Hydrothermal carbonization using subcritical water (250 °C, 8–10 MPa and 15 min reaction time) was investigated to recover solid carbonaceous fuel i.e. sludge char (HT-SL) from urban sewage sludge. The carbonaceous HT-SL had an energy–density of 15.82 MJ kg−1. For achieving maximum waste-to-energy conversion, the co-combustion of HT-SL with low rank Indonesian coal (LRIC) and hydrothermally upgraded LRIC (HT-LRIC) was investigated using a thermogravimetric analyzer (TGA) and the emission characteristics of gaseous pollutants were determined by using coupled Fourier transform infrared spectroscopy (FT-IR). To gain insights into the physico-chemical and microstructure properties, carbonaceous fuel were characterized by proximate, ultimate, field emission scanning electron microscopy, FT-IR, X-ray diffraction, Brunauer, Emmett and Teller and inductively coupled plasma optical emission spectrometry analysis. Conventional TGA and kinetic parameters such as activation energy of various LRIC, HT-LRIC and HT-SL blends were also determined. This fundamental study provides a basic insight into co-combustion of HT-SL with LRIC and HT-LRIC, which forms a scientific basis for the efficient utilization of sewage sludge as an energy source while minimizing greenhouse gas emissions.