial perspectives. The minimum products sale values that ensure the feasibility of the process along with a flexible heat and energy autonomy were identified at 9.85 €/kg of H2, 0.45 €/kg of H2SO4 and 0.277 €/kWh of produced electricity. Partial subsidy on the total fixed capital investment can further result in a substantial improvement of the investment's operating profitability.