Development and application prospect of flue gas d

2022-07-30
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The development and application prospect of coal-fired flue gas desulfurization technology

Abstract This paper introduces the limestone/lime discarding method, limestone/gypsum method, double alkali method, magnesium oxide method, Wellman lode method, ammonia method, seawater desulfurization method and other wet desulfurization methods, as well as the rotary spray drying method, furnace calcium injection tail humidification activation method, circulating vulcanization bed desulfurization technology, charged dry jet desulfurization method, electron beam irradiation method Pulse corona plasma method and other dry and semi dry flue gas desulfurization technologies. Finally, various flue gas desulfurization methods are compared

key words flue gas desulfurization wet desulfurization dry desulfurization

1 preface

China's energy composition is dominated by coal, and its consumption accounts for about 70% of the total primary energy consumption. This situation will not change for a long time in the future. Thermal power plants use coal as the main fuel for power generation. Direct combustion of coal releases a large amount of SO2, resulting in atmospheric environmental pollution. With the increase of installed capacity, SO2 emissions are also increasing. Strengthening environmental protection is an important guarantee for China to implement the strategy of sustainable development. Therefore, it is very urgent and necessary to strengthen SO2 control in thermal power plants. There are three ways to control SO2: desulfurization before combustion, desulfurization during combustion and desulfurization after combustion, namely flue gas desulfurization (FGD). At present, flue gas desulfurization is considered to be the most effective way to control SO2. Flue gas desulfurization mainly includes dry/semi dry and wet methods

2 development and application of various flue gas desulfurization technologies

2.1 wet flue gas desulfurization technology

the so-called wet flue gas desulfurization is characterized in that the desulfurization system is located at the end of the flue and behind the dust collector, and the reaction temperature in the desulfurization process is lower than the dew point, so the flue gas after desulfurization needs to be heated again before it can be discharged. As it is a gas-liquid reaction, it has fast desulfurization reaction speed, high efficiency and high utilization rate of desulfurizer. For example, when lime is used as desulfurizer, when CA/s=1, the desulfurization rate can reach 90%, which is suitable for flue gas desulfurization of large-scale coal-fired power plants. However, wet flue gas desulfurization has the problems of wastewater treatment, large initial investment and high operation cost

2.1.1 limestone/lime discarding method

the slurry of limestone or lime is used as the desulfurizer to spray and wash the SO2 flue gas in the absorption tower, so that the SO2 in the flue gas reacts to generate CaCO3 and CaSO4. The key to this reaction is the formation of a2+ during the maintenance of C hydraulic universal testing machine. The production of ca2+ in limestone system is related to the concentration of h+ and the existence of CaCO3; In the lime system, the production of ca2+ is related to the existence of Cao. The optimum operating pH value of limestone system is 5.8-6.2, while the optimum pH value of lime system is about 8 (US National Environmental Protection Agency)

the main device of limestone/lime discarding method is composed of desulfurizer preparation device, absorption tower and waste treatment device after desulfurization. The key equipment is absorption tower. For the limestone/lime discarding method, scaling and plugging are the biggest problems. The main reasons are: the evaporation of water in the solution or slurry leads to the deposition of solids: the deposition or crystallization of calcium hydroxide or calcium carbonate; Reaction product calcium sulfite or crystallization precipitation of calcium sulfate, etc. Therefore, the absorption scrubber should have the characteristics of large liquid holding capacity, high relative velocity between gas and liquid, large gas-liquid contact surface, few internal components and low resistance. Scrubbing towers mainly include fixed filling type, rotary table type, turbulent tower, venturi scrubbing tower and Doyle scrubbing tower, which have their own advantages and disadvantages. Those with high desulfurization efficiency often have the worst reliability of operation. The treatment of solid waste after desulfurization is also a big problem of limestone/lime discarding method. At present, there are mainly backfilling method and impervious storage method, both of which need to occupy a large land area. Due to the above shortcomings, the limestone/lime discarding method has been replaced by the limestone/gypsum method

