Utilization of sewage-sludge-derived hydrochars toward efficient cocombustion with different-rank coals: Effects of subcritical water conversion and blending scenarios

In the absence of prior drying, dewatered sewage sludge (DSS) was directly converted to hydrochars with superior fuel characteristics in subcritical water. Hydrochar derived at 320°C and 12.0 MPa (SHC-320) was screened for systematic cocombustion with different-rank coals. The results suggest that SHC-320 reduced the activation energy of the blends and altered the main combustion profiles. Meanwhile, blending of SHC-320 induced greater heat loss for higher-rank coals, whereas a higher portion of SHC-320 further improved the ignition reactivity of high-rank coal blends. In the high-temperature region, the value of the pre-exponential factor increased with increasing coal/SHC-320 ratio, resulting in more intense synergistic effects in blends. At a low coal/SHC-320 ratio (30:70), intense antisynergistic effects occurred in cocombustion with low- or high-rank coals. As a result of distinct synergistic interactions, cocombustion with moderate-rank coal achieved the best combustion efficiency among the blends. These findings benefit efficient utilization of DSS as a hydrochar solid fuel in existing cofiring power plants.