Judgment method for high temperature corrosion degree of boiler water cooling wall in large power station

[0025] The Shimen Power Plant in Hunan Province installed 2×300MW units. During the overhaul of the No. 1 boiler in 1999, serious high-temperature corrosion was found in the water wall around the high-load area of ​​the burner. The wall thickness measurement results show that the wall thickness of the water-cooled pipe with a design specification of φ63.5×8mm is only 6.8~7.1mm, which has been thinned by 0.9~1.2mm, and the corrosion rate is about 0.7~1.3mm/(10 4 )h. If the development continues at this speed, a large-scale water wall explosion accident may occur in recent years. Therefore, it is urgent to analyze and study its high-temperature corrosion mechanism and main influencing factors as soon as possible, seek the best technical way to solve the problem, and improve the safety and reliability of the heating surface of the boiler.

[0026] First, determine the four major factors affecting high-temperature corrosion: Standard coal converted to sulfur content S b ar; Co content near the wall; equivalent tangent circle diameter D sj; Tube wall temperature T. Then make the following provisions for the determination of the four major influencing factors:

[0027] 1. Measuring position of equivalent tangent circle diameter:

[0028] The actual tangent circle measurement position in the furnace is within the range of the lowermost primary air pulverized coal burner center elevation +0~100m, which is different from the measurement position of the furnace fullness and tangential circle diameter in the conventional cold aerodynamic field test. There should be no human interference.

[0029] 2. Test conditions:

[0030] When measuring the combustion atmosphere near the water wall in the high-load area, the test load must be greater than 80% of the rated load, the main combustion area in the furnace maintains a reasonable amount of oxygen, the oxygen content of the main control room is greater than 4.5%, and the measurement conditions are not less than three loads Working conditions.

[0031] 3. Testing equipment and measurement requirements:

[0032] The range of the test instrument must meet the requirements, passed the verification, and within the validity period, at least two different types of instruments with the same function should be used for measurement to improve the measurement accuracy; in the test of each working condition, each measuring point The number of measurements should not be less than three, and the stabilization time of each measurement should be greater than 5 minutes, and the average value of the measurement should be taken.

[0033] 4. Technical requirements for measuring point installation:

[0034] Use the optimization method to determine the location and quantity of the measuring points. Install the measuring points on the furnace wall where the high temperature corrosion of the water wall is the most serious, and install 7 measuring points on the corrosion concentrated parts. The main purpose is to measure the CO content of the most corroded position. The number of points is much less than the number of measuring points for conventional tests.

[0035] The sampling port in the measuring point furnace is flush with the high temperature corrosion part of the water wall, that is, h=1/2φ, h=34mm, and the total length of the measuring point is greater than 760mm. The main consideration is the cooling effect of the sampling tube on the sampling flue gas outside the furnace. Click to install see figure 1. Among them, 1-water wall tube, 2-fins, 3-water wall tube corrosion part, 4-corrosion area combustion atmosphere measurement point. And the measuring point sampling tube adopts φ8×1.5 stainless steel tube.

[0036] 5. The test result of the total sulfur content of the coal is converted to the sulfur content S in the standard coal according to the following formula b ar In order to determine the degree of corrosion accurately.

[0037] S b ar =29300×S ar /Q net.ar

[0038] Where: S arIs the sulfur content of the coal actually burned by the boiler, Q net.ar It is the received base low calorific value of the coal actually burned by the boiler.

[0039] The specific values ​​and grading standards of the four major influencing factors of high temperature corrosion in Shimen Power Plant are shown in Table 1, and the calculated values ​​of high temperature corrosion index and grading standard processing values ​​are shown in Table 2.

[0040] Table 1 Classification standard for high temperature corrosion tendency of water wall

[0041]

[0042] Factor

[0043] The high temperature corrosion tendency and corrosion rate standards of the water wall are:

[0044] (1) The degree of corrosion is slight or not corroded <0.5mm/10 4 h;

[0045] (2) The degree of corrosion is moderate or relatively obvious 0.5~1.5mm/10 4 h;

[0046] (3) The degree of corrosion is serious >1.5mm/10 4 h;

[0047] The following uses the grey clustering method to comprehensively evaluate the high temperature corrosion tendency of the water wall. The specific steps are as follows:

[0048] (1), establish clustering factor set

[0049] Combine the factors that affect the judgment object into a set X = {x 1 , X 2 , Λ, x j }, j=1, 2, Λ, p, p is the number of factors. The main reasons are summarized as four major influencing factors: Standard coal converted to sulfur content S b ar , CO content near the wall, equivalent tangent circle diameter D sj , Tube wall temperature T.

