An on-line prediction method for slagging of heating surface of coal-fired boiler in power plant

Abstract

The present application belongs to the technical field of coal-fired power generation, and discloses an online prediction method for slagging of heating surface of coal-fired boiler in power station. The method comprises the following steps: S1, detecting real-time gas-phase sodium concentration at different positions and different time points in the furnace to obtain gas-phase sodium mass at different positions and different time points in the furnace; S2, using the gas-phase sodium concentration at different positions and different time points in the furnace, calculating the mass fraction of gas-phase sodium oxide in the furnace at different positions and over a period of time, combining the mass fraction of sodium oxide in ash, and calculating the total amount of sodium oxide in the coal entering the furnace at different positions and different time points; S3, setting a slagging judgment principle, and according to the distribution of total amount of sodium oxide in the coal entering the furnace at different positions and different time points obtained in step S2, obtaining the slagging tendency at different positions in the furnace to realize online prediction of the slagging tendency distribution in the furnace. Through the present application, the accuracy and real-time performance of prediction of slagging of heating surface of coal-fired boiler are improved.

Description

Technical Field

[0001] This invention belongs to the technical field of coal-fired power generation, and more specifically, relates to an online prediction method for slagging on the heating surface of a coal-fired boiler in a power plant. Background Technology

[0002] The heating surfaces of a coal-fired boiler in a power plant, including the furnace water-cooled walls, superheaters, reheaters, economizers, and air preheaters, are crucial equipment for transferring the heat energy released from combustion to the working fluid. High-temperature sintering ash buildup occurs on the heating surfaces of the screen-type and convection-type superheaters and reheaters located in the upper part of the furnace and horizontal flue. The fundamental reason is that gaseous alkali metal compounds such as sodium and potassium produced during combustion in the furnace condense on the tube walls of the superheaters and reheaters at slightly lower temperatures, forming a thin white ash layer. This ash layer then adheres to fly ash and continues to thicken. Especially when burning Zhundong coal or blending with high-alkali fuels such as biomass, severe slagging and fouling are more likely to occur in the high-temperature superheaters and reheaters, resulting in a decrease in outlet air temperature and an increase in outlet flue gas temperature, leading to higher boiler exhaust gas temperature and reduced unit economics. Therefore, accurately predicting the degree of slagging on the heating surfaces of a coal-fired boiler in a power plant is helpful in taking better measures to prevent severe slagging, which is of great significance for further improving the economics of coal-fired power generating units.

[0003] The standard GB / T 39836-2021, "Determination Method of Slagging Index of Coal Combustion," implemented in 2021, specifies the determination method, test apparatus, test instruments and methods, data processing and calculation methods for the slagging index of coal combustion. This standard primarily involves conducting pulverized coal combustion tests in a one-dimensional furnace. Through ash sampling and analysis, static monitoring indicators such as softening temperature, silica-alumina ratio, alkali-acid ratio, and silica ratio are obtained. Then, the slagging characteristics of coal combustion are classified into slight slagging, moderate slagging, and severe slagging using the coal ash probe slagging discrimination index, the comprehensive coal ash slagging discrimination index, and the coal ash clustering slagging discrimination index. Therefore, the ash content of the coal entering the furnace is an internal factor causing slagging in the furnace, a static factor, but only one of the determining factors. Slagging on the heating surfaces in the furnace is also affected by boiler operating parameters, which are external factors and dynamic factors. The actual slagging situation caused by the same coal in different furnace types and under different combustion methods will also be different. Therefore, some researchers have proposed to consider dynamic monitoring indicators such as cleanliness factor, thermal efficiency coefficient, ash and fouling thermal resistance, heat transfer efficiency, and critical fouling rate, and to establish a method for online monitoring or evaluation of slagging on the furnace heating surface based on the above dynamic monitoring data.

