In-situ analysis of the effect of CaO/Fe2O3 addition on ash melting and sintering behavior for slagging-type applications

•CaO/Fe2O3 addition to SiO2- and Al2O3-rich ash proves the synergy for ash fusion.•The addition significantly affects mineral phase composition at high temperatures.•Combination of TMA and HT-SEM facilitates in-situ analysis of ash fusion process.•CaO/Fe2O3 ratio defines stages and temperatures of sintering, swelling, and fusion. Slagging-type technologies, particularly entrained-flow gasification, have been quickly developed and industrialized due to their inherent advantages. The ashes of many feedstocks are rich in SiO2 and Al2O3. Therefore, a high operating temperature is required, making the process less efficient. The use of pure additives such as limestone does not meet the requirements. In consequence, binary additives of CaO/Fe2O3 are studied to improve the fusibility. The CaO/Fe2O3 ratio is varied in order to optimize cost efficiency and the operation and control of processes. Ash fusion temperatures (AFTs) and X-ray diffraction (XRD) combined with in-situ methods of thermomechanical analysis (TMA) and high-temperature scanning electron microscopy (HT-SEM) are used to explore the effect of CaO/Fe2O3 on ash fusibility. The CaO/Fe2O3 ratio affects the ash fusion behavior due to changes in the mineral phase composition. The AFTs reveal an optimum combination for the joint addition of CaO and Fe2O3 compared to blends with the same amount of one single additive, while sintering is less strongly affected. Based on the in-situ analyses, the blends of ash and additive are categorized into two or three regimes of sintering, swelling, and fusion in the high temperature range. Depending on the applied additive ratio, different regimes of ash fusion are detected, which affect the slag properties.