Research Of Thermal Expansion Behavior Of High-temperature Coal Ash In Reducing Atmosphere

Two coal samples (referred to as YS and NTT) with the different Si/Al ratio and similar Fe/Ca were selected and added with1%to5%(air dry coal base) SiO2and Al2O3to explore the thermal expansion behavior of the coal ash by using the high-temperature melting device in weak reducing atmosphere. The effect of SiO2and Al2O3on ash melting characteristics and ash thermal expansion behavior was studied by ash melting instrument, X-ray difractometer and thermal analysis instrument. Combining with melt structural characteristics, the apparent morphology, mineral transformation behavior, thermal stability of slag were investigated to illustrate the mechanism of high-temperature thermal expansion behavior on coal ash.Main conclusions of paper are as following:(1) Ash melting temperature can be effectively increased by adding SiO2and Al2O3. Ash melting temperature was not linear correlation with Si/Al ratio when different Si/Al ratios of YS coal ash were prepared.(2) The results of ImageJ2X software calculation show clearly that NTT raw coal ash and YS, NTT ashes added SiO2have experienced the process of contraction deformation and expansion. YS raw coal ash only experienced shrinkage deformation and without expansion, which inferred that SiO2may lead to thermal expansion behavior at high temperature. There are bubbles on the surface and a large amount of microspores internally in the slag melts, which indicates that gas generation is the main reason of coal ash thermal expansion behavior at high temperature.(3) The XRD results show that the crystal minerals decrease gradually and amorphous minerals increase significantly in NTT coal ash, NTT coal ash with SiO2and YS coal ash with SiO2, which can be a dominant reason of thermal expansion phenomenon. With the addition of Al2O3in coal ash, the crystalline minerals don’t reduce significantly with the increase of temperature and some new minerals appear. So thermal expansion behavior of coal ash has been restrained by the crystalline minerals.(4) The results of melting structure calculation and thermal analysis show that the increase of slag viscosity, the decline of slag stability and the formation of low-temperature eutectic are the main factors leading to coal ash thermal expansion behaviors.