A method for judging the replacement time of SCR catalyst wear

Abstract

The invention relates to a method for judging the wear replacement timing of an SCR catalyst. The invention compares the calculated wear amount with the actual wear amount, corrects the constant in the wear amount calculation formula, makes the calculation of the wear amount more in line with reality, and improves the measurement accuracy; a database is built based on different load sections, different positions, different time weights, and different parameter types, and the actual wear data after the system is implemented is accurately predicted through a large amount of measurement data before the system is implemented. Once the data collection is completed and the system is implemented, the calculation of the wear amount will present a real-time feature, which is closer to the actual site than the laboratory wear test, and can reflect real-time performance compared with the numerical simulation method; after the method of the invention is implemented, the acquisition of the wear amount is more convenient and is only related to time and load, and the wear condition of any position can be displayed, so as to obtain the wear parameters of different positions in real time, and provide the replacement timing of the catalyst.

Description

Technical Field

[0001] The invention relates to a method for treating the wear of a catalyst in an SCR reactor, in particular to a method for judging the time of replacement of the SCR catalyst after wear. Background Art

[0002] With the implementation of the emission limit of 50mg / m3 of nitrogen oxides in my country's air pollutant emission standards, the denitrification system (SCR) has become an essential supporting facility for power plant boilers, and the importance of reliable operation of the denitrification system has become increasingly prominent.

[0003] A domestic power plant has 2×660MW ultra-supercritical transformer DC furnaces manufactured by Shanghai Boiler Factory, equipped with an SCR denitration system from a Shanghai Environmental Protection Engineering Co., Ltd. The denitration system was put into use in 2010. In 2011, the catalyst was found to have wear problems. In early 2013, the furnace was shut down for inspection again and it was found that the catalyst had been worn over a large area, and the wear showed a certain regularity. The catalyst wear area and dust accumulation area were counted, and the wear and dust accumulation areas were as follows: Figure 7 As shown (the percentage in the figure indicates the severity of wear, 100% means penetration).

[0004] It is generally believed that the mechanism of catalyst dust accumulation and wear is that dust accumulation first appears in the denitrification reactor, which then leads to a reduction in the flue flow area, and then an increase in the flue gas flow rate in the non-dust accumulation area, which ultimately leads to increased wear in the non-dust accumulation area.

[0005] The worn-out catalyst will then form a flue gas corridor, which will lead to a significant reduction in SCR performance and increased ammonia slip, causing air preheater blockage, which will in turn increase the energy consumption of the induced draft fan and increase carbon emissions, adversely affecting the realization of my country's dual carbon goals.

[0006] Under reasonable design, if the flue gas field of the entire cross section of the SCR catalyst is relatively uniform, the wear of the catalyst will also tend to be uniform, so that after the catalyst reaches the end of its service life, the overall replacement will not affect the performance of the SCR. However, inside the SCR reactor, the distribution of the flue gas field across the entire cross section cannot be uniform, which will lead to different service lives of the catalyst inside the reactor, which will cause low local reaction efficiency, local ammonia escape height, and air preheater blockage problems. However, the degree of catalyst wear is difficult to monitor during operation, and the timing of its replacement is even more difficult to control. Therefore, it is extremely urgent to study the operating reliability and safety of the SCR catalyst. Summary of the invention

[0007] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for determining the timing of SCR catalyst wear replacement, which can effectively solve the problem of accurately determining the timing of catalyst wear replacement in advance to meet the emission requirements of nitrogen oxides and effectively control ammonia slip.

[0008] In order to achieve the above object, the technical solution of the present invention is:

[0009] A method for determining when to replace an SCR catalyst due to wear comprises the following steps:

[0010] Step 1: Determine the wear calculation formula

[0011] A. Determine the formula

[0012] The catalyst wear amount is determined using the following formula:

[0013] Ea=λC0×Ug 3.4 ×D i 3.3

[0014] Where: Ea is the catalyst wear, λ is a constant, C0 is the fly ash concentration in the flue gas, U g is the flue gas velocity in the reactor, D i is the average particle size of fly ash in flue gas;

