A method for removing ash from a heat storage element in an air preheater

Abstract

This invention belongs to the field of air preheater technology, specifically relating to a method for removing ash from heat storage elements inside an air preheater, comprising the following steps: (S1) ultrasonically pretreating the heat storage element to be cleaned in a non-calcium chloride salt solution to obtain a pretreated element; (S2) subjecting the pretreated element to a first acid treatment using a first acid solution to obtain a first treated element; (S3) subjecting the first treated element to a second acid treatment using a second acid solution to obtain a cleaned heat storage element; wherein the first acid solution is selected from at least one of polycarboxylic acid, organophosphonic acid, and hydrochloric acid; and the second acid solution is selected from at least one of polycarboxylic acid and organophosphonic acid, and does not contain hydrochloric acid. This invention can efficiently remove ash accumulation on the heat storage elements of an air preheater and reduce the subsequent ash accumulation rate of the treated element, effectively solving the problems of incomplete ash removal, easy damage to the substrate, and rapid secondary ash accumulation in existing ash removal technologies.

Description

Technical Field

[0001] This invention belongs to the field of air preheater technology, specifically relating to a method for removing ash from the heat storage element of an air preheater. Background Technology

[0002] Air preheaters are core heat exchange components in thermal equipment such as power plant boilers and industrial boilers. Their internal heat storage elements (such as corrugated plate or honeycomb elements) transfer heat through alternating contact between high-temperature flue gas and low-temperature air, directly affecting the boiler's thermal efficiency and energy consumption. During long-term operation, dust carried by the flue gas (mainly containing calcium sulfate, silica, iron oxide, and small amounts of alkaline impurities) gradually adheres to the surface and internal pores of the heat storage elements, gradually forming a dense ash layer. Among these, ash with a calcium sulfate content ≥10% has low solubility and easily forms a crystalline hard shell, becoming a key issue affecting the performance of the heat storage elements.

[0003] The continuous accumulation of ash can cause multiple hazards: on the one hand, ash stratification significantly increases thermal resistance, leading to a 5% to 20% decrease in the heat exchange efficiency of the air preheater, which indirectly increases boiler energy consumption; on the other hand, dense ash can easily clog the corrugated gaps or honeycomb channels of the heat storage elements, increasing the resistance to flue gas flow. This not only affects the normal operation of the equipment, but may also induce local overheating, substrate corrosion and other malfunctions, requiring frequent shutdowns for ash cleaning, which seriously reduces production continuity.

[0004] Existing dust removal technologies are mainly divided into three categories, but all of them have obvious shortcomings: Firstly, mechanical cleaning technologies (such as rapping and high-pressure gas blowing) rely on physical impact to remove ash, which is effective for loose ash, but the removal rate of dense calcium sulfate ash is less than 60%. Moreover, high-frequency rapping can easily cause deformation and cracking of thin-walled heat storage elements (thickness < 1 mm), shortening the service life of the elements. High-pressure blowing is difficult to penetrate into the honeycomb channels, resulting in obvious cleaning blind spots.

[0005] Second, single chemical cleaning techniques (such as using inorganic acids or organic complexing agents alone): If high-concentration inorganic acids (such as hydrochloric acid) are used, although they can dissolve some alkaline impurities, they cannot destroy the calcium sulfate crystal structure and will accelerate the corrosion of carbon steel and stainless steel substrates; if a single organic complexing agent is used, although it can combine with calcium ions, it lacks ash dispersion ability, and loose particles are easy to re-agglomerate and adhere, and the surface of the component after treatment has no anti-adhesion modification, and secondary ash accumulation will occur within 1 to 2 weeks.

[0006] 3. Dust removal process without pretreatment: Some solutions directly treat the dust-accumulated components with acid, but due to the dense layering of calcium sulfate ash, the acid is difficult to penetrate to the interface between the ash and the substrate, resulting in low dust removal efficiency (requiring extended treatment time to more than 360 minutes), and a large amount of ineffective acid consumption, resulting in high treatment costs and making it difficult to meet the needs of industrial batch operations.

