An Asymmetric Gradient Partitioning and Ammonia Injection Control Method

Abstract

The present invention discloses an asymmetric gradient zoning and ammonia injection control method, which includes the following steps: First, the flue is zoned according to the flue gas flow velocity distribution at the inlet flue, so that the flue gas flow rates in each zone are basically the same, and then the ammonia injection amounts in each zone are calculated in combination with the inlet NOx concentration, realizing the rapid adjustment of the ammonia injection amounts in each zone. The asymmetric gradient zoning and ammonia injection control method provided by the present invention zones non-uniformly according to the gradient of the flue gas flow velocity distribution, making the flue gas flow rates in each zone basically consistent. On the basis of the asymmetric zoning, by measuring the NOx concentration at the inlet of the SCR reactor in each zone, the rapid adjustment of the ammonia injection amounts in each zone is realized. Compared with the traditional zoning method, the adjustability of the ammonia injection grid is greatly improved, and at the same time, favorable conditions are provided for the rapid zoning ammonia injection control. Compared with the traditional zoning feedback regulation, the response speed and regulation effect are greatly improved, having important practical value.

Description

Technical Field

[0001] The present invention belongs to the technical field of ammonia injection control, and particularly relates to an asymmetric gradient zoning and ammonia injection control method. Background Art

[0002] Most domestic coal-fired power plant SCR denitration devices adopt AIG type ammonia injection grids. The ammonia injection grid is installed in the inlet flue of the SCR reactor and is usually divided into n groups of ammonia injection branch pipes along the width direction of the boiler and 2 - 4 ammonia injection branch pipes along the depth direction to cover the entire flue cross-section. The denitration reducing agent is sprayed into the flue through the ammonia injection branch pipes of the ammonia injection grid and is fully mixed with NOx in the flue gas, and NOx removal is achieved under the action of the catalyst. Since the ammonia injection grid is arranged at equal distances in the flue cross-section, the zoning of ammonia injection is also evenly and equally spaced. However, due to different boiler combustion and flue structure conditions, the flue gas flow velocity distribution and NOx concentration distribution in the cross-section of the SCR reactor inlet flue are also uneven. In some areas of the cross-section, the flow velocity is very high, in some areas it is very low, in some areas the NOx concentration is very high, and in some areas the NOx concentration is very low. Although the flow rate of the denitration reducing agent in the corresponding area can be adjusted by adjusting the valve opening of the ammonia injection branch pipe, in many cases, simply adjusting the valve opening still cannot meet the demand for the reducing agent in the corresponding area, and even when the valve is adjusted to the maximum opening, the required reducing agent flow rate cannot be achieved. When the distribution of the reducing agent flow rate in different areas does not match the demand, it will lead to problems such as uneven NOx concentration distribution at the outlet of the SCR reactor, deviation in the representativeness of CEMS measurement, and high local ammonia slip concentration, and further cause blockage of downstream equipment such as air preheaters. Especially in the current situation where coal-fired power plants are facing further ultra-low emissions and flexible peak shaving, this problem is further highlighted.

[0003] Currently, the flow rate of the reducing agent (usually ammonia) in different areas at the inlet of the SCR reactor is mainly controlled by zoned ammonia injection control. The control mainly relies on the measurement of the NOx concentration value in the corresponding area at the outlet of the reactor, and feedback adjustment is carried out according to the measurement value. When the NOx concentration in the area is higher than the average value, the opening of the corresponding valve is increased accordingly. When the NOx concentration in the area is lower than the average value, the opening of the corresponding valve is decreased accordingly. However, this control method does not consider the influence of the flow velocity distribution and concentration distribution, and it is also difficult to meet the situation of rapid change of the current unit load. Summary of the Invention

[0004] In order to solve the technical problems existing in the prior art, the purpose of the present invention is to provide an asymmetric gradient zoning and ammonia injection control method.

[0005] In order to achieve the above purpose and reach the above technical effects, the technical solution adopted by the present invention is as follows:

[0006] An asymmetric gradient partitioning and ammonia injection control method, comprising the following steps:

[0007] First, partition the flue gas duct according to the flue gas velocity distribution at the inlet flue gas duct, so that the flue gas flow rates in each partition are basically the same, and then calculate the ammonia injection amounts for each partition in combination with the inlet NOx concentration, so as to achieve rapid adjustment of the ammonia injection amounts for each partition.

