Modeling and Development of Compact Onboard Fuel Processors for PEM Fuel Cell Applications.

摘要

Liquid hydrocarbons are high energy density fuels, and micro-reactor based fuel processors are viable alternatives to generate hydrogen for portable fuel cell applications. Micro-reactors have high mass and high heat transfer rates due to their small length scales. However, they suffer from thermal heat retention problems due to large surface area to volume ratios. This thesis attempts to explore methodologies to make micro-reactors thermally efficient. The approach is by use of a counterflow heat exchanger configuration for heat recuperation, and quantifying parameters by which operation of the system can be controlled and optimized.;In the first part, a simulation of the preferential oxidation (PrOx) catalytic reaction is setup in a cylindrical channel. The channel is contained within a counterflow heat exchanger to recuperate excess heat. Performance of the system is evaluated by using the concept of reactive length and thermal efficiency. The ratio of the channel used for 95% of net conversion of CO is defined as reactive length. Thermal efficiency is defined as the ratio of heat recuperated to the total heat available. The PrOx reaction model is verified by comparing simulation with experimental data.;A parametric study then is performed using parameters such as mass flow rates, inlet temperatures, thermal conductivity, PrOx selectivity, mass of catalyst and inlet concentrations. Significant parameters by which the system can be controlled effectively are identified. Parameters such as the inlet temperatures that directly affect the systems enthalpy are found to be the most effective. The remaining parameters have a smaller effect, but can be used to fine tune the operation of the system. The control mass flow rate can be used as an active control during operation.;In the second part, simulation is employed to understand the flow and thermal characteristics of two types of micro-reactors: silicon based and channel based micro-reactors. For silicon substrate micro-reactor, it is found that the flow mixes and redistributes itself due to high mass transfer limits. The temperature is uniform in the substrate due to high thermal conductivity of silicon. CFD simulation is able to predict these temperatures within the silicon micro-reactor to 5% accuracy. The silicon micro-reactor is compared against a packed-bed reactor and is found to operate comparable due to similar length and time scales. A 1-D reaction model is also able to predict the conversion trends in both the packed-bed and silicon micro-reactors. Collapsing a 2D temperature field to an average temperature underestimates the reaction rates and conversion since the Arrhenius kinetics are exponential with temperature.;The channel based micro-reactor design consists of three generations of fuel processors that are constructed and demonstrated by the University of Michigan fuel processor team. The first generation is a proof of concept for channel flow micro-reactors, while the second generation uses discrete reactors (ATR, WGS & PrOX) for each processor stage. The second generation processor performed to specifications but required external power to maintain operation. The third generation fuel processor combines these separate stages into a single physical package giving a thermally integrated fuel processor with internal heat recuperation. A continuous self-sustaining operation of the third generation fuel processor is demonstrated over extended periods of time. Thus showing that a thermally integrated iso-octane fuel processor can be built with self-sustaining capabilities. There are potential applications for such micro-reactor based fuel processors as portable electronics, military hardware, quick recharge devices and more.