Applications of desulfurization gypsum

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Current Status and Recommendations for the Comprehensive Utilization of Desulfurization Gypsum in China Editorial Note: In recent years, **controls on sulfur dioxide emissions from power plants have been tightened, leading to rapid development of the flue gas desulfurization industry in coal-fired power plants. By the end of 2005, the capacity of flue gas desulfurization units that were in operation amounted to 53 million kilowatts, of which over 90% used the wet desulfurization process. The rapid increase in desulfurization gypsum makes its utilization an urgent issue to address. To date, there are over 10 production lines for the resource utilization of desulfurization gypsum that have been built or are under construction across the country. Shandong Taihe Group is one of the large-scale enterprises in China that produce gypsum board, and it has been actively promoting the use of desulfurized gypsum in recent years. This article selectively presents, for reference, reports by Comrades Jia Tongchun and Jiao Wenbo from Shandong Taihe Group on the comprehensive utilization of desulfurization gypsum in China. The Current Status and Recommendations for the Comprehensive Utilization of Desulfurization Gypsum in China – Shandong Taihe Group, Jia Tongchun, Jiao Wenbo. With the rapid development of China’s economic construction, the prevention and control of air pollution has become a prominent issue. In China’s sulfur dioxide emissions, flue gas emissions from thermal power plants account for about half. Since 2000, the **10th Five-Year Plan for the Prevention and Control of Acid Rain and Sulfur Dioxide Pollution in the Two Control Areas** was formulated. In these areas designated for acid rain control and sulfur dioxide control, efforts to reduce the total amount of sulfur dioxide emissions focused on pollution generated by thermal power plants, and a range of measures were taken. “The \"two control areas\" include 4 municipalities directly under the Central Government, 4 special economic zones, and 21 provincial capitals, whose gross domestic product accounts for 67% of the total national GDP. The plan clearly states that by 2005, sulfur dioxide emissions in the \"two control areas\" are to be reduced by 20% compared to 2000 ； The annual average concentration of sulfur dioxide in urban air reaches **the second-level standard for environmental air quality in 80% of cases. **A number of desulfurization projects should be prioritized in areas with high-sulfur coal, regions with excessive emissions, and large power plants located near cities. By the end of 2005, the capacity of flue gas desulfurization units that had been built and put into operation reached 53 million kilowatts. In these power plants, over 90% use the wet flue gas desulfurization process for sulfur removal. The capacity of desulfurization units with a capacity of 100,000 kilowatts or more amounts to 44 million kilowatts. The average sulfur content in the coal used by these units is 1.23%, and the average desulfurization efficiency is 92.5%. This results in an annual reduction of 2.3 million tons of sulfur dioxide. There is more than 10 million tons of desulfurization gypsum that needs to be dealt with; if left untreated, it will accumulate and pollute the environment ； If utilized comprehensively, waste can be turned into treasure. I. Basic Properties of Desulfurization Gypsum 1. Fundamental Characteristics of Desulfurization Gypsum Desulfurization gypsum, also known as flue gas desulfurization gypsum, sulfur gypsum, or FGD gypsum, has the same main component as natural gypsum, namely calcium sulfate dihydrate. Flue gas desulfurization gypsum appears as fine particles, with an average particle size of about 40–60 μm. The particles are short cylindrical in shape, with a diameter-to-length ratio ranging from 1.5 to 2.5. Their color is gray or yellow. It contains a high amount of calcium sulfate dihydrate, usually over 90%, while the free water content is generally between 10% and 15%. In addition, it contains impurities such as fly ash, organic carbon, calcium carbonate, calcium sulfite, and soluble salts composed of sulfates or chlorides of sodium, potassium, and magnesium. 2. Comparison between flue gas desulfurization gypsum and natural gypsum: The chemical composition and fineness of natural gypsum and flue gas desulfurization gypsum are shown in Tables 1 and 2. Table 1 Chemical composition and fineness parameters of natural gypsum and desulfurized gypsum
Item: CaO, SiO2, Al2O3, SO3, Fe2O3, MgO, Loss on sieve (45 μm)/%
Natural gypsum: 31.5, 4.3, 1.7, 34, 1.1, 1.15, 1.30, 17.2, 8.8
Desulfurized gypsum: 31.6, 2.7, 0.7, 42.4, 0.5, 1.0, 19.2, 1.0

Table 2 Particle size distribution
Particle size/μm: 80, 60, 50, 40, 30, 20, 10, 5
Loss on sieve/% for natural gypsum: 10.9, 4.7, 9.5, 4.9, 14.4, 15.5, 20.0, 12.7
Loss on sieve/% for desulfurized gypsum: 5.0, 15.5, 8.3, 21.9, 31.0, 15.7, 1.7, 0.4
