Co-Electrolysis Performance and Process Analyses for Synthetic Liquid Fuel Production

Abstract

Synthetic liquid production with solid oxide electrolysis cells (SOECs) are expected to enhance a system efficiency via the Fischer-Tropsch synthesis (FTS) by 20% compared to conventional water electrolysis. Co-electrolysis performances were obtained using solid oxide cells, which were a single cell, a 6-cell-stack, and a 25-cell-stack optimized for fuel cell operation. Low partial pressure of the reactant H2O/CO2 resulted in larger concentration overvoltages. Gas composition and flowrate of the produced syngas with the 25-cell-stack agreed by 0.5% or less to calculated values in thermodynamic equilibrium at 800 ºC and 80% reactant utilization, leading to 99.8% current efficiency. Process simulations for an SOEC-FTS combined system clarified that fuel-electrode offgas recycle at 800 ºC will enhance a power-to-oil system efficiency by 8 percentage points. Flat profile of co-electrolysis and oil production efficiencies at 0.2-0.7 A/cm2 were observed. SOEC operative voltages should be optimized to avoid large temperature gradient in the stack.