Continuous Emission Monitoring System (CEMS) is widely used in various boilers and kilns in industries such as thermal power generation, chemical, petrochemical, steel, waste incineration, coking, cement, etc. It is used for online monitoring of flue gas emissions from fixed pollution sources in industrial production processes and guiding the operation and control of flue gas desulfurization and denitrification systems.

In recent years, new CEMS systems based on differential absorption spectroscopy (DOAS) technology have gradually become the mainstream technology route. The fiber optic spectrometer, as the core component of the CEMS system, is mainly used to convert the optical signals in the system into electrical signals, and then process the signals after photoelectric conversion to accurately measure the content of SO2, NO, NO2 and other gases in the flue gas before and after denitrification and desulfurization.

We will focus on introducing the typical testing system structure, spectrometer parameter selection, and their impact on the testing system for gas concentration inversion using fiber optic spectrometer and DOAS.

Typical DOAS testing system

A typical DOAS gas concentration measurement system consists of a continuous light source (xenon lamp, deuterium lamp, etc.), a gas chamber (or open light path), and a detector. Figure 1 shows the system schematic diagram:

DOASThe experimental device for measuring the concentration of flue gas is shown in Figure 2. The experimental device is divided into two parts: the gas path and the light path. The gas circuit mainly consists of four parts: gas source, gas distribution cabinet, measuring tank, and exhaust gas recovery tank. The optical path mainly consists of a light source, collimating/focusing lens, gas cell, and fiber optic spectrometer. High purity nitrogen and SO₂、NO、NO₂Standard gases enter the mixing pipe for mixing after passing through their respective pipelines and mass flow controllers. The flow rate can be adjusted and displayed through a flow display instrument. The mixed gas enters the measuring cell through a heat tracing tube, and the two ends of the measuring cell are sealed with quartz lenses. A thermal resistance is installed at the outlet of the measuring cell to measure the temperature of the gas flowing through the measuring cell, and a differential pressure gauge is installed in the middle of the measuring cell to measure the gas pressure. The gas from the measuring tank will enter the exhaust gas recovery tank for absorption.

Spectrometer selection and its impact on test results

The main performance parameters of a miniature fiber optic spectrometer include spectral range, spectral resolution (FWHM), stray light, optical path stability, nonlinear effects, etc.

1.spectral range

Determine the wavelength range of the spectrometer based on the required analysis of gas type characteristics and absorption spectral lines, while also considering resolution. The wider the spectral range, the lower the resolution.

2.spectral resolution

Spectral resolution, also known as spectral bandwidth, is usually expressed in nm or cm-1. The higher the resolution, the stronger the ability to distinguish spectral details. The following figure shows the test results of the spectrometer at different resolutions,

3.stray light

Scattered light refers to all non signal light detected in the optical path. The presence of stray light comprehensively affects the detection limit, signal-to-noise ratio, and concentration measurement accuracy of CEMS systems. The smaller the stray light, the smaller the system error.

4.Optical path stability

Optical path stability mainly refers to wavelength and amplitude drift caused by optical platform deformation due to temperature and external acceleration. The stability of the optical path has a significant impact on the concentration measurement results. The AvaSpec ULS series spectrometer greatly reduces stray light and improves mechanical and temperature stability. It has a dual built-in mode eliminator and a multi-stage composite parabolic mirror, which increases mechanical strength by 10 times.

5.nonlinear

Nonlinear is defined as the ratio of the actual amplitude measured by a spectrometer to the theoretical amplitude. As a systematic error, nonlinear effects can affect the shape, value, and proportion of the curve in measuring absorbance.

4、 Conclusion

The ultraviolet differential spectroscopy method has inherent advantages for the continuous emission system of flue gas in wet desulfurization and denitrification processes. Domestic commercial CEMS systems based on DOAS algorithm are constantly improving and upgrading according to different needs and application environments. The miniature fiber optic spectrometer has the advantages of small size, no scanning devices, and strong stability, making it the preferred spectral detector for CEMS systems.