(97f) Understanding Critical Heat Transfer Issues in Thermal Integration of Planar Microreactor Components of a Fuel Processor

Microreactors seem suited to small scale power generation, as their compactness should enable integration by stacking of miniature fuel processor and fuel cell components with fuel delivery system. The microscale dimensions which result in extremely small heat transport resistance, make them especially attractive in applications like fuel processing, where it is desired to thermally couple highly endothermic and exothermic processes in an efficient manner. The fuel processor needs effective thermal coupling to allow transfer of energy from the heat producing combustor to the endothermic steam reformer. Coupling endothermic and exothermic components of the fuel processor and minimizing losses can achieve a high thermal efficiency. However, such coupling must be accomplished in a manner that permits the maintenance of specific temperatures in the various components and maintains the surface of the package near room temperature. Microreactors generally offer high heat transfer rates mainly because of high surface-to-volume ratio and short conduction paths. This characteristic results in efficient heat extraction but at the same time results in higher heat losses to the ambient. Therefore, heat management offers a dual challenge of opposing the heat losses from the system that arise from high surface-to-volume ratio in conjunction with maintaining temperature gradients within the system to allow desired conditions in the unit reaction steps. In this study, a silicon microreactor-based catalytic methanol steam reforming reactor was designed and fabricated in the context of complete thermal integration to directly address the heat management issue. The design is made where vacuum packaging chips, thin film heater, and temperature sensors are directly embedded with the microreactor to simulate an integrated steam reformer in an overall fuel processing scheme. Detailed experiments are carried out to quantify heat losses through various pathways from the planar microreactor structure. The result provides fundamental insight in understanding of critical thermal transfer issues of an integrated microreactor system such as transfer of heat between reactor components, control of temperature, insulation, and heat losses. Based on this understanding, suggestions are made for scale up of reactor components and a packaging scheme for reduction of convective and radiative losses.