In recent years, the limestone - gypsum wet flue gas desulfurization technology of thermal power plants has become more and more mature and widely used. The unit capacity of the application is about 90% of the total capacity of the desulfurization installed capacity of the power station, and the applied unit capacity has reached 1300MW . However, due to the many limestone-gypsum wet flue gas desulfurization ( FGD ) subsystems, the FGD system includes a limestone slurry preparation system, a flue gas system, and a flapper door sealing air system. SO2 absorption system, gypsum slurry dewatering system, emptying system, compressed air system, process water system. In the system operation, equipment failure often occurs and affects the operational reliability of the FGD . The factors affecting the reliability of FGD system mainly include design conditions, chemical process, mechanical equipment and operation management. Based on this, relevant prevention and control measures are proposed to improve the reliability of FGD system operation.
The FGD reliability is expressed as the ratio of the difference between the load hours and the forced load operation hours of the device operation hour deduction device and the actual required device operation hours.
I. Factors affecting the reliability of the FGD system
(1) The influence of actual conditions of desulfurization system on the reliable operation of FGD system
1. Influence of flue gas characteristics on the reliability of desulfurization system
Brown coal combustion flue gas temperature of the flue gas temperature is usually generated by burning bituminous coal higher than 20-25%, and the moisture content of lignite combustion flue gas is relatively high, corrosion of the flue gas to create favorable conditions, therefore the desulfurization tower The requirements for lining anti-corrosion materials are also high.
2. Influence of burning coal quality on the reliability of desulfurization system
The concentration of sulfur dioxide in the flue gas produced by the unit using high-sulfur coal is relatively high, and some exceed the desulfurization design value. The high sulfur flue gas is relatively corrosive, and because of the large flow rate, high desulfurization efficiency, and a large amount of desulfurized solid product, it is necessary to increase the equipment capacity, and at the same time bring certain difficulties to the solid by-product (desulfurization gypsum). The required chemical process parameters are relatively high, and any design errors or improper equipment capacity and type selection will affect the reliability of the desulfurization system. If the forced oxidation device is not sufficiently oxidized due to improper design, not only will fouling occur, but also the desulfurization efficiency and gypsum purity will be affected, and the output of the desulfurization device will be reduced.
The high ash content and chloride content of coal will increase the fly ash content and HCL content in the flue gas, and these substances will eventually enter the circulating absorption slurry. Fly ash will increase the abrasiveness of the slurry and reduce the service life of the equipment. The complex formed by AL3 + and F- in the slurry will affect the activity of limestone. The CL- content in the slurry will not only increase the slurry. Corrosive, affecting the quality of gypsum and material selection, and affecting the solubility of limestone, thus affecting the efficiency of desulfurization.
(II) Influence of process factors on the reliability of FGD system
1. Effect of oxidation degree of calcium sulfite on the reliability of desulfurization system
The degree of oxidation of calcium sulfite is an important control parameter for limestone wet desulfurization process. Insufficient oxidation of calcium sulfite will form a large amount of highly adherent calcium sulphite / calcium sulfate hard scale in the internal surface of the absorption tower, affecting the desulfurization system. Performance and longevity.
For low-sulfur coal FGD system, it can achieve better oxidation degree. For high-sulfur coal and FGD system with large flue gas volume, the design of oxidation equipment is often unreasonable, such as large diameter of reaction tank and uneven distribution of oxidizing air. . Or the equipment arrangement in the reaction tank area is unreasonable and other factors make the oxidation insufficient. In this case, a large amount of scale will still occur, and the blockage of the nozzle, the blockage of the small-diameter pipe or the scaling will reduce the flow area of ​​the pipe. This will lead to frequent failures of the desulfurization system, accident shutdown or output. In addition, insufficient oxidation will affect system performance such as desulfurization efficiency, limestone utilization, and gypsum quality.
2 , the effect of defogger flushing water on the reliability of desulfurization system
Calcium sulfite / calcium sulfate hard scale plugging the defogger caused a drop in the availability of the FGD system. In the system of flushing the defogger with desulfurization and recovery water, special attention must be paid to ensure that the relative saturation of the rinsing water is less than 50% , in order to avoid fouling and clogging of the defogger blade due to the quality of the rinsing water, and finally forcing the desulfurization device. Out of service.
