llmenite and Nickel as Catalysts for Upgrading of Raw Gas Derived from Biomass Gasification

摘要

Two metal oxides, naturally occurring ilmenite (iron titanium oxide) and manufactured nickel oxide supported on an α-Al_2O_3 matrix (NiO/Al_2O_3), were compared as catalysts for secondary biomass gas upgrading. The experiments were conducted in a chemical-looping reforming (CLR) reactor, which combines biomass gas upgrading with continuous regeneration of coke deposits. The CLR system was fed with a tar-rich producer gas from the Chalmers 2-4-MVV biomass gasifier, and the possibilities to reduce the tar fraction and to increase the yield of hydrogen were evaluated for temperatures between 700 and 880 ℃. A system-wide molar balance was established to enable calculations of tar removal efficiency on a mass basis; these results were further compared with those for the more widely used tar-to-reformed gas ratio, yielding tar concentrations in units of grams of tars per normal cubic meter of product gas (g_(tar)/Nm~3_(gas)). Both materials exhibited activity with respect to tar decomposition and increased the yield of hydrogen. In addition, both tar removal and hydrogen production were increased with increasing temperature. All the phenolic compounds and a large proportion of the one-ring branched tars were decomposed at 800℃ by the two catalysts, despite the fact that the tar load in the raw gas was as high as 30 g_(tar)/Nm~3_(gas). Results from the mole balance snowed that it is important to specify on what basis the tar removal efficiency is calculated. The tar removal efficiency was calculated as 95% for the Ni/Al_2O_3 catalyst at 880℃ and as 60% for the ilmenite catalyst at 850℃ on tar-to-reformed gas basis. When the produced permanent gases were removed from the reformed gas, the same calculations yielded tar removal efficiency of 86% and 42%, respectively. The testing of serial samples of the effluent stream from the regeneration reactor for carbon oxides showed that coke was removed from the catalyst, and no deactivation by coke deposits was detected during the 8 h of operation of the CLR reactor.