Feedwater treatment method for once-through boilers

Abstract

The application provides a feed water treatment method of a once-through boiler, comprising the following steps: in a stage where the operation load of the once-through boiler is greater than or equal to 0% and less than 50%, the pH of the feed water of the once-through boiler is controlled to be 8.80-9.10, and the ammonia mass concentration of the feed water is controlled to be 0.25-0.45 mg / L; in a stage where the operation load of the once-through boiler is greater than or equal to 50% to 100%, the pH of the feed water of the once-through boiler is controlled to be 8.90-9.15, and the ammonia mass concentration of the feed water is controlled to be 0.30-0.60 mg / L; and dissolved oxygen is added to the feed water to make the dissolved oxygen content of the feed water be 15-35 μg / L. The feed water treatment method can reduce reagent consumption, reduce pipeline corrosion, improve corrosion balance, and has good corrosion prevention effect under various load working conditions.

Description

Technical Field

[0001] This application relates to the field of feedwater technology for once-through boilers, and in particular to a feedwater treatment method for once-through boilers. Background Technology

[0002] Supercritical and ultra-supercritical thermal power units all adopt a once-through boiler structure, without a steam drum water buffer regulation link. The feedwater quality index directly determines the corrosion, scaling, and salt accumulation status of the economizer, water-cooled walls, high and low pressure feedwater pipelines, and turbine flow passage components. The chemical control accuracy of the water-steam system directly affects the unit's safety, efficiency, and maintenance cycle.

[0003] Currently, most once-through boilers employ the traditional AVT (All Volatile Treatment) process, which relies on high-dose ammonia injection to raise the pH value of the feedwater and achieve passivation protection for carbon steel pipelines. However, this existing process suffers from numerous technical shortcomings, including high chemical consumption for feedwater corrosion prevention, uneven corrosion control, poor water quality during start-up and shutdown, and a high risk of salt buildup in equipment. Therefore, the industry urgently needs a new, refined feedwater treatment process that balances low chemical consumption, low corrosion, full-load stability, and adaptive start-up and shutdown capabilities. Summary of the Invention

[0004] Based on this, this application provides a feedwater treatment method for a once-through boiler, which can reduce chemical consumption, reduce pipeline corrosion, improve corrosion resistance balance, and has good corrosion resistance under various load conditions.

[0005] According to a first aspect of this application, a method for treating feedwater in a once-through boiler is provided, comprising the following steps:

[0006] During the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH of the once-through boiler is controlled at 8.80~9.10 and the feedwater ammonia concentration is controlled at 0.25mg / L~0.45mg / L.

[0007] During the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, the pH of the feedwater of the once-through boiler is controlled at 8.90~9.15 and the ammonia concentration of the feedwater is controlled at 0.30mg / L~0.60mg / L; and dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 15μg / L~35μg / L.

[0008] In some implementations, during the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH of the once-through boiler is controlled at 8.85~9.05 and the feedwater ammonia concentration is controlled at 0.30mg / L~0.40mg / L.

[0009] In some implementations, during the period when the operating load of the once-through boiler is greater than or equal to 50% to reach 100%, the feedwater pH of the once-through boiler is controlled at 8.95~9.10 and the feedwater ammonia concentration is controlled at 0.35mg / L~0.50mg / L.

[0010] In some implementations, during the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 22 μg / L to 32 μg / L.

[0011] In some implementations, the following steps are also included: after the operating load of the once-through boiler reaches 100%, the corrosion index of the water-steam system of the once-through boiler is continuously monitored. When the corrosion index exceeds the preset value, the feedwater pH, feedwater ammonia concentration and feedwater dissolved oxygen content of the once-through boiler are adjusted to control the corrosion index within the preset value.

[0012] In some implementations, corrosion indicators include one or more of the following: iron content in the feedwater at the economizer inlet, iron content in the steam in the steam-water separator, and iron content in the superheated steam.

[0013] In some implementations, the preset value includes at least one of the following three:

[0014] (1) The iron content of the feedwater at the economizer inlet is less than or equal to 2 μg / L;

[0015] (2) The iron content in the steam in the steam-water separator is less than or equal to 1.5 μg / L;

[0016] (3) The iron content in the superheated steam is less than or equal to 1 μg / L.

[0017] In some embodiments, when the corrosion index exceeds a preset value, the feedwater pH, feedwater ammonia concentration, and feedwater dissolved oxygen content of the once-through boiler are adjusted, including the following steps: when at least one of the iron content in the feedwater at the economizer inlet, the iron content in the steam in the steam-water separator, and the iron content in the superheated steam exceeds a preset value, the feedwater pH is adjusted to 9.10~9.15, the feedwater ammonia concentration is adjusted to 0.22mg / L~0.28mg / L, and the feedwater dissolved oxygen content is adjusted to 25μg / L~35μg / L; the corrosion index of the water-steam system of the once-through boiler is monitored until the corrosion index falls back to within the preset value.