The absence of a portion of the industrial water in the recovered water is generally a method for controlling the relative saturation of calcium sulfate in the recovered water. Even if the quality of the rinse water is good, the surface of the demister blade cannot be completely cleaned. The well-designed demister rinsing range, rinsing duration and rinsing frequency are the key to keeping the slab clean and avoiding clogging. In terms of maintaining the cleanliness and workability of the demister, it is more important to keep the surface of the demister blade moister than the online high-pressure water rinsing during operation. Therefore, low hydration and long rinsing time are used to maintain defogging. Blade cleaning is a more effective measure.
When the fouling of the defogger blade is not complete and the sludge is deposited in a large amount, the system pressure will be obviously improved. Therefore, by detecting the change of the system pressure, it is helpful to grasp the operating state of the defogging, and find the problem and deal with it in time. .
In addition, in the operation management, the flushing water pressure, flow rate, and timing check whether the flushing valve is opened and closed in the order of program control, and avoiding excessive flue gas flow is also an important measure to prevent clogging of the defogger. In recent years, some FGD commissioning systems have been discovered. The operating personnel do not understand or pay attention to the rinsing action of the defogger. Some power plants have not even flushed the defogger for a long time, causing the defogger to block and collapse. On the one hand, due to the poor running effect of the precipitator, the high concentration of flue gas enters the desulfurization system, causing a large amount of soot to accumulate on the demisters. The normal defogger flushing has not achieved the cleaning effect, and it also exceeds the defogging. Cleaning range. With the accumulation of a large amount of soot, eventually the demister blades collapse and the lower support of the demister is damaged.
For the wet desulfurization system without GGH installed, since the flue gas directly enters the chimney after passing through the mist eliminator, the defogger operates poorly and has certain harm to the downstream equipment. Since the exhaust gas temperature is not low, the smoke diffusion ability is weak, which will directly cause the gypsum slurry droplets carried by the flue gas to land near the chimney, forming a so-called "gypsum rain" phenomenon. When the temperature is low, white chimney will appear.
3. Effect of slurry chloride concentration on the reliability of desulfurization system
The slurry mainly cause corrosion damage to the material is a metal chloride concentration, pH and temperature, wherein the chloride concentration in the broadest range, difficult to select the metallic material caused by corrosion, due to the material failure becomes the FGD system One of the main reasons for utilization. With the improvement of the understanding of the corrosion characteristics and material properties of the slurry in this environment, the new FGD system has basically eliminated the system due to the CL- concentration of the slurry by selecting appropriate structural materials and strictly controlling the lining and welding quality during the installation process. Availability.
(three) Â Mechanical equipment (4) Impact on system reliability
1. Influence of flue gas system equipment on system reliability
( 1 ) booster fan
The booster fan is mainly used to overcome the resistance of the absorption tower and related equipment, and is arranged on the swim side of the FGD system. The booster fan must meet the normal and stable operation of the desulfurization tower.
( 2 ) flue
The working environment for conveying high-temperature flue gas and low-temperature flue gas is very different. The inlet flue for conveying high-temperature flue gas has no special technical requirements for the reliability of the FGD system. However, the flue conveying low-temperature flue gas and the flue conveying low-temperature wet flue gas after desulfurization should be selected according to the corrosive environment in which they are suitable, and the appropriate hydrophobic discharge facilities should be designed. In particular, the flue at the dry and wet junction near the inlet of the absorption tower is in a harsh corrosive environment characterized by high temperatures, alternating dry / wet, deposits on the flue surface and high concentrations of chloride.
The FGD system is now equipped with a bypass flue to reduce the impact of the FGD system on the reliability of the generator set. At present, the FGD system is a furnace, a tower, or a two-furnace and a tower, and both use rubber and resin lining as the main anti-corrosion material. When the unit starts or the boiler combustion is unstable, it is not allowed to put into operation the FGD system. In order to ensure safe and stable operation of the unit, a bypass flue must also be provided.
The flue gas produced by coal combustion is acidic. Since the temperature of the flue gas after desulfurization is much lower than the rated discharge temperature of the boiler, it is generally below the acid dew point and has different degrees of corrosion on the inner wall of the chimney. Therefore, the appropriate anticorrosive material for the inner wall of the chimney is selected. It is very important.
( 2 ) baffle
The baffle of the FGD system is mainly used for isolation. When the unit is in operation and the FGD is forced to stop, the flue gas bypass baffle is opened to allow the flue gas to be directly discharged to the chimney to ensure safe operation of the unit. Therefore, ensuring the flexibility and reliability of the flue gas bypass baffle switch is essential to ensure the safe and stable operation of the FGD system and the unit.
