Morphology, mineralogy and rietveld texture analysis of ash deposits from a moving grate boiler

Abstract

The release and transformation of inorganic species during combustion will cause severe ash-related issues, often leading to serious operational problems. To better understand these issues, several ash deposit samples collected from a biomass-fired moving grate boiler were characterized by inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, scanning electron microscope with energy dispersive spectroscopy and synchrotron radiation X-ray diffraction. The Rietveld refinement method was used to determine the mineral phases and texture information of ash deposits. The results showed that the fuel had a high slagging tendency and ash deposits were enriched in alkali and alkaline metal species. The petrology and morphology analysis indicated that they all presented layered structures and displayed heterogeneity in sizes and shapes with spherical, fibrous, prismatic and irregular morphologies and with the predominant surface compositions of O, Al, Si, Ca, Fe, K, Na and C. The Rietveld phase analysis of ash deposits demonstrated that several mineral types were identified, including oxides, silicates, phosphates, sulfides, sulfates and carbonates. The possible chemical reactions and mineral transformation during combustion were investigated. The texture exhibited in a sample can be classified as random, planar and fiber textures. The Rietveld texture analysis of minerals in the ash deposits showed that the texture indexes of most mineral phases were larger than one, indicating that the orientation distributions of these minerals were not random. Some of the pole figures of the minerals showed a weak orientation of densities, while some of them exhibited strong orientation of densities. Due to some peak overlaps existing between minerals, the texture of some mineral phases showed a fiber-like characteristic with several fiber components while some other minerals presented planar-like or biaxial-like texture.