As a result of low reaction rates large reactors are

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Unformatted text preview: trary to digestion, supercritical water gasification of biomass can lead to complete conversion of the feedstock. As a result of low reaction rates, large reactors are required in digestion processes. Another aspect of digestion is that the bacteria cannot handle all feedstocks and can loose activity as a result of poisoning. For supercritical water gasification conventional fluid bed technology is identified as the best reactor concept[9], and the product gas is available at high pressures. Supercritical water gasification is in an early stage of development. Due to its potential with respect to possible conversion of waste materials to a valuable gas, the laboratory research is developing rapidly. At present there are two pilot plants being operated in the world. The largest plant, in operation since the beginning of 2003, is the one of Forschungszentrum Karlsruhe (FzK) in Germany[12]. It has a design capacity of 100 l/hr, and was built to demonstrate supercritical gasification of wet residues from wine production. The second one is the process development unit (PDU) at the university of Twente (Enschede, the Netherlands)[13], with a capacity of 5-30 l/hr, and designed for operating temperatures up to 650°C and pressures of around 300 bar. A simplified scheme of this PDU is presented in figure 1. As first feedstocks “simple” components like ethanol and glycerol have been used, while later trials are intended for the more difficult feedstock types like starch and, eventually, real biomass. The feedstock is pumped to a pressure of about 300 bar. After heat exchange with the reactor effluent in a simple double-walled tube heat exchanger, the feedstock will reach a temperature of 400-550°C, passing the critical point of water. The reactor is operated at a temperature of 600-650°C, and a residence time of 0.5-2 min. for complete carbon ECN-RX--04-085 6 conversion. The two-phase product stream from the reactor arrives in a high- pressure (HP) gas-liquid separator (P = 300 bar, T = 25-...
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