Fusibility characteristic and flow properties of semi-char from industrial circulating fluidized bed gasification

The carbon conversion of fluidized bed gasification (FBG) has been restricted by the entrainment of high-carbon-containing semi-chars. Thus, the concept of FBG combined with combustion melting system was proposed to make effective use of the gasified semi-char. However, long-term running of the melting furnace was confined by severe slag plugging. The objective of this research is to understand the ash fusibility and flow properties of the semi-char, and provide some guidance for the design and operation of the integrated system. Two typical semi-chars from industrial circulating fluidized bed gasifier were chosen as raw materials, and the major results are as follows. The ash fusion temperatures (AFTs) of Liaocheng semi-char (LCSC) are about 100 °C lower than Suqian semi-char (SQSC), mainly caused by its higher basic element content. As the particle size increases, the AFTs of SQSC ash first increase sharply, and then rise smoothly, whereas no obvious differences are found for LCSC ash. The effect of atmosphere on ash fusibility was also studied. The AFTs appear in the order of oxidizing > inert > reducing, which should be related to the variations in Fe2+/Fe3+ ratios under different atmospheres. The viscosity-temperature curves show that the LCSC ash tends to form glassy slag when completely melted, which turns out to be plastic slags for SQSC ash, probably resulting from mullite crystallization. Heating stage microscopy has been considered an effective way to in situ observe the melting and crystallization process of ashes. The preliminary results indicate that dendrite particles tend to strengthen internal bonding in the molten slags, promote the formation of solid phases, and accelerate the growth in viscosity.