Method for on-line monitoring of total iron in water vapor of power plant

Abstract

The application provides a kind of on-line monitoring method for total iron in power plant water vapor, belongs to the technical field of water quality analysis of power plant, which can at least partially solve the problem of long digestion time of atomic absorption method in prior art and only offline detection, narrow range of on-line iron meter and insufficient trace accuracy.The application comprises: filtering the water sample in multiple stages and adjusting to a preset flow rate to obtain a pretreated water sample; after quantitatively adding an acidic mixed digestion solution and a reducing agent, the water sample is sent into a microfluidic closed digestion channel, and the suspended iron, colloidal iron and trivalent iron are converted into free iron under closed conditions; the target atomization mode is determined between the flame atomization mode and the graphite furnace atomization mode according to the range determination parameter; the absorbance generated by atomization is collected and substituted into the corresponding standard curve to calculate the total iron concentration and output.The application realizes wide-range on-line monitoring of total iron covering high-concentration rusted iron and trace iron.

Description

Technical Field

[0001] This invention relates to the field of power plant water quality analysis technology, specifically to an online monitoring method for total iron in power plant water vapor. Background Technology

[0002] Total iron content is one of the most important chemical monitoring indicators for power plant water-steam systems. Iron in these systems primarily originates from corrosion products in boiler and turbine equipment such as feedwater pipes, economizers, water-cooled walls, and superheaters. Its concentration directly reflects the corrosion status of these equipment and is a core criterion for evaluating the quality of the unit's water and steam. With the increasing demands on water and steam quality from supercritical, ultra-supercritical, and nuclear power units, the total iron concentration in feedwater during steady-state operation needs to be controlled at the microgram per liter level. However, during the cold flushing phase of unit startup, rust products precipitated in the pipelines can raise the total iron concentration to the milligram per liter level. Thus, the total iron concentration spans more than two orders of magnitude between different operating stages of the unit.

[0003] Currently, there are two main technical approaches for detecting total iron in power plant steam and water. The first approach is laboratory atomic absorption spectrophotometry. This technique relies on manual sampling at the sampling point, transferring the water sample to the laboratory, and then heating and digesting it on a hot plate for an extended period. The process involves step-by-step reduction, volume adjustment, and instrumental analysis, with the testing cycle for a single sample typically exceeding 30 minutes. Because each step—sampling, transfer, and digestion—requires manual intervention, this technique can only be performed offline. The test results lag significantly behind the actual state of the steam and water system, failing to reflect the dynamic changes in the unit's steam and water quality in real time, and also unable to provide continuous quantitative data during periods of rapid concentration changes, such as start-up flushing.

[0004] The second category is online iron meter detection. Most existing commercial online iron meters use the o-phenanthroline colorimetric method, converting iron concentration through photometric measurement after a colorimetric reaction. This technology typically covers a detection range of only 30 μg / L to 500 μg / L, with a detection limit of approximately 30 μg / L. For trace iron at the microgram per liter level during the steady-state operation of supercritical units, its sensitivity and accuracy are insufficient. Furthermore, the photometric measurement using the colorimetric method is easily affected by water turbidity and suspended impurities, with measurement deviations further increasing under high turbidity and multi-iron form conditions during startup. In addition, the high concentration of iron at the milligram per liter level during startup flushing exceeds its upper limit. Therefore, existing online iron meters cannot accurately measure trace iron under steady-state conditions, nor can they measure high-concentration iron under startup conditions, failing to cover the chemical monitoring needs of the entire unit operation cycle.

[0005] Therefore, how to provide an online monitoring method for total iron in power plant water and steam that can cover both high-concentration iron during the start-up flushing stage and trace iron under steady-state conditions, while taking into account both detection accuracy and timeliness, and without the need for manual intervention, is a technical problem that urgently needs to be solved in the field of power plant water quality analysis technology. Summary of the Invention

[0006] The present invention aims to at least solve one of the technical problems existing in the prior art, and to provide a method for online monitoring of total iron in water and steam in power plants.

[0007] To achieve the above objectives, the present invention provides a method for online monitoring of total iron in water vapor of a power plant, comprising: Step S1: Perform multi-stage filtration on the water sample from the power plant's water-steam system, and adjust the filtered water sample to a preset flow rate to obtain a pretreated water sample; Step S2: Add acidic mixed digestion solution and reducing agent to the pretreated water sample according to the preset addition ratio to form a mixed solution. Send the mixed solution into a microfluidic closed digestion channel and digest it under closed conditions at a preset digestion temperature and a preset digestion time to convert the suspended iron, colloidal iron and ferric iron in the mixed solution into free iron, and obtain a digested water sample. Step S3: Obtain the range determination parameter that characterizes the concentration range of the total iron concentration in the digested water sample. When the range determination parameter indicates that the total iron concentration is in the first concentration range, determine the flame atomization mode as the target atomization mode. When the range determination parameter indicates that the total iron concentration is in the second concentration range, determine the graphite furnace atomization mode as the target atomization mode. The upper limit of the second concentration range is not higher than the lower limit of the first concentration range. Step S4: The digested water sample is introduced into the atomizer corresponding to the target atomization mode for atomization, and the absorbance is collected at the characteristic wavelength of iron. Step S5: Substitute the absorbance into the pre-established standard curve corresponding to the target atomization mode, calculate and output the total iron concentration in the water sample.

[0008] Further, in step S1, the multi-stage filtration includes a primary filtration and a secondary filtration performed sequentially. The primary filtration traps particulate impurities in the water sample, and the secondary filtration traps colloidal impurities in the water sample after the primary filtration. The pore size of the secondary filtration is smaller than that of the primary filtration. The pore size of the primary filtration is 30μm~80μm, and the pore size of the secondary filtration is 0.2μm~0.8μm. The preset flow rate is 15mL / min~25mL / min, and the filtered water sample is stabilized at 80kPa~120kPa before being output.

[0009] Further, in step S2, the acidic mixed digestion solution is a hydrochloric acid-nitric acid mixed digestion solution formed by mixing hydrochloric acid solution and nitric acid solution according to a preset acid ratio, and the reducing agent is ascorbic acid solution; the preset addition ratio includes: the ratio of the added volume of the hydrochloric acid-nitric acid mixed digestion solution to the volume of the pretreated water sample is 1:80~1:120, the concentration of the ascorbic acid solution is 8g / L~12g / L, and the ascorbic acid solution is added at a dosage of 4μL~6μL per milliliter of the pretreated water sample.

[0010] Further, the hydrochloric acid solution is a solution formed by mixing concentrated hydrochloric acid and water at a volume ratio of 1:1, and the nitric acid solution is a solution formed by mixing concentrated nitric acid and water at a volume ratio of 1:1. The preset acid ratio is 1:1 by volume. The inner diameter of the microfluidic closed digestion channel is 1mm~3mm, and the length is 100mm~200mm. The preset digestion temperature is 120℃~130℃, the preset digestion time is 60s~180s, and the digestion pressure in the microfluidic closed digestion channel during digestion is 150kPa~200kPa. After digestion, the digested water sample is cooled to below the preset injection temperature before proceeding to step S3.

