A new method for controlling the silica content of boiler water

Abstract

This invention relates to a novel method for controlling the silicon content in boiler water, belonging to the field of water chemistry technology for thermal power plants. The method dynamically determines the allowable silicon content in the boiler water under the current operating conditions based on the real-time boiler operating pressure and a preset negative correlation between boiler pressure and the allowable silicon content in the boiler water. The measured silicon content in the boiler water is compared with this dynamic allowable value, and boiler blowdown is only performed when the measured value exceeds the allowable value. This invention utilizes the physical characteristic that the solubility and carry-over coefficient of silicon in boiler water by saturated steam decreases with decreasing boiler pressure. Under low-load conditions, the control index for silicon content in boiler water is reasonably relaxed, thereby significantly reducing the frequency and amount of boiler blowdown while ensuring steam quality and turbine safe operation. This reduces boiler heat loss and demineralized water consumption, improving boiler thermal efficiency and overall unit economy.

Description

Technical Field

[0001] This invention relates to the field of water chemistry technology in thermal power plants, specifically a new method for controlling the silicon content in boiler water. Background Technology

[0002] During operation, boiler water inevitably contains silica. Saturated steam has a certain capacity to dissolve and carry silica in the boiler water, and this capacity increases with increasing boiler pressure. When the silica content in the saturated steam exceeds the allowable value, silica will deposit on the surface of the turbine blades, forming salt deposits, reducing the flow area and decreasing the turbine's economic efficiency. Therefore, the current national standard "Water and Steam Quality Standard for Thermal Power Generating Units and Steam Power Equipment" (GB / T 12145-2016) sets upper limits for the silica content in boiler water for boilers with different parameters: ≤2 mg / L for high-pressure boilers, ≤0.45 mg / L for ultra-high-pressure boilers, and ≤0.1 mg / L (i.e., 100 μg / L) for subcritical boilers. These standard limits are fixed control indicators established to ensure that the silica content in the steam does not exceed the limit when the boiler is operating under rated conditions.

[0003] However, with the increasing demands of the power grid on the peak-shaving capacity of generator units, subcritical parameter drum boilers are frequently operating under non-rated load conditions. When the unit load decreases, the boiler pressure drops accordingly, and the dissolution and carry-over coefficient of silicon in saturated steam decreases significantly. Under the condition of boiler water pH = 9-9.7, when the boiler pressure drops from 17.64 MPa to 12.74 MPa, the dissolution and carry-over coefficient drops from approximately 8% to approximately 2.8%. Under this low-pressure condition, even if the silicon content in the boiler water exceeds 100 μg / L, the actual silicon content entering the saturated steam is often far below the safety limit, and will not lead to the deterioration of turbine salt deposits. If boiler blowdown is still mechanically carried out according to the national standard of 100 μg / L, it will cause unnecessary boiler water heat loss and waste of demineralized water, reducing the boiler's economic efficiency. Therefore, the contradiction in the existing technology lies in the need to control the silicon content in the boiler water to prevent turbine salt deposits on the one hand, and to reduce boiler blowdown to improve operating economics on the other hand. How to flexibly adjust the control index of silicon content in boiler water according to the actual operating conditions of the boiler while ensuring steam quality is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] In order to solve the problems of the prior art, the present invention provides a new method for controlling the silicon content of boiler water.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution: Firstly, a novel method for controlling the silicon content of boiler water, applied to a subcritical parameter drum boiler, wherein the pH value of the boiler water is controlled between 9 and 9.7, includes the following steps: Obtain real-time operating parameters of the boiler drum, including unit load and / or boiler pressure; Based on the real-time operating parameters, and according to the preset correspondence between boiler pressure and allowable silicon content in boiler water, the allowable silicon content in boiler water is dynamically determined; wherein, the allowable silicon content in boiler water is negatively correlated with the boiler pressure. Obtain the actual silicon content of the boiler water; The actual silicon content of the boiler water is compared with the dynamic allowable silicon content of the boiler water. If the actual silicon content of the boiler water is greater than the allowable silicon content of the boiler water, then boiler blowdown operation is performed.

[0006] In one specific implementation of the first aspect, the preset correspondence includes: When the boiler pressure is within the first pressure range, the allowable silicon content in the boiler water is set to the first allowable value; When the boiler pressure is within the second pressure range, the allowable silicon content in the boiler water is set to the second allowable value. Wherein, the upper limit of the second pressure range is less than or equal to the lower limit of the first pressure range, and the first allowable value is less than the second allowable value.