[0007] In summary, existing technologies cannot simultaneously meet the comprehensive requirements of "efficiently removing ash mainly composed of calcium sulfate, avoiding substrate damage, reducing subsequent ash accumulation, and adapting to industrial production," and there is still significant room for optimization in related ash removal solutions. Summary of the Invention

[0008] The purpose of this invention is to solve the problems mentioned in the background art and to provide a method for removing ash from the heat storage elements inside an air preheater, the method comprising the following steps: (S1) The heat storage element to be cleaned is subjected to ultrasonic pretreatment in a non-calcium chloride salt solution to obtain a pretreated element. (S2) The pretreatment element is subjected to a first acid treatment using a first acid solution to obtain a first treatment element; (S3) The first processing element is treated with a second acid solution to obtain a heat storage element after cleaning. The first acid solution is selected from at least one of polycarboxylic acid, organophosphonic acid and hydrochloric acid; preferably, it contains organophosphonic acid, polycarboxylic acid and hydrochloric acid simultaneously. The second acid solution is selected from at least one of polycarboxylic acid and organophosphonic acid, and does not contain hydrochloric acid; preferably, it contains both organophosphonic acid and polycarboxylic acid, and does not contain hydrochloric acid.

[0009] Beneficial effects I. This invention adopts a three-step dust removal process of "ultrasonic pretreatment with salt solution → first acid treatment → second acid treatment". Through the synergistic effect of pretreatment to loosen dense ash, first acid treatment to deeply remove ash, and second acid treatment to optimize the surface characteristics of the elements, it can not only efficiently remove the ash accumulation on the heat storage elements of the air preheater, but also avoid damage to the element substrate. At the same time, it reduces the subsequent ash accumulation rate of the treated elements, effectively solving the problems of incomplete ash removal, easy damage to the substrate, and rapid secondary ash accumulation in existing dust removal technologies.

[0010] Second, the present invention uses an inorganic salt solution of soluble chloride (preferably sodium chloride), which can ensure that the ions in the solution can fully penetrate into the interior of the ash and efficiently destroy the ash structure. It can also avoid the problems of ineffective pretreatment due to too low concentration and corrosion of the substrate or salt residue due to too high concentration. While improving the pretreatment effect, it reduces the processing difficulty of subsequent processes and lowers the overall ash removal cost.

[0011] Third, the present invention uses a first acid solution containing organophosphonic acid, polycarboxylic acid and low-concentration hydrochloric acid. Through the synergistic effect of organophosphonic acid complexing ash metal ions, polycarboxylic acid dispersing loose particles, and hydrochloric acid dissolving alkaline impurities, the ash removal efficiency can be greatly improved, and the corrosion of the substrate caused by excessive hydrochloric acid concentration can be avoided. This ensures the ash removal effect while extending the service life of the heat storage element.

[0012] Fourth, the present invention uses a second acid solution containing organophosphonic acid and polycarboxylic acid (excluding hydrochloric acid), which can form a dense protective film on the substrate surface through the synergistic adsorption of organophosphonic acid and polycarboxylic acid, and avoid hydrochloric acid from damaging the coordination bond and hydrogen bond structure of the protective film, effectively enhancing the anti-adhesion ability of the component surface, further reducing the subsequent dust accumulation rate of the treated component, and extending the dust removal cycle.

[0013] Fifth, the present invention employs a drying process before and after the second acid treatment. This process can remove residual first acid and moisture from the surface of the component before the second acid treatment, avoiding acid superposition that could corrode the substrate or affect the formation of the protective film. It can also fix the protective film after the second acid treatment, enhancing the adhesion between the film and the substrate, preventing the film from peeling off during subsequent use, and ensuring long-term anti-dust accumulation effect. Detailed Implementation

[0014] The technical solution and its effects of the present invention will be further described below with reference to specific embodiments / examples. The following embodiments / examples are merely illustrative of the present invention and are not intended to limit it. Simple modifications made to the present invention based on the concept of the present invention are all within the scope of protection claimed by the present invention.

[0015] The cleaning method for the air preheater heat storage element provided by the present invention includes the following steps: (S1) The heat storage element to be cleaned is subjected to ultrasonic pretreatment in a non-calcium chloride salt solution to obtain a pretreated element. (S2) The pretreatment element is subjected to a first acid treatment using a first acid solution to obtain a first treatment element; (S3) The first processing element is treated with a second acid solution to obtain a heat storage element after cleaning. The first acid solution is selected from at least one of polycarboxylic acid, organophosphonic acid and hydrochloric acid; preferably, it contains organophosphonic acid, polycarboxylic acid and hydrochloric acid simultaneously. The second acid solution is selected from at least one of polycarboxylic acid and organophosphonic acid, and does not contain hydrochloric acid; preferably, it contains both organophosphonic acid and polycarboxylic acid, and does not contain hydrochloric acid.

