Performance Evaluation of an Anode-Supported Solid Oxide Fuel Cell Short-Stack Operating with Different Hydrogen-Natural Gas Blends as Stationary Combined Heat and Power System

Abstract

One of the most promising characteristics of Solid Oxide Fuel Cells (SOFCs) is the possibility to process a wide variety of fuels with high electrical efficiencies, when compared to other types of fuel cells. Considering the prospected transition of the Natural Gas (NG) grid to hydrogen to decarbonize the gas sector, SOFCs could play a crucial role as efficient and clean stationary Combined Heat and Power (CHP) systems.In this work a 15-cell SOFC short-stack (Anode Supported, cell area 121 cm2) has been analysed experimentally under different compositions resembling possible short-, medium- and long-term gas grid scenarios (100% NG, 67%H2 blend in NG and 100%H2) which have been obtained by a preliminary system-level simulation which takes into account operating conditions of the stack and fine-tuning of the system parameters (e.g. Recirculation Rate RR, once through Fuel Utilization FUot, global Fuel Utilization FUtot) to optimize the overall system-level efficiency. Several tests were run at different temperatures (640-710 °C), Fuel Utilization (FU, 50%-70%) and at current loading (15-30 A - part/full load), characterizing in detail the stack response from an electrochemical (polarization curves and short-term stability tests) and chemical conversion point of view (gas chromatographic analyses in OCV and under load), as well as mapping its performances in stationary conditions (output power and stack-level efficiency). The conversion efficiency has been assessed in detail with different approaches at both stack and cell level, considering both once-through (stack-level) and total (system) FU rates.After having confirmed that the stack performances in reference testing conditions are fully aligned to the stack manufacturer data, confirming the test bench setup validity, the foreseen testing campaign has been carried out showing compatibility of the SOFC stack in the investigated conditions. The results show that from a pure performance point of view the 100%H2 composition obtains highest voltage and power at nominal conditions due to losses of the reformate compositions due to lower OCV values and kinetic behaviour of the chemical reaction chains. Instead, from an efficiency point of view the reformate conditions obtain higher stack-level efficiency (72.02% for the 67%H2+NG; 69.45% for the 100%NG) respect to the 100%H2 compositions (68.61%) due to the lower input fuel calorific value. The comparison of the different efficiency definitions provides further insight on the process and system level. By comparing reference testing conditions before and after the testing campaign it is possible to confirm that no significant degradation has occurred at global stack level in all the compositions. Figure 1