Thermally integrated ceramic microreactors for hydrogen production

The aim of this project has been to demonstrate a new design approach for realizing heat-integrated ceramic microchannel networks for the autothermal production of hydrogen for potential application in portable power systems. The design strategy involves two unique routes to materials processing: (i) precision machining that allows the creation of complex flow distribution patterns and (ii) ceramic extrusion that enables cost-effective production of microchannel networks for thermal and/or mass integration of several unique chemical processes. ^ One-dimensional analysis demonstrated that the use of high thermal conductivity materials (e.g. silicon, stainless steel) significantly limits thermal efficiency owing to axial conduction losses, ultimately leading to isothermal-slab behavior. Low thermal conductivity materials (e.g. cordierite ceramics, glass) yield superior thermal efficiencies, resulting from development of temperatures gradients along the solid phase axial length. Therefore low thermal conductivity materials are necessary to achieve reasonable thermal efficiency. ^ Starting with an initial prototype for integrating combustion and steam reforming of methanol for autothermal production of hydrogen, the capability of this new device was demonstrated. In the absence of any external insulation, stable reforming of methanol to hydrogen at conversions \u3e90% and hydrogen yields \u3e70% were achieved at a maximum reactor temperature of 400°C, while simultaneously maintaining a packaging temperature \u3c50°C. ^ Subsequently, three brass prototype microreactor designs were successfully fabricated and tested in order to investigate the influence of scale-up, individual process flow rates and radial distribution schemes upon thermal gradients and overall system performance in the absence of any external insulation. Results demonstrated that three separate two-dimensional radial distribution patterns in cost-effective, cartridge-based ceramic microchannel are capable of creating one and three-dimensional thermal gradients that enhance hydrogen yields. However, in the absence of external insulation, maximum hydrogen yields were limited to ∼32% for all three architectures. ^ Attempts were made to achieve miniaturization of the three proposed architectures by using Si-MEMS technology. While the functionality of the flow distributors was not demonstrated, the main goal of reducing weight and volume was accomplished with an overall size reduction of 98%. Further improvements of the microfabrication steps will enable the construction of functional silicon flow distributors for the autothermal production of hydrogen from methanol.