Integrated Autothermal Reformer, Heat Exchanger and Solid Oxide Fuel Cell in Single-Stack for Aircraft Gas-Turbine Applications
Authors: Lei Fuqiong , Gu Yifan , Ashar Akhil , Boushehri Cyrus , Vincent Tyrone L , Pratt Lucas M , Cadou Christopher P , Wachsman Eric D , Braun Rob J , Jackson Greg S 
Published: December 2024

To take advantage of carbon-neutral aviation fuels such as synthetic liquified natural gas, aircraft engines must increase their efficiency through novel approaches, such as hybrid electric gas-turbine/solid oxide fuel cells (GT/SOFCs). To date, most hydrocarbon-fueled SOFC stack designs utilize rigid architectures and independent pre-reformers that require complex manifolding and rigid sealing. To enable SOFCs to operate effectively and robustly within an aircraft GT engine flow path upstream of a combustor, our team is developing an innovative integrated SOFC stack with an inline autothermal reformer/heat exchanger (ATR/HX) to provide adequate operating conditions for high-power (W/cm2) performance. The ATR/HX, integrated upstream of the stack, provides preheating of the cathode air through mildly exothermic reforming of the fuel with a bleed of combustor air and recycling of some anode exhaust. The exothermic ATR provides adequate heat to the cathode air to allow intermediate-temperature SOFCs, with either gadolinium-doped ceria (GDC) electrolytes or thin-film yttria-stabilized zirconia (YSZ) electrolytes, to operate on GT compressor outlet temperatures just above 400 °C. To enable rapid thermal response of the integrated ATR/HX/SOFC, the stack design eliminates rigid seals to mitigate the risks of SOFC failure due to thermomechanical stresses. This paper presents the design and preliminary testing of the integrated ATR/HX/SOFC under rapid heating conditions to suggest the potential for SOFCs for next generation hybrid-electric aircraft application.

The ATR/HX/SOFC stack design is supported by 441 stainless steel plates with electrochemically etched, air-flow channels through the upstream HX and the SOFC cathode. The plates also include a pocket for the ATR, which consists of a woven metal-mesh with an Al2O3-washcoat supported Pt catalyst that can light off with CH4/bleed air/H2O inlet temperatures of 400 °C. The SOFC membrane electrode assembly (MEA), 10 cm*10 cm with 81 cm2 active cathode area, rest within a frame that supports the MEA as well as a silver mesh cathode collector and a nickel mesh anode current collector. The cell is sealed by compressing a thermiculite seal that extends over the full area of the stack and compresses on the exposed electrolyte area bordering the cathode. Testing at operating temperatures indicated minimal leakage (<1%) from the anode and from the cathode to the external environment. This design, which lacks rigid seals or confinement of the MEA, enables ease of assembly and disassembly and minimizes external stresses on the MEA to enable rapid heating during ATR light-off.

Integrated ATR/HX/SOFC stack has been initially incorporated into a low-pressure test facility shown in Figure 1a), which provides steam generation for the ATR inlet and premixing and preheating for the ATR inlet and air-side HX inlet flows. The ATR/HX/SOFC stack is preheated to 400 °C or more to achieve rapid light-off that rapidly preheats the SOFC inlet to desirable temperatures. For a 3-cell ATR/HX/SOFC short stack, light-off test to SOFC operating temperatures of 550 °C for thin-film YSZ-electrolyte MEAs from Elcogen are achieved in < 1 h. Such start-up times are expected to be reduced for larger ATR/HX/SOFC stacks with reduced heat loss per stack volume. Tests to date have only achieved maximum MEA power densities of 0.26 W/cm2 at 0.65 V/cell operating on the ATR outlet. Simulated performance shows pathway to higher power densities > 1.0 W/cm2 with higher temperature operation that will be achieved with coated interconnect materials. Tests to date show that the thin-film YSZ cells avoid cracking after undergoing multiple assembly and disassembly cycles, as well as high-temperature ATR light-off and fuel cell testing. These results indicate that both mechanical and thermal stresses remain sufficiently low to prevent cell cracking throughout start-up, normal operation, and shut-down processes, thus affirming the robustness and reliability of our integrated stack design.

Figure 1