The viscosity of molten slags is one of the most significant parameters to predict the flow properties of ash in coal gasification process.To estimate the viscosity of molten slags

a model based on the structure of slags was established.NBO/T (number of non-bridging oxygen/tetragonally-bonded oxygen) and P/M (polymer/modifier)

as the structural parameters of slags

provide description of the polymerization and bond strengths of molten slags

respectively.The model presented the dependency on both temperature and structural parameters with the viscosity in the SiO2-Al2O3-CaO-MgO-Fe2O3-K2O-Na2O-TiO2 slag systems.The accuracy of the model was verified by the experimental data of literatures.The results indicate that the viscosity decreases with the increase of NBO/T due to the reduction of polymerization in the molten slags

whereas the viscosity rises with the increase of P/M because of the strength weaken of molten slags.A good agreement is obtained from the calculated and measured values of the viscosity within a certain range.The deviation between the prediction and experiment results is within 10% when NBO/T < 0.5 and 25% when NBO/T > 0.5.