[0008] An asymmetric gradient partitioning and ammonia injection control method disclosed by the present invention specifically comprises the following steps:

[0009] 1) Obtain the flue gas velocity distribution of the cross section of the inlet flue gas duct;

[0010] 2) According to the flue gas velocity distribution, perform partitioning in the width direction and the depth direction respectively to ensure that the flue gas flow rates in each partition are basically the same;

[0011] 3) According to the defined partitions, arrange ammonia injection branch pipes in each partition, the flow rate of each ammonia injection branch pipe is adjustable, measure the inlet NOx concentration of each partition, calculate the ammonia injection amounts for each partition, and then adjust the flow rates of the ammonia injection branch pipes in each partition to achieve rapid adjustment of the ammonia injection amounts for each partition.

[0012] Further, in step 1), obtain the flue gas velocity distribution of the cross section of the inlet flue gas duct of the SCR reactor through measurement or numerical simulation methods.

[0013] Further, in step 2), according to the flue gas velocity distribution, first perform partitioning in the width direction. The principle of partitioning is to ensure that the flue gas flow rates in each area are basically the same. Increase the number of partitions in the area with high flue gas velocity and reduce the number of partitions in the area with low flue gas velocity. After completing the partitioning in the width direction, then perform partitioning in the depth direction.

[0014] Further, when performing partitioning in the width direction, divide the 300MW unit into 6 - 8 groups, divide the 600MW unit into 8 - 10 groups, and divide the 1000MW unit into 10 - 12 groups.

[0015] Further, when performing partitioning in the width direction, determine that the number of partitions in the width direction is n, the average flue gas velocity is v, and the total area of the flue gas duct is S. Then the flue gas flow rate q for each partition in the width direction = v×S / n. Thus, according to the average flue gas velocity v of a certain partition n calculate the area S of the partition n = q / v n .

[0016] Further, after completing the partitioning in the width direction, due to the existence of velocity deviation along the depth direction of the boiler flue gas duct, perform partitioning in the depth direction within each partition. When performing partitioning in the depth direction, determine the number of partitions in the depth direction as x. Then the flue gas flow rate q for a single partition in the depth direction x= q / x, from which the average flue gas velocity v in a certain depth partition is used to x calculate the partition area S in the depth direction x = q x / v x .

[0017] Furthermore, in step 3), measuring the NOx concentration at the inlet of each partition, calculating the ammonia injection amount of each partition, and then adjusting the flow rate of the ammonia injection branch pipes of each partition to achieve rapid adjustment of the ammonia injection amount of each partition includes the following steps:

[0018] Set the total number of partitions to m, the control value of the NOx concentration of the net flue gas emission to C, and measure the NOx concentrations of each inlet partition through a flue gas analyzer as c1, c2...c m , then the NOx concentration to be removed in each partition is c1 - C, c2 - C...c m - C, calculate the sum SUM = (c1 - C) + (c2 - C) +...(c m - C);

[0019] The total ammonia injection amount is Q, and the ammonia injection amount of each partition is rapidly distributed based on the measured NOx concentration of the partition on the basis of the total ammonia injection amount. Thus, the ammonia injection amounts of each partition are Q*(c1 - C) / SUM, Q*(c2 - C) / SUM...Q*(c m - C) / SUM respectively. The flow rate of each ammonia injection branch pipe is rapidly adjusted to the set value according to the distributed ammonia injection amount, so as to achieve rapid partition ammonia injection automatic control.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] The present invention discloses an asymmetric gradient partition and ammonia injection control method, which divides the partition unevenly according to the gradient of the flue gas velocity distribution, so that the flue gas flow rate of each partition is basically the same. On the basis of the asymmetric partition, by measuring the NOx concentration at the inlet of the SCR reactor in each partition, rapid adjustment of the ammonia injection amount in each partition is realized. Compared with the traditional partition method, the present invention greatly improves the adjustability of the ammonia injection grid, and at the same time provides favorable conditions for rapid partition ammonia injection control. In addition, based on the method of measuring the NOx concentration at the inlet of the SCR reactor in each partition, the rapid distribution of the ammonia injection amount is realized by using the principle of material conservation. Compared with the traditional partition feedback regulation, the response speed and regulation effect are greatly improved, and it has important practical value. Description of the Drawings