[0007] In one specific embodiment of the first aspect, the correspondence between the boiler pressure and the permissible silicon content of the boiler water is as follows: When the boiler pressure is 16.66MPa~17.64MPa, the allowable silicon content in the boiler water is ≤100μg / L; When the boiler pressure is 15.19MPa~16.66MPa, the allowable silicon content in the boiler water is ≤130μg / L; When the boiler pressure is 13.72MPa~15.19MPa, the allowable silicon content in the boiler water is ≤160μg / L; When the boiler pressure is ≤13.72MPa, the allowable silicon content in the boiler water is ≤200μg / L.

[0008] In one specific embodiment of the first aspect, obtaining the real-time operating parameters of the boiler drum includes: real-time monitoring of boiler pressure or unit load.

[0009] In one specific embodiment of the first aspect, the actual silicon content of the boiler water is obtained by online silicon metering or offline sampling and analysis.

[0010] In one specific implementation of the first aspect, under the rated load condition of the unit, the allowable silicon content in the boiler water is controlled to be ≤100μg / L; when the unit is operating below the rated load, the allowable silicon content in the boiler water is relaxed to a corresponding value greater than 100μg / L.

[0011] The beneficial effects of this invention are as follows: 1. The core technology of the boiler water silicon content control method proposed in this patent lies in: abandoning the mechanical application of a single fixed limit on boiler water silicon content, and instead dynamically adjusting the allowable silicon content of the boiler water according to the real-time operating pressure of the boiler drum (or unit load), and utilizing the physical characteristic that the silicon dissolution and carrying coefficient of saturated steam decreases as the boiler pressure decreases, thus reasonably relaxing the control index of boiler water silicon content under low-pressure conditions. Through this technology, when the unit participates in peak shaving and the boiler pressure is lower than the rated value, it can significantly reduce the trigger frequency and amount of boiler blowdown while ensuring that the silicon content of saturated steam does not exceed the limit and does not aggravate salt accumulation on the turbine blades. The direct technical effects include: reducing the heat loss associated with high-temperature boiler water discharge, reducing the consumption of demineralized water replenishment, thereby effectively improving the boiler's thermal efficiency and the overall economic efficiency of the unit.

[0012] 2. By establishing a dynamic correlation between boiler pressure and the permissible silicon content in boiler water, precise control of boiler blowdown behavior is achieved—blowdown is initiated only when the actual silicon content in the boiler water exceeds the permissible value corresponding to the current pressure, avoiding ineffective blowdown caused by overly strict fixed limits. Compared to the existing technology that blindly follows a single national standard limit for blowdown, this invention establishes a "pressure-adaptive" control logic, ensuring that blowdown operations truly serve the safe operation requirements of the steam turbine, rather than being rigid constraints detached from actual operating conditions. In summary, this patent maximizes the energy-saving potential of the boiler system while ensuring steam quality and long-term safe operation of the steam turbine, reducing water and coal consumption for power generation enterprises, demonstrating clear technological advancement and industrial practical value. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the method flow of the present invention.

[0014] Figure 2 This is a schematic diagram illustrating the relationship between boiler pressure and permissible silicon content in boiler water according to the present invention.

[0015] Figure 3 This is a schematic diagram showing the relationship between boiler pressure and dissolution-carrying coefficient according to the present invention. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0017] like Figures 1 to 3 This paper presents a new method for controlling the silicon content in boiler water.

[0018] I. Overview of the Overall Technical Solution This specific embodiment provides a method for controlling the silica content in boiler water of a subcritical parameter drum boiler. The core idea of ​​this method is not to mechanically adhere to the single limit of ≤100 μg / L for silica content in boiler water as specified in the national standard (GB / T 12145-2016), but rather to dynamically adjust the allowable silica content in the boiler water based on the real-time boiler pressure (or unit load). Since the silica dissolution and carry-over coefficient of saturated steam decreases significantly with decreasing boiler pressure, even under low-load (low-pressure) conditions, the silica content in the saturated steam will not exceed the allowable value, even if the silica content in the boiler water is high. This safely reduces the frequency and volume of boiler blowdowns, improving the unit's economic efficiency.

[0019] The prerequisites for this method are: the boiler is a subcritical drum boiler (rated pressure approximately 17.64 MPa), and the pH value of the boiler water is controlled between 9 and 9.7 (usually adjusted by adding sodium hydroxide or phosphate). Within this pH range, silicon mainly exists in the form of silicate ions, and its solubility and carrying capacity are stable and verifiable.

[0020] II. Specific Steps of the Method Step 1: Obtain real-time operating parameters Input: Operating status of the steam drum boiler.

