Optimization analysis of wet flue gas desulfurizat

2022-08-24
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Optimization analysis of wet flue gas desulfurization process in coal-fired power plants

in view of the common problems of wet flue gas desulfurization system in coal-fired power plants that have been put into operation, combined with relevant regulations, this paper analyzes the problems that should be paid attention to in the decision-making and process selection of flue gas desulfurization projects in thermal power plants from the perspective of technical reliability and economy, and puts forward suggestions to optimize the flue gas desulfurization process

wet flue gas desulfurization is a complex chemical and physical reaction process, including sulfur dioxide absorption, limestone dissolution, gypsum crystallization and other stages. Reactants, temperature, pH, residence time and other conditions affect the reaction. The adjustment of desulfurization chemical reaction process is to optimize and control these reaction conditions

wet flue gas desulfurization (FGD) system

1 ensure the quality of reaction raw materials

in addition to the raw flue gas, the substances involved in the desulfurization reaction also include desulfurizer limestone and process water, which directly affect the reaction or cooperate with other substances

the characteristics of desulfurizer limestone are mainly reflected in two aspects: particle size and reaction activity. Generally, the fineness of limestone powder requires more than 90% to pass 250 mesh. At the initial stage of operation of the mill in a power plant, the fineness of limestone powder often failed to meet this requirement, resulting in low utilization rate of limestone, and the CaCO3 content in gypsum is often more than 3%. After the operation adjustment of the mill, the fineness has been improved, the ability to regulate the pH of the slurry has been enhanced, and the CaCO3 content in gypsum has gradually become normal

limestone activity is an indicator that is easy to be ignored. Measured by the reaction rate, that is, when the pH is 5.5, the time when the limestone conversion fraction reaches 80%, the shorter the time, the better the reaction. According to the actual test results in recent years, when the reaction rate exceeds 20000s, the dissolution of ca2+ in limestone will be affected, which will lead to the decline of limestone utilization. Through the follow-up analysis of the limestone quality, we found that when the limestone activity was poor, we informed the power plant to replace the limestone raw materials in time to ensure that qualified limestone powder participated in the desulfurization reaction

desulfurization process water is evaporated and concentrated after entering the absorption tower. High concentration of inorganic ions will affect the dissolution of limestone and desulfurization reaction, so the water quality of desulfurization process must be strictly controlled, especially conductivity, COD, SS and other indicators

in order to save water consumption, a power plant carries out waste water recycling, and replenishes the domestic sewage treated by the power plant to the desulfurization process pool. After a series of laboratory static and dynamic tests, the conductivity of the treated domestic sewage is required to be less than 500us/cm, and the water volume is less than 800m3/d. Another power plant will mix the treated slag water with raw water as desulfurization process water. It is required that the effluent ca2+ concentration of the slag water treatment system be controlled within 700mg/l, cl- 1200mg/l, and turbidity 20ntu, so as not to have a bad impact on the composition of the slurry of the absorption tower, the automatic control of pH, and the quality of lapis lazuli

2 reasonably control the pH of side paddle liquid

the pH control of absorption tower slurry is the core of limestone gypsum wet desulfurization reaction, which is affected by unit load, SO2 concentration of raw flue gas, desulfurization efficiency control value, limestone quality and other conditions

within a certain range, the higher the pH value, the more conducive to SO2 absorption and improve the desulfurization rate. However, when the pH is greater than 5.8, the dissolution rate of ca2+ in limestone slows down, and the compliance of SO3 products will be tested; The oxidation of 2-is also inhibited, which is not conducive to the crystallization of gypsum; On the contrary, the lower the pH is, the more conducive it is to the dissolution of limestone, but the absorption of SO2 is inhibited, and the desulfurization efficiency will decline. Therefore, keeping the pH of absorption tower slurry stable during operation and controlling it within a suitable range (generally 5.2-5.6) is the premise for effectively controlling SO2 absorption reaction and obtaining stable desulfurization rate and stone quality

ph control has the characteristics of high nonlinearity, time variability, time delay and various uncertainties. It is also affected by the measurement accuracy of pH meter, limestone slurry densimeter and flue gas flow meter. It is difficult to stabilize the slurry pH value at a certain value in desulfurization operation. When the flue gas volume or SO2 concentration of raw flue gas (i.e. SO2 load) rises or falls suddenly, pH is easy to fluctuate. At this time, if the PID control performance is not good. 3. The maintenance of the oil source of the tensile testing machine: good or the relevant meter measurement is not accurate, pH will be out of control

