Synchrotron-based X-ray absorption spectroscopy study of vanadium redox speciation during petroleum coke combustion and gasification

Abstract

Vanadium in petroleum coke can have environmental and technological impacts in gasification and combustion processes. The multitude of vanadium oxidation and coordination states has made it difficult to characterize vanadium-rich materials, particularly for materials where the vanadium is associated with an amorphous state and cannot be characterized by X-ray diffraction. This study presents two methods whereby vanadium is characterized by synchrotron-based X-ray absorption spectroscopy, which applies to both crystalline and amorphous phases. The first method, comparison of main edge energies, is relatively straightforward but presents multiple limitations. The other method, linear combination fitting, overcomes many of these limitations but relies on the availability of suitable reference materials. These methods, applied to pilot plant gasification and combustion samples, indicate various trends in vanadium speciation, such as higher oxidation states for combustion samples relative to gasification samples. Analysis of synthetic pseudo-binary materials, which is important for the development of thermodynamic equilibrium models, indicated that V5+ is stable with high oxygen partial pressures encountered during combustion. With a lower oxygen partial pressure, representative of gasification conditions, V5+ is stable if there is enough calcium present; otherwise, V3+ is stable.