[0011] Furthermore, in step S3, the range determination parameter includes at least one of the following: unit operating condition stage identifier, pre-detection absorbance, and historical concentration trend value; When the range determination parameter includes the unit operating condition stage identifier, if the unit operating condition stage identifier indicates that the unit is in the start-up flushing stage, then the total iron concentration is determined to be in the first concentration range; if the unit operating condition stage identifier indicates that the unit is in the steady-state operation stage, then the total iron concentration is determined to be in the second concentration range. When the range determination parameter includes the pre-detection absorbance, a portion of the digested water sample is first pre-detected using the flame atomization mode to obtain the pre-detection absorbance. If the pre-detection absorbance is not lower than a preset switching threshold, the total iron concentration is determined to be in the first concentration range; otherwise, the total iron concentration is determined to be in the second concentration range. When the range determination parameter includes the historical concentration trend value, the total iron concentration calculated in the previous execution of step S5 will be used as the historical concentration trend value. The concentration range of the total iron concentration in this monitoring will be determined based on the concentration range of the historical concentration trend value. If it is the first monitoring, the total iron concentration will be determined to be in the first concentration range. The first concentration range is 0.1 mg / L to 5 mg / L, and the second concentration range is 1 μg / L to 100 μg / L.

[0012] Further, in step S4, when the target atomization mode is the flame atomization mode, the atomizer corresponding to the target atomization mode is a flame atomizer, and a flame formed by mixing acetylene and air is supplied to the flame atomizer, with the volume ratio of acetylene to air being 1:5 to 1:7, and the burner height of the flame atomizer being 6mm to 10mm; the characteristic wavelength of the iron element is 248.3nm.

[0013] Further, in step S4, when the target atomization mode is the graphite furnace atomization mode, the atomizer corresponding to the target atomization mode is a graphite furnace atomizer. The digested water sample is injected into the graphite furnace atomizer at a preset injection volume, and atomized according to a stepped temperature increase program including a drying section, an ashing section, an atomization section, and a purification section. The drying section includes a first drying step and a second drying step. The temperature of the first drying step is 90°C, the heating time is 10s, and the holding time is 20s. The temperature of the second drying step is 130°C, and the heating time is... The ashing section has a heating time of 10s and a holding time of 20s; the ashing section has a temperature of 1100℃, a heating time of 10s, and a holding time of 20s; the atomization section has a temperature of 2400℃, a heating time of 1s, and a holding time of 5s; the purification section has a temperature of 2600℃, a heating time of 1s, and a holding time of 3s; the protective gas for the graphite furnace atomizer is argon, the flow rate of the protective gas is 200mL / min~300mL / min, the operating current of the iron hollow cathode lamp is 10mA~14mA, and the spectral bandpass is 0.2nm.

[0014] Further, in step S5, the standard curve is pre-established as follows: for each atomization mode in the flame atomization mode and the graphite furnace atomization mode, an iron standard solution including at least five concentration points, including a blank concentration point, is prepared. Each concentration point is measured at least three times and the average value is taken to obtain the average absorbance of each concentration point. The average absorbance of each concentration point is linearly fitted with the corresponding concentration to obtain the standard curve corresponding to the atomization mode, and the linear correlation coefficient of the standard curve is not lower than the preset correlation coefficient threshold. The fitting equation for the standard curve is: ; In step S5, the total iron concentration is calculated according to the following formula: ; in, The absorbance is [value missing]. The intercept of the standard curve is... The slope of the standard curve is... The total iron concentration is denoted as .

[0015] Furthermore, the method also includes an automatic calibration step: zero-point calibration is performed with deionized water every first preset period; span calibration is performed with an iron standard solution of a preset calibration concentration every second preset period; when the measurement deviation of the zero-point calibration or the span calibration is greater than a preset calibration deviation threshold, the calibration is deemed unqualified, the standard curve is re-established, and step S5 is performed with the re-established standard curve; when the measurement deviation is not greater than the preset calibration deviation threshold, the calibration is deemed qualified.

[0016] Furthermore, the method also includes step S6: after the total iron concentration is output, the injection pipeline and the microfluidic closed digestion channel are automatically cleaned, and the process returns to step S1 continuously according to a preset monitoring cycle; a total iron concentration trend curve is generated based on the total iron concentration output in each monitoring cycle, and an over-limit warning message is output when the total iron concentration is greater than the preset concentration limit.

[0017] The beneficial effects of this invention are as follows: This invention automatically determines the target atomization mode between flame atomization mode and graphite furnace atomization mode based on the range determination parameter. The flame atomization mode covers the first concentration range with higher concentrations during the unit start-up flushing stage, while the graphite furnace atomization mode covers the second concentration range with trace levels under hot and steady-state conditions. The two modes together cover the total iron concentration variation range throughout the entire operating cycle of the unit, overcoming the shortcomings of existing online iron meters with narrow ranges and inability to monitor all operating conditions.

[0018] This invention places the digestion process of quantitatively adding acidic mixed digestion solution and reducing agent within a microfluidic closed digestion channel. The microfluidic closed digestion channel has a small inner diameter, a large specific surface area, and rapid heating. The closed conditions suppress boiling and acid mist volatilization loss during the digestion process, enabling suspended iron, colloidal iron, and ferric iron to be completely converted into free iron in a short time. This transforms the time-consuming offline digestion pretreatment of traditional atomic absorption spectrometry into online rapid digestion, significantly shortening the single detection cycle and making atomic absorption spectrometry suitable for online real-time monitoring scenarios.

[0019] This invention uses multi-stage filtration to trap particulate and colloidal impurities in water samples and adjusts the water sample to a preset flow rate. This eliminates the interference of high turbidity, multiple impurities, and multiple iron forms in the water sample during the start-up phase on the detection. At the same time, it provides a stable volume reference for quantitatively adding digestion solution and reducing agent according to preset ratios, ensuring the repeatability of absorbance measurement and improving the accuracy of trace iron detection.

[0020] The sampling and filtration, quantitative addition, closed digestion, pattern determination, atomization detection and concentration calculation steps of this invention are automatically executed in a seamless manner, eliminating the need for manual sampling and digestion, and meeting the needs of unmanned intelligent chemical monitoring in power plants. Attached Figure Description

[0021] Figure 1 This is an overall flowchart of the online monitoring method for total iron in water vapor of power plants according to the present invention; Figure 2 This is a schematic diagram of the structure of the online monitoring system for total iron in water and steam in a power plant according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the dual-mode switching and cyclic monitoring process according to an embodiment of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and beneficial effects of this application clearer, the following detailed description, in conjunction with the accompanying drawings and specific embodiments, further illustrates this application. It should be understood that the specific embodiments described in this specification are merely for explaining this application and are not intended to limit it.

[0023] The overall technical solution of the online monitoring method for total iron in water and steam in power plants according to the present invention will be described below.

[0024] See Figure 1 The online monitoring method for total iron in water vapor of power plants according to the present invention includes steps S1 to S5, and each step is described in detail below.

[0025] Step S1 involves performing multi-stage filtration on water samples from the power plant's steam-water system and adjusting the filtered water samples to a preset flow rate to obtain pretreated water samples. The water samples are taken from sampling points within the power plant's steam-water system, including but not limited to feedwater sampling points, boiler water sampling points, saturated steam condensate sampling points, and condensate sampling points. The water samples are introduced into the monitoring process of this invention after being cooled and depressurized by a sampling rack. The multi-stage filtration refers to performing at least two stages of filtration on the water samples sequentially, with the pore size of each stage decreasing progressively. Its function is as follows: the pre-stage filtration uses a larger pore size to trap particulate impurities in the water sample, preventing them from clogging the subsequent filter media and the subsequent microfluidic closed digestion channel; the subsequent filtration uses a smaller pore size to trap colloidal impurities in the water sample, preventing colloidal impurities from causing scattering interference to subsequent absorbance measurements. The commonalities of multi-stage filtration methods with different numbers of stages and different filter media are: they are all arranged in series according to a progressively decreasing pore size, all remove non-dissolved interfering substances from the water sample through physical interception, and all do not change the total amount of iron in the water sample. In a preferred embodiment, the multi-stage filtration includes a primary filtration and a secondary filtration performed sequentially. The pore size of the primary filtration is 30μm~80μm, and the pore size of the secondary filtration is 0.2μm~0.8μm. The primary filtration can use a sintered stainless steel filter element, and the secondary filtration can use a polytetrafluoroethylene (PTFE) precision filter membrane. In alternative embodiments, the primary filtration can also use a quartz sand filter column, and the secondary filtration can also use a hollow fiber ultrafiltration membrane. The choice of filter media does not affect the subsequent digestion and detection execution logic.