[0018] In some implementations, the following steps are also included: after the operating load of the once-through boiler reaches 100%, continuously monitor the instantaneous iron ion content and pH fluctuation of the feedwater of the once-through boiler; when the instantaneous iron ion content of the feedwater is greater than 5 μg / L or the pH of the feedwater is less than 9.05 or greater than 9.25; adjust the dissolved oxygen content of the feedwater to 40 μg / L~50 μg / L and adjust the pH of the feedwater to 9.10~9.20 within 20 minutes.

[0019] In some embodiments, after adjusting the dissolved oxygen content of the feedwater to 40 μg / L~50 μg / L and the pH of the feedwater to 9.10~9.20, the following steps are further included: when the iron content of the feedwater at the economizer inlet is less than or equal to 2 μg / L, the iron content of the steam in the steam-water separator is less than or equal to 1.5 μg / L, the iron content of the superheated steam is less than or equal to 1 μg / L, and the pH of the feedwater is stable within 9.05~9.25 and maintained for 30 minutes; the dissolved oxygen content of the feedwater is adjusted to 15 μg / L~35 μg / L and the pH of the feedwater is adjusted to 8.90~9.15 within 30 minutes.

[0020] Compared with traditional technologies, this application has at least the following beneficial effects:

[0021] The feedwater treatment method for a once-through boiler disclosed in this application combines differentiated feedwater pH and ammonia concentration control with varying loads and stages, along with low-ammonia and trace oxygenation coupling to form a film. This significantly reduces the amount of ammonia added while constructing a dense, stable, and low-shedding iron-based passivation film. This achieves low-corrosion operation of the steam-water system under all operating conditions—start-up, low load, and steady-state full load—of the once-through boiler. It solves the technical pain points of traditional processes, such as uneven corrosion prevention, high chemical consumption, poor start-up and shutdown water quality, and significant salt accumulation in the turbine. This method improves the precision of steam-water chemical monitoring in once-through boilers and enhances the long-term safe operation capability of the equipment.

[0022] Furthermore, after the once-through boiler is running at full load, by continuously monitoring the corrosion indicators of the steam-water system of the once-through boiler, continuously monitoring the instantaneous iron ion content and pH fluctuation of the feedwater, and making corresponding corrections; and by coupling the abnormal operating condition correction steps with the phased differentiated feedwater pH and ammonia concentration control, low ammonia and trace oxygenation film formation methods to form a closed loop, it is more conducive to improving the corrosion prevention effect of the once-through boiler. Detailed Implementation

[0023] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, a detailed description of specific embodiments of this application is provided. Many specific details are set forth in the following description to provide a thorough understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. Unless otherwise specifically stated, all raw materials, reagents, instruments, and equipment used in this application are commercially available or can be prepared by existing methods.

[0025] Currently, the traditional AVT (Activated Vapor Transmission) process is commonly used for feedwater treatment in once-through boilers. This process relies on high-dose ammonia injection to raise the pH value of the feedwater and achieve passivation protection for carbon steel pipelines. However, the traditional AVT process has long suffered from numerous inherent technical shortcomings, including high energy and chemical consumption, equipment corrosion and salt buildup, unstable passivation during unit start-up and shutdown, severe iron ion contamination, and poor corrosion protection uniformity. These are detailed below:

[0026] (1) Large ammonia addition and high energy and chemical consumption during operation. In order to maintain a high alkaline anti-corrosion environment, the traditional AVT process requires the feed water to be kept at a high ammonia concentration, resulting in huge annual ammonia consumption, high operating costs, and a large workload for chemical dosing, preparation, and manual maintenance, which is not economical.

[0027] (2) Ammonia enrichment in the system leads to corrosion and salt deposits in low-pressure equipment. Excessive ammonia migrates with the steam to the low-pressure cylinder, low-pressure heater, and condensate system, causing local ammonia concentration enrichment, which leads to ammonia corrosion and scouring corrosion of copper alloy and carbon steel equipment. At the same time, ammonium salts are prone to deposit and form salts in the low-pressure stage of the steam turbine, reducing the flow efficiency of the unit and increasing the cost of maintenance and descaling.

[0028] (3) Unstable passivation and serious iron ion exceedance during unit start-up and shutdown. During unit start-up and shutdown and load fluctuation, the oxide film formed by traditional high ammonia passivation is loose, has poor adhesion and is easy to peel off, resulting in a significant exceedance of iron ions in feedwater and steam during the start-up phase. This leads to an increase in the deposition of corrosion products on the boiler heating surface, which in turn induces problems such as scaling on water-cooled walls, increased pressure difference and decreased heat exchange efficiency in the long term.

[0029] (4) The corrosion protection of a single alkaline anti-corrosion mode is poor. The traditional pure ammonia high pH anti-corrosion method relies solely on alkalinity to inhibit corrosion, which cannot repair the defects of flow-accelerated corrosion (FAC) formed in the local oxygen-deficient and turbulent areas of the pipeline. Under high load and high flow velocity conditions, local corrosion of the pipeline is still prominent.

[0030] (5) Traditional process control is crude and cannot be applied stably under all operating conditions. The conventional water treatment process parameters on the market are simple and lack segmented control logic. It cannot adapt to the switching of multiple operating conditions such as unit start-up and shutdown, low load, variable load and steady-state operation, and is prone to problems such as pitting corrosion, uneven film layer and large water quality fluctuation.

