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.

Review of Phosphorus Chemistry in the Thermal Conversion of Biomass: Progress and Perspectives

“…Humans lack the ability to digest phytic acid, and a smaller fraction of the phosphorus content is therefore present in this form . However, most of the phosphorus is present as inorganic phosphorus, often aluminum- and/or iron-phosphates because iron- and aluminum-based additives are used as coagulation or precipitation agents in wastewater treatment processes. , Using chemical fractionation methods (the SMT protocol), it was shown that sewage sludge normally contain more than 70% inorganic phosphorus, out of which 60–95% was reported to be present as nonapatite phosphorus. ,− This includes, for example, Al- and Fe-phosphates. Solution state 31 P NMR analysis has shown similar results, and the phosphorus extracted for analysis was confirmed to be mainly orthophosphate. ,, However, some chemical fractionation studies (using the Hedley protocol) showed that more than 50% of the phosphorus is found in the acid-leachable (HCl) fraction, indicating phosphorus is associated as apatite in these sewage sludge samples. − …”

“…The decomposition of organic phosphorus species in biomass has been studied primarily using solution state 31 P NMR − ,,,,,, and chemical fractionation following the SMT protocol. ,− ,− …”

“…Normally, XRD analysis is only performed on the final biomass residues, and does therefore not provide a lot of information on the transformation pathways, but it gives an indication of the fate of phosphorus in thermal conversion of biomass. In an attempt to investigate what crystalline phosphates are most commonly found in biomass residues, the reported phases detected in 384 samples, from 65 references (refs , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , – , , , – , , , , – ) were compiled in this work. The result of the compilation is shown in Figure .…”

“…The phosphorus release from sewage sludge, either reported directly as release or retention in ash, ,,,,, or calculated using data on ash composition, ,,,, in different studies is shown in Figure . Generally, there appears to be no, or only minor, release of phosphorus up to 1100 °C.…”

Co-Gasification Synergistic Characteristics of Sewage Sludge and High-Sodium Coal

“…Sewage sludge is a byproduct generated in the process of sewage treatment, which contains many organic pollutants, heavy metals, parasites, pathogens, and other substances harmful to the environment and human health. − Due to rapid economic and social developments globally, the quantity of sludge produced has dramatically increased. Many countries have developed relevant policies regarding sludge treatment and disposal to encourage a reduction in its yield in a controllable way and render it harmless, with an eye toward transformation to useful products.…”