The failure of the baffle is that the blade and the baffle frame are stuck, or the blade shaft and the bearing are corroded and the action is unsuccessful; the dust at the bottom of the flue makes the baffle door switch not in place; the flue is deformed, so that the baffle switch is not in place, affecting Strictness; the flapper door displacement during operation, the limit switch action, and the flapper door closing signal cause the system to stop. The flue gas bypass baffle is more likely to be rejected due to long-term non-operation.
( 4 ) slurry circulation pump
The slurry circulation pump is the key equipment in the FGD system. The corrosion of the gypsum solid phase in the transport medium and the high content of CL- is very high for the service life, operation efficiency, safety and reliability of the pump and the convenience of maintenance. The absorption tower circulates the slurry solid-liquid two-phase medium, about 20% of the limestone and gypsum solid in the slurry, and the acidic mixture of sulfuric acid, hydrochloric acid and the like formed during the circulation of the flue gas, the high-speed flowing and complex medium-to-slurry The material used in the pump poses demanding requirements. The corrosion resistance and wear resistance of the slurry circulating pump overcurrent components are important indicators for determining the service life of the pump.
At present, during the operation of the FGD system, the probability of failure of the slurry circulation pump is relatively high, and the imported slurry circulation pump is better than the domestic one. The tops of the blades are serrated and grooved, reaching the deepest point.
The reason for this phenomenon is that the dust collector treatment effect does not meet the design requirements, and the high concentration of soot entrained in the flue gas enters the circulating slurry, which increases the brushing and abrasion of the slurry circulation pump blade. In addition, due to the indication or non-display of the desulfurization system pH meter, the desulfurization system does not rule out that it is sometimes operated under strong acidic conditions. The high concentration of soot entrained in the slurry causes the blades of the slurry circulation pump to be very severe from the brush and wear, forcing desulfurization. The system is out of service, this problem must be paid attention to by relevant managers, and relevant measures are formulated to improve the reliability of FGD operation.
( 5 ) slurry pipeline
The limestone slurry pipelines are all rubber-lined pipes, and their main purpose is to prevent corrosion of the slurry conveying pipeline by CL- , sulfuric acid, sulfurous acid, F-, etc. in the slurry. The main reason for the phenomenon that the pipeline lining of the desulfurization system falls off is that the quality of the pipeline anti-corrosion construction is not related to the cold wall effect of the pipeline. . The temperature formed by the temperature difference between the surface of the rubber layer and the outer side of the coating wall accelerates the formation of the rubber foam, which is also called the "cold wall effect".
The pipeline lining layer belongs to the semi-permeable protective layer. One of the reasons for the failure of these protective layers is that the moisture penetrates into the lining layer and the pipe matrix to cause the aging layer to fall off. In addition, the quality of the pipeline anti-corrosion construction is also caused by the pipeline lining. A major cause of gel loss. Any negligence in the construction may cause the lining of the pipeline to fall off, such as the treatment of the base sand eye can not meet the requirements, sandblasting and rust removal is not complete. Since the pipe lining is broken, it must be treated to ensure the reliability of the FGD operation.
(4) absorption tower equipment
(5) Impact on system reliability
At present, the FGD system is basically a furnace, a tower or a two furnace and a tower. With the advancement of technology, the multi-furnace and one tower has become the development direction of the third-generation FGD system. Therefore, the reliability of the absorption tower equipment is proposed. High requirements.
The main faults of the absorption tower equipment are damage to the inner liner, leakage of the perforation of the tower wall; blockage and wear of the nozzle; blockage of the fixed pipe network oxidized cloth; if the gas pipe fixing parts loosen or fall off, the gas pipe will be shaken or even broken; the absorption tower slurry circulation Pump overcurrent wear; liquid column nozzle wear through and demister clogging.
(6) Absorbent slurry equipment
1. Impact on system reliability
FGD system limestone slurry preparation Some ball mills are used. Although the wet grinding preparation absorbent slurry has good economic benefits, the ball mill is also a familiar equipment of the power plant. However, the wet grinding pulping system is still a high failure rate of the FGD system. The noise is relatively large, and the wear is serious, which brings certain hidden dangers to the stable operation of FGD .
2 , the impact of solid by-product processing equipment on system reliability
The FGD solid by-product (desulfurization gypsum) treatment equipment consists of a first-stage dewatering equipment and a second-stage dewatering equipment.
The first-stage dewatering equipment can generally use a hydrocyclone. Although the primary dewatering equipment can be interrupted for several hours without restricting the operation of the FGD system, usually the equipment must work continuously for 24 hours . Hydrocyclones are widely used in FGD systems due to their low investment cost, low footprint, high concentration ratio and high reliability. This device has no moving parts and the main fault is the wear of the cyclone liner. In addition, fouling may occur in the slurry distribution tank and the overflow slurry tank of the cyclone separator and the piping connected thereto.