[0004] However, existing methods and standards do not consider the impact of high alkali metal content in coal on slagging, and are not applicable to coal types with a total alkali metal content greater than 3%, including Zhundong coal. As mentioned earlier, the fundamental factor causing high-temperature sintering ash accumulation on the heating surfaces of superheaters and reheaters is the gaseous alkali metal compounds produced during combustion. Furthermore, existing methods and standards do not predict the distribution of slagging tendency on heating surfaces or specific areas of heating surfaces (such as the lower section of water-cooled walls); slagging predictions only provide a rough estimate of the slagging tendency for the entire furnace. Therefore, it is necessary to propose an online prediction method for the distribution of slagging tendency on the heating surfaces of coal-fired power plant boilers that considers gaseous alkali metals, to further improve the accuracy and real-time performance of slagging monitoring on the heating surfaces or specific areas of coal-fired power plant boilers, especially for those burning high-alkali coal or co-fired biomass. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, this invention provides an online prediction method for slagging on the heating surfaces of coal-fired power plant boilers, solving the problems of low prediction accuracy and poor real-time performance for slagging on the heating surfaces of gaseous alkali metal coal-fired power plant boilers.

[0006] To achieve the above objectives, according to one aspect of the present invention, an online prediction method for slagging on the heating surface of a coal-fired boiler in a power plant is provided, the method comprising the following steps:

[0007] S1 performs coal quality analysis on the coal type and collects the amount of coal fed into the furnace, thereby calculating the flue gas flow rate in the furnace and detecting the real-time gaseous sodium concentration at different locations and times in the furnace, thereby calculating the gaseous sodium mass at different locations and times in the furnace.

[0008] S2 uses the gaseous sodium concentration at different locations in the furnace at different times to calculate the mass fraction of gaseous sodium oxide at different locations in the furnace over a period of time. Combined with the mass fraction of sodium oxide in the ash, the total amount of sodium oxide from the coal type fed into the furnace at different locations and at different times is calculated.

[0009] S3 sets the slagging judgment principle. Based on the distribution of the total sodium oxide content of the coal type entering the furnace at different locations and time periods obtained in step S2, the slagging tendency at different locations in the furnace is predicted online.

[0010] More preferably, in step S1, the mass of gaseous sodium at different locations and times within the furnace is calculated according to the following formula:

[0011] M Na,g =C Na V

[0012] Among them, C Na V is the current concentration of sodium in the gaseous phase, V is the actual volume of flue gas in the furnace, and M is the concentration of sodium in the gaseous phase at that moment. Na,gIt represents the real-time mass of sodium in the gas phase at the current moment.

[0013] More preferably, in step S2, the total amount of sodium oxide in the coal fed into the furnace is determined according to the following steps:

[0014] S21 calculates the mass of sodium oxide burned at different locations and times based on the mass of gaseous sodium obtained in step S1 at different locations and times.

[0015] S22 calculates the mass fraction of sodium oxide over a period of time.

[0016] More preferably, in step S21, the mass of sodium oxide burned at different positions and at different times is calculated according to the following formula:

[0017]

[0018] in, M is the mass of sodium oxide burned at the current moment. Na,g It represents the total amount of gaseous sodium at the current moment.

[0019] More preferably, in step S22, the mass fraction of sodium oxide over the specified time period is calculated according to the following formula:

[0020]

[0021] Where t1 is the start time of the calculation, and t is the current time. It is the cumulative value of dynamic Na2O content at various locations over a period of time. It is the cumulative value of the amount of coal fed into the furnace over a period of time, A ar It is the ash content of the received basis in industrial analysis. It is the percentage of total sodium oxide burned over a period of time relative to the amount of coal fed into the coal mine.

[0022] More preferably, in step S2, the total amount of sodium oxide fed into the furnace at each location is calculated according to the following formula:

[0023]

[0024] Where w(Na2O) represents the mass fraction of Na2O in the ash analysis, and γ is the total amount of sodium oxide in the coal entering the furnace at each location. It is the percentage of total sodium oxide burned over a period of time relative to the amount of coal fed into the coal mine.

[0025] More preferably, in step S3, the slag determination principle is as follows:

[0026] When γ < M1, the tendency to slag is low;

[0027] When M1≤γ<M2, the tendency to slagging is moderate;

[0028] When M2≤γ<M3, the tendency to slag is high;

[0029] When γ > M3, the tendency to slag is severe.

