MODELING & SIMULATION OF GASIFICATION OF INDIAN COAL

Abstract

In 21St century, most of the countries including India are facing energy crisis due to accrued energy demand and decline in production of liquid and gaseous fuels. Most of the conventional oil and gas fields have already been exploited and efforts are on now to exploit sources like coal, oil sands, gas hydrates etc. for energy production. India has fourth largest reserves of coal in the world which can be used as a shield against the overwhelming energy crisis. However, most of the coal available in our country is low grade coal, having very high ash content and low sulfur content. In general, they are not amenable to the "standard" gasification process based on commercial entrained flow gasifiers. Instead,'they are amenable to 'less developed fluidized bed gasification technology. Design, optimization & scale up of fluidized bed coal process have been a challengeable problem due to its complex reaction and transfer mechanism. Therefore, efforts are required to develop the fluidized bed gasification technology through pilot plant studies and numerical modeling. Numerical modeling can be used for tailoring the flow patterns and temperature profiles inside the gasifiers to optimally match the demands made by the kinetics of gasification reactions. The present work focuses on the modeling and simulation of a pilot scale bubbling fluidized bed gasifier (BFBG) for the gasification of high ash Indian coal. For this purpose, a two dimensional model with quadrilateral cells is developed using FLUENT 12.0 software, taking into account the drying, volatilization, gasification and combustion processes. In the model, the exchange of mass, momentum and energy between solid (secondary phase) and gaseous phase (primary phase) has been described using Eulerian —Eulerian approach. The solid phase is described by kinetic theory of granular flows. The reaction system inside the gasifier involves 4 heterogeneous and 4 homogeneous reactions covering 6 species in gaseous phase (CO, CO2, H2, N2, 02 and 1-120) and coal in solid phase. The kinetics for the homogeneous reactions is described using eddy dissipation model available in FLUENT while for heterogeneous reactions, a user defined function (UDF) code with Arrhenius kinetics is written in C. The calibration and validation of the model has been done using experimental data generated in a pilot scale BFBG at Central Institute of mining & fuel research (CIMFR), Dhanbad, India. The computed exit gas composition as well as temperature profile inside the gasifier is in good agreement (error within 10 %) with experimental data. The flow behaviors, volume fraction and velocity profiles of gas and solid phases in the bed and freeboard zones have been predicted using this model. Also, this model has been used to study the effect of superficial gas velocity, temperature and pressure on the performance of the gasifier. Temperature & CO2 concentration were found to decrease while CO & H2 concentration increased along the length of gasifier. The concentration of combustible gases like CO & H2 increased with temperature while that of H2O and CO2 decreased with temperature. The effect of pressure was not very significant and a small decrease in concentration of CO & H2 and increase in concentration of CO2 was observed with pressure. In order to scale up the bubbling fluidized bed gasifier facility at CIMFR, Dhanbad, further case study simulations with various Indian coals having different ash content is required. Also, a disperse particle model (DPM) study of the model is necessary to determine the effect of air distributor holes on the bubble formation and hydrodynamics of the gasifier. iv