According to statistics, there are 1984 189 SO2 control processes, and there are more than 200 at present. It can be mainly divided into four categories: (1) pre-combustion control-raw coal purification (2) in-combustion control-fluidized bed combustion (CFB) and post-combustion control-flue gas desulfurization (4) new processes (such as coal gasification/combined cycle system and liquid slag burner), in which most countries adopt post-combustion flue gas desulfurization process. Limestone/gypsum wet desulfurization process is the mainstream of flue gas desulfurization.
Since 1930' s, a large number of wet limestone/gypsum methods have been researched and developed, and since the end of 1960' s, some devices have been put into commercial operation. ABB's first practical wet flue gas desulfurization system was put into use in the United States in 1968. 1977 bischoff company made the first demonstration device of lime/limestone gypsum method in Europe. Ji zi Thermal Power Plant 1 Unit 2 adopts the first set of large-scale desulfurization device 1976 of IHI (Ishikawa Island Seed Mill), and adopts the limestone-gypsum mixed desulfurization method with venturi tube and two towers. Mitsubishi Heavy Industries completed the first set of equipment in 1964, and developed a flue gas desulfurization device according to its operation performance.
The first generation flue gas desulfurization system: the United States and Japan began to install it in the 1970s. The early FGD system includes the following processes: lime-based fluid; Sodium-based solution; Limestone-based fluid; Alkaline fly ash-based fluid; Double alkali (lime and sodium); Magnesium-based fluid; Velman-Lord process. There are many types of absorption, including ventilation type, vertical countercurrent jet tower and horizontal jet tower, and some internal structures such as trays, packing and glass balls are used to strengthen the reaction.
The desulfurization efficiency of the first generation FGD is generally 70%~85%.
Except for a few, by-products have no commercial value and can only be discharged as waste. Only magnesium process and Velman-Lord process produce sulfur and sulfuric acid with commercial value. Its characteristics are low initial investment, high operation and maintenance cost and low system reliability. Scaling and material failure are the biggest problems. With the increase of experience, the process has been improved, the operation and maintenance cost has been reduced, and the reliability has been improved.
The second generation flue gas desulfurization system
Installation began in the early 1980s. In order to overcome the scaling and material problems in the first generation system, the dry jet absorber appeared, and the lime and limestone injected into the furnace and flue were also close to commercial operation. However, the mainstream FGD technology is still a wet cleaning method based on lime and limestone, and the ventilation cleaning method using fillers and glass balls has disappeared. The most common ones are improved jet tower and shower tray. Different processes have different efficiencies. The initial dry spray desulfurization can reach 70%~80%, 90% in some improved cases, and 30%~50% in furnace and flue injection, but the reactant consumption is large. With the improvement of process and operation experience, the efficiency can reach 90%. All by-products of the second generation FGD system in the United States are discharged as waste. However, in Japan and Germany, the solid by-products are forcibly oxidized by wet cleaning with limestone as the main method to obtain gypsum with commercial value in some industrial and agricultural fields. The second generation flue gas desulfurization system has made progress in operation and maintenance costs and system reliability.
Third Generation Flue Gas Desulfurization System
The injection technology of furnace and flue was improved, and LIFAC and fluidized bed technology were developed. By widely using forced oxidation and passivation technology, the scaling problem affecting the reliability of lime and limestone system has been basically solved. With the further understanding of chemical processes and the use of additives such as DBA, the reliability of these systems can reach more than 95%. The second generation flue gas desulfurization system adopts passivation technology and DBA to solve the existing problems. Many of these systems have desulfurization efficiency of 95% or higher. The solid by-products of some systems can be used in agriculture and industry. In Germany and Japan, gypsum production has become a routine project of power plants. With the improvement of equipment reliability, the necessity of setting redundant equipment decreases, and the flue gas treatment capacity of a single reactor increases. In the 1970s, it had a bad reputation in thermal power plants because of its large investment, high operating cost, corrosion, scaling and blockage. After 15 years of practice and improvement, the working performance and reliability have been greatly improved, and the investment and operating expenses have been greatly reduced, which makes its following advantages stand out: (1) It has long-term application experience in thermal power plants; (2) The desulfurization efficiency and absorption utilization rate are high (when the Ca/S of some units approaches 1, the desulfurization rate exceeds 90%); (3) Good availability (the availability of recently installed units has exceeded 90%). People's concept of wet method has changed.