Prediction of slagging potential of sewage sludge ash using Fu and AFI indices

Abstract The co-combustion of sewage sludge with coal offers environmental and operational benefits, but also presents significant challenges related to the slagging and fouling of heat exchanger surfaces. This study presents a comprehensive physicochemical characterisation of sewage sludge ash, focusing on chemical composition and ash fusion behaviour under oxidizing and semi-reducing conditions. Two empirical indices — the Fouling Index (Fu) and the Ash Fusibility Index (AFI) — were applied to assess slagging and fouling risks. The results indicate that Fu may underestimate slagging tendencies due to its omission of phosphorus pentoxide (P 2 O 5 ), which plays a critical role in lowering ash fusion temperatures. In contrast, AFI, based on direct measurement of ash behaviour, provides a more helpful assessment of operational risks. The study highlights the necessity of using fusion-based indices for accurate evaluation of alternative fuels like sewage sludge.