In a pre-proof article posted in the journal Measurement, researchers reported a novel cavity-enhanced spectroscopy sensing device that operates in the spectral range of 430-450 nm and allows for real-time monitoring and on-site evaluation of nitrogen dioxide (NO2) content in flue gas measurement.
Study: Continuous measurement of NO2 in flue gas employing cavity-enhanced spectroscopy sensing system. Image Credit: Rokas Tenys/Shutterstock.com
The device realized the system's compactness by acting on the receiver and transmitter via optical fiber coupling. The Rayleigh scattering cross-sections of helium and nitrogen were evaluated to accurately calibrate the reflectivity of the cavity mirrors.
The adsorption of gaseous NO2 in the optical cavity was studied using a qualitative model theory. The performance of the integrated NO2 flue gas measurement device was enhanced by optimizing flow response time, and the stability and repeatability of the system were also assessed.
The gas detection system's viability was verified by real-time monitoring of a coal-fuelled chemical plant’s NO2 concentration for a day near the plant’s flue gas measurement sampling port. The results were assessed and contrasted with the change of O2 and NO in the flue gas.
Optical Sensing System and its Significance in NO2 Emission Control
NO2 is a significant atmospheric trace gas that is primarily produced by:
- Nitric acid generation
- Motor vehicle exhaust emissions
- Boiler combustion
- Decomposition of soil and oceanic organic matter
- Lightning
NO2 is one of the primary elements contributing to the development of acid rain. A sustained high atmospheric NO2 concentration can cause severe photochemical smog. It even harms the human respiratory system and destroys the ozone layer.
Coal combustion exhaust is the most significant source of NO2 in the atmosphere and the focal point of environmental monitoring. Since coal fuel's organic nitrogen is readily decomposed and oxidized, more NO2 is produced during combustion. Therefore, a reliable and real-time monitoring system for the amount of nitrogen oxide in the flue gas measurement is crucial for limiting NO2 emissions.
Various detection methods have been used in atmospheric NO2 measurement. Even though conventional techniques such as chemiluminescence and wet chemistry have a high detection resolution, a delayed response speed and internal chemical reagents can divert the results from the correct value.
Therefore, in the present work, the researchers demonstrated optical sensing system technology based on absorption spectra to be an efficient and reliable method for detecting trace gases due to its selectivity, high sensitivity, and broad dynamic detection range.
A compact and inexpensive incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) optical sensing system was built using a 440 nm LED to analyze and detect the polluted gas in the reaction of the process industry.
The findings of this study confirm that a highly integrated NO2 flue gas measurement system is more effective in real-time monitoring and controlling the level of NO2 emissions.
Experimental Setup of the Novel Optical Sensing System
The layout of the optical sensing system consisted of the optical cavity, a light source, and the detection module. The customized integrated optical sensing system measurements were 105 cm ´ 15 cm ´ 12 cm and helped enhance instrument stability and specifications. The light source was powered by a current controller (DC4104) and was coupled with a heat sink to reliably deliver a constant current of 300 mA and reduce the impact of current variations.
Other gases such as SO2 and NO did not show discernible absorption because NO2 displayed maximum absorption near 440 nm, and water vapor had minimal interference on spectral measurements. A blue LED focused at 440 nm was selected as the light source. It was directly connected to an SMA multimode fiber and efficiently injected into the cavity via an optical collimator.
The transmitted light was captured by a FiberPort coupler and connected using the same SMA fiber to an Ocean Optics spectrometer with a spectral resolution of roughly 0.4 nm. Finally, the self-written LabVIEW software was used for implementing spectral analysis and data acquisition operations.
A contemporary coal chemical production company in western China was chosen to measure NO emission in the plant’s flue gas sampling port for real-time monitoring and flue gas measurement.
A dehydration device extracted condensed water vapor from the flue gas introduced from the chimney's sampling point. A 3 mm micropore filter screen was utilized as a gas filter before the entrance to prevent contamination of the instrument by large-diameter soot particles. Finally, real-time monitoring was conducted to assess the situation of the flue gas measurement through remote control to ensure regular operation.
Real-time monitoring was conducted for 24 hours to evaluate the situation of the flue gas measurement and identify the NO2 concentration. The results of the present study demonstrated the efficiency of the on-line flue gas measurement using IBBCEAS.
Novel System Promises More Effective Monitoring and Control of NO2 Emissions
The present paper demonstrated a state-of-the-art, customized integrated IBBCEAS optical sensing system with an effective absorption path of 2.7 km for real-time monitoring and on-line flue gas measurement to identify NO2 concentration in the flue.
The stable and compact structure of the optical sensing system makes optical coordination easier and increases the effectiveness of the light that enters the cavity. The system can detect low-concentration atmospheric NO2 after switching to a high reflectivity mirror, such as 99.99%. A lower reflectivity mirror was chosen in the present study to reduce production costs.
The impact of flow response time, reflectivity, and stability assessment of the system were also studied in the present work on NO2 gas measurement. Continuous real-time monitoring and evaluation of NO2 in the flue gas measurement were conducted to illustrate the efficiency of real-time measurement, which serves as an indicator for the polluting gas emission content. The researchers believe that the optical sensing system has tremendous potential in applications such as product assessment in industrial processes and atmospheric monitoring.
References
X. Bian, S. Zhou, X. Sun, B. Yu, and J. Li, Continuous Measurement of NO2 in Flue Gas Employing Cavity-Enhanced Spectroscopy Sensing System. 2022. Measurement. https://www.sciencedirect.com/science/article/pii/S0263224122009344
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