Note: The 8.8% of particles larger than 80 μm are not included in the calculations for natural gypsum. 3. Conclusion The chemical composition of desulfurized gypsum is not very different from that of natural gypsum, and their quality is comparable ； However, the impurities in natural gypsum are mainly clay minerals, and the particles remain large after grinding; as a result, they generally cannot participate in the hydration reaction, which means its properties are, to some extent, inferior to those of desulfurized gypsum. 　　II. Comprehensive Utilization of Desulfurization Gypsum in China
The comprehensive treatment and application of desulfurization gypsum have already begun in China. The use of this material presents significant market opportunities. In regions such as Jiangsu, Zhejiang, and Guangdong, where natural gypsum is scarce, the abundance of desulfurization gypsum creates business opportunities for enterprises that use gypsum as a raw material. Desulfurized gypsum can be used to manufacture building materials such as gypsum blocks, plastering gypsum, molding gypsum, gypsum board, and cement; however, at present, its widespread use is limited to the production of gypsum board and as a retarder in cement. 1. Used for manufacturing gypsum blocks. Although our country strongly encourages the use of gypsum blocks instead of solid clay bricks in exterior wall construction, the largest gypsum block production facility currently has a capacity of only 300,000 square meters, enabling the processing of about 20,000 tons of desulfurized gypsum per year (on a wet basis). Most production facilities are smaller, with capacities of less than 100,000 square meters; as a result, the amount of gypsum used is limited and the market has not yet been fully developed. 2. Used to manufacture putty gypsum and plastering gypsum. Currently, the nationwide usage of putty gypsum amounts to only around 100,000 tons, with a similar amount being used for plastering gypsum. The low level of usage means that there is little value in promoting its use. 3. Used to make mold plaster. China has a large market for mold gypsum, but currently there are still issues related to the technical processes involved in using desulfurized gypsum to produce mold gypsum, as well as concerns regarding the whiteness of this material. The relevant technologies are not yet mature, and further development of new techniques is needed. 4. Used as a cement retarder. Currently, when using desulfurization gypsum as a cement retarder, it first needs to be transformed into spherical form, which increases production costs and reduces economic efficiency. A more economical approach is to develop process technologies for direct utilization; to develop such technologies, the issue of wet-based desulfurization gypsum caking in ball mills must first be addressed. For the cement industry, serving as a cement retarder is the main application of desulfurization gypsum. Desulfurization gypsum in our country is mainly concentrated in the eastern coastal areas. In 2006, China’s cement production was around 1.3 billion tons, of which 800 million tons came from the eastern region alone. Based on an addition rate of 4%, if all natural gypsum is replaced by desulfurization gypsum as a retarding agent, 32 million tons of desulfurization gypsum can be utilized per year, which is more than sufficient to address the current issue of utilizing desulfurization gypsum. 5. Used in the manufacture of gypsum board. Manufacturing gypsum board is another major application for desulfurization gypsum; abroad, desulfurization gypsum is generally used to produce gypsum board as well. It is estimated that this year China’s total production and consumption of gypsum board will reach 750 million square meters. Assuming 10 kilograms of wet-base desulfurized gypsum is used per square meter, 7.5 million tons of desulfurized gypsum can be utilized each year. The demand for gypsum board in our country is still growing at a rate of around 20% per year, and there is great potential for utilizing desulfurized gypsum. There are successful precedents in the gypsum board production lines built in our country for using desulfurized gypsum as a substitute for natural gypsum. Since 1999, Shandong Taihe Group has been researching the technology for producing gypsum board using industrial by-product gypsum. The two gypsum board production lines in Jiangyin, Jiangsu, with an annual capacity of 30 million square meters each, can utilize 600,000 tons of desulfurized gypsum per year to produce 60 million square meters of gypsum board. Taihe has also collaborated with several power plants to invest in the construction of four production lines with an annual output of 25–30 million square meters, utilizing the desulfurization gypsum from those power plants. Some foreign companies are also planning to build production facilities in China to make extensive use of desulfurization gypsum. 6. Used as a soil conditioner. There are reports from places such as Shanghai of using it for the improvement of saline-alkali soil in beaches, but no reports of actual implementation have been seen. Although this method is theoretically feasible, there are still many practical issues that need to be resolved. III. Recommendations for accelerating the comprehensive utilization of desulfurization gypsum. Although desulfurization gypsum has quality comparable to natural gypsum, to speed up its comprehensive utilization, it is necessary to strengthen publicity efforts, increase research and development in related application and treatment technologies, and foster a viable market for such products. To this end, it is recommended that: 1. Clay bricks be further restricted, and new types of building materials be vigorously promoted. Plaster for painting, putty plaster, gypsum blocks, and the like are all excellent new types of building materials, but the market for them is small, making large-scale production impossible. 