[0031] Therefore, in response to the industry pain points of traditional once-through boiler feedwater corrosion prevention processes, such as high chemical consumption, uneven corrosion control, poor water quality during start-up and shutdown, and high risk of salt accumulation in equipment, there is an urgent need to develop a new refined feedwater treatment process that takes into account low chemical consumption, low corrosion, full-load stability, and adaptive start-up and shutdown.

[0032] In response, one embodiment of this application provides a feedwater treatment method for a once-through boiler. This method includes the following steps: during a period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, controlling the pH of the feedwater to 8.80~9.10 and the ammonia concentration to 0.25mg / L~0.45mg / L; during a period when the operating load of the once-through boiler is greater than or equal to 50% and reaches 100%, controlling the pH of the feedwater to 8.90~9.15 and the ammonia concentration to 0.30mg / L~0.60mg / L; and adding dissolved oxygen to the feedwater to achieve a dissolved oxygen content of 15μg / L~35μg / L.

[0033] The feedwater treatment method for the once-through boiler described in this application eliminates the high-ammonia shock passivation and high-pH pipe flushing operations during the startup phase of traditional once-through boiler feedwater treatment methods, and adopts a gentle, low-ammonia start-up film formation control method. During the period when the once-through boiler's operating load is greater than or equal to 0% and less than 50%, the feedwater pH is controlled within the range of 8.80 to 9.10, and the feedwater ammonia concentration is controlled within the range of 0.25 mg / L to 0.45 mg / L; this reduces ammonia dosage by more than 30% compared to traditional processes. The low-ammonia, weakly alkaline water environment prevents the formation of a loose, porous initial oxide layer on the pipe walls, effectively reducing the iron ion concentration in the feedwater during startup, providing a good substrate for the growth of a dense passivation film in subsequent stages, and solving the problem of excessive iron peaks during the startup phase of once-through boilers.

[0034] During the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, ultra-low ammonia fine control is implemented to stabilize the feedwater pH value at 8.90~9.15 and maintain the feedwater ammonia concentration at a constant 0.30mg / L~0.60mg / L. By controlling the parameters stably within a narrow range, large fluctuations in feedwater pH are avoided, mitigating local over-alkalization and ammonia salt enrichment. This reduces the risk of ammonia corrosion in low-pressure heaters, condensate pipes, and condensers from the source, reduces the amount of ammonia carried by steam, and lowers the salt deposition rate in the low-pressure stage of the turbine.

[0035] Furthermore, this application, within an operating load range of 50% to 100%, adds trace amounts of dissolved oxygen to the feedwater in a low-ammonia, weakly alkaline water environment, controlling the dissolved oxygen content to be 15 μg / L to 35 μg / L. Utilizing this low-ammonia, low-disturbance, and micro-oxygen coupled environment, a dense, low-solubility, and highly adhesive composite passivation film, primarily composed of ferric oxide, is preferentially formed on the inner metal surface of the pipe, replacing the loose magnetite film layer formed under traditional high-ammonia feedwater treatment processes. This effectively inhibits flow-accelerated corrosion in high-velocity areas and significantly improves the corrosion resistance of the pipe wall.

[0036] The feedwater treatment method for the once-through boiler described in this application combines differentiated feedwater pH and ammonia concentration control with varying loads and stages, along with low-ammonia and trace oxygenation coupling to form a film. This significantly reduces ammonia dosage while constructing a dense, stable, and low-shedding iron-based passivation film. It achieves low-corrosion operation of the steam-water system under all operating conditions—start-up, low load, and steady-state full load—of the once-through boiler. This method addresses the technical pain points of traditional processes, such as uneven corrosion prevention, high chemical consumption, poor start-up and shutdown water quality, and significant salt accumulation in the turbine. It also improves the precision of steam-water chemical monitoring in once-through boilers and enhances the long-term safe operation capability of the equipment.

[0037] It should be noted that boilers are essential thermal energy equipment in thermal power plant generator sets. Boilers are generally classified into drum boilers and once-through boilers according to their steam-water circulation methods. Drum boilers have a large steam drum (boiler drum). Feedwater is heated in the drum to generate high-pressure steam. The steam drives the turbine to power the generator. The exhaust steam condenses into water and is returned to the boiler for circulation. Steam-water separation and water circulation are achieved in the steam drum. Drum boilers are mainly suitable for thermal power generator sets with subcritical and below pressures.

[0038] A once-through boiler has no steam drum. Feedwater enters the economizer and flows through the water-cooled walls and superheater in a single pass, where it is completely heated, evaporated, and superheated to generate high-temperature, high-pressure steam, which is then sent to the turbine to drive the generator. The feedwater only flows through the once-through boiler once; there is no circulating water flow inside the boiler. Once-through boilers are mainly suitable for supercritical and ultra-supercritical thermal power generating units.

[0039] The operating load of a once-through boiler refers to the ratio of the boiler's current actual steam output to the maximum steam output that the boiler can achieve during long-term safe and stable operation. That is, current actual steam output / maximum steam output × 100%. For example, if the boiler's current actual steam output is 1000 t / h and its maximum steam output is 2000 t / h, then the boiler's operating load is 50%.

