(567g) Thermodynamic Analysis and Kinetic Modeling of Steam Gasification of Biomass

Fluidizable Ni/La₂O₃-γAl₂O₃ catalysts were prepared using incipient wetness technique for steam gasification of biomass producing high-quality and tar free synthesis gases at temperature below 700 ⁰C. Physicochemical characterizations of the prepared catalysts were performed to evaluate its surface structure and to predict its stability at the expected operating conditions of a fluidized bed gasifier unit.

Steam gasification of cellulose (glucose) and lignin (2-methoxy-4-methylphenol) surrogates were performed by contacting them with a fluidizable Ni/La₂O₃-γAl₂O₃ catalyst in a novel mini-fluidized CREC Riser Simulator varying temperature, steam/biomass ratio (S/B), reaction time and catalyst/biomass ratio (Cat/B). The gaseous products were analyzed in a GC/MS with a TCD and a mass spectrometer. Glucose gasification results showed that H₂, CO, CO₂, CH₄ and H₂O are mainly present in the product gas with negligible C2+ species and coke deposited on catalyst surface. On the other hand, gasification of 2-methoxy-4-methylphenol produced a small fraction of higher hydrocarbons as well as cokes.

The steam gasification of biomass involves a complex network of heterogeneous reactions. A thermodynamic model of biomass steam gasification was developed considering the significant gaseous products and cokes to determine the equilibrium composition of the product gas. Water gas shift, steam and dry reforming of methane, heterogeneous water-gas-shift, Boudouard equilibrium and hydrogenating gasification reactions were considered as the dominant reactions involved in steam gasification of biomass. It was found that at a constant S/B increase in C/H resulted in decreased coke formation. Dry gas yield, C-conversion and product composition were calculated using the model developed and compared with the experimental results. The trends of gasification products with the variation of temperature, pressure and S/B are consistent with the thermodynamics predictions. However, the difference between experimental results and thermodynamic predictions indicates that the process is actually kinetically controlled. A Langmuir-Hinshelwood based kinetic model was developed for steam gasification of biomass considering all significant products and reactions involved. Kinetic parameters for glucose gasification were estimated using the experimental results and the model is validated using experimental data at different operating conditions.