Effect of CaO/Na2O on slag viscosity behavior under entrained flow gasification conditions

The entrained flow gasification is becoming a predominant and efficient way for coal clean utilization. And the smooth operation of entrained gasifiers strongly depends on steady and reliable removal of the ash slag. In order to reduce the additive amount of flux and improve gasification efficiency, the influence of CaO/Na2O (weight ratio) at constant total content of CaO/Na2O on the viscosity-temperature behavior was studied in this work. Slag structure at high temperatures, and crystallization behavior of the coal ash slag with different CaO/Na2O were investigated in detail. As the CaO/Na2O of the slag decreased, the viscosity increased at same temperatures. Meanwhile, the slag viscosity which exhibited a rapid increase in viscosity transformed into the behavior of a glassy slag, of which the viscosity gradually increases as the temperature decreased. The slag structure depended on network polymerization degree and charge compensation effect of Na+ on Al3+. The polymerization degree of the slag was intensified with the decreasing CaO/Na2O, and the charge compensation effect of Na2O on Al3+ was promoted by CaO. The formation of anorthite was attributed to the rapid increase of viscosity at higher CaO/Na2O, while the decrease of CaO/Na2O was not favor of anorthite crystallization during cooling. In addition, the best ratio of CaO/Na2O in the slag was 6:4, and the slag viscosity behavior was suitable for the smooth operation of the entrained flow gasifiers.