[0022] Figure 1 is the flow chart of the present invention;

[0023] Figure 2 is the example diagram of the flue gas velocity distribution in Embodiment 1 of the present invention;

[0024] Figure 3 Width partition diagram of Embodiment 1 of the present invention;

[0025] Figure 4 Depth partition diagram of Embodiment 1 of the present invention. Detailed implementation manners

[0026] The present invention will be elaborated in detail below so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making the protection scope of the present invention more clearly defined.

[0027] The following gives a brief overview of one or more aspects to provide a basic understanding of these aspects. This overview is not an exhaustive survey of all contemplated aspects, and is neither intended to identify key or decisive elements of all aspects nor to attempt to define the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects in a simplified form as a prelude to a more detailed description to follow.

[0028] An ammonia injection grid is arranged at the inlet flue of the SCR denitration device of a coal-fired unit to inject a reducing agent into the flue. Since the cross-sectional area of the flue is relatively large, the flue needs to be partitioned. One ammonia injection branch pipe is arranged corresponding to each area. h groups of ammonia injection branch pipes are arranged along the width direction, and each group is further divided into n along the depth direction. A total of h×n ammonia injection branch pipes correspond to h×n partitions.

[0029] The amount of ammonia injection required for the flue gas in each partition is related to the total amount of NOx in the area. Total amount of NOx = NOx concentration × flue gas flow rate × partition area. Since the distribution and magnitude of the NOx concentration are usually related to the combustion conditions, it is generally difficult to predict, and its distribution and magnitude are generally obtained by on-line measurement; the distribution of the flue gas flow rate is usually related to the boiler design structure, and its distribution can generally be obtained by methods such as measurement or numerical simulation, and the magnitude of the flue gas flow rate changes linearly with the change of the boiler load.

[0030] Based on the above conditions, the present invention adopts a method of partitioning according to the distribution of the flue gas flow rate and combines on-line measurement of the NOx concentration in the partition to improve the effect and speed of partition ammonia injection control.

[0031] As Figures 1-4 shown, a non-symmetric gradient partitioning and ammonia injection control method proposed by the present invention includes the following steps:

[0032] 1) First, obtain the distribution of the flue gas flow rate at the cross-section of the inlet flue of the SCR reactor by measurement or numerical simulation method;

[0033] 2) According to the flue gas velocity distribution, grouping in the width direction is carried out first. Limited by economy and installation conditions, the number of partitions cannot be too many. Considering the flue gas velocity distribution, additional partitions are added in the areas with high flue gas velocity and reduced in the areas with low flue gas velocity. Usually, 300MW units are divided into 6 - 8 groups, 600MW units are divided into 8 - 10 groups, and 1000MW units are divided into 10 - 12 groups. Determine the number of partitions in the width direction as n, the average flue gas velocity as v, and the total flue duct area as S. The principle of partitioning is to ensure that the flue gas flow rate in each group area is basically the same. The flue gas flow rate q in each width - direction partition is q = v×S / n. Thus, based on the average flue gas velocity v of a certain group of partitions n Calculate the area S of the partition n = q / v n . Thus, the partitioning in the width direction is completed;

[0034] Since there are still velocity deviations along the depth direction of the boiler flue duct, therefore, within each group of partitions, further partitioning is carried out in the depth direction. Usually, according to the depth of the flue duct, 2 - 5 depth partitions are set. First, determine the number of partitions x in the depth direction, then the flue gas flow rate q in a single depth - direction partition x = q / x. Thus, based on the average flue gas velocity v of a certain depth partition x Calculate the area S of the depth - direction partition x = q x / v x . The above - mentioned method is used to carry out depth - direction partitioning for each width partition, completing the partitioning of the entire flue duct cross - section. This partitioning method is based on asymmetric splitting according to the velocity gradient, ensuring that the flow rate in each partition is basically the same, providing a basis for rapid partitioning ammonia injection control.