[0026] Adjusting the filtered water sample to a preset flow rate means maintaining the flow rate of the water sample entering subsequent steps at a constant set value through flow regulation. A constant water flow rate provides a stable volume reference for the quantitative addition of digestion solution and reducing agent in step S2 according to a preset ratio: under constant flow conditions, the volume of water sample entering the digestion process per unit time is determined, and the dosage of digestion solution and reducing agent can be accurately measured according to a fixed ratio with the water sample volume, thereby ensuring the stability of acidity and reducing agent concentration in the digestion system, and thus ensuring the completeness of digestion and the accuracy of concentration calculation. Flow regulation can be achieved through a constant pressure and constant flow unit, i.e., first stabilizing the water sample pressure within a preset pressure range, and then outputting a constant flow rate; it can also be achieved through direct quantitative delivery using a constant flow pump. In a preferred embodiment, the preset flow rate is 15 mL / min to 25 mL / min, and the stabilized water sample pressure is 80 kPa to 120 kPa. The pretreated water sample obtained after multi-stage filtration and flow regulation is then transported to step S2.

[0027] Step S2 involves quantitatively adding an acidic mixed digestion solution and a reducing agent to the pretreated water sample according to a preset addition ratio to form a mixed solution. The mixed solution is then sent into a microfluidic closed digestion channel, where it is digested under closed conditions at a preset digestion temperature and for a preset digestion time. This process converts suspended iron, colloidal iron, and ferric iron in the mixed solution into free iron, thereby obtaining a digested water sample.

[0028] Iron in power plant steam exists in multiple forms, including suspended particulate rust iron, colloidal hydrated iron oxide, and dissolved ferric and ferrous iron. Atomic absorption spectrometry requires the analyte to exist in a uniformly dispersed dissolved state in the sample solution; heterogeneity in form leads to poor sample representativeness and absorbance fluctuations. The purpose of this step is to completely convert the above-mentioned iron forms into dissolved free iron, eliminating the influence of iron form differences on the detection results. The conversion mechanism is as follows: the acidic mixed digestion solution provides a strongly acidic environment, dissolving iron oxides and hydroxides and destroying the complex structure formed by iron and organic matter; the reducing agent undergoes a reducing dissolution effect on iron oxide particles, accelerating the dissolution of particulate and colloidal iron, and reducing the dissolved ferric iron to ferrous free iron; the closed heating conditions increase the reaction temperature and suppress the volatilization loss of the reaction system, shortening the time required for complete conversion.

[0029] The acidic mixed digestion solution refers to a digestion solution formed by mixing two or more acid components, possessing both acid dissolution and oxidative complex-breaking capabilities. Applicable acidic mixed digestion solutions include hydrochloric acid-nitric acid mixed digestion solutions and nitric acid-sulfuric acid mixed digestion solutions. The common characteristics of different acid combinations are: they all contain acid components that can provide a high concentration of hydrogen ions to dissolve iron oxides and iron hydroxides, and they all contain acid components with oxidizing capabilities to destroy organic complex structures. In a preferred embodiment, the acidic mixed digestion solution is a hydrochloric acid-nitric acid mixed digestion solution formed by mixing hydrochloric acid solution and nitric acid solution according to a preset acid ratio. The hydrochloric acid component has a strong dissolving ability for iron oxides, and the nitric acid component's oxidizing ability can destroy organic complexes in the water sample. Furthermore, this combination does not leave any non-volatile matrix salts during subsequent atomization, minimizing interference with the matrix in graphite furnace atomization.

[0030] The reducing agent refers to a water-soluble reducing agent capable of reducing ferric iron to ferrous iron and driving the reducing dissolution of iron oxide particles, without introducing iron contamination itself or its reaction products. Suitable reducing agents include ascorbic acid solution and hydroxylamine hydrochloride solution. Both share the following characteristics: sufficient reduction potential to reduce ferric iron to ferrous iron; both are stable under acidic conditions; and both do not contain iron and do not interfere with the atomic absorption measurement of iron. In a preferred embodiment, the reducing agent is ascorbic acid solution. Ascorbic acid is non-toxic and non-corrosive, facilitating long-term storage and automatic metering in online systems.

[0031] The preset dosage ratio is set based on the volume of the pretreated water sample, including the ratio of the volume of the acidic mixed digestion solution to the volume of the pretreated water sample, and the dosage of the reducing agent per unit volume of the pretreated water sample. In a preferred embodiment, the ratio of the volume of the hydrochloric acid-nitric acid mixed digestion solution to the volume of the pretreated water sample is 1:80 to 1:120, the concentration of the ascorbic acid solution is 8 g / L to 12 g / L, and it is added at a dosage of 4 μL to 6 μL per milliliter of the pretreated water sample. The basis for setting this dosage ratio range is that when the volume ratio is less than 1:120, the acidity of the digestion system is insufficient, and the conversion of particulate iron is incomplete; when it is greater than 1:80, the acid content is excessive, which not only increases reagent consumption but also aggravates the acid mist background in the subsequent atomization process.

[0032] The microfluidic closed digestion channel refers to a closed flow channel structure with an inner diameter on the order of millimeters. Compared with traditional open digestion using electric heating plates and laboratory digestion vessels, the commonalities and advantages of the microfluidic closed digestion channel are as follows: First, the channel has a small inner diameter, resulting in a large specific surface area of ​​the mixture within the channel and a short heat transfer path. The mixture can be heated to the preset digestion temperature within tens of seconds, with a heating rate far exceeding that of open container heating methods. Second, the channel is in a closed state, allowing the digestion temperature to be higher than the boiling point of the water sample at normal pressure without boiling. This also suppresses acid mist volatilization and water evaporation loss, ensuring a constant volume of the mixture before and after digestion, thereby guaranteeing the accuracy of the concentration calculation benchmark in step S5. Third, the channel has a small volume, resulting in a small amount of mixture digested per digestion, low reagent consumption, and a short heating and cooling cycle, making it suitable for the cycle time requirements of online continuous monitoring. The specific flow channel form of the microfluidic closed digestion channel can be a coiled flow channel, a serpentine zigzag flow channel, or a straight pipe flow channel. The common feature of different flow channel forms is that their inner diameters are all on the order of millimeters and they are all under closed pressure. The choice of flow channel form does not affect the execution logic of digestion and transformation. In a preferred embodiment, the inner diameter of the microfluidic closed digestion channel is 1mm~3mm, and its length is 100mm~200mm.

[0033] The preset digestion temperature and preset digestion time together determine the degree of complete conversion. In a preferred embodiment, the preset digestion temperature is 120℃~130℃, the preset digestion time is 60s~180s, and the digestion pressure in the microfluidic closed digestion channel during digestion is 150kPa~200kPa. This range is set based on the following: when the digestion temperature is below 120℃, the dissolution rate of particulate iron decreases, requiring a longer digestion time for complete conversion, which is not conducive to shortening the detection cycle; when the digestion temperature is above 130℃, the improvement in the degree of complete conversion is limited, and it increases the pressure burden on the closed channel; the digestion pressure of 150kPa~200kPa is higher than the saturated vapor pressure of water samples at 120℃~130℃, which can suppress boiling of the mixture. After digestion, the digested water sample is cooled to below the preset injection temperature before proceeding to step S3. This is to prevent high-temperature samples from affecting the accuracy of injection measurement and the stability of atomization conditions. Cooling can be automatically completed by a cooling coil, and the preset injection temperature can be set to 25°C. The obtained digested water sample is then transported to steps S3 and S4.

