Popularize knowledge on desulfurization technology – come and learn!

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This post was last edited by I am kind on 2017-4-20 09:24. A comprehensive overview of desulfurization technology – come and learn more! 　　 　　Desulfurization technologies At present, there are 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. The wet flue gas desulfurization technology is relatively mature, efficient, and easy to operate. The traditional limestone/lime-gypsum flue gas desulfurization process 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. The double-alkali flue gas desulfurization technology was developed to overcome the scaling problem associated with the limestone-lime method. 　　I. Main sources: In recent years, with the increase in motor vehicles, vehicle exhaust has become a major source of air pollution, leading to more frequent acid rain, which severely damages buildings, soil, and the living environment for humans. Therefore, countries around the world have established higher standards for oil quality, imposing further restrictions on the sulfur, olefin, and benzene content in oils in order to better protect the living environment for humans. 　　With the increase in the processing of sulfur-containing crude oil and the widespread use of catalytic cracking of heavy oil, problems such as excessive sulfur content in petroleum products and poor stability are becoming increasingly severe. Due to the limitations in funds and hydrogen sources for hydrodesulfurization, it is of great significance for small and medium-sized refineries to conduct research on non-hydrodesulfurized refining. This article provides a brief overview of the progress in non-hydrodesulfurization technologies and their future development trends. 　　II. Distribution of sulfur: There are hundreds of sulfur-containing hydrocarbons in crude oil; to date, over 200 of them have been identified and their structures determined. These sulfur-containing hydrocarbons are distributed to varying degrees among the various distillates during the processing of crude oil. 　　Sulfur in fuel oil exists mainly in two forms: sulfides that can directly react with metals are known as \"active sulfur,\" including elemental sulfur, hydrogen sulfide, and thiols ； Sulfides that do not react directly with metals are called \"inactive sulfur\", including thioethers, disulfides, thiophenes, etc. For gasoline fractions, sulfur-containing hydrocarbons are mainly thiols, sulfides, and monocyclic thiophenes, which primarily originate from catalytic cracking (abbreviated as FCC) gasoline. Therefore, to make gasoline meet the standards for low-sulfur gasoline, it is necessary to pre-treat the FCC gasoline feedstock or post-treat the FCC gasoline product. The sulfur-containing hydrocarbons in diesel fractions include thiols, sulfides, thiophenes, benzothiophenes, and dibenzothiophenes. In the case of dibenzothiophenes, the presence of alkyl groups at the 4,6 positions makes desulfurization extremely difficult due to the steric effect of these alkyl groups. Moreover, as the boiling point of petroleum fractions increases, the structure of sulfur-containing compounds becomes increasingly complex. 　　III. Production methods: 1. Acid-base refining Acid-base refining is a traditional method that is still used by some refineries today. Since the acid and alkali residues resulting from acid-base purification are difficult to handle and cause significant losses of oil, this technology will inevitably be phased out in the long run. 　　2. Acid refining: This method uses inorganic acids such as sulfuric acid or hydrochloric acid of certain concentrations to remove sulfides and thiophenes from petroleum products, thereby achieving desulfurization. The reaction is as follows: R2S + H2SO4 → R2SH+ + HSO4-. 3. Alkaline purification: An aqueous NaOH solution can be used to extract some acidic sulfides; adding polar solvents such as sulfoxides or lower alcohols to the alkaline solution, or increasing the concentration of the alkali, can improve the extraction efficiency. Using 40% NaOH can remove over 60% of mercaptans and 90% of thiophenol from diesel fuel; thiophenol has a significant impact on the stability of the oil. 　　4. Catalytic method In the phthalocyanine catalyst method, the catalysts that are widely used in industry at present are polyphthalocyanine cobalt (CoPPC) and sulfonated phthalocyanine cobalt (CoSPc). This catalyst is used to treat oils in alkaline solutions, allowing for the removal of thiols from them. Xia Daohong believes that cobalt polyphthalocyanine (CoPPC) and sulfonated cobalt phthalocyanine (CoSPc) have poor solubility in alkaline solutions, which reduces the efficiency of these catalysts. To address this issue, a new type of catalyst with better water solubility was synthesized – quaternary ammonium sulfonated cobalt phthalocyanine (CoQAHPc)n. This catalyst contains both oxidation centers and basic centers, and the synergistic effect between them results in a significant increase in its catalytic activity. Furthermore, both the metal chelating agent method and the acidic catalyst method can convert organic sulfides into hydrogen sulfide, thereby effectively removing sulfides from refined oils. 　　