Process Modeling of an Innovative Power to LNG Demonstration Plant

The continuous increase in electricity production from renewable energy sources (RESs) introduces the intrinsic fluctuating characteristic of RESs in the electric power grid, causing nontrivial grid management issues (e.g., grid congestion). In this work, an innovative power to liquefied methane concept was developed, and process simulations for a 200 kW(el) demonstration plant were carried out. The proposed concept is based on water electrolysis to produce hydrogen, CO2 capture from air using solid adsorption materials, catalytic CO2 methanation, gas separation, and a single mixed refrigerant (SMR) liquefaction process. The gas separation unit produces an exhaust stream, rich in not only hydrogen and carbon dioxide but also methane, that is recycled to the methanation unit inlet. A thermodynamic analysis excluded the possibility of carbon deposition formation in the methanation reactor due to methane recirculation. The gas separation system was designed using a combination of temperature swing adsorption techniques (stream dehumidification) and membrane separation (CO2 separation). After a screening of different polyimide-type membranes, a two-stage layout was selected and dimensioned. Subsequently the liquefaction unit was developed, optimizing the SMR composition and pressures to minimize the total work required. Hence, the minimum work required for the liquefaction resulted in being 0.57 kWh(el)/kg(LNG). Finally, the thermal integration was performed to minimize the external heat requirement. The heat produced by the electrolyzer and methanation unit is greater than the thermal energy requirement by the CO2 capturing unit during desorption. A process efficiency up to 46.3% (electric to chemical) resulted from the study. The process modeling results also evidenced that the impact of the gas pretreatment and liquefaction process on the plant energetics is 4% of the total power input.