A.Desulfurization technology
At present, there are dozens of technical process types for flue gas desulfurization and denitrification. The special pump for boiler dust removal, desulfurization and denitration is divided into three types: wet, semi-dry, and dry according to whether water is added to the desulfurization and denitration process and the dry and wet form of the desulfurization products Large-scale desulfurization process. The wet desulfurization technology is relatively mature, high in efficiency and simple in operation.
1. Wet flue gas desulfurization technology
Advantages: Wet flue gas desulfurization technology is a gas-liquid reaction, with fast reaction speed, high desulfurization efficiency, generally higher than 90%, mature technology, and wide application. The wet desulfurization technology is relatively mature, and its production and operation are safe and reliable. Among the many desulfurization technologies, it always occupies a leading position.
Classification: Commonly used wet flue gas desulfurization technologies include limestone-gypsum method, indirect limestone-gypsum method, lemon absorption method, etc.
2. Dry flue gas desulfurization technology
Advantages: The dry flue gas desulfurization technology is a gas simultaneous reaction. Compared with the wet desulfurization system, the equipment is simple, the area is small, the investment and operating costs are lower, the operation is convenient, the energy consumption is low, and the product is easy to dispose of. Sewage treatment system, etc.
Disadvantages: But the reaction speed is slow, the desulfurization rate is low, and the advanced ones can reach 60-80%. But at present, this method has low desulfurization efficiency, low absorbent utilization, serious wear and scaling, and it is difficult to maintain the equipment. The stability and reliability of the equipment operation are not high, and the service life is short, which limits Application of this method.
Classification: Commonly used dry flue gas desulfurization technologies include activated carbon adsorption method, electron beam radiation method, charged dry absorbent injection method, metal oxide desulfurization method, etc.
A typical dry desulfurization system sprays a desulfurizing agent (such as limestone, dolomite or slaked lime) directly into the furnace. Taking limestone as an example, when calcined at high temperature, the desulfurizer is calcined to form porous calcium oxide particles, which react with SO2 in the flue gas to form calcium sulfate to achieve the purpose of desulfurization.
Dry flue gas desulfurization technology has been applied to large converters and blast furnaces in the iron and steel industry, but this method is not suitable for small and medium blast furnaces. The advantages of dry desulfurization technology are simple process, no sewage and acid treatment problems, low energy consumption, especially the higher flue gas temperature after purification, which is conducive to the diffusion of chimney exhaust, and does not produce "white smoke". The latter flue gas does not require secondary heating and is less corrosive; its disadvantage is that the desulfurization efficiency is low, the equipment is huge, the investment is large, the area is large, and the operation technology requirements are high. Common dry desulfurization technologies are as follows
a. Activated carbon adsorption method:
Principle: SO2 is adsorbed by activated carbon and catalytically oxidized to sulfur trioxide (SO3), and then reacts with water to generate H2SO4. The saturated activated carbon can be regenerated by washing or heating, and at the same time, dilute H2SO4 or high-concentration SO2 is generated. By-products H2SO4, liquid SO2 and elemental sulfur can be obtained, which can effectively control the emission of SO2 and recover sulfur resources. The technology has been improved by Xi'an Jiaotong University on activated carbon, and developed ZL30 and ZIA0 with low cost and strong selective adsorption performance, which further improved the activated carbon process, so that the SO2 adsorption rate in the flue gas reached 95.8%, which reached the national emission standard.
b. Electron beam radiation method:
Principle: The flue gas is irradiated with high-energy electron beams to generate a large amount of active substances, and the SO2 and nitrogen oxides in the flue gas are oxidized to SO3 and nitrogen dioxide (NO2), and H2SO4 and nitric acid (NaNO3) are generated in one step, and the ammonia ( NH3) or limestone (CaCO3) absorbent.
c. Charged dry absorbent jet desulfurization method (CD.SI):
Principle: The absorbent flows through the high-voltage electrostatic corona charging area generated by the spray unit at high speed, so that the absorbent is electrostatically charged. When the absorbent is sprayed into the flue gas flow, the absorbent repels each other due to the same kind of charge. Full exposure greatly improves the desulfurization efficiency. This method is dry treatment, no equipment pollution and scaling, no waste water or waste residue, by-products can also be used as fertilizer, no secondary pollutants, desulfurization rate greater than 90%, and the equipment is single, and the adaptability is relatively wide. However, this method of desulfurization relies on electron beam accelerators to generate high-energy electrons; for general large-scale enterprises, high-power electron guns are harmful to the human body, so radiation shielding is also required, so operation and maintenance requirements are high. An electronic desulfurization device was built in Chengdu Thermal Power Plant in Sichuan, and the desulfurization of SO2 in the flue gas reached the national emission standard.
