ABSTRACTIn this study, the effect of hydrothermal treatment (HTT) on co-combustion of paper sludge with subbituminous coal was investigated. The paper sludge was treated by the hydrothermal pilot plant at a temperature of 197°C and a pressure of 1.9 MPa for 30 min. Thermal degradation during co-combustion was studied by thermogravimetric analysis. When burning hydrothermally treated paper sludge (HTT-PS) with coal, multiple decomposition behaviors were observed. The ignition temperature of the mixture was reduced since the paper sludge was more reactive than coal. NO emissions performance was tested by a batch-type fixed bed combustor. Results showed that the NO emissions could be reduced by 26–31% and, therefore, the mixture of coal and HTT-PS yielded lower NO emissions compared to the original mixture. NO conversion of HTT-PS was also lower. X-ray photoelectron microscopy suggested that HTT dissolved or protonated amide and simultaneously condensed the nitrogen functional compounds resulting in lower NO emissions. The ash composition of paper sludge was positively altered by HTT and slagging and fouling indices of HTT-PS were improved.