Principle of flue gas desulfurization---principle of lime method flue gas desulfurization

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1. The necessity and importance of flue gas desulfurization. my country’s energy structure is dominated by coal burning.* * , coal production has ranked first in the world, with annual output reaching more than 1.2 billion tons. It will reach 1.5 billion tons in 2000 and 1.8 billion tons in 2010. Coal accounts for 75% of total primary energy consumption. Atmospheric pollution caused by coal burning includes powdered lime/limestone-stone dust, SO2, NOX and CO2. As coal consumption continues to grow, sulfur dioxide emissions from coal burning have also continued to increase, exceeding 20 million tons for many years in a row, ranking first in the world. As a result, acid rain and sulfur dioxide pollution in my country have become increasingly serious. According to the polluting industrial sectors, the order is thermal power plants, chemical plants and smelting plants. Among them, the emission of pollutants from coal-fired power plants accounts for about 50% of the total industrial emissions (it may reach more than 90% in some areas). In 1998, the country's installed power generation capacity reached 277 million kilowatts, an increase of 9.07% over the previous year, and power generation reached 1,157.7 billion kilowatt hours, an increase of 2.07% over 1997. Among them, the thermal power installed capacity was 209.88 million kilowatts, accounting for 75.7%, and the thermal power generation capacity was 938.8 billion kilowatt hours, accounting for 81%. According to preliminary estimates, the sulfur dioxide emissions from thermal power plants nationwide in 1998 were approximately 7.8 million tons, accounting for 37.3% of the national sulfur dioxide emissions. According to my country’s electric power long-term plan: By 2000 and 2010, my country's installed power capacity will reach 289 million kilowatts respectively. Among them, 130 million kilowatts of coal-fired units were added from 1990 to 2000, and another 220 million kilowatts of coal-fired units were added from 2000 to 2010. It is predicted that coal consumption in my country's coal-fired power plants will reach 510 million tons in 2000 and 900 million tons in 2010 respectively. The power industry will be a large user of coal. If such a large amount of coal is discharged into the atmosphere without treatment, my country's total SO2 emissions will rank first in the world. Based on my country's coal production of 1.5 billion tons in 2000 (the coal sulfur content is calculated as an average of 1.2%), the total SO2 emissions reached 18 million tons. Air pollution will cause serious environmental problems. One of the most important problems is "environmental acidification". “"Environmental acidification" is closely related to the discharge of SO2 and NOX into the atmosphere. They enter the ground in two ways: Wet deposition - SO2 and NOX in the atmosphere are removed by rainwater to the ground ； Dry deposition - SO2 and NOX in the atmosphere fall directly to plants or moist ground surfaces. At present, acid rain in my country has developed from a few areas in the southwest in the 1980s to most areas south of the Yangtze River, east of the Tibetan Plateau and the Sichuan Basin. The area with a precipitation pH value less than 5.6 (the international standard for evaluating acid rain) already accounts for 30% of the country's land area. The level of acid rain pollution in central China has exceeded that of the southwest, which was the most polluted in the 1980s, and the frequency of acid precipitation exceeds 90%. Sulfur dioxide pollution in the air in many cities in my country is very serious. At present, 62% of the cities have average concentrations of sulfur dioxide in the air exceeding * * "Ambient Air Quality Standards" secondary standard. The daily average concentration exceeds * * "Ambient Air Quality Standards" Level 3 standard. According to the 1998 China Environmental Status Bulletin: “my country's atmospheric environmental pollution is still dominated by soot, and the main pollutants are sulfur dioxide and soot. The problem of acid rain remains serious. The total amount of sulfur dioxide emissions in 1998 was 20.9 million tons, of which emissions from industrial sources were 15.93 million tons, accounting for 76.2%. ； Emissions from domestic sources were 4.97 million tons. Among the sulfur dioxide emitted by industry, industrial enterprises at or above county level emitted 11.72 million tons, accounting for 73.6%; township enterprises emitted 4.21 million tons. ”Therefore, controlling sulfur dioxide emissions has become an urgent requirement for sustainable social and economic development and is imperative. 