Effects of atmosphere on mineral transformation of Zhundong coal during gasification in CO2/H2O conditions

Abstract

Coal gasification has been regarded as a promising method for the thermochemical utilization of Zhundong (ZD) coals. However, ZD coals are characterized by the high content of alkali metal, which may cause a series of running problems such as ash accumulation, slag blocking and fouling on heating surface of the gasifier and waste heat boiler. To improve the efficiency of fuels utilization, the evolution behavior of minerals of ZD coals – Hongshaquan (HSQ) coal and Jiangjunmiao (JJM) coal was investigated by experiment and FactSage calculation in this work. The results gained by low temperature ashing method show that the minerals of both HSQ coal and JJM coal are basically similar mainly including quartz, kaolinite, calcite, halite and of pyrite. It was found that the minerals transformation of coal samples is greatly influenced by gasification atmosphere (CO2/H2O). For HSQ coal, the minerals of slag are mainly composed of quartz, nepheline, gehlenite and periclase under pure H2O or pure CO2 atmospheres, while the minerals of slag are mainly lime and periclase under the mixing CO2/H2O (CO2: H2O = 1:1) atmosphere. For JJM coal, the minerals of slag under the pure H2O atmosphere are obvious different from that under the pure CO2 atmosphere, and mainly include gehlenite, lime, magnesia, and nepheline in pure H2O atmosphere, and the pleonaste and merwinite minerals are detected under pure CO2 and CO2/H2O (CO2: H2O = 1:1) mixing atmosphere. Furthermore, the FTIR results for the two coal samples demonstrate that the more silicate minerals exist in coal slag, which might be caused by the factor that H2O promotes the melting and polycondensation of SiO2 and Al2O3 to increase the quantity of silicate. In addition, according to the calculation results of FactSage, iron is formed during the evolution of iron-containing minerals with water vapor in atmosphere which may be due to the promotion effect of water on the decomposition of pyrite in coal slag. Moreover, water vapor can promote the migration of the alkali metal sodium in the coal slag to the gas phase, reducing the sodium content in the slag, and thus results in a higher viscosity of the molten slag under the water vapor atmosphere than under the pure CO2 atmosphere. The order of slag viscosity is: η0(H2O) > η0(CO2/H2O) > η0(CO2). This work provides a theoretical information for the effect of CO2/H2O gasification conditions on the mineral’s evolution behavior of ZD coal and offers a guidance for the operation of gasifier.