In situ absorption sensor for NO in combustion gases with a 5.2 μm quantum-cascade laser

Abstract A new mid-infrared absorption sensor for in situ detection of nitric oxide in combustion exhaust gases has been developed and demonstrated for temperatures up to 700 K. A novel external-cavity quantum-cascade diode laser, which can be wavelength-tuned over the R-branch of the fundamental absorption band near 5.2 μm, was utilized, enabling critical evaluation of the interference absorption by H 2 O in combustion exhaust gases. The water vapor absorption spectrum was measured over the range 1880 cm −1 to 1951 cm −1 at 633 K (680 F), typical of conditions in the economizer region of coal-fired power plants. Based on the data for water vapor interference and laser performance, four candidate fundamental-band transitions (R10.5 (1/2, 3/2) and R15.5 (1/2, 3/2)) were selected as optimum and subsequently investigated in detail. A laboratory combustion exhaust rig with a 1.79 m constant-temperature line-of-sight path was then used to validate the sensor for NO concentrations between 20 and 95 ppm at 600 K; an NO detection limit of −1/2 was obtained. The sensor was subsequently applied in successful real-time measurements of NO across a 3 m path in the exhaust of a pulverized-coal-fired power plant, including capture of a 30% transient change in NO concentration at the economizer exit that occurred during the shutdown and recovery of the SNCR NO x control system. The sensor shows excellent promise for monitoring NO in practical combustion exhausts of coal-fired power plants at temperatures up to at least 700 K.