Impact of Residual Carbon on Ash Fusibility of Semi-char from an Industrial Circulating Fluidized Bed Gasifier

Because an industrial circulating fluidized bed gasifier has a low operating temperature, a mass of carbon-rich fine chars is carried away by the syngas, making the total carbon conversion lower. Because gasified semi-char has a short residence time and poor reactivity, residual carbon of gasified semi-char is difficult to convert completely in the smelting furnace. However, the smooth operation of the smelting furnace highly depends upon steady and reliable removal of liquid slag, and the deterioration of the melting characteristics of coal ash results in difficult slag discharge, blocking of slag-drip opening, water wall destruction by falling solid chunks of slag, and even melting system emergency shutdown. However, inherent carbon in the slag has an important and unclear impact on the flow characteristics of the slag in the semi-char melting process. Using ash fusion determinator (AF700, LECO Corporation, Saint Joseph, MI, U.S.A.), scanning electron microscopy, X-ray diffraction, and FactSage 6.1 analyses, we examined the ash samples with various carbon contents, prepared to investigate the fusibility and transformation of minerals in the melting process of gasified semi-char. As the results suggest, residual carbon in ash significantly impacted ash melting points and melting behavior as a result of the carbothermal reactions of residual carbon with mullite or free silica. First, quartz was reduced to moissanite between 1200 and 1400 °C through the following reaction: 3C(s) + SiO2(s) → SiC(s) + 2CO(g). Subsequently, if mullite and residual carbon existed in ash over 1400 °C, the following reaction would occur: 3Al6Si2O13(s) + 6C(s) → 3Al2O3(s) + 2SiC(s) + 4CO(g). The crystalline structure of moissanite tended to be destroyed at a relatively low temperature in the presence of iron. Moissanite would not be detected until iron was converted to ferrosilicon completely.