Study on waste heat recovery from sludge drying exhaust gas based on High-Temperature heat pump coupled with steam compression

To address the high energy consumption and heat loss in sludge thermal drying, this study proposed a novel waste heat cascade utilization (WHCU) system for sludge drying exhaust gas based on high temperature heat pump steam compression technology. Process simulation of the high temperature heat pump was conducted through Aspen Plus to investigate the performance of five typical working fluids (R245fa, R245ca, R600, R1234ze(Z) and R1233zd(E)). R245fa was identified as the optimal working fluid, considering factors like performance, environmental impact, cost, and availability. Further process simulation and experimental investigation of WHCU system using R245fa were conducted based on a sludge thermal drying system with capacity of 1.06 t-DS/h. The results show that a system coefficient of performance (COP2) of 2.64 and an energy conversion efficiency of 66 % could be achieved at evaporator and condenser temperatures of 65 degrees C and 110 degrees C, respectively. A direct benefit of 72.60 CNY/t of wet sludge, an energy saving (expressed in standard coal equivalents) of 21.17 kg/h, and a CO2 reduction of 52.78 kg/h could be achieved under the optimal operating condition. The WHCU technology offers substantial benefits in energy conservation, emission reduction, and economic viability, with promising application prospects in sludge thermal drying.