Coal ash slag viscosity is the most important parameter for the stable running of entrained flow gasifier. The precipitation of metallic iron in slag is often observed, which would increase viscosity of slag and block the gasifier. The reason that result in precipitation of metallic still not clear, thus, coal ash with more than 8% Fe₂O₃ is not suitable for Shell gasifier. In this work, The effect of residual char and reducing atmosphere on precipitation of metallic iron was investigated. The thermodynamic calculation (FactSage) results shown that 90% iron can be reduced by residual char though carbothermal reaction while there is no metallic iron in slag at high temperature under strong reducing atmosphere (80CO/20CO₂), besides, the reducibility of residual char decrease due to the reaction (C+CO₂=2CO) under strong reducing atmosphere. For dynamic of precipitation of metallic iron, kinetics of carbothermal reaction between Fe-char was established by non-isothermal method, and kinetics of reaction between iron and reducing atmosphere was analyzed by isothermal method though switching atmosphere at 1400 °C. Based on above result, A model between char-slag under entrained flow gasification conditions was established to illustrate the precipitation of metallic iron in ash with slag with different Fe₂O₃ content(8%, 13% and 18%). To validate the model results, mineral transformation experiment was carried out. The XRD result shown that reaction between Fe₂O₃ and residual char in slag lead to formation of FeSi, Fe₃C and little metallic iron. Meanwhile, more metallic iron was observed under the strong reducing atmosphere. It can be include that the reducing atmosphere is the main reason that lead to formation of metallic iron. Morphology and distribution of metallic iron in slag under different reducing condition was observed by BSE-EDX. The precipitation of metallic iron reduced by strong reducing atmosphere is more disperse than that by residual char, thus, metallic iron particles in slag with residual char is easily to aggregate and form the large one.
 

Coal gasification; Residual carbon, Metallic iron, Carbothermal reaction.