The process of removing NOx from the flue gas emitted from a smoke-tube boiler was investigated using an ozonizer, which is used for carrying out the indirect oxidation of NO, and a Na2SO3 chemical scrubber. The flow rate of the flue gas was in the range of 410 - 1480 N·m3/h, its temperature was in the range of 185°C- 325°C, and the concentration of NOx was approximately 40 ppm during the combustion of city gas. Operational and performance data were obtained from the experiments. The efficiency of the NOx removal process was clearly dependent on the oxidation reduction potential (ORP), and it increased with a decrease in the ORP of the scrubbing solution. To effectively remove the requisite amount of NOx, it is essential to maintain the ORP by controlling the injection rates of the Na2SO3 solution and maintaining an ORP of less than 0 mV and the rate of change in ORP to less than 0 mV/min. More than 4 kg/h of Na2SO3 must be added to the scrubber in order to sustain the aforementioned ORP condition of the scrubbing solution. The amount of NOx emitted from the flue gas was maintained as less than 4 ppm at a flue gas flow rate of 410 N ·m3/ h during a 300-min continuous system operation by maintaining an ORP of -30 mV and a pH of more than 7.8 for the scrubbing solution.