Integrated System for the Treatment of Oxides of Nitrogen from Flue Gases SANJOY BARMAN AND LIGY PHILIP* Department of Civil Engineering, Indian Institute of Technology, Madras, India 600 036 A novel and effective system was developed for the complete treatment of NO x from flue gases. The system consisted of photocatalytic or ozone oxidation of NO x , followed by scrubbing and biological denitrification. Maximum photocatalytic oxidation of NO x was achieved while using powdered TiO 2 at a catalytic loading rate of 10 g/ h, relative humidity of 50%, and a space time of 10 s. The used catalyst was regenerated and reused. A total of 72% of oxidized NO was recovered as HNO 3 /HNO 2 in the regeneration process. Stoichiometrically, 10% excess ozone was able to affect 100% oxidation of NO to NO 2 . Presence of SO 2 adversely influenced the oxidation of NO by ozone. The scrubbing of NO was effective with distilled water. Heterotrophic denitrifiers were able to denitrify the leachate with an efficiency of 90%, using sewage (COD 450 mg/L) as electron donor. The new integrated treatment system seems to be a promising alternative for complete treatment of NO x from flue gases. Introduction In general, oxides of nitrogen (NOx) refer collectively to six compounds of nitrogen and oxygen. However, it often refers to the major species: nitric oxide (NO) and nitrogen dioxide (NO2). NOx is responsible for troposphere ozone and urban smog through photochemical reactions. NOx, together with SO2, is the major contributor to acid rain that harms forest crops, buildings, as well as aquatic life (1, 2). The rapid economic growth and ever increasing consumption of fossil fuels have resulted in large emissions of NOx (3, 4). Concern for environmental and health issues have forced the envi- ronmental regulatory agencies to enforce stringent NOx emission standards. For example, the 1990 Clean Air Act Amendments have led to regulations requiring significant NOx emission reduction from stationary sources in the U.S. (5). The recently promulgated Clean Air Interstate Rule (CAIR) enforces a large reduction in NOx emissions. CAIR plans for 60% NOx and 70% SO2 reduction from 2003 levels within the next 10 years (6). Various methods exist to reduce NOx emission. Combus- tion modification and selective catalytic reduction (SCR) methods are probably the most widely used techniques to control NOx emissions from industries (7-9). However, reduction in NOx is often limited in the combustion modification methods, while SCR systems can be expensive. Other new technologies such as nonthermal plasma and pressure swing adsorption appear to be efficient and cost- effective for the removal of higher concentrations of NOx, but they are still expensive for the treatment of huge volumes of flue gases (10, 11). Thus, there is a need for environmentally friendly and cost-effective alternatives for comprehensive treatment of NOx from flue gases. Biological removal of NOx from contaminated gas stream is emerging as a novel treatment method. Biofiltration, or the use of microorganisms to treat air streams, seems to be a more promising alternative to conventional air pollution control technologies (12, 13). It has been reported that NO2 and SO2 can be removed effectively using a biotrickling filter/ scrubber within a contact time of 6 s, due to their high solubility in water (14, 15). However, it may be noted here that NO represents 85-95% of the total NOx generated in the combustion process. A few attempts have been made in the past to find efficient biological methods for NOx removal from flue gases (16-18). They employed denitrification or nitrification processes. Davido et al. (18) demonstrated the potential of nitrifying bacteria for the removal of NO. The system required a long residence time of 13.7 min to remove 90% NO from a 100 ppm contaminated stream. Up to 96% removal of NO was observed with Thiobacillus denitrificans for a gas stream containing 5000 ppm of NO. However, simultaneous SO2/NOx removal from flue gas was not technically feasible by a combined system with Desulfovibrio desulfuricans and T. denitrificans due to the NO inhibition to D. desulfuricans (19, 20). The presence of 3-8% oxygen in the flue gases of utility boilers adversely affected the anoxic denitrification process, whereas the nitrification process needed a very high empty bed contact time (EBRT) on the order of 10-14 min. Due to the large volume of NO generated and its very low solubility (62 mg in 1 kg of water at 20 °C and an NO pressure of 760 mmHg) (21), neither biotrickling filters nor scrubbers seem to be a viable option for the removal of NO within practical contact times. Accordingly, conversion of NO to NO2, or any other soluble form using a suitable technique, followed by scrubbing and denitrification seems to be a viable research area that could lead to a cost-effective and practical NOx reduction alternative. Titanium dioxide is gaining importance as a photocatalyst for treatment of a wide range of organic and inorganic pollutants (22-25). It has been reported that NOx can be effectively oxidized to the soluble form/forms by means of photocatalytic oxidation using TiO2 (24-26). However, optimization of the operating parameters and assessment of the maximum conversion efficiency of this approach need to be addressed. Chemical oxidation is another promising process which can effectively oxidize most of the compounds. Ozone (O3) is one of the most reactive oxidizing agents. The feasibility and plausible mechanisms of this process were demonstrated by Puri (27). In the present study, development of a new integrated system for the complete treatment of NOx from flue gases is described. The treatment system consisted of advanced oxidation (photocatalytic oxidation using TiO2 and oxidation by ozone) followed by a scrubber and/or a denitrification process. The proof of concept was demonstrated at the bench- scale. The performance of the system was monitored under selected operating conditions. An attempt was also made to recover the reusable byproducts of the process. Materials and Methods Photocatalytic Reactor. A schematic of the reactor is shown in Figure 1a. The rectangular shaped reactor (dimension: 50 cm × 12 cm × 12 cm) was made of acrylic sheets of 6 mm thickness. Baffles (12 cm × 8 cm × 0.6 cm) were provided on both sides of the reactor at 5 cm spacing to get adequate * Corresponding author phone: +91-44-257-4274; fax: +91-44- 257-4252; e-mail: ligy@iitm.ac.in. Environ. Sci. Technol. 2006, 40, 1035-1041 10.1021/es0515102 CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 3, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1035 Published on Web 12/21/2005