[0015] B. Parameter acquisition

[0016] A measurement position is selected above the first layer catalyst of the SCR reactor, and the flue area corresponding to the gradually contracting and expanding section of the flue is avoided as the candidate area of ​​the measurement position. The selected measuring hole position is any measuring hole in the candidate area, and the flue gas velocity, fly ash concentration and fly ash particle size at the measurement position are measured through the measuring hole;

[0017] C. Correct the wear calculation formula by the actual wear of the catalyst

[0018] According to step B, obtain the test data of flue gas velocity, fly ash concentration and fly ash particle size, use the wear calculation formula to calculate the wear, take advantage of the downtime opportunity to check the actual wear of the catalyst at the corresponding position, compare it with the wear empirical formula, substitute the actual wear into the wear calculation formula, correct the constant λ in the wear calculation formula, calculate λ1, repeat the above process, and get λ2, λ3... in turn. At least 3 corrections should be made, and the average value is obtained to get λn, and the final wear calculation formula E is obtained as follows:

[0019] Ea=λnC0×Ug 3.4 ×D i 3.3

[0020] Step 2: Obtain the SCR full-section flue gas field parameters

[0021] A. Selection of measurement section and measurement point location

[0022] Select a section within the range of 500-1000 mm from the top surface of the catalyst as the measurement section;

[0023] Use longitude and latitude lines to divide the cross section into several squares, and the intersection of the diagonals of each square is the measurement point location;

[0024] B. Establishing the original database of measurement parameters

[0025] According to the wear calculation formula, it is necessary to measure the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of the SCR inlet cross section. Since the load of the unit changes at any time, the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution at one load point cannot be used to characterize the wear characteristics of the entire time period. Therefore, it is necessary to test the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of each load section;

[0026] Define the test load section to be selected from 100% ECR-30% ECR, select at least 3 test load sections, measure and record the flue gas velocity, fly ash concentration and fly ash particle size at each measuring point under each test load section, and establish an original database of measurement parameters;

[0027] Step 3: Calculate the wear coefficient E

[0028] Substitute the actual flue gas parameters of the full-load section into the wear coefficient calculation formula to calculate the wear coefficient E:

[0029]

[0030] Where:

[0031] T is the running time of the full load section, △T is the running time between two load sections, C 0i is the fly ash concentration in the operating load section, U gi is the flue gas velocity in the operating load section, D ii is the average particle size within the operating load section. The above fly ash concentration, flue gas velocity and average particle size are all obtained through the original database of measured parameters. When the operating load is between two adjacent test load sections, the data under the larger load section is substituted into the formula for calculation; for example, when the operation is under 98% ECR for 5 hours, the flue gas parameter C is selected. 0i , U gi , D ii The parameters are under 100% ECR condition, the time is 5 hours, and so on;

[0032] Step 4: Catalyst replacement decision

[0033] According to the calculation results of the catalyst wear coefficient E at each load section and each wear position, it is judged whether the catalyst at the corresponding position needs to be replaced:

[0034] a. If E>0.3, the catalyst needs to be replaced;

[0035] b. If E < 0.3 and the catalyst service life is greater than 20,000 hours, the catalyst needs to be replaced;

[0036] c. If E < 0.3 and the service life of the catalyst is less than 20,000 hours, there is no need to replace the catalyst;

[0037] The larger the E value, the more severe the wear.

[0038] Preferably, when establishing the original database of measurement parameters in step 2, the test load segments are defined as 100% ECR, 95% ECR, 90% ECR, 85% ECR, 80% ECR, 75% ECR, 70% ECR, 65% ECR, 60% ECR, 55% ECR, 50% ECR, 45% ECR, 40% ECR, 35% ECR, and 30% ECR, and the flue gas velocity, fly ash concentration, and fly ash particle size at each measuring point are measured and recorded under each test load segment condition to establish the original database of measurement parameters.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] (1) The present invention compares the calculated wear amount with the actual wear amount and corrects the constant in the wear amount calculation formula, so that the calculation of the wear amount is more in line with reality, can be adapted to different power plants, and improve the measurement accuracy.

