Coking equipment: A comprehensive overview of flue gas desulfurization and denitration technologies

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:Part 1: Desulfurization Technologies. There are currently dozens of types of flue gas desulfurization technologies. Based on whether water is used in the desulfurization process and on the dry or wet state of the desulfurization products, flue gas desulfurization can be divided into three main categories: wet process, semi-dry process, and dry process. Wet flue gas desulfurization technology is relatively mature, efficient, and easy to operate. I. Advantages of wet flue gas desulfurization technology: Wet flue gas desulfurization is a gas-liquid reaction process with a fast reaction rate and high desulfurization efficiency, generally exceeding 90%. It is a mature technology with wide applicability. Wet flue gas desulfurization technology is relatively mature, with safe and reliable operation; it has always held a dominant position among various desulfurization technologies, accounting for over 80% of the total installed capacity for desulfurization. Disadvantages: The product is a liquid or sludge, making it difficult to handle; the equipment is highly corrosive; the flue gas after washing needs to be reheated, resulting in high energy consumption; it requires a large amount of space; and the investment and operating costs are high. With complex systems, large-scale equipment, high water consumption, and high initial investment, it is generally suitable for large power plants. Classification: Common wet flue gas desulfurization technologies include the limestone-gypsum method, the indirect limestone-gypsum method, and the lime absorption method. A. Limestone/lime-gypsum method: Principle: It involves using limestone or lime slurry to absorb SO2 from flue gas, resulting in the formation of calcium sulfite. The separated calcium sulfite (CaSO3) can be discarded, or it can be oxidized to calcium sulfate (CaSO4), which is then recovered in the form of gypsum. It is the most technologically advanced and stable desulfurization process in the world today, with a desulfurization efficiency of over 90%. At present, the traditional limestone/lime-gypsum flue gas desulfurization process is widely used in the Chinese market. It uses calcium-based desulfurizers to absorb sulfur dioxide, resulting in the formation of calcium sulfite and calcium sulfate. Due to their low solubility, these substances tend to cause scaling and blockages within the desulfurization towers and pipelines. Compared with the limestone-based flue gas desulfurization technology, the double-alkali flue gas desulfurization technology overcomes the scaling problem associated with the limestone-lime method. B. Indirect limestone-gypsum process: Common indirect limestone-gypsum processes include the sodium-alkali double-alkali method, the alkaline aluminum sulfate method, and the dilute sulfuric acid absorption method, among others. Principle: Sodium alkali, basic alumina (Al2O3·nH2O), or dilute sulfuric acid (H2SO4) absorb SO2; the resulting absorbent solution is regenerated by reacting with limestone, producing gypsum. This method is simple to operate, causes little secondary pollution, has no problems with scaling or blockage, and offers high desulfurization efficiency; however, the quality of the gypsum produced is poor. C. Lemon absorption method: Principle: Citric acid (H3C6H5O7·H2O) solutions possess good buffering properties. When SO2 gas passes through the citrate solution, the SO2 in the flue gas reacts with H ions in the water to form H2SO3 complexes, resulting in an SO2 absorption rate of over 99%. This method is only suitable for flue gases with low concentrations of SO2, and not for the absorption of SO2 gases with high concentrations; therefore, its range of application is relatively limited. In addition, there are wet flue gas desulfurization technologies such as seawater desulfurization method, phosphate-ammonium compound fertilizer method, and liquid-phase catalysis method. II. Advantages of dry flue gas desulfurization technology: Dry flue gas desulfurization is a gas-phase reaction; compared to wet desulfurization systems, it features simpler equipment, less floor space required, lower investment and operating costs, easier operation, reduced energy consumption, easier disposal of by-products, and no need for a wastewater treatment system. Disadvantages: However, the reaction speed is slow, and the desulfurization rate is low; advanced methods can achieve 60–80%. However, at present this method has a low desulfurization efficiency, low utilization rate of the absorbent, severe wear and scaling issues, difficulties in equipment maintenance, low stability and reliability in operation, and a short service life, all of which limit the application of this method. Classification: Common dry flue gas desulfurization technologies include activated carbon adsorption, electron beam irradiation, charged dry absorbent injection, and metal oxide desulfurization. A typical dry desulfurization system involves directly injecting a desulfurizing agent (such as limestone, dolomite, or slaked lime) into the furnace. Taking limestone as an example, when calcined at high temperatures, the desulfurization agent forms porous calcium oxide particles after calcination; these particles react with SO2 in the flue gas to produce calcium sulfate, thereby achieving desulfurization. Dry flue gas desulfurization technology has