Mathematical Modeling of Coal Gasification in a Fluidized Bed Reactor Using a Eulerian Granular Description

Abstract

A three-dimensional computational model was developed to describe the coal-gasification processes inside fluidized-bed reactors. The commercial multi-purpose CFD code FLUENT 6.3 was employed, taking into account drying, volatilization, combustion and gasification processes. Both gas phase and solid phase were described using a eulerian approach to model the exchanges of mass, energy and momentum between phases. The disperse phase was described using the kinetic theory of granular flows. The chemical model involved five heterogeneous and five homogeneous chemical reactions, tracking seven species in the gas phase (CO2, CO, H2O, CH4, H2, O2 and N2) and one specie in the solid phase (C(s)). Drying and volatilization rates were estimated by mass conservation. Heterogeneous reaction-rates were determined by combining an Arrhenius kinetic-rate and a diffusion rate using the kinetics/diffusion Surface Reaction Model; the model was implemented within FLUENT through UDFs (User Defined Functions). Homogeneous reaction-rates were described by a turbulent mixing rate using the Eddy Dissipation Model available in FLUENT. Calibration and validation were performed by using existing experimental data from a benchmark coal-gasification case available in the literature. Results are in good agreement with experimental data, capturing known phenomena like fluidization-bed height, temperature distribution and species concentrations. The main contribution of the present work was implementing the necessary sub-models within the FLUENT code in order to handle reactive fluidized-beds in complex geometries. This allowed combining the flexibility of a commercial CFD code with the accuracy of simplified models developed in academic frameworks.

Computational Modeling of the Combustion and Gasification Zones in a Downdraft Gasifier

“…The gasification products are vented through the side near the top of the gasifier, being carried with the "updraft", while the ash and char are deposited out the bottom. (1.1) (Cornejo & Farias, 2011). These reactions take place around 773K.…”

“…Since there is no data on the exact distribution of the volatiles in biomass, they were predicted using Equation 1.4, where VOL indicates the total volatile content of the biomass, based on the dry proximate analysis of each biomass (Cornejo & Farias, 2011).…”

“…The combustion zone operates at approximately 1123K (Atnaw & Sulaiman, 2009) and is governed by Equations 1.5 -1.8 (Cornejo & Farias, 2011). These include, in order, the water-gas shift reaction, CO combustion, H 2 combustion, and CH 4 combustion.…”

“…MODELING 2.1 Design ANSYS FLUENT and Gambit software was used to model the down-draft gasifier system to be installed at UIOPP. While some researchers have utilized the multiphase model in FLUENT (Cornejo & Farias, 2011;Yimlaz, et al, 2010), others have modeled the process with mathematical user defined functions (Watanabe & Otaka, 2006;Paes, 2005;Huang & Ramaswamy, 2009;Cuoci, et al, 2009;Wen & Chaung, 1979). Still others have broken down their modeling by zone, i.e.…”

“…drying (Ciegis & Starikovicius, 2002) or pyrolysis (Babu & Chaurasia, 2003). Although most work has focused on the gasification of coal particles (Watanabe & Otaka, 2006;Wen & Chaung, 1979;Cuoci, et al, 2009;Cornejo & Farias, 2011;Yilmaz, et al, 2010), some have focused on biomass materials (Paes, 2005;Huang & Ramaswamy, 2009;Ciegis & Starikovicius, 2002;Babu & Chaurasia, 2003). The model chosen for this research focuses on the combustion zone of biomass in a down-draft gasifier.…”