[0035] 3) According to the defined partitions, ammonia injection branch pipes are set in each partition. The flow rate of each ammonia injection branch pipe is adjustable. At the same time, partition measurement sampling pipes and flue gas analyzers are set, which can measure the NOx concentration in the partition. The above matters are all completed when the boiler is out of service. When the boiler is operating normally, samples are taken through the partition measurement sampling pipes and then analyzed by the flue gas analyzer to obtain the NOx concentration in the partition. By quickly measuring each partition, the NOx concentration distribution of all partitions can be obtained. Assume that there are a total of m partitions, and the net flue gas emission NOx concentration is controlled at C. The NOx concentrations of each partition at the inlet of the SCR reactor measured by partitioning are c1, c2…c m , then the NOx concentration to be removed in each partition is c1 - C, c2 - C…c m - C. Calculate the sum SUM=(c1 - C)+(c2 - C)+…(c m-C). The total amount of ammonia injection Q required for the entire reactor is feedback-regulated by the automatic control system based on the NOx concentration control value C of the net flue gas emission. The ammonia injection amounts for each zone are quickly distributed based on the measurement of the NOx concentration in the zone on the basis of the total ammonia injection amount, and the ammonia injection amounts for each zone are obtained as Q*(c1 - C) / SUM, Q*(c2 - C) / SUM... Q*(c m -C) / SUM respectively. The flow rate of each ammonia injection branch pipe is quickly adjusted to the set value according to the distributed ammonia injection amount, so as to realize the automatic control of rapid zone-by-zone ammonia injection.

[0036] Example 1

[0037] First, the flue gas velocity distribution at the inlet flue cross-section of the SCR reactor is obtained by measurement or numerical simulation methods. Figure 2 Fig. is the flue gas velocity distribution diagram of Example 1. The distributions of different boilers will be significantly different. The upper left area is the high-velocity area with a velocity range of 16 - 24 m / s; the middle area is the medium-velocity area with a velocity range of 8 - 16 m / s, and the right area is the low-velocity area with a velocity range of 0 - 8 m / s.

[0038] First, grouping is carried out in the width direction according to the velocity distribution. Limited by economy and installation conditions, the number of zones cannot be too many. Considering the velocity distribution, the number of zones is increased in the high-velocity area and decreased in the low-velocity area. The principle of zoning is to ensure that the flue gas flow rates of each group of areas are basically the same. The number of zones in the width direction is determined as n, the average flue gas velocity is v, the total area of the flue is S, and the flue gas flow rate q of each group of zones = v×S / n. Thus, according to the average flue gas velocity v of a certain group of zones n the area S of the zone is calculated n = q / v n . According to the flue gas velocity distribution in the above example, Figure 3 zones can be obtained, and there are 6 groups of zones in the width direction. So far, the zoning in the width direction is completed.

[0039] Since there are still velocity deviations along the depth direction of the boiler flue, further zoning is carried out in the depth direction within each group of zones. Usually, according to the depth of the flue, 2 - 5 depth zones are set. When carrying out the zoning in the depth direction, first determine the number x of depth zones, then the flue gas flow rate q of a single depth zone x = q / x. Thus, according to the average flue gas velocity v of a certain depth zone x the area S of the zone is calculated x = q x / v x . This method is used to carry out the zoning in the depth direction for each width zone to complete the zoning of the entire flue cross-section, as shown in Figure 4As shown, there are 3 deep reverse partitions. This partitioning method is based on the asymmetric segmentation of the flow velocity gradient to ensure that the flow rate of each partition is basically the same, providing a basis for rapid partition ammonia injection control.

