April 11, 2022

Introduction to the process of flue gas desulfurization

With the development of industry and the improvement of people’s living standards, the thirst for energy is also increasing, and SO2 in coal-fired flue gas has become the main cause of air pollution. Reducing SO2 pollution has become the top priority of today’s atmospheric environment governance. Many flue gas desulfurization processes have been widely used in industry, and it also has important practical significance for the treatment of exhaust gas from various boilers and incinerators.

Flue gas desulfurization (FGD) is an effective desulfurization method widely used in industrial industries. According to the form of sulfide absorbent and by-products, desulfurization technology can be divided into dry, semi-dry and wet. The dry desulfurization process mainly uses a solid absorbent to remove SO2 in the flue gas. Generally, fine limestone powder is sprayed into the furnace, which is decomposed into CaO by heating, absorbs SO2 in the flue gas, and generates CaSO3, which is dedusted together with fly ash. collected or discharged through the chimney. Wet flue gas desulfurization is a gas-liquid reaction of liquid absorbent under ionic conditions to remove SO2 in flue gas. The equipment used in the system is simple, the operation is stable and reliable, and the desulfurization efficiency is high. The biggest advantage of dry desulfurization is that there is no discharge of waste water and waste acid in the treatment, which reduces secondary pollution; the disadvantage is that the desulfurization efficiency is low and the equipment is huge. Wet desulfurization uses liquid absorbent to wash flue gas to remove SO2, and the equipment used is relatively simple, easy to operate, and high desulfurization efficiency; however, the flue gas temperature after desulfurization is lower, and the corrosion of equipment is more serious than that of dry method.

Limestone (lime)-gypsum wet flue gas desulfurization process
Limestone (lime) wet desulfurization technology has been widely used in the field of wet FGD due to the cheap and easy availability of absorbents.
The reaction mechanism with limestone as absorbent is:
Absorption: SO2(g)→SO2(L)+H2O→H++HSO3-→H+ +SO32-
Dissolution: CaCO3(s)+H+ → Ca2++HCO3-
Neutralization: HCO3- +H+ →CO2(g)+H2O
Oxidation: HSO3-+1/2O2→SO32-+H+
SO32- +1/2O2→SO42-
Crystallization: Ca2++SO42- +1/2H2O →CaSO4·1/2H2O(s)
The characteristics of this process are high desulfurization efficiency (>95%), high utilization rate of absorbent (>90%), adaptability to high concentration SO2 flue gas conditions, low calcium-sulfur ratio (generally <1.05), and comprehensive utilization of desulfurized gypsum, etc. . The disadvantage is that the capital construction investment cost is high, the water consumption is large, and the desulfurization wastewater is corrosive.

seawater flue gas desulfurization
The seawater flue gas desulfurization process is a desulfurization method that utilizes the alkalinity of seawater to remove sulfur dioxide from flue gas. The desulfurization process does not need to add any chemicals, and does not generate solid waste, the desulfurization efficiency is >92%, and the operation and maintenance costs are low. After the flue gas is dedusted by the dust collector, it is sent to the gas-gas heat exchanger by the booster fan for cooling, and then sent to the absorption tower. In the desulfurization absorption tower, in contact with a large amount of seawater from the circulating cooling system, the sulfur dioxide in the flue gas is removed by absorption reaction, and the seawater is discharged after oxidation. The flue gas after removing sulfur dioxide is heated up by the heat exchanger and discharged from the flue.
The seawater flue gas desulfurization process is limited by regions and is only suitable for projects with abundant seawater resources, especially for thermal power plants where seawater is used as circulating cooling water. , anti-corrosion problems of chimneys, aeration tanks and aeration devices. Its process flow is shown in Figure 1.

spray drying process
The spray drying process (SDA) is a semi-dry flue gas desulfurization technology, and its market share is second only to the wet process. In this method, the absorbent slurry Ca(OH)2 is sprayed in the reaction tower, the mist droplets are evaporated by the hot flue gas while absorbing SO2 in the flue gas, and solids are generated and collected by the dust collector. When the calcium-sulfur ratio is 1.3-1.6, the desulfurization efficiency can reach 80%-90%. Semi-dry FGD technology combines the general characteristics of dry and wet processes. The main disadvantage is that the use of slaked lime milk as the absorbent, the system is easy to scale and block, and special equipment is required for the preparation of the absorbent, so the investment cost is too large; the desulfurization efficiency and the utilization rate of the absorbent are not as high as those of the limestone/gypsum method.
Spray drying technology is widely used in small and medium-capacity units burning low-sulfur and medium-sulfur coal. In January 1990, a set of medium-sized test equipment was built in Baima Power Plant in China. Later, many units also adopted this desulfurization process, and the technology has basically matured.