d. Metal oxide desulfurization method:
Principle: According to the characteristics of SO2 being a relatively active gas, oxides such as manganese oxide (MnO), zinc oxide (ZnO), iron oxide (Fe3O4), copper oxide (CuO) have strong adsorption to SO2, At room temperature or low temperature, metal oxides adsorb SO2, and at high temperature, metal oxides and SO2 chemically react to form metal salts. Then the adsorbate and metal salt are regenerated by thermal decomposition method, washing method, etc. This is a dry desulfurization method. Although there is no sewage, waste acid and no pollution, this method has not been promoted, mainly because the desulfurization efficiency is relatively low, the equipment is huge, the investment is relatively large, the operation requirements are high, and the cost is high. . The key to this technology is the development of new adsorbents.
The above SO2 flue gas treatment technologies are currently widely used. Although the desulfurization rate is relatively high, the process is complicated, the operating cost is high, the pollution prevention is not thorough, and the secondary pollution is caused. It is in line with my country to achieve the harmonious development of the economy and the environment. The policy is not compatible, so it is necessary to explore and research new desulfurization technologies.
3. Semi-dry flue gas desulfurization technology
Semi-dry desulfurization includes spray drying desulfurization, semi-dry and semi-wet desulfurization, powder-particle spouted bed desulfurization, and flue jet desulfurization.
a. Spray drying method:
The spray-drying desulfurization method is to use the force of machinery or air flow to disperse the absorbent into extremely fine mist droplets. The mist droplets and the flue gas form a relatively large contact surface area, which is a kind of heat generated between the gas and liquid phases. Exchange, mass transfer and chemical reaction desulfurization methods. Commonly used absorbents are lye, lime milk, limestone slurry, etc. At present, most devices use lime milk as absorbent. Under normal circumstances, the desulfurization rate of this method is 65%-85%. Its advantages: desulfurization is carried out in the three-phase state of gas, liquid and solid, the process equipment is simple, the product is dry CaSO, CaSO, easy to handle, there is no serious equipment corrosion and blockage, and the water consumption is relatively small. Disadvantages: The automation requirements are relatively high, the amount of absorbent is difficult to control, and the absorption efficiency is not very high. Therefore, choosing and developing a reasonable absorbent is to solve the new problem faced by this method.
b. Semi-dry and semi-wet method:
The semi-dry and semi-wet method is a desulfurization method between wet and dry methods, and its desulfurization efficiency and desulfurization agent utilization rate are also between the two. This method is mainly suitable for the flue gas treatment of small and medium boilers. . The characteristics of this technology are: low investment, low operating costs, although the desulfurization rate is lower than that of wet desulfurization technology, it can still reach 70% tn, and it is less corrosive, less occupied, and reliable. Compared with the wet desulfurization system, the semi-dry and semi-wet desulfurization system commonly used in the industry eliminates the pulping system, and sprays the Ca(OH) in the wet desulfurization system: the aqueous solution is sprayed into CaO or Ca(OH) ): Powder and water mist. Compared with the dry desulfurization system, it overcomes the disadvantages of low reaction efficiency and long reaction time of the calcium injection method of SO2 and CaO in the furnace, improves the utilization rate of the desulfurizer, and the process is simple, which has a good development prospect.
d. Powder-particle spouted bed half-thousand method flue gas desulfurization method:
Technical principle: The flue gas containing SO2 enters the spouted bed of powder particles through the preheater, and the desulfurizer is made into powder and mixed with water in advance, and continuously sprayed into the bed from the top of the spouted bed in the form of slurry, with the spouted particles Fully mixing, with the help of contact with hot flue gas, desulfurization and drying are carried out at the same time. The product after the desulfurization reaction is blown out from the separator in the form of dry powder. This kind of desulfurization technology uses limestone or slaked lime as a desulfurizing agent. It has a high desulfurization rate and desulfurization agent utilization rate, and has little impact on the environment. However, there are strict requirements between the inlet temperature, the relative humidity in the bed, and the reaction temperature. If the moisture content of the slurry and the reaction temperature are not properly controlled, the desulfurization agent will stick to the wall.
d. Flue injection semi-dry flue gas desulfurization:
The method uses the flue between the boiler and the dust collector as a reactor for desulfurization, and does not require an additional absorption vessel, so that the process investment is greatly reduced, the operation is simple, and the space required is small, and it is suitable for development and application in my country. Semi-dry flue gas injection flue gas desulfurization means spraying absorbent slurry into the flue, and the droplet reacts while evaporating, and the reaction product exits the flue as a dry powder.