[0041] (2) The present invention builds a database based on different load segments, different positions, different time weights, and different parameter types. Through a large amount of measurement data before the system is implemented, the actual wear data after the system is implemented is accurately predicted. Once the data collection is completed and the system is implemented, the calculation of the wear amount will present real-time characteristics. Compared with laboratory wear tests, it is closer to the actual field, and compared with numerical simulation methods, it can reflect real-time performance.

[0042] (3) After the method of the present invention is implemented, the amount of wear can be obtained more conveniently and is only related to time and load. The wear condition at any position can be displayed, thereby obtaining the wear parameters at different positions in real time and providing the timing for catalyst replacement. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a logic diagram of the method for determining the timing of SCR catalyst wear replacement according to the present invention.

[0044] Figure 2-4 The schematic diagram of the parameter acquisition measurement position selection of the present invention is shown in FIG. Figure 2 For the main view, Figure 3 is the left view, Figure 4 This is a top view.

[0045] Figure 5-6 This is a schematic diagram of selecting the cross section and measuring point positions for the full cross section smoke field parameters of the SCR of the present invention. Figure 5 Select the schematic diagram (main view) for measuring the section position. Figure 6 Schematic diagram of the locations of measuring points in the measurement section (top view).

[0046] Figure 7 This is a statistical chart of catalyst wear and dust accumulation in a power plant.

[0047] Figure 8 Schematic diagram of catalyst wear in a power plant.

[0048] Fig. 9 This is a schematic diagram of the locations of measuring points in the measurement section of an application example of the present invention. DETAILED DESCRIPTION

[0049] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples.

[0050] like Figure 1 As shown, a method for judging the timing of SCR catalyst wear replacement of the present invention comprises the following steps:

[0051] Step 1: Determine the wear calculation formula

[0052] A. Determine the formula

[0053] The catalyst wear amount is determined using the following formula:

[0054] Ea=λC0×Ug 3.4 ×D i 3.3

[0055] Where: Ea is the catalyst wear, λ is a constant, C0 is the fly ash concentration in the flue gas (unit: g / m 3 ), U g is the flue gas velocity in the reactor (unit: m / s), D i is the average particle size of fly ash in flue gas (unit: μm);

[0056] B. Parameter acquisition

[0057] A measurement position is selected above the first layer catalyst of the SCR reactor, avoiding the flue area corresponding to the gradually contracting and expanding section of the flue and the eddy flow area as an alternative area for the measurement position, and the selected measuring hole position is any measuring hole in the alternative area, and the flue gas velocity, fly ash concentration and fly ash particle size at the measuring position are measured through the measuring hole;

[0058] The selection of measurement location follows the following principles:

[0059] Since the flue at the SCR ammonia injection grid is often irregular, if a specific position parameter is selected as the flue gas field parameter before judgment execution, it is necessary to select valuable points and avoid the low-speed eddy area. The area with small rotating eddy and high speed can quickly obtain the catalyst wear characteristics. The specific position selection principle is:

[0060] (1) The selected location should avoid the gradually contracting and expanding section of the flue and the vortex area. The area less affected by the vortex area should be selected as the candidate area. When selecting, it is necessary to avoid the flue area corresponding to the gradually contracting and expanding section of the flue;

[0061] (2) The specific location is above the first layer of catalyst in the SCR reactor;

[0062] (3) The specific selected measuring hole position is any measuring hole in the candidate area;

[0063] (4) The principle for selecting the specific depth is to avoid being more than 1m away from the flue.

[0064] like Figure 2-4 As shown, the position selected is the first layer catalyst of the SCR reactor, avoiding the gradually contracting and expanding sections of the flue and the vortex area, selecting the area less affected by the vortex area, determining the right area on the top of the SCR reactor as the alternative area, and selecting the measuring hole in the alternative area according to the measuring hole position. After selecting the measurement position, it is necessary to measure the flue gas velocity, fly ash concentration and fly ash particle size at that position through the measuring hole.