2. Accelerate the development of application technologies for desulfurization gypsum. Products such as mold gypsum have a very wide range of market applications, but mold gypsum is mainly produced from bulk gypsum, and there is currently no better solution to this issue ； Research and development also need to be accelerated to address the issue of desulfurization gypsum sticking to ball mills in cement production. 3. Increase policy support for the utilization of desulfurization gypsum, and further improve and implement tax incentives for the comprehensive utilization of resources. 4. Strengthen monitoring to ensure the proper operation of the desulfurization equipment in power plants, thereby guaranteeing the supply of raw materials for enterprises that utilize resources in a comprehensive manner. There are several reasons why desulfurization equipment in power plants does not operate properly. First, the quality of companies that manufacture such equipment varies; some of these products have poor construction quality, which leads to malfunctions and the cessation of desulfurization processes. Additionally, there is a high reliance on foreign technologies and equipment, which makes it difficult to repair faults promptly, resulting in interruptions in desulfurization activities ； Second, electricity price policies fail to be implemented in a timely manner ； Third, environmental protection enforcement is not strict; there is a lack of supervision or insufficient oversight over the daily operation of desulfurization facilities ； Fourth, some power plants deliberately shut down their desulfurization facilities to gain economic benefits. Technical Issues and Desulfurization Efficiency of Lime-Stone-Gypsum Wet Flue Gas Desulfurization Technology Tian Bin Abstract: This paper explains the principle of the lime-stone-gypsum wet flue gas desulfurization process, as well as the existing technical issues and solutions, and discusses the main factors affecting desulfurization efficiency. Keywords: wet flue gas desulfurization ； Technical issues ； Desulfurization efficiency: Current desulfurization technologies are being widely adopted in newly built, expanded, or renovated large-scale coal-fired industrial and mining enterprises, especially coal-fired power plants. The limestone-gypsum wet desulfurization method is the most mature technology, suitable for China’s conditions, and is the most commonly used efficient desulfurization process in the country. However, in practical applications, if technical issues such as scaling, blockage, and corrosion are not properly addressed based on specific circumstances, the desired desulfurization results cannot be achieved. This article provides a brief discussion on the technological principles of this method, the technical issues encountered in practice, the solutions for these issues, and the main factors affecting desulfurization efficiency. 1. The limestone-gypsum wet flue gas desulfurization process and its principle: The flue gas exiting the electrostatic precipitator enters the heat exchanger GGH via a booster fan BUF; after being cooled, it enters the absorption tower Abs where it mixes with the limestone slurry. Some of the water in the slurry evaporates, further cooling the flue gas. By washing the flue gas with circulating lime slurry, over 95% of the sulfur in the flue gas can be removed. It can also remove nearly 100% of the hydrogen chloride in the flue gas. At the top of the absorber, the flue gas passes through the demister Me to remove suspended water droplets. After leaving the absorber, and before entering the chimney, the flue gas passes through the heat exchanger once again to be heated. The outlet temperature of the absorption tower is generally 50–70°C, which mainly depends on the type of fuel being burned. The lowest gas temperature in the chimney is often specified by **emission standards**. In our country, with GGH-based desulfurization, the minimum temperature at the chimney is generally 80°C; without GGH-based desulfurization, this temperature is around 50°C. Most desulfurization flues are equipped with bypass dampers (which are normally closed). In emergency situations or during startup, the bypass damper opens to allow flue gases to bypass the sulfur dioxide removal unit and be discharged directly into the chimney. The limestone-gypsum slurry is pumped from the absorption tower sedimentation tank into the nozzle manifold installed at the top of the tower. As the limestone-gypsum slurry descends through the spray tower, it comes into contact with the rising flue gas. The SO2 in the flue gas dissolves into the aqueous solution, where it is neutralized by the alkaline substances present, thereby removing