Morphology and phosphate distribution in bottom ash particles from fixed-bed co-combustion of sewage sludge and two agricultural residues.

The purpose of this study was to provide detailed knowledge of the morphological properties of ash particles, including the volumetric fractions and 3D distributions of phosphates that lay within them. The ash particles came from digested sewage sludge co-combusted with K- and Si-rich wheat straw or K-rich sunflower husks. X-ray micro-tomography were combined with elemental composition and crystalline phase information to analyse the ash particles in 3D. Analyses of differences in the X-ray attenuation enabled calculation of 3D phosphate distributions that showed high heterogeneity in the slag particles. This is underscored by a distinct absence of phosphates in iron-rich and silicon-rich parts. The slag from silicate-based wheat straw mixtures had lower average attenuation than that from sunflower husks mixtures, which contained more calcium. Calculated shares of phosphates between 7 and 17 vol% were obtained, where the highest value for a single assigned phosphate was observed in hard slag from wheat straw with 10 % sewage sludge. The porosity was notably higher for particles from pure wheat straw combustion (62 vol%), compared to the other samples (15-35 vol%). A high open pore volume fraction (60-97 vol%) indicates that a large part of the pores can be accessed by the surroundings. For all samples, more than 60 % of the discrete (closed) pores had an equivalent diameter < 30 μm, while the largest volume fraction consisted of pores with an equivalent diameter > 75 μm. Slag from sunflower husk mixtures had larger pore volumes and a greater relative number of discrete pores >75 µm compared to wheat straw mixtures.