[0040] According to the defined partitions, ammonia injection branch pipes are set in each partition, and the flow rate of each branch pipe is adjustable. At the same time, partition measurement sampling pipes and flue gas analyzers are set to measure the NOx concentration of the partition. The above matters are completed when the boiler is out of service. When the boiler is running normally, samples are taken through the partition measurement sampling pipes and then analyzed by the flue gas analyzer to obtain the NOx concentration of the partition. By quickly measuring each partition, the NOx concentration distribution of all partitions can be obtained. Assume that there are m partitions in total, and the net flue gas emission NOx concentration is controlled at C. The NOx concentrations of each partition at the inlet of the SCR reactor measured by partition are c1, c2... c m , then the NOx concentration to be removed from each partition is c1 - C, c2 - C... c m - C, calculate the sum SUM = (c1 - C)+(c2 - C)+...(c m - C). The total ammonia injection amount Q required for the entire reactor is feedback-regulated by the automatic control system according to the net flue gas emission NOx concentration control value C. The ammonia injection amount of each partition is quickly distributed based on the partition NOx concentration measurement on the basis of the total ammonia injection amount. Thus, the ammonia injection amount of each partition is Q*(c1 - C) / SUM, Q*(c2 - C) / SUM... Q*(c m - C) / SUM, and the flow rate of each branch pipe is quickly adjusted to the set value according to the allocated ammonia injection amount, so as to realize the automatic control of rapid partition ammonia injection.

[0041] Example 2

[0042] The size of a certain flue is: width 8m, depth 4m. Table 1 gives the flue gas flow velocities of each area. The flue gas flow velocity measurement grid is 0.5m * 0.5m. If the flue gas flow velocity deviation is large, the grid density can be increased. The flue gas flow velocity distribution can be obtained through numerical simulation or actual measurement. Assume that there are 8 groups of partitions in the width direction and 2 partitions in the depth direction. According to the traditional partitioning method, each 1m in the width direction is a group of partitions, and each 2m in the depth direction is a partition. According to the gradient partitioning method, first calculate the average flow velocity of the flue cross-section as 15m / s, and the flue cross-sectional area is 32m 2 , then the total flue gas flow rate is 480m 3 / s. For 8 partitions in the width direction, the flow rate of each partition is 60m 3 / s.

[0043] Table 1

[0044]

[0045] Calculate the flue gas flow under the original grid distribution. Divide it into groups of 0.5 m in the width direction. The flue gas flows in the area are 41, 40, 38, 37, 36, 35, 34, 33, 32, 30, 28, 27, 22, 19, 16, 15 m 3 / s, as shown in Table 2. Calculate according to the flow rate of 60 m 3 / s for each group partition in the width direction. Calculate the width of each group partition proportionally. For example, the width of the first group partition is the width of the first group of the grid 0.5 m + the width calculated proportionally for the second group of the grid 0.5*(60 - 41) / 40 m = 0.74 m. By analogy, obtain the widths of 8 group partitions in the width direction. When using the numerical simulation calculation method, the grid density can be further increased to improve the accuracy of the partition. When the grid is fine enough, it is the approximate integration method.

[0046] Table 2

[0047]

[0048] Taking the first width partition in the depth direction as an example, calculate the flue gas flows of each group of grids in the depth direction within this partition, which are 8.4, 7.7, 7.6, 7.4, 7.3, 7, 7, 6.9 m 3 / s. Since the depth direction needs to be divided into 2 zones, the flue gas flow of each zone is 60 / 2 = 30 m 3 / s. Thus, the depth of the first partition in the depth direction is the depth of the first group of the grid 0.5 m + the depth of the second group of the grid 0.5 m + the depth of the third group of the grid 0.5 m + the width calculated proportionally for the fourth group of the grid 0.5*(30 - 8.4 - 7.7 - 7.6) / 7.4 m = 1.93 m. The depth of the second partition in the depth direction is 4 - 1.93 = 2.07 m. By analogy, obtain the depth partitions of each group of partitions. When using the numerical simulation calculation method, the grid density can be further increased to improve the accuracy of the partition. When the grid is fine enough, it is the approximate integration method. The above calculation example is based on the manual calculation method. The actual operation can be based on the integration method of the computer, that is, the integration of each area to the given flow rate is the dividing line. Tables 3 and 4 show 8 group partitions in the width direction and 2 partitions in the depth direction, as shown in the underlined part.

[0049] Table 3

[0050]

[0051] Table 4

[0052]

[0053] For the parts or structures not specifically described in the present invention, existing technologies or existing products can be adopted, and no further elaboration will be made here.

