A diode-laser absorption system having the potential of simultaneous determination of NO and NO2 concentrations in the exhaust jets from gas turbines has been being developed. The sensitivities of the detection units at a typical exhaust gas temperature of 800 K were estimated as 30 ppmv-m and 3.7 ppmv-m for NO and NO2, respectively. Experiments using simulated exhaust gas flows have shown that CO2 do not have any interference with the NO and NO2 measurements. The detection limits in ppm of the system were considerably lowered by using a multi-pass optical system. A pair of off-axis parabola mirrors was useful to prevent the laser beam from straying from the detection area of the sensor due to the beam steering in the exhaust gas. Furthermore, the multi-path optical duct fabricated with 14 mirrors on the inner wall was effective in the measurement of NO and NO2 in the exhaust gas from gas turbines.

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