Although the above-mentioned catalytic methods exhibit relatively high desulfurization efficiency, they all have drawbacks such as high catalyst investment costs, stringent preparation conditions, and easy loss of catalytically active components. At present, the economic benefits for refineries using this method are not very good; to apply catalytic desulfurization technology on a large scale, several technical challenges still need to be overcome. 　　5. Solvent extraction Appropriate solvents can be used through extraction to effectively remove sulfides from oils. Generally speaking, the extraction method can effectively extract thiols from oils, and then the extraction solvent and the thiols can be separated through distillation to yield thiol by-products with high added value, while the solvent can be reused. During the extraction process, alkaline solutions are commonly used as the extractants. However, the distribution coefficient of organic sulfides between alkaline solutions and the refined oil is not high. To improve the desulfurization efficiency during extraction, a small amount of polar organic solvents such as MDS, DMF, and DMSO can be added to the alkaline solution, which helps to **enhance the desulfurization efficiency during extraction. Xia Daohong and others proposed the MDS-H2O-KOH chemical extraction method. Experiments were conducted on FCC gasoline using these three extractants to determine the extraction and recovery rates. The results showed that this method enables the extraction of thiol compounds from oil products using the same setup, while also allowing for the efficient recovery of individual thiols as well as mixed thiol compounds, thereby producing thiol by-products of high purity. This approach offers significant economic and social benefits. Fujian Refining and Chemical Company combined the extraction and alkaline washing processes, using a methanol-alkaline washing composite solvent extraction method to significantly improve the storage stability of FCC diesel; the extraction solvent can be recycled after methanol is recovered through distillation. This method requires low investment, has high desulfurization efficiency, and holds great application value. 　　6. Catalytic Adsorption The catalytic adsorption desulfurization technique involves the use of solid adsorbents with good adsorption selectivity and renewability, to reduce the sulfur content in petroleum products through chemical adsorption. It is a newly developed method that can effectively remove sulfides from FCC gasoline. Compared to conventional gasoline hydrodesulfurization, its investment costs and operating expenses can be reduced by more than half. It can also efficiently remove impurities such as sulfur, nitrogen, and oxides from petroleum products, with a desulfurization rate exceeding 90%. Therefore, it is highly suitable for the current situation of domestic oil refineries. Since adsorption desulfurization does not affect the octane rating or yield of gasoline, this technology has attracted significant attention both domestically and internationally. 　　Konyukhova et al. used some natural zeolites (such as mordenite, calcium crossite, offretite, etc.) after acid activation to adsorb ethanethiol and dimethyl sulfide in oils, while ZSM-5 and NaX zeolites were used for the adsorption of thioethers and thiols, respectively. Tsybulevskiy studied the catalytic adsorption properties of oils after modification using X or Y-type molecular sieves. Wismann investigated the catalytic adsorption properties of activated carbon for oils. In these studies, it is common that the desulfurization depth is insufficient, the sulfur capacity of the adsorbents is low, the service life of the desulfurizing agents is short, and their regeneration performance is poor; thus, this **limits their industrial application. It is reported that the adsorption desulfurization technology developed by Phillips Petroleum was applied in a plant with a capacity of 258 kt/a in 2001; the average sulfur content in the gasoline produced after treatment was around 30 μg/g. This was the first industrial facility to use adsorption methods for removing sulfides from gasoline, and this technology is set to be applied to diesel desulfurization as well. 　　Catalytic adsorption desulfurization technology in China is still in the research phase. Xu Zhida, Chen Bing, etc. used polyacrylonitrile-based activated carbon fibers (NACF) to adsorb mercaptans in oil products; however, this method could only remove a portion of the mercaptans from the oil. Zhang Xiaojing et al. used 13X molecular sieve as an adsorbent to study the whole fraction and heavy fraction (>90°C) of FCC gasoline. Preliminary results showed that after refining the whole fraction and heavy fraction of gasoline with a sulfur content of 1220 μg/g, the resulting products were of better quality compared to the unrefined light fraction. In summary, catalytic adsorption desulfurization technology can effectively remove sulfides from oil products without affecting their properties, and its investment and operating costs are much lower than those of other desulfurization methods such as hydroprocessing, solvent extraction, and catalytic oxidation. Therefore, studying catalytic adsorption desulfurization technology is of great significance. 　　