Systematic Determination of Optimal Design-Points of Fully Integrated Power-to-SNG Process Chains Via Detailed Simulation of SOEC Stacks
Authors: Furst Oscar , Wehrle Lukas , Schmider Daniel , Dailly Julian , Deutschmann Olaf 
Published: September 2023

The target of carbon neutrality by 2050 set in the 2015 Paris agreement requires ambitious investments in renewable energy plants. Most potential lies in solar and wind energy, though their intermittent nature needs to be compensated by means of large-scale energy storage technologies. Power-to-SNG (Synthetic Natural Gas) enables the production of the versatile energy carrier CH4 through the use of electrical energy, allowing long-term storage of energy in chemical form. SNG produced using captured CO2 and green hydrogen is carbon-neutral and could therefore help to reduce the greenhouse gas emissions of the transportation and chemical industry [1].
In a Power-to-SNG plant comprising a CO2 capture unit, an electrolyzer and methanation reactors, electrical power is the largest contributor to the levelized cost of the produced SNG [2]. This makes the optimization of the system efficiency a critical step towards achieving commercial viability. Since the water splitting reaction is the largest energy sink in the cascade of conversion steps, the use of Solid Oxide Electrolysis Cells (SOEC) is promising because their energy requirement can be partially covered by high temperature heat provided by the methanation reactors. When optimizing the performance of the plant, the operating conditions of the electrolyzer must therefore be considered in conjunction with other parameters contributing to the thermal balance of the plant, which calls for dedicated system modeling approaches.
In this work, multiple Power-to-SNG process chains with integrated SOEC module are developed by means of detailed, yet highly customizable plant and component models. Optimal design points of each plant configuration are determined and comparatively assessed. The different process chains are generated by varying CO2 sources and methanation technologies, whilst processes are resolved down to the individual pumps, heat exchangers, cleaning and conditioning steps. Good comparability between configurations and operating conditions of the plant is guaranteed through ideal thermal integration using the pinch method [3]. The SOEC module is simulated based on high-accuracy, multivariate performance maps obtained from a multi-physics 3D stack simulation tool [4,5] computed on a high-performance cluster. For the simulations, two types of SOEC technologies are considered: Ni-GDC/3YSZ/LSCF-electrolyte supported cells and Ni-YSZ/8YSZ/LSC cathode-supported cells.
The plant configurations are optimized independently with a simplex algorithm and thereby, the results of investigations focusing on standalone SOEC stacks or using low level-of-detail stack models in system simulations are ascertained. Results demonstrate the importance of balance-of-plant trade-offs in order to achieve best possible efficiencies. For example, in cases with direct air capture (DAC) of CO2, best results are obtained with endothermal operation of the stack and elevated air flow through the anode. Thus, using electrolyte supported cells, power-to-SNG efficiencies (based on HHV) of 67% (fixed-bed methanation) to 69% (fluidized-bed methanation) can be reached.

References

[1] M. Sterner and I. Stadler, Energiespeicher - Bedarf, Technologien, Integration (Springer Berlin Heidelberg, Berlin, Heidelberg, 2014)
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Figure: Temperature distribution and hydrogen mole fraction in the electrolyte-supported SOEC stack. Stack is operated at 1 bar and 0.3 A cm-2 with inlet gases at 1173 K such that 90% steam conversion is reached, which corresponds to efficiency-optimized conditions in plants with direct air capture. The species mole fractions along the fuel channel as well as the current density computed for the central repeating unit is shown in more detail.

Figure 1