[0030] More preferably, M1, M2, and M3 are obtained according to the following steps:

[0031] S31 conducts combustion experiments on at least three types of coal in a boiler to obtain the total sodium oxide distribution and sodium oxide mass fraction in ash analysis for each type of coal, calculates the average total sodium oxide for each type of coal, and thus obtains the average sodium oxide mass fraction and total sodium oxide for each type of coal.

[0032] S32 constructs a coordinate system with sodium oxide mass fraction as the abscissa and the average value of total sodium oxide as the ordinate. The average values ​​of sodium oxide mass fraction and total sodium oxide obtained for each coal type in S31 are reflected in the coordinate system and fitted into a curve. The values ​​of M1, M2 and M3 are determined according to the preset sodium oxide threshold for ash analysis.

[0033] In summary, the technical solutions conceived by this invention have the following beneficial effects compared with the prior art:

[0034] 1. This invention proposes an online prediction method for slagging on the heating surface of a coal-fired power plant boiler with gaseous alkali metals. It takes into account the impact of high alkali metal content in coal on slagging and is applicable to coal types with a total alkali metal content greater than 3%, including Zhundong coal, etc.

[0035] 2. By detecting the distribution of gaseous sodium in the furnace and processing historical data, the future slagging degree of a single heating surface or heating surface area can be accurately predicted online, thereby further improving the accuracy and real-time performance of slagging prediction for the heating surface of coal-fired boilers, especially for coal-fired boilers in power plants that burn high-alkali coal or co-fire biomass.

[0036] 3. When calculating the total amount of sodium oxide in the coal fed into the furnace at each location, this invention takes into account the total amount of sodium oxide burned over a period of time. Compared with the prior art, which only uses the sodium oxide content in ash analysis to replace the total sodium oxide content, this invention has higher calculation accuracy, is more in line with the actual situation, and improves the accuracy of the final prediction results.

[0037] 4. The present invention uses a method of fitting curves from multiple coal types to obtain the predicted range of slagging tendency, without the need for slagging experiments. The method is simple and easy to implement. Attached Figure Description

[0038] Figure 1 This is a flowchart of an online prediction method for slagging on the heating surface of a coal-fired boiler in a power plant, constructed according to a preferred embodiment of the present invention.

[0039] Figure 2 This is a graph showing the change in the amount of coal fed into the furnace, constructed according to a preferred embodiment of the present invention.

[0040] Figure 3 This is a graph showing the change in gaseous sodium concentration at the 19m center position over 12 hours, constructed according to a preferred embodiment of the present invention.

[0041] Figure 4 This is a curve showing the change in the total amount of Na2O entering the furnace at the center position of 19m, constructed according to a preferred embodiment of the present invention.

[0042] Figure 5 It is a fitting curve of the interval threshold value constructed according to a preferred embodiment of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0044] An online prediction method for slagging on the heating surface of a coal-fired power plant boiler, taking into account the release of gaseous alkali metals during combustion, includes the following steps:

[0045] (1) The coal type is analyzed and the amount of coal fed into the furnace is collected. The flow rate of flue gas in the furnace is calculated. The real-time concentration of gaseous sodium at different locations and times in the furnace is detected. The mass of gaseous sodium at different locations and times in the furnace is calculated.

[0046] (2) Using the gaseous sodium concentration at different locations in the furnace at different times, calculate the mass fraction of gaseous sodium oxide at different locations in the furnace over a period of time. Combined with the mass fraction of sodium oxide in the ash, calculate the total amount of sodium oxide of the coal type entering the furnace at different locations and at different times.

[0047] (3) Set the slagging judgment principle. Based on the distribution of the total sodium oxide of coal type in different locations and time periods obtained in step S2, the slagging tendency at different locations in the furnace is realized to achieve online prediction of the slagging tendency distribution in the furnace.

[0048] Preferably, step (1) includes the following sub-steps:

[0049] S1: Perform coal quality analysis (on received basis) on the coal type, including: industrial analysis, elemental analysis and ash analysis. Calculate the flue gas flow rate V in the furnace based on the coal quality analysis, collect the amount of coal fed into the furnace m1, and record the relevant results.

