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NOx removal from the flue gas of a smoke tube boiler using an ozonizer for NO oxidation and a Na2SO3 aqueous solution scrubber (diameter: 0.6 m; height: 3.7 m) was investigated. In this paper, in order to determine the correlation NOx removal performance, the height of the packing material layer in the scrubber was varied from 2.1 to 0 m. The performance tests were carried out using both a compressed natural gas (CNG) and a heavy oil. The flue gas flow rates were in the range of 455-1440 Nm3/h under CNG firing and 675-1330 Nm3/h under heavy oil firing, and the boiler NOx emission concentrations were approximately 40 and 100 ppm, respectively. A total packing height of 0.75 m resulted in NOx removal efficiencies of 70% and 60% for CNG and heavy oil, respectively. Moreover, the Na2SO3 consumption rate at 0.75 m was 10%-20% less than that at 1.2 m, although the NOx removal efficiency was nearly the same. The correlation between the NOx removal performance and the gas retention time in the scrubber was obtained, providing the design information of the scrubber for industrial applications.