[0021] Operation: The boiler's steam pressure (unit: MPa) or unit load (unit: MW) is acquired in real time through a distributed control system (DCS). Steam pressure is typically taken from the pressure measuring point at the steam drum outlet or main steam pipeline; unit load can be obtained through a generator power transmitter.

[0022] Output: Current boiler pressure value Or the corresponding load value.

[0023] Data Relationship: For a given subcritical boiler, the unit load and boiler pressure have an approximately linear relationship. In this implementation, boiler pressure is preferentially used as the basis for judgment because the dissolved carryover coefficient is directly determined by pressure. If the load is used directly, the load-pressure correspondence curve needs to be calibrated in advance.

[0024] Step 2: Dynamically determine the allowable silicon content in the boiler water Input: Real-time boiler pressure obtained in step 1 .

[0025] Operation: Determine the allowable silicon content of the boiler water under the current operating conditions according to the preset "Pressure-Allowable Silicon Content Correspondence Table" (Table 1). .

[0026] This correspondence table is calculated based on the dissolution and carry-over coefficient of silicon in saturated steam (Table 2) and the limit on silicon content in steam for safe operation of steam turbines (usually ≤20μg / L).

[0027] Table 1. Correspondence between boiler pressure and allowable silicon content in boiler water (boiler water pH = 9-9.7).

[0028] Note: When the pressure is exactly equal to the interval boundary value, it is assigned to the interval with the higher pressure value (i.e., the more stringent allowable value) to ensure safety.

[0029] If the boiler pressure is below 13.72 MPa (e.g., deep peak shaving to high-pressure boiler parameters), the most lenient range will still apply, meaning the boiler water silicon content is allowed to be ≤200 μg / L. In other words, regardless of whether the pressure is in the range of 12.74 MPa to 13.72 MPa or below 12.74 MPa, the same limit (≤200 μg / L) applies to ensure the integrity and safety of the control logic.

[0030] If the boiler pressure is higher than 17.64 MPa (overload operation), the most stringent limit (≤100 μg / L) must be followed, and it is recommended to strengthen the blowdown.

[0031] Output: Permissible silicon content in boiler water under current operating conditions. .

[0032] Step 3: Obtain the actual silicon content of the boiler water Input: Boiler water sample.

[0033] Operation: Obtain the actual silicon content of the boiler water using one of the following two methods. ; Online monitoring: Samples are taken from the boiler drum downcomer or continuous blowdown pipe, cooled, and then sent to an online silicate meter (such as a silicate analyzer).

[0034] Offline sampling: Sampling shall be conducted at least once per shift, and the samples shall be determined in the laboratory using spectrophotometry (silica molybdenum blue method).

[0035] Output: Current actual silicon content of boiler water (Unit: μg / L)

[0036] Step 4: Comparison and Pollution Discharge Decisions enter: .

[0037] Operation: Compare the sizes of the two: like No sewage will be discharged; the system will continue to operate normally.

[0038] like If so, then boiler blowdown operation will be performed.

[0039] Specific procedures for sewage discharge: Open the boiler continuous blowdown valve (usually a throttle valve), and control the blowdown flow rate between 0.3% and 1.0% of the boiler's evaporation rate. Continue blowdown until the silica content of the boiler water drops to a certain level. When the water level is below 80%, then close the valve.

[0040] Output: Drainage command and drainage duration / flow rate.

[0041] III. Summary of Logical Chains Physical basis: The dissolution and carrying coefficient of silicon by saturated steam decreases as the boiler pressure decreases (see Table 2, data from "Water Treatment in Thermal Power Plants" (Volume 2)).

[0042] Table 2. Dissolution and carryover coefficients under different pressures (boiler water pH = 9–10). To ensure the safety of steam turbines, it is required that... (National standard requirements for water vapor quality).

[0043] Dynamic adjustment logic: When the pressure decreases, Decrease This increases the capacity, allowing the boiler water to maintain a high silicon content without the need for blowdown.

[0044] IV. Examples Example 1: Peak-shaving operation of Guofeng Coal-fired Power Plant's 300MW subcritical parameter unit Background: Guofeng Coal-fired Power Plant Unit 1 is a subcritical unit (main steam rated pressure 17.5 MPa, unit rated load 300 MW), and the boiler water pH is stable at 9.3–9.6. According to national standards for steam and water quality, boiler water with a silicon content exceeding 100 μg / L requires blowdown.

[0045] Operating conditions: The power grid requires the unit to participate in deep peak shaving, the load drops from 300MW to 10MW, and the boiler pressure drops from 17.5MPa to about 12MPa.

