Through the analysis and research of desulfurization technology at home and abroad and the introduction of the pilot plant of desulfurization technology in domestic power industry, the current desulfurization methods can be roughly divided into three categories: desulfurization before combustion, desulfurization during combustion and desulfurization after combustion.
Among them, flue gas desulfurization (FGD) is also called. In flue gas desulfurization technology, according to the types of desulfurizers, it can be divided into the following five methods: calcium method based on CaCO3 (limestone), magnesium method based on MgO, sodium method based on Na2SO3, ammonia method based on NH3 and organic alkali method based on organic alkali. The commercial technology widely used in the world is calcium method, accounting for more than 90%. According to the wet and dry state of absorbent and desulfurization products during desulfurization, desulfurization technology can be divided into wet method, dry method and semi-dry (semi-wet) method. Wet flue gas desulfurization technology is to use absorbent solution or slurry to desulfurize and treat desulfurization products in wet state. This method has the advantages of fast desulfurization reaction, simple equipment and high desulfurization efficiency, but there are many problems such as serious corrosion, high operation and maintenance cost and easy to cause secondary pollution. Desulfurization absorption and product treatment of dry flue gas desulfurization technology are carried out in dry state. This method has the advantages of no waste water, no acid discharge, little equipment corrosion, no obvious temperature drop of flue gas during purification, high temperature of flue gas after purification, good exhaust diffusion of chimney and less secondary pollution, but there are some problems such as low desulfurization efficiency, slow reaction speed and huge equipment. Semi-dry flue gas desulfurization technology refers to flue gas desulfurization technology in which desulfurizer is desulfurized in dry state, regenerated in wet state (such as water-washed activated carbon regeneration process), or desulfurized in wet state, and desulfurized products are treated in dry state (such as spray drying method). Especially, the semi-dry method of wet desulfurization and dry treatment of desulfurization products has the advantages of fast wet desulfurization reaction and high desulfurization efficiency, and the advantages of dry method that there is no waste acid discharge from sewage and desulfurization products are easy to treat, which has attracted wide attention. According to the use of desulfurization products, it can be divided into two methods: discard method and recovery method.
Several desulfurization processes of 1. 1
(1) limestone-gypsum flue gas desulfurization process
Limestone-gypsum desulfurization process is the most widely used desulfurization technology in the world, and about 90% of flue gas desulfurization devices used in thermal power plants in Japan, Germany and the United States adopt this process.
Its working principle is that limestone powder is added with water to make slurry, which is pumped into the absorption tower as an absorbent to fully contact and mix with flue gas. Sulfur dioxide in flue gas reacts with calcium carbonate in slurry and air blown from the lower part of the tower to generate calcium sulfate. When the calcium sulfate reaches a certain saturation, it crystallizes to form gypsum dihydrate. The gypsum slurry discharged from the absorption tower is concentrated and dehydrated to make its water content less than 10%, and then sent to the gypsum storage bin for stacking by the conveyor. The desulfurized flue gas passes through a demister to remove fog droplets, then is heated by a heat exchanger, and then is discharged into the atmosphere through a chimney. Because the absorbent slurry in the absorption tower repeatedly contacts the flue gas through the circulating pump, the utilization rate of absorbent is high, calcium and sulfur are low, and the desulfurization efficiency can be greater than 95%.
(2) rotary spray drying flue gas desulfurization process
In the spray drying desulfurization process, lime is used as desulfurization absorbent, and after digestion, water is added to make deashing emulsion. The emulsified liquid after lime removal is pumped into the atomization device in the absorption tower. In the absorption tower, the absorbent atomized into fine droplets is mixed and contacted with flue gas, and reacts with SO2 in flue gas to generate CaSO3, and SO2 in flue gas is removed. At the same time, the moisture brought by the absorbent quickly evaporates and dries, and the flue gas temperature decreases. Desulfurization reaction products and unused absorbent are taken out of the absorption tower with flue gas in the form of dry particles, and then enter the dust collector for collection. The flue gas after desulfurization is discharged after dust removal by a dust collector. In order to improve the utilization rate of desulfurization absorbent, some dust collectors are generally added to the pulping system for collection and recycling. There are two different atomization forms to choose from in this process, one is rotary spray wheel atomization, and the other is gas-liquid two-phase flow.