[0050] S2: Detect the concentration C of gaseous sodium at different locations and times within the furnace. Na The change is denoted as Δt between two adjacent detections, and the gas phase sodium concentration C at each location and time is used. Na Multiply by the actual flue gas flow rate V in the furnace to obtain the mass M of gaseous sodium at each location and time. Na,g (Unit: mg):

[0051] M Na,g =C Na V

[0052] Preferably, step (2) includes the following sub-steps:

[0053] S1: Convert the mass of vapor sodium at each location and time in the furnace into the mass of Na2O at each location and time in the furnace according to the following formula.

[0054]

[0055] In the formula, 2 represents that one Na2O molecule contains 2 Na atoms, 23 is the atomic mass fraction of Na, and 62 is the molecular mass fraction of Na2O, with the unit being mg.

[0056] S2: Calculate the mass fraction of sodium oxide over a period of time using the following formula.

[0057]

[0058] In the formula, t1 represents the start time of the calculation; t represents the current time. The cumulative distribution of dynamic Na2O content at various locations over a period of time is represented by mg. This represents the cumulative amount of coal fed into the furnace over a period of time, expressed in kg; 10 2 The purpose is to Unit conversion to %; A ar Ash content of the received basis in industrial analysis, in %; Based on ash mass, the unit is %.

[0059] S3: The total sodium oxide γ at different times and locations of the coal entering the furnace is calculated using the following formula:

[0060]

[0061] In the formula, w(Na2O) represents the mass fraction (%) of Na2O in the ash analysis;

[0062] Preferably, step (3) includes the following sub-steps:

[0063] S1: Considering that slagging is a long-term process, the prediction interval for slagging is proposed as follows:

[0064] When γ < M1, the tendency to slag is low; when M1 ≤ γ < M2, the tendency to slag is moderate; when M2 ≤ γ < M3, the tendency to slag is high; when γ > M3, the tendency to slag is severe.

[0065] The values ​​of M1, M2, and M3 are determined through the following process: Combustion experiments are conducted on at least three types of coal in a boiler to obtain the total sodium oxide distribution and sodium oxide mass fraction in ash analysis for each coal type. The average value of the total sodium oxide for each coal type is calculated. A coordinate system is constructed with the sodium oxide mass fraction in ash analysis as the abscissa and the average value of the total sodium oxide for each coal type as the ordinate. The average value of the sodium oxide mass fraction and the total sodium oxide for each coal type is plotted in the coordinate system, and a fitting curve is plotted. The values ​​of M1, M2, and M3 are determined based on a preset sodium oxide threshold in ash analysis.

[0066] S2: Based on the total Na2O distribution γ in each region, predict the slagging tendency distribution of each heat exchange surface from the current time t to the future time t2, and display the judgment results in real time, so as to achieve the goal of online prediction of the slagging tendency distribution of the heat exchange surface during the combustion process of coal fed into the furnace.

[0067] The present invention will be further described below with reference to specific embodiments.

[0068] Figure 1 The present invention is illustrated in the flowchart below. The present invention will be further explained in detail through an embodiment in conjunction with the flowchart.

[0069] This example uses a 660MW coal-fired boiler with opposing front and rear walls (using Zhundong coal) as a case study. A method for online prediction of slagging on the heating surface of a power plant coal-fired boiler includes the following steps:

[0070] (1) Coal quality analysis calculation and detection: coal quality analysis was performed on the coal receiving basis to determine the coal type by slag formation, and the industrial analysis, elemental analysis and ash analysis of the receiving basis were obtained. The relevant results are recorded as shown in Table 1.

[0071] Table 1

[0072]

[0073] Based on the elemental analysis of the coal and formula (1), the dry air consumption V per unit mass of the coal for complete combustion is calculated. 0 (Unit: m) 3 ):

[0074] V 0 =0.0889(C ar +0.375S ar )+0.265H ar-0.0333O ar (1)

[0075] Among them, C ar S ar H ar With O ar The percentages of carbon, sulfur, hydrogen, and oxygen in the coal obtained in step (1) are given. Substituting the data from Table 1 into formula (1), we obtain the volume of dry air required for the complete combustion of 1 kg of Zhundong coal, V. 0 =7.06m 3 / kg. Then, using the following formulas, calculate the volumes of nitrogen, carbon dioxide, and water vapor produced in the flue gas from burning 1 kg of coal, assuming an equivalence ratio of 1:

[0076]

[0077] Substituting the data from Table 1 into formula (2), we obtain the volume of nitrogen in the flue gas produced by burning 1 kg of Zhundong coal under the condition of an equivalence ratio of 1. carbon dioxide volume With water vapor volume In actual combustion processes, the amount of air used for combustion often exceeds the amount required for the combustion reaction. Therefore, an excess air coefficient needs to be introduced to correct for the volume of dry flue gas and water vapor.

