Modeling of Supercritical Water Gasification of Xylose to Hydrogen-Rich Gas in a Hastelloy Microchannel Reactor

Microchannel reactors offer high rates of heat transfer that intensify biomass gasification in supercritical water by sustaining the reaction temperature in the presence of endothermic reforming reactions and providing rapid fluid heating. Furthermore, the large ratio of surface area to volume in the microchannels enhances "unintentional" catalytic activity from the reactor wall for reactors comprised of nickel alloys such as Hastelloy. In this study, a parallel-channel Hastelloy C-276 microreactor was used to gasify xylose, a hemicellulose model compound, at 650 degrees C and 250 bar. The reactor consisted of 14 parallel microchannels, each measuring 127 mu m x 1000 mu m, integrated into a contiguous reactor block using scalable microfabrication techniques. The channels were configured in a serpentine design, which resulted in temperature gradients within the channels during fluid heating isolated from those in subsequent channel passes. Complete conversion of a 4.0 wt % aqueous xylose solution to hydrogen-rich gas was achieved with an average fluid residence time of 1.4 s. Computational fluid dynamics (CFD) modeling was used to simulate xylose gasification in the microchannel reactor and investigate temperature gradients produced by the heat of reaction for xylose gasification. Additional CFD simulations were used to show the effect of short residence times (less than 1.0 s) on the reacting fluid temperature while in the reactor. The results from this study suggest that parallel-channel Hastelloy microreactors potentially offer a unique way to improve biomass gasification by supercritical water.