Exploring influence of MgO/SiO2 on viscosity-temperature property of coal ash slags under entrained flow gasification condition

Abstract

To realize smooth slag discharge of Zhundong coals in entrained-flow-bed gasifier, the viscosity-temperature characteristics of slags with different MgO/SiO2 ratios were studied. The results show that low MgO/SiO2 ratio causes high BO content in liquid slag, resulting in excessive polymerization degree and viscosity. As the MgO/SiO2 increases, the activation energy of liquid slag increases, occurrence form of oxygen transforms from BO to NBO, more structural depolymerization reactions occur, and [SiO4] tetrahedra obtains more O2− and show more NBO. NBO is more likely to react with Mg2+, Ca2+, Al3+, and Fe2+ to form crystals, thus increasing the crystallization reaction activity of slag and cause the initial crystallization temperature and crystal content increasing. An increasing MgO/SiO2 causes a characteristic transition of slag from weak crystallinity and high polymerization degree to strong crystallinity and low polymerization degree, meanwhile, the poor viscosity-temperature characteristics with high viscosity of liquid slag is first improved, and then the problem of strong crystallinity leading to poor viscosity-temperature characteristics with high Tcv occurs. Therefore, the viscosity-temperature characteristics first improve and then deteriorate. The crystal size increases as MgO/SiO2 increases, which also cause Tcv increasing. Large ΔT and low Tp25 make 0.28MS become the most suitable for slag discharge.