Synthetic natural gas via integrated high-temperature electrolysis and methanation: Part I—Energy performance

In this two-part paper the production of synthetic natural gas (SNG) through integrated plants featuring high temperature electrolysis and subsequent syngas methanation is analyzed. Part one focuses on plant con fi guration and performance evaluation. Part two focuses on cost for an economic assessment. Two different ways to produce SNG have been analyzed: the fi rst option features a plant that integrates steam electrolysis with methanation (Sabatier reaction); the second one considers co-electrolysis of water and carbon dioxide coupled with TREMP TM (Topsøe recycle energy-ef fi cient methanation process). In both cases high temperature electrolysis with solid oxide cells (SOEC) technology has been employed. A power input of 10MWe was taken as the DC electricity input for both plant SOEC generators, based on power-to-gas plants now under construction in Europe. Sensitivity analyses were used to evaluate the impact of selected operating parameters on the plant performance. Especially the pressurization of the SOEC brings a reduction of the over all electrical input required to run the plant. By operating a pressurized SOEC, post-electrolysis syngas compression (that would be required otherwise because methanation takes place at 33bar) is partly replaced by liquid water pumping before electrolysis. Thermal integration based on the pinch analysis methodology was also applied in order to calculate the minimum external (thermal) energy requirement. Notably, most of the heat required for vaporizing and super-heating the electrolysis watercan be recovered from the exothermic methanation section. Hence, a good thermal integration is available between SOEC and syngas upgrade catalytic section. This boosts the electricity-to-SNG ef fi ciency to values as high as 80%. The co-electrolysis plant shows a LHV ef fi ciency of 81.4% that is more than fi ve percentage points higher than the steam electrolysis case (76%): notably exothermic in-stack methanation – that occurs in the case of high pressure co-electrolysis assessment – allows for a reduction of the electricity input to the SOEC for the same amount of syngas produced among the two plants.