How to digest the slurry in the accident slurry tank of desulfurization system and the matters needing attention in use

1. Technical characteristics of magnesium oxide desulfurization process 1. Mature technology. Magnesium oxide desulfurization technology is a kind of desulfurization technology whose maturity is second only to that of calcium method. Magnesium oxide desulfurization technology has a lot of application achievements all over the world, including more than 1 projects in Japan, 95% of power stations in Taiwan Province use magnesium oxide method, and it has been applied in the United States, Germany and other places, and has been applied in some areas of China at present. 2. Sufficient sources of raw materials. The reserves of magnesium oxide in China are very considerable. At present, the proven reserves of magnesium oxide are about 16 billion tons, accounting for about 8% of the world. Its resources are mainly in Liaoning, and magnesium oxide can be used as a desulfurizer in the desulfurization system of power plants. 3, the desulfurization efficiency is high. In terms of chemical reactivity, magnesium oxide is much larger than calcium-based desulfurizer, and its molecular weight is smaller than that of calcium carbonate and calcium oxide. Therefore, the desulfurization efficiency of magnesium oxide is higher than that of calcium method under the same other conditions. In general, the desulfurization efficiency of magnesium oxide can reach more than 95~98%, while the desulfurization efficiency of limestone/gypsum method only reaches about 9~95%. 4. Low investment cost Because magnesium oxide has its unique advantages as desulfurization itself, the structural design of the absorption tower, the amount of circulating slurry, the overall scale of the system and the power of the equipment can all be relatively small, so that the investment cost of the whole desulfurization system can be reduced by more than 2%. 5. Low operating cost. The main factors that determine the operation cost of desulfurization system are the consumption cost of desulfurizer and water, electricity and steam. The price of magnesium oxide is higher than that of calcium oxide, but the amount of magnesium oxide to remove the same SO2 is 4% of that of calcium carbonate. In terms of power consumption such as water, electricity and steam, the liquid-gas ratio is a very important factor, which directly affects the desulfurization efficiency of the whole system and the operating cost of the system. For limestone gypsum system, the liquid-gas ratio is generally above 15L/m3, while magnesium oxide is below 5 L/m3, so the magnesium oxide desulfurization process can save a large part of the cost. At the same time, the sale of by-products of magnesium oxide process can offset a large part of the cost. 6, reliable operation. Compared with the calcium method, the biggest advantage of magnesium desulfurization is that the system will not have the problem of equipment scaling and blocking, which can ensure the safe and effective operation of the whole desulfurization system. At the same time, the PH value of magnesium desulfurization is controlled between 6. and 6.5, and the equipment corrosion problem has been solved to some extent under this condition. Generally speaking, the safety performance of magnesium desulfurization in practical engineering has a very strong guarantee. 7. The comprehensive benefit is high. Because the reaction products of magnesium desulfurization are magnesium sulfite and magnesium sulfate, the comprehensive utilization value is very high. On the one hand, we can produce magnesium sulfate by forced oxidation, and then concentrate and purify it to produce magnesium sulfate heptahydrate for sale. On the other hand, we can also directly calcine it to produce sulfur dioxide gas with high purity to produce sulfuric acid. 8. The utilization prospect of by-products is broad. We know that sulfuric acid is called "the mother of chemical industry" and sulfur dioxide is the raw material for producing sulfuric acid. China is a country with a relative shortage of sulfur resources. The annual import of sulfur exceeds 5 million tons, equivalent to 7.5 million tons of sulfur dioxide. In addition, magnesium sulfate is widely used in food, chemical industry, medicine, agriculture and many other aspects, and the market demand is also relatively large. Magnesium desulfurization makes full use of existing resources and promotes the development of circular economy. 9. There is no secondary pollution. In the common wet desulfurization process, there is inevitably a problem of secondary pollution. For magnesium oxide desulfurization technology, the follow-up treatment is relatively perfect, SO2 is regenerated, and the problem of secondary pollution is solved. Second, the reaction mechanism of magnesium oxide desulfurization is similar to that of calcium oxide. Both alkaline oxides react with water to generate hydroxide, and then react with sulfurous acid solution generated by dissolving sulfur dioxide in water for acid-base neutralization. Magnesium sulfite and magnesium sulfate generated by magnesium oxide reaction are reused after recovering SO2, or all of them are forcibly oxidized into sulfate to make magnesium sulfate heptahydrate. The main chemical reactions in the desulfurization process are MgO+H2O = mg (OH) 2mg (OH) 2+SO2 = MgSO 3+H2O+SO2 = mg (HSO 3) 2mgsO3+1/2o2 = MgSO 4. The main reactions in the magnesium oxide regeneration stage are MgSO 3 → MgO+SO 2 MgSO 4 → MgO+SO 3 mg (. 