[0040] It is understandable that during the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the pH value of the feedwater can be 8.80, 8.81, 8.82, 8.83, 8.84, 8.85, 8.86, 8.87, 8.88, 8.89, 8.90, 8.91, 8.92, 8.93, 8.94, 8.95, 8.96, 8.97, 8.98, 8.99, 9.00, 9.01, 9.02, 9.03, 9.04, 9.05, 9.06, 9.07, 9.08, 9.09, 9.10, or any value within the range formed by any two of the above values. The ammonia concentration in the feedwater can be 0.25 mg / L, 0.26 mg / L, 0.27 mg / L, 0.28 mg / L, 0.29 mg / L, 0.30 mg / L, 0.31 mg / L, 0.32 mg / L, 0.33 mg / L, 0.34 mg / L, 0.35 mg / L, 0.36 mg / L, 0.37 mg / L, 0.38 mg / L, 0.39 mg / L, 0.40 mg / L, 0.41 mg / L, 0.42 mg / L, 0.43 mg / L, 0.44 mg / L, 0.45 mg / L, or any value within the range formed by any two of the above values.

[0041] During the period when the operating load of the once-through boiler is greater than or equal to 50% to reach 100%, the pH value of the feedwater can be 8.90, 8.91, 8.92, 8.93, 8.94, 8.95, 8.96, 8.97, 8.98, 8.99, 9.00, 9.01, 9.02, 9.03, 9.04, 9.05, 9.06, 9.07, 9.08, 9.09, 9.10, 9.11, 9.12, 9.13, 9.14, 9.15, or any value within the range formed by any two of the above values. The ammonia concentration in the feedwater can be 0.30 mg / L, 0.31 mg / L, 0.32 mg / L, 0.33 mg / L, 0.34 mg / L, 0.35 mg / L, 0.36 mg / L, 0.37 mg / L, 0.38 mg / L, 0.39 mg / L, 0.40 mg / L, 0.41 mg / L, 0.42 mg / L, 0.43 mg / L, 0.44 mg / L, 0.45 mg / L, 0... 0.46 mg / L, 0.47 mg / L, 0.48 mg / L, 0.49 mg / L, 0.50 mg / L, 0.51 mg / L, 0.52 mg / L, 0.53 mg / L, 0.54 mg / L, 0.55 mg / L, 0.56 mg / L, 0.57 mg / L, 0.58 mg / L, 0.59 mg / L, 0.60 mg / L, and any value within the range formed by any two of the above values.

[0042] In some embodiments, during the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH is controlled at 8.85~9.05 and the feedwater ammonia concentration is controlled at 0.30mg / L~0.40mg / L. Further controlling the feedwater pH and ammonia concentration within the above ranges during the period when the operating load is greater than or equal to 0% and less than 50% is more conducive to reducing the feedwater iron ion concentration during the start-up phase.

[0043] In some embodiments, during the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, the feedwater pH is controlled at 8.95~9.10 and the feedwater ammonia concentration is controlled at 0.35mg / L~0.50mg / L. Further controlling the feedwater pH and ammonia concentration within the above range during the period when the operating load is greater than or equal to 50% to 100% is more conducive to mitigating excessive local alkalization and ammonia salt accumulation, reducing the risk of ammonia corrosion in low-pressure heaters, drain pipes, and condensers, reducing the amount of ammonia carried in steam, and lowering the salt deposition rate in the low-pressure stage of the turbine.

[0044] In some embodiments, during the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 22μg / L~32μg / L. This is more conducive to promoting the preferential formation of a dense, low-solubility, and highly adhesive composite passivation film mainly composed of ferric oxide on the inner metal surface of the pipe, which helps to inhibit flow-accelerated corrosion in high-flow-rate areas and improve the corrosion resistance stability of the pipe wall.

[0045] In some embodiments, the feedwater treatment method for a once-through boiler further includes the following steps: when the operating load of the once-through boiler reaches 100%, continuously monitor the corrosion index of the steam-water system of the once-through boiler; when the corrosion index exceeds a preset value, adjust the feedwater pH, feedwater ammonia concentration and feedwater dissolved oxygen content of the once-through boiler to control the corrosion index within the preset value.

[0046] Thus, during the full-load steady-state operation of the once-through boiler, the corrosion index of the water-steam system of the once-through boiler is continuously monitored. By adjusting the feedwater pH, feedwater ammonia concentration and feedwater dissolved oxygen content, the corrosion index is controlled within the preset value, which helps to maintain the continuous integrity, uniformity and non-detachment of the composite passivation film, and achieve a long-term low-corrosion and low-salt-accumulation operation state of the water-steam system of the once-through boiler.

[0047] In some embodiments, the corrosion indicators of the once-through boiler's steam-water system include one or more of the following: iron content in the feedwater at the economizer inlet, iron content in the steam-water separator, and iron content in the superheated steam. That is, by monitoring and controlling one or more of these indicators, the corrosion indicators are kept within preset values, thereby achieving a long-term, low-corrosion, low-salt-accumulation operating state for the once-through boiler's steam-water system.