1. Legal requirements: The "Atmospheric Pollution Prevention Law of the People's Republic of China" revised in 1995: “Thermal power plants and other large and medium-sized enterprises that emit sulfur dioxide in acid rain control areas and sulfur dioxide pollution control areas cannot use low-sulfur coal for new projects. They must build supporting desulfurization and dust removal devices or take other measures to control sulfur dioxide emissions and dust removal. For existing enterprises that do not use low-sulfur coal, they must adopt measures to control sulfur dioxide emissions and dust removal. * * Enterprises are encouraged to adopt advanced desulfurization and dust removal technologies. ” 2. * * Requirements for pollutant emission standards The "Air Pollutant Emission Standards for Thermal Power Plants" (GB13223-1996) puts forward different control requirements for sulfur dioxide in thermal power plants according to different periods. For new, expanded, or reconstructed thermal power plants (the third period) whose environmental impact reports are pending review and approval starting from January 1, 1997, on the basis of the total emission control of the entire plant, the chimney sulfur dioxide emission concentration limit is increased, and is linked to the "two control zones" and the sulfur content of the coal. If the sulfur content of coal is greater than 10%, the maximum allowable emission concentration is 1200mg/m3N, and if it is less than or equal to 1%, the maximum allowable emission concentration is 2100mg/m3N. That is to say, power plants located in the "two control areas" are required to desulfurize when the sulfur content of coal fired is greater than 1%, otherwise they will not be able to meet emission standards. For power plants with a coal sulfur content below 1%, whether to desulfurize must be determined after passing an environmental impact assessment based on the power plant's total allowable emissions, total regional control volume, and local environmental quality requirements. 3. * * * The requirements for sulfur dioxide control in thermal power plants in the "two control areas" are based on the " * * * "Reply on Issues Related to Acid Rain Control Areas and Sulfur Dioxide Pollution Control Areas" (Guo Han No. 5) puts forward clear requirements for sulfur dioxide emissions from thermal power plants, that is, thermal power plants in the "two control areas" are required to meet: To meet emission standards by the end of 2000 ； Except for thermal power plants that determine electricity based on heat, it is prohibited to build new coal-fired thermal power plants in urban areas and near-efficiency areas of large and medium-sized cities. ； Newly built or renovated power plants with coal-fired sulfur content greater than 1% must build desulfurization facilities ； Power plants that currently burn coal with a sulfur content greater than 1% must take emission reduction measures before 2000 ； Before 2010, desulfurization facilities will be built in phases and batches or other measures to reduce sulfur dioxide emissions with corresponding effects will be adopted. Section: Judging from the pollution situation of pollutants: Ash pollution is "point" pollution ； Wastewater pollution is “watershed” pollution ； Exhaust gas pollution is "global" pollution. Air pollution is global and will spread across national borders, causing disputes between countries. “"Acid rain" and SO2 pollution have become increasingly important issues for mankind. 2. Introduction to flue gas desulfurization process In 1927, in order to protect the high-rise buildings in London, the United Kingdom first adopted the limestone desulfurization process at the Battuan Fuan and Banchiside power plants (a total of 120MW) on the banks of the Thaws River. According to statistics, there were 189 SO2 control processes in 1984, and now there are more than 200. Mainly can be divided into four categories: (1) Pre-combustion control - raw coal purification (2) Control during combustion - sulfurized bed combustion (CFB) and absorbent injection in the furnace (3) Post-combustion control - flue gas desulfurization (4) New technologies (such as coal gasification/combined cycle system, liquid slagging burner) most of them * * Adopt post-combustion flue gas desulfurization process. For flue gas desulfurization, the wet limestone/gypsum desulfurization process is the mainstream. Name Wet method Semi-dry method Dry method 1 Lime/limestone-gypsum circulating fluidized bed (CFB) 2 Seawater desulfurization spray drying method 3 Basic aluminum sulfate method In-furnace calcium spray humidification activation method Electron beam irradiation method (EBA) 4 Water and dilute acid absorption method 5 Double alkali method 6 Acid method 7 Sodium salt circulation method, magnesium oxide method, seawater method A large number of wet limestone/gypsum method research and development have been carried out since the 1930s, and devices have been put into commercial operation in the late 1960s. ABB's first practical-scale wet flue gas desulfurization system was put into use in the United States in 1968. In 1977 Bischoff built the first lime/limestone gypsum method demonstration device in Europe. IHI (Ishikawashima Harima)'s first large-scale desulfurization device was applied in Units 1 and 2 of Isogo Thermal Power Plant in 1976, using the limestone-gypsum mixed desulfurization method in the Venturi tube 2 tower. Mitsubishi Heavy Industries completed the first set of equipment in 1964 and developed a flue gas desulfurization device based on its