The fluidized-bed reactor is one of the commonly applied types in thermochemical conversion processes, such as combustion, pyrolysis, and gasification, of multiple solid feedstocks, like coal, biomass, petroleum coke, and waste materials. In fluidized-bed gasifiers using the internal circulation (INCI) principle, like the COORVED gasifier, agglomerates are formed in vicinity of the central gas jet due to the high temperatures above the initial sintering temperature of the ash. When these ash-rich agglomerates are interconnected by sintering reactions, they are no longer fluidized and forming the fixed-bed for post-gasification, which are finally discharged from the reactor. Thus, for a detailed understanding of the agglomeration mechanisms of the feedstocks those chemical reactions have to be investigated for a reliable operation of INCI-type gasifiers. Four biomass feedstocks, including sewage sludge, rice husks, corn straw, and municipal waste, with a certain bandwidth in composition are investigated in the present study. All feedstocks are ashed at medium temperatures of 450 and 550 °C. The jet region inside the gasifier is simulated by a rapid heating procedure up to high temperatures of the ashes. In parallel, these conditions are reproduced and in-situ studied in a heating chamber inside a scanning electron microscope. The agglomerated particles and related chemical reactions are investigated by various experimental methods, including XRF, XRD, SEM-EDX, AFT, and cold compression strength tests. These results are supported by thermochemical calculations using the software package FactSage™. The four feedstocks have demonstrated distinct differences in their agglomeration behavior. Based on the applied methods, the underlying chemical reactions in the ashes causing agglomeration are identified. These mineral reactions are compared to the reactions calculated by FactSage™ in similar temperature and atmosphere ranges. In summary, the origin of the agglomeration of different feedstocks is concluded by the application and cross-comparison of the comprehensive methods. Those feedstocks are finally evaluated for their applicability in an INCI-type gasifier.