Spray drying desulfurization process has the characteristics of mature technology, simple process flow and high system reliability, and the desulfurization rate can reach above 85%. This process has a certain application range (8%) in the United States and some countries in Western Europe. Desulfurization slag can be used for brick making and road construction, but it is mostly abandoned in ash yard or backfilled with waste ore.
(3) Flue gas desulfurization process of ammonium phosphate fertilizer
The flue gas desulfurization technology of ammonium phosphate fertilizer belongs to recovery method, which is named after its by-product ammonium phosphate. The process is mainly composed of adsorption (desulfurization of activated carbon to produce acid), extraction (decomposition of phosphate rock with dilute sulfuric acid to extract phosphoric acid), neutralization (preparation of ammonium phosphate neutralization solution), absorption (desulfurization of ammonium phosphate solution to produce fertilizer), oxidation (oxidation of ammonium sulfite), concentration and drying (preparation of solid fertilizer) and other units. It is divided into two systems:
Flue gas desulfurization system-The dust content of flue gas is less than 200mg/Nm3 after passing through a high-efficiency dust collector, and the pressure of flue gas is increased to 7000Pa by a fan. The flue gas after primary desulfurization is cooled and humidified by spraying water, and then enters the activated carbon desulfurization tower group with four parallel towers (one of which is periodically switched and regenerated), and the primary desulfurization rate is controlled to be greater than or equal to 70%, so as to prepare sulfuric acid with a concentration of about 30%. The flue gas after primary desulfurization enters the secondary desulfurization tower for phosphorus removal.
Fertilizer preparation system-In a conventional single-tank multi-slurry extraction tank, dilute sulfuric acid prepared by desulfurization at the same level decomposes phosphate rock powder (P2O5 content is more than 26%), and dilute phosphoric acid (its concentration is more than 10%) is obtained after filtration, and ammonium phosphate is prepared after neutralization by adding ammonia, which is used as a secondary desulfurizer, and the slurry after secondary desulfurization is concentrated and dried to prepare ammonium phosphate compound fertilizer.
(4) Lime spraying in the furnace and tail humidification flue gas desulfurization process.
In order to improve the desulfurization efficiency, the process of calcium injection in the furnace and humidification activation of tail gas desulfurization is based on the process of calcium injection in the furnace, and a humidification section is added at the tail of the boiler. In this process, limestone powder is mainly used as absorbent. Limestone powder is pneumatically injected into the furnace at the temperature of 850~ 1 150℃. Limestone is decomposed into calcium oxide and carbon dioxide by heating, and calcium oxide reacts with sulfur dioxide in flue gas to produce calcium sulfite. Because the reaction is carried out between gas and solid phases, the reaction speed is slow and the utilization rate of absorbent is low due to the influence of mass transfer process. In the tail humidification activation reactor, humidified water is sprayed out in mist, which contacts with unreacted calcium oxide to generate calcium hydroxide, and then reacts with sulfur dioxide in flue gas. When the calcium-sulfur ratio is controlled at 2.0~2.5, the desulfurization rate of the system can reach 65~80%. Due to the addition of humidified water, the flue gas temperature drops. In general, the outlet flue gas temperature is controlled above the dew point temperature 10~ 15℃. Humidified water evaporates rapidly due to the heating of flue gas temperature, and unreacted absorbent and reaction products are discharged with flue gas in dry state and collected by dust collector.
The desulfurization process has been applied in Finland, the United States, Canada, France and other countries, and the maximum single unit capacity using this desulfurization technology has reached 300,000 kilowatts.
(5) Flue gas circulating fluidized bed desulfurization process
Flue gas circulating fluidized bed desulfurization process consists of absorbent preparation, absorption tower, desulfurization ash recovery, dust collector and control system. In this process, dry hydrated lime powder is generally used as absorbent, and other dry powder or slurry with absorption and reaction ability to sulfur dioxide can also be used as absorbent.
