Isothermal hot air drying behavior of municipal sewage sludge briquettes coupled with lignite additive

• Effects of lignite on convective drying of sewage sludge briquette were conducted.
• Influence of hot air temperatures on drying kinetics of sewage sludge were studied.
• Midilli model showed perfect prediction for sewage sludge/lignite thermal drying.
• Lignite could improve the drying behavior of sewage sludge briquette over 120 °C.

Isothermal drying behaviors of the municipal sewage sludge briquettes were investigated based on a laboratory-scale hot air forced convective dryer at 1.5 m s−1 air speed. The influences of drying temperatures (60–180 °C) and lignite as a typical additive with 10% dosage on the drying kinetics of the sewage sludge were highlighted. A considerable shrinkage and cracking of the sewage sludge briquettes presented in the drying process, and there were many hollow cavities formed in the final drying stages. The average drying rates of the sewage sludge/lignite blend in the whole falling rate periods were higher than that of the raw sewage sludge. The Midilli models gave good agreement between experimental and predicted moisture ratios of the raw sewage sludge or sewage sludge/lignite blend with the highest R2. The apparent activation energies of the raw sewage sludge or sewage sludge/lignite blend in the first falling rate periods were higher than that in the second falling rate periods. Adding lignite could obviously improve the drying behavior of the sewage sludge briquettes over 120 °C.