2.1.2 limestone/gypsum method

the difference between this technology and the abandonment method is that air is pumped into the slurry of the absorption tower to force CaSO3 to be oxidized to CaSO4 (gypsum), and the by-product of desulfurization is gypsum. At the same time, more uniform slurry is produced by blowing air, which is easy to achieve 90% desulfurization rate, and easy to control scaling and plugging. Because limestone is cheap and easy to transport and preserve, it has become the main desulfurizer for gypsum process since the 1980s. At present, limestone/gypsum forced oxidation system has become the preferred wet flue gas desulfurization process when choosing flue gas desulfurization equipment in thermal power plants at home and abroad

the main advantages of limestone/gypsum method are: wide range of applicable coals, high desulfurization efficiency (when ca/s=1 in some units, the desulfurization efficiency is greater than 90%), high utilization rate of absorbent (can be greater than 90%), high equipment operation rate (up to 90%), high working reliability (the most mature flue gas desulfurization process at present), rich and cheap sources of desulfurizer limestone. However, the disadvantages of limestone/gypsum method are also obvious: the initial investment cost is too high, the operation cost is high, the floor area is large, the system management operation is complex, the wear and corrosion phenomenon is serious, the by-product gypsum is difficult to treat (it can only be stacked due to marketing problems), and the waste water is difficult to treat

the limestone/gypsum FGD process is widely used in China, and the typical one is Chongqing Luohuang Power Plant. The plant 2 × In 1990, two sets of limestone/gypsum FGD systems of Mitsubishi Corporation of Japan were introduced for 360MW units, and all of them were completed and put into operation in 1993. The main technical parameters of the desulfurization process are as follows: the desulfurization efficiency is greater than 95%, the SO2 concentration of imported flue gas is 10010mg/nm3, the annual consumption of limestone is about 130kt, the purity of by-product gypsum is not less than 90%, and the annual output is about 400kt. At present, only a small amount is sold, and most of them are stacked in the ash yard

limestone/gypsum desulfurization process is a perfect system, which includes flue gas heat exchange system, absorption tower desulfurization system, desulfurizer slurry preparation system, gypsum dehydration system and wastewater treatment system. The perfect and relatively complex system is also the reason for the relatively high one-time investment in the wet desulfurization process. Among the four major subsystems of the above desulfurization system, only the absorption tower desulfurization system and the desulfurizer slurry preparation system are essential for desulfurization; The flue gas heat exchange system, gypsum dehydration system and wastewater treatment system can be simplified or cancelled according to the specific conditions of each project. There are similar practices in foreign countries. For power plants that do not need to recover gypsum by-products, gypsum dehydration system and wastewater treatment system can be omitted, and gypsum slurry can be directly pumped into the storage site. The simplification of wet desulfurization process can reduce the investment to varying degrees. According to the preliminary calculation, after the wet desulfurization process is simplified, the maximum investment can be reduced by about 50%, the absolute investment can be reduced to the level of the simple desulfurization process, and the comprehensive economic benefits of the wet desulfurization process can be further improved

liquid column jet flue gas desulfurization and dust removal integrated technology is a new flue gas wet desulfurization technology independently developed by Tsinghua University and the crystallization of more than ten years' scientific research achievements of the State Key Laboratory for clean coal combustion of Tsinghua University. The technology has the following characteristics: high desulfurization efficiency; Low initial investment cost; Low operation cost; Low system resistance; The desulfurization product is gypsum, which is easy to treat; Good adaptability of desulfurizer; Good adaptability to sulfur content of coal

the integrated system of liquid column jet flue gas desulfurization and dust removal is mainly composed of desulfurization reaction tower, desulfurizer preparation system, desulfurizer product treatment system, control system and flue system, of which the core device of liquid column jet desulfurization reaction tower (which can also be transformed by using water film dust collector) is. As shown in the figure below, the flue gas enters tangentially from the lower part of the desulfurization reaction tower and contacts with the circulating liquid of the desulfurizer during the rising process in the reaction tower. SO2 in the flue gas reacts with the desulfurizer to remove SO2, and the pure flue gas is discharged from the top of the reaction tower. The circulating liquid of desulfurizer is sprayed upward from the nozzle arranged at the lower part of the desulfurization reaction tower, scattered and dropped at the upper part. In the process of spraying up and down, high-efficiency gas-liquid contact is formed to promote the removal of SO2 in the flue gas, and further improve the dust removal efficiency