The untreated flue gas discharged from the boiler enters from the bottom of the absorption tower (i.e. fluidized bed). There is a venturi device at the bottom of the absorption tower. After the flue gas flows through the Venturi tube, its speed is accelerated, and it is mixed with fine absorbent powder, and particles, gas and particles are violently rubbed to form a fluidized bed. Under the condition of spraying uniform water mist to reduce the temperature of flue gas, the absorbent reacts with sulfur dioxide in flue gas to produce CaSO3 and CaSO4. Flue gas containing a large number of solid particles after desulfurization is discharged from the top of the absorption tower, enters the recovery dust collector, and the separated particles return to the absorption tower through the middle ash bin. Because solid particles are recycled for hundreds of times, the utilization rate of absorbent is high.
The by-product of this process is dry powder, and its chemical composition is similar to that of spray drying desulfurization process. It is mainly composed of fly ash, CaSO3, CaSO4 and unreacted absorbent Ca(OH)2, which is suitable for backfilling waste ore and road foundation.
Typical flue gas circulating fluidized bed desulfurization process, when the sulfur content of coal is about 2% and the calcium-sulfur ratio is not more than 1.3, the desulfurization rate can reach above 90% and the exhaust temperature is about 70℃. This process is currently used in foreign10 ~ 200,000 kW units. Because of its small floor space and low investment, it is especially suitable for flue gas desulfurization of old units.
(6) Seawater desulfurization process
Seawater desulfurization process is a desulfurization method that uses the alkalinity of seawater to remove sulfur dioxide from flue gas. In the desulfurization absorption tower, a large amount of seawater sprays and washes the coal-fired flue gas entering the absorption tower, and the sulfur dioxide in the flue gas is absorbed and removed by seawater. The purified flue gas is demisted by demister, heated by flue gas heat exchanger and discharged. Seawater that has absorbed sulfur dioxide is mixed with a large amount of seawater that has not been desulfurized, and then aerated in the aeration tank to oxidize SO32- in seawater into stable SO42-, and then discharged into the sea after adjusting the PH value and COD of seawater to meet the discharge standard. Seawater desulfurization process is generally suitable for coastal power plants with good diffusion conditions, using seawater as cooling water and burning low-sulfur coal. Seawater desulfurization technology is widely used in flue gas desulfurization of industrial furnaces such as aluminum smelters and oil refineries in Norway, and more than 20 sets of desulfurization devices have been put into operation. In recent years, the application of seawater desulfurization technology in power plants has made rapid progress. The biggest problem of this process is the possible heavy metal deposition after flue gas desulfurization and its impact on the marine environment. It takes a long time to draw a conclusion, and it needs to be carefully considered in areas with sensitive environmental quality and high environmental protection requirements.
(7) Electron beam desulfurization process
The process flow consists of flue gas pre-dust removal, flue gas cooling, ammonia filling, electron beam irradiation and by-product capture. The flue gas discharged from the boiler enters the cooling tower after coarse filtration by the dust collector, and cooling water is sprayed in the cooling tower to cool the flue gas to a temperature suitable for desulfurization and denitrification (about 70℃). The dew point of flue gas is usually around 50℃, and the cooling water sprayed in mist completely evaporates in the cooling tower, so there is no waste water. Flue gas passing through the cooling tower flows into the reactor, and a certain amount of ammonia, compressed air and soft water are mixed and sprayed at the entrance of the reactor. The amount of ammonia added depends on the concentration of SOx and nitrogen oxides. After electron beam irradiation, SOx and nitrogen oxides generate intermediate products of sulfuric acid (H2SO4) and nitric acid (HNO3) under the action of free radicals. Then sulfuric acid and nitric acid react with ammonia stored in * * * to generate powdery particles (mixed powder of ammonium sulfate (NH4)2SO4 and ammonium nitrate NH4NO3). Some of these powdery particles are deposited at the bottom of the reactor, discharged by the conveyor, and the rest are separated and collected by the by-product dust collector, and then sent to the by-product warehouse for storage after granulation. The purified flue gas is discharged into the atmosphere from the chimney through the desulfurization fan.