[0034] Step S3 is to obtain a range determination parameter that characterizes the concentration range of the total iron concentration in the digested water sample. When the range determination parameter indicates that the total iron concentration is in the first concentration range, the flame atomization mode is determined as the target atomization mode. When the range determination parameter indicates that the total iron concentration is in the second concentration range, the graphite furnace atomization mode is determined as the target atomization mode. The upper limit of the second concentration range is not higher than the lower limit of the first concentration range.

[0035] This step is the core decision-making process for achieving wide-range coverage. Flame atomization and graphite furnace atomization (GFA) modes complement each other in detection capabilities: Flame atomization uses continuous suction-jet injection, offering a wide linear range and high concentration tolerance, suitable for detecting higher concentrations of iron, but its sensitivity is limited; GFA uses micro-injection and programmed temperature ramping to atomize the entire sample, resulting in a longer residence time of atomic vapor within the graphite tube, and its sensitivity is more than two orders of magnitude higher than Flame atomization, suitable for trace iron detection, but high-concentration samples will exceed its linear range. By matching the first concentration range with the linear range of Flame atomization and the second concentration range with the high-sensitivity detection capability of GFA, each detection can be performed under atomization conditions appropriate to the sample concentration level. In a preferred embodiment, the first concentration range is 0.1 mg / L to 5 mg / L, covering the high concentration of corroded iron during the unit start-up flushing stage; the second concentration range is 1 μg / L to 100 μg / L, covering trace iron under both hot and steady-state operating conditions.

[0036] The range determination parameter refers to a parameter that characterizes the concentration range of total iron in the digested water sample before formal testing. Possible range determination parameters include at least one of the following: unit operating condition stage indicator, pre-detection absorbance, and historical concentration trend value. The commonality among these three is that they can all determine the concentration range of total iron before formal testing, thereby ensuring that the digested water sample is introduced into an atomizer matching its concentration level, preventing high-concentration samples from exceeding the range of graphite furnace atomization mode or trace samples from falling below the sensitivity limit of flame atomization mode. The determination mechanisms of the three range determination parameters are explained below.

[0037] The unit's operating condition stage identifier is provided by the power plant's distributed control system (DCS) or operating instructions, indicating whether the unit is currently in the start-up flushing stage or the steady-state operation stage. During the cold flushing stage of unit start-up, a large amount of rust products accumulated in the pipelines precipitate out, and the total iron concentration in the water sample is at the milligrams per liter level, corresponding to the first concentration range. After the unit enters the hot and steady-state operation stages, the water vapor undergoes purification and circulation, and the total iron concentration drops to the micrograms per liter level, corresponding to the second concentration range. A stable correspondence exists between the operating condition stage and the concentration range; therefore, the unit's operating condition stage identifier can be directly used as the basis for determining the concentration range.

[0038] The pre-detection absorbance is obtained as follows: a portion of the digested water sample is first pre-detected using flame atomization mode to obtain the pre-detection absorbance. According to Beer-Lambert's law, absorbance is positively correlated with the concentration of the analyte; therefore, the pre-detection absorbance can directly characterize the total iron concentration level. If the pre-detection absorbance is not lower than a preset switching threshold, the total iron concentration is determined to be within the first concentration range, and formal detection is performed directly using flame atomization mode; otherwise, the total iron concentration is determined to be within the second concentration range, and detection is performed using graphite furnace atomization mode. The preset switching threshold is set as the absorbance value corresponding to the lower limit concentration of the first concentration range in flame atomization mode. This method does not require external operating condition information and is suitable for scenarios where operating condition information is missing or where operating conditions are rapidly switched.

[0039] The historical concentration trend value utilizes the characteristic that the total iron concentration in the water vapor system changes continuously and gradually between adjacent monitoring periods: the total iron concentration calculated in the previous execution of step S5 is used as the historical concentration trend value, and the concentration range of the total iron concentration in this monitoring is determined based on the concentration range in which the historical concentration trend value is located. When the historical concentration trend value is close to the boundary between two concentration ranges, the result of the pre-detection absorbance can be combined to verify the judgment, so as to avoid misjudgment when the concentration changes across ranges.

[0040] In a preferred embodiment, when the concentration determined according to the range determination parameter is in the transition concentration band between the first concentration range and the second concentration range, it is first classified into the first concentration range and detected using flame atomization mode according to the higher concentration principle. If the detected total iron concentration is lower than the lower limit of the first concentration range, it is automatically retested using graphite furnace atomization mode according to the second concentration range, thereby ensuring that samples of any concentration level can obtain effective detection results. The determined target atomization mode is output to steps S4 and S5.

[0041] Step S4 involves introducing the digested water sample into the atomizer corresponding to the target atomization mode for atomization, and collecting the absorbance at the characteristic wavelength of iron.

[0042] When the target atomization mode is flame atomization mode, the corresponding atomizer is a flame atomizer. The digested water sample is introduced into the flame atomizer via a continuous suction-jet method, where it is desolvated, evaporated, and dissociated into ground-state iron atoms in a flame formed by a mixture of acetylene and air. The volume ratio of acetylene to air determines the reducing power and temperature of the flame, while the burner height determines the region of maximum atomic concentration through which the light beam passes; both factors jointly affect the absorption sensitivity and measurement stability. In a preferred embodiment, the volume ratio of acetylene to air is 1:5 to 1:7, and the burner height is 6 mm to 10 mm.

[0043] When the target atomization mode is graphite furnace atomization, the corresponding atomizer is a graphite furnace atomizer. The digested water sample is injected into the graphite tube of the graphite furnace atomizer at a preset injection volume, and atomization is performed according to a stepped temperature increase program including a drying section, an ashing section, an atomization section, and a purification section: In the drying section, the solvent water in the sample is removed stepwise at a temperature below to slightly above the boiling point of water to avoid sample boiling and splashing; in the ashing section, acid mist and matrix components in the sample are decomposed and removed at an intermediate temperature to reduce background absorption; in the atomization section, the temperature is instantly increased to the atomization temperature of iron, causing the iron group to decompose into ground-state atomic vapor; in the purification section, the residue in the graphite tube is removed at a higher temperature to avoid memory effect. The graphite furnace atomizer is purged with protective argon gas throughout the stepped temperature increase to prevent the graphite tube from oxidizing and ablating at high temperatures and to prevent the matrix vapor from being carried away. The hollow iron cathode lamp emits characteristic spectral lines of iron, which are selectively absorbed by the ground-state iron atomic vapor, and the degree of absorption is converted into absorbance by the detector. In a preferred embodiment, the preset injection volume is 20 μL, the flow rate of the protective gas is 200 mL / min to 300 mL / min, the operating current of the iron hollow cathode lamp is 10 mA to 14 mA, and the spectral bandpass is 0.2 nm.

[0044] In a preferred embodiment, the characteristic wavelength of iron is 248.3 nm, which is the resonant absorption line of iron and has the highest absorption sensitivity. In an alternative embodiment, for high-concentration samples near the upper limit of the first concentration range, a less sensitive absorption line of 248.8 nm or 302.1 nm can be selected to extend the measurement upper limit of the flame atomization mode. The selection of the characteristic wavelength does not change the execution logic of absorbance acquisition and concentration calculation. The acquired absorbance is output to step S5.

