Waste heat recovery system was designed based on flash evaporation. The flash evaporation includes one-stage and two-stage flash evaporation. The one-stage evaporation pressure is 0.12 MPa

the first stage pressure of the two-stage evaporation is 0.4～1.0 MPa

and the final stage pressure of the two-stage flash evaporation is 0.12 MPa. The hydrothermal reaction temperature is 200～260 ℃. The mass and energy balance of the whole process of sludge hydrothermal carbonization(HTC)-waste heat recovery were studied

and the effects of flash stage

flash pressure and hydrothermal reaction temperature on slurry waste heat recovery were analyzed. The results show that

the energy consumption of the two-stage flash system was slightly lower than that of the one-stage flash system

but the number of flash stages and the first-stage flash pressure of the two-stage flash system had little impact on the preheated temperature of the feedstock. By increasing the HTC temperature

the feedstock can be heated to a higher temperature and the energy saving efficiency was substantially improved due to the increased waste heat recovery. In detail

when the HTC temperature was 200

230 and 260 ℃

the feedstock can be heated to 99.30

125.85 and 154.35 ℃

and the energy saving efficiency was about 36%

41% and 45%

respectively. However

the net energy yield of the system(the difference of gross energy yield of hydrochar and energy consumption) decreased with the increase of HTC temperature. In the case of dewatering the sludge to a moisture of 30%

HTC-mechanical coupling method could save 76.5% of energy consumption compared with the thermal drying method.