operating performance. First generation FGD system: Installed since the 1970s in the United States and Japan. Early FGD systems included the following processes: Lime-based liquid ； Sodium based solution ； limestone based fluid ； Alkaline fly ash based fluid ； Double base (lime and sodium) ； magnesium based fluid ； Wellman-Lord process. A wide range of absorption types are used, including ventilation type, vertical counterflow spray tower, horizontal spray tower, and some internal structures such as trays, fillers, glass balls, etc. are used to enhance the reaction. The efficiency of the first generation FGD is generally 70%~85%. Except for a few, the by-products have no commercial value and can only be discharged as waste. Only the magnesium-based method and the Wellman-Lord method produce sulfur and sulfuric acid with commercial value. The characteristic is that the initial investment is not high, but the operation and maintenance costs are high and the system can * Sex is low. Fouling and material failure are the biggest problems. With the growth of experience, improvements have been made to the process, reducing operation and maintenance costs and increasing the possibility of * sex. Second generation FGD system: Installation began in the early 1980s. To overcome scaling and material problems in first-generation systems, dry injection absorbers emerged, and furnace and flue injection of lime and limestone also approached commercial operation. However, the mainstream FGD technology is still the lime-based and limestone-based wet cleaning method, and the ventilation cleaning method using fillers and glass balls has disappeared. Improved spray towers and shower towers are the most common. Different processes have different efficiencies. The initial dry injection FGD can reach 70%~80%, and in some improvement cases it can reach 90%. The furnace and flue injection method can reach 30%~50%, but the reagent consumption is large. With the improvement of the process and the improvement of operating experience, an efficiency of 90% can be achieved. The by-products of all second-generation FGD systems in the United States are discharged as waste. However, in Japan and Germany, solid by-products are forced oxidized in limestone-based wet cleaning methods to obtain gypsum that is commercially valuable in certain industrial and agricultural fields. The second generation FGD system has lower operation and maintenance costs and system availability. * Sexual progress has been made. Third generation FGD system: Furnace and flue injection processes were improved, and LIFAC and fluidized bed technologies were developed. Through the widespread use of forced oxidation and passivation technologies, lime and limestone-based systems can * The problem of sexual scaling has basically been solved. With greater understanding of the chemical processes and the use of additives such as dibasic acids (DBA), the potential for these systems * The performance can reach more than 95%. Both passivation technology and DBA are applied to second-generation FGD systems to solve existing problems. Many of these systems achieve desulfurization efficiencies of 95% or higher. The solid by-products of some systems have applications in agriculture and industry. In Germany and Japan, the production of gypsum has become a routine project in power plants. With the device available * With the improvement of efficiency, the necessity of setting up redundant equipment is reduced, and the flue gas treatment capacity of a single reactor is getting larger and larger. In the 1970s, it had a bad reputation among thermal power plants due to large investments, high operating costs, and problems such as corrosion, scaling, and clogging. After 15 years of practice and improvement, the working performance is as good as the * The performance has been greatly improved, and the investment and operating costs have been greatly reduced, making the following advantages more prominent:: (1) Have long-term application experience in thermal power plants ； (2) High desulfurization efficiency and absorption utilization rate (some units have a desulfurization rate exceeding 90% when Ca/S is close to 1) ； (3) Good availability (the availability of recently installed units has exceeded 90%). People's concept of wet method has thus changed. At present, it is the most widely used and technically mature process, and it can operate * , The maintenance cycle is long, economical, practical and cheap limestone fine powder is used as the absorbent, which reacts with SO2 in the flue gas, and after several reaction steps, the by-product gypsum is generated. According to statistics, among the existing flue gas desulfurization devices in the world, wet methods account for about 85% (of which limestone/gypsum systems account for 36.7% and other wet methods 48.3%), spray drying systems account for 8.4%, absorbent regeneration systems 3.4%, and absorbent spraying in the flue 1.9%. 