Non-catalytic conversion of biomass under these conditions (230-400°C) is very susceptible to the formation of carbonaceous deposits (see also Figure 20.2). In fact without a catalyst, the product distribution consists only for ca. 10 wt% of permanent gases (primarily CO2) and 90 wt% condensed products. This is mainly caused by sugars and their decay products as they can easily polymerize in hot compressed water.58 Pacific Northwest National Laboratory (US) developed a catalytic process for the destruction of organic waste at ca. 350°C while producing a methane rich gas.59-61 Tests were carried out at laboratory and pilot scale focusing on both catalyst and process development. Ruthenium on rutile titania, ruthenium on carbon and stabilized nickel catalysts showed the highest activity and the best stability. With these catalysts, nearly 100% gasification of model components (1-10 wt% organics in water) was achieved. The gas produced consisted of nearly only CH4 and CO2, as dictated by the overall thermodynamic equilibrium. The catalytic process was carried out in a series of fixed bed reactors. When using feedstock materials with the tendency to produce char/coke, a continuous stirred-tank reactor (CSTR) was required before the fixed bed to soften the feed and to prevent the buildup of solids. Pilot plant runs using complex feeds like potato waste and manure were carried out. The required liquid hourly space velocity (LHSV) was in the range of 1.5-3.5 Nm3feed/m3cat/h. For a waste disposal process these LHSVs are acceptable, but for the production of gaseous energy carriers from biomass the activity is rather low. Waldner62 reported high extents of gasification and equilibrium methane yield of concentrated (up to 30 wt%) wood sawdust slurries using Raney Nickel as catalyst at 400°C. For complete gasification, 90 minutes reaction time was required in their batch reactor.

The catalysts employed accelerate the rate of the gasification reaction relative to the rate of poly condensation/polymerization reactions, or they are able to gasify the formed polymers, or a combination of both. However, after comparing reaction rates it can be argued that the majority of the gas is produced via gasification of partially polymerized components: in non-catalytic experiments with monomer sugars as feed maximal oil (polymerized components) yields are obtained for reaction times of 2-5 minutes,58 whereas in catalytic test 30 up to 90 minutes reaction time62 are needed to achieve complete gasification. Van Rossum et al3 proposed a simplified lumped reaction path scheme for the conversion of small carbohydrates (< C6) in hot compressed water (see Figure 20.8). Savage63 and Kruze57 reported extensive reviews on catalysis and reactions in supercritical water.

Huber et al.15 and Cortright et al.6 reported interesting catalysis around 230°C for the production of hydrogen rich gas from small oxygenated hydrocarbons. They were able to decrease the methane formation rate via C-O bond cleavage and methanization (hydrogenation) while maintaining the high rates of C-C bond cleavage and shift for hydrogen production. Cortright used a Pt catalyst, Huber a Raney nickel catalyst promoted with tin. High hydrogen yields were obtained for methanol, ethylene glycol and glycerol. However, with sorbitol and glucose as feedstock already significant amount of methane were being produced next to hydrogen. Though in an embryonic stage, the methodology of decelerating methane producing reactions at catalytic sites while keeping a high rate of catalytic hydrogen production seems promising to produce hydrogen rich gas at conditions for which overall chemical equilibrium dictates a methane rich gas, viz. at sub critical temperature and at the combination of high temperature and high concentration of organics. In this concept, it will be important to decrease

homogeneous reactions to undesired by-products (oil/char/CH4) and to increase the reaction rate. This is quite a challenge for both catalyst and reactor design.