Numerical Simulation Of Coal Combustion Process In Circulating Fluidized Bed With Bidisperse Particles

Advantages of circulating fluidized bed (CFB) combustion technology such as the abilityto burn wide variety of fuels efficiently and environmentally friendly have led to a steadyincrease in its commercial use over the past decades. Due to complex coupling of densegas-solid flow, homogeneous and heterogeneous chemical reactions and heat and masstransfer process in the CFB furnace, comprehensive modeling of coal combustion processwith computational fluid dynamics (CFD) in CFB combustors is rather difficult. Two fluidsmodel based on kinetic theory of granular flow has been widely used to simulate the gas-solidflow process. In the previous model of gas particle flow in the CFB, the kinetic theory ofgranular flow based on monodisperse assumption is widely used to describe the particlevelocity fluctuation. Comprehensive CFD models of coal combustion using Eulerianapproach of particle phase did not consider the variance of particle size and density and theparticle breaking process during combustion. However, recent studies show that thedifferences of particles’ properties and particle breaking process have a significant impact onthe flow characteristics of gas-solid flow and coal combustion process. Therefore, it isnecessary to develop the mathematical model to simulate the combustion process in the CFB,considering the differences of particle properties and particle breaking during combustion.Based on the previous study on bed materials and computational load, only two particlephases are considered to represent the solid phase with the differences in the diameter, densityand hardness based on related researches.Based on the bidisperse particles assumption in the model, the governing equationsrepresenting the conservation of mass, momentum and energy in gas-solid two phase flow andkinetic theory of granular flow are presented. The coarse particle phase consists of fourcomponents: coal, coarse particle char, CaCO3and coarse particle ash, while the fine particlephase is composed of four components: fine particle char, CaO, CaSO4and fine particle ash.Sub-models with chemical reactions are incorporated to consider the process in the coalcombustion: devolatization, char particles combustion, volatiles combustion. Meanwhile, theprimary fragmentation of particles during the devolatization and the secondary fragmentationof particles during the char particles combustion are considered in the heterogeneous reactionmodel. Heat transfer between phases and radiative heat transfer models are also adopted in themodel. Sub-model with reactions related with pollution gases(NOxå'ŒSO2) are considered.In order to validate the reliability of the mathematical model based on two particle phases,2D numerical simulation of two pilot CFB experiments are conducted to study thegas-particle flow characteristics and coal combustion process. The temperature profiles andconcentrations of CO CO2and O2in the outlet agree well with the experiments. Thedistribution of velocity and concentration of two particle phases in a circulating fluidized bedcombustor is predicted. Distributions of gas species and related reaction rates in the riser werealso obtained. To study the emission of NOx and SO2, reactions related are also considered.The distribution of NOx and SO2and reaction rates are obtained. Effects of excess air andCa/S on SO2emission are analyzed.The combustion model based on binary particle phases was applied to simulate the gassolid flow and combustion characteristics in an116MW CFB combustor. The flowcharacteristics of gas and particles in the CFB furnace are obtained with concentration of twoparticle phases at different section of furnace. And coal combustion reactions are studied withgas species concentration, reaction rates and temperature profiles of the furnace. Distributionof NOx and SO2are provided and their related reactions of production and reduction areanalyzed. To investigate the impact of operating conditions on combustion performance in theCFB furnace, different primary air velocities, secondary air velocities and coarse particledensities are considered in different simulations.