[0048] In some specific examples, the preset values ​​of the above corrosion index include at least one of the following three items: (1) the iron content of the feedwater at the economizer inlet is less than or equal to 2 μg / L; (2) the iron content of the steam in the steam-water separator is less than or equal to 1.5 μg / L; (3) the iron content of the superheated steam is less than or equal to 1 μg / L.

[0049] If the iron content in the feedwater at the economizer inlet exceeds the preset value, it indicates excessive iron at the feedwater source; if the iron content in the steam in the steam-water separator exceeds the preset value, it indicates excessive corrosion in the boiler water-cooled wall section; if the iron content in the superheated steam exceeds the preset value, it indicates corrosion and / or scale detachment in the superheater and pre-flow components. Through real-time monitoring and dynamic control of these corrosion indicators, the stability of the water chemical environment can be precisely maintained at different operating stages, avoiding localized corrosion or deposition problems caused by water quality fluctuations.

[0050] It should be noted that once-through boilers are typically equipped with economizers, water-cooled walls, external steam-water separators, superheaters, and other equipment. Feedwater is heated sequentially by the economizer and water-cooled walls before entering the external steam-water separator. Steam enters the superheater to absorb heat from the flue gas and is heated into high-temperature, high-pressure superheated steam, which is then sent to the high-pressure cylinder of the turbine to perform work.

[0051] Furthermore, when the corrosion index of the once-through boiler's steam-water system exceeds the aforementioned preset values, the feedwater pH, feedwater ammonia concentration, and feedwater dissolved oxygen content of the once-through boiler are adjusted, including the following steps: when at least one of the following exceeds the preset values: feedwater iron content at the economizer inlet, steam iron content in the steam-water separator, and iron content in the superheated steam, the feedwater pH is adjusted to 9.10~9.15, the feedwater ammonia concentration is adjusted to 0.22mg / L~0.28mg / L, and the feedwater dissolved oxygen is adjusted to 25μg / L~35μg / L; the corrosion index of the once-through boiler's steam-water system is monitored until all corrosion indexes fall back to within the preset values.

[0052] In some embodiments, the feedwater treatment method for a once-through boiler further includes the following steps: after the operating load of the once-through boiler reaches 100%, continuously monitor the instantaneous iron ion content and pH fluctuation of the feedwater; when the instantaneous iron ion content of the feedwater is greater than 5 μg / L or the pH of the feedwater is less than 9.05 or greater than 9.25; adjust the dissolved oxygen content of the feedwater to 40 μg / L~50 μg / L and adjust the pH of the feedwater to 9.10~9.20 within 20 minutes.

[0053] When the instantaneous iron ion concentration in the feedwater exceeds 5 μg / L, or the feedwater pH is less than 9.05 or greater than 9.25, it indicates a significant load fluctuation or water quality disturbance in the once-through boiler. By rapidly increasing the dissolved oxygen in the feedwater to 40-50 μg / L and slightly adjusting the pH to 9.10-9.20, locally damaged or exposed metal surface passivation films can be quickly repaired. This facilitates the establishment of an automatic film repair control logic for abnormal water quality conditions, enabling timely correction of abnormal operating conditions. By coupling the above abnormal operating condition correction steps with the aforementioned staged differentiated feedwater pH and ammonia concentration control, and low-ammonia and trace oxygenation film formation methods to form a closed loop, the corrosion prevention effect of the once-through boiler is further improved.

[0054] In some embodiments, after adjusting the dissolved oxygen content of the feedwater to 40 μg / L~50 μg / L and the pH of the feedwater to 9.10~9.20, the feedwater treatment method for the once-through boiler further includes the following steps: when the iron content of the feedwater at the economizer inlet is less than or equal to 2 μg / L, the iron content of the steam in the steam-water separator is less than or equal to 1.5 μg / L, the iron content of the superheated steam is less than or equal to 1 μg / L, and the pH of the feedwater is stable within 9.05~9.25 and maintained stably for 30 minutes; within 30 minutes, the dissolved oxygen content of the feedwater is adjusted to 15 μg / L~35 μg / L, and the pH of the feedwater is adjusted to 8.90~9.15.

[0055] In this way, after correcting abnormal operating conditions and quickly repairing locally damaged and exposed metal surface passivation films, once the water quality indicators return to steady-state standards, they are immediately lowered back to the low ammonia and micro-oxygen baseline control parameters of 15μg / L~35μg / L dissolved oxygen content and 8.90~9.15 pH value, avoiding the risk of secondary corrosion caused by long-term high oxygen and high alkalinity water supply.

[0056] In summary, the feedwater treatment method for once-through boilers in this application utilizes ammonia to regulate the basic pH of the feedwater, creating an alkaline passivation environment, while trace amounts of oxygen synergistically construct an oxidizing condition. This dual-parameter coupling generates a high-temperature resistant, anti-stripping, and corrosion-resistant metal passivation film. By controlling the dissolved oxygen content and pH of the feedwater, the ammonia and oxygen dosages can be smoothly and synchronously adjusted during peak shaving and start-up / shutdown conditions, avoiding passivation film detachment and the combined damage from oxygen and ammonia corrosion of thermal equipment caused by sudden parameter changes. Employing a low-ammonia operation mode fundamentally inhibits ammonia corrosion of copper pipes and ammonia poisoning of resin, while the oxygenation process suppresses pitting corrosion and under-deposit corrosion. The synergistic effect of these two methods achieves low ammonia dosage, low corrosion across the entire system, and long-term, high-efficiency feedwater passivation.

