Discussion on the cause of scaling in wet desulfurization system of large coal-fired power plants

The flue gas desulfurization method of large coal-fired power plants is generally limestone/gypsum wet method. But many equipments of this wet desulfurization system during daily operation often cause scaling problems. At present, more literature think s this scale is caused by the mixing of desulfurized gypsum and fly ash in flue gas. However, this paper analyzes the scale composition and appearance of several power plants in China, which shows that the scale is not a simple mixture of gypsum and fly ash, and is a smelting reaction of fly ash in a complex environment and forms a hard scale of gypsum/limestone-fly ash gel. This kind of scale will endanger the safe and stable operation of the desulfurization system, specific operational and maintenance recommendations are proposed for reference.

2 removing device in the flue gas is washed, and the droplets carried in the flue gas are remov ed while passing through the mist eliminator at the top of the absorption tower. The limeston e slurry is prepared in a preparation system, stored in a limestone slurry tank, and sent to the absorption tower through a limestone supply pump. The limestone slurry in the tower is sent to the spray layer through a circulation pump, and the slurry sprayed from the nozzle is for med under a certain pressure. The misty droplets are in gas-liquid contact with the flue gas t o form calcium sulfite. The oxidation fan sends air into the slurry tank at the bottom of the absorption tower to further react calcium sulfite to form calcium sulfate (gypsum), and then th e gypsum slurry is pumped to the gypsum cyclone station through gypsum transfer for prelimi nary solid-liquid separation, and then flows to the vacuum belt. The dewatering machine perfo rms vacuum dehydration to make the gypsum water content less than 10%, and then enters th e gypsum storage bin and is transported by car. Through the absorption tower, most of the so ot, sulfur dioxide of the flue gas is removed.Finally,the treated flue gas is discharged through the chimney.

