NO(x) removal from an actual boiler flue gas is investigated using an indirect plasma and chemical hybrid system comprising a commercial ozonizer and an Na(2)SO(3) chemical scrubber. A high NO(x) removal efficiency in the continuous operation of a commercial-scale apparatus is achieved by controlling pH and oxidation reduction potential (ORP) and injecting additional Na(2)SO(3) and NaOH aqueous solutions into the scrubber sump when required. Ozone injection is demonstrated to be extremely effective for NO oxidation. The relation between the ORP and NO, removal efficiency, which is essential for the optimal operation of this system, is obtained. (C) 2007 Elsevier B.V. All rights reserved.