[0057] Compared with the traditional AVT high ammonia volatilization treatment process for once-through boilers, the feedwater treatment method of this application has the following advantages:

[0058] (1) Significantly reduce ammonia water reagent consumption and significantly improve the economic efficiency of unit operation. This application adopts low ammonia control throughout the process, and the overall ammonia dosage is significantly reduced compared with the traditional process, which greatly reduces the cost of reagent procurement, preparation and maintenance, reduces the labor intensity of daily dosing, and has outstanding energy-saving and consumption-reducing effects.

[0059] (2) Thoroughly improve the problem of water quality exceeding standards during start-up and shutdown, and reduce the risk of deposition on the heating surface. This application abandons the traditional high-ammonia loose film formation method and adopts mild low-ammonia start-up film formation. The peak value of iron ions during the start-up of the once-through boiler is greatly reduced, the start-up water quality qualification time is shortened, and the deposition of corrosion products on the boiler water-cooled wall and superheater is effectively reduced, thereby reducing the rise of boiler pressure difference and scaling rate.

[0060] (3) Effectively inhibits flow-accelerated corrosion, making the corrosion protection of the entire system more balanced and stable. This application forms a dense and stable Fe2O3 passivation film on the pipe wall through low ammonia and micro-oxygen coupling, which significantly inhibits flow-accelerated corrosion in high-velocity areas, bends, and diameter changes of water supply pipelines. This helps to solve the problem of weak local corrosion control in traditional high ammonia processes and helps to extend the service life of water and steam pipeline equipment.

[0061] (4) Eliminate the hidden dangers of ammonia corrosion in the low-pressure system and salt accumulation in the turbine. This application can significantly reduce the ammonia enrichment level in the system, reduce the amount of ammonium salt carried by the steam, fundamentally alleviate the ammonia corrosion scouring problem in the system, reduce the rate of salt accumulation and scaling in the low-pressure stage of the turbine, stabilize the flow efficiency of the unit, and reduce the frequency of shutdown for cleaning and maintenance.

[0062] (5) It is self-adaptive under all operating conditions, has strong anti-disturbance capabilities, and high operational safety. This application covers all operating conditions of DC boiler start-up and shutdown, low load, steady-state full load, load fluctuation, and water quality abnormality repair. It sets up an automatic film repair step for abnormalities, has strong stability, no pitting corrosion, and no risk of film layer detachment, and is suitable for various supercritical and ultra-supercritical DC boiler units.

[0063] (6) Minimal equipment modification, highly practical, and highly valuable for promotion. This application only optimizes chemical operation control parameters and process logic, requiring no new equipment, no pipeline modification, and no large investment. It can be implemented based on existing chemical dosing systems and online monitoring instruments, making it suitable for promotion and application in similar units across the industry.

[0064] The present application will be further described below with reference to specific embodiments and comparative examples, but it should not be construed as a limitation on the scope of protection of the present application. In Embodiment 1, Comparative Example 1 and Comparative Example 2, a 350MW supercritical once-through boiler was used, with a feedwater flow rate of 1080t / h and a feedwater temperature of 261.2℃, and continuous operation for 90 days was carried out for comparative monitoring.

[0065] Example 1:

[0066] The water treatment method adopted in this application is as follows:

[0067] During the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH of the once-through boiler is controlled at 8.80~9.10 and the feedwater ammonia concentration is controlled at 0.25mg / L~0.45mg / L.

[0068] During the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, the pH of the feedwater of the once-through boiler is controlled at 8.90~9.15 and the ammonia concentration of the feedwater is controlled at 0.30mg / L~0.60mg / L; and dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 15μg / L~35μg / L.

[0069] After the operating load of the once-through boiler reaches 100%, continuously monitor the iron content in the feedwater at the economizer inlet, the iron content in the steam in the steam-water separator, and the iron content in the superheated steam. If at least one of the following preset values ​​is not met, adjust the feedwater pH, feedwater ammonia concentration, and feedwater dissolved oxygen content to control the iron content in the feedwater at the economizer inlet, the iron content in the steam in the steam-water separator, and the iron content in the superheated steam within the preset values. The preset values ​​are: (1) the iron content in the feedwater at the economizer inlet is less than or equal to 2 μg / L; (2) the iron content in the steam in the steam-water separator is less than or equal to 1.5 μg / L; and (3) the iron content in the superheated steam is less than or equal to 1 μg / L.

[0070] Furthermore, once the once-through boiler reaches 100% operating load, the instantaneous iron ion content and pH fluctuation of the feedwater are continuously monitored. When the instantaneous iron ion content in the feedwater exceeds 5 μg / L or the feedwater pH is less than 9.05 or greater than 9.25, the dissolved oxygen content in the feedwater is adjusted to 40 μg / L~50 μg / L within 20 minutes, and the feedwater pH is adjusted to 9.10~9.20. Further, when the iron content in the feedwater is less than or equal to 2 μg / L, the iron content in the steam in the steam-water separator is less than or equal to 1.5 μg / L, the iron content in the superheated steam is less than or equal to 1 μg / L, and the feedwater pH remains stable within 9.05~9.25 for 30 minutes, the dissolved oxygen content in the feedwater is adjusted to 15 μg / L~35 μg / L within 30 minutes, and the feedwater pH is adjusted to 8.90~9.15.