7. Chelation method Treating sulfur-containing oils with a DMF solution of metal chlorides enables electron pair interactions between organic sulfides and metal chlorides, resulting in the formation of water-soluble chelates that can be removed. There are many metal ions that can form complexes with organic sulfides, among which CdCl2 yields the best results. The order of reactivity in the complexation reactions between various metal chlorides and organic sulfides is as follows: Cd2+ > Co2+ > Ni2+ > Mn2+ > Cr3+ > Cu2+ > Zn2+ > Li+ > Fe3+. Since complexation cannot remove acidic components from oil products, in practical applications, a combination of complexation extraction and alkali washing refining is often employed. This method yields highly significant desulfurization effects; the resulting oil products exhibit good stability and offer considerable economic benefits. 　　IV. Biological desulfurization: Biological desulfurization, also known as biocatalytic desulfurization (abbreviated as BDS), is a new technology that utilizes aerobic and anaerobic bacteria to remove bound sulfur from sulfur-containing heterocyclic compounds in petroleum under normal temperature and pressure. As early as 1948, the United States had patents for biological desulfurization, but there were no successful examples of removing hydrocarbon sulfides, mainly due to the inability to effectively control bacterial activity. Several successful reports on \"microbial desulfurization\" have appeared since then, but they hold little practical value; this is because although microorganisms remove sulfur from the oil, they also consume a large amount of carbon in the oil, thereby reducing its calorific value. Scientists have been conducting in-depth research on this topic. In 1998, researchers at the Institute of Gas Technology (IGT) in the United States successfully isolated two special strains capable of selectively removing sulfur from dibenzothiophene. Subsequently, industrial models for removing heterocyclic sulfur compounds from oil products were developed. In 1992, two patents related to this technology were filed in the United States (Nos. 5002888 and 5104801). American company Energy BioSystems Corp (EBC) was granted the rights to use these two strains. Based on this, the company not only succeeded in producing and regenerating biological desulfurization catalysts, but also reduced the cost of producing these catalysts while extending their service life. In addition, the company isolated a bacterium of the genus Micrococcus, which is capable of breaking C-S bonds, thereby achieving desulfurization without loss of hydrocarbons in the oil. Today, EBC Company has become the company in the world that has conducted the most extensive research on biological desulfurization technology. In addition, the Institute of Life Engineering Industrial Technology at the Japan Industrial Technology Agency, in collaboration with the Petroleum Industry Revitalization Center, has developed a new strain for diesel desulfurization. This strain is capable of removing both dibenzothiophene and benzothiophene sulfur from diesel; sulfur in these two sulfides is difficult to remove using other methods. 　　The BDS process involves an oxidative reaction between aerobic bacteria naturally present in nature and organic sulfides; through selective oxidation, the C-S bonds are broken, and the sulfur atoms are oxidized to sulfates or sulfites and transferred to the aqueous phase, while the backbone structure of DBT is oxidized to hydroxybiphenyls which remain in the organic phase, thereby achieving the removal of sulfides. BDS technology has been developing for several decades since its inception, and it is still in the research and development phase to this day. Due to the many advantages of BDS technology, it can be integrated seamlessly with existing HDS units, which not only helps to significantly reduce production costs but also makes BDS more economically competitive than HDS, given the higher added value of organic sulfur products. At the same time, BDS can also be combined with catalytic adsorption desulfurization, representing an effective method for achieving deep desulfurization of fuel oil. Therefore, BDS technology has broad application prospects, and industrial-scale installations are expected to appear around 2010. 　　V. New desulfurization methods:
1. Oxidative desulfurization technology