been applied in the steel industry for large converters and blast furnaces; however, it is less suitable for small and medium-sized blast furnaces. The advantages of dry desulfurization technology are its simple process, no need to deal with wastewater or acidic waste, low energy consumption. In particular, the temperature of the purified flue gas is high, which facilitates the dispersion of the exhaust gases from the chimney and prevents the formation of \"white smoke\". The purified flue gas does not require further heating, and it has low corrosivity ； Its disadvantages are lower desulfurization efficiency, large equipment size, high investment costs, large floor space required, and high demands on operational skills. Common dry desulfurization technologies include. A. Principle of activated carbon adsorption method: SO2 is adsorbed by activated carbon and catalytically oxidized to sulfur trioxide (SO3), which then reacts with water to produce H2SO4. The saturated activated carbon can be regenerated through washing or heating, thereby producing either dilute H2SO4 or high-concentration SO2. By-product H2SO4, liquid SO2, and elemental sulfur can be obtained, which allows for effective control of SO2 emissions while also enabling the recovery of sulfur resources. Through improvements made to activated carbon by Xi’an Jiaotong University, ZL30 and ZIA0 were developed – materials with low costs and strong selective adsorption capabilities. This further refined the production process for activated carbon, enabling an SO2 adsorption rate of 95.8% in flue gas, thus meeting the **emission standards. B. Principle of electron beam radiation method: High-energy electron beams are used to irradiate the flue gas, generating a large amount of reactive substances that oxidize SO2 and nitrogen oxides in the flue gas to SO3 and nitrogen dioxide (NO2). Further, H2SO4 and nitric acid (NaNO3) are formed, which are then absorbed by absorbents such as ammonia (NH3) or limestone (CaCO3). C. Principle of the Charged Dry Absorbent Injection Desulfurization Method (CD.SI): The absorbent flows at high speed through the high-voltage electrostatic corona charging zone generated by the injection unit, thereby acquiring an electric charge. When this charged absorbent is injected into the flue gas stream, particles with the same charge repel each other, allowing the surface of the absorbent to be fully exposed and thus significantly improving the desulfurization efficiency. This method is a dry-process treatment; it avoids equipment contamination and scaling, produces no wastewater or waste residues, and the by-products can be used as fertilizer. There are no secondary pollutants generated, the desulfurization rate is over 90%, and the equipment is simple with broad applicability. However, this method relies on an electron beam accelerator to generate high-energy electrons for desulfurization ； For typical large enterprises, high-power electron guns are required, which are harmful to the human body; therefore, radiation shielding is also needed, resulting in high demands in terms of operation and maintenance. A digital desulfurization system has been installed at the Chengdu Thermal Power Plant in Sichuan, enabling the desulfurization of SO2 in flue gas to meet **emission standards. D. Principle of metal oxide desulfurization: Taking into account the fact that SO2 is a relatively reactive gas, oxides such as manganese oxide (MnO), zinc oxide (ZnO), iron oxide (Fe3O4), and copper oxide (CuO) have a strong adsorption capacity for SO2. At normal or low temperatures, these metal oxides adsorb SO2; at high temperatures, they undergo a chemical reaction with SO2 to form metal salts. Then, the oxides of the adsorbents and metal salts are regenerated through methods such as thermal decomposition and washing. This is a dry desulfurization method; although it generates no wastewater or waste acid and thus does not cause pollution, it has not been widely adopted. The main reasons are its relatively low desulfurization efficiency, large-sized equipment, high investment costs, stringent operational requirements, and overall high expenses. The key to this technology is the development of new adsorbents. The above-mentioned SO2 flue gas treatment technologies are currently widely used. Although they achieve a high desulfurization rate, they have drawbacks such as complex processes, high operating costs, incomplete pollution prevention, and the occurrence of secondary pollution. These issues are not in line with China’s policy of achieving harmonious economic and environmental development; therefore, it is necessary to explore and research new desulfurization technologies. III. Semi-dry flue gas desulfurization technologies Semi-dry desulfurization includes spray drying desulfurization, semi-dry and semi-wet desulfurization, powder-particle spouted bed desulfurization, and flue injection desulfurization. A. Spray drying method: The spray drying desulfurization method utilizes mechanical or airflow forces to disperse the absorbent into extremely fine mist droplets. These mist droplets create a large contact surface area with the flue gas, enabling heat exchange, mass transfer, and chemical reactions between the gas and liquid phases as a means of desulfurization. Commonly used absorbents include alkaline solutions, lime milk, and limestone slurry; currently, the vast majority of installations use lime milk as the absorbent. Under normal circumstances, the desulfurization