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

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 H2O 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 <60 ppb m (Hz)−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 NOx 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.