(8) Ammonia washing desulfurization process
In this desulfurization process, ammonia water is used as absorbent and ammonium sulfate fertilizer is a by-product. The flue gas discharged from the boiler is cooled to 90~ 100℃ by the flue gas heat exchanger, and then enters the pre-scrubber, where hydrogen chloride and hydrogen fluoride are removed after washing. The washed flue gas passes through a droplet separator to remove water droplets, and then enters a pre-scrubber. In the pre-scrubber, ammonia water is sprayed from the top of the tower to wash the flue gas, and SO2 in the flue gas is absorbed and removed by washing. After the washed flue gas is discharged, the water droplets carried by it are removed by the droplet separator and enter the desulfurization scrubber. In this scrubber, the flue gas is further washed, and the mist droplets are removed by the demister at the top of the scrubber and enter the desulfurization scrubber. Then it is heated by the flue gas heat exchanger and discharged from the chimney. The ammonium sulfate solution with a concentration of about 30% generated in the washing process is discharged from the washing tower, and can be sent to a chemical fertilizer plant for further treatment or directly sold as liquid nitrogen fertilizer, or further concentrated, evaporated and dried, and processed into granular, crystalline or blocky chemical fertilizer for sale.
1。 2 Desulfurization before combustion
Desulfurization before combustion is to remove sulfur from coal before combustion. Desulfurization technology before combustion mainly includes physical coal washing method, chemical coal washing method, coal gasification liquefaction, coal water slurry technology and so on. Coal washing is to clean the raw coal used in boilers by physical, chemical or biological means, so as to remove sulfur from coal, purify coal and produce products with different quality and specifications. Microbial desulfurization technology is also a chemical method in essence. Coal powder is suspended in bubble liquid containing bacteria, and enzymes produced by bacteria can promote sulfur oxidation to sulfate, thus achieving the purpose of desulfurization. At present, desulfurization bacteria commonly used in microbial desulfurization technology are: Thiobacillus ferrooxidans, Thiobacillus thiooxidans, archaea, thermosulfide leaf fungi and so on. Coal gasification refers to the process of using steam, oxygen or air as oxidant to react with coal at high temperature to generate combustible mixed gases (called gas) such as H2, carbon monoxide and methane. Coal liquefaction is an advanced clean coal technology that converts coal into clean liquid fuels (gasoline, diesel, aviation kerosene, etc.). ) or chemical raw materials. Coal water slurry is made by grinding raw coal with ash content less than 65,438+00%, sulfur content less than 0.5% and high volatile content into fine coal powder of 250-300 μ m, and adding 65%-70% coal, 30%-35% water and about 65,438+0% additives. During combustion, the coal water slurry is sprayed from the nozzle at high speed, atomized into droplets of 50~70μm, quickly evaporated in the furnace preheated to 600~700℃, and mixed with micro-explosion. Coal volatilizes and catches fire, and its ignition temperature is lower than that of dry pulverized coal.
Among the desulfurization technologies before combustion, physical coal washing technology is mature, most widely used and most economical, but only inorganic sulfur can be removed; Biological and chemical desulfurization can remove both inorganic sulfur and organic sulfur, but the production cost is expensive and it is far from industrial application. The gasification and liquefaction of coal need further research and improvement; Microbial desulfurization technology is under development; Coal water slurry (CWS) is a new low-pollution fuel to replace petroleum. It not only keeps the original physical characteristics of coal, but also has the same fluidity and stability as oil. It is called liquid coal product, which has great market potential and has been commercialized.
Although there are still various problems in desulfurization technology before burning coal, its advantages are that it can remove ash at the same time, reduce transportation volume, reduce pollution and wear of boilers, reduce ash treatment capacity of power plants and recover part of sulfur resources.
1.3 desulfurization in combustion, also known as in-furnace desulfurization.
