Diesel Steam Reforming for PEM Fuel Cells

Abstract

Different applications for the decentralized stationary or mobile power supply require the usage of liquid hydrocarbons such as fuel oil, diesel, or gas oil in fuel cell systems. Reducing the sulfur content of conventional liquid fuels such as diesel or gasoline below 10ppm, the usage of these fuels in fuel cell applications becomes increasingly promising. The first process step represents thereby the reforming, which can be carried out in different ways. One of the commonly favored gas process technologies is the steam reforming process, which is state of the art for natural gas applications. Using a proton exchange membrane (PEM) fuel cell requires a complex gas cleanup system. Using a pressurized steam reforming process offers a significant reduction of the whole system size by efficiently compressing the liquid educts. Complete PEM systems with steam reformers tend to have a higher efficiency than, for example, systems using the autothermal reforming process. Advanced diesel steam reformers for industrialization still have to be developed and improved. The Oel-Wärme-Institut gGmbH has successfully carried out research on steam reforming with variations of important parameters using a sulfur free reference fuel and desulfurized diesel. During the experiments, several parameters such as steam to carbon ratio, reformer inlet temperatures, catalysts, and fuels were varied. While running the process, a continuous product gas measurement was taken. The reformer is equipped with several thermocouples. Three of them are moveable to measure the temperature profile of the catalyst. The experiments show that product gas concentrations reach a nearly equilibrium concentration with reformer inlet temperatures ϑ>700°C of a reference fuel∕steam mixture. Hydrogen concentrations over 70% were feasible. Constant inlet temperatures of ϑ=850°C and a variation of the steam to carbon ratio only have a noticeable effect on the water gas shift equilibrium. After all experiments, carbon deposits were found in the steam reformer system and under some circumstances on the catalysts. Experiments with operating times of more than 20h were performed at a steam to carbon ratio of 4.5. The application of continuously desulfurized diesel fuel indicates a degradation of the catalyst after a few hours. For the overall system design of PEM fuel cell applications, an operation mode at a reduced steam to carbon ratio has to be developed.