Research status of double circulation desulfurization gypsum single tower

2016-02-03 10:05:05

Author: Lv Hongyun Wang Peng Jiang Fan
Abstract

Gypsum method is currently the mostwidely used flue gas desulfurization (FGD) technology. Single tower with doublecirculation of the technology by gypsum method is gradually improved on thebasis of traditional gypsum technology, which overcome the disadvantage oftraditional gypsum method of single tower with single circulation— lowefficiency, high energy consumption, and reducing the content of SO₂in flue gas to meet the latest national emission standards. This paperintroduce the mechanism, development process, and research status of desulfurizationtechnology; meanwhile prospect the development and the trend of the technologyusing single tower with double circulation by gypsum method.

Keywords：gypsummethod；Singletower with double circulation; desulfurization; development process

preface
The problem of air pollution has been listed as the ten global environmental problems first, among them, SO2 pollution was listed as the first [1]. SO2 is a colorless gas with a pungent odor, is one of the main air pollutants,
Is the main cause of acid rain. At present, all countries in the world to the SO2 concentration in the atmosphere as a measure of an important indicator of the degree of atmospheric pollution. SO2 in the atmosphere mainly from sulfuric acid plant tail gas, sintering flue gas and flue gas. Due to coal and oil usually contain sulfur compounds, so the combustion generates sulfur dioxide. So, is still the largest source of pollution from coal-fired flue gas, especially in coal or heavy oil as fuel gas power plants and industrial boiler emissions as the emissions of SO2 accounted for about 50% of the country's total emissions of about [2]. Although the emission of SO2 in our country have been from a single concentration control to concentration, rate and total triple control, but according to the statistics of the state environmental protection departments, in 2013, the total discharge amount of SO2 is still as high as 2278.6 million T, including power industry emissions about 1124 million tons, ranking first in the world.
[2013]37, the State Council issued the "air pollution prevention action plan" clearly pointed out: first, to accelerate the desulfurization and dust removal desulfurization denitration renovation project construction in key industries. In 2013 release No. 14 "on the implementation of air pollutant emission value of the notice" clearly pointed out, is located in the "three zone 10 groups" in the thermal power generating units in the period of 45, P (SO2) must reach 50mg / Nm3 below the emission standards [3]. Therefore, the future power plant flue gas desulfurization efficiency more and more urgent, desulfurization technology has been developed can be divided into pre combustion desulfurization, furnace desulfurization and flue gas desulfurization (FGD) three. Flue gas desulfurization (FGD) can be divided into two major categories of dry desulphurization and wet desulphurization, due to the wet flue gas desulfurization technology has the following advantages: (1) absorber and wide range of application; (2) the fuel for a wide range; (3) the variation range of the fuel sulfur adaptability; (4) good adaptation to unit load change; (5) desulfurization efficiency is high; (6) sorbent utilization rate is high; (7) by-product of high purity. The wet flue gas desulfurization technology is one of the [4] SO2 emission control technology of coal used most widely and most efficient in the world. Which due to the limestone - gypsum wet flue gas desulfurization (FGD) technology of absorbent rich source, low cost, high efficiency, reliable operation (shutdown rate of less than 3%), wide scope of application, technology is mature, the by-product has the economic benefit and so on, so it has been gradually extended, become the most widely used in the desulfurization technology, occupies an important position in the market. Until the end of 2010, China's coal-fired power plant flue gas desulfurization capacity reached 565 million kilowatts, the use of limestone - gypsum wet desulphurization process accounted for 92%, the process has become the leading technology of desulfurization technology for thermal power plants in China [5]. In the process of limestone - gypsum wet flue gas desulfurization, single tower and double cycle technology under the precondition of ensuring the desulfurization efficiency, advantage in technical and economic aspects of the has greater application prospects.
1 limestone gypsum flue gas desulfurization process
1.1 wet limestone gypsum FGD principle
The working principle of limestone - gypsum wet desulphurization process is due to the absorption tower absorbent slurry through a circulating pump repeated contact with the flue gas and absorbent utilization rate is very high, relatively low calcium sulfur, desulfurization efficiency is greater than 95%[6]. The following chemical reactions occurring in the process of desulfurization:
1.SO2 + H2O - H2SO3 (absorption)
2.CaCO3 + H2SO3, CaSO3 + CO2 = H2O + (neutralization)
CaSO3 + H2SO3, Ca (HSO3) 2 (neutral)
3.Ca (HSO3) 2 + 1 /2 O2, CaSO4+H2SO3 (oxidation)
4.CaSO3 + 1 /2 H2O - CaSO3 - 1 /2H2O (crystal)
5.CaSO4 + 2H2O - CaSO4 - 2H2O (crystal)
1.2 typical limestone gypsum wet flue gas desulfurization process of single tower and single cycle.
A relatively simple typical limestone - gypsum with single tower and single cycle desulfurization technology is the limestone slurry made as the absorbent by pump sent to the absorption tower in the tower and a flue gas and at the lower part of the tower drum into the air from the full contact mixed, the SO2 in flue gas and air in the O2 and grout of CaCO3 for oxidation reaction of CaSO4, CaSO4 saturated, crystallization form a gypsum slurry. The absorbing tower discharge gypsum slurry by thickening, dewatering, so that the water content is less than 10%, and then sent to the gypsum storehouse stacking; desulfurization of tail gas after removal demister after removal of the droplets discharged into the atmosphere through the chimney, the process as shown in Figure 1.

