Modification of lignite ash fusion temperatures by the addition of different types of sewage sludge

•Lignite ash fusion temperatures (AFTs) decrease as a consequence of sludge addition.•Sludge addition to lignite increases SiO2, Al2O3, Fe2O3 and P2O5 content in ashes.•B/A(+ P) and SiO2–Al2O3–CaO diagram predict adequately AFT changes and slagging.•CaO, Al2O3 and SiO2 form silicates, which can produce low-melting eutectics.•P2O5 from sludge forms phosphates, which reduce AFTs.

The effect of the addition of three types of sludge (A, V and F) on the slagging behaviour of a brown lignite was investigated. It was evaluated by determining ash fusion temperatures (AFTs) of lignite, sludge samples and coal-sludge blends of 10 and 50 wt.% of each sludge, together with their chemical and mineral composition. The addition of sludge to lignite produces a decrease in the AFTs, the extent of which depends on the percentage of sludge added to the lignite, as well as the sludge additives used (organic polyelectrolyte, lime and/or FeCl3) and their phosphorus content. As a consequence, the addition of sludge to lignite produces an increase in the slagging tendency. Those lignite–sludge blend ashes whose base to acid ratio (B/A(+ P)) ranges from 0.7 to 2.0 have the lowest AFTs. In this range, most of the ash compositions are near to those of SiO2–Al2O3–CaO eutectics. Also, an increase in the phosphorus content of the blends, as well as the formation of amorphous silicates and phosphates, reduce the AFTs.