A method for controlling pH value of wet desulfurization spray slurry

Abstract

The invention discloses a method for controlling the pH value of a wet desulfurization spray slurry, which belongs to the field of wet desulfurization process control and is applied to a limestone-gypsum wet desulfurization control system. The method obtains a large hysteresis characteristic expression of the pH value of the slurry in the absorption tower during the adjustment and change process according to desulfurization data in the desulfurization process in the absorption tower, as well as a nonlinear characteristic expression between the opening degree of a slurry supply regulating valve and the slurry supply flow rate. A pulsed intermittent slurry supply mode is constructed according to the two expressions. Under the action of the pulsed intermittent slurry supply mode, the influence of the large hysteresis characteristic on the detection of the pH value of the slurry is eliminated, and the nonlinear characteristic problem of the pH value of the slurry during the change process is solved, so that the control effect of the pH value of the slurry meets the production process requirements.

Description

Technical Field

[0001] The invention belongs to the field of wet flue gas desulfurization process control, and relates to a spray slurry pH value control technology for a limestone-gypsum wet flue gas desulfurization control system in a coal-fired power plant, specifically a wet flue gas desulfurization spray slurry pH value control method based on a pulsed intermittent slurry supply mode. Background Art

[0002] In the limestone-gypsum wet flue gas desulfurization control system, the change of the pH value of the slurry in the absorption tower is one of the important factors affecting the desulfurization efficiency of the entire desulfurization system. When the pH value of the slurry is too high, it helps to accelerate the chemical reaction between the slurry in the absorption tower and the original flue gas, but the excessively high pH value of the slurry will hinder the precipitation of calcium ions in the slurry and reduce the solubility of limestone. When the pH value of the slurry is too low, the solubility of limestone increases. However, according to the chemical reaction mechanism, the absorption of flue gas by the slurry is inhibited in an acidic environment. Therefore, whether the pH value of the slurry is too high or too low, it is not conducive to the absorption of the original flue gas by the spray slurry in the absorption tower. It has been found that the pH value of the spray slurry in the absorption tower is maintained in the range of 5.2 to 5.8, which is the most ideal for the desulfurization rate of the entire wet flue gas desulfurization control system. Under special circumstances, the pH value of the spray slurry is allowed to be adjusted to 6.2, but the time shall not exceed two minutes.

[0003] When fresh slurry is added to the absorption tower, the change in the pH value of the spray slurry until it is detected by the pH meter has a large hysteresis characteristic; and because the slurry supply equipment itself has obvious dead zone characteristics, there is a nonlinear characteristic between the opening degree of the slurry supply valve and the slurry supply flow, resulting in the inability to accurately control the slurry flow entering the absorption tower. Therefore, the wet flue gas desulfurization control system for spraying slurry pH value control in the absorption tower is a complex control system with large hysteresis and nonlinearity. Traditional control methods usually use PID feedback closed-loop control, or introduce fuzzy control technology to achieve the control of the pH value of the spray slurry. Although the desulfurization indicators required by the national environmental protection policy can be achieved in the end, the overshoot of the pH value during adjustment is inevitable due to the large hysteresis characteristics of the slurry pH value adjustment. At the same time, the adjustment of the pH value of the slurry in the absorption tower is unidirectional, and the slurry pH value can only be artificially increased, and the slurry pH value cannot be controlled by artificial means. Therefore, the reduction of the slurry pH value can only rely on the chemical reaction between the inlet original flue gas and the slurry. In this way, the over-adjusted slurry pH value cannot be reduced in time, making it difficult to meet the control effect. The existence of nonlinear characteristics also exacerbates the over-adjustment phenomenon of slurry pH value under the continuous slurry supply adjustment mode. Summary of the invention

[0004] In order to solve the shortcomings of traditional control methods in controlling the pH value of slurry, the present invention provides a wet flue gas desulfurization spray slurry pH value control method based on a pulsed intermittent slurry supply mode, which is applied to the limestone-gypsum wet flue gas desulfurization control system. It can eliminate the nonlinearity and large lag problems of the spray slurry pH in the control process, effectively realize the precise control of the slurry pH value, make the slurry pH value control effect meet the production process requirements, and at the same time improve the utilization rate of the slurry and reduce the production cost.

[0005] The technical solution adopted by the present invention to solve its technical problems is: a method for controlling the pH value of wet desulfurization spray slurry, which obtains the large hysteresis characteristic expression of the slurry pH value in the absorption tower during the adjustment and change process according to the desulfurization data in the desulfurization process in the absorption tower, and the nonlinear characteristic expression between the slurry supply valve opening and the slurry supply flow rate, and constructs a pulsed intermittent slurry supply mode based on the above two expressions. Under the action of the pulsed intermittent slurry supply mode, the influence of the large hysteresis characteristic on the slurry pH value detection is eliminated, and the nonlinear characteristic problem of the slurry pH value in the change process is solved.

