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Modeling the hydrodynamics of dense-solid gas flows is strongly affected by the wall boundary condition and in particular, the specularity coefficient φ which characterizes the tangential momentum transfer from the particles to the wall. The focus of this study is to investigate the impact of φ on the fluidization hydrodynamics using a fully Eulerian description of the solid and gas phases in 3D cylindrical coordinates. In order to quantify this impact, tools for characterizing the bubbling dynamics and solids circulation are developed and applied to both lab-scale (diameters 10 cm and 14.5 cm) and pilot-scale (diameter 30 cm) cylindrical beds. Comparison of simulation predictions with experimental data for different fluidization regimes and particle properties suggests that values of φ in the range [0.01,0.3] are suitable for simulating most dense solid-gas flows of practical interest. It is also shown that for this range of φ, the fluidization hydrodynamics are not significantly dependent on the choice of φ especially as the bed diameter is increased. Additionally, 3D validation of the variable φ model by Li and Benyahia [1] shows the bubble diameter predictions to be in excellent agreement with experiment and the average value of φ predicted within the range [0.01,0.3]. Quantifying the impact of φ and establishing an appropriate range is not only important for accurate simulations at both lab and pilot scales but also validation of models and sub-models for a better understanding of the fluidization phenomenon. Finally, a comparison of the Gidaspow and Syamlal-O'Brien gas-solids drag model shows that the former is more applicable to homogeneous bubbling fluidization (U/U mf <4) while the latter is only suitable for high velocities (U/U mf >4) associated with larger bubbles and slugs.

This study presents the effect of biomass origin on the yield, nanostructure and reactivity of soot. Soot was produced from wood and herbaceous biomass pyrolysis at high heating rates and at temperatures of 1250 and 1400 • C in a drop tube furnace. The structure of solid residues was characterized by

The thermochemical conversion of biomass via gasification offers a promising approach to producing fungible substitutes for petroleum-derived fuels and chemicals. The kinetic study of the gas-phase reactions of biomass gasification is key to understanding fluidized bed biomass gasification (FBBG). Under typical operating conditions for air-blown FBBG (700−1000 °C), tars exist in the product gas in significant quantities (2−50 g/Nm 3 ). Predicting the formation and evolution of tars in a FBBG reactor model is particularly important as they introduce several operational and cleanup challenges in practice. However, such predictions require implementation of detailed chemical kinetic mechanisms due to the large number of species and competing conversion pathways involved. A detailed gas-phase mechanism has been proposed by the CRECK modeling group at Politecnico di Milano encompassing the secondary pyrolysis, cracking, and oxidation reactions of the devolatilization species of biomass, as well as the oxidation and combustion reactions of the resultant gas-phase hydrocarbon species. In this work, a one-dimensional reactor network model (RNM) of an air-blown fluidized bed gasifier utilizing this detailed chemistry model is developed and validated for the prediction of major gas-phase species and tar compounds. It is found that this RNM is able to accurately predict the syn-gas production and total tar concentration given a modification of water gas shift and/or CO oxidation kinetics to account for catalytic effects of the biomass ash and char. Additionally, validation of the predicted tar composition is attempted against available experimental measurements. Good agreement is achieved for single-ring aromatic and oxygenated tar compounds, while it is found that polycyclic aromatic hydrocarbons are underpredicted by more than an order of magnitude. Finally, the conversion pathways of representative devolatilization products of biomasslevoglucosan, xylofuranose, and p-coumarylare analyzed in the context of syn-gas and tar formation routes.

ABSTRACT:Based on the evolution of volatile and solid products predicted by a previous model for torrefaction (Bates and Ghoniem, 2012) a model has been developed which describes their thermal, chemical, and physical properties as well as the rates of heat release. The first stage of torrefaction, associated with hemicellulose decomposition, is exothermic releasing between 40-280 kJ/kg initial . The second stage is associated with the decomposition of the remaining lignocellulosic components, completes over a longer period, and is predicted to be either endothermic or exothermic depending on the temperature. Increasing torrefaction severity, as quantified by the mass loss, is predicted to cause greater heat release. The rate of mass loss and rate of heat release increase with higher temperatures. The higher heating value of volatiles produced during torrefaction was estimated to be between 4.4 and 16 MJ kg -1 increasing with the level of mass loss. Highlights: Model developed to describe the energy balance during willow torrefaction  First stage is exothermic releasing 40-280 kJ/kg  Second stage is either exothermic or endothermic  Higher torrefaction temperatures result in increased reaction rate and heat release rate