An experimental study of thermal de-NOx using NH3 as reductant in O-2/CO2 atmosphere with the effect of SO2 and different additives was performed in a drop tube furnace. Results show that the optimum temperature window is 841-1184 degrees C, and the optimum reaction temperature is about 900 degrees C with a de-NOx efficiency of 95.4%. A certain amount of SO2 has an inhibiting effect on NO reduction. The effect of additives, including Na2CO3, C2H5OH and FeCl3, on NO reduction by NH3 is also explored. The addition of Na2CO3 and FeCl3 is useful to widen the temperature window and shift the reaction to lower temperature for the efficiency is increased from 30.5% to 74.0% and 67.4% respectively at 800 degrees C. Qualitatively, the modeling results using a detailed kinetic modeling mechanism represent well most of the process features. The effect of Na2CO3, C2H5OH and FeCl3 addition can be reproduced well by the Na2CO3, C2H5OH and Fe(CO)(5) sub-mechanism respectively. The reaction mechanism analysis shows that the effects of these additives on NO reduction are achieved mainly by promoting the production of OH radicals at lower temperature. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.