The characterization of non-volatile impurities in mixed carbon feedstock and their interactions with gasifier liner materials

Carbon, slag chemistry, temperature, and partial pressure of oxygen affect material reactions in a gasification chamber. Corresponding thermodynamic parameters are critical as they determine the types of crystals that form in slags, weight fractions of the phases, and corrosive interactions between slags and refractory materials. The crystal formation then effectively changes physical characteristics of slags, impacting slag viscosity. In order to study interactions between slags and refractory materials, it is important to assess thermodynamic data for the slags and the timing of crystal formation as opposed to slag residence time in the gasification chamber. The current study focuses on the thermodynamic analysis of impurities contained in carbon feedstock used in gasification, its kinetic behavior, and the effects of carbon feedstock chemistry on the degradation of gasifier refractory materials. Utilizing qualitative and quantitative techniques (XRD, SEM-EDX/WDX, TEM-EDX and ICP-OES); a thermodynamic phase diagram encompassing industrial slags based upon coal/petcoke ash mixture compositions was constructed using equilibration conditions obtained in samples after 72 hours of exposure at 1500 °C in an atmosphere of 64%CO/36%CO₂. This CO/CO₂ ratio corresponded to a simulated gasification environment of Po2 = 10-8 atm. Mullite crystal was found in Al₂O₃ rich slags, while V₂O₃ crystal was present in slags with high V₂O₃ (content corresponding to slags with high petcoke content). No crystalline phases were observed in SiO₂ rich slags at low V₂O₃ content (slag with high content of coal ash). The formation kinetics of crystals in coal/petcoke slags was also studied from 1200 - 1700°C in an atmosphere of 64%CO/36%CO₂, and a Time-Temperature-Transformation diagram constructed. This diagram exhibited two distinct crystallization events; with higher than 10 wt.% additions of petcoke slag to coal slag promoting V₂O₃ crystallization in the slag above 1350°C, and with additional crystalline phases such as V-rich spinel forming at temperatures lower than 1350°C. Experimental results indicated that the V₂O₃ crystal would form in the slag as fast as 35 seconds. Finally, an evaluation of the interfacial reactions between synthetic slags (average coal and petcoke slag chemistry) and refractory materials (90 wt.% Cr₂O₃ - 10 wt.% Al₂O₃ and 100 wt.% Al₂O₃) was conducted utilizing a Confocal Scanning Laser Microscope. Ground slag samples at specific locations on refractory coupons were heated at 1500 °C in a 64%CO/36%CO₂ gas mixture using a gold-image heating chamber. Characterization of the slag/refractory interface by the SEM-EDX indicated that slag penetration occurred into porous areas of the refractory microstructure, and that refractory dissolution into slag occurred through the refractory surface and in the porous areas penetrated by slag, which promoted de-bonding of refractory material. It was noted that VOx-containing crystalline phases precipitated at the slag/refractory interface in coal/petcoke slag mixtures. Chemical spalling of the Cr-containing refractory grains along with chemical dissolution of the grain in the slag facilitated surface degradation of the refractory.