[0045] Step S5 involves substituting the absorbance into a pre-established standard curve corresponding to the target atomization mode to calculate and output the total iron concentration in the water sample.

[0046] The theoretical basis for concentration calculation is the Beer-Lambert law: ; in, Absorbance The molar absorptivity is 1. Optical path length The absorbance is the concentration of the solution to be tested. Under fixed atomization and optical conditions, the absorbance and concentration are linearly related. Therefore, the measured absorbance can be converted into the total iron concentration using a pre-established standard curve.

[0047] The standard curve is pre-established for each atomization mode: An iron standard solution with at least five concentration points, including a blank concentration point, is prepared. Each concentration point is measured at least three times, and the average value is taken to obtain the average absorbance of each concentration point. A linear fit is performed between the average absorbance of each concentration point and the corresponding concentration to obtain the standard curve corresponding to that atomization mode. The linear correlation coefficient of the standard curve is not lower than a preset correlation coefficient threshold, which can be set to 0.999. The fitting equation for the standard curve is: ; Total iron concentration is calculated using the following formula: ; in, The absorbance is [value missing]. The intercept of the standard curve is... The slope of the standard curve is... The total iron concentration is given. Since the target atomization mode has been determined in step S3, this step automatically calls the standard curve corresponding to the target atomization mode to perform inverse calculation, ensuring that the absorbance matches the range of the standard curve. The calculated total iron concentration is output in the form of a numerical value, a real-time concentration curve, and a trend report for use by chemical monitoring personnel and the unit operation control system.

[0048] The above describes the overall technical solution of this invention. It can be seen that the pretreated water sample output from step S1 provides a sample basis with constant flow and impurity retention for the digestion and conversion in step S2; the digested water sample output from step S2 normalizes all forms of iron into free iron, providing a uniformly morphologically homogeneous analyte for the range determination in step S3 and the atomization detection in step S4; the target atomization mode output from step S3 determines the atomizer used in step S4 and the standard curve called in step S5; the absorbance output from step S4 provides direct input for the concentration back-calculation in step S5. The outputs of each step serve as the basis for subsequent steps, collectively achieving online quantitative monitoring of total iron covering both high concentrations and trace amounts.

[0049] The following is combined Figure 2 An embodiment of an online monitoring system for total iron in water and steam in a power plant, implementing the method of the present invention, is described. (See also...) Figure 2The system includes a primary water sample pretreatment unit 1, a secondary water sample pretreatment unit 2, a water sample pressure stabilization and constant flow unit 3, a sample inlet pump 4, a hydrochloric acid-nitric acid mixed digestion solution tank 5, a hydrochloric acid-nitric acid mixed digestion solution metering pump 6, an ascorbic acid reducing agent tank 7, an ascorbic acid reducing agent metering pump 8, a microfluidic online closed digestion unit 9, a dual-mode atomic absorption detection unit 10, and a control analysis and calculation unit 11. The water sample inlet is sequentially connected to the primary water sample pretreatment unit 1 and the secondary water sample pretreatment unit 2. The outlet of the secondary water sample pretreatment unit 2 is connected to the water sample pressure stabilization and constant flow unit 3. The primary water sample pretreatment unit 1 and the secondary water sample pretreatment unit 2 jointly perform multi-stage filtration in step S1, and the water sample pressure stabilization and constant flow unit 3 performs flow regulation in step S1. The inlet of the water sample injection pump 4 is connected to the outlet of the water sample stabilizing constant flow unit 3 for quantitative extraction of the pretreated water sample; the inlet of the hydrochloric acid-nitric acid mixed digestion solution metering pump 6 is connected to the hydrochloric acid-nitric acid mixed digestion solution tank 5, and the inlet of the ascorbic acid reducing agent metering pump 8 is connected to the ascorbic acid reducing agent tank 7. The outlets of the water sample injection pump 4, the hydrochloric acid-nitric acid mixed digestion solution metering pump 6, and the ascorbic acid reducing agent metering pump 8 are all connected to the inlet of the microfluidic online closed digestion unit 9 to perform quantitative addition and mixing in step S2; the microfluidic online closed digestion unit 9 is equipped with the microfluidic closed digestion channel and its heating and cooling structure for performing closed digestion and cooling in step S2. The outlet of the microfluidic online closed digestion unit 9 is connected to the dual-mode atomic absorption detection unit 10. The dual-mode atomic absorption detection unit 10 is equipped with a flame atomizer, a graphite furnace atomizer, an iron hollow cathode lamp, and a detector, and is equipped with a switching valve for performing atomization and absorbance acquisition in step S4. The control analysis and calculation unit 11 is electrically connected to the water sample stabilizing constant flow unit 3, the water sample injection pump 4, the hydrochloric acid-nitric acid mixed digestion solution metering pump 6, the ascorbic acid reducing agent metering pump 8, the microfluidic online closed digestion unit 9, and the dual-mode atomic absorption detection unit 10, respectively. It is used to perform range determination and target atomization mode determination in step S3, control the switching valve to introduce the digested water sample into the corresponding atomizer, perform standard curve retrieval and concentration calculation in step S5, and perform automatic calibration scheduling, automatic cleaning scheduling, and over-limit warning output.

[0050] The method of the present invention will be described in detail below through specific embodiments.

[0051] Example 1 This embodiment uses the online monitoring of trace iron in feedwater during the steady-state full-load operation of a 1000MW ultra-supercritical unit as an application scenario, combined with... Figure 1 and Figure 2 The specific execution process of the method of the present invention is described.

[0052] Step S1: The water sample is drawn from the unit's feedwater sampling point, cooled and depressurized by the sampling rack, and then introduced into the monitoring process. The primary filtration uses a sintered stainless steel filter element with a 50μm pore size to remove particulate impurities from the water sample; the secondary filtration uses a polytetrafluoroethylene (PTFE) precision filter membrane with a 0.45μm pore size to remove colloidal impurities from the water sample after primary filtration. The water sample pressure stabilization and constant flow unit 3 stabilizes the pressure of the filtered water sample at 100kPa and the flow rate at a preset flow rate of 20mL / min, obtaining a pretreated water sample.

[0053] Step S2: The sample pump 4 quantitatively extracts 100 mL of pretreated water sample; the hydrochloric acid-nitric acid mixed digestion solution metering pump 6 quantitatively adds 1 mL of hydrochloric acid-nitric acid mixed digestion solution from the hydrochloric acid-nitric acid mixed digestion solution tank 5, that is, the ratio of the volume of hydrochloric acid-nitric acid mixed digestion solution added to the volume of pretreated water sample is 1:100. In the added hydrochloric acid-nitric acid mixed digestion solution, the hydrochloric acid solution is a solution formed by mixing concentrated hydrochloric acid and water at a volume ratio of 1:1, and the nitric acid solution is a solution formed by mixing concentrated nitric acid and water at a volume ratio of 1:1. The hydrochloric acid solution and the nitric acid solution are mixed at a volume ratio of 1:1 according to the preset acid ratio; the ascorbic acid reducing agent metering pump 8 simultaneously quantitatively adds 0.5 mL of ascorbic acid solution with a concentration of 10 g / L from the ascorbic acid reducing agent tank 7, that is, adding 5 μL of pretreated water sample per milliliter. The resulting mixture is fed into the microfluidic closed digestion channel within the microfluidic online closed digestion unit 9. This channel has an inner diameter of 2 mm and a length of 150 mm. The preset digestion temperature is 125℃, the preset digestion time is 100 s, and the digestion pressure under closed conditions is 180 kPa. Under closed digestion conditions, suspended iron, colloidal iron, and ferric iron in the mixture are completely converted into ferrous free iron. The preset injection temperature is set to 25℃. After digestion, the mixture is cooled to below 25℃ by the cooling structure of the microfluidic online closed digestion unit 9 to obtain a digested water sample.

