(559y) Improved Design of a Diesel-Driven Solid Oxide Fuel Cell System for Auxiliary Power Unit Applications

Bae, M., KAIST
Bae, J., Korea Advanced Institute of Science and Technology (KAIST)
Katikaneni, S. P., Saudi Aramco
In this study, a diesel-driven solid oxide fuel cell system has been developed for auxiliary power unit applications. Among various hydrocarbon fuels, diesel fuel has many advantages such as high energy density and versatility, and it is often used for heavy-duty applications and remote areas. Diesel itself can be used to generate electricity with a diesel generator, but it produces high pollution and noise, with low energy efficiency. In another way, a diesel autothermal reformer can be introduced to generate hydrogen-rich reformate that can be used for a solid oxide fuel cell stack to generate electricity. With the diesel reformer, the diesel fuel inside a fuel tank of the heavy-duty truck can be directly used to generate hydrogen for the fuel cell system.

With the integration of the diesel autothermal reformer and the solid oxide fuel cell stack, a fuel cell system for the auxiliary power unit application has been developed. For the heavy-duty truck applications, the power output target of the auxiliary power unit was set to one kilowatt to satisfy the electrical needs. Structured catalysts were introduced for the diesel reformer to ensure the mechanical strength of the component. During the development of the diesel reformer, a long-term test for the catalyst component was conducted to ensure its catalytic stability. 2,000 hours of operation was achieved with the structured catalyst for the autothermal reforming.

In commercial diesel fuel, a small amount of sulfur content exists. To protect the fuel cell stack, desulfurization with ZnO based adsorbent was followed by the autothermal reforming process. The temperature of the reformate has to be controlled within the range of the operating window of the desulfurizer to reduce the sulfur content to ppm level. A heat network analysis for the reformer module including the desulfurization process was done and required specifications for heat exchangers were obtained.

Two versions of the integrated fuel cell system were designed with different assumptions for the thermal stability of the solid oxide fuel cell stack. The solid oxide fuel cell stack needs a high operating temperature around 700 deg C, and the thermal stability of the stack determines overall system start-up time. The first version of the system was designed with meeting the requirement of the slow heat-up of the stack. The temperature of the fuel cell stack is increased with the heat from the reformate oxidation in the first design, so it takes some time to reach the operating temperature. However, the start-up time of the fuel cell system can be shortened if the fuel cell stack has enough thermal stability that can sustain the fast heat-up process. Therefore, the system configuration can also be modified at the improved version by using external heat sources during the start-up process. In result, shorter start-up time and simplified start-up process were obtained with the enhanced design of the fuel cell system.