Rheology and thixotropic properties of slurry fuel prepared using municipal wastewater sludge and coal

Large amounts of municipal wastewater sludge (MWS) result in serious environmental problems and huge disposal investments. As a type of biosolid with available calorific value, MWS can be directly combined with coal, water, and additives to produce coal–sludge slurry (CSS). Burning/gasifying this liquid fuel not only solves environmental problems, but also maximizes the heat value of sludge. In this study, the slurryability, rheology, and thixotropy of several CSSs were examined, and the effects of sludge type, sludge mixing proportion (α), and sludge particle size on rheology and thixotropic properties were assessed. Compared with coal–water slurry, CSSs behaved as more obvious pseudoplastic fluids with considerable thixotropic response, indicating that the sludge enhances the pseudoplasticity and thixotropy of the slurry. The rheological behavior well fitted the Herschel–Bulkley model so that almost all the correlation coefficients were higher than 0.99. As solid concentration increased, yield stress τy and consistency coefficient K increased; the thixotropy of CSSs also strengthened. The reduction in sludge particle size decreased the apparent viscosity and improved the slurryability of CSSs. In the experiment, the fixed-viscosity concentration increased by 1–2.5 percentage points. In addition, this study showed that thixotropy is positively correlated with yield stress τy and stability of CSSs. ► Sludge enhanced the pseudoplasticity and thixotropy of the coal–water slurry. ► The rheological behavior well fitted the Herschel–Bulkley model for CSSs (R2>0.985). ► The thixotropy of CSSs strengthened with increasing the solid concentration. ► The reduction in sludge particle size improved the slurryability of CSSs. ► Thixotropy was positively correlated with yield stress τy and stability of CSSs.