Experimental Study On Microscopic Characteristics Of Fly-ash In Circulating Fluidized Bed Boilers Based On Fractal Theory

As a clean coal combustion technology,the circulating fluidized bed boilers are developed rapidly in China because of dual pressures of the energy and environment.However there are some problems in the actual operation,for example high carbon content in fly-ash.In this paper,the author takes a circulating fluidized bed boiler in capacity of 480t/h as an object to study the distribution characteristics of the fly-ash samples on different operating parameters.With the help of fractal theory,the micro-pore structures of the fly-ash samples are further studied when the particle sizes are between 37 and 41 μm.Thus,the physical nature of the carbon content in fly-ash is explored from the microscopic point of view.The distribution characteristics of the different fly-ash samples shows that the fly-ash particles have both high carbon content and high percentage of quality in general for the fly-ash of 37 to 41μm in diameter.This phenomenon influences the boiler efficiency.So it is necessary to analyze the effect of coal and operating parameters on the carbon content and pore structure of the fly-ash.To further explore the essential reason of high carbon content in the fly-ash,the fractal theory has been introduced.In this paper,the author investigates the fractal characteristics of the pore distribution which acquired from the experiment of mercury injection.The experimental results show that there exist two distinct regions to all of the fly-ash samples in the measurement accuracy range of mercury injection.In Region One,the pore radii are between 3525.128nm and 9975.897nm.The fractal dimension is close to 2 and there exists a linear relationship between specific surface and specific volume.So the pore structure is similar to smooth plane.In Region Two,the pore radii are between 18.572nm and 3376.897nm,and the pore structure is of good fractal characteristics.In view of the fractal characteristics,the fractal feature of pore distributions in Region Two is further discussed in this paper.The fractal relationship between specific surface and specific volume is acquired through fitting the experimental data,and the relationship is InS∝DlnV.At the same time,it is discovered that the trend of specific volume is contrary to the fractal dimension in the range of experimental study.Base on the above analysis the bed temperatures greatly influence the fly-ash characteristics.The relationship between the bed temperature and fractal characteristics of fly-ash is also discussed in this paper.The results show that the fractal characteristics of fly-ash from burning Coal One are satisfied the relationship:S×exp(-D)∝T-1 when the bed temperature changes evenly.It is indicated that the influence of the bed temperature is contrary to the fractal dimension on the combustion reactions.In this paper,the relationship among carbon content,bed temperature and fractal dimension is explored.Four samples from burning Coal One satisfy the relationship:C∝TSexp(-D)on the whole.So there is a certain relationship between carbon content and fractal dimension.