Design, fabrication, and characterization of a micro fuel processor

Abstract

The development of portable-power systems employing hydrogen-driven solid oxide fuel cells continues to garner significant interest among applied science researchers. The technology can be applied in fields ranging from the automobile to personal electronics industries. In order for fuel cell systems to outperform batteries, a method of chemically converting high-energy-density combustible fuels to hydrogen while maintaining high thermal efficiency must be developed. This thesis focuses on developing microreaction technology that minimizes thermal losses during the conversion of fuels - such as light end hydrocarbons, their alcohols, and ammonia - to hydrogen. Critical issues in realizing high-efficiency devices capable of operating at high temperatures have been addressed: specifically, thermal management, the integration of materials with different thermophysical properties, and the development of improved packaging and fabrication techniques. A new fabrication scheme for a thermally insulated, high temperature, suspended-tube microreactor has been developed. The new design improves upon a monolithic design proposed by Leonel Arana. In the new modular design, a high-temperature reaction zone is connected to a low-temperature package via the brazing of pre-fabricated, thin-walled glass tubes. The design also replaces traditional deep reactive ion etching (DRIE) with wet potassium hydroxide (KOH) etching, an economical and time-saving alternative. A glass brazing method that effectively accommodates the difference in thermal expansion between the silicon reactor and the glass tubes has been developed. The material used in this procedure is stable at temperatures up to 710 °C. Autothermal combustion of hydrogen, propane, and butane in excess oxygen has been demonstrated in ambient atmosphere and under vacuum. Hot spot temperatures of up to 900 °C have been measured during autothermal combustion of propane in ambient and vacuum conditions.

(cont) Experimental temperature measurements have been compared to steady-state temperature estimates, and show good agreement. Finally, a computational fluid dynamics (CFD) model has been developed to study the heat transfer properties of the microreactor. Using simplified reaction schemes from the literature, the model has successfully reproduced the results observed in the laboratory.