Numerical simulation of coal fluidization and gasification in fluidized beds

Abstract

The primary objective of this study is to conduct numerical simulation of coal fluidization and gasification in fluidized bed gasifiers. Simulations involve Eulerian-Eulerian multi-phase flow model which is carried out using the Multiphase Flow with Interphase eXchanges (MFiX) computational flow dynamic code. An investigation of coal fluidization is carried out and the influence of numerical diffusion on accuracy of fluidized bed simulations is studied. This is due to the importance of accurate prediction of bubble dynamics and gas-solid mixing in bubbling fluidized beds. The fluidization process is simulated using various numerical schemes, including First Order Upwind (FOU) as well as higher order Total Variation Diminishing (TVD) schemes. Simulations are conducted using wide range of grid resolution and the effect of mesh resolution on the results is studied. It is shown that using higher order discretization schemes is essential to capture correct shape of bubbles, bed height and particle dynamics in the bed. Comparison is also made of computational performance of all numerical schemes considered. The TVD schemes are shown to yield quite different computation times caused by parallelization efficiency on distributed memory platforms. In the gasification simulations, the chemical reaction effects are taken into account using a time-splitting scheme in which the corresponding source terms are directly integrated in a separate step via a stiff ordinary differential equation solver. Simulations are carried out of counterflow and crossflow gasifiers. In the counterflow configuration, bituminous coal is fed into the reactor from the top by gravity and steam serves as the gasifying media which enters form the bottom. Simulation results are compared with the experimental data. Gasification occurs following devolatilization and cracking processes as incoming coal particles heated rapidly to the gasification temperature. Subsequently, gasification process is carried out in an isothermal fashion. As a result, no energy balance is considered in the simulations. Two four-step global mechanisms are used to describe the char gasification and water-gas shift reactions. Comparison is made of the results obtained using these two kinetic models. In the crossflow reactor, sub-bituminous coal enters the gasifier from the side while an upward stream of nitrogen from the bottom is used to fluidize the bed. The devolatilization and gasification processes are described by an eight-step reaction mechanism consisting of three reaction steps to model the devolatilization and cracking processes, as incoming coal particles heated to the gasification temperature; and five reaction steps to represent the char gasification, CO methanation and water-gas shift reactions. In these simulations, energy equation is solved to find the temperature distribution within the reactor. To assess the performance of the time-splitting method, the chemistry effects are also incorporated using the non-splitting method originally implemented in MFiX. It is shown that the splitting scheme, introduced in this study, results in reduction in computation time.