A NEW APPROACH FOR TREATING NITROGEN OXIDES CONTAINING OFF-GAS STREAMS IN AN AEROBIC BIOFILTER SYSTEM

摘要

Electricity generation from coal has increased with the economic development in China. Developing cost-effective strategies to control nitrogen oxides (NOx) from coal-fired power plant combustion gases is urgent. A bench-scale biofiltration system was developed to evaluate the NOx removal efficiency under high oxygen concentration. In general, the denitrification process occurs only under an anaerobic condition, oxygen inhibits the activities of denitrifying bacteria in NOx removal. A special method was employed to screen and characterize natural aerobic denitrification cultures that produce N_2gas under oxic conditions. Then well cultivated nitrifying multi-species were added with the denitrification cultures, and used to inoculate the biofilter reactor later. The reactor was continuously loaded with artificial flue gas containing NO, O_2 and N_2 The system had been running for 120 days and kept on high NOx removal rate steadily. A stable NOx removal with an efficiency of more than 80% from the gas phase can be obtained by the bioreactor concept, when treating flue gas containing NO (400-600 ppmv) and a certain O_2concentration (0~20%). Adsorption studies were conducted comparing NO removal in a cultivated experiment and a blank experiment. In the blank experiment, less than 35% NO was removed as oxygen increased. The tendency of the three curves about NO removal rate with various O_2concentrations was mainly similar but some differences in the highest and lowest removal rate happened in the definite O_2concentration range. It seems no link in the NOx removal and the NO concentration in terms of the changing oxygen concentration. Oxygen was shown to have a significant effect on NOx removal during the first two or three days when oxygen concentration increased sharply. It took 2 days for the NOx removal rate to recover (300ppmv), while it took 3days for 400 ppmv NO, and 3.5 days for 500ppmv NO to recover. The higher the concentration the NO which the influent gas contained, the longer time the microflora need to regain activities. Compared with a humidifier, the microbial regenerator which was incorporated in the biofilter can improve aerobic denitrifying bacteria activity by applying alternating oxic-anoxic conditions in the presence of nitrate and nitrite. Oxidation-Reduction Potential (ORP) and Dissolved Oxygen (DO) were used to control the carbon source dose and trace element. The concentration of oxygen was an important factor, but a high concentration of oxygen was found not to markedly inhibit microflora activities in biofilm as well as high NOx removal rate.