[0050] (2) Determine the evaluation level

[0051] The various evaluation levels that may be made by the judged object are formed into a set V, and the corrosion tendency is defined as three degree levels: slight, medium, and severe.

[0052] (3) Provision of clustering rights

[0053] According to different evaluation levels, determine the relative weight of each factor of the corresponding level, which can be calculated as follows: η jk = ( 1 / λ jk ) / Σ j = 1 p ( 1 / λ jk ) , Among them, λ jk Is the grading standard value, j = 1, 2, Λ, p, p is the number of factors, k = 1, 2, Λ, m, and m is the number of levels.

[0054] (4), specify the whitening function

[0055] The whitening function is specified for the set evaluation level, and the evaluation level is divided into 3 levels (m=3).

[0056]

[0057]

[0058]

[0059] (5), calculate the clustering coefficient δ k

[0060] Solve the clustering weight of a sample to different levels δ k = Σ j = 1 p f jk ( λ jk ) η jk , Among them, j = 1, 2, Λ, p, p is the number of factors; k = 1, 2, Λ, m, m is the number of levels; λ jk It is the grading standard value.

[0061] (6) Determine the propensity level and confidence level according to the maximum principle

[0062] Determine various whitening function values ​​f k , The function value is linearly interpolated according to the above whitening function, and the maximum value is 1. Determine the clustering coefficient η k , The coefficient of the k-th category, δ k = Σ j = 1 4 f jk ( λ jk ) η jk , Where k=1, 2, 3.

[0063] According to the above calculation method, through the compilation of calculation software, the cluster coefficient δ of high temperature corrosion of the boiler water wall of Shimen Power Plant k :

[0064] The clustering coefficient for minor corrosion is δ 1 = 0.309;

[0065] The clustering coefficient of moderate corrosion is δ 2 = 0.632;

[0066] The clustering coefficient of severe corrosion is δ 3 =0.05;

[0067] Construct a clustering vector, and determine the classification according to the principle of maximum vector:

[0068] The clustering row vector of Shimen Power Plant is δ k = (0.309, 0.632, 0.05), where δ 2 = 0.632 is the maximum, so it is judged to be moderate high temperature corrosion.

[0069] The cluster row vector is normalized, in order to compare the tendency of the high temperature corrosion of the water wall,

[0070] Need to normalize the clustering row vector, that is, use δ 1 +δ 2 +δ 3 Divide the sum of, and get: δ k = (0.312, 0.638, 0.05), therefore, the high temperature corrosion tendency of the boiler water wall of Shimen Power Plant is medium, and the confidence level is 0.638, which is 63.8%, so it is judged as medium high temperature corrosion. The original corrosion rate is 0.7~1.3mm/10 4 h is in the middle corrosion range, which is consistent with the original actual corrosion situation of the boiler.

[0071] Based on the above conclusions, it is recommended that Shimen Power Plant give priority to adopting the equipment modification plan with less on-site modification work-the dual-channel burner "wedge body modification technology". In the cold aerodynamic field test, the combustion after the installation of the wedge body is tested The cold flow field at the outlet of the device was measured by German anemometer and the long streamer was used to observe the air flow trajectory. The results showed that the speed of the primary air outlet increased, and the air flow attenuation and deflection characteristics were significantly improved. The primary air rotated in the direction of the main air flow, and no brush was found. Wall phenomenon; when the electric load of the hot state unit is 270MW and the oxygen content of the flue gas at the outlet of the economizer is >4.5~5%, the combustion atmosphere near the water wall in the high load area: O 2 = 2.1 to 5.5%, CO = 580 to 915 ppm, there is basically no reducing atmosphere, and the effect is good after implementation.

[0072] Practical results show that, on the basis of equipment modification, through the adoption of supporting boiler operation optimization and adjustment and fuel quality control measures, the problem of high temperature corrosion of the water wall of Shimen Power Plant #2 boiler has been basically solved.