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 °C 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.

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Previous work has shown that very high yields of charcoal are obtained when pyrolysis of the biomass feedstock is conducted at elevated pressure in a closed vessel, wherein the pyrolytic vapors are held captive and in contact with the solid products of pyrolysis. In this paper, we show that, for some biomass species, the yield of carbon produced by this process effectively attains the theoretical value predicted to exist when thermochemical equilibrium is realized. Various agricultural wastes (e.g., kukui nut, macadamia nut, and pecan shells) and tropical species (e.g., eucalyptus, leucaena, and bamboo) offer higher yields of carbon than the hardwoods traditionally employed by industry in the U.S. and Europe. Moreover, the yields of carbon from oat and rice hulls and from sunflower seed hulls are nearly as high as the yields of carbon from hardwoods. There is a correlation between the yield of carbon and the acid-insoluble lignin content of the feed. Charcoal briquettes made from agricultural wastes and lump charcoal from tropical species are promising sources of renewable carbon for use in the smelting of metal ores.