High-temperature microfluidic systems for thermally-efficient fuel processing

Abstract

Miniaturized fuel cell systems have the potential to outperform batteries in powering a variety of portable electronics. The key to this technology is the ability to efficiently process an easily-stored, energy-dense fuel. In many cases, use of these fuels requires a fuel processor-a high temperature chemical reactor that generates a hydrogen-rich stream for use by the fuel cell. In high-temperature microfluidic systems, where heat transfer rates are often very high, thermal management is a major challenge. This thesis investigates the use of silicon microfabrication technology to fabricate high-temperature submillimeter-scale fuel processors designed to maximize thermal efficiency. A prototype MicroElectroMechanical Systems (MEMS) chemical reactor/heat exchanger for fuel processing has been designed and fabricated. The fuel processor, measuring 8x 10x 1.5 mm, consists of thin-walled silicon nitride tubes and a suspended silicon reaction zone. This structure couples the energy between catalytic combustion and decomposition or steam reforming reactions to produce hydrogen. The design enables a high level of thermal isolation of the reaction zone while allowing heat exchange between process streams. Thermal management in the fuel processors has been characterized up to 825⁰C through experimental testing using integrated resistive heaters and temperature sensors and through finite element modeling. Catalyst localization, for controlled catalytic combustion of premixed fuels in the reaction zone, has been achieved using passive fluidic stop valves. Ammonia decomposition (cracking) and combustion of various fuels over washcoated supported-metal catalysts have been investigated.

(cont.) Using the energy provided by the integrated heater to drive the reaction, up to 1.6 WLHV (based on the lower heating value) of hydrogen has been produced by catalytic ammonia decomposition at temperatures exceeding 800⁰C. Hydrogen burns stably in stoichiometric mixtures with air to >99% conversion for flow rates of hydrogen between 2 and 12 ml-min-1 and steady-state reactor temperatures between 400 and 930⁰C. At higher hydrogen flow rates and reactor temperatures, homogeneous combustion has been observed. Self-sustained (autothermal) combustion of butane at atmospheric pressure and ammonia under reduced ambient pressure (down to 4 Pa) have also been demonstrated. Hydrogen has been produced via ammonia decomposition using energy from hydrogen, butane, and ammonia combustion to drive the reaction.