OCR Text |
Show (7) 4 NH3 + 6 NO 5 N2 + 6 H20 (8) 2 NH3 + 8 NO (9) At atmospheric pressure with temperatures below 500 K, the forward reaction in Reaction 7 is thermodynamically favored, while above 1,000 K, decomposition of N02 is likely. In either case, the possible occurrence of this reaction does not interfere with the NH3 measurements since the chemi1uminescent analyzer detects both NO and N02. The occurrence of Reactions 8 and 9 in the laboratory system was not investigated experimentally. However, at high conversion levels, the contribution of these reduction reactions to overall conversion of NH3 is expected to be within the experimental error associated with the measurements (particularly in the AIG troubleshooting field program). Dependence of NH3 conversion on reaction temperature is illustrated in Figure 3, which includes individual data points representing runs at a velocity of 0.17 m/sec as well as average values for the repeat experiments at the same velocity. Although data are scattered, an increase in conversion with temperature is observed for both NH3 inlet levels. As indicated in Table 1, conversion levels as high as 98 percent were achieved at temperatures above 1,200 K. In Figure 3, the steep increase in conversion in the kinetically limited region (reaction rate strongly influenced by temperature) is followed by a more gradual rise in the mass transfer limited region at high temperatures, as has been reported in literature. 5,11 In Reference 5, conversion levels similar to those measured in this study have been obtained on a stainless steel converter using a gas mixture containing 43 ppm NH3 in 02/N2' As shown in Figure 3, the measurements conducted in the laboratory indicate that operation at 1,200 K will result in high NH3 conversion efficiencies in the reactor, which should allow for accurate determination of NH3 concentrations in the field. The presence of mass transfer effects is indicated in Figure 4 as well, where gas velocity is plotted against conversion for runs conducted at an NH3 inlet concentration of 96 ppm. For replicate test runs, average values for conversion are included in the figure. Conversion appears to be a function of gas velocity; a sharp increase in conversion with increased velocity is followed by a gradual increase at gas velocities greater than 0.1 m/sec. Additional data will be collected in Phase II both to further evaluate mass transfer effects and refine the optimal operating envelope for the prototype probe design upgrade. For NH3 concentrations <100 ppm, gas velocities greater than 0.2 m/sec are expected to give the NH3 conversion efficiencies (80-95+ percent) required for a field troubleshooting program. Although data are limited, Figure 5 presents the effect of inlet NH3 concentration on conversion efficiency. At a gas velocity of 0.12 m/sec, an increase in conversion efficiency is observed when the inlet NH3 concentration is varied from 57 to 96 ppm. This can be interpreted as an indication that the observed reaction kinetics is not first order for the system under consideration, since first order kinetics is characterized by independence of conversion on inlet reactant concentration. Although Golodets et al., have represented oxidation of NH3 as a first order reaction on 7 |