Electrochemical Energy Storage and Synthetic Natural Gas Production Based on Reversible Molten Carbonate Cells

Abstract

One of the biggest issues associated to Carbon Capture and Utilisation (CCU) applications involves the exploitation of the captured CO2 as a valuable consumable. An interesting application is the conversion of CO2 into renewable fuels via electrochemical reduction at high temperature. Still unexplored in the literature is the possibility of employing a Molten Carbonate Electrolysis Cell (MCEC) to directly converting CO2 and H2O into H2, CO and eventually CH4, if a methanation process is envisaged. The introduction of this concept into a reversible system — similarly to the process proposed with reversible solid-oxide cells — allows the creation of a cycle which oxidises natural gas to produce CO2 and then employs the same CO2 and excess renewable energy to produce renewable natural gas. The result is a system able to perform electrochemical storage of excess renewable energy (from wind or solar) and if/when required sell renewable natural gas to the grid. In this work, a simulation of a reversible Molten Carbonate Cell (rMCC) is proposed. The reference MCFC technology considered is that from FuelCell Energy (USA) whose smaller stack is rated at 375 kW (DC). A simplified 0D stack model is developed and calibrated against experimental data. The Balance of Plant (BoP) is in common between the two operation modes MCFC and MCEC. In the former case, natural gas is electrochemically oxidised in the fuel compartment which receives carbonate ions (CO32−) from the air compartment, fed with air enriched with CO2 produced during electrolysis mode. The CO2 in the anode off gas stream is then purified and stored. In electrolysis mode, the stored CO2 is mixed with process H2O and sent to the fuel compartment of the MCEC; here, electrolysis and internal methanation occur. An external chemical reactor finalises the production of methane for either natural gas grid injection or storage and reuse in fuel cell mode. A thermodynamic analysis of the system is performed the yearly round-trip efficiency is assessed considering an assumed availability operating time of 7000 h/y. Finally, the overall green-house gas emission is assessed.