Oxidative desulfurization technology involves using an oxidant to convert thiophene-based sulfides into sulfoxides and sulfones. Subsequently, sulfoxides and sulfones are removed from the oil by solvent extraction. The oxidant is regenerated and then reused. Current low-sulfur diesel is produced using hydrogenation technology. Since the dimethyldibenzothiophene structure in diesel is stable and difficult to desulfurize through hydrogenation, achieving a sulfur content of 10 μg/g in the fuel requires higher reaction pressures and lower space velocities, which undoubtedly increases the investment costs and production expenses associated with hydrogenation technology. Oxidative desulfurization technology can not only meet the requirement of 10 μg/g for diesel fractions, but also allows for the installation of simple and practical desulfurization devices at distribution points, making it an excellent approach to ensuring the quality of oil products sold to end-users. 　　2. ASR-2 Oxidative Desulfurization Technology The ASR-2 oxidative desulfurization technology is a new type of desulfurization method developed by Unipure Company. It offers advantages such as low investment and operating costs, mild operating conditions, no need for a hydrogen source, low energy consumption, no pollution emissions, the ability to produce ultra-low sulfur diesel, and flexible plant design. It provides refineries and distribution centers with an economical and reliable way to meet the requirements regarding sulfur content in petroleum products. 　　During the experimental process, this technology can reduce the sulfur content in diesel from 7000 μg/g to 5 μg/g. In addition, this technology can also be used to produce ultra-low sulfur diesel as a blending component for petroleum products, thereby meeting the needs of the petroleum processing and sales markets. Currently, the design work for pilot and industrial trials of ASR-2 technology is in progress. The process flow is as follows: sulfur-containing diesel, an oxidant, and an aqueous solution of catalyst are mixed in a reactor; under near-atmospheric pressure and mild temperatures, thiophene-based sulfur compounds are oxidized to sulfones ； The aqueous phase containing the raw catalyst and sulfone is then separated from the organic phase and sent to the regeneration section, where the sulfone is removed and the catalyst is regenerated ； The oil phase containing sulfones is sent to the extraction system to achieve the separation of the sulfones from the oil phase ； The sulfones obtained from the aqueous and organic phases are sent together to a processing system to produce high-value chemical products. 　　Although the ASR-2 desulfurization technology has been researched for many years, it has not been put into industrial application, mainly because several technical issues such as catalyst regeneration and recycling, and removal of oxides remain unresolved. ASR-2 technology can reduce the sulfur content in diesel products to 5 μg/g, which is much lower than the sulfur contents of 30 μg/g and 15 μg/g for diesel products produced using hydroprocessing technologies; as a result, both the sulfur content and the total treatment costs are significantly reduced. Therefore, if some technical issues can be properly resolved, the ASR-2 oxidation desulfurization technology will have very broad market prospects. 　　3. Ultrasonic oxidation desulfurization technology The Ultrasonic Oxidation Desulfurization (SulphCo) technology is a new type of desulfurization method developed jointly by USC and SulphCo Company. The chemical principles of this technology are basically the same as those of the ASR-2 technology; the difference is that the SulphCo technology utilizes an ultrasonic reactor, which enhances the reaction process and leads to more satisfactory desulfurization results. The process is described as follows: The raw materials are mixed in a reactor with an aqueous phase containing an oxidant and a catalyst. Under the action of ultrasonic waves, small bubbles are rapidly formed and destroyed, thereby causing intense mixing between the oil phase and the water phase. In addition, ultrasonic waves can quickly increase the local temperature and pressure within the mixture, and hydrogen peroxide is generated within it to participate in the reactions involving sulfides ； Sulphones and sulfates are removed through solvent extraction; after the solvent is regenerated, it is reused, and the sulphones and sulfates can be used to produce other chemical products. 　　After completing the laboratory work, SulphCo carried out pilot-scale scale-up experiments, achieving satisfactory results: the sulfur content in diesel with different sulfur levels could all be reduced to below 10 μg/g through oxidation desulfurization technology. Currently, Bechtel is carrying out industrial trials of the SulphCo technology. 　　4. Light and plasma desulfurization technologies 　　Institutions such as the National Institute for Environmental Studies in Japan and the University of Tübingen in Germany are researching desulfurization using ultraviolet light irradiation and plasma technology. The mechanism is as follows: disulfides form free radicals through the breaking of S-S bonds, while thioethers and thiols form free radicals through the breaking of C-S and S-H bonds respectively, and they react in the following ways:
Reactions in the absence of an oxidant:
CH3S- + CH3 → CH4 + CH2 ==== S
CH3S- + CH3CH2R → CH3SH + CH2 ==== SCH2R
CH3S- + CH3S- → CH3SSCH3
CH3S- + CH2 ==== S → CH3SCH2S- + CH3 → CH3SCH2SCH3
Reactions in the presence of an oxidant:
CH3S- + O2 → CH3SOO- + RH → CH3SOOH + R-
SO3 + CH3 → CH3SO3H
CH3SOOH + R- → CH3SO- + OH-
CH3SO- + RH → CH3SOOH + R-
3CH3SOOH → CH3SOOSCH3 + CH3SO3H
