Fixation of Phosphorus in Ash during Cocombustion of Sewage Sludge and Coals: Influence of Coal and Steam

Abstract

Sewage sludge (SS) incineration technology has become a primary method to recover and reuse SS, and the cocombustion with coal is a more feasible process due to the limited heating value of wet SS and the difficulty of drying SS. Being that phosphorus is a major element in SS, its migration during cocombustion with coal was unclear but vital. Phosphorus migration during the cocombustion of wet SS and various coals was investigated by drop tube furnace in this study. More phosphorus was fixed as apatite in ash by calcic and magnesian minerals at 1000 °C than at 800–900 °C. Calcium was taken up by sulfur in Tianchi coal (TC) and subsequently captured phosphorus ineffectively at 800 °C, while the competition of sulfur on calcium was alleviated over 900 °C. Silicic and aluminic minerals in Xiheishan coal (XHS) exhibited a good capacity for phosphorus capture at 800 °C. The outstanding calcic proportion in Xiaolongtan coal (XLT) created the highest rate of phosphorus fixation in ash, and the relative enrichment (RE) of phosphorus achieved up to 93.29% at 1000 °C. Steam generally deteriorated phosphorus capture at 1000 °C. For SS cocombustion with XHS, the effect of sulfur was alleviated by steam, and the impact of steam was mainly caused by Ca5(PO4)3OH. For SS cocombustion with TC or XLT, 10% steam still hindered phosphorus capture, while 5% steam promoted phosphorus capture. The RE of phosphorus in ash was positively correlated with coal content and increased from 69.64% with 30% XLT to 93.29% with 70% XLT. TC and XHS hindered phosphorus fixation in ash, and their content hardly affected the RE of phosphorus.