OCR Text |
Show - (1) - (2) The converter that is commonly employed for N02 dissociation is a stainless steel coil. If NH3 is present in the sample gas, stainless steel will also oxidize NH3 to NO. While this may be an interference in quantification of NOx (typically alleviated via gas conditioning systems, NH3 scrubbers, etc ., to remove the ammonia), it is this fact that has led to the observation that NH3 can be measured indirectly but quantitatively with chemiluminescent analyzers. 7 ,8 Several researchers have successfully applied chemiluminescence as a differential technique to monitor NH3 and total NOz continuously, reporting NH3 conversion efficiencies up to 100 percent at temperatures greater than 920 K for NH3/air mixtures using stainless steel converters in commercially available chemiluminescence analyzers. 3-6 According to Janssen et al., this differential technique is applicable and reliable for streams where the concentration of NH3 in the gas is of the same order of magnitude as the concentration of NOx , such as upstream of the SCR catalyst. s Although there has been less than satisfactory performance in some cases (e.g., sample stream under fuel-rich conditions9 ), the challenge in applications of interest to the Radian study involve reliable measurement of NH3 and NOz in air both at high and low concentrations, and where relative concentrations greatly vary. NH3 Oxidation In an effort to define the critical design variables for the probe prototype and the laboratory reactor system, literature on oxidation of NH3 was reviewed. The following summary is intended to provide a framework for discussion of the results obtained from the laboratory study and field trials . The catalytic oxidation of NH3 can proceed via three main paths ; (3) (4) (5) All of these reactions are essentially irreversible over the range of temperatures under consideration by this study. In the oxidation of NH3 over the metal oxides, as well as over metals, primarily N2 is formed at low temperatures. At higher temperatures, N20 appears with its yield passing through a maximum. At temperatures greater than 700 K, NO begins to form. The temperature at which specific products are formed depends on the catalyst. In the presence of many catalysts at 700-800 K, all three reactions may occur. However, selective catalysts accelerate only one of the reactions. Reaction 5, which is the basis of nitric acid manufacture, is carried out on platinum catalysts at temperatures greater than 1,000 K, with 96-99 percent conversion. Metal oxide catalysts containing C0304 or Fe30./Cr203 systems are also employed to catalyze Reaction 5, which is of primary interest in this study. lJDo/148 4 |