Simulation of Stack-Integrated Autothermal Reformer and Solid Oxide Fuel Cells for Operation in Aircraft Engines
Authors: Gu Yifan , Ashar Akhil , Lei Fuqiong , Zhu Huayang , Braun Rob J , Jackson Greg S 
Published: December 2024

Renewable liquified natural gas provides a potential sustainable aviation fuel, but with a volumetric energy density around 70% of conventional jet fuel. As such, alternative aircraft power plants such as hybrid gas turbine (GT) / solid oxide fuel cells (SOFCs) have been proposed to increase fuel conversion efficiency and support hybrid electric propulsion concepts. To enable reliable SOFC operation within aircraft gas turbine flow paths, stack architectures must be identified that sustain very high-power densities with direct inline air feeds from the gas turbine compressor outlet. Typical aircraft compressor outlet temperatures are below SOFC operating temperatures but high enough to light-off a catalytic autothermal reformer (ATR). The ATR combined with an air heat exchanger (HX) can preheat anode and cathode streams to desirable inlet temperatures above 500 °C for intermediate-temperature SOFCs that utilize gadolinia-doped ceria (GDC) electrolytes or thin-film, yttria-stabilized zirconia (YSZ) electrolytes. By packaging the inline ATR/HX within the SOFC stack and mixing partial anode exhaust recycle and bleed air with the ATR fuel stream, a volumetrically efficient inline stack architecture is being designed for demonstration at conditions simulating an aircraft gas-turbine compressor outlet. The combination of the ATR/HX and the high-power-density SOFC imposes significant temperature gradients within the stack that can impact performance. To assess suitable operating conditions and preferred designs for the ATR/HX/SOFC operating on renewable natural gas, a 3-D multi-physics CFD model has been developed. Simulation results for SOFCs with thin-film YSZ or GDC electrolytes show the challenge maintaining high-power densities while maintaining the full stack within expected temperature limits for expected cell materials.

The ATR/HX/SOFC stack design includes the upstream ATR/HX, 3.6 cm in length. The ATR flow path is filled by a porous mesh with a washcoat-supported Pt catalyst for light off of CH4/bleed air/H2O inlet mixtures. The air-side flow paths in the ATR/HX are etched parallel channels that extract heat from the exothermic ATR reactions. The ATR/HX outlets feed the SOFC anode and cathode flow paths respectively. For this study, the SOFC membrane electrode assembly (MEA) 10 cm in length, consists of a porous Ni-YSZ anode-support (400 mm thick), dense YSZ or GDC electrolytes (≤ 20 mm thick), and a perovskite-YSZ composite cathode (35 mm thick). The anode flow passages over the support involve a high-porosity mesh that enables current collection. The cathode flow channels are etched ribs that enable high air flow rates to support high power density with adequate SOFC stack cooling. The ATR/HX/SOFC design has been incorporated into an ANSYS Fluent model to explore how integration of the ATR/HX in the stack can enable high-power performance of the SOFC while supporting light-off of the ATR fuel stream at aircraft compressor outlet conditions. The ANSYS Fluent model uses customized user-defined functions to handle the heterogeneous catalytic CH4oxidation surface chemistry for the ATR (supported Pt catalyst [1]) and for internal CH4 reforming in the Ni/cermet anode support in the SOFC [2]. The model simulates non-isothermal operation of a single ATR/HX/SOFC cell, representative of a full stack, at various inlet flow temperatures, pressures, flow rates, and fuel stream compositions, which are set by anode exhaust recycle fraction.

CFD model results for thin-film YSZ-electrolyte MEAs illustrate a strong dependence of SOFC power density on the anode recycle fraction and cathode bleed air splits into the ATR fuel stream. For a 400 °C air inlet, the short ATR/HX is capable of achieving near-equilibrium CH4 conversion to an H2-rich reformate while preheating the cathode inlet up to 500 °C. Simulations over a range of anode recycle fractions show that lower recycle fractions increase ATR outlet temperatures and H2 mole fractions with SOFC power densities reaching 2.0 W cm-2, but at the risk of excessive anode temperatures and large temperature gradients across the length of the MEA. Increased cathode excess air ratios can lower the temperatures at the expense of stack power density. Higher anode recycle fractions decrease the temperature rise across the length of the MEA, but also at the expense power density. Further studies with GDC electrolytes based on recent GDC SOFC model developments [3] will explore how high-power densities of GDC can be supported within its operating temperature constraints.

References

[1] R. Quiceno, J. Pérez-Ramírez, J. Warnatz, O. Deutschmann, Appl. Catal. A: Gen., 303(2), 166–176, 2006.

[2] E. S. Hecht, G. K. Gupta, H. Zhu, A. M. Dean, R. J. Kee, L. Maier, O. Deutschmann, Appl. Catal. A: Gen., 295(1), 40–51, 2005.

[3] H. Zhu, A. Ashar, R. J. Kee, R. J. Braun, G. S. Jackson, J Electrochem. Soc., 170(10), 104506, 2023.

Figure 1