[0054] Step S3: In this embodiment, the range determination parameter adopts the unit operating condition stage identifier. The control analysis and calculation unit 11 reads the unit operating condition stage identifier provided by the DCS. This identifier indicates that the unit is in the steady-state operation stage, and determines that the total iron concentration is in the second concentration range of 1μg / L~100μg / L, and determines that the graphite furnace atomization mode is the target atomization mode.

[0055] Step S4: The control analysis and calculation unit 11 controls the switching valve of the dual-mode atomic absorption detection unit 10 to inject the digested water sample into the graphite furnace atomizer at a preset injection volume of 20 μL. Atomization is performed according to the following stepped heating program: the temperature of the first drying step is 90℃, the heating time is 10s, and the holding time is 20s; the temperature of the second drying step is 130℃, the heating time is 10s, and the holding time is 20s; the temperature of the ashing stage is 1100℃, the heating time is 10s, and the holding time is 20s; the temperature of the atomization stage is 2400℃, the heating time is 1s, and the holding time is 5s; the temperature of the purification stage is 2600℃, the heating time is 1s, and the holding time is 3s. Argon gas is introduced as a protective gas throughout the stepped heating process, with a flow rate of 250 mL / min. The operating current of the iron hollow cathode lamp is 12 mA, the spectral bandpass is 0.2 nm, and the absorbance is collected at the characteristic wavelength of iron, 248.3 nm.

[0056] Step S5: Call the pre-established standard curve corresponding to the graphite furnace atomization mode. This standard curve is pre-established as follows: Prepare iron standard solutions with concentrations of 0 μg / L, 2 μg / L, 10 μg / L, 30 μg / L, 60 μg / L, and 100 μg / L, where 0 μg / L is the blank concentration. Measure each concentration point three times and take the average value to obtain the average absorbance for each concentration point. Perform linear fitting between the average absorbance of each concentration point and the corresponding concentration to obtain the standard curve corresponding to the graphite furnace atomization mode. The linear correlation coefficient r of the standard curve should not be less than 0.999. Substitute the absorbance obtained in step S4 into this standard curve, calculate the total iron concentration in the feed water using the aforementioned concentration back-calculation formula, and output the result.

[0057] Implementation Results: In this embodiment, the feedwater concentration during the steady-state operation of the unit was continuously monitored online for 72 hours. The total iron concentration output in each monitoring cycle remained stable at around 3.2 μg / L, and each monitoring cycle, including digestion, did not exceed 10 minutes. Measurement repeatability was characterized by relative standard deviation (RSD), calculated according to the following formula: ; in, The relative standard deviation, The standard deviation of repeated measurements. This is the average of multiple repeated measurements. The relative standard deviation of continuous monitoring over 72 hours should not exceed 3.0%.

[0058] The accuracy of the test was verified by the spiked recovery rate, which was calculated using the following formula: ; in, To determine the spiked recovery rate, The total iron concentration is the value measured after adding a standard iron solution of known concentration to a water sample. This represents the background value of total iron concentration in the water sample before spiked addition. These are theoretical spike concentrations. The spike recovery rate in this embodiment is 90%–110%, and the limit of detection (LOD) in graphite furnace atomization mode is 0.2 μg / L. These indicators meet the requirements for high-quality steam-water chemical monitoring in ultra-supercritical units.

[0059] Example 2 This embodiment uses the online monitoring of high-concentration rusted iron during the cold flushing phase of a 660MW supercritical unit's start-up as an application scenario. During the cold flushing phase, a large amount of rust products accumulated in the feedwater pipelines and boiler equipment precipitate out, resulting in high turbidity in the water sample, a high proportion of particulate and colloidal iron, and a total iron concentration in the milligrams per liter range. This embodiment combines... Figure 1 and Figure 2 This section describes the specific execution process of the method of the present invention in this scenario.

[0060] Step S1: The water sample is drawn from the condensate sampling point of the unit, cooled and depressurized by the sampling rack, and then introduced into the monitoring process. The primary filtration uses a sintered stainless steel filter element with a pore size of 50μm to trap large particles of rust residue in the rinsing water sample; the secondary filtration uses a polytetrafluoroethylene precision filter membrane with a pore size of 0.45μm to trap colloidal impurities and eliminate scattering interference from high-turbidity water samples on subsequent absorbance measurements. During the start-up phase, the pressure and flow rate of the water sample at the sampling point fluctuate significantly. The water sample pressure stabilization and constant flow unit 3 stabilizes the pressure of the filtered water sample at 110kPa and the flow rate at the preset flow rate of 20mL / min, providing a stable volume reference for the quantitative addition in step S2, thus obtaining a pretreated water sample.

[0061] Step S2: The dosage ratio and digestion conditions are the same as in Example 1: the volume ratio of the hydrochloric acid-nitric acid mixed digestion solution to the pretreated water sample is 1:100; the concentration of ascorbic acid solution is 10 g / L, and it is added at a dosage of 5 μL per milliliter of pretreated water sample; the inner diameter of the microfluidic closed digestion channel is 2 mm and the length is 150 mm; the preset digestion temperature is 125℃; the preset digestion time is 100 s; and the digestion pressure is 180 kPa. The rinse water sample contains a high proportion of suspended particulate rust iron. Under closed digestion conditions, the strong acid dissolution effect of the acidic mixed digestion solution and the reducing dissolution effect of ascorbic acid work synergistically to completely convert particulate and colloidal iron into free iron within 100 s. After digestion, the sample is cooled to below 25℃ to obtain the digested water sample.

[0062] Step S3: In this embodiment, the range determination parameter adopts the unit operating condition stage identifier. The control analysis and calculation unit 11 reads the unit operating condition stage identifier provided by the DCS. This identifier indicates that the unit is in the start-up flushing stage, and determines that the total iron concentration is in the first concentration range of 0.1 mg / L to 5 mg / L, and determines the flame atomization mode as the target atomization mode.

[0063] Step S4: The control analysis and calculation unit 11 controls the switching valve of the dual-mode atomic absorption detection unit 10 to introduce the digested water sample into the flame atomizer in a continuous suction and spray manner. The flame atomizer is supplied with a flame formed by mixing acetylene and air, with a volume ratio of acetylene to air of 1:6 and a burner height of 8 mm. The absorbance is collected at the characteristic wavelength of iron element, 248.3 nm.

[0064] Step S5: Call the pre-established standard curve corresponding to the flame atomization mode. This standard curve is pre-established as follows: Prepare iron standard solutions with concentrations of 0 mg / L, 0.5 mg / L, 1 mg / L, 2 mg / L, 4 mg / L, and 5 mg / L, where 0 mg / L is the blank concentration point. Measure each concentration point three times and take the average value to obtain the average absorbance for each concentration point. Perform linear fitting between the average absorbance of each concentration point and the corresponding concentration to obtain the standard curve corresponding to the flame atomization mode. The linear correlation coefficient r of the standard curve should not be less than 0.999. Substitute the absorbance obtained in step S4 into this standard curve, calculate the total iron concentration in the condensate using the aforementioned concentration back-calculation formula, and output the result.