2 → MgO+H2O+2So2SO2+1/2o2 → SO3SO3+H2O → H2SO4 When the by-product is forcibly oxidized to make MgSO4 7H2O for sale, MgSO3+1/2o2 → MgSO4 MgSO4+7H2O → MgSO4 7H2O 3, Brief Introduction to the Process of Magnesium Oxide Desulfurization Process; Among the wet desulfurization processes that have been commercialized at present, magnesium oxide desulfurization technology is a promising desulfurization technology. This process is mature, with less investment, simple structure and good safety performance, and can reduce secondary pollution, recycle desulfurizer, reduce desulfurization cost and bring certain economic benefits. Compared with calcium desulfurization, it avoids a series of problems existing in simple wet desulfurization, such as pipeline blockage, low flue gas temperature, flue gas carrying water and secondary water pollution, etc. At the same time, compared with the relatively complete limestone/gypsum method, it has the advantages of small floor space, low operating cost, greatly reduced investment and greatly improved comprehensive economic benefits. The whole process flow of magnesium process can be divided into two kinds: sulfuric acid production by-products and magnesium sulfate heptahydrate production, and the processes are described as follows: (1) The flue gas from the boiler for sulfuric acid production is mostly above 14℃, which contains a lot of carbon dioxide, dust and sulfur dioxide, as well as acid gases such as hydrofluoric acid, hydrochloric acid and sulfur trioxide. Flue gas first enters the dust removal system, and more than 99% of the dust is collected by electrostatic precipitator or bag precipitator and sold to cement plants and other related enterprises as building materials, which can not only increase the income of enterprises, but also avoid blocking the nozzle and reducing the desulfurization efficiency due to dust particles. After dust removal, the flue gas enters the desulfurization reaction tower from the bottom of the desulfurization tower. At the flue gas inlet of the desulfurization tower, there is a device for spraying water to reduce the temperature of flue gas, which is suitable for SO2 chemical reaction. A layer of cyclone plate is installed above the flue gas inlet to slow down the flue gas flow rate, increase the reaction time and achieve the effect of uniform distribution of flue gas in the tower. There are three layers of nozzles on the swirl plate to spray desulfurizer slurry continuously, which makes reverse contact with flue gas from bottom to top and fully reacts. In order to reduce the structural blockage of equipment and the excessive pressure loss in the tower to ensure the smooth flow of flue gas, there is no support or maintenance frame in the tower. After washing, the humidity of flue gas is relatively high, so it needs to be dehydrated. Generally, two demisters are installed above the spray layer in the absorption tower. At the same time, an automatic process water flushing system is installed on the demister to timely treat the dust accumulated on the demister after running for a period of time. The temperature of flue gas coming out of the desulfurization tower is generally around 55~6℃, and the flue gas still contains a little moisture, so direct discharge is easy to cause the fan blade and chimney to be corroded with water. Therefore, in front of the fan, the temperature of flue gas is increased by heating before being discharged, so that the corrosion of the chimney of the fan can be avoided. In order to ensure that the normal operation of the boiler will not be affected when the equipment in the desulfurization tower is overhauled, a bypass system is added, and the direction of flue gas is controlled through the baffle door, which is used to protect the desulfurization system and will not have any adverse impact on the operation of the boiler. For magnesium oxide, it reacts with sulfur dioxide in the absorption tower and becomes magnesium sulfite, and part of it is oxidized by oxygen in flue gas to become magnesium sulfate. The mixed slurry is dehydrated and dried to remove the surface moisture and crystal water of the solid. The dried magnesium sulfite and magnesium sulfate are roasted in the regeneration process to decompose them, so that magnesium oxide can be obtained and sulfur dioxide can be precipitated at the same time. The roasting temperature has a great influence on the properties of magnesium oxide, and the roasting temperature suitable for magnesium oxide regeneration is 66~87℃. When the temperature exceeds 12℃, magnesium oxide will be sintered and can no longer be used as desulfurizer. The concentration of sulfur dioxide in the roaster exhaust gas is 1~16%, which can be used to produce sulfuric acid after dust removal, and the regenerated magnesium oxide can be recycled for desulfurization. 1. Flue gas system The flue gas system refers to several flue gas treatment systems including pre-precipitator, bypass, flue gas heating device and chimney. In this system, the flue gas from the boiler is adjusted to suitable reaction conditions after dust removal and cooling treatment. At the same time, the flue gas can pass through the bypass when the equipment fails or the system runs abnormally, ensuring the normal operation of the whole power plant system. The purpose of flue gas heating is to reduce the moisture content of flue gas and facilitate the flue gas discharged from the chimney to spread as soon as possible. 