in order to avoid and reduce the occurrence of such situations, we conduct research on stabilizing the pH of the slurry. First, we reset the PID parameters involved in the pH adjustment of the slurry in some desulfurization plants that have been put into operation for a long time, and dynamically track and adjust the relevant control parameters in time. The second is to formulate the relationship curve between SO2 load and the amount of limestone slurry according to theoretical calculation. The operator can closely monitor the amount of limestone slurry according to the SO2 load at that time, so that generally there will be no excess or shortage of limestone slurry, and there will be no ups and downs of slurry pH. after taking these two measures, the stability of desulfurization pH in most power plants has been greatly improved than before

cause analysis and Countermeasures of abnormal pH

after the general limestone gypsum wet desulfurization system is put into operation, abnormal pH of slurry has occurred more or less, and most of them are shown as the blind area of desulfurization reaction, which is an abnormal situation in the SO2 absorption reaction process, that is, the desulfurization efficiency and slurry pH do not increase with the addition of limestone slurry, which shows that the dissolution of limestone is blocked, and the slurry pH and desulfurization rate decline

according to relevant literature reports, the desulfurization reaction blind zone is usually divided into two situations: one is CaSO3 blinding, which occurs when CaSO3 is produced in large quantities and not completely oxidized; Second, aluminum fluoride complex (AlFx) is formed in the slurry, which is adsorbed on the surface of limestone particles and hinders the dissolution of limestone. In view of this situation, we studied and analyzed the mechanism and characteristics of the blind zone, and summarized the general conditions for the blind zone: the sudden change of SO2 load in the raw flue gas, the poor quality of limestone, the high content of medium components in coal combustion, the high content of dust in the raw flue gas, and the low discharge of desulfurization wastewater

on this basis, we have carried out research and Practice on the prevention and treatment methods of desulfurization blind areas in many power plants. When there are precursors of desulfurization blind areas, many power plants will take the following measures in time to greatly reduce the incidence of desulfurization blind areas and shorten the time to deal with them

① strictly control the amount of limestone slurry, try to put the slurry pH automatic, and formulate the relationship curve between SO2 load of raw flue gas and the amount of limestone slurry, so as to ensure that the limestone slurry is not excessive

② strengthen the chemical analysis during the abnormal pH of the slurry. When the CaC03 content in the slurry exceeds 3%, it is necessary to adjust the relevant process control parameters, and increase the detection of slurry f- if necessary, so as to find out the cause of the abnormality as soon as possible

③ if CaSO3 causes blindness, reduce or stop the addition of limestone slurry immediately, and slowly increase the pH value when the pH drops to about 4.0. In case of blindness caused by aifx, the boiler shall be replaced immediately

④ after the blind area occurs, take operational measures such as increasing the circulating pump, increasing the oxidation fan, increasing the discharge of wastewater, and reducing the slurry density of the absorption tower in time

4 practice of improving the economy of desulfurization operation

slurry recirculation pump is the main power consumption equipment of SO2 absorption system. Increasing the number of circulating pumps put into operation means that China's R & D and production capacity is still relatively weak. High liquid gas ratio can improve the desulfurization rate, reduce the pH of slurry and improve the utilization rate of limestone. On the contrary, the pH of the slurry must be increased, which is detrimental to the operation of the system

during desulfurization operation, it is very important to correctly grasp the relationship between liquid gas ratio and desulfurization efficiency, slurry pH and limestone utilization rate for the operation reliability and economy of desulfurization system. In practice, we should make full use of the logical relationship between these process parameters to adjust and optimize the operation number of circulating pumps according to the load of SO2 in the inlet flue gas

different from the conventional equipment of thermal power plants, the performance and service life of desulfurization system equipment are particularly affected by the operating environment, and are prone to scaling, blockage, corrosion, wear and other phenomena. The purpose of equipment maintenance is to delay or reduce the occurrence of these phenomena. For example, in order to reduce the differential pressure of GGH, wash it regularly with compressed air or high-pressure water, and wash the demister regularly at the same time; In order to improve the efficiency of the oxidation fan, the inlet filter of the fan should be cleaned regularly. In order to reduce the wear of equipment, the slurry density of absorption tower shall be reduced as much as possible during operation

wet desulfurization has many special thermal and chemical instruments, such as CEMS, pH meter, density meter, liquid level meter, flowmeter, etc. the accuracy and effectiveness of these instruments are very important for process control and economy. For example, the monitoring value of SO2 concentration in CEMS directly affects the control of desulfurization rate and equipment adjustment. The effectiveness of pH meter side volume is related to desulfurization rate, limestone utilization rate and gypsum quality. The accuracy of slurry densitometer can effectively monitor the wear of equipment and reduce their maintenance costs. Therefore, strengthening the calibration and maintenance of desulfurization instruments and discussing the practical significance of this topic is self-evident. Improving its accuracy and effectiveness is an important premise to ensure the reliable and economic operation of desulfurization system

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