The secondary dewatering equipment usually adopts the vacuum belt dewatering machine. The vacuum belt dewatering machine often has the defects that the belt and the belt are biased. The dewatering performance of the solid by-products is deteriorated or the water content of the filter cake is high, and the lower hopper is blocked. When it is broken, it will cause the solid content of the filtrate to increase or even block the filtrate receiving tank; when the dehydration performance of the gypsum is poor or the belt scraper is not well adjusted, too much gypsum will adhere to the cloth belt, causing the filter cake to rinse. Tanks, pumps, and pipes that are closed first.
The three-stage dewatering equipment is a facility for stabilizing and immobilizing solid by-products. The current FGD system does not have such solid waste treatment equipment. A few of the FGD systems using low-coal units, due to the low daily output of desulfurization gypsum or due to the geographical location is not allowed to be disposed of, all the gypsum after desulfurization is dehydrated and directly exported. Quite a few FGD systems can only recover part of the desulfurized gypsum due to the lack of desulfurization gypsum market, and some or most of the gypsum is discharged to the ash storage yard. However, the landfill pit must be treated with anti-seepage treatment. If it is not treated with anti-seepage treatment, it will inevitably cause secondary pollution. These non-standard and non-compliant environmental protection policies will be prohibited by increasingly strict environmental protection regulations. Therefore, the power plant should take a long-term view and actively explore the market for desulfurization and gypsum recycling. Consider the environmentally-friendly desulfurization waste residue treatment method as soon as possible, or leave the site and add appropriate treatment equipment at an appropriate time.
(VII) Impact of operation management on the reliability of FGD system
Due to the large number of FGD subsystems and complex equipment, operators are required to be proficient in the performance and operation methods of each device to improve the ability to handle emergencies. If the operating personnel cannot strictly enforce the operating procedures, it will definitely affect the reliability of the FGD operation.
Second, prevention and treatment
(1) Prevention and control measures for design conditions
1. For the FGD system with high-sulfur coal, pay special attention to the ratio of liquid to gas, the uniformity of smoke distribution in the absorption tower, the volume of the reaction tank (the residence time of the slurry solids), the selection of the oxidation unit, and the capacity of the oxidation fan. The prevention and control suppliers have unilaterally pursued economic benefits, and selected design parameters with smaller or even lower margins, leaving hidden dangers for the safe and stable operation of the FGD system in the future.
2. All equipment (such as absorber nozzles, pumps, etc.) that are prone to wear, wear, or failure of the desulfurization system and therefore affect the performance of the unit are designed for easy replacement, service, and maintenance. For the heavily worn parts of the FGD system, such as absorber nozzles, mist eliminators, demister flushing systems, absorption tower mixing equipment, all hydraulic swirler sets that contact the slurry are designed with imported equipment.
The main components of the desulfurization device are selected from foreign imported equipment. First, the desulfurization system can ensure efficient and stable operation. Second, the service life of the desulfurization device is prolonged and the reliability of the FGD system is improved. Other equipments are made of domestically produced equipment. Under the premise of meeting the process requirements of the desulfurization system, some of the equipment is localized, which reduces the construction cost to a certain extent.
3. The FGD system adopts the microprocessor-based advanced distributed control system DCS , and has a complete protection system to ensure automatic safe shutdown or manual shutdown under critical conditions.
4. Strictly follow the quality control procedures of each link as stipulated in the quality management system to ensure the quality of construction and installation projects.
(two) Â Prevention and control measures of chemical process factors
1. Increase the degree of oxidation of calcium sulfite
The absorption tower is operated by an empty tower, and a forced oxidation process is adopted to eliminate the threat of scale formation in the tower to the stable operation of the desulfurization device. A well-designed FGD system should have a forced oxidation level close to 100% .
2 , improve the operation of the defogger
( 1 ) Strengthen the monitoring of the pressure difference of the defogger, reasonably control the flushing water volume and flushing frequency of the defogger, improve the water quality of the flushing water, avoid the fouling of the defogger and operate the FGD system.
( 2 ) The operation of the flue gas bypass baffle should be avoided or partially opened to avoid the increase of the flue gas flow rate of the absorption tower caused by the change of FGD flue gas caused by flue gas recirculation, which affects the operation effect of the FGD system.
( 3 ) To avoid excessive or too small amount of flushing water in the primary defogger, the frequency of flushing after the secondary defogger and the lower the water volume, the better.
