Hydrothermally upgraded chars with improved density and friable characteristics were produced from oil-palm empty fruit bunch (EFB) at three temperatures (150,250, and 350 degrees C-denoted as H-150, H-250 and H-350). These chars were co-combusted with low rank Indonesian coal and with hydrothermally upgraded coal (HT-coal). The composition of major gaseous pollutants released from the co-combustion process with specific reference to CO, CO2, CH4, NO, and SO2 was studied in real-time using a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometry. Combustion characteristic factor (CCF) was determined for a systematic analysis of the thermal decomposition process. In the co-combustion of hydrothermally treated biomass with coal and HT-coal, H-250 (50%)/HT-coal (50%) (CCF = 4.1 x 10(-7)) fuel blend showed the highest CCF values i.e. the most efficient co-combustion process. Further analysis of the emission profiles of gaseous pollutants revealed that the co-combustion of 50% H-350 with 50% HT-coal by mass produced the lowest levels of gaseous pollutant emissions. Overall, a systematic combustion carried out in this study showed that co-combustion of hydrothermally upgraded EFB biochar with coal and HT-coal leads to environmental benefits, specifically reduced emissions of toxic (CO), acidic (NO and SO2) and greenhouse (CH4 and CO2) gases. (C) 2013 Elsevier B.V. All rights reserved.