In-furnace desulfurization is to add sulfur-fixing agent such as CaCO3 into the furnace during combustion, so that the sulfur in coal is converted into sulfate and discharged with slag. Its basic principles are:
CaCO3→CaO+CO2↑
CaO+SO2→CaSO3
Calcium sulfate+1/2×O2→ calcium sulfate
(1) calcium injection technology for edge furnace
As early as the late 1960s and early 1970s, research on desulfurization technology by injecting sulfur-fixing agent into the furnace has been carried out. However, because the desulfurization efficiency is lower than 10% ~ 30%, it can neither be compared with wet FGD nor meet the requirement of 90% removal rate. Once left out. However, in 198 1, the US Environmental Protection Agency studied the desulfurization technology of multi-stage combustion of calcium injection in the furnace to reduce nitrogen oxides, and gained some experience. When Ca/S is greater than 2, the desulfurization rate can reach 40% and 60% respectively with limestone or hydrated lime as absorbent. For the desulfurization of medium and low sulfur coal, as long as it can meet the requirements of environmental protection, it is not necessary to adopt flue gas desulfurization technology with high investment cost. The desulfurization process with calcium injection in the furnace is simple and the investment cost is low, which is especially suitable for the transformation of old plants.
(2) LIFAC flue gas desulfurization process
LIFAC process is to spray limestone powder into a suitable temperature zone in a coal-fired boiler, and add an activation reactor after the boiler air preheater to remove SO2 from the flue gas. The desulfurization process developed by Tampera and IVO in Finland was put into commercial operation for the first time in 1986. The desulfurization efficiency of LIFAC process is generally 60% ~ 85%.
The most advanced coal-fired power plant in Canada, Shand Power Station, adopts LIFAC flue gas desulfurization process. The operation results of 8 months show that the desulfurization process has good performance, and the desulfurization rate and equipment availability have reached the level of some mature SO2 control technologies. The introduction of LIFAC desulfurization process in Xiaguan Power Plant in China has the advantages of less process investment, small floor space and no waste water discharge, which is beneficial to the transformation of old power plants.
1.4 desulfurization after combustion, also known as flue gas desulfurization (FGD).
Coal-fired flue gas desulfurization technology is the most widely used and most efficient desulfurization technology at present. For coal-fired power plants, flue gas desulfurization will be the main method to control SO2 emission for a long time to come. At present, the main development trends of flue gas desulfurization technology in thermal power plants at home and abroad are: high desulfurization efficiency, large installed capacity, advanced technical level, less investment, less land occupation, low operating cost, high degree of automation and good reliability.
1.3. 1 dry flue gas desulfurization process
This process was used for flue gas desulfurization in power plants in the early 1980s. Compared with the traditional wet washing process, it has the following advantages: low investment cost; Desulfurization products are dried and mixed with fly ash; No need to install demister and reheater; Equipment is not easy to corrode, scale and block. Its disadvantages are: the utilization rate of absorbent is lower than that of wet flue gas desulfurization process; When used in high sulfur coal, the economy is poor; The mixture of fly ash and desulfurization products may affect the comprehensive utilization; The control requirements of drying process are very high.
(1) spray dry flue gas desulfurization process: spray dry flue gas desulfurization (hereinafter referred to as dry FGD) was first developed by JOY Company of the United States and Niro Atomier Company of Denmark. It was developed in the mid-1970s, and rapidly popularized and applied in the electric power industry. In this process, the atomized lime slurry contacts the flue gas in the spray drying tower, and the lime slurry reacts with SO2 to generate a dry solid reactant, which is finally collected by the dust collector together with the fly ash. In China, Sichuan Baima power plant has carried out pilot test of rotary spray dry flue gas desulfurization, and gained some experience, which provides a basis for the design of optimal parameters of rotary spray dry flue gas desulfurization for 200 ~ 300 MW units.
(2) Dry flue gas desulfurization technology with fly ash: Japan began to study dry flue gas desulfurization technology with fly ash as desulfurizer from 1985, and completed industrial practical test by the end of 1988. 199 1 At the beginning of the year, the first dry flue gas desulfurization equipment with a flue gas treatment capacity of 644,000 nm3/h was put into operation. Its characteristics: the desulfurization rate is as high as 60% or more, and its performance is stable, reaching the level of general wet desulfurization performance; The desulfurizer has low cost; Less water consumption, no need for drainage treatment and flue gas reheating, and the total equipment cost is lower than that of wet desulfurization1/4; The fly ash desulfurizer can be reused; No slurry, convenient maintenance, simple and reliable equipment system.