sulfur from the flue gas. Calcium carbonate in limestone reacts with sulfur dioxide and oxygen (oxygen in the air), ultimately producing gypsum, which precipitates out of the solution in a sedimentation tank. The gypsum slurry is drawn from the precipitation tank of the absorption tower, stored after being concentrated, dehydrated, and washed, and then transported away from the site. 2. Scaling and blockage in the desulfurization system and solutions 2.1 Mechanisms of scaling and blockage 1) When the concentration of the gypsum product exceeds the absorption limit of the slurry, gypsum begins to deposit in crystalline form. Once the relative saturation concentration reaches a certain level, gypsum crystals will grow on the surface of existing gypsum crystals in the suspension. At even higher saturation levels, crystal nuclei are formed; simultaneously, crystals also grow on the surfaces of various other objects, leading to scaling on the inner walls of the absorption tower. 2) Under conditions of low oxidation level in the system, or even in the absence of oxidation, a reactant consisting of Ca(SO3)0.8(SO4)0.21/2H2O can be formed; this is known as CSS-scale, and it causes scaling in the system, which can even lead to blockages. 3) Sudden changes in the pH value of the absorption solution: at low pH values, the solubility of sulfites increases sharply, while the solubility of sulfates decreases slightly. This results in a large amount of gypsum being produced and precipitating in a short time, leading to the formation of hard deposits. At high pH values, the solubility of sulfites decreases, leading to their precipitation and the formation of soft scale. Operation at an alkaline pH value will result in calcium carbonate scale. 2.2 Solutions 1) Adopt a forced oxidation process to ensure complete oxidation, keeping the oxidation rate of calcium sulfite above 95%, and maintaining a sufficient density of gypsum seed crystals in the slurry. 2) Ensure strict dust removal to prevent nozzle clogging. 3) Control the evaporation rate and amount of water in the absorption solution, ensuring that the supersaturation of gypsum in the solution does not exceed 130% during operation. 4) Control the pH value of the solution, especially avoiding sudden changes in pH during operation. 5) Calcium sulfate dihydrate or calcium sulfite seeds are added to the absorption solution. 6) Additives such as magnesium ions and ethanedioic acid are added to the absorption solution. 7) Appropriately increasing the liquid-to-gas ratio is also an important technical measure to prevent scaling and blockage in the system. 3. Corrosion and anti-corrosion of sulfur systems 3.1 Corrosion mechanisms 1) Acidic gases such as SO2, HCl, and HF in flue gas generate corresponding acids upon contact with liquids; the ions SO32‑, Cl‑, and SO42‑ are highly corrosive to metals and also cause significant diffusion and penetration damage to anti-corrosion linings. 　　2) The metal surface undergoes electrochemical corrosion in the presence of water and electrolytes, which is more evident at the welds. 　　3) Crystal corrosion: Sulfates and sulfites in the solution penetrate into the anti-corrosion lining and its capillaries as the solution moves through these areas. When the system is shut down, the absorption tower gradually dries out, causing the sulfates and sulfites in the solution to precipitate and crystallize. This leads to an increase in volume, which generates stress on the anti-corrosion lining and results in delamination and damage. 　　4) Influence of ambient temperature. Due to GGH (regenerative heat exchanger) failures or faults in the circulating fluid system, the flue gas temperature inside the tower rises, and the allowable stress of its anti-corrosion materials decreases sharply as the temperature increases. 　　5) Due to the presence of solid particles in the slurry, it exerts a certain scouring effect on the materials inside the tower as it falls. 3.2 Corrosion prevention techniques 1) Control the pH value of the slurry appropriately. 　　2) Select an appropriate flue gas inlet temperature for the FGD (desulfurization equipment), and choose a corresponding anti-corrosion lining; it is a fatal mistake to use a lining material that does not match the inlet flue gas temperature or the designed temperature inside the tower. 3) Strictly control the construction quality of the anti-corrosion lining. 　　4) During the on-site construction of the absorption tower, it is necessary to ensure that all welds are fully welded, with smooth and defect-free seams. For internal supports and frames, round bars and square bars should be used instead of angle bars, channel bars, or I-beams. External pipes shall not be welded but connected using flanges. 　　5) Select appropriate anti-corrosion materials. 