On May 18, Xiling Gorge of the Yangtze River after rain, with high gorges and flat lakes, green mountains and green waters, and swirling clouds, formed a beautiful natural ecological beauty. Abstract: Mercury pollution is becoming a serious pollution faced by the world, especially coal-fired power plants account for a large share of it. This article mainly combed the mercury pollution situation at that time and the monitoring requirements of governments in various countries, and introduced the calibrators, catalytic conversion methods, measurement methods, key points and precautions in the system planning of the mercury continuous emission monitoring system CEMS.
Key words: total mercury measurement; Mercury calibrator; Mercury CEMS
It is estimated that the anthropogenic mercury emission source is about 1960 tons per year, during which industrial pollution sources account for 85% of the total emissions. There are three main forms of mercury in the atmosphere: ultra-low emission dust instrument element mercury, ionic mercury and particulate mercury. These shapes have different characteristics: elemental mercury is a global pollution source because of its long-distance transmission; Ionic mercury has poor mobility, is soluble in water and enters the water source with rainwater. Mercury ions in the bottom sediment may become highly toxic methylmercury, which will accumulate to the concentration of biological death under the biological enrichment. Methylmercury will return to the atmosphere again in the process of mercury re emission.
Because mercury is extremely toxic, people have been systematically eliminating its source. The Minamata Convention signed by 102 countries in 2013 is a ding level arrangement of the world mercury emission reduction mission. Scientists have been studying the removal of mercury with the intention of effectively eliminating mercury emissions. In recent years, one of the research focuses on mercury emissions is the measurement of each component of total mercury. An important prerequisite for effective emission reduction of mercury is to determine the content of various components.
The US EPA has issued the Technical Indicators and Detection Methods of the Continuous Monitoring System for Total Gaseous Mercury Discharge from Fixed Sources, and has formulated the corresponding on-site monitoring methods and specifications. On January 1, 2012, China also implemented the GB-13223-2011 Code for the Discharge of Air Pollutants from Thermal Power Plants, during which a limit value of 0.03mg/m3 was set for mercury and its compounds discharged from coal-fired boilers. At present, the monitoring of total mercury emissions by domestic coal-fired power plants has not been widespread, and there are few manufacturers producing online mercury monitoring CEMS. With the increasingly stringent requirements of the state on environmental emissions, the market for mercury emission monitoring has huge potential. This paper will introduce several key points in mercury CEMS.
1. Calibrator
The oxidation control, incineration control and total mercury differentiation measurement of mercury in flue gas are crucial for mercury emission reduction. The successive emission monitoring system needs calibrators to ensure the accuracy of measurement results.
1.1 Element mercury calibrator
There are several methods to complete the elemental mercury calibrator: full steam pressure method, immersion tube method, standardized cylinder method, chemical reagent method. The saturated vapor pressure method is widely used in the ultra-low CEMS system industry. This method mainly uses the corresponding relationship between the temperature and the saturated vapor pressure of mercury. Under the condition of maintaining a certain temperature, a certain amount of mercury can reach a dynamic balance with its saturated vapor, and can be taken out of the lazy carrier gas to form a stable standard gas. The immersed tube method uses the principle that the penetration rate of the immersed tube equipped with mercury immersion source remains unchanged under constant temperature and pressure, and the inert carrier gas can get standard gas of different concentrations through the dispersion tank. This method has high requirements for temperature, pressure, and the stability of the device. If a condition changes, long-term re balancing is required to meet the requirements for stable generation. The continuous and stable preparation is not easy to complete. As long as the standardized cylinder method is available in the United States, the price is expensive and the useful life is only six months. At the same time, regular traceability is required, which is costly and complex to operate. The chemical reagent method is completed by introducing HgCl2 solution into the reducing agent and reducing it to elemental mercury vapor, which is carried out by carrier gas. This process requires more and is manual operation, which is not conducive to continuous preparation.
1.2 Ionic mercury calibrator
The principle of the ionized mercury calibrator is to take the HgCl2 solution out of the corresponding carrier gas through the evaporator to form the ionized mercury gas. Some scholars also used ultrasonic atomization to generate the ionized mercury standard gas, which has been tested in the laboratory and on the site. The cost can be controlled at about 7000 yuan, while the price of the Tekran3315 ionized mercury calibrator is about 250000 yuan, with significant advantages.
2. Catalytic conversion
Now all kinds of mercury analyzers measure gaseous elemental mercury, and the ionic mercury emitted in actual production needs to be transformed accordingly. One way to convert ionic mercury into elemental mercury is to complete the conversion at high temperature of 800 ℃. However, this method requires high requirements for other components in the system, costs high and is not widely used; The other method is to make the sample gas pass through the catalyst for catalytic conversion at about 300 ℃. The catalyst used in this method needs to be replaced regularly, and the conversion efficiency of the catalyst used for a long time cannot be guaranteed; The other method is chemical reduction method, which converts the sample gas through reducing agent. This method requires a lot of chemicals and regular manual replacement. It is very unfriendly for automatic on-site operation.
3. Measurement method
The primary methods for measuring elemental mercury are: cold vapor atomic absorption spectrometry (CVAAS), cold vapor atomic fluorescence spectrometry (CVAFS), Zeeman modulated atomic absorption spectrometry (ZAAS), ultraviolet differential absorption spectrometry (UV-DOAS), etc. CVAAS and CVAFS are widely used.
3.1 Cold steam atomic absorption spectrometry
This method is to use the characteristic spectral line announced by the mercury lamp at 253.7 nm to pass through the elemental mercury vapor, which is absorbed by the ground state atom of mercury in the vapor. After measuring the degree of weakening of the radiation light intensity, the content of mercury in the sample gas is obtained. Other elements in the spectral line of this band will also be absorbed to disturb the measurement of mercury, so the influence of disturbing components will be removed by gold amalgam preconcentration or dual channel optical path method.
3.2 Cold vapor atomic fluorescence spectrometry
This method is to use the ground state atom of mercury to absorb the radiation of a specific frequency and then be excited to a high energy state. During the excitation process, the fluorescence of a characteristic wavelength is emitted in the form of light radiation, and the concentration of mercury is obtained by measuring the fluorescence intensity. Due to the high detection sensitivity of this method, the corresponding dilution probe can be selected for the measurement of high concentration flue gas. However, in the measurement process, the excited mercury atom will also collide with other irrelevant particles, resulting in fluorescence quenching, reducing the detection sensitivity.
4. System planning
Mercury exists as a metal liquid under normal temperature and pressure, and has good adsorptivity on the metal surface. Therefore, its adsorption on the surface should be eliminated as much as possible in the system planning. Including sampling pipeline, transmission pipeline, and sample gas flow path, it is required to use PTFE lined pipes or corresponding flexible pipes. For the light source gas chamber in the spectrometer, it is required to use quartz raw materials. Under the special high temperature working condition of VOCs online monitoring system, glass lined pipes are required for various pipelines. For the working condition with large humidity content of sample gas, the whole process heat tracing is required to avoid adsorption on liquid droplets. The exhaust gas at the outlet also needs to be adsorbed by activated carbon or amalgam to ensure no secondary pollution.
5. Conclusion
In fact, the mainstream mercury online monitoring system only counts elemental mercury and ionic mercury, and a small number of particulate mercury among them are not covered. The measurement of this part needs further development. The mercury emission in the atmosphere has been paid more and more attention by all parties. The mercury monitoring data has an important guiding role in the directional elimination of mercury pollution sources, the linkage of upstream process flow, and the development of shopping malls. It is urgent to monitor the mercury emission, especially the monitoring of coal-fired power plants.