This technology is applied to various organic sulfides and crude gasoline; depending on their molecular structures, reactions occur through the methods mentioned above. The products include alkanes, alkenes, aromatics, as well as sulfides or elemental sulfur, with a desulfurization rate of 20% to 80%. If air is introduced simultaneously during irradiation, the desulfurization rate can be increased to 60%–100%, and sulfur is converted into SO3, SO2, or sulfur itself, which can be removed by washing. 　　5. High-efficiency atomization-based desulfurization and dust removal technology
The high-efficiency atomization-based desulfurization and dust removal technology primarily involves studying the chemical composition and physical movement characteristics of harmful substances such as soot and sulfur dioxide. By applying principles from fluid mechanics, aerodynamics, chemistry, and mechanics, it integrates various high-tech processes—including solid spray technology, atomization washing technology, coalescence atomization technology, impingement turbulence technology, filtration and absorption technology, and mist separation technology—into a multidisciplinary and multi-process environmental protection technology. Its main advantages include a long service life, high efficiency with low resistance and energy consumption, minimal space requirements, low construction costs, low operating expenses, minimal maintenance needs, ease of management, closed-loop circulation of ash-laden water, and no secondary wastewater or dust pollution. After treatment, the various parameters of the smoke and dust meet the **environmental protection emission standards**, in line with the policies aimed at promoting the development of environmentally friendly industries that are efficient, durable, have low resistance, and require low costs. This approach enables the simultaneous achievement of effective dust removal, desulfurization, denitrification, and mist removal, thus achieving the goal of controlling and purifying air pollution. It offers significant environmental, social, and economic benefits in reducing harmful substances such as acid rain, sulfur dioxide, nitrogen oxides, fluorides, dust, and inhalable particulates, thereby improving the quality of the atmospheric environment; it also has excellent market prospects. 　　The process flow is as follows: The dust-laden gas first enters a high-efficiency solid spray scrubbing chamber, where it is cooled and saturated by an alkaline solution; large-particle dust and sulfur dioxide are initially absorbed. Subsequently, the three-phase stream consisting of flue gas, water mist, and dust undergoes mass transfer due to differences in mass and inertia, and all enter a high-efficiency coalescence and atomization scrubbing chamber. There, through processes such as contraction, rapid aggregation, and dispersion, desulfurization and dust removal occur for a second time. After that, the flue gas and water mist... The three-phase dust-laden gas stream impacts at a certain velocity against the high-efficiency circulating fluidized filtration chamber filled with alkaline solution. Through mechanisms such as thorough flushing, turbulence, agitation, filtration, and mass transfer, it undergoes desulfurization and dust removal for the third time. At this point, the relatively clean flue gas enters the high-efficiency upper stable swirl counter-mass-transfer washing chamber via a tangential or volute path; there, counter-current mass transfer occurs due to the upward-flowing alkaline liquid film and mist, resulting in final desulfurization and dust removal. The purified gas then proceeds along a tangential or volute path into the high-efficiency lower stable swirl dewatering and demisting chamber, where gas-liquid separation takes place. Finally, it is conveyed by an induced draft fan to the chimney for discharge into the atmosphere. The gray water is discharged from the automatic overflow seals at the bottom of the high-efficiency solid spray washing chamber, the high-efficiency circulating fluidized filtration chamber, and the high-efficiency upward stable swirl counter-mass transfer washing chamber into the circulation tank, where it undergoes neutralization and precipitation in alkaline water; the alkaline wastewater is then recovered for use in the desulfurization dust collector. The clean water flows out from the drainage outlet at the bottom of the high-efficiency downward stable swirl dehydration and demisting chamber, thus completing the entire process of smoke elimination, desulfurization, denitrification, dust removal, dehydration, and demisting. 　　Technical advantages:
a. A technical design that integrates smoke elimination, desulfurization, denitrification, dust removal, and dehydration into one process. It features a simple and compact structure, a reasonable process flow, resistance to scaling and blockage internally, and a design wherein flue gas contains no moisture ； 　　b. The effective internal area of the equipment is designed to be utilized at 100%, with the smoke and dust being completely dissolved in an alkaline aqueous solution throughout the purification process, thereby achieving efficient mass transfer ； 　　c. An efficient splash jet atomization design is employed, along with a design that eliminates vulnerable components inside the equipment, to ensure the most effective desulfurization and dust removal ； 　　d. To achieve the most efficient desulfurization and dust