Transformation of apatite phosphorus and non-apatite inorganic phosphorus during incineration of sewage sludge

•A novel approach to P recovery from sewage sludge was studied by XRD and TGA–DSC.•High temperature and CaO could effectively promote transformation from NAIP to AP.•The AP percentage in total phosphorus reaches 99% at 950°C in sewage sludge ash.•Phosphorus components emerged during the thermal treatment was measurements.•The reactions between NAIP and CaO at different temperatures were concluded.