High-Temperature Ash Melting and Fluidity Behavior upon the Cocombustion of Sewage Sludge and Coal

Wastewater treatment produces a large amount of sludge, where the minimizing of the disposed sludge is essential for environmental protection. The co-combustion of sludge with coal is a preferable method for sewage sludge disposal from the economic and environmental perspective. The co-combustion of sludge has been widely used in the industry with the advantages of large processing capacity. The melting characteristics of ash are an important criterion for the selection of the co-combustion methods and furnace types. In this study, two types of sludge and four types of coal with different ash melting points were selected, where the ash melting behavior upon co-combustion is investigated by experimental and thermodynamical approaches. Especially, the slag fluidity upon co-combustion is explored via a modified inclined plane method. It has been found that the presence of SiO₂ and CaO in sludge substantially enhances its fusion temperature owing to the high content of CaO, while SiO₂ acts as a solvent, facilitating the co-melting of other oxides and raising the sludge fusion temperature. Fe₂O₃ exhibits a specific mass fraction within the range of 10-20%. Furthermore, the presence of CaO and SiO₂ prohibits the flow ability of the slag at high temperatures, and Fe₂O₃ promotes the flow ability for sludge at high temperatures. With increasing base/acid ratio, the sludge flow velocity increases remarkably and peaks at 1.6. The interaction between Fe-Ca and Si-AI significantly affects the fluidity significantly. The findings are expected to optimize the condition of co-combustion and desirable furnace design for the incineration of sludge.