3. Main features of limestone (lime)-gypsum wet desulfurization (1) High desulfurization efficiency. The desulfurization rate of the limestone (lime)-gypsum wet desulfurization process is as high as more than 95%. The flue gas after desulfurization not only has a very low sulfur dioxide concentration, but also has a low dust content. * * reduce. Large units adopt wet desulfurization technology, which removes large amounts of sulfur dioxide, which is beneficial to regions and power plants in implementing total volume control. (2) The technology is mature and operational * Good sex. The operation rate of limestone (lime)-gypsum wet desulfurization devices in foreign thermal power plants can generally reach more than 98%. Due to its long development history, mature technology and rich operating experience, the normal operation of the boiler will not be affected by the desulfurization equipment. In particular, newly built large units adopt wet desulfurization technology, which has a long service life and can achieve good investment returns. (3) Strong adaptability to changes in coal types. This process is suitable for flue gas desulfurization of any type of coal with sulfur content. Whether it is high-sulfur coal with a sulfur content greater than 3% or low-sulfur coal with a sulfur content less than 1%, the limestone (lime)-gypsum wet desulfurization process can be adapted. (4) It covers a large area and requires a relatively large one-time construction investment. The limestone (lime)-gypsum wet desulfurization process occupies a larger area than other processes, so it is difficult for existing power plants to adopt this process without reserved desulfurization sites, and its one-time construction investment is also higher than other processes. (5) Absorbents are rich in resources and cheap. Limestone, used as an absorbent in the limestone (lime)-gypsum wet desulfurization process, is widely distributed in my country and is rich in resources. The grade of limestone in many areas is also very good, with a calcium carbonate content of more than 90%, and the best ones can reach more than 95%. Among the various absorbents used in the desulfurization process, limestone is the cheapest, is easier to crush and grind, and has a higher calcium utilization rate. (6) Desulfurization by-products are easy to be comprehensively utilized. The desulfurization by-product of the limestone (lime)-gypsum wet desulfurization process is dihydrate gypsum. The annual output of desulfurized gypsum in Japan and Germany is about 2.5 million tons and 3.5 million tons respectively, which can basically be used comprehensively. The main uses are for the production of building materials products and cement retarder. The comprehensive utilization of desulfurization by-products can not only increase the efficiency of power plants and reduce operating costs, but also reduce the disposal costs of desulfurization by-products and extend the service life of ash fields. (7) Technological progress is rapid. In recent years, foreign countries have conducted in-depth research and continuous improvement on the limestone (lime)-gypsum wet process. For example, the absorption device has been combined into one tower from the original three towers of cooling, absorption, and oxidation. The flow rate in the tower has been greatly increased, and the performance of the nozzle has been further improved. Through technological progress and innovation, it is expected that the problems of large area and high cost of this process will gradually be properly solved. Project Limestone/gypsum wet desulfurization process Double alkali desulfurization process Seawater desulfurization process Spray drying desulfurization process Ammonia desulfurization process Circulating fluidized bed desulfurization process Electron beam desulfurization process Process type Wet wet wet semi-dry dry dry dry desulfurizer limestone magnesium base and sodium base lime seawater lime ammonia limestone ammonia by-product status wet wet dry dry dry coal sulfur content no limit applicable to high sulfur coal 1% Left and right low sulfur coal, medium and low sulfur coal, high sulfur coal, medium and low sulfur coal, medium and low sulfur coal, high desulfurization rate, general, high, general applicable range, large capacity, maximum installed capacity 1000MW, medium capacity test, medium capacity, maximum 200MW unit, medium and small capacity, small industrial trial stage, investment, medium, low, medium, low, medium, medium, medium, medium, medium, low, medium, low, medium, high, medium, low, high, low, medium, high. 4. Flue gas desulfurization principle Since the 1970s, flue gas desulfurization tests, research, and development in my country: 1) From 1974 to 1976, Shanghai Zhabei Power Plant conducted engineering tests on limestone-gypsum flue gas desulfurization, with a test scale of 2500 m3/h. 2) In 1977, Shanghai Shinan Power Plant conducted a test of dilute acid catalytic oxidation method with a scale of 500 m3/h. 3) In 1978, Hunan 300 Power Plant conducted a sodium sulfite method test with a scale of 5000 m3/h. 4) In 1979, Hubei Songmuping Power Plant conducted a small trial of the iodine-containing ammonium fertilizer method, with a scale of 5000 m3/hour. 5) In 1982, Sichuan Chengdu Power Plant conducted a phosphorus activated carbon method test with a scale of 1359 m3/h. In 1988, Sichuan Douba Power Plant was put into pilot testing with a scale of 5000 m3/h. From 1994 to 1996, Sichuan Douba Power Plant engineering test was carried out, with a scale of 80,000 to 100,000 m3/h. * * Key projects, investment 8.1 to 8.9 million yuan. 6) In 1984, the rotary spray drying method test was conducted at Baima Power Plant in Neijiang, Sichuan, with a scale of 5000 m3/h. Pilot testing was carried out in 1988, with a scale of 70,000 m3/h and an investment of 11 million yuan. * * identification. It was originally decided to carry out a demonstration project during the Eighth Five-Year Plan, but it was not arranged for some reasons. 