Experimental Demonstration of Nitric Oxide Measurement at Elevated Temperature in the Coal-Fired Boiler Exhaust

Oxides of Nitrogen (NOx) are among the most critical pollutants, leading to a variety of environmental concerns. This paper describes the demonstrates a high-temperature Quantum Cascade Laser-based (QCL) probe-based system for in-situ measurement of the pollutants in the flue gas. The system demonstrates a sensitivity of less than one ppm and a fast system response time of 60 seconds at high temperature (>300°C) and high dust-load (70 g/m
³
) flue gas from a coal-fired boiler. The system uses a pulsed 2f wavelength modulation spectroscopy technique for the NO concentration measurement at the core. The paper also elaborates on a novel optical design using hollow-core fibres to couple the laser and launch the laser beam into the measurement zone. A unique fibre cooling mechanism was adopted to shield the fibres from excessive temperatures. The filter arrangements and shielding mechanism on the measurement chamber prevent the dust from entering the measurement zone and maintain the stoichiometry of the flue gas during measurement. The probe was tested in a lab setup at 300°C for its sensitivity, response time and linearity. Field-test demonstrates designed performance and repeatability in a coal-fired power plant at an air-preheater (APH) inlet. The gas temperature was below 350°C. In conclusion, this paper demonstrats the useof a QCL based probe for in-situ gas concentration measurement in harsh conditions.