(235g) Structural Optimization-Based Design of an Integrated Biogas – SOFC System for Sustainable Electricity Production

Biogas has a strong potential for renewable power generation. Conventional route consists of biogas combustion to generate high pressure steam, which subsequently drives a turbine to generate electricity. Recently, an alternate route of converting biogas (methane) into hydrogen via methane steam reforming, and driving a fuel cell to generate electricity is gaining popularity. This new approach poses better conversion efficiency and potential for synergy between the component steps. Existing literature consists of multiple design alternatives around biogas enrichment, recycle of fuel cell effluents and energy integration between endothermic reforming and exothermic fuel cell [1-3]. However, no formal design optimization and subsequent comparison has been pursued. Motivated by this, the present work aims at designing an optimal process for the biogas reforming + fuel cell route using structural optimization.

Based on the existing literature, a flowsheet superstructure, as shown in the figure below, is constructed. It incorporates the design alternatives like biogas enrichment via CO2 removal, recycle of anode gas to improve hydrogen utilization, recycle of cathode gas to reduce air intake, provision for pre-reformer and direct internal reforming and additional power generation through steam turbine driven by effluent heat. Initially, a feed minimization problem is solved to come up with an optimal flowsheet in the absence of heating cost. Using pinch analysis, two heat exchanger network alternatives are designed. Subsequently, an operating cost minimization problem is solved to come up with an overall optimal flowsheet. Lastly, the robustness of the proposed flowsheet is analyzed via sensitivity studies with respect to key parameters.

The proposed study provides key insights into the selection of various design variables (such as extent of external/internal reforming, fuel utilization, etc.), processing routes (anode gas recycle, biogas enrichment, etc.), and heat and power integration potential (steam turbine power versus reformer heat demand) of the system. The performance of the proposed flowsheet is compared with literature.

References:

[1] Galvagno, A., Chiodo, V., Urbani, F., & Freni, F. (2013). Biogas as hydrogen source for fuel cell applications. International Journal of Hydrogen Energy, 38(10), 3913-3920.

[2] Siefert, N. S., & Litster, S. (2014). Exergy & economic analysis of biogas fueled solid oxide fuel cell systems. Journal of Power Sources, 272, 386-397.

[3] Baldinelli, A., Barelli, L., & Bidini, G. (2017). Upgrading versus reforming: An energy and exergy analysis of two Solid Oxide Fuel Cell-based systems for a convenient biogas-to-electricity conversion. Energy Conversion and Management, 138, 360-374.