rate of this method is 65% to 85%. Its advantages are: desulfurization takes place in gas, liquid, and solid phases; the process equipment is simple; the products formed are dry CaSO4 and CaSO3, which are easy to handle; there is no severe equipment corrosion or clogging, and water consumption is also relatively low. Disadvantages: High levels of automation are required, it is difficult to control the amount of absorbent used, and the absorption efficiency is not very high. Therefore, selecting a suitable absorbent is the key to overcoming the new challenges faced by this method. B. Semi-dry and semi-wet method: The semi-dry and semi-wet method is a desulfurization technique that lies between the wet and dry methods; its parameters such as desulfurization efficiency and the utilization rate of the desulfurizing agent also fall between those of the two other methods. This approach is mainly suitable for treating flue gases from small and medium-sized boilers. This technology is characterized by low investment and operating costs; although its desulfurization rate is lower than that of wet desulfurization techniques, it can still reach 70% of the target value. It also has low corrosivity, requires less space, and offers reliable performance. Compared with wet flue gas desulfurization systems, the semi-dry and semi-wet flue gas desulfurization systems commonly used in industry eliminate the pulp preparation system, and replace the injection of Ca(OH)2 aqueous solutions in wet flue gas desulfurization systems with the injection of CaO or Ca(OH)2 powder along with water mist. Compared with dry desulfurization systems, it overcomes the disadvantages of low reaction efficiency and long reaction time for the SO2-CaO reaction in the furnace using calcium injection, improves the utilization rate of the desulfurizing agent, has a simple process, and holds great potential for development. C. Technical principle of the powder-particle spouted bed semi-halftank flue gas desulfurization method: Flue gas containing SO2 passes through a preheater and enters the spouted bed. The desulfurizing agent, in powdered form, is premixed with water and then continuously injected into the bed from the top in the form of a slurry, where it mixes thoroughly with the particles in the bed. Through contact with the hot flue gas, desulfurization and drying occur simultaneously. The product after the desulfurization reaction is blown out of the separator in the form of a dry powder. This desulfurization technology uses limestone or slaked lime as the desulfurizing agent. It features a high desulfurization rate and utilization rate of the desulfurizer, while having minimal impact on the environment. However, there are strict requirements regarding the inlet temperature, relative humidity inside the bed, and reaction temperature; when the moisture content of the slurry and the reaction temperature are not properly controlled, the desulfurizer may stick to the walls. D. Flue gas injection semi-dry flue gas desulfurization: This method uses the flue gas duct between the boiler and the dust collector as a reactor for desulfurization; no additional absorption vessels are required, which reduces the investment needed for the process. It is simple to operate, requires less space, and is suitable for development and application in China. Semi-dry flue gas desulfurization involves injecting an absorbent slurry into the flue; as the slurry droplets evaporate, they react, and the reaction products exit the flue in the form of dry powder. IV. Emerging flue gas desulfurization methods In recent years, technology has advanced by leaps and bounds, and environmental issues have been elevated to a legal level. Scientists and engineers in our country have developed some new desulfurization technologies, but most of them are still in the experimental stage and require further verification through industrial application. 1. The caustic sulfide desulfurization method: Developed by Outokumpu, this method primarily uses industrial-grade sodium sulfide as a raw material to absorb SO2 from industrial flue gases; the end product is sulfur. The reaction process is quite complex; substances such as Na2SO4, Na2SO3, Na2S2O3, S, and Na2Sx are produced. From these products, it can be seen that the process requires a significant amount of energy, and the value of the by-products is low. Shi Lin from South China University of Technology conducted research showing that the concentrations of various sulfur compounds during the process vary depending on the reaction conditions. By maintaining the pH of the solution between 5.5 and 6.5 and adding a small amount of the oxidizing additive TFS, the main product formed is Na2S2O3. Through filtration and evaporation, 5H2O·Na2S2O3 with high added value can be obtained. Moreover, the desulfurization rate reaches as high as 97%. The reaction equation is: SO2 + Na2S = Na2S2O3 + S. This new desulfurization technology has passed pilot testing and is currently being promoted for wider use. 2. The membrane absorption method, a membrane separation technology represented by organic polymer membranes, is a new gas separation technique developed in recent years and has been widely applied, especially in water purification and treatment. Researchers such as Jin Mei from the Dalian Institute of Physical Chemistry, Chinese Academy of Sciences, have creatively used membranes to absorb the released SO2 gas, achieving relatively significant results with a desulfurization rate of 90%. The process is as follows: They utilize a polypropylene hollow fiber membrane absorber, with an NaOH solution as the absorbing liquid, to remove SO2 gas. Its characteristic lies in using a porous membrane to separate the gaseous SO2 from the NaOH absorbing liquid. The SO2 gas passes through the pores in the porous membrane and reaches the gas-liquid interface, where it reacts rapidly with NaOH, thereby achieving the purpose of desulfurization. This method is a new technology that combines membrane separation technology with absorption technology; it features low energy consumption, simple operation, and low investment costs. 