[0006] Furthermore, the pulsed intermittent slurry supply mode is specifically as follows: in view of the large hysteresis characteristic, the detection period of the pH value of the spray slurry is set to be longer than the time required for the hysteresis of the change of the pH value of the slurry in the absorption tower; in view of the nonlinear characteristics, at the beginning of the control cycle, a timed high pulse is given to increase the opening of the slurry supply valve from a reference value, and then the opening of the slurry supply valve is adjusted back, and then the increase in slurry put into the absorption tower by the slurry supply system is determined by adjusting the pulse width.

[0007] Furthermore, the large hysteresis characteristic expression is:

[0008]

[0009] Where: ΔpH is the increment of pH of slurry in the absorption tower; is the increment of slurry flow; τ is the pure delay time parameter of wet flue gas desulfurization system; C in_so2 is the concentration of SO2 in the inlet flue gas; Q smoke is the flue gas flow entering the absorption tower.

[0010] Furthermore, the nonlinear characteristic expression is:

[0011]

[0012] Where: v is the opening degree of the slurry supply valve; It represents the slurry flow rate entering the absorption tower when the slurry supply valve opening is v; v1 and v2 represent different slurry supply valve openings; Q_0, Q_1 and Q_2 represent the corresponding slurry flow rate values ​​at different slurry supply valve openings.

[0013] Compared with the prior art, the beneficial effects of the present invention are embodied in: adopting a pulsed intermittent slurry supply mode, at the beginning of the control cycle, the slurry supply valve opening is increased by a timed high pulse and then adjusted back to the reference valve position, which can eliminate the dead zone problem existing in the slurry supply system. After being adjusted back to the reference valve position, the slurry supply valve opening and the slurry supply flow rate can be approximately linearly related, and the pulse width is set as the opening time of the slurry supply valve opening at the current value. Therefore, by adjusting the pulse width to determine the slurry increment provided by the slurry supply system, that is, the pulse area after being adjusted back to the reference valve position, quantitative control of the amount of spray slurry placed in the absorption tower is successfully achieved, and the nonlinear problem of slurry pH value in the control process is solved. In order to eliminate the influence of large hysteresis characteristics on the detection of slurry pH value, a delayer with a delay time greater than the slurry pH value hysteresis time is set. When the pH value of the spray slurry is detected after the delay time ends, the obtained spray slurry pH value is the true value of the spray slurry. Therefore, the pulsed intermittent slurry supply mode can completely eliminate the nonlinearity and large hysteresis problems existing in the control process of the pH value of the spray slurry, so that the control effect of the slurry pH value meets the production process requirements.

[0014] The present invention expands the scope of slurry pH control in the field of wet desulfurization process control, is simple to operate, highly practical, and can be applied to different production conditions of wet desulfurization. It provides an economical and practical method for adjusting slurry pH for wet desulfurization process systems, and has high promotion value. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a flow chart of a method for controlling pH value of wet desulfurization spray slurry based on a pulsed intermittent slurry supply mode;

[0016] Figure 2 This is the large hysteresis characteristic diagram of the spray slurry pH value when it is adjusted;

[0017] Figure 3 This is the nonlinear characteristic diagram of the spray slurry pH value when it is adjusted;

[0018] Figure 4 To use the linear regression method to obtain the linear relationship between the opening degree of the slurry supply valve and the slurry supply flow rate;

[0019] Figure 5 This is the principle diagram of pulsed intermittent slurry supply mode;

[0020] Figure 6 This is the control effect diagram of the pH value of the spray slurry under the pulsed intermittent slurry supply mode. DETAILED DESCRIPTION

[0021] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0023] The overall technical idea of ​​the present invention is: by establishing a mathematical model expression for the change of the pH value of the spray slurry, the time lag of the change process of the pH value of the spray slurry is obtained in combination with the wet desulfurization process data. A nonlinear relationship between the slurry supply regulating valve opening and the slurry supply flow rate is established based on the collected desulfurization data, and a linear regression method is used to perform linear fitting on the linear region in the nonlinear relationship to obtain a nonlinear mathematical expression between the slurry supply regulating valve opening and the slurry supply flow rate. By analyzing the nonlinear and large hysteresis characteristics of the pH value of the spray slurry during the control process, a pulsed intermittent slurry supply mode is designed. The use of a pulsed intermittent slurry supply mode can overcome the complex control problems such as nonlinearity and large hysteresis in the existing technology in the process of adjusting the pH value of the slurry, and can effectively achieve precise control of the pH value of the slurry, while improving the utilization rate of the slurry and reducing production costs.