[0016] In some embodiments, in step (S1), the ash removal method of the present invention is applicable to various air preheater heat storage elements for power plant boilers and industrial boilers. The structural types include corrugated plate type, honeycomb type, and shell-and-tube type. The element substrate is selected from one or more of carbon steel, stainless steel, and low alloy heat-resistant steel. The above-mentioned mainstream substrates are all compatible with the acid treatment conditions of this solution and will not cause significant damage during the ash removal process.

[0017] In some embodiments, in step (S1), the ash adhering to the surface of the air preheater heat storage element is adhesive ash, meaning it cannot be removed by simple pretreatment such as conventional compressed air purging or low-pressure water rinsing, or it is a dense, hard shell of ash that remains even after the aforementioned pretreatment. The mass of this adhesive ash accounts for 1-30 wt% of the total mass of the heat storage element to be treated, preferably 2-15 wt%, for example 3 wt%, 4 wt%, 6 wt%, 8 wt%, 11 wt%, 12 wt%, or 14 wt%; corresponding to commonly used 5mm thick carbon steel elements in industry, the density of the ash surface is 50-200 mg / cm³ of the total mass. 2 (e.g., 80 mg / cm) 2 100 mg / cm 2 Or 150 mg / cm 2 The main components of the adhesive ash are calcium sulfate, silicon dioxide, aluminum oxide, iron oxide, and a small amount of alkaline impurities. Among them, calcium sulfate, due to its low solubility and tendency to form a dense crystalline shell, is the core component that causes the ash to adhere strongly to the substrate and is difficult to remove using conventional cleaning methods. It is also the key target of the cleaning method of this invention.

[0018] Those skilled in the art will understand that the calcium sulfate content in the ash of the air preheater is mainly determined by factors such as the sulfur content of the coal, boiler combustion conditions, desulfurization process, and equipment operating time, and varies significantly in different industrial scenarios, for example, 7.88% (Example 12), 13.67% (Example 1), 29.83% (Example 11), and 33.48% (Example 10). The inventors have found that this solution is particularly effective compared to conventional ash removal methods for difficult-to-clean ash scenarios where the calcium sulfate content in the ash is ≥10 wt%, for example, when the calcium sulfate content is 10~90 wt%, preferably 10~50 wt%, such as 15 wt%, 20 wt%, 30 wt%, 35 wt%, or 40 wt%. Within this content range, the Cl... - The osmotic loosening effect works synergistically with the complexing and scavenging effect of subsequent acid treatment to completely destroy the calcium sulfate crystal structure and remove ash, thus solving the problem of incomplete removal of high calcium sulfate ash by conventional methods.

[0019] Step (S1) Ultrasonic pretreatment In some embodiments, in step (S1), the chloride salt is selected from commonly used inorganic salts in the art other than calcium chloride, such as soluble chlorides, for example, at least one of sodium chloride, potassium chloride, magnesium chloride, and zinc chloride. The reason for selecting this type of chloride salt is that its dissociated Cl... - It can quickly penetrate into the interior of the ash and form soluble complexes (such as CaCl2) with the calcium sulfate in the ash. 4H2O), combined with the "cavitation effect" of the ultrasonic process, can physically loosen the interface between ash and the substrate of the heat storage element, opening up a penetration channel for subsequent acid treatment; if non-chlorinated compounds such as sulfates and nitrates are used, their anions easily react with Ca in the ash. 2+ The formation of new insoluble precipitates (such as secondary precipitation of calcium sulfate) can actually exacerbate ash adhesion and increase the difficulty of ash removal.

[0020] Preferably, the solute in the chloride salt solution is at least one selected from sodium chloride, potassium chloride, and magnesium chloride, more preferably sodium chloride. The reasons are: sodium chloride has low industrial cost, produces no heavy metal pollution, has good compatibility with the subsequent first acid solution, and will not introduce interfering ions; simultaneously, the Na+ released from sodium chloride... + With Cl - The synergistic ultrasonic cavitation effect efficiently disrupts the dense crystal structure of calcium sulfate, which is more conducive to weakening the bonding force between ash and substrates (such as carbon steel and stainless steel).

