The Effect Of Mineral Addictives On Ash Fusion And Viscosity-temperature Characteristics Of Lignite

Lignite is a kind of abundant resources in China. Owning to its characteristics of high inherent water and low heat value, the combustion of lignite trends to cause slagging and ash deposition in furnace so that the large-scale utilization of lignite is limited. In this dissertation, ash fusion characteristics and viscosity-temperature characteristics of lignite were studied by the use of low-temperature ashing instrument, high-temperature pipe furnace, high temperature viscosimeter, X-ray diffractometer and thermodynamic software Factsage. On one hand, Kaolin, bauxite and corundum were added into coal samples in order to study the effect of mineral additives on melting properties of ash and mechanism of additives on ash melting characteristics. On the other hand, the effect of mineral additives on viscosity-temperature of ash was studied through the mixture of Kaolin and coal sample; Additionally the temperature curve of solid-liquid ratio was simulated by Factsage, which explained the reason of the change of ash viscosity.According to the XRD spectrum of low-temperature ash samples, the common minerals in coal ash are mainly kaolin, quartz, etc. Because the original mineral compositions occupy very few percentages in ash, primary mineral compositions from different kinds of coal ashes have no obvious regular but have a lot to do with coal regions.In the ashing process, minerals in coal ash go through a complex transformation. Mineral compositions of coal ashes have important influence on ash melting point. The main crystals of Zhundong coal in high temperature are anhydrite, dicalcium silicate and fayalite. Baorixile coal has the similar ash compositions with Yimin coal, and they both have pyroxene in high-temperature coal ash. Mullite appears in high-temperature ash of Ping8coal, which leads to the highest melting point.In the range of0.5～2, the ash melting points of coal samples drop with the the increase of SiO2/Al2O3The ash deformation temperature of Zhaotong coal changes a lot, while the softening temperature, hemisphere temperature and flowing temperature change a little. The ash melting point of Baorixile coal changes within50℃. The ash melting point of Zhundong coal has an obvious change, especially when the ratio changes from3:2to1:1. Adding mineral additives, pyroxene in ash of Baorixile and Zhaotong coal melts and calcium feldspar precipitates. If bauxite continues to be added, the ash melting point has no obvious change, showing that Al2O3does not participate in the minerals reaction in coal ash. After Kaolin and corundum were added into samples, ettringite was generated, contributing to generate high meting temperature minerals such as grossular, showing that the ratio can obvious enhance the ash melting point of Zhundong coal. The same result is obtained by the simulation of Factsage.Viscosity of four kinds of coal ashes improves with the decrease of temperature. Baorixile and Yimin coal have no obvious critical point of viscosity, indicating they belong to vitreous slag or nearly vitreous slag. The ash viscosity of Zhundong increases slowly firstly, and after reaching the critical point, increases rapidly once again. Therefore, Zhundong ash belongs to plastic slag. Zhaotong coal has a clear critical point of viscosity, therefore, belonging to crystal slag. The different ash compositions lead to different sizes of network structure in ash melting liquid, which causing the different difficulties of crystal precipitation during the cooling process, finally resulting in the different kinds of slag.After kaolin is added into coal samples, the viscosity-temperature curve of Baorixile and Yimin coal ash shift to the high-temperature region. The shape of viscosity-temperature curve of Baorixile coal ash changes a little, while that of Yimin coal ash, when the amount of additive surpasses50%, shows it transforms into crystal slag. When there is large amount of crystal contained in coal ash melting liquid, it can not be treated as Newtonian fluid. Factsage simulation of crystallization process during cooling shows that the temperature curve of solid-liquid ratio of Baorixile coal ash and Yimin coal ash presents three stages, slow crystallization stage in high-temperature zone, rapid crystallization stage in low-and-medium-temperature zone. When kaolin is added, rapid crystallization stage prolongs and crystal precipitation rate accelerates in medium-temperature zone, while in low temperature zone, crystal precipitation rate slows down.