Modeling Study On The Influence Of External Heat Exchanger To Circulating Fluidized Bed Boiler On The Material Balance

In large-scale process of circulating fluidized bed boiler,external heat exchanger can solve heating surface arrangement and the heat problem,playing an important role in the future development of circulating fluidized bed boiler.This article studied operation of the external heat exchanger influenced the material balance of circulating fluidized bed boiler system through mathematical model in detail.On the basis of existing one-dimensional mass balance model,added the material flow module of separator and external heat exchanger,set up the one dimensional multiple granularity material balance model with loop.Discussion on the same returning materials distribution ratio,the fluidized performance of external heat exchanger(such as fluidized wind velocity and porosity)changed the material balance of the system and the tapered valve opening of external heat exchanger(such as returning materials distribution ratio)changed material balance of the furnace.First of all,analyzed the modeling thought of the furnace,separator and external heat exchanger through the mechanism research,simplified the external heat exchanger as a cell with an import and an export,built a zero-dimensional model of circulating fluidized bed boiler system with external heat exchanger.Then,according to the flow characteristics,the furnace was divided into a plurality of cells,considering the ash formation and attrition,bed material stratification,particles age and other factors to establish a one-dimensional model of circulating fluidized bed boiler system with external heat exchanger.Finally,the model was applied in a 350 MW supercritical circulating fluidized bed boiler to study the effects of the operation of the external heat exchanger on material balance of circulating fluidized bed boiler system.Fluidized wind velocity of external heat exchanger increased,the fly ash flow rate increased,the discharge flow rate decreased.The residence time change was not a single,residence time of finer particles and coarser particles became longer,residence time of the middle-sized particles became shorter.The average particle size of the fly ash and the circulating ash became finer,the average particle size of the slag became coarser.The average ash particle size of the furnace became coarser,the average ash particle size of the separator and external heat exchanger became finer.The upper furnace pressure drop and the concentration of material reduced.In the process of actual operation,under the premise to ensure circulating fluidized bed boiler system safe and smooth operation,we should reasonably choose fluidized wind velocity of external heat exchanger according to the boiler load needs.Circulating ash share of external heat exchanger increased,the fly ash flow rate increased,the discharge flow rate decreased.Residence time of finer particles became shorter,residence time of coarser particles became longer.The average particle size of the fly ash and the circulating ash became finer,the average particle size of the slag became coarser.The average ash particle size of the furnace became coarser,the average ash particle size of the separator and external heat exchanger became finer.The upper furnace pressure drop and the concentration of material reduced.In the process of actual operation,under the premise to ensure circulating fluidized bed boiler system safe and smooth operation,we should reasonably choose circulating ash share of external heat exchanger according to the boiler load needs.