[0021] It should be noted that this scheme does not use calcium chloride as a solute, for two main reasons: firstly, calcium chloride itself contains Ca... 2+ Firstly, calcium chloride may recombine with unreacted sulfate ions in the ash to form new calcium sulfate precipitates, leading to secondary solidification of the ash and reducing the pretreatment effect. Secondly, calcium chloride has strong hygroscopic properties. If it remains on the surface of the heat storage element, it easily absorbs moisture from the air to form a viscous liquid film, which not only increases the difficulty of subsequent acid treatment but may also accelerate local corrosion of the substrate and affect the service life of the element.

[0022] In some embodiments, in step (S1), the concentration of the chloride salt solution is 0.05~2 mol / L, such as 0.1 mol / L, 0.5 mol / L, 0.8 mol / L, 1.0 mol / L, 1.5 mol / L, or 1.8 mol / L. If the concentration is too low, the Cl in the solution will be too high. - Insufficient concentration results in weak complexation and penetration with calcium sulfate, requiring a significant extension of ultrasonic time to achieve the desired loosening effect; excessively high concentration leads to a significant increase in solution osmotic pressure, which can easily cause local dissolution of the oxide film on the surface of the heat storage element (such as the Fe3O4 film on the surface of carbon steel), increasing the risk of substrate corrosion. At the same time, excessive salt residue will increase the dissolution load of subsequent acid treatment, indirectly increasing the treatment cost.

[0023] Preferably, the concentration of the chloride salt solution is 0.2~2 mol / L. At this concentration, Cl... -The concentration is sufficient to quickly react with calcium sulfate in the ash, and even for thick (≥0.5 mm) dense ash layers, it can destroy the structure in a short time. At this concentration, the residual salt can still be completely dissolved by hydrochloric acid in the first acid solution, without the need for additional water rinsing. This simplifies the process and further shortens the overall cleaning cycle.

[0024] In some embodiments, the ultrasonic treatment in step (S1) has a power of 20-500 W and a duration of 5-120 min. This power can be matched to the type of heat storage element; for thin-walled heat storage elements, the power should be appropriately reduced to avoid deformation or cracking; for thick-walled heat storage elements, the power should be appropriately increased to enhance the cavitation effect and rapidly penetrate the ash layer. The ultrasonic treatment time can be adjusted accordingly based on the ash thickness.

[0025] Preferably, the ultrasonic treatment power is 50~300 W and the time is 10~60 min. These parameters are compatible with most air preheater heat storage elements on the market (ash thickness is usually 0.1~1 mm, substrate thickness is 0.5~2 mm), and ordinary industrial ultrasonic cleaning machines can meet the requirements, reducing equipment investment.

[0026] Step (S2) First acid treatment In some embodiments, the first acid treatment in step (S2) follows the effect of the ultrasonic pretreatment in step (S1), i.e., the ultrasonic pretreatment has been passed through Cl - The process involves penetrating and loosening the ash structure, forming partially soluble complexes, and then using a first acid treatment to achieve deep ash removal. The first acid solution is selected from at least one of organophosphonic acid, polycarboxylic acid, and hydrochloric acid, each with its own specific function: organophosphonic acid can specifically react with Ca in the ash. 2+ Fe 3+ The process involves the formation of stable complexes by metal ions, disrupting the residual crystal structure of calcium sulfate. Polycarboxylic acid, through its steric hindrance effect, disperses the ash particles (including SiO2 and Al2O3) that remain attached after ultrasonic loosening in the acid solution, preventing re-agglomeration and deposition. Hydrochloric acid dissolves both alkaline impurities (such as calcium carbonate and magnesium hydroxide) in the ash and rapidly dissolves residual salts (such as sodium chloride) after ultrasonic pretreatment, preventing the salts from combining with the ash to form a new adhesion layer. When these three agents work synergistically, the ash removal rate is increased by 20%–30% compared to a single acid solution, without conflicting with residual pretreatment components or causing significant substrate corrosion.

[0027] In some embodiments, the total concentration of organic acid (i.e., at least one of polycarboxylic acid and / or organophosphonic acid) in the first acid solution is 5-20 g / L, preferably 8-15 g / L. If the concentration is too low, the effective components are insufficient, which is not conducive to fully complexing metal ions and dispersing particles in the ash, and may reduce the ash removal rate; if the concentration is too high, it will not only increase the raw material cost, but may also cause excessive adsorption of acid solution on the substrate surface, increasing the subsequent drying burden and corrosion risk.

[0028] In some embodiments, the amount of acid used in the first acid treatment is a ratio of (100~300) mL:1 g to the total mass of ash in step (S1), preferably (150~200) mL:1 g. Simultaneously, the volume of the first acid used must be sufficient to submerge the heat storage element, ensuring that the ash on the element surface and within the pores can contact the acid.

