Study On Behavior Of Anhydrite In Fluidized Bed Combustion Ashes

Fluidized bed combustion (FBC for short) technology is one of the advanced coal combustion technology that meets the demands of energy conversation and environmental protection. However, further application of this technology was blocked due to the basic research deficiency and limited understand to the characteristics of FBC ashes—the byproduct of the technology, leading to lack of effective ways to utilize the ashes.FBC ashes can be widely used in building materials for they are one kind of incinerated-clay minerals essentially. Nonetheless, FBC ashes containing a lot of solid sulfides—anhydrite, which can not only activate the pozzalanic activity of FBC ashes, but also cause expansion out of control, adding many special characteristics to the ashes that hampered their utilization as building materials.This thesis is focus on anhydrite solidified in FBC ashes. Micro-morphology and particle distribution of anhydrite were investigated by X-ray diffraction (XRD), electron scanning microscope (SEM) and energy dispersive scanning (EDS); the function that anhydrite played during hydration of FBC ashes was studied by chemical analysis.Based on which, the influence on activity and activation behavior of FBC ashes with the change of anhydrite were studied and a method to evaluate activation of FBC ashes was proposed aimed at eliminating disadvantage influence of anhydrite. Besides, the restraint of anhydrate expansion in FBC ashes was studied and a way of their utilization as building materials was put forward.The results are listed as follows: (1) XRD and chemical analysis results testified that the sulphates mineral solidified in FBC ashes is mainlyⅡ-anhydrite. (2) Micro-morphology and particle distribution of anhydrite in FBC ashes was studied through new methods of density-separation and sieving. The anhydrite in FBC ashes are concentrated in particles of FBC ashes with small size and high density. (3) SEM and EDS analysis results showed that the compacted and loose particles are co-existed both in FBC fly ashes and bed ashes. The protrudent crystals on the surface of compacted particles of FBC fly ashes are mainly anhydrite and some clay minerals while loose particles are mainly unburnt carbon; and for FBC bed ashes, the protrudent crystals on the surface of compacted particles are mainly anhydrite while the column crystals on the surface of loose particles are clay minerals as well as some absorbed anhydrite. (4) The results of reaction rate constant K and dissolution curve showed that the reactive activity and dissolution capability of anhydrite calcined at 850℃for 2h are similar to that of anhydrite solidified in FBC ashes. Anhydrite calcined at 850℃for 2h can be added to FBC ashes to investigate the influence on anhydrite properties of FBC ashes with different dosage. (5) The results of pH value and content of ettringite and chemical combined water indicated that anhydrite takes part in the hydration process and effects the hydration of FBC ashes as well as activation behavior and expansion properties. (6) Although the content of anhydrite had no effect on the degree of FBC ashes'activity, it obviously influenced the activation behavior of FBC ashes. When the content of anhydrite was at the range of 2~4% (SO3), the pozzolanic activity of FBC ashes was most effectively activated. While content of anhydrate was below this rage, the activating effect became worse, and if anhydrite was excessive, the delay-ettringite and dihydrate gypsum formed at later age would reduce the strength because of uncontrolled expansion. (7) A method aimed at eliminating disadvantage influence of anhydrite was proposed to evaluate activation of FBC ashes, namely the 28d compressive strength ratio of cement clinker mortar. This new method can value activity of available FBC ashes objectively, in which the content of SO3 and water content were respectively controlled by gypsum content and mortar fluidity. This method is adapted to FBC ashes with content of SO3 below 11.6%. The average content of SO3 in domestic FBC ashes is 7.8%, which meets the sulphur content demand mentioned above. (8) Anhydrite was the main factor that leads to expansion of FBC ashes during hydration. Additives can restrain the expansion through promoting the dissolution and hydration of anhydrite, such as CaCl2 and Na2O3. The linear expansion ratio of FBC ashes mortar can reduce by 19% after adding 1% of CaCl2 or Na2CO3.The results also showed that, FBC ashes can be used as additive for cement and concrete under restricted amount. (9) The expansion of anhydrite was eliminated by way of autoclave curing, which explored a new way for utilization of FBC ashes. The linear expansion ratio of FBC ashes pastes was below 5×10-4 after autoclave curing. There was tobermorite formed when FBC ashes pastes being autoclave cured, and no ettringite can be found.