[0065] Implementation Results: The initial total iron concentration measured during flushing was 2.85 mg / L. As flushing progressed, the total iron concentration output in each monitoring cycle gradually decreased to 0.32 mg / L. The change in total iron concentration from high to low was continuously and quantitatively captured. A single monitoring cycle, including digestion, did not exceed 10 minutes. The detection limit in flame atomization mode was 0.03 mg / L, and the relative standard deviation of repeated measurements was no greater than 3.0%. Based on the continuously output total iron concentration, when the total iron concentration dropped below the preset flushing qualification limit, the cold flushing was deemed qualified, and the process proceeded to the next start-up stage. This provided real-time quantitative basis for determining the flushing endpoint, preventing insufficient flushing that could lead to rust products entering the furnace, and also preventing excessive flushing that could prolong the start-up time.

[0066] Example 3 This embodiment uses the entire start-up and shutdown process of a supercritical unit from cold flushing to grid-connected steady-state operation as an application scenario. It focuses on illustrating the execution process of dual-mode automatic switching, cyclic monitoring, automatic calibration, and out-of-range early warning. (See reference...) Figure 3 In this embodiment, the values ​​of filter aperture, preset flow rate, dosage ratio, and digestion conditions for water sample pretreatment are the same as in Embodiment 1.

[0067] The dual-mode automatic switching judgment strategy: In this embodiment, the range judgment parameter adopts a combination of pre-detection absorbance and historical concentration trend value. In each monitoring cycle, a portion of the digested water sample is first pre-detected using flame atomization mode to obtain the pre-detection absorbance; the preset switching threshold is set to the absorbance value corresponding to the lower limit of the first concentration range of 0.1 mg / L on the standard curve of flame atomization mode; if the pre-detection absorbance is not lower than the preset switching threshold, the total iron concentration is determined to be in the first concentration range, and the formal detection is directly completed in flame atomization mode; if the pre-detection absorbance is lower than the preset switching threshold, the total iron concentration is determined to be in the second concentration range, and the detection is switched to graphite furnace atomization mode. When the total iron concentration calculated for multiple consecutive monitoring cycles is consistently within the same concentration range and the difference between the total iron concentration and the boundary of the concentration range is greater than the preset margin, the total iron concentration calculated in the previous execution of step S5 will be used as the historical concentration trend value in subsequent monitoring cycles. The concentration range of the total iron concentration in this monitoring will be determined based on the concentration range of the historical concentration trend value, and the pre-detection step will be skipped to shorten the monitoring cycle. When the difference between the historical concentration trend value and the boundary of the concentration range is not greater than the preset margin, the result will be reverted to be verified by the pre-detection absorbance.

[0068] During the transition period from the flushing stage to hot operation, the total iron concentration is in the transition zone between the first and second concentration ranges. According to the principle of choosing the higher concentration, it is first classified into the first concentration range and detected by flame atomization mode. If the detected total iron concentration is lower than the lower limit of the first concentration range by 0.1 mg / L, the control analysis and calculation unit 11 automatically switches to graphite furnace atomization mode for retesting, ensuring that samples of any concentration level during the transition period can obtain valid test results.

[0069] Step S6: After the total iron concentration is output for each monitoring cycle, the control analysis and calculation unit 11 drives the automatic cleaning of the sample injection pipeline and the microfluidic closed digestion channel with deionized water to remove residual acid and iron components, avoid cross-contamination and memory effect between adjacent monitoring cycles, and then returns to step S1 to continue execution according to the preset monitoring cycle. In this embodiment, the preset monitoring cycle is set to 10 minutes.

[0070] Automatic calibration: Zero-point calibration is performed every first preset cycle (2 hours) using deionized water; span calibration is performed every second preset cycle (24 hours) using an iron standard solution of a preset calibration concentration. The preset calibration concentration for flame atomization mode is 2 mg / L, and for graphite furnace atomization mode is 50 μg / L. The preset calibration deviation threshold is set to 3.0%. If the measurement deviation of zero-point or span calibration exceeds 3.0%, the calibration is deemed unqualified, a new standard curve for the corresponding atomization mode is established, and step S5 is executed using the newly established standard curve. If the measurement deviation is not greater than 3.0%, the calibration is deemed qualified, and the original standard curve continues to be used.

[0071] Trend and Early Warning: The control analysis and calculation unit 11 generates a total iron concentration trend curve based on the total iron concentration output in each monitoring cycle, allowing chemical monitoring personnel to view the changes in total iron concentration throughout the unit's start-up and shutdown process. The preset concentration limit is set according to the unit's steam and water quality standards. In this embodiment, the preset concentration limit for the steady-state operation phase is set to 10 μg / L. When the output total iron concentration exceeds the preset concentration limit, an over-limit warning message is output.

[0072] Implementation Results: Throughout the start-up and shutdown process, during the flushing phase, the concentration decrease was continuously tracked in milligram per liter mode using flame atomization mode. After the unit transitioned to hot and steady-state operation, it automatically switched to graphite furnace atomization mode to track trace concentrations in micrograms per liter. The entire process operated continuously for 72 hours without human intervention. The detection results from each monitoring cycle were consistent and seamless, with no monitoring points failing due to concentrations exceeding the range of the atomization mode used. During steady-state monitoring, the output total iron concentration was consistently below the preset concentration limit, and no exceedance warning was issued. After spiked into the water sample to exceed the preset concentration limit, the control analysis and calculation unit 11 immediately issued an exceedance warning, demonstrating the effectiveness of the exceedance warning function.

[0073] The three embodiments described above address three typical scenarios: steady-state trace monitoring, high-concentration monitoring during start-up flushing, and dual-mode switching monitoring throughout the start-up and shutdown process. These embodiments verify the adaptability of the method to various concentration levels throughout the entire operating cycle of the unit. The common technical approach of the three embodiments follows the execution flow of steps S1 to S5. The core differences lie in the following: the selection method for the range determination parameter differs; Embodiments 1 and 2 determine the range based on the unit's operating condition stage identifier, while Embodiment 3 primarily uses pre-detected absorbance and supplements it with historical concentration trend values; the target atomization mode differs, corresponding to graphite furnace atomization mode, flame atomization mode, and automatic switching between the two atomization modes, respectively.

[0074] In summary, the embodiments of the present invention have at least the following technical effects: This invention automatically switches between flame atomization mode and graphite furnace atomization mode based on range determination parameters. The flame atomization mode covers the first concentration range of 0.1 mg / L to 5 mg / L, and the graphite furnace atomization mode covers the second concentration range of 1 μg / L to 100 μg / L. Together, they cover a total iron concentration variation range spanning more than two orders of magnitude. In contrast, the detection range of existing online iron meters only covers 30 μg / L to 500 μg / L. This invention achieves full online monitoring coverage of total iron throughout the entire operating cycle from unit start-up flushing to steady-state operation.

[0075] This invention uses a microfluidic closed digestion channel to perform online rapid digestion. The single monitoring cycle, including digestion, does not exceed 10 minutes, which is significantly shorter than the single sample detection cycle of no less than 30 minutes for traditional laboratory atomic absorption spectrometry. This transforms atomic absorption spectrometry from offline sampling to online real-time monitoring, and the detection results can reflect the dynamic changes in the unit's water vapor quality in real time.

[0076] The detection limit of the graphite furnace atomization mode of this invention is 0.2 μg / L, and the detection limit of the flame atomization mode is 0.03 mg / L. The relative standard deviation of repeated measurements is not greater than 3.0%, and the spiked recovery rate is 90%~110%. The trace detection capability is better than the detection limit of about 30 μg / L of the existing colorimetric online iron table, and it takes into account the detection sensitivity under trace conditions and the detection accuracy under high concentration conditions.

[0077] The present invention automatically connects and executes each step of sampling and filtration, flow regulation, quantitative addition, closed digestion, mode determination, atomization detection, concentration calculation, automatic cleaning and automatic calibration, eliminating the need for manual sampling and digestion, and meeting the needs of unmanned intelligent chemical monitoring in power plants.

