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Show , At temperatures greater than 1,000 K, the rate for Reaction 5 is diffusion controlled. With excess 02 in the reaction mixture, the diffusion of NH3 becomes the rate-determining step . In this region, NO formation on platinum catalysts at atmospheric pressures has been described as first order in NH3: 10 where : rHO - rate of formation of NO k - reaction rate constant C~3 - concentration of NH3 ( 6 ) However, in a recent study conducted on activated stainless steel , the oxidation reaction was found to be weakly dependent on NH3 and 02 concentrations. 11 Conversion leve i s as high as 80 percent were achieved at 1 ,040 K for an NH3/He/02 mixture with the NH3:02 ratio at 1:2.5 . EXPERIMENTAL Laboratory Reactor System The bench-scale laboratory reactor system used to develop des~gn characterization data for the Phase I prototype probe is shown in Figure 2. The stainless steel flow reactor used during the experiments was 0 . 53 m long and 25.4 mm in diameter , and contained a bundle of seven 6 .4-mm 00 stainless steel tubes. The grade of stainless steel used was 316, with a nominal composition of 18 percent Cr, 11 percent Ni, 2 .5 percent Mo, and 68.5 percent of Fe . 12 The reactor was heated using a tube furnace, and the temperature was manually controlled with a Variac. A Type K thermocouple was placed on the surface of the reactor for monitoring and controlling reaction temperature . The concentrations of certified NH3 gases used in the experiments were 56.7, 57.2, and 96 ppm NH3 (in air) . Zero air was used to purge the system between runs. Gas flow rates were monitored using needle valves and calibrated rotameters. NOx concentrations were monitored via a Thermo Electron (TECO) Model 10AR analyzer. Calibration of the analyzer was conducted using certified gases with 90 and 50 .5 ppm NOx (in N2). Sample gas line to the NH3 oxidation reactor and from the reactor exit to the NOx analyzer was heat traced to simulate actual field conditions and also to minimize the potential for side reactions (e.g . , N2 and N20 formation). (The effects from the presence of moisture were not evaluated in Phase I.) Prototype NH3 Monitoring Probe System/Field Trials During the field demonstration program, two TECO Model 10AR analyzers were employed along with two Exemplar gas conditioning systems. The stainless steel NH3 oxidation reactor and the temperature-controlled heater were mounted at the back end of the sample probe equipped with a thermocouple and a pitot tube. The sample gas lines from the probe/reactor to gas conditioners were heat traced and maintained above 300°F to avoid condensation. Poor temperature control of the reactor resulted in runs that covered a temperature range from approximately 1,050 to 1,225 K. Before each run (e.g., horizontal traverse at a given sample port), zero air was used to establish the baseline for the NOx monitors . The lmo/148 5 |