(477b) Development of a Supercritical Water Microchannel Reactor for Reforming Biomass to Hydrogen Gas

Thermochemical conversion of renewable, CO₂ neutral biomass feed stocks to hydrogen gas or synthesis gas is a promising technology route for the production of chemicals and fuels that may significantly contribute to the worlds energy supply. Supercritical water (SCW) at temperatures greater than 374ºC and pressures greater than 220 bar is an effective medium for reforming biomass to hydrogen rich gas. However, SCW gasification of biomass in traditional flow through reactors is limited by heat transfer to the reacting fluid, resulting in long residence times, > 20 sec, and the formation of recalcitrant byproducts and coke. Microchannel reactors can intensify heat transfer to endothermic reforming reactions, and minimize the fluid heating period due to small diameter reactor channels (< 100 µm). A SCW Hastelloy C-276 microchannel reactor was developed to gasify soluble and liquefied biomass constituents including hemicellulose/lignin rich alkaline pretreatment streams and biocrude. The microreactor was designed with the aid of extensive heat transfer modeling and constructed using microfabrication techniques. The reactor architecture consists of 14 parallel rectangular channels (127 µm by 1000 µm) that serpentine 15 times throughout the device. The performance of the device was benchmarked with biomass constituent model compounds. For example, xylose, a hemicellulose model compound, was stoichiometrically reformed to hydrogen rich gas (67% H₂, 33% CO₂) within a 1 second residence time at 650ºC and 250 bar without any byproduct formation. A hydrogen yield of 10 moles of hydrogen per mole of xylose reacted was obtained. The maximum theoretical hydrogen yield from xylose is 5 moles of hydrogen, thus 5 moles of hydrogen in the product gas are liberated from water. This study shows that microchannel reactors have considerable promise for intensifying thermochemical conversion of biomass to chemicals and fuels.