With the development of industry and the improvement of people's living standards, increasing its thirst for energy, in coal-fired flue gas SO2 has become the main cause of air pollution. Reduce SO2 pollution has become a pressing matter of the moment in atmospheric environmental governance. A lot of flue gas desulfurization technology has been widely used in industry, it also has important practical significance to the treatment of various boiler and exhaust gas incinerator. This paper discusses several commonly used in flue gas desulfurization technology.
1 common flue gas desulfurization technology
Flue gas desulfurization (FGD) is an effective method of desulfurization industry, large-scale application. In accordance with the sulfide and byproducts form absorbent, desulfurization technology can be divided into dry, semi dry and wet three. Dry desulfurization process is mainly the removal of SO2 from flue gas by using solid sorbents, the limestone powder is sprayed into the furnace, which is decomposed into CaO, the absorption of SO2 in flue gas, generating CaSO3, and fly ash together in the dust collection or through a chimney. Wet flue gas desulfurization is the gas-liquid reaction in ionic liquid absorbent conditions, thereby removing SO2 in flue gas system, the equipment is simple, stable and reliable operation, high desulfurization efficiency. The biggest advantage is no dry desulfurization wastewater, waste acid discharged governance, reduce pollution two; the disadvantage is low desulfurization efficiency, large equipment. The flue gas wet desulfurization liquid absorbent washing to remove SO2, with relatively simple equipment, easy operation, high desulfurization efficiency; but after desulfurization flue gas temperature is low, the equipment corrosion is more serious dry.
1.1 limestone (lime) - gypsum wet flue gas desulfurization process
(Shi Hui) limestone wet FGD technology as absorbent cheap, widely used in the field of wet FGD.
Using limestone as absorbent for reaction mechanism:
Absorption: SO2 (g), SO2 (L) +H2O - H++HSO3- - H+ +SO32-
Solution: CaCO3 (s) +H+, Ca2++HCO3-
HCO3-: +H+, CO2 and +H2O (g)
Oxidation: HSO3-+1/2O2 to SO32-+H+
SO32- +1/2O2, SO42-
+1/2H2O, CaSO3: Ca2++SO32- crystal 1/2H2O (s)
The process is characterized by high desulfurization efficiency (>95%), high absorbent utilization rate (>90%), can adapt to the high concentration of SO2 in flue gas conditions, low calcium sulfur ratio (<1.05), the comprehensive utilization of desulfurization gypsum can. The disadvantage is the high cost of investment in infrastructure, water consumption, desulfurization wastewater is corrosive etc..
1.2 seawater flue gas desulfurization
Seawater flue gas desulfurization process is a desulfurization method using seawater alkalinity to SO2 removal in flue gas. The desulfurization process does not need to add any chemicals, do not produce solid waste, >92% desulfurization efficiency, lower operation and maintenance cost. The flue gas deduster, a pressurized fan into the gas cooling heat exchanger, and then into the absorption tower. In the desulfurization absorption tower, contact with a lot of water from the circulating cooling system, sulfur dioxide in the flue gas is absorbed by the oxidation reaction of removal, water discharge. After the removal of sulfur dioxide in flue gas heat exchanger by heating, flue emission.
The seawater FGD process subject to geographical restrictions, applies only to the rich water resources engineering, especially suitable for sea water as circulating cooling water in power plant, but the need to properly solve the internal absorption tower, absorption tower, drainage ditch and rear flue chimney, aeration and aeration device corrosion problems. The process shown in figure 1.

1.3 spray drying process
The spray drying process (SDA) is a kind of semi dry flue gas desulfurization technology, and its market share after wet. This method is the absorbent Ca (OH) 2 in the reaction tower spray, droplet evaporation in hot flue gas by absorption of SO2 in flue gas at the same time, and the formation of solid dust catcher. When the calcium sulfur ratio is 1.3 ~ 1.6, the desulfurization efficiency can reach 80% ~ 90%. The general characteristics of semi dry and dry and wet the FGD technology. The main disadvantage is the use of lime milk as absorbent, easy fouling and clogging, and require specialized equipment for the preparation of sorbents, and the investment cost is too large; the desulfurization efficiency and sorbent utilization rate is not high limestone / gypsum method.
Application of spray drying technology in small and medium capacity unit and sulfur burning low sulfur coal. China in 1990 January in Baima power plant built a pilot plant. Later, many units also adopt the desulfurization process, the technology has matured.
