A potential route to synthesis gas from biomass is gasification under conditions in which water is in the liquid or fluid phase at elevated temperature and hydrostatic pressure. Exploratory research has been done in a laboratory-scale, plug-flow reactor with solutions of glucose, the monomeric unit of cellulose, in pure water without addition of any potential catalyst (Klass, Kroenke, and Landahl, 1981; Ng, 1979). Some of the results are summarized in Table 9.7. Gasification experiments carried out below the critical temperature for water (374°C) indicated little or no gasification. At temperatures above 374°C, con­version to a relatively clean synthesis gas began to occur, as shown in this table. Char was not observed. Hydrogen yield and concentration in the product gas and the molar ratio of H2: CO exhibited significant increases with increasing temperature. Biomass gasification under these conditions might be expected to offer unique opportunities for homogeneous catalysis at lower capital and operating costs than heterogeneously catalyzed systems.

Heterogeneous catalysts have been found to be effective for the low — temperature, elevated-pressure gasification of 2 to 10% aqueous biomass slur­ries or solutions that range from dilute organics in wastewater to waste sludges

from food processing (Elliott et ah, 1991, 1993). Continuous, fixed-bed cata­lytic reactor systems have been operated on three scales ranging from 0.03 to 33 L/h. The residence time in the supported metallic catalyst bed is less than 10 min at 360°C and 20,365 kPa at liquid hourly space velocities of 1.8 to 4.6 L of feedstock/L of catalyst/h depending on the feedstock. Aqueous effluents with low residual COD (chemical oxygen demand) and a product gas of medium-energy quality have been produced. Catalysts have been demonstrated to have reasonable stability for up to six weeks. Ruthenium appears to be a more stable catalyst than nickel. The product gas contains 25 to 50 mol % carbon dioxide, 45 to 70 mol % methane, and less than 5 mol % hydrogen with as much as 2 mol % ethane. The by-product water stream carries residual organics and has a COD of 40 to 500 ppm. The medium-energy product gas is produced directly in contrast to medium-energy, gas-phase processes that require either oxygen in place of air or the dual reactor system to keep the nitrogen in air separated from the product.