[0078]

[0079] In the formula, V gy For dry flue gas volume, Let α be the volume of water vapor and α be the excess air coefficient, which can be easily obtained from the oxygen monitoring equipment in the power plant. In the boiler furnace, α = 1.15. Substituting the calculated data into formula (3), the dry flue gas volume V is calculated. gy =7.97mg 3 / kg, water vapor volume mg 3 / kg. Calculate the volume of flue gas produced by the complete combustion of 1 kg of coal using the following formula:

[0080] V y =V gy +V H2O (4)

[0081] The volume of flue gas V produced by the complete combustion of 1 kg of coal was calculated. y V y =8.77m 3 / kg. The change in coal input over 12 hours, as shown by boiler DCS system data, is as follows: Figure 2 As shown, the amount of coal fed multiplied by the volume of flue gas produced by the complete combustion of 1 kg of coal is V.y The furnace flue gas flow rate V can then be obtained, in meters. 3 .

[0082] The change in gaseous sodium concentration over time at various locations within the furnace was obtained using a spectral probe. Na The measurement time interval Δt = 1s, and the change curve of gaseous sodium concentration at the center position at an elevation of 19m over 12 hours is shown below. Figure 3 As shown. In this embodiment, only the distribution of vaporized sodium along the height direction on the central axis of the boiler is considered. The mass M of vaporized sodium at different locations and times in the furnace is calculated according to the following formula. Na,g (Unit: mg):

[0083] M Na,g =C Na V (5)

[0084] (2) Calculate the mass of sodium oxide burned at different locations and times according to the following formula.

[0085]

[0086] In the formula, 2 represents that one Na₂O molecule contains 2 Na atoms, 2³ is the atomic mass fraction of Na, and 6² is the molecular mass fraction of Na₂O. The unit is mg. Calculate the mass fraction of sodium oxide over a period of time using the following formula.

[0087]

[0088] In the formula, This represents the cumulative dynamic Na2O content at various locations over a period of time, expressed in mg. A represents the cumulative amount of coal fed into the furnace over a period of time, expressed in kg; ar Ash content of the received basis in industrial analysis, in %; 10 2 The purpose is to The units are converted to %; t1 represents the initial time of calculation; t represents the current time, and we take t-t1 = 3600s. The total sodium oxide γ at different positions and time periods of the coal entering the furnace is calculated using the following formula:

[0089]

[0090] In the formula, w(Na2O) represents the mass fraction (%) of Na2O in the ash analysis. The result of the γ change at a height of 19m is as follows: Figure 4 As shown.

[0091] (3) Considering that slagging is a long-term process, the impact of gaseous alkali metal release on slagging during combustion needs to be reflected over a longer time scale. Combustion experiments were conducted on a boiler using three different types of coal: high-ash-fusion-point Shenhua coal, low-ash-fusion-point Shenhua coal, and Zhundong coal. The total sodium oxide distribution and sodium oxide mass fraction in ash analysis were obtained for the three coal types. The average total sodium oxide content of the three coal types was calculated. A coordinate system was constructed with the sodium oxide mass fraction in ash analysis as the abscissa and the average total sodium oxide content as the ordinate. The sodium oxide mass fraction and the average total sodium oxide content of the three coal types were plotted on the coordinate system, and a fitting curve was created. The curve is shown below. Figure 5 As shown, select 2mg / m 3 6mg / m 3 With 8mg / m 3 The values ​​of M1, M2, and M3, which are used as the three thresholds for judgment, are 4.25, 8.82, and 11.52, respectively.

[0092] The final judgment principle is as follows: when γ < 4.25, the tendency to slag is low; when 4.25 ≤ γ < 8.82, the tendency to slag is moderate; when 8.82 ≤ γ < 11.52, the tendency to slag is high; when γ > 11.52, the tendency to slag is severe.

