Gypsum obtained from wet flue gas desulfurization

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In recent years, with the continuous strengthening of air pollution control in China, the control of sulfur dioxide emission in coal-fired flue gas of power plants has become more and more strict. According to China's requirements for the control of sulfur dioxide emissions in the flue gas of power plants, new power plants must meet the national standards for sulfur dioxide emissions when they are put into operation. By 2015, the flue gas desulfurization transformation project of all old power plants will be completed. According to incomplete statistics, for a 600000 kW power plant, the FGD unit produces 100000 tons of FGD gypsum every year. If all the capacities of domestic power plants are desulfurized, 30million tons of desulfurized gypsum will be produced every year. How to make rational use of desulfurization by-products to make them become resources and effectively use them will produce great economic and social benefits

due to the short time of FGD gypsum production technology in China, the application value and market understanding of FGD gypsum have just started, and the performance, processing technology and equipment of FGD gypsum paste according to the content of the press release have not been systematically studied. There are still great differences between FGD gypsum and natural gypsum in material transportation, dust treatment, calcination and other treatment processes and equipment. In terms of physical properties, FGD gypsum is also different from natural gypsum, such as high purity, large free moisture, fine particle size, uniform particle size distribution, poor grading and large water consumption for standard consistency. Because the sources of these two kinds of gypsum are different, the main impurity in desulfurized gypsum is unreacted calcium carbonate, and it also contains a small amount of unoxidized calcium sulfite. In the production of flue gas desulfurization, the quality of desulfurized gypsum will be directly affected by factors such as the difference of use technology, the different composition of limestone raw materials and the different fineness of limestone. Therefore, in the application and processing of desulfurization gypsum, we must combine the specific reality, adopt scientific means, and conduct in-depth research. The desulfurized gypsum after washing and filtration contains about 10% - 20% free water, the particles are fine and loose, the color is almost white and slightly yellow, and sometimes because of the unstable desulfurization operation, more impurities such as coal ash are brought in, and the color will be black. The whiteness of better desulfurized gypsum is more than 80%

1 application status of desulfurization gypsum at home and abroad

1 1 application status of FGD gypsum abroad

FGD gypsum has long been widely used abroad, the earliest and best used is Japan, followed by Germany, France, the Netherlands and other European countries. Since 1977, Japan has used almost all gypsum, an industrial by-product, to produce paper gypsum board and fiber gypsum board, which are also used in a large amount in the cement industry. Nearly 100% of desulfurized gypsum has been applied, of which more than 95% is used in gypsum board and cement. In Germany, more than 1.5 million tons of desulfurized gypsum are used to produce plastering gypsum every year; The United Kingdom, France, the Netherlands and other countries are also widely used in the production of plastering gypsum, gypsum plasterboard, gypsum block, self leveling gypsum and other gypsum products

1. 2 application status of domestic desulfurization gypsum

compared with developed countries, China has generally insufficient understanding of the application of desulfurization gypsum, and the research on the performance and application technology of desulfurization gypsum has just started. The utilization of desulfurized gypsum is mainly concentrated in Chongqing power plant, Luohuang Power Plant, Beijing No.1 Thermal Power Plant, Taiyuan No.1 Thermal Power Plant, Wuhan Huaneng Yangluo power plant, etc. These factories not only effectively solve the problem of stacking desulfurization gypsum, but also are very popular in the market because of their good product quality and low price

Shanxi Taiyuan No.1 Thermal Power Plant has built a comprehensive utilization production line of desulfurization gypsum near the power plant to completely digest the desulfurization gypsum of the plant into gypsum products. The first phase of the project was officially put into operation in 1998, with an annual treatment of 60000 tons of desulfurization gypsum. The products produced are cement additives, hemihydrate gypsum and plaster

the desulfurization gypsum of Beijing No.1 Thermal Power Plant is also treated by itself. The comprehensive utilization device of desulfurization gypsum and the desulfurization device of power generation flue gas are planned, installed and put into operation at the same time. The comprehensive utilization device of desulfurized gypsum adopts German sasky equipment to produce gypsum blocks, with a design capacity of 380000 m2/A. Wuhan Huaneng Yangluo power plant cooperated with Northeast University to build a production plant with an annual output of 100000 tons of building gypsum powder, which has become a model project of Huaneng company and was completed and put into operation in October 2007. Beijing Xingda desulfurized gypsum Co., Ltd. and Northeast University have also put into production a 50000 ton/year production line of building gypsum powder

2 comprehensive utilization of desulfurization gypsum

2 1 prospects for comprehensive utilization of FGD gypsum

the advantages of comprehensive utilization of FGD gypsum with the development of new wall materials and the improvement of people's understanding of the "green" performance of gypsum, gypsum building materials have developed rapidly in recent years, such as gypsum board, gypsum block, plaster and other products have increased rapidly. If the state introduces reasonable policy guidance, the development of gypsum building materials will be further accelerated with the expansion of the utilization field of desulfurization gypsum, and the price of steel billets in Shandong and Tianjin will rise by 10 yuan/ton

(1) as FGD gypsum is a gypsum resource by-product of industry, it can completely replace natural gypsum, and gypsum has good quality and low price, which can effectively reduce the production cost of gypsum building materials, improve the market competitiveness and share with similar products, and cultivate the market of gypsum building materials

