12 Catalytic Conversion of High-Moisture Biomass to Synthetic Natural Gas in Supercritical Water
Part 2. Heterogeneous Catalysis
Published Online: 15 JUL 2010
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Handbook of Green Chemistry
How to Cite
Vogel, F. 2010. Catalytic Conversion of High-Moisture Biomass to Synthetic Natural Gas in Supercritical Water. Handbook of Green Chemistry. 2:281–324.
- Published Online: 15 JUL 2010
Methane produced from waste biomass is a renewable and clean biofuel that can be distributed using the existing natural gas infrastructure. It can be used for heat and power generation and as a transportation fuel. High-moisture biomass is a relatively untapped resource with a significant energetic potential and attractive costs. However, new technologies are needed for converting high-moisture biomass efficiently into methane and recovering the nutrients for use as a fertilizer. Gasification of the biomass in a hydrothermal environment is an emerging technology that offers many advantages over gas-phase conversion processes or anaerobic digestion. Heterogeneous catalysis is the key to a successful hydrothermal gasification process for the synthesis of methane. Only a few metals, including Ru, Ni, Rh and Pt, are useful under these conditions. Pd and Co catalysts might also be suitable but conclusive data are lacking. Alloying is another approach that holds promise to yield active and stable catalysts. The choice of hydrothermally stable supports is limited to some insoluble oxides and carbon. Some of these oxides have not yet been tested as catalyst supports (e.g. Nb2O5, Ta2O5 and UO2) and might prove useful. The mechanism for the gasification of the organic compounds to CO and H2 is likely to follow a Mars–van-Krevelen redox cycle with two oxides of the catalytic metal involved. The strongest evidence for such a mechanism was found for RuO2, but specific in situ studies are needed for corroborating this hypothesis and determining the actual oxidation states involved in the mechanism. Deactivation in hydrothermal gasification follows the same modes as in gas-phase and liquid-phase catalysis. Coke deposition is not a primary cause of deactivation due to the high partial pressure of water and the high solubility of coke precursors in near- and supercritical water. Salts play a crucial role in catalyst deactivation. Sulfate was found to be a strong poison for Ru catalysts, but the actual poison might be sulfide, formed by reduction of the sulfate with hydrogen or organic compounds. Based on this knowledge, a continuous catalytic hydrothermal gasification process is under development at PSI featuring continuous on-line salt precipitation and removal before the catalytic reactor.
- natural gas;
- catalytic hydrothermal gasification;
- supercritical water;
- heterogeneous catalysis