3.Discussion on the cause of scaling
According to the operation of limestone/gypsum wet desulfurization equipment in recent years, the fouling phenomenon of absorption towers, GGH, demisters and other equipment is quite common. The scaling of some coal-fired power plants is very serious, which seriously affects the desulfurization equipment and even The host is running safely. The scale of the desulfuriz ation system is morphologically divided into soft scale, crystal scale and hard scale. The soft scale is soft and easy to be washed by water. Most of them are simple accumulation of gyps um, which is easy to appear in the mist eliminator, the bottom of the absorption tower and th e flue. The crystal scale is hard, and it is also mixed with powdery substance. There are tran slucent crystals on the surface and inside, which appear below the surface of the slurry on th e inner wall of the absorption tower. It is not easy to appear around the stirrer. Although the crystal scale is hard, the structure is loose and easy to peel off. The hard scale is hard and dense, and cannot be washed away by water. The hard scale is easy to appear on the desulfu rization GGH, the absorption tower wall and the tower beam. At present, there are many repo rts on scaling problems in desulfurization systems. The common view of scaling is the crystal lized matter of gypsum and the product of the mixing of gypsum and smoke in the flue gas [1][2].
In the analysis of the hard scale in the desulfurization absorption tower of Maoming Thermal power plant, the main components of the hard scale were found as follows: 12.8 The scale is dense and hard, the compressive strength is large, it is not easy to be broken, and it is easy to block the absorption tower nozzle. See Figure 2 for detail. Through the data of the scale sample, it is found that the main components in the scale sa mple are smoke, gypsum and the like. The amount of gypsum in the scale sample has a cert ain amount, it is not the main component, but the content of the main components of the fly ash, Al2O3, SiO2 and Fe2O3, is Very high, from the perspective of the hardness and density of the scale, these mixtures are not simply mixed together. Kang An [3] found that there were new crystals in hard and soft scales when studying the hard and soft scales of Dingzhou Power Plant and Zhangzhou Power Plant. This also indicat es that the gypsum and fly ash in the scale are not simply mixed. together. In addition, the h ard scales of Shaoguan and Huangpu Power Plants are analyzed in Table 3.  The scale composition of the wet desulfurization system of five power plants in Table 1 to Table 3, the content of SiO2, Al2O3 and Fe2O3 in hard scale is very high, while the conte nt of SO3 representing gypsum content is relatively small, the highest is 18.2%, the smallest Only 5.7%, the content of SO3 in the soft scale of Dingzhou Power Plant in Table 2 is as h igh as 30%, which indicates that the formation of soft scale is mainly due to the existence of a large amount of desulfurization gypsum, which is mainly calcium sulfate dihydrate crystal, gypsum Low strength and poor water resistance [4], when there is a large amount of gypsum in the scale, it is easy to be washed away by water. Fly ash is dominant in hard scale. In terms of composition, the fly ash belongs to the Ca O-SiO2-Al2O3 series, which has potential chemical activity, that is, the fly ash alone does not have hydraulic activity when mixed with water. When the CaO content is high, the fly ash meets water. Can be hardened alone [5], when the CaO content reaches 20%, the pure fly ash is hydrated after adding water to form hydrated calcium silicate, hydrated calcium aluminate or ettringite [5]. The fly ash in the hard scale of the wet desulfurization tower accounts for a large proportion. The main components are SiO2, CaO٠SiO2٠H2O, Al2O3, Fe2O3, CaCO3, e tc., and a small amount of Ca(OH)2 [6].
The GGH blockage occurred in Unit 5 of BeiLun Power Plant Phase II. The composition analysis of fly ash and scale samples showed that the content of CaO in fly ash was 5.13%, the fly ash content in hard scale accounted for the majority, and the gypsum content was 9-1 6% [7].
When the boiler flue gas of a thermal power plant passes through the GGH of the desulfu rization system, the absorption tower, etc., the temperature of the flue gas is lowered so that SO2, SO3, HCl and HF in the flue gas are condensed on the fly ash, and these acids can re 5 act with the fly ash. The reaction of H2SO4 with fly ash has the best effect on the conversio n of Fe. Under the environment of 50-150°C, there will be crystals on the surface of fly ash. The higher the temperature, the more complex the crystals are, making the fly ash adhere to each other [8]. Even if the mass content of Ca is 0.4%, Ca ions are more likely to be preci pitated after treatment with HCl. After the strong acid-treated fly ash, many channels and a s mall number of depressions appear on the surface [9]. The compressive strength of the cement of H2SO4 pretreated fly ash has been significantly improved. The higher the temperature, the higher the compressive strength when the temperature is 30-90°C [10]. In the hard scale of t he desulfurization equipment, higher levels of CaSiF6٠2H2O, KFe3(SO4)2(OH)6, NaMgAlF6.H 2O, Fe2SiO4 and other hydrates were found [11], and when the calcium oxide content was fo und to be small, It also forms hard scale [12]. Fluoride and sulfate ions are present in the hyd rate,which indicates that strong acid substances such as HF and SO3 in the flue gas adhere to and react with the fly ash, destroying the fly ash. The vitreous surface of the surface release s active substances such as Fe2O3, alkali metal oxides and alkaline earth metal oxides in the fly ash, and hydrates in response to water.
During the process of desulfurization absorption tower slurry, many substances in the flue gas, such as fly ash, are absorbed. These substances are continuously mixed and reacted, so t hat the surface tension of the slurry becomes smaller and smaller, and the slurry is more likel y to adhere to the tower wall of the absorption tower. On the top of the tower beam, etc., th e gypsum and fly ash in the slurry are deposited on the tower wall beam and continuously a ggregate and aggregate. In the acidic solution, Al2O3 reacts with the acidic substance, and the surface of the fly ash forms a colloid such as Al(OH)3. These colloids have strong flocculati on effect, So that the gypsum and limestone with small particle size in the slurry are adsorbe d on the surface of the fly ash to form crystal nuclei and continuously crystallize. In addition, in the acidic environment, the fly ash also undergoes hydration reaction. Under the higher te mperature of desulfurization environment (>50°C), the micro-grained gypsum and limestone are filled with voids, so that the fly ash-gypsum system forms a hard and compact gel on the t ower wall and the tower beam.
The content of chloride ion in the desulfurization absorption tower is high, which also affe cts the fouling. 3CaO٠Al2O3٠CaCl2٠10H2O is formed in the fly ash-lime system with CaCl2, and the excitation effect of CaCl2 on low calcium fly ash is better than that. High calcium fl y ash is evident [13].
Early studies have suggested that limestone powder is an inert material that is not active a nd primarily has a filling effect in the gel. However, some studies have suggested that limesto ne powder not only has a filling effect, but its fineness has a certain influence on the hydrati on of silicate materials. The finer the fineness, the stronger the effect, and it will react with hydration components to form carbon aluminate. Calcium enhances the early strength of silicat e materials [14].During the desulfurization operation, the limestone powder is continuously adde d to the slurry of the absorption tower, and the particle size is small, and the average particle size is 40μm, which creates conditions for the scale in the absorption tower, so that the earl y strength of the scale is enhanced, and it is not easy to be broken. The scale continues to i ncrease.
Limestone/gypsum desulfurization of coal-fired power plants In addition to soft and crystalli ne scales, there are some hard scales. This is mainly because in the strong acid environment such as H2SO4, HCl, HF in the flue gas, the glass body on the surface of the fly ash under goes structural changes, and the active silicon oxide, alumina, CaO in the fly ash reacts with water,which causes the fly ash to react with various impurities such as gypsum and limestone of fine particle size to form a dense, hard gel which is not easily broken.This kind of hard scaling brings great harm to the operation of the desulfurization equipment. Because the gel a ccumulates thicker on the tower wall and the tower beam, it will eventually rupture and fall off, and the running agitator collides with the debris. The agitator is damaged, and the debris 6 is sucked into the slurry circulation pump, colliding with the impeller, causing it to break, a nd clogging the nozzle of the spray layer, causing the entire spray layer to become clogged a nd unable to operate.

4.Suggestion
The following measures should be taken for reducing the scaling of limestone/gypsum desulfur ization systems in coal-fired power plants: 1.The sulfur content of the coal into the furnace should be reduced. The combustion of hig h-sulfur coal increases the concentration of sulfur trioxide in the flue gas and reduces the dew point temperature. The fly ash easily reacts with acidic substances. 2.Improve the dust removal efficiency of the dust collector and reduce the dust concentratio n as much as possible. Since the fine particle size fly ash is not easily caught, and the small er the particle size, the greater the activity of the fly ash, the fly ash accumulates in the desu lfurization absorption tower and deposits on the tower wall and the tower beam to form hard scale.
3.Discharge the desulfurization wastewater in time. The density of fly ash is relatively sm all compared to gypsum and limestone. After the absorption tower slurry passes through the g ypsum cyclone station, all the lighter materials return to the absorption tower. The fly ash wil l accumulate in the absorption tower, causing the slurry to foam and form a hard. dirt. Theref ore, the desulfurization wastewater should be discharged in time to discharge the fly ash.
4.Control the concentration of chloride ions in the absorption tower. The high concentration of chloride ions not only causes corrosion of the equipment, but also makes it easier to hydr ate the fly ash.