removal, it is necessary to ensure the fullest possible mass transfer between the flue gas and the alkaline solution ； 　　e. The manufacturing material can be made of natural, wear-resistant granite, addressing the long-standing issues of environmental protection equipment such as poor wear resistance, lack of corrosion resistance, and short lifespan ； 　　f. A 25% dilute alkali solution, which ensures proper liquid-gas conversion, a stable absorption rate for sulfur dioxide, and maintains a pH value around 10, is used as the sulfur dioxide absorbent. It is non-volatile and causes minimal loss, resulting in high desulfurization efficiency and stable results. It also effectively solves the problems of ash accumulation and scaling inside the equipment ； 　　g. The equipment features a well-structured flue gas passage internally, with no dead corners in the flow path of the flue gas; this reduces the thermal resistance of the flue gas, ensures optimal performance under design conditions, and does not affect the operation of combustion equipment such as boilers ； 8. The principle of the simple and efficient cyclic double-alkali desulfurization method makes full use of the waste alkali solution produced in factories, achieves waste-to-waste conversion and comprehensive utilization, reduces operating costs, enables closed-loop reuse of alkaline water, achieves 100% wastewater utilization rate, and eliminates any secondary wastewater pollution emissions. 　　VI. Negative impacts: The low sulfur content in gasoline and diesel **reduces environmental pollution; in particular, there is a consensus among countries regarding policies to reduce sulfur levels in fuel oils. However, during the process of reducing the sulfur content in fuel oil, unforeseen negative effects emerged, primarily manifested as: a decline in lubrication properties and increased wear on equipment. In 1991, Sweden found that when using diesel with a sulfur content of 0.00%, the sintering and wear caused by the fuel pumps were even worse than those caused by regular diesel. Japan also conducted bench tests on diesel fuels with different sulfur contents, and the results confirmed the issue of reduced lubricating performance of the diesel. The main reason is that the natural polar compounds with lubricating properties present in the oil are removed simultaneously during desulfurization, which leads to a decline in lubricating performance and increased wear of the equipment. 　　The stability of diesel deteriorates, and the color of the fuel worsens. When the sulfur content in diesel drops below 0.05%, the increase in peroxides accelerates the formation of gels and precipitates, affecting the proper operation of equipment and leading to poor exhaust quality. The main reason is that the natural antioxidant components originally present in diesel are also removed during desulfurization. At the same time, as the sulfur content in diesel decreases, the color of the oil becomes darker, giving off an unpleasant appearance. 　　VII. Conclusions and Recommendations: Given the widespread use of petroleum products in production and daily life, it is very important to remove the harmful sulfur from them. The non-hydrodesulfurization methods currently used in industry include acid-base refining, solvent extraction, and adsorption desulfurization, all of which have their own defects and limitations. Among them, acid-base purification generates large amounts of waste acid and waste alkali solutions, causing severe environmental pollution ； The solvent extraction desulfurization process is energy-intensive and results in a low yield of oil products ； In the adsorption method, the adsorbent has a low capacity for adsorption and requires frequent regeneration. Other non-hydrodesulfurization technologies are still in the experimental stage; among them, biological desulfurization, oxidative desulfurization, and desulfurization using light and plasma show great promise and could be effective methods for producing clean fuel oils in the future. Since reducing the sulfur content in fuel oil and minimizing air pollution is a complex process, various factors must be taken into account during implementation to enhance the reliability of the technology, thereby achieving the best economic and environmental benefits. 　　Lime (stone) – Wet flue gas desulfurization using gypsum process. Lime (stone) – Gypsum FGD is the dominant desulfurization technology in the domestic market at present, and its core technology has been successfully applied by many companies in China. 　　VIII. Technical Principles: 1. Absorption of SO2 and SO3 SO2 + H2O → H+ + HSO3-; SO3 + H2O → H2SO4. The key to the absorption of SO2 and SO3 is to increase their solubility in water. The higher the pH value, the greater the surface area of the water, and the higher the turbulence in the gas phase, which in turn leads to an increased amount of SO2 and SO3 that can be dissolved. 　　2. Reaction with limestone slurry: CaCO3 + 2H+ + HSO3- → Ca2+ + HSO3- + H2O + CO2; CaCO3 + H2SO4 → CaSO4 + H2O + CO2; CaCO3 + 2HCl → CaCl2 + H2O + CO2. The key to this step is to increase the solubility of CaCO3 – the lower the pH value, the greater the solubility. 　　