Removal of nitric oxide by activated ammonia generated by vacuum ultraviolet radiation

Abstract

To broaden and lower the narrow temperature window of the selective non-catalytic reduction (SNCR) of nitric oxide (NO), the use of activated ammonia as the reduction agent was examined. A 172nm VUV (vacuum ultraviolet) excimer lamp was employed as the excitation source for molecular ammonia. Activated ammonia generated by VUV radiation in a photochemical reactor was injected into a model flue gas (NO/O2/N2) at room temperature. The effects of reaction temperatures, oxygen concentrations, and NH3/NO molar ratios (MRs) on NO removal were investigated in a lab-scale plug flow reactor. Temperatures ranged from 500°C to 850°C. A temperature window enlargement of 150°C was achieved at the lower boundary of the temperature window. The results are the first clear observations of low temperature SNCR by activated ammonia. Above 600°C, NO removal was affected by injection of activated ammonia, while around 750°C, conventional SNCR by injection of molecular ammonia was effective. An approximate 80% NO removal was attained at 700°C with an MR=2.0 and 8.3% O2. Although the chemical compositions of activated ammonia remained unknown during the present study, the results suggested that effective chemical species for NO removal such as NH2 and H2 were produced from activated ammonia above 600°C. The number of photons emitted per mole of molecular ammonia, Np, was proposed to evaluate the effects of NH3 concentrations and residence times of NH3 gas in the photochemical reactor. The optimum Np range was found to be the point at which maximum NO removal was attained.