Characteristics Of Fly Ash And Slag In Circulating Fluidized Bed Boiler

As an important part of fossil fuels,coal is the material basis of our country's industrialization process and rapid development of economic.And the low calorific value fuel —— a by-product of coal mining,is in the state of random stacking for a long time.With the energy situation and environment pollution intensified,people began to pay attention to the use of low calorific value coal.Circulating fluidized bed?CFB?boiler as a new technology that has little pollutant emission to air,plays an important role in low-grade coal.However,the amounts of CFB fly ash increase with the consumption of low calorific value coal.And CFB fly ash has not yet been fully utilized due to the difference from traditional fly ash in physical and chemical properties.Especially the higher content of CaO and SO₃ restrict the large-scale use of CFB fly ash.Aimed to current situation,the properties of fly ash of different combustion conditions using coal gangue as materials were carried out.While the distribution regularity of different forms of calcium in actual CFB boiler fly ash was analyzed.The main conclusions are as follows:?1?There is no difference in the composition of the fly ash from the laboratory and the actual CFB boiler fly ash.The mineral species is not affected by atmosphere.And the dispersion has a great influence on the decomposition of calcium carbonate.Briquette combustion has a large effect on the mineral content of fly ash and doesn't affect the mineral species.After molding of sample,the contact between mineral were more closely and minerals were more likely to react.Molding of sample is effective to reduce the content of free calcium oxide in fly ash,which is beneficial to the utilization of CFB boiler fly ash.?2?The distribution of particle size of slag is more dispersed compared to fly ash.The mineral species in fly ash do not vary with the particle size.The content of SiO2 and Al2O₃ in slag decreases with the decrease of particle size.The content of CaO and SO₃ are on the contrary.The content of Fe₂O₃ and MgO do not change with the particle size.From the chemical composition and mineral composition,slag is more promising than fly ash in gradient utilization.?3?CFB boiler load has little effect on the particle size distribution of fly ash,the chemical composition and mineral composition of fly ash,which has great influence on the dissolution rate of alumina and the microscopic morphology of particles.The alumina are in the form of amorphous,and do not participate in the formation of crystalline minerals.The dissolution rate of alumina in slag is higher than that of fly ash.The dissolution rate is related to the microstructure of the particles.The more pores and cracks are on the surface of the particles,the easier the dissolution of alumina.The dissolution rate of aluminum in slag increases gradually with the decrease of the particle size,but it is lower than 30% in the hydrochloric acid solution.And the overall activity is poor.?4?Amorphous matter is in majority in CFB boiler fly ash,it's main chemical composition is SiO2,Al2O₃ and CaO.Calcium oxide in fly ash mainly has three forms: anhydrite,lime and amorphous phase,in which the proportion of lime is the lowest,not more than 20%,unreacted calcium oxide mainly in amorphous form.Alumina is more likely to enter the fly ash,calcium oxide,quartz and anhydrite are easier to enter the slag.Calcium oxide,calcium sulfate contents increase with the particle size decreases.?5?The change of relative contents of calcium that in different forms is not consistent.Because the relative content is not only related to the mass fraction of itself,but also to the change of the total calcium oxide content in fly ash.Differences in change rate lead to no unified rule which can describe the variation of relative contents of calcium in different forms.