An energy-efficient approach to sludge drying: Simulation and optimization of a heat pump assisted low-temperature belt dryer

Sludge drying is the most energy-intensive and carbon-emitting stage in the sludge treatment and disposal process. The thermal drying technologies currently employed generally exhibit high energy consumption, which increases the overall cost and energy demand of sludge management. To solve this problem, this study designed a novel heat pump-assisted low-temperature belt dryer with a center-return air configuration to enhance drying uniformity and efficiency. Computational fluid dynamics (CFD) simulations were used to guide the drying chamber design. The results supported installing baffles at both ends to reduce local airflow velocities and positioning fans centrally to improve airflow uniformity. Thermodynamic modeling was used to identify optimal operating parameters. Results indicated an optimal refrigerant charge of 9-12 kg, and that the waste heat exchanger could meet system requirements under lower water flow and larger temperature difference conditions. The highest specific moisture evaporation rate (SMER) and lowest energy consumption per ton of sludge were achieved in the evaporation-temperature simulations at an evaporation temperature of 36°C, with values of 3.84 kg water/kWh and 173.6 kWh per ton of sludge, respectively. Lower return air temperature improved energy efficiency while meeting dryness requirements, and increasing the air flow ratio reduced energy use but required trade-offs with drying performance. Engineering-scale monitoring showed that the outlet sludge moisture content remained below 40% and the measured SMER ranged from 3.5 to 4.5 kg water/kWh during the observation period.