Biomass gasification in supercritical water

Biomass feedstocks, including corn- and potato-starch gels, wood sawdust suspended in a cornstarch gel, and potato wastes, were delivered to three different tubular flow reactors by means of a "cement" pump. When rapidly heated to temperatures above 650 degreesC at pressures above the critical pressure of water (22 MPa), the organic content of these feedstocks vaporized. A packed bed of carbon within the reactor catalyzed the gasification of these organic vapors in the water; consequently, the water effluent of the reactor was clean. The gas was composed of hydrogen, carbon dioxide, methane, carbon monoxide, and traces of ethane. Its composition was strongly influenced by the peak temperature of the reactor and the condition of the reactor's wall. Extraordinary yields (>2 L/g) of gas with a high content of hydrogen (57 mol %) were realized at the highest temperatures employed in this work. Irrespective of the reactor geometry and method of heating, all three reactors plugged after 1-2 h of use with feedstocks that contained 15 wt % organic material. Lower loadings of organics lengthened the time before plugging occurred. The plug resulted from accumulations of ash and small amounts of char formed by coking reactions involving the biomass vapors. A method for removing plugs from the reactor was developed and employed during an 8-h gasification run involving potato wastes. Extensive corrosion of each reactor's inner wall occurred during these tests. Nickel and other metals were leached from the reactor and deposited in the carbon catalyst. Nickel alloy tubes are not suitable for use in this application.