Field deployable laser sensor for calibration-free and real-time measurement of NO_x from stationary sources

Nitrogen oxides (NO_x) are common pollutants emitted from stationary sources, such as industrial boilers and power plants, with significant adverse impacts on the environment and biosphere. The abatement of NO_x from anthropic industrial processes requires reliable and sensitive monitoring systems. To address this challenge, we report a sensitive, robust, and compact quantum cascade laser (QCL)-based sensor to measure NO and NO₂ emissions from industrial exhausts. This sensor uses two quantum cascade lasers, which take advantage of the intense absorption spectra of NO at 1929 cm⁻¹ and NO₂ at 1599 cm⁻¹. A high-temperature multipass cell is designed to withstand harsh environments, which increases the light path length. The NO_x concentrations are directly determined from the wavelength modulation spectroscopic model without the need for calibration. We conducted several laboratory experiments to evaluate the sensor's performance, with a focus on accuracy, sensitivity and selectivity. In the end, we showcased the field deployment of our sensor by continuously measuring NO_x concentrations in a 2.8-MW gas-fired industrial boiler.