4. Analysis of factors affecting desulfurization efficiency 4.1 pH value of the absorption solution When SO2 in the flue gas comes into contact with the slurry in the absorption tower, the following chemical reactions occur: SO2 + H2O = HSO3- + H+; CaCO3 + H+ = HCO3- + Ca2+; HSO3- + 1/2 O2 = SO42- + H+; SO42- + Ca2+ + 2H2O = CaSO4·2H2O. It is clear from these reaction processes that a high pH of the slurry facilitates the absorption of SO2, while a low pH promotes the precipitation of Ca2+, and these two effects are contrary to each other. At a pH value of 6, the absorption efficiency of sulfur dioxide is optimal, but scaling and blockage tend to occur at this point. A low pH value facilitates the oxidation of calcium sulfite; while it increases the solubility of limestone, it hinders the absorption of sulfur dioxide, resulting in a **reduced desulfurization efficiency. When pH = 4, the absorption of sulfur dioxide is almost impossible, and the absorbent solution becomes acidic, which can also cause corrosion to the equipment. The most appropriate pH value should be determined after testing, but it is generally between 4 and 6. 4.2 Liquid-to-gas ratio and slurry circulation rate An increase in the liquid-to-gas ratio means an increased probability of gas-liquid contact, thereby leading to a higher desulfurization rate. However, there is a gas-liquid equilibrium between sulfur dioxide and the absorption liquid; once the liquid-to-gas ratio exceeds a certain value, the desulfurization rate will no longer increase. After the fresh limestone slurry is sprayed and comes into contact with flue gas, the reaction between gases such as SO2 and limestone is not complete; therefore, continuous circulation of the reaction is necessary. Increasing the circulation rate of the slurry increases the opportunities for contact and reaction between CaCO3 and SO2, thereby improving the removal efficiency of SO2. 4.3 Contact time between flue gas and desulfurization agent: After entering the absorption tower from the gas-gas heater, the flue gas flows from bottom to top, where it comes into contact with the limestone slurry droplets sprayed downward; the longer the contact time, the more complete the reaction becomes. Therefore, operating the circulation pump corresponding to the high-position spray tray for a long period of time facilitates full reaction between the flue gas and the desulfurization agent, resulting in a higher desulfurization efficiency. 4.4 Limestone particle size and purity The finer the limestone particles, the greater their surface area, which leads to more complete reactions, a faster absorption rate, and a higher utilization efficiency of the limestone. The general requirement is that 90% should pass through a 325-mesh or 250-mesh sieve, and the purity of limestone is generally required to be above 90%. 4.5 The amount of oxygen O2 in the oxidizing air participates in the chemical process of flue gas desulfurization, causing 4HSO3‑ to be oxidized to SO42‑. As the oxygen content in the flue gas increases, the formation of CaSO4·2H2O accelerates, and the desulfurization rate also shows an upward trend. Operating more oxidation fans can increase the desulfurization rate. 4.6 The fly ash present in the original flue gas to some extent hinders the contact between SO2 and the desulfurization agent, reducing the dissolution rate of Ca2+ in limestone; meanwhile, certain heavy metals that continuously dissolve from the fly ash inhibit the reaction between Ca2+ and HSO3-. If the dust content in the flue gas continuously exceeds the design limit, the desulfurization efficiency will drop significantly, and the nozzles will become clogged. It is generally required that the dust content at the FGD inlet be less than 200 mg/m3. The lower the flue gas temperature at the point of entry into the absorption tower, the easier it is for SO2 gas to dissolve in the slurry and form HSO3‑; in other words, low temperatures facilitate absorption while high temperatures facilitate desorption. Usually, the flue gas is cooled to 60. It is most appropriate to carry out the absorption operation at around C. A higher absorption temperature will reduce the efficiency of SO2 absorption. 4.8 Cl‑ content Chlorine exists in the system mainly in the form of calcium chloride; it is difficult to remove, which affects the desulfurization efficiency and complicates the subsequent treatment processes. During operation, the Cl‑ content in the system should be strictly controlled (generally kept below 20,000 ppm) to ensure that it remains within the limits permitted by the design specifications (which are usually around 40,000 ppm). 5. Conclusion: By using the methods mentioned above, the technical issues related to desulfurization in practice can be basically resolved, enabling the desulfurization efficiency to meet the design requirements. This ensures that China can effectively protect its living environment while developing its economy, and that the standard of living for its people can be improved comprehensively! References: . Environmental Engineering, Chengdu University of Science and Technology Press. Industrial Desulfurization Technologies, Chemical Industry Press. Clean Coal Power Generation Technologies, China Electric Power Press. Flue Gas Desulfurization and Denitrification Technologies and Engineering Examples, Publishing House of Environmental Science and Engineering. Author’s profile: Tian Bin, male, engineer; has many years of experience in the construction of environmental protection projects such as dust removal, desulfurization, and denitrification in large thermal power plants. E-mail: tb6171@yahoo.com.cn