3. Microbial desulfurization technology utilizes microorganisms for flue gas desulfurization, taking advantage of their role in various processes of the sulfur cycle and their ability to obtain energy. The mechanism involves, under aerobic conditions, the indirect oxidation by desulfurizing bacteria of SO2 in the flue gas into sulfuric acid, with the bacteria gaining energy from this process. Compared with traditional chemical and physical desulfurization methods, biological desulfurization requires no external conditions such as high temperature, high pressure, or catalysts; it operates at normal temperature and pressure, has a simple process flow, and does not cause secondary pollution. Abroad, it was based on geothermal power plants removing 5 tons of H2S per day ； The total cost of microbial desulfurization is 50% of that of conventional wet methods. Whether it is organic sulfur or inorganic sulfur, combustion generates inorganic sulfur SO2, which can be indirectly utilized by microorganisms; therefore, developing microbial flue gas desulfurization technology holds great potential. Wang An and others from Sichuan University used Bacillus ferrooxidans in laboratory conditions to conduct desulfurization studies, achieving a desulfurization rate of 98% at a low liquid-to-gas ratio. 4. Development trends in flue gas desulfurization technologies: The various technologies currently available each have their own advantages and disadvantages; a specific analysis is required when applying them, and a suitable desulfurization technology should be chosen by taking into account factors such as investment, operation, and environmental protection. With the advancement of technology, the emergence of any new technique involves many different disciplines. Therefore, keeping track of the latest developments and research findings in other fields and applying them to flue gas desulfurization technologies is an important way to develop new such technologies. New desulfurization methods such as microbial desulfurization and electron beam desulfurization have great potential for development due to their unique characteristics. With the growing emphasis on environmental management and the continuous increase in industrial flue gas emissions, desulfurization technologies that require low investment and operating costs, offer high desulfurization efficiency, make good use of desulfurizing agents, generate little pollution, and cause no secondary pollution will surely become the main trend in the development of flue gas desulfurization technologies in the future. Various flue gas desulfurization technologies have achieved certain economic, social, and environmental benefits in the removal of SO2, but they still have some shortcomings. With the continuous development of biotechnology and high-tech, a range of advanced and highly effective desulfurization techniques such as electron beam desulfurization and biological desulfurization will replace traditional desulfurization methods. Part Two: Common Denitration Technologies. Among the common denitration technologies, based on the mechanism of nitrogen oxide formation, the technical measures for reducing nitrogen emissions can be divided into two categories: one is to address the issue at its source. Control the generation of NOx during calcination. Its technical measures: ① Use low-nitrogen burners ； ②Segmented combustion in the decomposer and pipelines to control combustion temperature ； ③Change the ingredient formula, use a mineralizer, and reduce the clinker firing temperature. Another category is end-of-pipe treatment. Technical measures to control NOx emissions in flue gas: ① \"Staged combustion + SNCR\", with pilot projects already underway in China ； ②Selective Non-Catalytic Reduction (SNCR) is already being piloted in China ； ③Selective Catalytic Reduction (SCR); currently, there are only three lines for experimentation in Europe ； ③SNCR/SCR combined denitration technology: there is still no successful experience in cement denitration in China ； ④Biological denitration technology (currently in the research and development stage). Domestic denitration technology is still in the exploratory and demonstration phase, and no scientific summary has been conducted yet. Are the various design processes, technical approaches, as well as equipment and facilities scientific and reasonable, and do they operate reliably? The denitration efficiency, operating costs, energy consumption, and emissions of secondary pollutants will all be put to the test in practice. Denitration technologies can be specifically classified as follows: Denitration before combustion: 1) Hydrogen-based denitration; 2) Washing. Denitration during combustion: 1) Low-temperature combustion; 2) Low-oxygen combustion; 3) FBC combustion technology; 4) Use of low-NOx burners; 5) Concentration