Emissions Monitoring Using UV Spectroscopy

INTRODUCTION

Emissions of polluting gases from industries and ships have brought severe air pollution. The main product of combustion includes sulfur dioxide (SO₂), nitrogen oxide (NO), carbon dioxide (CO₂), and particles. As a toxic gas, SO₂ is responsible for many deleterious effects on human health, the environment, and the climate. Furthermore, SO₂ emissions contribute to the formation of sulfate aerosols and small particles, which may penetrate deeply into human lungs. In sufficient quantities, it can contribute to health problems. However, its far distance and low concentration make gas emissions difficult to measure by using the existing techniques. The development of remote sensing techniques based on ultraviolet (UV) absorption spectroscopy for simultaneous concentration measurements of SO₂ and NO emitted has emerged. This method is increasingly used due to its ability to remotely measure the gas concentrations in real-time.

METHOD

Ultraviolet (UV) absorption spectroscopy is an analytical technique used to measure the absorbance of UV light by a sample. This technique is based on the principle that molecules absorb light at specific wavelengths, which causes electronic transitions within the molecules. Many gases such as SO₂ and NO have strong absorption lines in the UV wavelength region, which makes the use of broadband UV spectroscopy suitable for detecting the concentrations of these gases. SO₂ has two absorption bands from 209 to 232 nm and from 272 to 312 nm, respectively, while NO has two absorption peaks at 215 and 226 nm. The maximum allowed SO₂ emission concentration in 1 hour is 125 µg/m³ and NO is 200 µg/m³.

Figure 1 The absorption cross sections (cm² ) of SO₂ and NO in the range of 209 to 230 nm

Figure 2 a. The absorption cross section (cm²) of SO₂ in the range of 270 to 312 nm, b. Absorption spectrum recording (a.u.) of NO and SO₂ in the range of 209 to 312 nm

The field measurement campaign was to evaluate emission coefficients and total emissions of SO₂ and NO, from a 220-ton.h^-1 circulating fluidized bed boiler (CFB). This measurement was performed at a thermal power plant in Shandong - one of the most heavily polluted provinces in China. The obtained experimental data set was analyzed using the intelligent algorithm.

The results of the measurement are shown in Figure 3. The SO₂ concentration varied between 750 mg.m^-3 and 1300 mg.m^-3. This variation can be explained by changes in boiler load or sulfur content in the fuel. This shows that on-line monitoring can timely optimize the desulfurization tower and improve the desulfurization efficiency, thereby reducing pollution and complying with national standards. In contrast to the SO₂ case, the NO concentration in CFB boilers is relatively low, varying between 100 and 220 mg.m^-3. In general, most of the NO is produced by thermal fixation of nitrogen in the combustion air. In addition, nitrogen in the fuel can also be oxidized to NO. Therefore, the combustion temperature and the nitrogen content in the fuel are identified as key factors for NO formation.

Figure 3 Measured SO₂ and NO emission concentrations (mg.m^-3 )

SYSTEM

To measure the concentration of SO₂ and NO, a filter was installed in the sampling probe to remove the fine particles in the sampled gas and finally pumped into the gas analysis system. A spectrometer (Ocean Optics HR2000+) was used, with a deuterium lamp DH-mini as a UV light source.

The HR2000+ Spectrometer provides optical resolution as good as 0.035 nm (FWHM). The HR2000+ is responsive from 200-1100 nm, but the specific range and resolution depends on your grating and entrance slit selections. With its capability of transferring 1 ms spectra continuously, the HR2000+ is the fastest spectrometer available from Ocean Optics. The HR2000+ is perfect for applications where fast reactions need to be monitored and high resolution is necessary, such as chemistry and biochemistry applications.