2. If the particle size of magnesium oxide purchased from the slurry preparation system meets the desulfurization requirements, it can directly enter the digestion device to make slurry with a concentration of 15~25% without crushing, and then it is sent to the absorption tower through the slurry pump to complete the desulfurization purpose. 3. The absorption tower of SO2 absorption system is the main place for SO2 absorption, and the material is mostly made of ordinary steel structure with anti-corrosion coating. The bottom of the tower is a slurry pool, the middle of the tower is a spray layer, and the top is a demister. The slurry circulates continuously in the tower, and when the slurry concentration reaches a certain level, it is discharged into the slurry treatment system through the slurry output pump. 4. Slurry treatment system The slurry coming out of the absorption tower is mainly magnesium sulfite and magnesium sulfate solution. When it is required to regenerate magnesium oxide, the solution should be purified first, then concentrated and dried. The dried magnesium sulfite is calcined at 85℃ in the presence of carbon to regenerate magnesium oxide and sulfur dioxide. The calcined magnesium oxide returns to the absorption system, and the collected sulfur dioxide gas with high purity is sent to a sulfuric acid plant to make sulfuric acid. (2) Preparation of magnesium sulfate heptahydrate This process is not much different from the above process, but it is different in the treatment method of desulfurizer slurry. Magnesium sulfite generated after the reaction of sulfur dioxide and magnesium hydroxide in the desulfurization tower enters the slurry pool at the bottom of the absorption tower, and is forcibly blown to the slurry pool by the blower to be oxidized into magnesium sulfate. The water containing magnesium sulfate is continuously recycled for desulfurization. When the concentration of magnesium sulfate in circulating water reaches a certain condition, it is pumped into the water collection tank and then sent to the magnesium sulfate impurity removal system. After removing impurities from desulfurization sewage by impurity removal equipment, magnesium sulfate solution is crystallized into magnesium sulfate heptahydrate by concentration equipment. After drying, the recovered magnesium sulfate heptahydrate is packaged and stored, and the water is extracted from magnesium sulfate heptahydrate (MgSO4? 7H2O) is separated and recovered and then transported to the desulfurization tower for recycling. Compared with the previous process, the main differences are as follows: 1. In order to improve the purity of magnesium sulfate, the absorption system needs to strengthen oxidation in the slurry tank of the absorption tower, so the structure of the absorption tower is different from that of the regenerated magnesium oxide tower, and the power consumption will be increased accordingly. 2. An impurity removal system is added. The slurry from the absorption tower contains many impurities, which will affect the quality of magnesium sulfate. Therefore, it is necessary to add an impurity removal system to purify the magnesium sulfate solution. 3. The magnesium sulfate solution purified by the concentration system needs to be concentrated to make the solution into a high-concentration concentrated solution, and then the excess water is removed to convert the magnesium sulfate solution into magnesium sulfate with seven crystal waters. Finally, different packaging methods can be selected according to different requirements of users for finished product treatment. (3) Abandonment method In many cases, the actual situation of the user enterprise itself does not allow the desulfurization by-products, especially the desulfurization of small and medium-sized boilers. Due to the small scale and the small amount of by-products, the abandonment method is mostly adopted. The flue gas system, absorbent preparation system, SO2 absorption system and flue gas reheating device of the disposal method are basically the same as the above two methods, but the difference is that most of the water is recovered from the reacted slurry after solid-liquid separation and the solids are discarded. Discarding method can greatly reduce the investment cost of the system, and the process is much simpler. At the same time, it can also avoid a series of problems such as equipment scaling and pipeline blockage, and the power consumption of the subsequent part can also be omitted, but the consumption cost of desulfurizer is high, and it is more troublesome to treat the waste solids, but it will not cause secondary pollution after centralized treatment. 4. Conclusion Through the above analysis, magnesium oxide desulfurization is a kind of desulfurization method which is feasible in theory and fully verified in practical application, and has a good market prospect in some areas, especially in areas rich in magnesium oxide. Because this method can recycle desulfurizer, by-products can also bring certain economic benefits, and at the same time, it can avoid many shortcomings of large-scale wet method, so magnesium oxide desulfurization technology will be gradually applied more widely.