[0071] Comparative Example 1:

[0072] The traditional AVT(R) reducing volatile organic compound (VOC) water treatment process is adopted, as detailed below:

[0073] Only ammonia water and hydrazine are added to the feedwater for deoxygenation, strictly maintaining a reducing environment; the dissolved oxygen in the feedwater is controlled to be less than 5 μg / L throughout the process; the pH of the feedwater is controlled at 9.3~9.6 by adding ammonia; the dosage of hydrazine is controlled at 20 μg / L~50 μg / L; uniform dosing parameters are used in the full load range of the unit from 0~100%, without segmented adjustment according to the load; dissolved oxygen is not actively introduced, but is eliminated by hydrazine, and a magnetic iron oxide passivation film is generated on the inner wall of the pipeline.

[0074] Comparative Example 2:

[0075] The traditional AVT(O) oxidizing total volatile water treatment process is adopted, as follows:

[0076] Only ammonia water is added, and hydrazine reducing agent is eliminated; the pH of the feed water is controlled at 9.0~9.3; relying on natural dissolved oxygen, the dissolved oxygen content of the feed water is maintained at 50μg / L~150μg / L; a fixed ammonia addition rate is adopted at full load, and a zoned adjustment strategy is adopted when there is no load; an α-Fe2O3 passivation film is formed on the inner wall of the pipeline, and no emergency control strategy for pH and oxygen content after water quality abnormalities is set throughout the process.

[0077] The following are performance testing methods:

[0078] (1) Average iron ion content in water supply

[0079] Online sampling port of the economizer inlet feedwater pipeline; testing timing: continuous operation of the unit at 100% stable full load for 72 hours; one water sample was collected every 2 hours, for a total of 36 samples; the iron ion content of all water samples was detected, and the arithmetic mean was calculated after removing abnormal and abrupt data. The iron ion content was tested using inductively coupled plasma mass spectrometry (ICP-MS), with an instrument detection limit of Fe: 0.1 μg / L and a relative error of ≤±10%.

[0080] (2) Iron deposition rate at the economizer inlet

[0081] Continuous and stable operation for 30 days; iron ion concentration in the economizer inlet and outlet feedwater was collected every 72 hours; the iron ion difference between inlet and outlet was calculated; the daily average iron deposition rate (μg / (m²)) was calculated based on the iron deposition mass per unit time and unit heat exchange area. 2 ·d)).

[0082] (3) Copper content in low-pressure heaters

[0083] Low-pressure heater condensate outlet; Testing timing: Sampling begins after 72 hours of stable operation at full load; Sampling is conducted every 4 hours for 3 consecutive days; The copper ion content of the condensate sample is measured, and the average value of all test data is taken. Copper ion content is measured using inductively coupled plasma mass spectrometry (ICP-MS), with an instrument detection limit of Cu: 0.05 μg / L and a relative error of ≤±10%.

[0084] (4) Main steam conductivity

[0085] Sampling point of the main steam pipeline at the end of the superheated steam; the unit operates at 100% rated load; hydrogen conductivity data are collected online every 2 hours; continuous monitoring for 72 hours, instantaneous fluctuation peaks are removed, and the steady-state average conductivity value (μS / cm) is calculated.

[0086] (5) Salt deposition rate of turbine blades

[0087] The system is shut down and disassembled to inspect the salt accumulation on the blades after 180 days of continuous operation.

[0088] Extremely low: Salt deposition rate < 0.2 mg / (cm³) 2 ·Year);

[0089] Moderate: 0.2~0.5 mg / (cm³) 2 ·Year);

[0090] High: >0.5mg / (cm) 2 ·Year).

[0091] (6) Risk of copper-ammonia corrosion in the system

[0092] Determined based on the average copper ion content of water samples from the low-pressure hydrophobic system:

[0093] Extremely high risk: Copper ions > 3 μg / L;

[0094] Moderate risk: 1~3 μg / L;

[0095] The risk has been largely eliminated: copper ions <1 μg / L.

[0096] The main process parameters and performance data of the above embodiments and comparative examples are shown in Table 1.

[0097] Table 1

[0098]

[0099] As shown in Table 1, the traditional AVT(R) reducing volatile feedwater treatment process in Comparative Example 1 relies on high ammonia to increase pH, which controls carbon steel corrosion to some extent. However, high ammonia causes ammonia corrosion of copper tubes in the low-pressure heater. Copper ions migrate with the steam and deposit in the turbine, resulting in a high salt deposition rate on the turbine blades and an extremely high risk of copper ammonia corrosion in the system.

[0100] In Comparative Example 2, the traditional AVT(O) oxidizing volatile feedwater treatment process reduced iron content through oxygenation and passivation, but still required a relatively high amount of ammonia to maintain pH. The passivation film had poor stability and was easily damaged under load fluctuations. The turbine blade salt deposition rate was moderate, and the system's copper-ammonia corrosion risk was moderate.

