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A CFD based combustion model of an entrained flow biomass gasifier

Department of Chemical Engineering, University of Sydney, Sydney, NSW 2006, Australia; Biomass Energy Services and Technology Pty. Ltd., 1 Davids Close, Somersby, NSW 2250, Australia
Applied Mathematical Modelling (Impact Factor: 2.16). 03/2000; DOI: 10.1016/S0307-904X(99)00025-6

ABSTRACT This paper contains the description of a detailed Computational Fluid Dynamics (CFD) model developed to simulate the flow and reaction in an entrained flow biomass gasifier. The model is based on the CFX package and represents a powerful tool which can be used in gasifier design and analysis. Biomass particulate is modelled via a Lagrangian approach as it enters the gasifier, releases its volatiles and finally undergoes gasification. Transport equations are solved for the concentration of CH4, H2, CO, CO2, H2O and O2 and heterogeneous reactions between fixed carbon and O2, CO2 and H2O are modelled. The model provides detailed information on the gas composition and temperature at the outlet and allows different operating scenarios to be examined in an efficient manner.

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    ABSTRACT: This work presents a volume-filtered formulation for describing chemically reacting flows in the presence of solid catalytic particles. The equations are discretized in a Eulerian–Lagrangian framework and applied to flows of isothermal, heterogeneously reacting chemical species in fully developed three- dimensional risers. The aim of this study is to identify and quantify the influence of particle clusters on heterogenous reactions. The Archimedes number, Ar, is varied from 500 to 12,500, and the Damköhler number, Da, from 0.1 to 10. To assess the multiphase dynamic effects on the chemistry, conversion times from the three-dimensional simulations are compared to a zero-dimensional model that solves for the temporal evolution of the species mass fraction and ignores all spatial variations. The conversion process associated with the three-dimensional simulations is shown to be significantly longer compared to the zero-dimensional solution, with an increasing effect for larger values of Da. The discrepancies can be fully attributed to the presence of clusters, which are accounted for in the zero-order equations by an additional term that contains the covariance between species mass fraction and particle volume fraction fluctuations, which needs to be modeled. To this purpose, contributions to the fluctuating chemical source term are evaluated from the three-dimensional data and discussed, and a presumed-shape probability distribution function (PDF) approach is investigated. This PDF approach models the fluctuating chemical source term by a product of a beta distribution for the species mass fraction and a lognormal distribution for the particle concentration, and yields a mean species solution that agrees very well with the three-dimensional results for the range of Ar and Da considered in this study.
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