Electron beam flue gas desulfurization process (EBA method)
Electron beam radiation technology desulfurization process is a dry desulfurization technology, which is a high-tech combining physical and chemical methods. The process flow of the process is composed of pre-dust removal, flue gas cooling, ammonia flushing, electron beam irradiation and by-product trapping processes. The flue gas discharged from the boiler enters the cooling tower after being roughly filtered by the dust collector, and sprays cooling water in the cooling tower to cool the flue gas to a temperature suitable for desulfurization and denitration treatment (about 70°C). The dew point of the flue gas is usually around 50°C. The flue gas after passing through the cooling tower flows into the reactor, and is injected with ammonia, compressed air and soft water in a near stoichiometric ratio. The amount of ammonia added depends on the concentration of SOx and NOx. After electron beam irradiation, SOx and NOx are in Under the action of free radicals, intermediate sulfuric acid and nitric acid are generated. Then sulfuric acid and nitric acid are neutralized with coexisting ammonia to form a mixture of powdered granular ammonium sulfate and ammonium nitrate. The desulfurization rate can reach more than 90%, and the denitrification rate can reach more than 80%. In addition, sodium-based, magnesium-based and ammonia can also be used as absorbents. The mixed particles of ammonium sulfate and ammonium nitrate generated by the general reaction are separated and captured by the by-product dust collector, and the purified flue gas is boosted and discharged into the atmosphere .

Flue gas circulating fluidized bed desulfurization process (CFB-FGD)
In the late 1980s, Germany’s LURGI company developed a new dry desulfurization process, which became the flue gas circulating fluidized bed desulfurization process (CFB-FGD). This process is based on the principle of circulating fluidized bed. Through the multiple recirculation of the absorbent, the contact time between the absorbent and the flue gas is more than half an hour, which greatly improves the utilization rate of the absorbent. Many advantages of the process, such as simple process, less land occupation, small investment and comprehensive utilization of by-products, etc., and can reach or even exceed the desulfurization of wet process at a very low calcium-sulfur ratio (Ca/S=1.1~1.2). Efficiency (above 95%).

CFB process
The CFB process flow consists of absorbent preparation, absorption tower, absorbent recycling, dust collector and control system. Untreated boiler flue gas enters from the bottom of the fluidized bed. A Venturi device is connected to the bottom of the fluidized bed, and the flue gas is accelerated after the Venturi tube and combined with very fine absorbent powder. Between particles, intense friction occurs between gas and particles. The absorbent reacts with SO2 to form calcium sulfite and calcium sulfate.
After desulfurization, the flue gas with a large number of solid particles is discharged from the top of the absorption tower and enters the absorbent recycling dust collector, which can be a mechanical type or a mechanical pre-dust collector before the electric precipitator. Most of the solid particles in the flue gas are separated and returned to the absorption tower through an intermediate ash bin. Since most particles are circulated many times, the residence time of the absorbent is very long, typically more than 30 minutes. A part of the ash in the intermediate ash silo is proportionally discharged from the solid recirculation loop according to the supply of absorbent and the efficiency of dust removal, and sent to the ash silo for external transportation.
If the flue gas discharged from the recirculating dust collector cannot meet the requirements of the emission standard, another dust collector needs to be installed. The clean flue gas after dust removal is discharged into the atmosphere through the induced draft fan and the chimney.
The absorbent is generally Ca(OH)2 dry powder with very fine particles, below 10μm. During desulfurization, the absorbent is fed into the fluidized bed absorption tower, and a certain amount of water is also injected to improve the desulfurization efficiency. In this way, the temperature of the flue gas after water spray is very close to the dew point of the water, and high desulfurization efficiency can be achieved under various operating conditions.
The by-product of the CFB process is in the form of dry powder, and its chemical composition is similar to that of the spray-drying process, mainly consisting of fly ash, CaCO3, CaSO4, and unreacted Ca(OH)2. The disposal methods are also basically the same as the by-products of the spray drying process. The by-product of the CFB process will solidify after adding water, the yield strength can reach 15-18N/mm2, the permeability is similar to clay, about 3×10-11, and the compacted density is 1.28g/cm3. If it can be further developed, it can become a Good building materials industry raw materials.
The composition of typical desulfurization ash fly is: fly ash is about 60% to 70%; CaCO3 is 7% to 12%; Ca(OH)2 is 2% to 4%; CaSO3 is 12% to 18%; CaSO4 is 2% ~5%; water <1%.
The CFB-FGD process has a good application prospect in the desulfurization industry due to its characteristics different from the traditional desulfurization process. The CFB-FGD process system is simple and has high reliability; high desulfurization efficiency; when the flue gas load changes, the system may still work normally; desulfurization by-products are in the form of dry powder, without a large amount of waste water, which is conducive to comprehensive utilization; basically there is no such thing as wet absorption. Serious corrosion, scaling and clogging problems in the tower; part of heavy metals can be removed, especially part of mercury can be removed, which is very meaningful for further treatment of flue gas.

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