[0065] C. Correct the wear calculation formula by the actual wear of the catalyst

[0066] According to step B, obtain the test data of flue gas velocity, fly ash concentration and fly ash particle size, use the wear calculation formula to calculate the wear, take advantage of the downtime opportunity to check the actual wear of the catalyst at the corresponding position, compare it with the wear empirical formula, substitute the actual wear into the wear calculation formula, correct the constant λ in the wear calculation formula, calculate λ1, repeat the above process, and get λ2, λ3... in turn. At least 3 corrections should be made, and the average value is obtained to get λn, and the final wear calculation formula E is obtained as follows:

[0067] Ea=λnC0×Ug 3.4 ×D i 3.3

[0068] Step 2: Obtain the SCR full-section flue gas field parameters

[0069] After obtaining the calculation formula for the final wear amount of the actual catalyst device, how to judge the catalyst wear condition of the entire section? At this time, it is necessary to test the flue gas field parameters of the catalyst inlet section. The measurement section position, measurement point position, number of measurement points, and measurement parameters are described below.

[0070] A. Selection of measurement section and measurement point location

[0071] like Figure 5 As shown, a section is selected within the range of 500-1000 mm from the top surface of the catalyst as the measurement section;

[0072] Use longitude and latitude lines to divide the cross section into several squares, and the intersection of the diagonals of each square is the measurement point location;

[0073] like Figure 6 As shown in the figure, the relationship between the side length L (or B) of the rectangular section and the number of measuring points N is defined as follows:

[0074]

[0075] In addition, due to the large size of the SCR denitrification inlet cross-section, the measuring instruments currently on the market cannot take into account such a large cross-section. Holes can be opened symmetrically on opposite sides of the SCR inlet cross-section. The hole opening principles are as above.

[0076] B. Establishing the original database of measurement parameters

[0077] According to the wear calculation formula, it is necessary to measure the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of the SCR inlet cross section. Since the load of the unit changes at any time, the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution at one load point cannot be used to characterize the wear characteristics of the entire time period. Therefore, it is necessary to test the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of each load section;

[0078] Define the test load section to be selected from 100% ECR-30% ECR, select at least 5 test load sections, measure and record the flue gas velocity, fly ash concentration and fly ash particle size at each measuring point under each test load section, and establish an original database of measurement parameters;

[0079] For example, when establishing the original database of measurement parameters, the test load sections can be defined as 100% ECR, 75% ECR, 50% ECR, and 30% ECR. The flue gas velocity, fly ash concentration, and fly ash particle size at each measuring point are measured and recorded under each test load section to establish the original database of measurement parameters.

[0080] The test load sections can also be defined as 100% ECR, 95% ECR, 90% ECR, 85% ECR, 80% ECR, 75% ECR, 70% ECR, 65% ECR, 60% ECR, 55% ECR, 50% ECR, 45% ECR, 40% ECR, 35% ECR, and 30% ECR. The flue gas velocity, fly ash concentration, and fly ash particle size at each measuring point are measured and recorded under each test load section, and an original database of measurement parameters is established. The denser the intervals of the selected test load sections, the higher the measurement accuracy.

[0081] Step 3: Calculate the wear coefficient E

[0082] Substitute the actual flue gas parameters of the full-load section into the wear coefficient calculation formula to calculate the wear coefficient E:

[0083]

[0084] Where:

[0085] T is the operating time of the full load segment (i.e. the entire time period during the assessment period), ΔT is the operating time between two load segments, C 0i is the fly ash concentration in the operating load section, U gi is the flue gas velocity in the operating load section, D ii is the average particle size within the operating load section. The above fly ash concentration, flue gas velocity and average particle size are all obtained through the original database of measured parameters. When the operating load is between two adjacent test load sections, the data under the larger load section is substituted into the formula for calculation; for example, when the operation is under 98% ECR for 5 hours, the flue gas parameter C is selected. 0i , U gi , D ii The parameters are under 100% ECR condition, the time is 5 hours, and so on;

[0086] The method for selecting fly ash particle size is as follows: fly ash samples are taken from each measuring point under each load section condition, and fly ash particle size screening and statistics are performed on the above fly ash samples respectively to obtain the distribution law of sample particle size between 0 and 500 μm. According to the size of the currently commonly used coal powder sieve, weight percentage statistics are performed according to 0-28 μm, 28-38 μm, 38-50 μm, 50-63 μm, 63-80 μm, 80-90 μm, 90-100 μm, 100-160 μm, 160-200 μm, 200-250 μm, 250-400 μm, 400-500 μm, and >500 μm. The average particle size is calculated according to the Rosin-Rammler model to obtain the average particle size of each measuring point under each test load section condition.