separation of coal powder; 6) Flue gas recirculation technology. Denitration after combustion: 1) Selective non-catalytic reduction denitration (SNCR); 2) Selective catalytic reduction denitration (SCR); 3) Activated carbon adsorption; 4) Electron beam denitration technology. The denitration efficiency of SNCR can reach 25%–40% in large coal-fired power plants, and up to 80% in smaller plants. Since this method is greatly influenced by the structural dimensions of the boiler, it is often used as a supplementary treatment method for low-nitrogen combustion technologies. It has a low construction cost, is easy to install, and requires little space; it is suitable for the renovation of existing factories, and can be used in conjunction with boiler designs in new factories. Selective Catalytic Reduction (SCR) is currently the most mature technology for flue gas denitration. It is a post-combustion denitration method that was first put into commercial use in Japan during the late 1960s and 1970s. This technology utilizes a reducing agent such as NH3 or urea, which, under the action of a metal catalyst, reacts selectively with NOx to produce N2 and H2O, rather than being oxidized by O2; hence the term “selective”. The currently popular SCR processes in the world are mainly divided into two types: ammonia-based SCR and urea-based SCR. Both of these methods utilize the reducing ability of ammonia to convert NOx – primarily NO – into N2 and water, substances that have little impact on the atmosphere, with NH3 serving as the reducing agent. Currently, most of the catalysts used in SCR systems use TiO2 as a carrier, with V2O5 or V2O5-WO3 or V2O5-MoO3 as the active component, and are available in three types: honeycomb, plate, and corrugated. SCR catalysts used in flue gas denitration can be divided into high-temperature catalysts (345°C–590°C), medium-temperature catalysts (260°C–380°C), and low-temperature catalysts (80°C–300°C); different catalysts are suitable for different reaction temperatures. If the reaction temperature is too low, the catalyst’s activity decreases, which leads to a reduction in denitration efficiency; moreover, if the catalyst continues to operate at low temperatures, it may suffer permanent damage ； If the reaction temperature is too high, NH3 is prone to oxidation, leading to an increase in NOx production. It can also cause phase changes in the catalyst material, resulting in a decline in the catalyst’s activity. Currently, most SCR systems at home and abroad use high-temperature catalysts, with a reaction temperature range of 315°C to 400°C. The advantages and disadvantages of this method in practical applications are as follows: Advantages: It features high denitration efficiency and relatively low cost; it is currently widely used in projects both domestically and internationally, and has become the mainstream technology for flue gas denitration in power plants. Disadvantage: The fuel contains sulfur, which can produce a certain amount of SO3 during combustion. After adding the catalyst, under aerobic conditions, the amount of SO3 produced increased significantly, and NH4HSO4 was formed together with the excess NH3. NH4HSO4 is corrosive and sticky, and can cause damage to the equipment in the tail flue. Although the amount of SO3 generated is limited, its impact cannot be underestimated. Additionally, catalyst poisoning cannot be ignored either. Part Three: Application of Desulfurization and Denitrification Technologies in Boiler Manufacturers. In domestic boiler manufacturers, coal or gas is commonly used as the combustion medium. For coal-fired boilers, the most maturely applied desulfurization technology in China is the FGD method (which uses absorbents or adsorbents to remove sulfur dioxide from flue gases), while for denitrification, the selective catalytic reduction SCR technology is primarily used. The SCR denitration technology is the most widely used method for removing NOx in the world. This approach involves adding a denitration unit to existing FGD processes; the catalytic process in which a reducing agent reacts with NO in an oxygen-containing atmosphere is known as selective catalytic reduction. Appropriate catalysts and reducing agents should possess the following characteristics: 1) The reducing agent should have high reactivity. 2) The reducing agent can selectively react with NOx without reacting with the numerous oxidizing substances present in the flue gas. 3) The reducing agent must be inexpensive to enable a low-cost operation of the removal process. 4) The catalyst should **lower the NOx reduction temperature. 5) The catalyst should possess high catalytic activity to facilitate the effective reduction of low-concentration NOx in flue gas. 6) The catalyst exhibits selectivity in reacting with the reducing agent to form N2, and remains inert toward the reaction of the reducing agent with other oxidizing substances in the flue gas. 7) The catalyst should have structural stability. 8) The catalyst is not poisoned by other pollutants in the flue gas. For the boiler industry, it is essential to study simultaneous desulfurization and denitrification technologies. At present, most systems in China use separate desulfurization and denitrification methods, which leads to redundant equipment installation, high energy consumption, as well as elevated labor and operational costs. Meanwhile, simultaneous desulfurization and denitrification technologies can help avoid such problems to a certain extent.