[0024] First, according to the actual value of the slurry pH under the current desulfurization condition obtained by the slurry pH detector in the absorber in the wet flue gas desulfurization system, the adjustment instruction for the slurry pH value is obtained by comparing the difference between the detected actual value of the slurry pH and the set value of the slurry pH under the current desulfurization condition, and judging whether to increase or decrease the slurry pH value. When the instruction to increase the slurry pH value in the absorber is obtained, the pulsed intermittent slurry supply mode is triggered. Under the action of the pulsed intermittent slurry supply mode, a high pulse instruction is first given to the slurry supply valve opening, the purpose of which is to eliminate the dead zone that occurs in the process of slowly increasing the opening degree when the slurry supply valve controls the slurry flow rate, that is, the nonlinear characteristics between the detected slurry supply valve opening value and the slurry supply flow rate. The high pulse instruction can solve the slurry adjustment dead zone problem caused by the slurry supply equipment to the greatest extent, and then adjust the slurry supply valve opening degree to return to the slurry supply valve opening degree position corresponding to the current desulfurization condition and maintain it for a period of time. For different desulfurization conditions, this time period and the corresponding slurry supply valve opening degree value after adjustment can be reasonably adjusted. The slurry supply valve opening is adjusted back to the reference value to ensure the minimum slurry flow index in the absorption tower, so that quantitative control of the amount of slurry put into the absorption tower can be achieved. After the slurry supply valve opening returns to the reference value and remains for a period of time, the slurry pH value is detected and the difference between it and its set value is determined to determine whether to trigger the pulsed intermittent slurry supply mode of the next cycle. In this way, the large hysteresis problem that occurs when the slurry pH detector in the absorption tower is detected is solved. The present invention not only fully considers the nonlinear and large hysteresis problems existing in the slurry pH control in the on-site wet desulfurization process, but also can apply the present invention to the precise control of the slurry pH value in the wet desulfurization system for generator sets of different capacities using limestone-gypsum wet desulfurization method, while also ensuring that the sulfur content of the flue gas at the system outlet meets the national environmental protection emission standards, so it is more operational and reliable in practice.

[0025] Based on the above technical ideas, the pH value control method of wet desulfurization spray slurry based on the pulsed intermittent slurry supply mode is specifically described as follows:

[0026] Figure 1 A flow chart of a method for controlling pH value of a wet desulfurization spray slurry based on a pulsed intermittent slurry supply mode of the present invention is provided, comprising:

[0027] Step S1: Determine the maximum lag time T1 generated when adjusting the pH value of the spray slurry according to the desulfurization data during the desulfurization process in the absorption tower, such as Figure 2 As shown;

[0028] Step S2: According to the analysis of desulfurization data, the nonlinear relationship between the slurry supply valve opening and the slurry supply flow rate is obtained, such as Figure 3 As shown; for the linear region, the linear expression between the two is determined by using linear regression, such as Figure 4 As shown;

[0029] Step S3: According to the lag time of the slurry pH value obtained during the adjustment process and the nonlinear mathematical expression between the slurry supply valve opening and the slurry supply flow rate, a pulsed intermittent slurry supply mode principle diagram is constructed, such as Figure 5 shown.

[0030] exist Figure 5 In the principle diagram of the pulsed intermittent slurry supply mode shown in , the horizontal axis represents the operating time of the desulfurization system, and the vertical axis represents the percentage of the slurry supply valve opening; 0 represents the slurry supply valve opening in a fully closed state, and 100 represents the slurry supply valve opening in a fully open state; h0, h1, and h2 represent different slurry supply valve opening values ​​respectively; S represents the area enclosed by the time period t2 and the slurry supply valve openings h0 and h1, and represents the slurry flow rate entering the absorption tower during the desulfurization process. In the t1 time period, the slurry supply valve opening is increased from the reference value h0 to h2 with a high pulse, in order to skip the dead zone of the slurry supply valve and enter the approximate linear region between the slurry supply valve opening and the slurry supply flow rate; in the t2 time period, the slurry supply valve opening is reduced to the target opening and remains unchanged; in the t0 time period, the slurry supply is ended, and the slurry supply valve is restored to the reference value and remains unchanged. It can be seen from step S2 that there will be a lag of T1 time period from the time the slurry enters the absorption tower to the time it takes effect. Therefore, within a pulsed intermittent slurry supply cycle, t0≥T1 should be set to ensure that the pH value of the slurry detected each time is the true value in the absorption tower.