[0087] Step 4: Catalyst replacement decision

[0088] According to the calculation results of the catalyst wear coefficient E at each load section and each wear position, it is judged whether the catalyst at the corresponding position needs to be replaced:

[0089] a. If E>0.3, the catalyst needs to be replaced;

[0090] b. If E < 0.3 and the catalyst service life is greater than 20,000 hours, the catalyst needs to be replaced;

[0091] c. If E < 0.3 and the service life of the catalyst is less than 20,000 hours, there is no need to replace the catalyst;

[0092] The larger the E value, the more severe the wear.

[0093] The present invention has achieved good technical effects through practical application, and the application examples are as follows:

[0094] A power generation company arranged appropriate measuring points according to the principle of selecting measuring positions and measuring holes in step 1 of the present invention, and carried out measurements. After correcting the wear calculation formula by obtaining parameters and correcting the actual wear of the catalyst, λ n =3.89×10 -9 , thus determining the calculation formula for the final wear amount E, the specific measurement results are as follows:

[0095] Selection of initial flue gas parameters

[0096]

[0097] (2) Measurement of flue gas parameters at full load section of catalyst inlet

[0098] A power generation company monitors the catalyst wear at 24 measuring points across the entire cross section. The measuring point layout is as follows: Fig. 9 As shown;

[0099] In order to reduce the amount of data, this application example uses data under four test load conditions to illustrate. The load conditions are 100% ECR, 75% ECR, 50% ECR, and 30% ECR. The measurement results are as follows. Based on the measurement results, an original database of measurement parameters can be established.

[0100] 100% ECR Condition Data Sheet

[0101]

[0102] 75% ECR Condition Data Table

[0103]

[0104]

[0105] 50% ECR Condition Data Table

[0106]

[0107] 30% ECR working condition data table

[0108]

[0109] (3) Calculation of wear coefficient E

[0110] According to the above load segment statistical time period, a total of 1000 hours are counted, and the results are as follows:

[0111] Working conditions 100-75% load 75-50% load 50-30% load Time (h) 200 550 250

[0112] (4) Substitute the wear coefficient calculation formula into the wear coefficient calculation (weighted time), and the results are shown in the following table:

[0113] Location Measuring hole 1 Measuring hole 2 Measuring hole 3 Measuring hole 4 Measuring hole 5 Measuring hole 6 Depth 1 0.0184 0.0415 0.1100 0.2068 0.2198 0.3799 Depth 2 0.0035 0.0080 0.0211 0.0396 0.0421 0.0712 Depth 3 0.0012 0.0027 0.0071 0.0132 0.0140 0.0241 Depth 4 0.0010 0.0028 0.0073 0.0104 0.0111 0.0181

[0114] According to the prediction results of the method of the present invention, the wear coefficient at the depth 1 of the measuring hole 6 is greater than 0.3, and the catalyst needs to be replaced.

[0115] (5) Result verification

[0116] According to the test results, the furnace was shut down for inspection and it was found that measuring holes 4, 5, 6 and depth 1 were all severely worn, with wear ratios of approximately 18%, 18% and 33%, respectively, which was consistent with the predicted results (it was found that the wear coefficients of measuring holes 4, 5 and depth 6 were 0.2068 and 0.2198, which were close to 0.3. Although no replacement was required, wear had already occurred, which was consistent with the situation where the larger the E value, the more severe the wear).

