The effect of hydrothermal treatment on attrition during the fluidized bed combustion of paper sludge

• Fluidized bed attrition of hydrothermally treated paper sludge (HTT-PS) was investigated.
• HTT-PS showed lower attrition propensity compared to raw sludge.
• HTT-PS was also more reactive than raw sludge and showed better combustion performance.
• Paper sludge ash would contribute significantly to the bed inventory of the fluidized bed.

A combination of experimental techniques was employed to test primary fragmentation and char particle attrition by abrasion during fluidized bed (FB) combustion of raw paper sludge (Raw-PS), hydrothermally treated paper sludge (HTT-PS), and a subbituminous coal (Sub-C), for comparison. The hydrothermal treatment (HTT) was conducted by a pilot-scale reactor at 197 °C (1.9 MPa) for 30 min. The results showed that all three samples extensively underwent primary fragmentation. Char attrition tests under inert conditions showed that Sub-C intensely experienced particle rounding off at the beginning, but after that it became very strong against mechanical abrasive attrition, followed by HTT-PS and Raw-PS, respectively. The oxidative char attrition tests showed that Sub-C exhibited an initial low amount of carbon elutriation rate followed by an attrition enhancement effect at later stages of burn-off, whereas for the Raw-PS and HTT-PS attrition was always lower than under inert conditions due to extensive postcombustion of fines. HTT-PS always produced a lower amount of elutriated carbon than Raw-PS and this indicates better combustion performance as well as lower unburned carbon emission. Finally, the Primary Ash Particle Size Distribution (PAPSD) of the three fuels was determined, showing that the paper sludge would contribute much more than coal to the ash bed inventory in a full-scale FB combustor.