( 4 ) Ensure the treatment effect of the dust collector and reduce the flushing load of the defogging system defogger. It also reduces equipment wear.
(3) Countermeasures against machinery and equipment
1. Improve the anti-corrosion quality of the flue lining
According to the environmental conditions of the high temperature flue and the low temperature flue, choose reasonable anti-corrosion materials, improve the anti-corrosion quality of the flue lining, and prevent the flue from being corroded for a long time and affect the safe operation of FGD . According to the actual situation, the FRP material with high temperature resistance is selected, and the anti-corrosion material such as high-temperature scale resin coating and heat-resistant modified polyurea is used for anticorrosion of the inner wall of the flue to achieve anti-corrosion purposes.
2 , regular operation of the flue bypass baffle switch test
The bypass baffle requires the FGD system to block the original flue gas from leaking to the net flue gas side during normal operation. When the FGD system fails, the bypass baffle can be opened in time and quickly to ensure that the normal operation of the generator set is not affected.
The main reason for the flue gas bypass baffle is that the material is improper and the car is caused by corrosion. Therefore, the baffle in a low-temperature corrosive environment should not only use corrosion-resistant alloys for the blades, shafts and seals, but also the same corrosion-resistant alloy as the structural materials. The shaft seal of the blade shaft should be designed to prevent solid particles such as corrosive gases and smoke from entering the bearing. To ensure the reliability of the bypass baffle operation, the blades can be divided into 2-3 groups and operated by separate operating mechanisms to reduce the risk of failure to open during an accident. When the boiler is under low negative pressure, try to turn the flapper door to avoid long-term non-operation and transfer to the component to be stuck. It is best to start the bypass door once a week, such as weekly or daily. If the baffle adopts an electric operating mechanism, the operating power supply should be absolutely reliable; if a pneumatic operating mechanism is used, a certain capacity of the gas tank should be set on the spot, and the bypass baffle can be quickly opened even when the air compressor stops supplying air.
3 , Â Absorption tower equipment
4 , prevention and control measures
Reasonable choice of corrosion-resistant, wear-resistant materials, with appropriate spare capacity. A well-designed defogger ensures efficient operation of the defogger.
To ensure the reliability of the slurry circulation pump, an alloy pump is recommended. The alloy pump has a simple structure, a long service life and a small maintenance amount.
5 , absorbent slurry equipment
6 , prevention and control measures
From the point of view of FGD system management, in the case of reliable limestone powder supply, it is better to choose the purchased limestone powder as the first is to reduce the ball mill wet grinding equipment and the second is to reduce the equipment noise.
7 , solid by-product processing equipment
8 , prevention and control measures
For primary dewatering equipment, polyurethane resin lining is usually used to reduce equipment wear and corrosion. The feed valve of the cyclone separator must be made of corrosion-resistant and wear-resistant materials to reduce or reduce the probability of failure of the valve core corrosion, wear or rust, and ensure safe and stable operation of the equipment.
Strictly implement the operation rules of the secondary dewatering equipment, regularly inspect the equipment, and regularly maintain the belt and the belt to ensure that the secondary dewatering equipment operates under the design conditions. If possible, remove the filter cake rinse tank and switch to the filter cake with industrial water.
(IV) Prevention and control measures for FGD system operation management
1. Strengthen technical training for operational personnel, strictly implement operational procedures and related systems, and give full play to the functional role of the quality management system.
2. Strengthen the maintenance of the desulfurization facilities, reduce the wear, scale and corrosion of the equipment as much as possible, and prevent the desulfurization facilities from being shut down.
3. Strengthen the maintenance of the online emission detection system for flue gas emissions, and regularly perform quantitative testing of equipment to ensure the reliability and stability of the online monitoring system for flue gas emissions.
4. Strengthen the management of desulfurization gypsum of desulfurization by-products, and do an event treatment plan for desulfurization by-product desulfurization gypsum that cannot be transported in time.
5. Do a good job in the management of spare parts for dust removal and desulfurization facilities to ensure timely disposal of facilities.
6. Exchange of experience and interoperability between the same industry to improve the reliability of FGD system operation.
Third, the conclusion
The limestone-gypsum wet flue gas desulfurization process is a mature and efficient flue gas desulfurization process, which can greatly reduce the sulfur dioxide emissions of thermal power plants, and has certain environmental, economic and social benefits. However, at the same time, the FGD system is relatively large and there are many subsystems. In order to ensure the safe and stable operation of FGD , it should focus on protection from several aspects such as design conditions, chemical process, machine equipment and operation management, and improve the reliability of FGD system.
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