Catalytic reforming of activated sludge model compounds in supercritical water using nickel and ruthenium catalysts

We report the catalytic supercritical water reforming of model compounds of activated sludge at 380 °C using Raney nickel, Ni/α-Al2O3, Ru/C, and Ru/γ-Al2O3 catalysts. The model compounds were glucose, glycine, glycerol, lauric acid and humic acid, representing carbohydrates, proteins, alcohols, fatty acids and humic substances, respectively. Using Raney nickel as the catalyst, the carbon conversions decreased with the following order: glycerol > glucose > glycine > lauric acid > humic acid. The conversion generally increased with reduction in the number of CC bonds presented per unit mass of the molecules, except for the glycine that contains a nitrogen atom in its structure. Comparison of the experimental yields of methane with the equilibrium values calculated at the corresponding conversions revealed that, in the presence of Raney nickel, methane is almost at a quasi-equilibrium state. Moreover, using binary mixtures of the above model compounds as feedstock, the interactions between these model compounds were investigated. At low catalyst loadings, the presence of humic acid in the binary mixtures resulted in lower carbon conversions compared to the expected values based on the rule of mixtures. Higher gas yields were obtained from the decomposition of these model compounds compared to an activated sludge feedstock. Addition of sludge ash decreased the gasification yields of glucose and glycerol and increased the yields obtained from glycine. Furthermore, since algae are comprised of lipids, proteins, and carbohydrates, the findings of this study also provide a better understating of the catalytic gasification of such feedstock in supercritical water.

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► Higher gas yields obtained from model compounds compared to sludge.
► Methane and carbon dioxide are almost at a quasi-equilibrium state.
► Humic substances are relatively more difficult to decompose.