4. Emerging flue gas desulfurization methods
In recent years, science and technology have advanced by leaps and bounds, and environmental issues have risen to legal heights. Our country's scientific and technological workers have developed some new desulfurization technologies, but most of them are still in the experimental stage and await further industrial application verification.
a. Sodium sulfide desulfurization method
The alkali sulfide desulfurization method developed by Outokumpu Company mainly uses industrial grade sodium sulfide as a raw material to absorb SO2 industrial flue gas, and the product is aimed at generating sulfur. The reaction process is quite complicated, with Na2SO4, Na2SO3, Na2S203, S, Na2Sx and other substances being generated. From the products, it can be seen that the process consumes high energy and the value of by-products is low. The stone forest of South China University of Technology has shown that various sulfur in the process The content of the compound changes with the change of reaction conditions. The pH value of the solution is controlled between 5.5-6.5, and a small amount of oxidizing additive TFS is added, and the product mainly produces Na2S203. Filtration and evaporation can obtain 5H0˙Na2S203 with high added value. , And the desulfurization rate is as high as 97%, and the reaction process is: SO2+Na2S=Na2S203+S. This new desulfurization technology has passed the pilot test and is being promoted and applied.
b. Membrane absorption method
Membrane separation technology represented by organic polymer membranes is a new gas separation technology developed in recent years and has been widely used, especially in water purification and treatment. Researchers such as Jin Mei of Dalian Institute of Physics and Chemistry, Chinese Academy of Sciences, creatively use membranes to absorb and remove SO2 gas, with a significant effect, with a desulfurization rate of 90%. The process is: they use a polypropylene hollow fiber membrane absorber, using NaOH solution as the absorbing liquid to remove SO2 gas, which is characterized by using a porous membrane to separate the gas SO2 gas from the NaOH absorbing liquid, and the SO2 gas reaches through the pores in the porous membrane. At the gas-liquid interface, SO2 reacts quickly with NaOH to achieve the purpose of desulfurization. This method is a new technology that combines membrane separation technology and absorption technology, with low energy consumption, simple operation and low investment.
c. Microbial desulfurization technology
According to the characteristics of microorganisms participating in the various processes of the sulfur cycle and obtaining energy, the mechanism of using microorganisms for flue gas desulfurization is: under aerobic conditions, through the indirect oxidation of desulfurization bacteria, the SO2 in the flue gas is oxidized to sulfuric acid , The bacteria get energy from it.
Compared with traditional chemical and physical desulfurization, biological desulfurization basically has no external conditions such as high temperature, high pressure, catalyst, etc. It is operated under normal temperature and pressure, and the process flow is simple and there is no secondary pollution. Foreign countries once used geothermal power stations to remove 5t of H:S per day; the total cost of microbial desulfurization was calculated to be 50% of the conventional wet method. Regardless of organic sulfur or inorganic sulfur, once burned, inorganic sulfur SO2 can be generated indirectly used by microorganisms. Therefore, the development of microbial flue gas desulfurization technology has great potential. Wang An of Sichuan University and others selected ferrooxidans for desulfurization research under laboratory conditions, and the desulfurization rate reached 98% at a lower liquid-to-gas ratio.
d. Development trend of flue gas desulfurization technology
Various existing technologies have their own advantages and shortcomings. Specific analysis should be made in specific applications, and a suitable desulfurization technology should be selected from various aspects such as investment, operation, and environmental protection. With the development of science and technology, the generation of a certain new technology will involve many different disciplines. Therefore, paying attention to the latest developments and research results of other disciplines and applying them to flue gas desulfurization technology is to develop new flue gas desulfurization technology. The important ways of developing, such as microbial desulfurization, electron beam desulfurization and other new desulfurization technologies, will have a lot of room for development due to their unique characteristics. As people pay more and more attention to environmental governance and industrial flue gas emissions continue to increase, desulfurization technology with low investment and operating costs, high desulfurization efficiency, high desulfurization agent utilization, less pollution, and no secondary pollution will surely become the future flue gas desulfurization technology. The main trend of the development of gas desulfurization technology.