System components — Flue gas system — Absorption tower system — Pulp preparation system — Slurry drainage system — Process water system — Gypsum dewatering, storage, and transportation system — Wastewater treatment system. Advantages of the limestone-gypsum wet flue gas desulfurization method: a. The technology is mature, with the maximum capacity of a single unit exceeding 1000 MW ； 　　b. High desulfurization efficiency ≥95%, Ca/S ≤1.03 ； 　　c. The system operates stably with an availability of ≥95% ； 　　d. Desulfurizer—limestone, inexpensive and readily available ； 　　e. The desulfurization by-product—gypsum, can be utilized comprehensively ； 　　f. No shutdown is required during construction. 　　Disadvantages: Complex system, large footprint ； High cost, large one-time investment ； High operation frequency and high operating costs, as well as issues with the handling of by-products. 　　3. Coal desulfurization technologies before combustion These mainly involve coal washing for desulfurization, that is, purifying the coal before combustion in order to remove part of the sulfur and ash content present in the raw coal. They are divided into physical methods, chemical methods, and microbial methods, etc. 　　a. Physical methods: These mainly refer to gravity separation of coal, which utilizes the difference in density between the organic matter in coal and pyrite to separate them. The influencing factors of this method mainly include the crushing particle size of coal and the state of sulfur, etc. The main methods include jigging coal separation, heavy medium coal separation, air classification, etc. 　　b. Chemical methods: These can be divided into physicochemical methods and pure chemical methods. The physicochemical method is flotation ； Chemical methods include alkali-based desulfurization, gas desulfurization, pyrolysis and hydrogenation desulfurization, and oxidation-based desulfurization. 　　c. Microbial method: A new biotechnological approach applied in the coal industry based on bacteria for leaching metals, which can remove organic and inorganic sulfur from coal. China’s current coal washing rate is relatively low, at around 20%, compared to 42% in the United States, 94.9% in the United Kingdom, 88.7% in France, and 98.2% in Japan. Increasing the washing rate of coal is expected to significantly reduce sulfur dioxide pollution from coal combustion. However, physical washing can only remove 80% of the inorganic sulfur in coal, which accounts for 15% to 30% of the total sulfur content in coal; it is therefore insufficient to meet the requirements for controlling sulfur dioxide pollution from coal combustion, and can only serve as an auxiliary method for coal desulfurization. 　　4. Coal desulfurization technology during combustion Limestone or dolomite is added as a desulfurizing agent during coal combustion; calcium carbonate and magnesium carbonate decompose upon heating to form calcium oxide and magnesium oxide, which react with sulfur dioxide in the flue gas to produce sulfates that are then discharged along with the ash. 　　There are mainly two types of desulfurization technologies used in combustion processes in China: briquette sulfur fixation and fluidized bed combustion desulfurization technology. 　　Type A coal briquette sulfur fixation technology: Different raw materials are screened and blended in certain proportions; after being crushed, they are mixed with pre-treated binders and sulfur fixation agents. Through mechanical pressing and drying, finished industrial coal briquettes with specific strength and shape are obtained. Sulfur-fixing agents mainly include limestone, marble, calcium carbide slag, etc., and their dosage depends on the sulfur content. Using briquetted coal can **reduce the concentrations of sulfur dioxide, carbon monoxide, and dust in the flue gases, as well as save coal; the economic and environmental benefits are considerable. However, in industrial applications, issues such as the delayed ignition of briquetted coal and shutdowns caused by improper operation must be addressed. 　　b. Fluidized bed combustion desulfurization technology: Coal and adsorbents are added to the bed in the combustion chamber; air is blown from the bottom of the furnace to suspend the bed and enable fluidized combustion, creating turbulent mixing conditions that increase the residence time and thereby improve combustion efficiency. The reaction process involves the combustion of sulfur in coal to produce sulfur dioxide; simultaneously, limestone is calcined and decomposed into porous calcium oxide. The sulfur dioxide reaches the surface of the adsorbent and reacts there, thereby achieving desulfurization. The main factors affecting flue gas desulfurization by fluidized bed combustion include the calcium-sulfur ratio, calcination temperature, particle size and pore structure of the desulfurizer, as well as the type of desulfurizer. To improve desulfurization efficiency, the following methods can be employed: ⑴ Improving the design and operating conditions of the combustion system; ⑵ Pre-calcining the desulfurizing agent; ⑶ Using additives such as sodium carbonate and potassium carbonate; ⑷ Developing new types of desulfurizing agents. 5. Flue gas desulfurization after combustion The basic principle of flue gas desulfurization is the acid-base neutralization reaction. Sulfur dioxide in flue gas is an acidic substance; by reacting with alkaline substances, it forms sulfites or sulfates, thereby removing sulfur dioxide from the flue gas. The most commonly used alkaline substances are limestone, quicklime, and slaked lime; other alkaline substances such as ammonia and seawater can also be used. It is divided into three categories: wet flue gas desulfurization technology, dry flue gas desulfurization technology, and semi-dry flue gas desulfurization technology, which are introduced as follows:
