Diode-laser-based sensor measurements of nitric oxide and carbon monoxide in combustion exhaust streams
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All-solid-state continuous-wave (cw) laser systems for ultraviolet (UV) absorption measurements of the nitric oxide (NO) molecule and mid-infrared (IR) absorption measurements of carbon monoxide (CO) were developed and demonstrated. For the NO sensor, 250 nW of tunable cw UV radiation at 226.8 nm is produced by sum-frequency-mixing in a beta-barium borate crystal. For the CO sensor, 2W of tunable cw IR radiation at 4.5 m is produced by difference-frequency mixing in a periodically-poled lithium niobate crystal. A tunable external-cavity diode laser (ECDL) provides one of the fundamental beams for both processes so that the wavelength of the generated UV/IR can be tuned over NO/CO absorption lines to produce a fully resolved absorption spectrum. The sensors were used for measurements in the exhaust stream of an operating auxiliary power unit (APU) gas turbine engine and a well-stirred reactor (WSR). During these tests, NO was measured in the exhaust at levels below 10 ppm. For measurements at levels above 20 ppm, the NO emission levels obtained using the new sensor agreed with the results of probe sampling chemiluminescent analyzer results to within 10%. A detection limit of 0.8 ppm of per meter path length at 1000 K is estimated for the NO sensor. Measurements with the CO sensor demonstrated an agreement with extractive probe sampling to within 15%. The estimated detection limit of the CO sensor is a few ppm per meter path length at 1000 K.