Various flue gas desulfurization technologies have achieved certain economic, social and environmental benefits in the process of removing SO2, but there are still some shortcomings. With the continuous development of biotechnology and high-tech, electron beam desulfurization technology and biological A series of high-tech and highly applicable desulfurization technologies such as desulfurization will replace traditional desulfurization methods.
B. Denitration technology
Among the common denitrification technologies, according to the formation mechanism of nitrogen oxides, technical measures for reducing nitrogen and reducing emissions can be divided into two categories:
One type is governance from the source. Control the formation of NOx during calcination. The technical measures: ①Use low-nitrogen burners; ②Sectional combustion in the decomposition furnace and pipelines to control the combustion temperature; ③Change the batching plan, use mineralizers, and reduce the clinker burning temperature.
The other is to manage from the end. Controlling NOx emissions in flue gas, its technical measures: ① "stage combustion + SNCR", domestic pilots; ②selective non-catalytic reduction (SNCR), domestic pilots; ③selective catalytic reduction (SCR) At present, there are only three lines of experiments in Europe; ③SNCR/SCR combined denitrification technology, domestic cement denitrification has no successful experience; ④Biological denitrification technology (in the research and development stage).
The domestic denitrification technology is still in the stage of exploration and demonstration and has not yet been scientifically summarized. Are various design process technical routes and equipment facilities scientific and reasonable, and whether they are reliable in operation? Denitrification efficiency, operating costs, energy consumption, and how much secondary pollutants are discharged will all be tested in practice.
The denitration technology can be divided into:
Denitrification before combustion: Hydrodenitration
Denitration during washing and combustion:
1) Low temperature combustion
2) Low oxygen combustion
3) FBC combustion technology
4) Adopt low NOx burner
5) Separation of pulverized coal concentration
6) Flue gas recirculation technology
Denitrification after combustion:
1) Selective non-catalytic reduction denitrification (SNCR)
2) Selective catalytic reduction denitrification (SCR)
3) Activated carbon adsorption
4) Electron beam denitration technology
Among them, the SNCR denitrification efficiency can reach 25%-40% in large coal-fired units and 80% in small units. Because this method is greatly affected by the size of boiler structure, it is mostly used as a supplementary treatment method for low-nitrogen combustion technology. Its low project cost, simple layout and small floor space are suitable for the transformation of the old factory, and the new factory can be used in conjunction with the boiler design.
The selective catalytic reduction technology (SCR) is currently the most mature flue gas denitration technology. It is a post-furnace denitration method. It was first commercialized in Japan in the late 1960s and 1970s. It used reducing agent (NH3, Urea) under the action of a metal catalyst, it selectively reacts with NOx to generate N2 and H2O instead of being oxidized by O2, so it is called "selectivity". At present, the popular SCR processes in the world are mainly divided into two types: ammonia SCR and urea SCR. Both of these methods use ammonia's NOx reduction function to reduce NOx (mainly NO) to N2 and water that have little effect on the atmosphere under the action of a catalyst. The reducing agent is NH3.
At present, most of the catalysts used in SCR use TiO2 as the carrier and V2O5 or V2O5-WO3 or V2O5-MoO3 as the active ingredient, and are made into three types: honeycomb, plate or corrugated. SCR catalysts used in flue gas denitrification can be divided into high temperature catalysts (345℃～590℃), medium temperature catalysts (260℃～380℃) and low temperature catalysts (80℃～300℃). Different catalysts have different suitable reaction temperatures. . If the reaction temperature is low, the activity of the catalyst will decrease, resulting in a decrease in denitrification efficiency, and if the catalyst continues to run at low temperatures, the catalyst will be permanently damaged; if the reaction temperature is too high, NH3 will be easily oxidized, and NOx production will increase. It will cause the phase change of the catalyst material and degrade the activity of the catalyst. At present, most SCR systems at home and abroad use high-temperature catalysts, and the reaction temperature range is 315°C to 400°C. The advantages and disadvantages of this method in practical applications are as follows.
Advantages: This method has high denitrification efficiency and relatively low price. It is currently widely used in domestic and foreign projects and has become the mainstream technology for power station flue gas denitration.
Disadvantages: The fuel contains sulfur, and a certain amount of SO3 can be generated during the combustion process. After adding the catalyst, under aerobic conditions, the amount of SO3 generated greatly increases, and the excess NH3 generates NH4HSO4. NH4HSO4 is corrosive and viscous, which can cause damage to the tail flue equipment. Although the amount of SO3 produced is limited, its impact cannot be underestimated. In addition, catalyst poisoning cannot be ignored.
