The hydrothermal (HT) treatment has been developed as a thermal pretreatment technology to improve sludge dewaterability or to produce solid fuel by controlling sludge moisture content. Nevertheless, almost nothing is known about the impact of HT pre-treatment on the NOx emission from sludge combustion. This work mainly focused on the NO emission behavior of HT pretreated and untreated sewage sludge employing five combustion modes, such as conventional combustion (CC), decoupling combustion (DC), separated combustion (SC) and air-staging combustion (AC) with two gas-supplying modes, under a temperature of 1073 K. The results show that the NO emission from the HT pretreated sludge (HTS) was lower than that from the raw sludge (RS), independent on combustion modes. In CC and AC, the HT pretreatment dramatically reduced NO emission with a reduction ratio of 50.7% and 56.4%, respectively. In order to investigate the mechanism of HT reducing NO emission in various combustion modes, the X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy examination were performed to investigate the nitrogen functional group existing in the sludge before and after the HT. Two bonds, the N-C bond in amide or amine with a binding energy (BE) of around 400 eV and the N-H bond in ammonia or protonated amine with a BE of about 402 eV were found to be the predominant functional group existing in sewage sludge. The transformation of the protein nature and the reduction of the binding energy between the bonds, have improved the devolatilization properties of sludge, thus enhancing the reaction of NO with NH3 similar to the DeNO(x) or SNCR process, which is identified as the main reason that the HT dramatically reduced the NO emission from sludge combustion. The data demonstrated that the HT is a promising pretreatment technology because it cannot only improve sludge dewaterability, but also effectively controls the NO emission from sludge combustion. (C) 2013 Elsevier Ltd. All rights reserved.