Fluidizable La2O3 promoted Ni/gamma-Al2O3 catalyst for steam gasification of biomass: Effect of catalyst preparation conditions

摘要

This study reports a new fluidizable La2O3 promoted Ni/gamma-Al2O3 catalyst for biomass steam gasification. Catalysts are prepared using a specially designed incipient wetness multi-step impregnation technique with subsequent reduction of metal precursors in a fluidized bed. Catalyst characterization shows that addition of La2O3 up to 5 wt% improves surface area, CO2 adsorption capacity, Ni reducibility and metal dispersion, as well as reduces support acidity. XRD shows undesirable LaAlO3 formation when catalysts with more than 10 wt% La2O3 are calcined above 1000 degrees C. This result points toward the increase of local catalyst bed temperature during exothermic reduction of metal nitrates. Higher reduction gas flow rates can control the rise of catalyst bed temperature by effectively removing the generated heat. This controlled reduction helps to minimize sintering/dehydroxylation of the metastable gamma-Al2O3, with octahedral and tetrahedral site ratio being used as an indicator of gamma-Al2O3 dehydroxylation. The octahedral and tetrahedral site ratios are estimated using H-2 TPR and NH3-TPD. Steam gasification experiments are performed in a CREC Riser Simulator under expected conditions of circulating fluidized bed gasifiers using glucose and 2-methoxy-4-methylphenol to represent cellulose and lignin, respectively. Gasification performance of the prepared catalyst is found to be a function of Ni dispersion and support basicity/acidity ratio. It is hypothesized that acid sites of gamma-Al2O3 are responsible for coke deposition via hydrocarbon cracking, whereas basic sites facilitate coke reforming. Based on these data, a 20% Ni/5% La2O3-gamma Al2O3 is developed, in this study, optimizing catalyst formulation and preparation conditions. This catalyst yields a 97% carbon conversion of glucose to permanent gases with no tar formation at 650 degrees C. In the case of 2-methoxy-4-methylphenol gasification, a 85.5% carbon conversion with only 8.3% tar formation is achieved. (C) 2014 Elsevier B.V. All rights reserved.