[0093] Based on the results of each position calculated in step (2), predict the slagging tendency of each heated surface from the current time t to the future time t2, and take t2-t=3600s.

[0094] The slagging prediction results for the water-cooled wall at position 19m within 12 hours were as follows: the slagging tendency is low in the time period from 26750s to 29420s, and moderate in the time period at other times. Similarly, the distribution of slagging prediction in the height direction can be obtained.

[0095] Those skilled in the art will readily understand that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant, characterized in that, The method includes the following steps: S1 performs coal quality analysis on the coal type and collects the amount of coal fed into the furnace, thereby calculating the flue gas flow rate in the furnace and detecting the real-time gaseous sodium concentration at different locations and times in the furnace, thereby calculating the gaseous sodium mass at different locations and times in the furnace. S2 Calculate the mass fraction of gaseous sodium oxide at different locations in the furnace over a period of time using the gaseous sodium concentration at different times. Combine this with the mass fraction of sodium oxide in the ash to calculate the total amount of sodium oxide in the coal fed into the furnace at different locations and time periods. S3 sets the slagging judgment principle. Based on the distribution of the total sodium oxide content of the coal type entering the furnace at different locations and time periods obtained in step S2, the slagging tendency at different locations in the furnace is predicted online. In step S2, the total amount of sodium oxide at different locations and time periods of the coal fed into the furnace is calculated according to the following formula: in, This indicates the mass fraction of Na2O in the ash analysis. This refers to the total amount of sodium oxide from the coal type fed into the furnace at each location. It is the percentage of total sodium oxide burned over a period of time relative to the amount of coal fed into the coal mine.

2. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 1, characterized in that, In step S1, the mass of gaseous sodium at different locations and times within the furnace is calculated according to the following formula: Among them, C Na V is the current concentration of sodium in the gaseous phase, V is the actual flue gas flow rate in the furnace, and M is the concentration of sodium in the gaseous phase at that moment. Na,g It represents the real-time mass of sodium in the gas phase at the current moment.

3. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 1, characterized in that, In step S2, the total amount of sodium oxide in the coal fed into the furnace is determined according to the following steps: S21 Calculate the mass of sodium oxide burned at different locations and times based on the mass of gaseous sodium obtained in step S1. S22 Calculates the mass fraction of sodium oxide over a period of time.

4. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 3, characterized in that, In step S21, the mass of sodium oxide burned at different locations and at different times is calculated according to the following formula: in, M is the mass of sodium oxide burned at the current moment. Na,g It represents the current mass of gaseous sodium.

5. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 3 or 4, characterized in that, In step S22, the mass fraction of sodium oxide over the specified time period is calculated according to the following formula: in, t 1 represents the start time of the calculation. t It is the current moment. It is the cumulative value of dynamic Na2O content at various locations over a period of time. It is the cumulative value of the amount of coal fed into the furnace over a period of time. It is the ash content of the received basis in industrial analysis. It is the percentage of total sodium oxide burned over a period of time relative to the amount of coal fed into the coal mine.

6. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 1, characterized in that, In step S3, the slag formation judgment principle is as follows: when M1 has a low tendency to slagging; When M1 M2, moderate tendency to slagging; When M2 M3 has a high tendency to slagging; when M3 has a severe tendency to slagging.

7. The method for online prediction of slagging on the heating surface of a coal-fired boiler in a power plant as described in claim 6, characterized in that, M1, M2, and M3 are obtained according to the following steps: S31 Conduct combustion experiments on at least three types of coal in a boiler to obtain the total sodium oxide distribution and sodium oxide mass fraction in ash analysis for each type of coal, calculate the average value of the total sodium oxide for each type of coal, and thus obtain the average value of sodium oxide mass fraction and total sodium oxide in ash analysis for each type of coal. S32 Construct a coordinate system with the sodium oxide mass fraction in ash analysis as the abscissa and the average value of the total sodium oxide as the ordinate. The average values ​​of the sodium oxide mass fraction and the total sodium oxide obtained in S31 for each coal type are reflected in the coordinate system and fitted into a curve. The values ​​of M1, M2 and M3 are determined according to the preset sodium oxide threshold for ash analysis.

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