Modeling of dense gas-solid reactive mixtures applied to biomass pyrolysisin a fluidized bed

A model is presented for mathematically describing the thermofluid dynamics of dense, reactive, gas-solid mixtures. The model distinguishes among mult iple particle classes, either on the basis of their physical properties (di ameter, density) or through their thermochemistry (reactive versus inert pa rticles). A multifluid approach is followed where macroscopic equations are derived from the kinetic theory of granular flows using inelastic rigid-sp here models, thereby accounting for collisional transfer in high-density re gions. Separate transport equations are constructed for each of the particl e classes, allowing for the description of the independent acceleration of the particles in each class and the interaction between size classes, as we ll as for the equilibration processes whereby momentum and energy are excha nged between the respective classes and the carrier gas. Aimed at high-dens ity suspensions, such as fluidized beds, the relations obtained for the str ess tensor are augmented by a model for frictional transfer, suitably exten ded to multiple-class systems. This model, previously derived, is here enla rged to include heat and mass transfer, as well as chemical reactions and i s therefore applicable to general gas-solid combustion systems. The notewor thy novelties of the model with respect to other derivations in the literat ure include: (i) a systematic and consistent derivation of the solids trans port equations and transport properties within the multifluid concept, allo wing for non-equilibrium effects between the respective particle classes, ( ii) the ability to explicitly account for the possibility of porous solid f uel particles, and (iii) the modeling of multiple chemical reactions in bot h gas and solid phases and the associated effects of heat and mass transfer . The model, which includes a separately validated chemistry model, is appl ied to high-temperature biomass particle pyrolysis in a lab-scale fluidized bed reactor and is used to obtain yield of reaction products. The results indicate that, at fixed initial particle size, the fluidizing gas temperatu re is the foremost parameter influencing tar yield. The biomass feed temper ature, the nature of the feedstock, and the fluidization velocity all have minor impact on the yield. It is also shown that the fluidizing gas tempera ture can be optimized for maximizing the tar yield. (C) 2001 Elsevier Scien ce Ltd. All rights reserved.