Experimental Study On Crystallization Process And Kinetics Of Non - Newtonian Coal Ash Slag

In entrained flow gasifiers, crystallization inside the liquid slag can increase the viscosity of the slag, affecting flow down the wall and resulting in operational problems. Although increasing attention has been paid to the influence of crystallization on viscosity, few studies have investigated crystallization kinetics in slag melts due to challenges associated with high temperature and experimental complexity. In order to gain a deep understanding of crystallization characteristics, different experimental methods have been used. Differential scanning calorimetry(DSC), single hot thermocouple technique(SHTT) and ahigh-temperature quenching furnace were set up to investigate the influence of temperature and cooling rates on crystallization kinetics. Five oxides have been made into synthetic slags with various Si/Al,Si+Al,Ca O and Fe2O3 ratios.A prediction model for the crystallization temperature has been established based on the synthetic slag system, and the model has been verified through real ash slags. The main conclusions are as follows:(1) The temperature-time transformation(TTT) and continuous-cooling transformation(CCT) curves of the slags have been obtained which can illustrate the influence of temperature and cooling rates respectively. As the temperature decreases, higher free energy accelerates the crystallization rates and shortens the residence time. However higher degree of supercooling will increase the viscosity and restrict the growth of crystals, leading to small size of crystals. The increase of cooling rates can decrease the initial crystallization temperature and crystal size. But the influence isn’t significant until the cooling surpasses certain rate. Kissinger method is used to calculate the non-isothermal kinetics and JMA is for isothermal kinetics.(2) Components and their ratiosare the intrinsic factor of crystallization characteristics. The influence of Si/Al, Si+Al, Ca O, Fe2O3 and the atmosphere have been investigated. Apart from the weakest crystallization tendency of small Si/Al, the influence of Si/Al in the range of 1.5 to 4.5 is not significant, but the temperature range of crystallization becomes narrow. Decrease of total amount of Si+Al can lead to a great crystallization tendency and accelerate the precipitation. The main crystals of high amount of Si+Al are anorthite and spinel while low amount of Si+Al favors diopside and mellite. The influence of Ca O between 5 to 45 wt.% is non-linear or non-proportional to the Ca O content because it crossed different crystalline phases. Slags with 15-35% Ca O had a higher crystallization tendency but lower crystallization temperature than slags with lower Ca O contents. Diopside and anorthite were the two main crystalline phases for slags with less than 35% Ca O. However, when the content of Ca O exceeded 40%, the formation of Ca2 Si O4can result in highercrystallization temperature. As for the iron oxide, the high ratio of Fe2O3 enhances the crystallization tendency and favors the formation of spinel. Crystallization tendency will become weak under the reduced atmosphere 60/40 CO/CO2, which requires longer incubation time and higher undercoolings.(3) A prediction model has been established based on the experimental results of synthetic slags, and the predicted results were verified by real ash slag. The slags with higher base/acid value than 0.7 and Si/Al in the range of 1 to 4 can display stronger crystallization peak, and the higher base/acid, the stronger crystallization tendency. The influence of Fe2O3 is more significant than Ca O. In addition, the initial crystallization temperature has a linear relationship with the liquidus temperature. And the predicted results of the linear correlations agree well with the experimental results.