Research On The Modelling And The Design Optimization Of Supercritical Circulating Fluidized Bed Boiler

Supercritical circulating fluidized bed boiler technology combines the advantages of the circulating fluidized bed combustion technology and the power generation technology with supercritical parameters. So it is one of the clean-coal utilization technologies in China. Supported by the "Eleventh Five-Year" Science and Technology Support Projects of China, the research on the overall mathematical model of large-scale supercritical circulating fluidized bed boiler and the design optimization of a600MW supercritical circulating fluidized bed boiler is carried out in this paper.Firstly, the necessity and the feasibility of the development of the supercritical circulating fluidized bed boiler are discussed. Then the worldwide development and studies of the supercritical circulating fluidized bed boiler and its mathematical model are reviewed.The overall mathematical model of the supercritical circulating fluidized bed boiler developed in this paper is built on the basis of the mathematical model which was previously developed for the small-and moderate-scale circulating fluidized bed boiler. This overall mathematical model considers all the physical and chemical processes in the circulating fluidized bed boiler comprehensively and is made up of the submodels of hydrodynamics, combustion, attrition, heat transfer, the formation and reduction of pollutants, cyclone, external heat exchanger and the tail flue gas duct of the boiler. Due to its modular structure, it is convenient to develop new submodels or renew the parameters which are suitable for the characteristics of large-scale supercritical circulating fluidized bed boilers.In order to describe the fragmentation submodel of coal particles in the fluidized bed combustion correctly, the experimental research on the influences of coal particle properties on the ash formation behaviors during fluidized bed combustion was conducted on a bench-scale fluidized bed combustor. Coal samples were divided into6ranks with different ash content. For every rank of coal sample,3size ranges were used in the experiments. Effects of coal particle ash content and particle size on the ash formation behaviors were studied, such as bottom ash mass fraction, ash size distribution and carbon content in the bottom residue and the fly ash. On the basis of the experimental results and the relevant studies in the references, an empirical model for the fragmentation of coal particles in the fluidized bed combustion is developed.For the numerical solution of the overall mathematical model of the large-scale supercritical circulating fluidized bed boiler, the boiler system is divided into a series of "cells" along the flow direction from the furnace to the tail flue gas duct. The performance of a300MW CFB boiler is used to validate the mathematical model. The result shows that the developed mathematical model is correct and can be used to predict the performance of large-scale supercritical circulating fluidized bed boilers.Then the overall mathematical model of the supercritical circulating fluidized bed boiler is applied to calculate the characteristics and the performances of six different design concepts of a600MW supercritical circulating fluidized bed boiler. Furthermore the structure of the furnace, separator, the external heat exchanger (EHE) and the layout of heat surfaces are discussed from different aspects. The design of a600MW supercritical circulating fluidized bed boiler with pant-leg furnace and6cyclones and6EHEs symmetrically arranged on the both sides of the furnace is recommended.Finally, the mathematical model of supercritical circulating fluidized bed boiler is used to predict the performances of the recommended600MW supercritical circulating fluidized bed boiler under full and part loads. The results are helpful for the study of the characteristics of supercritical circulating fluidized bed boiler.