Claims

1. A method for determining the timing of replacement of an SCR catalyst due to wear, characterized in that: The following steps are involved: Step 1: Determine the wear calculation formula A. Determine the formula The catalyst wear amount is determined using the following formula: Where: Ea is the catalyst wear, λ is a constant, C0 is the fly ash concentration in the flue gas, U g is the flue gas velocity in the reactor, D i is the average particle size of fly ash in flue gas; B. Parameter acquisition A measurement position is selected above the first layer catalyst of the SCR reactor, avoiding the flue area corresponding to the gradually contracting and expanding section of the flue and the eddy flow area as an alternative area for the measurement position, and the selected measuring hole position is any measuring hole in the alternative area, and the flue gas velocity, fly ash concentration and fly ash particle size at the measuring position are measured through the measuring hole; C. Correct the wear calculation formula by the actual wear of the catalyst According to step B, obtain the test data of flue gas velocity, fly ash concentration and fly ash particle size, use the wear calculation formula to calculate the wear, take advantage of the shutdown opportunity to check the actual wear of the catalyst at the corresponding position, compare it with the wear empirical formula, substitute the actual wear into the wear calculation formula, correct the constant λ in the wear calculation formula, calculate λ1, repeat the above process, and get λ2, λ3... in turn. At least 3 corrections should be made, and the average value is obtained to get λn, and the final wear calculation formula Ea is obtained as follows: Step 2: Obtain the SCR full-section flue gas field parameters A. Selection of measurement section and measurement point location Select a section within the range of 500-1000 mm from the top surface of the catalyst as the measurement section; Use longitude and latitude lines to divide the cross section into several squares, and the intersection of the diagonals of each square is the measurement point location; B. Establishing the original database of measurement parameters According to the wear calculation formula, it is necessary to measure the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of the SCR inlet cross section. Since the load of the unit changes at any time, the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution at one load point cannot be used to characterize the wear characteristics of the entire time period. Therefore, it is necessary to test the flue gas velocity distribution, fly ash concentration distribution and fly ash particle size distribution of each load section; Define the test load section to be selected from 100% ECR-30% ECR, select at least 3 test load sections, measure and record the flue gas velocity, fly ash concentration and fly ash particle size at each measuring point under each test load section, and establish an original database of measurement parameters; Step 3: Calculate the wear coefficient E Substitute the actual flue gas parameters of the full-load section into the wear coefficient calculation formula to calculate the wear coefficient E: Where: T is the running time of the full load section, ΔT is the running time between two load sections, C 0i is the fly ash concentration in the operating load section, U gi is the flue gas velocity in the operating load section, D ii is the average particle size within the operating load section. The above fly ash concentration, flue gas velocity and average particle size are all obtained through the original database of measurement parameters. When the operating load is between two adjacent test load sections, the data under the condition of the larger load section is substituted into the formula for calculation; Step 4: Catalyst replacement decision According to the calculation results of the catalyst wear coefficient E at each load section and each wear position, it is judged whether the catalyst at the corresponding position needs to be replaced: a. If E>0.3, the catalyst needs to be replaced; b. If E < 0.3 and the catalyst service life is greater than 20,000 hours, the catalyst needs to be replaced; c. If E < 0.3 and the service life of the catalyst is less than 20,000 hours, there is no need to replace the catalyst; The larger the E value, the more severe the wear.

2. The method for determining the timing of SCR catalyst wear replacement according to claim 1, characterized in that: In the step 2 of establishing the original database of measurement parameters, the test load sections are defined as 100% ECR, 95% ECR, 90% ECR, 85% ECR, 80% ECR, 75% ECR, 70% ECR, 65% ECR, 60% ECR, 55% ECR, 50% ECR, 45% ECR, 40% ECR, 35% ECR, and 30% ECR. The flue gas velocity, fly ash concentration, and fly ash particle size at each measuring point are measured and recorded under each test load section condition to establish the original database of measurement parameters.

3. The method for determining the timing of SCR catalyst wear replacement according to claim 1, characterized in that: When the original database of measurement parameters is established in the step three, the method for selecting the fly ash particle size is as follows: fly ash samples are taken from each measuring point under each load section condition, and fly ash particle size screening statistics are performed on the above fly ash samples to obtain the distribution law of sample particle size between 0 and 500 μm, and weight percentage statistics are performed according to 0-28 μm, 28-38 μm, 38-50 μm, 50-63 μm, 63-80 μm, 80-90 μm, 90-100 μm, 100-160 μm, 160-200 μm, 200-250 μm, 250-400 μm, 400-500 μm, and >500 μm, and the average particle size is calculated according to the Rosin-Rammler model to obtain the average particle size of each measuring point under each test load section condition.

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