Effects Of Operating Parameters Of Circulation Fluidized Bed Boilers On Characteristics Of Fly-ash And Slag

Circulating fluidized bed boilers (CFBB), as a kind of relatively mature clean coal combustion technology, is actively developed due to its advantages and adaptability to China. But there are many problems in the operation, such as coking, fouling and high carbon content in fly ash, which have close relationship with operation parameters.The author takes a CFBB in capacity of480t/h located in a power plant as the object. According to the problems in the boiler, combustion adjustment tests are carried out, and the distribution of carbon content in fly ash is also studied. By mercury porosimetry, scanning electron microscope (SEM) and fractal, the author explores experimentally the microcosmic characteristics of coal, fly ash and slag under different conditions. This thesis provides the effect of operating parameters on the fractal dimensions and of fly ash and slag.According to the combustion adjustment tests the author finds out the optimum operating condition under different loads and the relationship between operating parameters and the carbon contents in fly ash is obtained. The results show that the carbon contents in fly ash may be reduced by increasing oxygen and reducing primary air. The choice for wind chamber pressures depends on the oxygen contents and primary air, and the higher wind pressure is suitable for the lower wind and lower oxygen.The carbon content in fly ash has a peak characteristics with particle sizes and reaches the maximum at37μm (greater than10%), and these particles occupy approximately50%in the total amount of fly ashes. The fly ashes of48-78μm in diameter have lower carbon contents (below7%). Therefore, the fly ash located in the peak zone is the main factor that results in mechanical incomplete combustion heat loss.The experimental results by mercury porosimetry show that for the fly ash the average porosity (50.461%in the peak zone and34.100%in the low carbon zone), specific volume (688.333mm3/g,410.400mm3/g), specific surface area (3.082m2/g,2.580m2/g)are significantly higher than that of the coal (7.208%,71.467mm3/g and0.576m2/g) and flag (14.277%,90.920mm3/gand0.881m2/g). The average pore size (9432.196nm)of the slag is much bigger than that of the coal (870.654nm) and the fly ash (2037.189nm in the peak zone and2079.635nm in the low carbon zone) since there is no micro pores in the slag.The coal, fly ash and slag are of fractal characteristics. The fractal dimension for the slag (average2.861) is generally larger than the coal (average2.527) and fly ash (2.227in the peak zone and2.694in the low carbon zone). And the fractal dimension for the fly ash in the low carbon zone is bigger than that in the peak zone. For the fly ash, the longer the residence time in the furnace is, the greater the fractal dimension is.Using SEM the author observes the morphology of the fly ash in the peak zone and low carbon zone. The former may be divided into a dense solid body and a vertebral body with a small amount of porosity. The latter may be divided into a honeycomb body, a whirlpool body and a porous irregular body with traces. Based Hausdorff the author calculates the fractal dimension according to SEM image. The results show that SEM images of the fly ash have good surface fractal characteristics, which is consistent with the fractal dimensions obtained from pressure mercury experiments.This thesis provides the relationships between the fractal dimensions and the carbon contents in the fly ash and the slag by considering the primary air, oxygen contents, bed temperature and chamber pressure.