Experimental Research On Microcosmic Characteristics Of Fly-ash In Circulating Fluidized Bed Boiler

In recent years, the circulating fluidized bed (CFB) boiler has been developing rapidly due to the superiority of CFB combustion technology in China even around the world. Therefore this technology provides a practicable way for the thermal power plants. However, the fact that the carbon contents in fly-ash from CFB boilers in our country is commonly higher than the designed values, which has become the bottleneck for rapid development in CFB boilers. Therefore, it is necessary to research on the microcosmic characteristics of fly-ash particles in CFB in order to reduce the carbon contents in fly-ash. By the way the combustion efficiency in CFB boiler may not only be improved, but also the fly-ash is comprehensively utilized. As an emerging cross-branching discipline, the fractal theory provides important guarantee to solve the complicated non-linear problems in nature. In addition, the fractal theory provides a new way of thinking for researching the characteristics of fly-ash. Besides fractal dimension may transform quantitative to qualitative when people descript complicated matters.The thesis mainly takes the CFB boiler in a thermal power plant with the capacity of 75t/h as the main experimental objects. According to the established experimental scheme, the author carries on the carbon content and microcosmic characteristics for different fly-ash in particle sizes. By the fractal theory, the microscopic characteristics of the fly-ash is analysed, and the relation between the parameters and the fractal dimension is explored.The author studies the particle distribution of coal as fired. The results show that the particle sizes of coal were relatively large so that the actual boiler load is lower than the designed value. Therefore, the proposal can be put forward that the crushing equipment is needed to guarantee proper particle distribution. There is a peak feature between particle sizes and their carbon contents through screening experiment and carbon content identification. And the carbon contents have peak values in 45-25μm (11.66%) and 100-80μm (9.94%).According to the above phenomena, the experiments are carried for fly-ash in different sizes with nitrogen adsorption, mercury porosimetry and scanning electron microscope (SEM). The results from nitrogen adsorption show that the adsorption isotherms approximate belong to the second type, and the hysteresis loops are the superposition of multiple types. The pores of the fly-ash are of three categories:tapered pore, parallel-plate slit pore and ink-bottle shaped pore. The pore distribution is widely enough without the existence of 2nm in diameter. The porous characteristic of macro-pore may effectively be determined by mercury porosimetry. The results show that pore size distribution is between several and ten thousand nanometers, and the average pore diameter is about 45μm in diameter. The specific volume consists of the pores whose diameters are greater than 100nm, especially 1000nm, and the specific surface area are composed of pores whose diameters are less than 100nm. By the SEM experiment, the morphology of fly-ash is classified as three kinds:close-grained type, layered type and porous type. And according to the carbon contents, the fly-ash is divided into three kinds:low carbon content (F1 and F2), medium carbon content (F3, F5 and F9), and high carbon content (F4, F6, F7 and F8).On the basis of the results achieved from nitrogen sdsorption mercury porosimetry and SEM, fractal dimensions are calculated, and the relationship between fractal dimension and particle sizes are determined. The relationship between the fractal dimension and carbon content is also observed through SEM.