[0046] Control process: Obtain real-time parameters: The DCS displays the current boiler pressure as approximately 12 MPa.

[0047] Dynamically determine the allowable silicon content: Referring to Table 1, 12MPa falls within the "≤13.72MPa" range, therefore the allowable silicon content in the boiler water is... .

[0048] Obtaining actual silicon content: The online silicon meter displays the actual silicon content of the boiler water. .

[0049] Comparison and decision: 135μg / L < 200μg / L, the requirements are met, and no discharge is required.

[0050] In contrast, according to the original national standard, 135 μg / L > 100 μg / L would trigger a discharge of pollutants. Using this method, a discharge was avoided in this instance.

[0051] Economic calculation: A single discharge (based on continuous discharge of 5t / h for 300 minutes) results in a loss of approximately 25 tons of demineralized water and a heat loss of approximately 2.5 × 10⁻⁶ tons. 7 Based on 200 peak-shaving operations per year, this would save 5,000 tons of demineralized water, approximately 200 tons of standard coal, and reduce CO2 emissions by approximately 500 tons annually.

[0052] Example 2: Operation under rated load Operating conditions: The unit operates at full load of 300MW, with a boiler pressure of 17.5MPa.

[0053] process: Pressure 17.5 MPa, from the table... .

[0054] Online silicon meter display .

[0055] 108 > 100 Perform sewage discharge by opening the continuous sewage discharge valve at a flow rate of 3 t / h for 60 minutes. Reduce the concentration to 85 μg / L and then close the valve.

[0056] Note: Under rated load, this method is consistent with national standards to ensure steam quality.

[0057] Example 3: Boundary Pressure Treatment Operating condition: The boiler pressure is exactly 15.19 MPa (range boundary).

[0058] Handling: Apply the higher pressure range, i.e., classify it as "15.19~16.66MPa". Instead of 160 μg / L, to ensure a safety margin.

[0059] In summary, this method can be directly embedded into the existing DCS control system of a power plant without additional hardware investment. Only the setpoints for the boiler water silica content alarm and blowdown interlock in the control logic need to be modified, changing the fixed values ​​to dynamic lookup functions based on pressure.

[0060] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A novel method for controlling the silicon content of boiler water, applied to subcritical parameter drum boilers, wherein the pH value of the boiler water is controlled between 9 and 9.7, characterized in that... Includes the following steps: Obtain real-time operating parameters of the boiler drum, including unit load and / or boiler pressure; Based on the real-time operating parameters, and according to the preset correspondence between boiler pressure and allowable silicon content in boiler water, the allowable silicon content in boiler water is dynamically determined; wherein, the allowable silicon content in boiler water is negatively correlated with the boiler pressure. Obtain the actual silicon content of the boiler water; The actual silicon content of the boiler water is compared with the dynamically determined allowable silicon content of the boiler water. If the actual silicon content of the boiler water is greater than the allowable silicon content of the boiler water, then boiler blowdown is performed.

2. The method according to claim 1, characterized in that, The preset correspondence includes: When the boiler pressure is within the first pressure range, the allowable silicon content in the boiler water is set to the first allowable value; When the boiler pressure is within the second pressure range, the allowable silicon content in the boiler water is set to the second allowable value. Wherein, the upper limit of the second pressure range is less than or equal to the lower limit of the first pressure range, and the first allowable value is less than the second allowable value.

3. The method according to claim 2, characterized in that, The specific relationship between boiler pressure and allowable silicon content in boiler water is as follows: When the boiler pressure is 16.66MPa~17.64MPa, the allowable silicon content in the boiler water is ≤100μg / L; When the boiler pressure is 15.19MPa~16.66MPa, the allowable silicon content in the boiler water is ≤130μg / L; When the boiler pressure is 13.72MPa~15.19MPa, the allowable silicon content in the boiler water is ≤160μg / L; When the boiler pressure is ≤13.72MPa, the allowable silicon content in the boiler water is ≤200μg / L.

4. The method according to claim 1, characterized in that, The acquisition of real-time operating parameters of the boiler drum includes: real-time monitoring of boiler pressure or unit load.

5. The method according to claim 1, characterized in that, The actual silicon content of the boiler water is obtained through online silicon meters or offline sampling and analysis.

6. The method according to claim 1, characterized in that, Under rated load conditions, the allowable silicon content in the boiler water is controlled to ≤100μg / L; when the unit is operating below rated load, the allowable silicon content in the boiler water is relaxed to the corresponding value greater than 100μg / L.

Images