An On-line Monitoring Technology Of SO₂ And NO Emissions From Coal-Fired Power Plants

The sulfur dioxide (SO₂) and nitrogen oxide (NO_x) which generated in the combustion process of coal plays an important role in the air pollutions. Substantial emissions of these gases can cause many environmental issues such as deterioration of air quality, destruction of soil, water bodies, woodland areas, building, etc. In a word, the increasing environmental pollutions not only lead to significant economical loss but also affect the health of human beings. China is a country with coal as its main energy source, and about 50% of total coal consumed today is used for power generation. Therefore, thermal power plants are major emission sources of air pollutions. In recent years, the demands for energy continued to increase rapidly have caused a corresponding rise in the numbers of power plants producing electricity. Therefore, many countries in the world are trying very hard to find possible ways and means to reduce the effect of sulfur dioxide and nitrogen oxide pollution on the environment. It is of great importance to accurately measure the concentrations of sulfur dioxide for environmental protection purposes.We present a novel workable and simple method for measuring emissions of SO₂ and NO from power plant based on ultraviolet (UV) absorption spectroscopy without any reference spectrum. The absorption spectrum of both gases is detected using a one broadband deuterium lamp light source as an emitter and a spectrometer as a detector.SO₂ has the same sensible absorption lines in the UV region around 300 nm, which has a strong characteristic absorption spectrum in the wavelength range of 277.4 311.54 nm. Thus, the UV spectral region is chosen for measurement of SO₂ concentrations. And the linear relationship between concentration of sulfur dioxide (SO₂) and optical parameter (OP) is established using the Beer-Lambert law. In order to reduce the impact of possible noise signals, all 720 data points in a particular section from 277.4 to 311.54 nm were superimposed, so that the noise signal offset each other, which makes the result more accurate than that obtained using two spectral signals in the previous method. Temperature may have its influence on the line shape of absorption spectrum because of the Doppler Effect and the measured result, therefore, in the experiments, we analyzed the influence from temperature on SO₂ detection, and temperature correction coefficient was added. In the combustion generated gases, NO₂ has relatively large cross section around 300 nm although its concentrations are far smaller than that of SO₂. In order to evaluate the cross sensitivity of measurement of NO₂ to SO₂, we performed measurements of a low concentration of SO₂ mixed with comparable concentrations of NO₂. The result of the evaluation was that the SO₂ concentration determination was not affected by the presence of NO₂. In order to verify the concentration formula, two standard concentrations of SO₂ gas were chosen for long term experimental tests, the experimental results are consistent with standard concentrations of SO₂, the measurement precision of this approach is better than±1%, and the minimum detection limit is below 1 ppm per meter.NO has several absorption lines between 200 nm and 230nm in the UV region. The concentration formula of NO was established using the Beer-Lambert law based on ultraviolet (UV) absorption spectroscopy. Long term experimental tests of NO with standard concentration were made for 7 hours at a room temperature. Finally, using the method described above, the measurement precision of the system were obtained that is is±1% of NO. Considering SO₂ also has the same sensible absorption lines around 226 nm, therefore we evaluated the sensitivity of SO₂ to NO concentration, and the correctional concentration formula was established.To validate the stability and capability of the measuring system, field measurements were conducted on-line at certain coal based thermal power plant in China. The emission concentrations of SO₂ and NO from circulating fluidized bed (CFB) boiler were monitored. The measured data of SO₂ and NO during the 12nd-19th field campaigns are presented. Results of the measurements finely indicate the actual changes of SO₂ and NO emissions. Finally, the obtained concentrations of SO₂ and NO allow us to calculate the total release and emission coefficient (per kg of coal and per KWh) from the power plant, which provide a reliable basic data for environmental evaluation and collection of the drain contamination expenses. The continuous measurement data obtained successfully demonstrated that the simple detection system had the ability to monitor exhaust gas concentrations of power plant accurately and quickly in an extremely adverse environmental condition. Furthermore, compared with commercial instrument for detection of SO₂ and NO, the system has far more superiorities in response time, cost and performance etc, more suitable for the monitoring requirement of power plant.