 

 

 

 

                           

 

Figure 1 Typical desulfurization process flow diagram

1.3 typical limestone gypsum wet process of single tower double circulation flue gas desulfurization process.
At present, in order to achieve the purpose of efficient desulfurization technology, to improve the efficiency by increasing the number of spraying layer of single tower traditional single cycle, thereby increasing the height of absorption tower, poor economic performance of the method. Compared with the traditional method of Twin Towers series technology efficiency is also improved, but the investment in equipment, its application is also restricted. In order to solve the problems both in technology and economy, the development of a single tower double circulation technology. Single tower double circulation desulfurization system mainly includes: absorption tower, flue gas system, limestone slurry preparation system, process water system, gypsum dewatering system, waste water treatment system. With single tower and single cycle process and the Twin Towers series process, in addition to the absorption tower has the obvious difference, basically the same as other systems. The tower is the core of the desulfurization device, as shown in figure 2.

 

 

 

 

 

 

Figure 2.Double cycle single tower process Schematic diagram of gypsum absorption tower

The technique quite in flue gas by the two independent of SO2 removal process, flue gas from the absorption tower entered the bottom of, first of all with slurry circulating spraying device sprays reverse contact after cooling, washing to remove part of SO2, and then through the set liquid bucket of guide vanes into circulation zone, flue gas in here and on the circulation spraying device sprays liquid slurry reverse contact, after washing to remove residual SO2. Desulfurization of flue gas cleaning by removing the mist eliminator after removal of the droplets, by absorption tower above the chimney, desulfurization process is complete, SO2 was almost divisible.
Liquid collecting bucket will desulfurization is divided into two loops, a. Under the circle from the slurry pool, a circulation pump, a spray layer etc.; on the loop of a liquid collecting bucket, absorption zone feeding groove, two circulation pump, spraying layer. Circulating slurry tank, two stage cycle are respectively provided with independent spray layer, according to different functions, each cycle has different operation parameters.
Desulfurization cycle, the cycle of desulfurization efficiency general 30-70% and circulating slurry pH control in 4.6-5.3, slurry residence time in about 4 minutes, the main function of the loop is to ensure excellent calcium sulfite oxidation effect, and plenty of gypsum crystallization time, according to data display, in the acidic environment PH=4.5, the oxidation efficiency is the highest. Especially for high sulfur coal, this acidity and oxidation air coefficient can be greatly reduced, thereby greatly reducing the power consumption of the oxidation wind machine, but also can greatly enhance the quality of gypsum.
After the circulation of flue gas directly into the secondary cycle, the cycle to achieve the main desulfurization scrubbing process, because there is no need to consider the oxidation of crystal, so pH can control in very high level to be 5.8-6.4. This can greatly reduce the amount of circulating slurry, generally the amount of circulating slurry can be reduced by about 20%.
Status of single tower double circulation flue gas desulfurization gypsum 2
In order to solve the desulfurization process and meet the needs of the new situation, the major domestic and foreign desulfurization companies are actively introducing and developing a single tower double circulation desulfurization process.
German noell company developed the first generation of single tower double circulation desulfurization tower, the equipment has the advantages of small occupation area, high economy. It is mainly used in the higher sulfur content of coal, the desulfurization efficiency can reach above 97% and the SO2 concentration in the range of large flue gas have good adaptability, due to its technical and economic advantages, get the extensive attention.
State power Longyuan introduction German noell of single tower and double cycle technology, and this technique was first used in domestic project in Guangzhou Hengyun thermal power plant limited liability company [7], the project is original desulfurization tower based on the transformation of the design of the first domestic gypsum method using single tower double circulation desulfurization technology equipment. The project in the 168h during the test run, the desulfurization tower entrance of the concentration of SO2 in 1800~4200mg/m3, tower outlet SO2 concentration stabilized below 50mg/m3, and achieved a certain effect of saving efficiency. This project in the 75% and 100% load (flue gas flow) under the condition of desulfurization system performance test set of flue gas desulfurization equipment, desulfurization efficiency reached 99.3%, the demonstration effect on the desulfurization market.
Ideal technology Co., Ltd. in Hangzhou [8] in the traditional single tower and double circulation device was modified in the original equipment on the basis of setting the trench and deposition basin, circulation system and circulatory system in their trench and sedimentation tank connected to form a loop, each loop in a separate circulating water structure, this transformation ensures the desulfurization device has no fouling scaling, prolongs the service life of the single tower and double cycle system, up to 20 to 30 years.
Jiangsu Feng Industry Environmental Protection Science and Technology Group Co., Ltd. [9], in the original single tower and double circulation process based on multi desulfurization tower by first turn on a spray system, double loop clapboard, secondary spray system and desulphurization tower export were adjustable design improvement, in different position setting double loop ring spacer, the equipment according to different desulfurization conditions and requirements of the corresponding optimization. This design not only makes flue gas desulfurization to meet the power plant flue gas discharge standards requirements at the same time, could be solved according to the change of coal change absorption desulfurization tower operation where the energy saving of the system, but also can reduce the cost of operation, this transformation also realize the absorption tower system within the uniform gas distribution, and avoids the loss of the wall flow effect brought about by the flue gas by escaping the desulfurization efficiency.

