Direct catalytic methanation of biogas – Part II: Techno-economic process assessment and feasibility reflections

Authors: Witte, J., Kunz, A., Biollaz, S. M. A., Schildhauer, T. J.

Six different processes for the direct methanation of biogas for biomethane production were evaluated regarding their economically optimised operating conditions, their absolute costs and profitability. Detailed process simulations turned out to be important to obtain a solid process design so that a realistic cost estimate is possible. This is necessary since the stated costs in literature vary by a factor of 10 due to the shortage of simulation details. In addition, cost data for the methanation from recent literature are often under-estimated due to the lack of a required second upgrading step (post-upgrading) to reach the demanded gas grid specifications and of additional process equipment. A fixed bed methanation reactor is two times more expensive than a bubbling fluidised bed reactor but due to the dominant share of costs of the electrolysis, costs differences between the whole processes of 5% are less pronounced. It turned out that a gas separation membrane and a second fixed bed, as a post-upgrading step, are equally expensive. However, a gas separation membrane seems to be technically more robust. Biomethane production costs of 10.2 Ct(epsilon)/kWh(total) for processes containing a fluidised bed and of 10.7 Ct(epsilon)/kWh(total) for processes containing a main fixed bed are obtained. As a consequence, the corresponding processes are profitable until an electricity price of 6.0 Ct(epsilon)/kWh(el) and 5.3 Ct(epsilon)/kWh(el) without considering the extra revenues from process heat and oxygen. Furthermore, the profitability of the processes increases with the methane content in the biogas. Considering extra revenues and a concentration increase of 10% for methane in the biogas, the maximum electricity price for a profitable process increases by 3 Ct(epsilon)/kWh(el).

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