Biological System for the Treatment of Nitrogen Oxides from Flue Gas

Oxides of nitrogen (NOx) is responsible for troposphere ozone and urban smog through photochemical reactions. The rapid economic growth and unlimited consumption of fossil fuels have resulted in large emissions of NOx. Due to the adverse environmental effects, environmental regulatory agencies are compelled to enforce stringent NOx emission standards. Recently promulgated Clean Air Interstate Rule (CAIR) demands a large reduction in NOX emissions. Combustion modification and selective catalytic reduction (SCR) methods are the most widely used techniques to control NOx emissions from point sources. Since SCR systems are expensive, reduction in NOx is mostly limited in the combustion modification methods. Other emerging technologies like nonthermal plasma and pressure swing adsorption appear to be efficient and cost effective for the removal of higher concentration of NOx. However, they are expensive for the treatment of huge volumes of flue gases. Hence, there is an urgent need for developing an environmental friendly and cost-effective alternative 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. A few attempts have been made in the past to find efficient biological methods for NOx removal from flue gases employing denitrification or nitrification processes. In this study, a novel bioscrubber was developed for the complete treatment of NOx from flue gases. As a first step, an autotrophic ANAMMOX system was developed using ammonia as the electron donor and nitrate/nitrite as electron acceptor generating nitrogen gas as the reaction product. Once an efficient ANAMMOX culture was developed, nitrate was replaced by NOx. Initially synthetic flue gas with a composition of 80% N2, 19% CO2 and 100 ppmv was fed to the bioscrubber with an EBRT of 60 sec. The system showed a NOx removal of 20-25 %. The synthetic flue gas with 3-5% oxygen also showed the same NOx removal efficiency. In the latter case, more ammonia consumption in the system was noted. However, there was no nitrate accumulation in the system in both the cases. In order to improve the NOX removal efficiency, NO in the flue gas was partially oxidized to NO2 with the help of ozone (one mole of ozone per mole of NO with an EBRT of 10 sec) and fed to the bioscrubber. The NOx removal efficiency in the system was improved to 73-75%. The bioscrubber was able to remove more than 90% of the generated NO3 . The reactor was operated at an EBRT of 30sec and 10sec. At lower EBRTs, performance of the reactor was affected. The removal efficiency was dropped to 60% at 30 sec and 40% at 10 sec. However, the nitrate accumulation in the system was not significant. A control scrubber without any ANAMMOX culture was also operated. The control system showed 65-72% NOx removal with an EBRT of 60 sec. However, there was considerable nitrate accumulation in the system. The new system developed seems to be a promising alternative for the complete treatment of NOx from flue gases in an environmentally friendly way.