Comparison of Power-to-Liquid processes using PEM electrolysis with RWGS and SOEC co-electrolysis

Kurzfassung

Future aviation, shipping and heavy load transportation will continue to depend on energy carriers with a high energy density. The “Power-to-Liquid” technology is an approach to produce such synthetic hydrocarbons. The German Aerospace Centre investigates and evaluates the technical and economic performance of these processes. Two process concepts were investigated. The first concept (P-PEM) is based on H2 from PEM electrolysis, which reacts with CO2 via the reverse water gas shift reaction to synthesis gas. In the second concept (P-SOEC) synthesis gas is produced by high temperature co-electrolysis. For both concepts the synthesis gas is then converted to liquid hydrocarbons in the Fischer-Tropsch reactor. A downstream product upgrading section allows the production of defined fractions for specific applications. The two process concepts were modelled with the flowsheeting simulation tool Aspen Plus. The capacity was set to a feed of 100 MWLHV of H2. In both models a recycle stream network was implemented to increase the conversion. 5.5 t/h (67.2 MWLHV) liquid hydrocarbons were generated with the P-PEM concept. A feed of 22.8 t/h of CO2 was required and 21.8 MW of steam at 225°C and 20 bar were generated as excess heat. The CO2 separation process was included into the process model to investigate a combined heat and material integration of CO2 separation from industrial sources. It could be shown, that the excess heat of the synthesis process satisfies the heat demand of the CO2 separation. The process performance is defined as the “Power-to-Liquid” efficiency, which is the fraction of electrical energy chemically bound into liquid hydrocarbons. “Power-to-Liquid” efficiency is 43%. The carbon conversion is 73%. Additionally, the effect of various operation conditions was investigated by sensitivity analyses. Increasing the chain growth probability, using oxygen generated by electrolysis in the burner and preheating thr supply air of the burner were identified as measures to increase the overall efficiency of the process. The measures could increase the efficiency to 48% and the carbon conversion rate to 81%.The capital and operation costs were estimated on the basis of the developed flowsheet. The effect of the capital costs of the electrolyzer on the production costs was investigated. The performance of the P-SOEC concept will be evaluated analogously to the investigation of the P-PEM concept. Both concepts will be compared regarding their technical and economic performance. The technical evaluation includes efficiencies, conversion extends and technical feasibility. The economical assessment comprises capital and operational cost.