Study On Combustion Mechanism Of Char Particles In Circulating Fluidized Bed

Circulation fluidized bed boiler plays very important role in the field of clean coal combustion by right of high efficiency and low environmental pollution. However, its combustion efficiency is commonly lower than the design value because of higher fly-ash carbon contents. There are few reports about char burnout. Therefore, it is necessary to research physic nature and combustion characteristics for char.The combustion parameters of fine char and the carbon content in fly ash are discussed in this paper. The author explores the microcosmic appearance and micro-pore parameters of fly-ash in CFB by means of scan electron microscope and mercury injection apparatus. Based on fractal theory, the combustion charicteristics for char is simulated. This method is used to diagnose and optimize the operation of the circulation fluidized bed boiler.The effects on combustion is investigated, such as operation condition of boiler, the organic macerals of coal, the fragmentation of coal, the pore fractal of the char and so on. The results show that char combustion depends on the physical makeup and the organic macerals of coal and char may break up while it burns. The pore structure for char is a kind of fractal structure and may change when burning take place.By fractal theory, the thesis analyzes the anomaly self similarity structure of char. A building cube blocks model is developed for the prediction of the distribution of pore area and pore volume in fly-ash during combustion process. Some factors affecting the model, for example side length, surface numbers, surface area, volume increments, are analyzed. The results m and n are the structure parameters which determinie space fabric and surface form. K, which means iteration, is the hierarchy parameter in the fractal model. Coal parameters are related to the char pore structure and K is related to burnout ratio of char.Based on fly-ash samples from practical circulation fluidized bed boiler, the particle size distribution, the pore parameters and the fractal dimension are discussed. The results show that the pore distribution properties are segmentation, multiple fractal dimensions. When the pore diameter rises the fractal dimension increases and then decreases. Obviously char is of high porosity and complex spore structure. The internal surface area of mesopore plays a determinant role in the total area and the pore volume of macropore in the whole volume.In order to investigate the char burnout trait of char, this study takes a CFB boiler in Taiyuan Chemical Fertilizer Limited Corporation as an example and gives some suggestions on optimize operation. The results show that the main problems of the boiler are the lower temperature in dilute zone,higher excess air ratio and serious air leakage.The reasons that result in high carbon content are lower furnace temperature, shorter residence time of fine particle in chamber and blocking in solids recycle system.