MODELLING AND SIMULATION OF BIOMASS GASIFICATION WITH CIRCULATING FLUIDIZED BED

Abstract

The use of biomass as a source of energy has been further enhanced in recent years and special attention has been paid to biomass gasification. Present work reported CFD simulation of circulating Fluidized bed as a gasifier. Report describe various types of gasifier used like Fixed bed, Fluidized bed and Entrained-flow type and shows the effects of various process parameter on their performance and also mentioned the advantage of circulating fluidized bed over all gasifier types. Report describe different biomass gasification models like devolatilization sub models, Mixture fraction model, Porous media, two-phase model, Hydrodynamics model, Heat transfer Model, and Mass transfer model. Biomass Gasification simulation is being carried out by taking biomass raw material (mostly wood) in Circulating fluidized bed reactor by using CFD Eulerian—Eulerian combustor model. The model used includes the Biomass particle processes steps such as drying, volatilization, homogeneous and heterogeneous reactions of combustion and gasification, particle drag and turbulent dispersion, as well as heat-up. The fundamental problem in modeling of these reactors is due to the complex motions of two phases where the interface is unknown and transient and to the interaction between the phases which is understood only for a limited range of conditions. Also, a large number of independent variables such as particle size, shape and density, as well as gas velocity and turbulence intensity, significantly affect the hydrodynamics behavior of fluidized bed reactors. For reaction different parameters related to frequency factor ,activation energy and corresponding temperature of individual reaction also plays an important role. In this work, to investigate hydrodynamics and heat transfer phenomenon in Gas-Solid Circulating fluidized bed a, gas-solid circulating fluidized bed system having a diameter of 3.5 m and height of 17.6 m containing beads of average particle diameter equal to 500 μm has been simulated using commercial computational fluid dynamics (CFD) code Fluent. For simulation of drag model Syamlal- Obrien drag model is used. for simulation of pressure velocity coupling simple phase coupled scheme is used with Green-Gauss cell based scheme for spatial discretization .Various results are obtained of solid volume fraction with varying with time, Chemical species mass fraction with reactor height, Temperature contour in three different diameter reactor and its steam function. Checking for mesh independent three mesh size reactor is used and compared its result for its suitability. Obtained results are compared with literature data by M. Grabner et al. (2007).