[0078] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A method for online monitoring of total iron in steam and water in power plants, characterized in that, include: Step S1: Perform multi-stage filtration on the water sample from the power plant's water-steam system, and adjust the filtered water sample to a preset flow rate to obtain a pretreated water sample; Step S2: Add an acidic mixed digestion solution and a reducing agent to the pretreated water sample in a preset ratio to form a mixed solution. Send the mixed solution into a microfluidic closed digestion channel and digest it under closed conditions at a preset digestion temperature and a preset digestion time to convert the suspended iron, colloidal iron and ferric iron in the mixed solution into free iron, thereby obtaining a digested water sample. Step S3: Obtain the range determination parameter that characterizes the concentration range of the total iron concentration in the digested water sample. When the range determination parameter indicates that the total iron concentration is in the first concentration range, determine the flame atomization mode as the target atomization mode. When the range determination parameter indicates that the total iron concentration is in the second concentration range, determine the graphite furnace atomization mode as the target atomization mode. The upper limit of the second concentration range is not higher than the lower limit of the first concentration range. Step S4: The digested water sample is introduced into the atomizer corresponding to the target atomization mode for atomization, and the absorbance is collected at the characteristic wavelength of iron. Step S5: Substitute the absorbance into the pre-established standard curve corresponding to the target atomization mode, calculate and output the total iron concentration in the water sample.

2. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S1, the multi-stage filtration includes a primary filtration and a secondary filtration performed sequentially. The primary filtration removes particulate impurities from the water sample, and the secondary filtration removes colloidal impurities from the water sample after the primary filtration. The pore size of the primary filtration is 30μm~80μm, and the pore size of the secondary filtration is 0.2μm~0.8μm. The preset flow rate is 15mL / min~25mL / min, and the filtered water sample is stabilized at 80kPa~120kPa before being output.

3. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S2, the acidic mixed digestion solution is a hydrochloric acid-nitric acid mixed digestion solution formed by mixing hydrochloric acid solution and nitric acid solution according to a preset acid ratio, and the reducing agent is ascorbic acid solution; The preset addition ratio includes: the ratio of the volume of the hydrochloric acid-nitric acid mixed digestion solution to the volume of the pretreated water sample is 1:80 to 1:120, the concentration of the ascorbic acid solution is 8 g / L to 12 g / L, and the ascorbic acid solution is added at a dosage of 4 μL to 6 μL per milliliter of the pretreated water sample.

4. The method for online monitoring of total iron in steam and water in power plants according to claim 3, characterized in that, The hydrochloric acid solution is a solution formed by mixing concentrated hydrochloric acid and water at a volume ratio of 1:1, and the nitric acid solution is a solution formed by mixing concentrated nitric acid and water at a volume ratio of 1:

1. The preset acid ratio is 1:1 by volume. The inner diameter of the microfluidic closed digestion channel is 1 mm to 3 mm, and the length is 100 mm to 200 mm. The preset digestion temperature is 120℃ to 130℃, the preset digestion time is 60 s to 180 s, and the digestion pressure in the microfluidic closed digestion channel during digestion is 150 kPa to 200 kPa. After digestion, the digested water sample is cooled to below the preset injection temperature before proceeding to step S3.

5. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S3, the range determination parameter includes at least one of the following: unit operating condition stage identifier, pre-detection absorbance, and historical concentration trend value; When the range determination parameter includes the unit operating condition stage identifier, if the unit operating condition stage identifier indicates that the unit is in the start-up flushing stage, then the total iron concentration is determined to be in the first concentration range; if the unit operating condition stage identifier indicates that the unit is in the steady-state operation stage, then the total iron concentration is determined to be in the second concentration range. When the range determination parameter includes the pre-detection absorbance, a portion of the digested water sample is first pre-detected using the flame atomization mode to obtain the pre-detection absorbance. If the pre-detection absorbance is not lower than a preset switching threshold, the total iron concentration is determined to be in the first concentration range; otherwise, the total iron concentration is determined to be in the second concentration range. When the range determination parameter includes the historical concentration trend value, the total iron concentration calculated in the previous execution of step S5 will be used as the historical concentration trend value. The concentration range of the total iron concentration in this monitoring will be determined based on the concentration range of the historical concentration trend value. If it is the first monitoring, the total iron concentration will be determined to be in the first concentration range. The first concentration range is 0.1 mg / L to 5 mg / L, and the second concentration range is 1 μg / L to 100 μg / L.

6. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S4, when the target atomization mode is the flame atomization mode, the atomizer corresponding to the target atomization mode is a flame atomizer. A flame formed by mixing acetylene and air is supplied to the flame atomizer, with the volume ratio of acetylene to air being 1:5 to 1:

7. The burner height of the flame atomizer is 6 mm to 10 mm. The characteristic wavelength of the iron element is 248.3 nm.

7. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S4, when the target atomization mode is the graphite furnace atomization mode, the atomizer corresponding to the target atomization mode is a graphite furnace atomizer. The digested water sample is injected into the graphite furnace atomizer at a preset injection volume, and atomized according to a stepped temperature increase program including a drying section, an ashing section, an atomization section, and a purification section. The drying section includes a first drying step and a second drying step. The temperature of the first drying step is 90°C, the heating time is 10s, and the holding time is 20s. The temperature of the second drying step is 130°C, and the heating time is 1... The ashing section has a temperature of 1100℃, a heating time of 10s, and a holding time of 20s; the atomization section has a temperature of 2400℃, a heating time of 1s, and a holding time of 5s; the purification section has a temperature of 2600℃, a heating time of 1s, and a holding time of 3s; the protective gas for the graphite furnace atomizer is argon, the flow rate of the protective gas is 200mL / min~300mL / min, the operating current of the iron hollow cathode lamp is 10mA~14mA, and the spectral bandpass is 0.2nm.

8. The method for online monitoring of total iron in steam and water in power plants according to claim 1, characterized in that, In step S5, the standard curve is pre-established as follows: For each atomization mode in the flame atomization mode and the graphite furnace atomization mode, an iron standard solution with at least five concentration points, including a blank concentration point, is prepared. Each concentration point is measured at least three times and the average value is taken to obtain the average absorbance of each concentration point. The average absorbance of each concentration point is linearly fitted with the corresponding concentration to obtain the standard curve corresponding to the atomization mode, and the linear correlation coefficient of the standard curve is not lower than the preset correlation coefficient threshold. The fitting equation for the standard curve is: ； In step S5, the total iron concentration is calculated according to the following formula: ； in, The absorbance is [value missing]. The intercept of the standard curve is... The slope of the standard curve is... The total iron concentration is denoted as .

9. The method for online monitoring of total iron in steam and water in power plants according to claim 8, characterized in that, It also includes an automatic calibration step: every first preset period, zero-point calibration is performed with deionized water; every second preset period, span calibration is performed with an iron standard solution of a preset calibration concentration; when the measurement deviation of the zero-point calibration or the span calibration is greater than a preset calibration deviation threshold, the calibration is determined to be unqualified, the standard curve is re-established, and step S5 is performed with the re-established standard curve; when the measurement deviation is not greater than the preset calibration deviation threshold, the calibration is determined to be qualified.

10. The method for online monitoring of total iron in steam and water in power plants according to any one of claims 1 to 9, characterized in that, The method also includes step S6: after the total iron concentration is output, the injection pipeline and the microfluidic closed digestion channel are automatically cleaned, and the process returns to step S1 continuously according to the preset monitoring cycle; a total iron concentration trend curve is generated based on the total iron concentration output in each monitoring cycle, and an over-limit warning message is output when the total iron concentration is greater than the preset concentration limit.

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