a. Wet flue gas desulfurization technology
Wet flue gas desulfurization technology refers to the use of liquids or slurries as absorbents. Since it is a gas-liquid reaction, the reaction speed is fast, the efficiency is high, and the utilization rate of the desulfurization agent is high. The main drawback of this method is the secondary pollution caused by desulfurization wastewater ； The system is prone to scaling and corrosion ； The initial investment cost for desulfurization equipment is high ； Operating costs are relatively high. 　　⑴Limestone-gypsum flue gas desulfurization technology: This technology uses limestone slurry as a desulfurizing agent to spray and wash the flue gas in an absorption tower, causing sulfur dioxide in the flue gas to react to form calcium sulfite. At the same time, air is blown into the slurry in the absorption tower to force the conversion of calcium sulfite into calcium sulfate; the by-product of this desulfurizing agent is gypsum. The system includes a flue gas heat exchange system, an absorption tower desulfurization system, a desulfurizer slurry preparation system, and a gypsum dewatering and wastewater treatment system. Due to its low cost, ease of transportation, and storage, limestone has become the primary desulfurizing agent in wet flue gas desulfurization processes; the limestone-gypsum flue gas desulfurization technique is thus the preferred method for this purpose. This method has a high desulfurization efficiency (greater than 95%) and high operational reliability, but it is prone to clogging and corrosion, and the desulfurization wastewater is difficult to treat. 　　⑵Ammonia-based flue gas desulfurization technology: The principle of this method is to use ammonia water as the desulfurization absorbent. Ammonia water comes into contact with flue gas in an absorption tower; sulfur dioxide in the flue gas reacts with ammonia water to form ammonium sulfite, which is then oxidized to produce an ammonium sulfate solution. After crystallization, dehydration, and drying, ammonium sulfate (a fertilizer) can be obtained. The reaction speed of this method is much faster than that of the limestone-gypsum method, and there are no issues related to structure or clogging. In addition, wet flue gas desulfurization technologies include the sodium method, double-alkali desulfurization method, and seawater flue gas desulfurization method, etc. The choice should be based on the source of the absorbent, local conditions, and the market prospects for by-products. 　　b. Semi-dry flue gas desulfurization technology Mainly introduces the rotary spray drying method. This method is a process jointly developed by the United States and Denmark. Compared with flue gas desulfurization processes, this method features simple equipment, low investment and operating costs, and a small footprint, while achieving a flue gas desulfurization rate of 75%–90%. This method utilizes the principle of spray drying to atomize the absorbent slurry and spray it into the absorption tower. Inside the absorption tower, the absorbent undergoes a chemical reaction with sulfur dioxide in the flue gas; at the same time, it absorbs the heat from the flue gas, causing the moisture in the absorbent to evaporate and dry out. The waste residue resulting from the desulfurization process is discharged in a dry state. The method consists of four steps: 1) Preparation of the absorbent ； 　　2) Atomization of absorbent slurry ； 　　3) The fog particles mix with the flue gas, absorb sulfur dioxide, and are dried ； 　　4) Discharge of desulfurization waste residue. 　　This method generally uses quicklime as the absorbent. Quicklime is matured to become slaked lime, which possesses good reactivity. The slaked lime slurry is sprayed into uniform mist droplets using high-speed spinners with speeds of 15,000–20,000 revolutions per minute; the diameter of these mist particles can be less than 100 microns, giving them a large surface area. Once in contact with flue gas, intense heat exchange and chemical reactions occur, rapidly evaporating most of the water and resulting in solid waste with very low moisture content. Lime stone-based desulfurization methods include wet flue gas desulfurization/semi-dry methods (Alstom’s NID)/dry methods (RCFB) and (CFB). As mentioned earlier, these methods are used in over 80% of thermal power plants. Ammonia-based methods are less commonly used, mostly in power plants in the petrochemical industry. Magnesium-based methods represent a technique for regenerating absorbents; it seems this method is not used in China. Sea water-based methods require installation near the sea. 　　Source: Petrochemical Bond