Gao Xiang of the Zhejiang University, [10] et al., on the grout pond process equipment double cycle single tower gypsum method was modified to design a high efficient single tower double circulation system and method in the circulatory system, limestone slurry tank side were stored in the main tower and the tower of the slurry pool outside, two a slurry tank are connected through pipelines, the formation of double loop pH control system of multi zone, because of zoning control of two pH slurry tank, different slurry spray layer provided by the two slurry pool, high pH value of slurry is conducive to the absorption of SO2, the desulfurization efficiency of desulfurization system in more than 99%, low pH value is good for the crystallization of sulfite oxidation and gypsum, gypsum dehydration rate and improve the purity of gypsum. This design through the pH control improves the desulfurization efficiency.
Guangzhou Hengyun enterprises Wang Guoqiang [11] et al single tower double circulation liquid collecting bucket part of the transformation, in the original effusion bucket device design guide plate and its application in 300MW Coal-fired Boiler, and the operation of single tower and double loop in the pH value, slurry density and other important parameters are adjusted, making tower gas by liquid collecting bucket rectifier, air distribution uniformity, good gas-liquid contact, reducing the single cycle often encountered in the corner, tower space utilization rate is improved.
Chongqing CPI as far as [12] Environmental Protection Engineering Co., Ltd. Nie Hua et al. The segmentation for the concurrent flow and counter flow cooling tower, the downstream tower set flue gas inlet at the upper part of the desulfurizing tower, the tower counter flow setting outlet flue gas, slurry pond located in CIS, countercurrent tower below, the slurry pool above for clockwise and counter flow tower flue gas channel, in order to form a relatively independent double circulation desulfurization system in a desulfurization tower, this design not only guarantees the high desulfurization efficiency (99%), also reduces the amount of circulating slurry and system energy consumption, and reduces the height of the desulfurization tower, simplifies the design of double circulation desulphurization system.
Sichuan Dongfang Electric Group, Dongfang Boiler Co., Ltd. Yang Zhizhong, [13] et al., developed a cis countercurrent double circulation device, is different from the traditional single tower and double circulation device, absorption the lower part of the tower was slurry separator separated into two independent circulating pool, the spray system; absorption the upper part of the tower is flue gas baffle plate is divided into two independent flue. The gas passes through the gas path through the circulating pool in series. In the desulfurization process, control of two level cyclic system pH value, improve the oxidation and absorption rate of two and two SO2 of flue gas desulfurization by-product of the crystallization rate. This design improves the efficiency of desulfurization desulfurization gypsum double circulation method of single tower.
Shandong University of Dong Yong [14] et al of two towers and a single cycle liquid collecting bucket is reformed, on the basis of the original added the cascade structure. This structure is divided into a stage cascade and the secondary cascade, two stage cascade staggered formation overlooking the annular structure, cascade structure of root and the liquid collecting bucket is connected, the liquid collecting bucket is communicated through the grout reflux pipe and the desulfurization tower, the slurry pool. This design is simple, small resistance loss, gas-liquid flow field distribution uniformity, good contact effect, strengthen the gas-liquid mass transfer between gas and slurry, promote the absorption of SO2 and improve the efficiency of desulfurization. In addition, this device can make the liquid collecting device to reduce the overall height, circulating spray layer installation position is reduced, and the water slurry circulating pump pressure reducing system, reduce equipment investment, improve economy.
3 summary and Outlook
Overall, the gypsum method single tower double circulation desulfurization technology is a suitable to different kinds of coal flue gas desulfurization, desulfurization efficiency is high, equipment covers an area of small, economical value of by-product high cost-effective method. It overcome, the single tower and single cycle technology for liquid gas is relatively high, Jiang Chirong product, the oxidation wind machine high pressure shortcomings. Also overcome the twin towers series process due to equipment covers an area of large area, high resistance of system and high investment shortcomings. In the twin towers in series process based on transformation, under the premise to reduce the cost of the equipment and floor space, to ensure the quality of the desulfurization, desulfurization efficiency is improved. Will gypsum method single tower double circulation desulphurization technology application to 2000MW and above on SO2 emissions requirements higher coal-fired power plants or other sulfur containing tail gas desulfurization projects, especially desulfurization efficiency transformation projects, under the premise of low economic operation and meet the requirements of environmental protection, the green industry requirements and has high application prospect.

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