[0101] In Example 1, a low-ammonia, trace oxygenation synergistic passivation method was used, which significantly reduced the amount of ammonia added and simultaneously suppressed both iron and copper corrosion. The passivation film was dense and stable, with strong adaptability to load fluctuations. The iron ion content in the feedwater, the iron deposition rate at the economizer inlet, and the copper content in the low-pressure heater were all significantly reduced; the salt deposition rate of the turbine blades was extremely low, and the risk of copper-ammonia corrosion in the system was basically eliminated.

[0102] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0103] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for treating feedwater in a once-through boiler, characterized in that, Includes the following steps: During the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH of the once-through boiler is controlled at 8.80~9.10 and the feedwater ammonia concentration is controlled at 0.25mg / L~0.45mg / L; During the period when the operating load of the once-through boiler is greater than or equal to 50% to reach 100%, the pH of the feedwater of the once-through boiler is controlled at 8.90~9.15 and the ammonia concentration of the feedwater is controlled at 0.30mg / L~0.60mg / L; and dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 15μg / L~35μg / L.

2. The feedwater treatment method for a once-through boiler according to claim 1, characterized in that, During the period when the operating load of the once-through boiler is greater than or equal to 0% and less than 50%, the feedwater pH of the once-through boiler is controlled at 8.85~9.05 and the feedwater ammonia concentration is controlled at 0.30mg / L~0.40mg / L.

3. The feedwater treatment method for a once-through boiler according to claim 1, characterized in that, During the period when the operating load of the once-through boiler is greater than or equal to 50% to reach 100%, the feedwater pH of the once-through boiler is controlled at 8.95~9.10 and the feedwater ammonia concentration is controlled at 0.35mg / L~0.50mg / L.

4. The feedwater treatment method for a once-through boiler according to claim 1, characterized in that, During the period when the operating load of the once-through boiler is greater than or equal to 50% to 100%, dissolved oxygen is added to the feedwater to make the dissolved oxygen content of the feedwater 22μg / L~32μg / L.

5. The feedwater treatment method for a once-through boiler according to any one of claims 1 to 4, characterized in that, It also includes the following steps: Once the operating load of the DC boiler reaches 100%, the corrosion index of the DC boiler's water-steam system is continuously monitored. When the corrosion index exceeds the preset value, the feedwater pH, feedwater ammonia concentration, and feedwater dissolved oxygen content of the DC boiler are adjusted to control the corrosion index within the preset value.

6. The feedwater treatment method for a once-through boiler according to claim 5, characterized in that, The corrosion indicators include one or more of the following: iron content in the feedwater at the economizer inlet, iron content in the steam in the steam-water separator, and iron content in the superheated steam.

7. The feedwater treatment method for a once-through boiler according to claim 6, characterized in that, The preset value includes at least one of the following three items: (1) The iron content of the feedwater at the economizer inlet is less than or equal to 2 μg / L; (2) The iron content in the steam in the steam-water separator is less than or equal to 1.5 μg / L; (3) The iron content in the superheated steam is less than or equal to 1 μg / L.

8. The feedwater treatment method for a once-through boiler according to claim 7, characterized in that, When the corrosion index exceeds the preset value, the feedwater pH, feedwater ammonia concentration, and feedwater dissolved oxygen content of the once-through boiler are adjusted, including the following steps: When at least one of the following exceeds the preset value: the iron content in the feedwater at the economizer inlet, the iron content in the steam in the steam-water separator, and the iron content in the superheated steam, adjust the feedwater pH to 9.10~9.15, adjust the feedwater ammonia concentration to 0.22mg / L~0.28mg / L, and adjust the feedwater dissolved oxygen content to 25μg / L~35μg / L; monitor the corrosion index of the water-steam system of the once-through boiler until the corrosion index falls back to within the preset value.

9. The feedwater treatment method for a once-through boiler according to any one of claims 1-4 and 6-8, characterized in that, It also includes the following steps: Once the operating load of the DC boiler reaches 100%, the instantaneous iron ion content and pH fluctuation of the feedwater of the DC boiler are continuously monitored. When the instantaneous iron ion content in the feedwater is greater than 5 μg / L or the pH of the feedwater is less than 9.05 or greater than 9.25, the dissolved oxygen content in the feedwater should be adjusted to 40 μg / L~50 μg / L within 20 minutes, and the pH of the feedwater should be adjusted to 9.10~9.

20.

10. The feedwater treatment method for a once-through boiler according to claim 9, characterized in that, After adjusting the dissolved oxygen content of the feedwater to 40 μg / L~50 μg / L and the pH of the feedwater to 9.10~9.20, the following steps are also included: When the iron content in the feedwater at the economizer inlet is less than or equal to 2 μg / L, the iron content in the steam in the steam-water separator is less than or equal to 1.5 μg / L, the iron content in the superheated steam is less than or equal to 1 μg / L, and the feedwater pH is stable within 9.05~9.25 for 30 minutes; within 30 minutes, adjust the dissolved oxygen content of the feedwater to 15 μg / L~35 μg / L and adjust the feedwater pH to 8.90~9.15.

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