the cost of liquid column injection flue gas desulfurization device accounts for about 6% of the total investment of the power plant. The technical and economic indicators it can achieve are: the desulfurization rate is more than 85%, the utilization rate of desulfurizer is more than 90%, and the dust removal efficiency is more than 95%; The operation cost is low, and the desulfurization cost is about 0.45 yuan/kg sulfur dioxide. The main desulfurization product is CaSO4, which can be used as building materials and transformation of saline alkali land. The technology is applicable to a wide range of flue gas volume of various scales. All kinds of coal-fired boilers can be applied from 35t/h to 300MW, and it has good adaptability to coal, including high, medium and low sulfur coal. This technology is also very suitable for the transformation of old plants. At present, it has been used for desulfurization of three 10t/h boilers in Shenyang chemical fertilizer plant. The three 10t/h boilers share a desulfurization reaction tower with a flue gas volume of 4 × 104nm3/h, sulfur content of coal is 1.7%

2.1.3 double alkali method

in fact, the double alkali desulfurization process is developed to overcome the shortcoming that the limestone/lime method is easy to scale and further improve the desulfurization efficiency. It first absorbs SO2 with aqueous solution of alkali metal salts such as sodium salt, then regenerates the SO2 absorbed absorption solution with limestone or limestone in another lime reactor, and the regenerated absorption solution is returned to the absorption tower for reuse. SO2 is still precipitated in the form of calcium sulfite and gypsum. Because the solid production process does not occur in the absorption tower, the scaling problem of limestone/lime method is avoided

2.1.4 magnesium oxide method

some metal oxides such as MgO, MnO2 and ZnO have the ability to absorb SO2, and their slurry or aqueous solution can be used as desulfurizer to wash flue gas desulfurization. The sulfite and sulphurous acid absorbing SO2 decompose at a certain temperature to produce SO2 gas, which can be used to produce sulfuric acid. The metal oxide formed by decomposition is regenerated and can be recycled. China is rich in magnesium oxide resources, so it can be considered that this method requires pre dedusting and dechlorination of flue gas, and 8% of MgO will be lost in this process, resulting in secondary pollution

2.1.5 Wellman Lord method

uses the absorption and regeneration cycle process of sodium sulfite solution to remove SO2 from flue gas, which is also called sub nano cycle method. The actual application effect is that when it is used for coal with sulfur content of 1% ~ 3.5%, the desulfurization efficiency can reach more than 97%. The flue gas resistance loss of the whole system is 4 ~ 7kpa, the system is reliable, and the availability rate is more than 95%. This method is suitable for high sulfur coal to recover sulfur by-products as much as possible

wellman Lord process is a sodium sulfite circulating absorption process developed by Davy McKee company in the United States in the late 1960s. At present, 31 sets of large-scale industrial units have been built in the United States, Japan and Europe. This process mainly uses NaOH generated by NaCl electrolysis to absorb sulfur dioxide in flue gas, produce NaHSO3 and Na2SO4, and recover liquid sulfur dioxide, sulfuric acid or elemental sulfur through different recovery devices. The main process methods are as follows:

the flue gas is pretreated by the venturi scrubber to remove 70% ~ 80% of fly ash and 90% ~ 95% of chloride. The pretreated flue gas is introduced into a three-stage packed tower, which is in full contact with sodium sulfite and supplementary sodium hydroxide solution in the reverse direction to remove more than 90% of sulfur dioxide and generate sodium bisulfite. The solution is refluxed section by section to increase concentration. After heating, the purified flue gas is discharged from the 121.9m chimney. The sodium bisulfite generated by washing enters the regeneration system - forced circulation evaporator, is heated to generate sodium bisulfite, and is released

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