[0054] The above are only embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structure or equivalent process transformation made by using the content of the specification of the present invention, or directly or indirectly applied in other related technical fields, shall be similarly included in the patent protection scope of the present invention.

Claims

1. An asymmetric gradient partitioning and ammonia injection control method, characterized in that First, partition the flue duct according to the flue gas velocity distribution at the inlet flue duct to make the flue gas flow rates in each partition basically the same, and then calculate the ammonia injection amount for each partition in combination with the inlet NOx concentration to achieve rapid adjustment of the ammonia injection amount for each partition. Specifically, it includes the following steps: 1) Obtain the flue gas velocity distribution at the cross-section of the inlet flue duct; 2) According to the flue gas velocity distribution, conduct partitions in the width direction and the depth direction respectively to ensure that the flue gas flow rates in each partition are basically the same; 3) According to the defined partitions, set ammonia injection branch pipes in each partition. The flow rate of each ammonia injection branch pipe is adjustable. Measure the NOx concentration at the inlet of each partition, calculate the ammonia injection amount for each partition, and then adjust the flow rate of the ammonia injection branch pipes in each partition to achieve rapid adjustment of the ammonia injection amount for each partition; In step 2), according to the flue gas velocity distribution, first conduct the partition in the width direction. The principle of partitioning is to ensure that the flue gas flow rates in each partition are basically the same. Increase the number of partitions in the area with high flue gas velocity and decrease the number of partitions in the area with low flue gas velocity. After completing the partition in the width direction, then conduct the partition in the depth direction.

2. The asymmetric gradient partition and ammonia injection control method according to claim 1, wherein In step 1), obtain the flue gas velocity distribution at the cross-section of the inlet flue duct of the SCR reactor through measurement or numerical simulation methods.

3. An asymmetric gradient partition and ammonia injection control method according to claim 1, characterized in that When conducting the partition in the width direction, divide the 300MW unit into 6 - 8 groups, the 600MW unit into 8 - 10 groups, and the 1000MW unit into 10 - 12 groups.

4. The asymmetric gradient partition and ammonia injection control method according to claim 1, wherein When partitioning in the width direction, if the number of partitions in the width direction is determined to be n, the average flue gas velocity is v, and the total flue area is S, then the flue gas flow rate q for each partition in the width direction is q = v × S / n. Thus, based on the average flue gas velocity v of a certain partition n the area S of the partition is calculated n = q / v n .

5. An asymmetric gradient partitioning and ammonia injection control method according to claim 1, characterized in that After completing the zoning in the width direction, due to the flow velocity deviation in the depth direction of the boiler flue, zoning in the depth direction is carried out within each zone. When carrying out the zoning in the depth direction, determine the number of zones x in the depth direction, then the flue gas flow rate q in a single depth zone x = q / x. Thus, based on the average flue gas flow velocity v in a certain depth zone x calculate the area S of the depth zone x = q x / v x .

6. The asymmetric gradient partitioning and ammonia injection control method according to claim 1, characterized in that In step 3), the steps of measuring the NOx concentration at the inlet of each partition, calculating the ammonia injection amount for each partition, and then adjusting the flow rate of the ammonia injection branch pipes in each partition to achieve rapid adjustment of the ammonia injection amount for each partition include: Set the total number of partitions to be m, and the control value of the NOx concentration in the net flue gas emission to be C. The NOx concentrations of each inlet partition measured by the flue gas analyzer are c1, c2... c m , then the NOx concentration to be removed from each partition is c1 - C, c2 - C... c m - C. Calculate the sum SUM = (c1 - C) + (c2 - C) +... (c m - C); The total ammonia injection amount is Q. The ammonia injection amounts in each zone are rapidly distributed based on the total ammonia injection amount combined with the measurement of NOx concentration in each zone. Thus, the ammonia injection amounts for each zone are respectively Q*(c1 - C) / SUM, Q*(c2 - C) / SUM…Q*(c m - C) / SUM. The flow rate of each ammonia injection branch pipe is rapidly adjusted to the set value according to the distributed ammonia injection amount, so as to achieve the automatic control of rapid zone-based ammonia injection.

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