(2) the emergence of desulfurized gypsum will change the uneven distribution pattern of original gypsum resources. The distribution of natural gypsum in China is relatively concentrated in economically underdeveloped areas, while FGD gypsum resources are mainly distributed in economically developed large and medium-sized cities. Therefore, the distribution pattern of gypsum resources has been adjusted and balanced to a great extent, so that gypsum building materials can develop in an overall and balanced way in a large range

(3) cement retarder, gypsum block, construction gypsum powder and its products made of desulfurized gypsum are very competitive with natural gypsum products in performance

2. 2 suggestions for comprehensive utilization of desulfurization gypsum (1) try to digest by yourself or build a factory nearby. Because the price and raw material cost of gypsum products are low, the transportation cost needs to be reduced as much as possible, so a plant should be built near the power plant. The advantage of the power plant building its own plant is that it can make full use of the low-cost power and heat resources of the power plant to reduce the processing cost of gypsum products. It can also realize comprehensive utilization with fly ash and other power plant wastes

(2) choose gypsum products with large output as the market direction. At present, the main product markets for the comprehensive utilization of FGD gypsum are construction gypsum powder, cement retarder, gypsum board and gypsum block. At this stage, China is mainly used as cement retarder, gypsum board (including all kinds of wallboards, decorative boards and insulation boards), gypsum block and plastering gypsum. Therefore, the comprehensive utilization of a large amount of desulfurized gypsum produced in the future will also focus on these aspects

in a word, the comprehensive utilization of desulfurization gypsum is a good thing for the country and the people, and turning waste into treasure. It is a project supported and actively encouraged by the state. Compared with natural gypsum, the raw material of construction gypsum products and cement retarder produced with desulfurized gypsum is industrial waste, which can be ignored the cost, eliminating the crushing process of gypsum. The production cost of the product is low, and the grade of gypsum is high. The performance of gypsum products is superior or equivalent to that of natural gypsum products, so it has good market competitiveness

3 calcination process and equipment of desulfurization gypsum

with desulfurization gypsum as raw material, the processing of various gypsum products is inseparable from the calcination process. Gypsum stone or gypsum exists in the form of calcium sulfate dihydrate, and there are two crystal waters in its molecular formula. Only when a semi crystalline water is removed can the calcium sulfate semi hydrate have the property of gel. The process of removing the crystalline water is the calcination of gypsum. The calcined building gypsum powder can be used to produce gypsum board, gypsum block, gypsum decorative board and other products

in the 1960s, foreign countries used traditional intermittent frying pans and external burning rotary kilns as basic equipment for calcining gypsum, which were mainly used for calcining natural gypsum. In the 1980s, air flow rapid calcination process was developed. In the mid-1990s, Peter mill based rapid calcination equipment was developed to complete drying, calcination and crushing in the same equipment. At the same time, various intermittent frying pans, internal and external combustion rotary kilns, boiling calciners and other calcination equipment have been developed, forming a diversified and large-scale

the research of large-scale modern calcination equipment in China is still in its infancy. Many enterprises are transforming from traditional calcination equipment of natural gypsum to industrial by-product gypsum. At present, the more advanced calcination process of desulfurization gypsum in China is the combination of drying and calcination

innovative management mode

as desulfurized gypsum contains about 10% surface water, it will be officially implemented on March 25 this year. It is difficult to produce stable and high-quality products by traditional calcination. According to the characteristics of desulfurized gypsum, the desulfurized gypsum should be dried before calcination. The drying equipment used mainly include rotary dryer, air dryer and Stede fast dryer. Heating methods include steam, coal, gas and heat transfer oil. The calcination methods mainly include rotary kiln, fluidized bed furnace and air flow calcination. This process is called two-step process

the Stede gypsum calciner developed by Northeast University Liaoning Dongda Powder Engineering Technology Co., Ltd. is a one-step process that involves drying, calcining and dispersing in the same equipment. The equipment has been put into use in China and is specially used to treat desulfurization gypsum

due to different heat sources, the products produced are also different. The selection of appropriate calcination process and equipment has a direct impact on energy consumption, product quality and environmental protection. Taking Wuhan Huaneng Power Plant as an example, the plant uses self-produced 280 ℃ and 1.7 MPa steam as the heat source. The two-step calcination process was developed in cooperation with Northeast University Liaoning Dongda Powder Engineering Technology Co., Ltd. The desulfurized gypsum is dried by fast drying, and the surface water is reduced to less than 1% to become a loose and uniform dihydrate gypsum powder. Then the built-in fluidized bed is used for calcination to generate hemihydrate gypsum. This process has formed a scale of 100000 tons of building gypsum powder per year, and all indicators are better than those required by the national building gypsum standard

Beijing Xingda desulfurization gypsum Co., Ltd. adopts the one-step process of Stede calciner with coal as the heat source, and takes the hot flue gas of coal as the heat source to complete the drying, dispersing, calcination and other processes in the same machine. It has built a desulfurization gypsum calcination production line with an annual output of 50000 tons. The construction gypsum powder produced is mainly used as cement retarder, gypsum block and construction gypsum powder, and has achieved good economic benefits

4 conclusion

as the comprehensive utilization of desulfurized gypsum has just started in China, new problems will inevitably appear. The method of natural gypsum cannot be completely copied in the production process. It is necessary to develop new methods, new processes and new equipment for the characteristics of desulfurized gypsum. Therefore, more energy should be invested in the research of equipment supporting, and the system solution of comprehensive utilization of desulfurized gypsum should also be developed and formulated

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