1.4 Electron Beam Flue Gas Desulfurization (EBA)
Electron beam irradiation technology of desulphurization technology is one kind of dry desulfurization technology, high technology is a combination of physical and chemical methods. The process is composed of smoke dust, flue gas, ammonia pre cooling into, electron beam irradiation and by-product trapping process. The smoke exhaust of the boiler, after processing coarse filter dust collector into the cooling tower, spray cooling water in the cooling tower, the flue gas is cooled to suitable for desulfurization, denitration treatment temperature (about 70 DEG C). The dew point of flue gas is usually about 50 DEG C. Flow into the reactor through the flue gas cooling tower after the injection, close to the stoichiometric ratio of ammonia, compressed air and water mixed injection, adding ammonia depends on the amount of SOx and NOx concentration, after electron beam irradiation, intermediates of sulfuric acid and nitric acid, the role of SOx and NOx in the formation of free radicals. Then, sulfuric acid and nitric acid with the coexistence of ammonia neutralization reaction, a mixture of powder particles to generate ammonium sulfate and ammonium nitrate. The desulfurization rate can reach above 90%, the denitrification rate was above 80%. In addition, still can use sodium, magnesium and ammonia as the absorbent, the general reaction of ammonium sulfate and ammonium nitrate mixed particles are byproducts of dust collector separation and collection, through the flue gas purification boost after discharge to the atmosphere.
2 new flue gas desulphurization technology of circulating fluidized bed desulfurization (CFB-FGD)
In twentieth Century 80 at the end of the 1990s, the German Lu Qi (LURGI) company has developed a new dry desulfurization process, a flue gas circulating fluidized bed desulfurization (CFB-FGD). This process is based on the principle of circulating fluidized bed, through multiple recycle absorbent, absorbent makes contact with the flue gas for more than half an hour, greatly improving the utilization of sorbent, which not only has many advantages such as dry process, simple process, less area, small investment and by-product can be comprehensive utilization, but also in very low calcium sulfur ratio conditions (Ca/S=1.1~1.2) to achieve desulfurization efficiency even more than the wet process (more than 95%).
2.1 introduction to the technique of CFB
The CFB process consists of absorbent preparation, absorption tower, absorbent recycling, dust collector and control system components, without processing boiler flue gas from the fluidized bed into the bottom. At the bottom of the fluidized bed with a Venturi device, the flue gas through the venturi tube after speed, combined with each other and with fine absorbent powder. Between the particles, produces intense friction between gas and particles. Absorbent and SO2 reaction, formation of calcium sulfite and calcium sulfate.
The flue gas desulfurization with a large amount of solid particles discharged from the top of the absorption tower, into the dust collector in the absorber recirculation, dust collector can be mechanical, can also be a mechanical type pre precipitator electric precipitator before. Most of the solid particles in the flue gas are separated, through an intermediate ash bin return absorption tower. Because most of the particles are circulating many times, so the residence time of the absorber is very long, usually up to more than 30 minutes. The middle part of the ash ash bin according to the absorbent supply and removal efficiency, according to the proportion of discharged solid recirculation loop, sent to the ash bin to be transported. The process is shown in Figure 2.

From the recirculation of the flue gas discharged from the dust as it does not meet the emission standards, you need to install a dust collector. After the dust in the clean flue gas passes through the fan, chimney into the atmosphere.
The absorbent is Ca (OH) 2 powder, very fine particle size, below 10 m. Desulfurization absorbent, input fluidized bed absorption tower, and also injected a certain amount of water to improve the desulfurization efficiency. This can make water after the flue gas temperature and water dew point is very close, to achieve high desulfurization efficiency under various operating conditions.
2.2 CFB technology products
A by-product of the CFB process is a dry powder, its chemical composition and by-product of spray drying process is similar, mainly composed of fly ash, CaCO3, CaSO4 and unreacted Ca (OH) 2. The disposal of by-products and the spray drying process is basically the same. A by-product of CFB process water after curing, the yield strength is 15 ~ 18N/mm2, and the permeability of clay is similar, about 3 * 10-11, the compaction density is 1.28g/cm3, if further development, can become a good building materials industrial raw materials.
The typical desulfurization ash fly fly ash composition: about 60% to 70%; CaCO3 is 7% ~ 12%; Ca (OH) 2 is 2% to 4%; CaSO3 is 12% to 18%; CaSO4 is 2% ~ 5%; water <1%.
The process characteristics of 2.3 CFB
The characteristics of CFB-FGD process, which is different from traditional desulfurization technology has good application prospect in the desulfurization industry. The CFB-FGD process is simple, high reliability; high desulfurization efficiency; flue gas load changes, the system can still work properly; desulfurization by-product is a dry powder, not a large number of wastewater, to comprehensive utilization; basically does not exist like severe corrosion, wet absorption tower in the node scaling and clogging problems; some heavy metals can be removed, especially the removal of a portion of the mercury, further control of flue gas is very meaningful.
The prospect of 3 desulfurization technology
The desulfurization technology has been an important topic of concern in the work of environmental protection.
The mainstream of the desulfurization process will be widely used at home and abroad. Restricted by the technical conditions and economic costs, limestone gypsum wet, spray drying process is the preferred process for a variety of desulfurization requirements. The electronic beam method and the seawater desulfurization process for under experimental study or application of geographical restrictions, so the market share is limited, but in the local area will be developed. CFB-FGD will become one of the important means of incineration technology in flue gas desulfurization. This technology in foreign countries has been successfully commercialized, market prospects.