7) Sichuan Chongqing Tianyuan Chemical Plant Self-Provided Power Plant 2 * A 35 ton/hour boiler was tested using the sodium sulfite method with an investment of 20 to 30 million yuan. It ran for 18 days and was forced to shut down due to system blockage. 8) Sichuan Chongqing Luohuang Power Plant 2 * The 360,000-kilowatt unit adopts imported limestone-gypsum technology from Mitsubishi Company of Japan. The equipment cost is US$36 million and has been put into operation. 9) Use Japanese grants to test power plants: ①Huangdao Power Plant in Qingdao, Shandong Province has carried out a pilot test of the rotary spray drying method and has started trial operation. Invested by Japan Power Development Corporation 36. 500 million yen (including 70 million yen for equipment costs), with a scale of 300,000 m3/hour. The equipment is manufactured and installed by Mitsubishi Corporation. ②Shaanxi Taiyuan No. 1 Thermal Power Plant Japan invested 3.6 billion yen in a simple wet limestone process engineering test. Scale: 300,000 m3/hour. ③Sichuan Chengdu Power Plant Japan invested US$11 million in electron beam ammonia engineering testing, with a scale of 300,000 m3/h. 10) Take advantage of Germany * * All soft loans are based on limestone-gypsum process. The participating power plants include Beijing Dongjiao 2×410T/H Thermal Power Plant, Hangzhou Banshan 2×12.5KW Power Plant, and Chongqing 2×200,000KW Power Plant. 11) Shenzhen Mawan Power Plant Phase II Project - The Western Power Plant uses AB Company's seawater desulfurization technology. The scale is a 300,000-kilowatt unit with an investment of US$15.7 million (excluding civil engineering installation). 12) The 50,000-kilowatt unit of No. 8 furnace of Guiyang Power Plant in Guizhou conducted engineering tests using the limestone discarding method. The investment is about 5 million yuan. It is the task of the "Eighth Five-Year Plan". 13) Wet limestone three-phase fluidized bed dust removal and desulfurization process developed by Wuhan University of Hydropower and Electric Power. 14) Nanjing Xiaguan Power Plant and Shaoxing Qianqing Thermal Power Plant use the Danish LAVIC desulfurization process of calcium injection in the furnace and flue humidification. 5. Principle of desulfurization reaction: When the absorption liquid is atomized and sprayed into the flue gas through the nozzle, the absorption liquid is dispersed into fine droplets and covers the entire section of the absorption tower. SO2 is absorbed when these droplets come into contact with the flue gas in countercurrent flow. In this way, SO2 is absorbed in the absorption zone, and the oxidation and neutralization reactions of the absorbent are completed in the liquid storage zone at the bottom of the absorption tower and eventually gypsum is formed. In order to maintain a constant pH value of the absorbent and reduce limestone consumption, the absorbent in the absorption tower is continuously stirred by the mixer, oxidizing air and absorption tower circulation pump. ·Chemical Process The chemical process of the forced oxidation system is described below.: 1. The absorption reaction flue gas and the circulating slurry sprayed from the nozzle are effectively contacted in the absorption tower. The circulating slurry absorbs most of the SO2. The reaction is as follows: SO2＋H2O→H2SO3 H2SO3⇋H＋＋HSO3－ 2. Oxidation reaction Flue gas inlet oxidation Air neutralization zone slurry pool absorption reaction Oxidation reaction HSO3－＋1/2O2→HSO4－ HSO4－⇋H＋＋SO42－ SO2＋H2O→H2SO3 H2SO3⇋H＋＋HSO3－ Neutralization reaction Ca2＋＋CO32－＋2H＋＋SO42－＋H2O→CaSO4·2H2O＋CO2↑ 2H＋＋CO32－→H2O＋CO2↑ Part of the HSO3 at the flue gas outlet of the absorption zone oxidation zone is oxidized by the oxygen in the flue gas in the absorption tower spray zone, and the other HSO3 is completely oxidized by the oxidizing air in the reaction pool. The reaction is as follows: Desulfurization reaction principle diagram HSO3-+1/2O2→HSO4- HSO4-⇋H+++SO42- 3. Neutralization reaction reactant slurry is introduced into the absorption tower to neutralize hydrogen ions and maintain a certain pH value of the absorption liquid. The neutralized slurry is recycled in the absorption tower. The neutralization reaction is as follows: Ca2＋＋CO32－＋2H＋＋SO42－＋H2O→CaSO4·2H2O＋CO2↑ 2H＋＋CO32－→H2O＋CO2↑ 4. Most impurities in other flue gases such as Cl, F and dust are washed away by the circulating slurry. A part of the circulating slurry containing gypsum, dust and impurities is pumped out and sent to the gypsum dehydration system.