Abstract: As a coal-consuming country in our country, coal-fired power plants have been the main source of SO2, NOx and smoke and air pollutants. With the promulgation of the Action Plan for Upgrading and Upgrading Coal-fired Energy-Saving and Emission-Reduction (2014-2020), the limestone-gypsum desulphurization system needs technological upgrading in order to meet the "ultra-clean emission" requirement of coal-fired power plants. This paper introduces the current advanced limestone-gypsum desulfurization technology, which provides a technical reference for the emission standard of "clean emission" of coal-fired power plants.
As a big country of energy consumption, coal occupies the absolute dominant position in China's energy consumption structure. According to the statistics of "BP World Energy Statistics 2013", the proportion of coal in China reaches 67.5% [1], and the proportion of coal consumption in the power industry Occupy about half. The SO2, NOx and respirable particulate matter generated from coal combustion are the major pollutants in China's atmospheric environment [2~3]. In 2013, China's SO2 and NO x emissions were 20.439 million tons and 22.274 million tons respectively, ranking the first in the world [4 ], The atmosphere prevention and control that China faces is still grim.
In order to ensure the sustainable development of society and economy, effectively control and control fog and haze weather, the state further strengthens the emission requirements of coal-fired power plants. The "Emission Standard of Air Pollutants for Thermal Power Plants" (GB13223-2011) promulgated in 2011 stipulates the key areas Of the coal-fired power plant smoke and dust, SO2, NOx emission concentrations were not higher than 20mg / Nm3, 50mg / Nm3, 100mg / Nm3, in 2014 by the State Council, the Ministry of Environmental Protection and the Bureau of Energy jointly issued the "coal-fired energy-saving emission reduction upgrades and transformation (2014-2020) "(Development and Reform Energy [2014] No.2093) further required that coal-fired power plants in the eastern part of the country discharge basically meet the emission standards of gas-fired power plants, that is, the emission concentrations of soot, SO2 and NOx should not exceed 5mg / Nm3, 35 mg / Nm3, 50 mg / Nm3. Therefore, "ultra-low emissions" become the inevitable choice for coal-fired power plants. At present, most of the existing coal-fired power plants in China use limestone-gypsum wet flue gas desulfurization equipment. Limestone-gypsum wet flue gas desulfurization has the advantages of high reliability and high desulfurization efficiency. The technology is the most mature and the most widely used [5] ~ 10], but limestone - gypsum desulfurization equipment is numerous, the process is complex, compared with denitration, dust removal, flue gas desulfurization need to meet the "ultra-clean emissions", technical difficulties and more difficult to transform, this article describes and analyzes the current advanced desulfurization process Technology, provide a reference for the "clean emission" route.
1 flue gas "clean emissions" limestone - gypsum desulfurization approach
Due to the change of coal supply in coal-fired power plants, the basic quality of coal used for combustion can not be controlled, and the sulfur content of coal tends to deviate from the designed value of the existing system. The existing desulfurization equipment needs to be reformed according to the entrance of absorption tower SO2 Technology route. However, the desulfurization facilities of new power plants need to select reasonable technical routes, optimize design targets, reduce energy consumption and material consumption so as to increase the economic viability of "ultra-clean emissions."
To meet the stringent environmental standards, truly "clean emissions", desulfurization system requires practical technical ways. Such as improving the fineness, purity and activity of limestone to improve the reaction efficiency and gypsum quality; research and development of efficient desulfurization synergist to reduce the "liquid film resistance" of SO2 absorption and improve the reaction rate; In addition, the development of new technology and equipment to improve Tower flue gas distribution, increased gas-liquid contact area, absorption tower absorption, mass transfer capacity is the main way to upgrade the desulfurization technology, but also "clean emissions" the main process route.
2 flue gas "clean emissions" limestone - gypsum desulfurization process
2.1 absorption tower tray technology
Pallet spray tower technology is a patented technology from Barwe. The technology of pallet tower refers to adding one or more perforated plate trays on the basis of counterflow spray to arrange the pallet in the cross section of the entire absorption tower. So that the flue gas into the absorption tower was evenly distributed in the entire cross-section of the flue gas after the tray and then into the spray area to clean, not only conducive to the uniform distribution of flue gas, and throttling through the tray orifice jet greatly increased the spray The turbulence degree of the flue gas in the gob area improves the gas-liquid mass transfer coefficient of the gas-liquid mass transfer conditions, and the stock solution on the tray can also remove SO2 and other pollutants in the flue gas. When the liquid-gas ratio is effectively reduced, High desulfurization efficiency [11 ~ 13].
After the tray is set up, the resistance of the absorption tower system is increased, which increases the power consumption of the booster fan. However, the decrease of the liquid-gas ratio reduces the flow of the slurry circulation pump, and the power consumption of the circulation pump decreases, reducing the overall power consumption of the system [13] . Tray tower technology is more suitable for site factors difficult to transform desulfurization system.
For the existing tray absorption tower, you can adjust the tray opening rate, the installation of second-tier tray further desulfurization efficiency. Dual-tray desulfurization system is based on the original single-layer tray add a layer of alloy tray, which play the role of desulphurization and efficiency. The advantage of the technology is more obvious when the desulfurization efficiency is higher than 98% or the high-sulfur coal is burned, and the double tray is more fully adjusted than the single tray gas-liquid phase homogeneously, the gas phase is evenly distributed and the desulfurization synergism is obvious
2.2 "single tower double loop" technology
"Single-tower double-loop" technology for the German patent FBE, the process for the flue gas through an absorption tower to achieve two SO2 removal process, after two stages of slurry circulation. The main function of the first cycle is to ensure the oxidation of calcium sulfite and the full dissolution of limestone, and to ensure adequate gypsum crystallization time, circulating slurry pH control in 4.6 ~ 5.0; second cycle to ensure the final SO2 removal efficiency, pH The value is high, reaching 5.8-6.0, reducing the liquid-gas ratio while achieving high desulfurization efficiency and low emission concentration. Secondary circulation is equipped with an independent circulation pool, spray layer. Depending on the function, each cycle has different operating parameters [15,16].
The secondary circulation in the "single-column double-cycle" technology is relatively independent and can control the functions of oxidation and absorption respectively, and can better adapt to the changes of coal types and fluctuations of the boiler load, and is particularly suitable for the desulfurization of power plants burning high-sulfur coal. At the same time in the second cycle of the absorption zone using a higher pH, can reduce the liquid-gas ratio, saving power consumption. The technology has been put into use in Guangzhou Hengyun Thermal Power Plant Liability Company, the desulfurization efficiency was stable at 98.7%, SO2 concentration of the outlet concentration of SO2 remained below 50mg / Nm3 at 1800 ~ 4200mg / Nm3 inlet desulfurization tower tower during the trial period , The minimum is less than 10mg / Nm3 [17].
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