Recyclable standardized sludge for performance evaluation of low-temperature drying units: Preparation and...

To address the inconsistency in energy efficiency evaluation of air-source heat pump sludge drying systems caused by direct use of actual sludge, an energy efficiency testing strategy based on the standardized sludge was proposed. Standardized sludge prepared using poplar wood powder (33 wt%) and limestone powder (2 wt%) according to the equivalent simulation principle, it exhibited equivalent characteristics to actual sludge in terms of volatile solids (49.18 ± 5%), calorific value (2424 cal/g ± 5%), pyrolysis behavior and moisture transport kinetics (fitted by a two-term exponential equation). Moreover, standardized sludge could be continuously reused over 15 cycles under fixed moisture contents (65 ± 3% inlet, 20 ± 2% outlet). A "fixed boundary-open conditions" black-box testing method was employed, with energy efficiency evaluated via Specific Moisture Extraction Rate (SMER). Pilot-scale tests demonstrated that energy efficiency varied with open conditions (air volume and temperature), with a variation of 12%. SMER exhibited high reproducibility (<5% variation) and successfully distinguished performance differences among drying units. The method also showed favorable applicability and economic viability, with a single-test cost of approximately 455 CNY. This study proposed a novel approach for energy efficiency testing of low-temperature sludge drying units based on standardized sludge.