1.3.2 wet flue gas desulfurization process
The process flow, form and mechanism of wet flue gas desulfurization in the world are similar, mainly using limestone (CaCO3), lime (CaO) or sodium carbonate (Na2CO3) as detergents to wash flue gas in the reaction tower, thus removing SO2 from flue gas. This process has a history of 50 years. After continuous improvement and perfection, the technology is mature, which has the advantages of high desulfurization efficiency (90% ~ 98%), large single machine capacity, strong coal adaptability, low operating cost and easy recovery of by-products. According to the statistics of Environmental Protection Agency (EPA), wet lime method accounts for 39.6%, limestone method accounts for 47.4% and two methods account for 87% of the wet desulfurization devices used in thermal power plants in the United States. Double alkali method accounts for 4. 1%, and sodium carbonate method accounts for 3. 1%. In countries all over the world (such as Germany and Japan). ), more than 90% of large thermal power plants adopt wet lime/limestone-gypsum flue gas desulfurization process.
The main chemical reaction mechanism of lime or limestone method is:
Lime method: SO2+Cao+ 1/2h2o → caso3? 1/2H2O
Limestone method: SO2+CaCO3+ 1/2h2o → caso3? 1/2H2O+CO2
Its main advantages are wide commercialization, rich absorbent resources and low cost. Waste residue can be discarded and recycled as commercial gypsum. At present, lime/limestone method is the most widely used flue gas desulfurization process in the world. For high sulfur coal, the desulfurization rate can reach more than 90%, and for low sulfur coal, the desulfurization rate can reach more than 95%.
The traditional lime/limestone process has its potential defects, mainly in equipment scaling, blockage, corrosion and wear. In order to solve these problems, various equipment manufacturers have adopted various methods to develop the second and third generation lime/limestone desulfurization process systems.
Wet FGD process is relatively mature: magnesium hydroxide method; Sodium hydroxide method; Wellman-Lord FGD process of Davy Mckee, USA; Ammonia method, etc.
In the wet process, the reheat of flue gas directly affects the investment of the whole desulfurization process. Because the flue gas temperature after wet desulfurization is generally low (45℃), mostly below the dew point, if it is directly discharged into the chimney without reheating, it is easy to form acid mist and corrode the chimney, which is not conducive to flue gas diffusion. Therefore, wet FGD devices are generally equipped with flue gas reheat system. At present, regenerative (rotary) flue gas heat exchanger (GGH) is widely used. GGH is expensive, accounting for a high proportion of the whole FGD process investment. In recent years, Japan's Mitsubishi Company has developed a leak-free GGH, which has solved the problem of flue gas leakage well, but the price is still high. The former German company Shu developed a new technology that can save the chimney. It installs a complete set of flue gas desulfurization device in the cooling tower of power plant, and uses the waste heat of circulating water in power plant to heat flue gas. It works well and is a very promising method.
1.5 plasma flue gas desulfurization technology
The research of plasma flue gas desulfurization technology began in 1970s, and there are two methods that have been developed on a large scale in the world at present:
(1) electron beam radiation
When the electron beam irradiates the flue gas containing water vapor, the molecules in the flue gas, such as O2 and H2O, will be excited, ionized or cracked, resulting in strongly oxidizing free radicals, such as O, OH, HO2 and O3. These free radicals oxidize SO2 and NO in flue gas into SO3 and NO2 or corresponding acids respectively. In the presence of ammonia, stable ammonium sulfate and ammonium sulfate solid are generated and captured by the dust collector to achieve the purpose of desulfurization and denitrification.
(2) Pulse Corona Method (PPCP)
The basic principle of pulse corona discharge desulfurization and denitrification is basically the same as electron beam irradiation desulfurization and denitrification. Many countries in the world have carried out a lot of experimental research and large-scale pilot projects, but there are still many problems to be studied and solved.
1.6 seawater desulfurization
Seawater is usually alkaline, and its natural alkalinity is about 1.2 ~ 2.5 mmol/L, which makes seawater have natural acid-base buffering capacity and SO2 absorption capacity. Some foreign desulfurization companies use this characteristic of seawater to develop and successfully apply seawater to wash SO2 in flue gas to achieve the purpose of flue gas purification.
Seawater desulfurization process is mainly composed of flue gas system, seawater supply and discharge system and seawater recovery system.