Transformation of mineral matter during pyrolysis, gasification and combustion of biosolid chars

During thermochemical processing of biosolids in sewage sludge, different forms of phosphorus-containing compounds are generated in biosolid chars (biochars). This study examines the effects of biochar processing conditions on the mineral compounds produced during the pyrolysis, gasification, and combustion of biosolids, and the major determinants of phase formation kinetics. Our results show identified phase transformations through experiments in a laboratory tube furnace followed by X-ray diffraction analysis (XRD). Additionally, a synchrotron powder diffraction study was conducted to verify XRD results of the laboratory processed samples and to observe in situ phase transformations in biochars. It was found that under a neutral atmosphere, crystalline oxide phases are formed at lower temperatures and much faster than iron phosphide, while gasification and combustion conditions led to the formation of crystalline phases in phosphate forms. These forms of phosphorus compounds in by-products of biosolids thermochemical treatment can be used as agricultural fertilizers.