[0031] Embodiment 1:

[0032] A method for controlling the pH value of wet flue gas desulfurization spray slurry based on a pulsed intermittent slurry supply mode is applied in the limestone-gypsum wet flue gas desulfurization process control system of a coal-fired power plant. Table 1 gives the values ​​of various parameters in the wet flue gas desulfurization operation.

[0033] Table 1: Values ​​of various parameters in wet flue gas desulfurization operation

[0034]

[0035] Based on the desulfurization working condition of the above-mentioned wet flue gas desulfurization system, when the difference between the actual pH value of the spray slurry and the set value is within the range of [0.1, 0.5], the pulsed intermittent slurry supply mode parameters are set at this time: the slurry supply valve opening degree h0 = 15%, h1 = 55%, h2 = 85%, and the adjustment time period t1 = 10s, t2 = 60s. Since in the on-site wet flue gas desulfurization monitoring process, when the slurry supply valve opening degree is opened to the maximum, the slurry pH value rises by 0.1, which takes about 100s, so t0 = 100s.

[0036] Embodiment 2:

[0037] When the pH meter in the absorption tower detects that the deviation between the actual pH value of the spray slurry and the set value is greater than 0.5, the parameter variables in the pulse intermittent slurry supply mode are set: h0=15%, h1=75%, h2=85%, t1=10s, t2=90s, t0=100s. The setting of the above parameter variables is determined by the deviation between the actual pH value of the slurry and the set value. When the deviation value is large, it indicates that the pH value of the slurry needs to be increased at this time to increase the desulfurization efficiency of the spray slurry. Corresponding to the principle diagram of the pulse intermittent slurry supply mode, it means increasing the area of ​​S. Because S represents the amount of fresh slurry flowing into the absorption tower.

[0038] The effect of pulsed intermittent slurry supply mode on the pH value of slurry is as follows: Figure 6 As shown, in Figure 6 It can be seen that with the increase of SO2 concentration in the outlet flue gas, the set value of the pH value of the spray slurry also increases. When the pH value of the spray slurry detected in the absorption tower and the slurry pH set value remain within the error range, the slurry supply valve opening is maintained at the reference value; when the pH value of the spray slurry detected in the absorption tower is lower than the slurry pH set value, the pulse intermittent slurry supply mode is triggered to adjust the pH value of the spray slurry until the pH value of the spray slurry reaches the slurry pH set value, and then no further adjustments are made and the slurry supply valve opening is maintained at the reference value.

[0039] Obviously, the above embodiments are merely examples for the purpose of clear explanation, and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived therefrom are still within the scope of protection of the invention.

Claims

1. A method for controlling pH value of wet desulfurization spray slurry, characterized in that: According to the desulfurization data in the desulfurization process in the absorber, the large hysteresis characteristic expression of the slurry pH value in the absorber during the adjustment and change process, and the nonlinear characteristic expression between the slurry supply valve opening and the slurry supply flow rate are obtained. According to the above two expressions, a pulsed intermittent slurry supply mode is constructed. Under the action of the pulsed intermittent slurry supply mode, the influence of the large hysteresis characteristic on the slurry pH value detection is eliminated, and the nonlinear characteristic problem of the slurry pH value in the change process is solved; The pulsed intermittent slurry supply mode is specifically as follows: in view of the large hysteresis characteristic, the detection period of the pH value of the spray slurry is set to be longer than the time required for the hysteresis of the change of the pH value of the slurry in the absorption tower; in view of the nonlinear characteristics, at the beginning of the control cycle, a timed high pulse is given to increase the opening of the slurry supply valve from a reference value, and then the opening of the slurry supply valve is adjusted back, and then the slurry increment put into the absorption tower by the slurry supply system is determined by adjusting the pulse width.

2. The method for controlling pH value of wet desulfurization spray slurry according to claim 1, characterized in that: The large hysteresis characteristic expression is: Where: ΔpH is the increment of pH of slurry in the absorption tower; is the increment of slurry flow; τ is the pure delay time parameter of wet flue gas desulfurization system; C in_so2 is the concentration of SO2 in the inlet flue gas; Q smoke is the flue gas flow entering the absorption tower.

3. The method for controlling pH value of wet desulfurization spray slurry according to claim 1, characterized in that: The nonlinear characteristic expression is: Where: v is the opening degree of the slurry supply valve; It represents the slurry flow rate entering the absorption tower when the slurry supply valve opening is v; v1 and v2 represent different slurry supply valve openings; Q_0, Q_1 and Q_2 represent the corresponding slurry flow rate values ​​at different slurry supply valve openings.

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