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Hydrothermal carbonization (HTC) is a promising and effective technology for upgrading the fuel quality of high‐moisture organic wastes. However, a large amount of energy is required to heat the feedstock to high temperature for HTC. Therefore, the maximum waste heat should be recovered to increase the energy efficiency of the HTC process. In this work, a flash‐organic Rankine cycle (FSPG‐ORC) system was coupled after HTC to recover the heat contained in the HTC slurry product. Various operating factors including the flashing temperature, the HTC temperature, the solid‐water ratio (S/W) of the feedstock, and the organic working fluids in the ORC section were studied to obtain their influences on thermodynamic performance of the coupled system. Results indicated that the net power (Wnet) of the FSPG‐ORC system increased from 185.64 to 774.30 kW through the increase of the HTC temperature from 160 to 280°C. In addition, the Wnet and exergy efficiency of the FSPG‐ORC system also increased with increasing the flashing temperature of HTC slurry product. The coupled FSPG‐ORC system converted waste heat to electric energy and increased the energy efficiency of the HTC process. [ABSTRACT FROM AUTHOR]