[0029] Preferably, the first acid solution simultaneously contains organophosphonic acid, polycarboxylic acid, and hydrochloric acid, forming a "complexation-dispersion-dissolution" treatment system. The mass ratio of polycarboxylic acid to organophosphonic acid is 1:(1~3). If the amount of organophosphonic acid is too low, the complexing ability is insufficient, and the ash crystals are not thoroughly destroyed; if the amount of organophosphonic acid is too high, the dispersion ability of polycarboxylic acid is insufficient, and the loosened particles after complexation are prone to agglomerate and settle on the surface of the element or in the pores, affecting the dust removal effect.

[0030] The concentration of hydrochloric acid in the first acid solution is ≤0.1 mol / L, preferably 0.005~0.08 mol / L, and more preferably 0.01~0.05 mol / L. This concentration can efficiently dissolve alkaline impurities and pretreatment residual salts, while controlling the substrate corrosion rate at an extremely low level. If the concentration is too high, it will accelerate substrate corrosion and may also increase ash adhesion during subsequent use.

[0031] More preferably, the mass ratio of polycarboxylic acid to organophosphonic acid in the first acid solution is 1:(1.5~2.5); the hydrochloric acid concentration is 0.02~0.05 mol / L. At this ratio, the hydrochloric acid can fully dissolve impurities and residual salts without causing excessive acidity due to excess. It forms a stable composite system with organophosphonic acid and polycarboxylic acid. Treatment at 50°C for 120 min can achieve an ash removal rate of over 98%, and the acid solution does not separate or decompose after storage at 20~80°C for 72 h. It is suitable for batch preparation and continuous connection with the ultrasonic pretreatment process.

[0032] In some embodiments, the organophosphonic acid in the first acid solution is selected from aminotrimethylphosphonic acid (ATMP), ethylenediaminetetramethylphosphonic acid (EDTMP), hydroxyethylidene diphosphonic acid (HEDP), diethylenetriaminepentamethylphosphonic acid (DTPMP), and potassium hexamethylenediaminetetramethylphosphonate (HDTMPA). At least one of K6). These organophosphonic acids not only have a strong ability to complex metal ions, but can also form weak adsorption with hydroxyl groups on the surface of the pretreated substrate, thus helping to protect the substrate and avoid direct corrosion by acid, which corresponds to the anti-corrosion requirements of ultrasonic pretreatment.

[0033] Preferably, the organophosphonic acid is HEDP and / or ATMP. These two compounds have a better complexing effect and can more quickly react with residual Ca after ultrasonic pretreatment. 2+ It combines [various technologies], and the cost is relatively low.

[0034] In some embodiments, the polycarboxylic acid in the first acid solution is selected from at least one of polyacrylic acid, polymethacrylic acid, acrylic acid-maleic anhydride copolymer (MA / AA), maleic anhydride-vinyl acetate copolymer, and polyaspartic acid. These polycarboxylic acids have a molecular weight range of 1000-10000 Da, ensuring good water solubility while also penetrating the loosened ash gaps after ultrasonic pretreatment through their long-chain structure, preventing small-molecule polycarboxylic acids from being adsorbed and deactivated by ash. Furthermore, they do not react adversely with organophosphonic acids or hydrochloric acid, ensuring the stability of the acid solution.

[0035] Preferably, the polycarboxylic acid is selected from acrylic acid-maleic anhydride copolymer and / or polyaspartic acid. Among them, acrylic acid-maleic anhydride copolymer has higher dispersion efficiency and can quickly disperse fine particles loosened after ultrasonic pretreatment (at the same concentration, its ability to disperse ash particles is 1.5 times that of polyacrylic acid), avoiding the redeposition of particles in the pores or gaps of the element; polyaspartic acid is a biodegradable material, suitable for scenarios with high environmental protection requirements such as power plant desulfurization and denitrification.

[0036] In some embodiments, in step (S2), the temperature of the first acid treatment is 20~80 °C and the time is 10~360 min. If the temperature is too low, the organophosphonic acid complexation reaction rate is slow, and the loosened ash after ultrasonic pretreatment cannot be treated in time, resulting in low removal efficiency; if the temperature is too high, polycarboxylic acid is prone to thermal degradation (e.g., the degradation rate of acrylic acid-maleic anhydride copolymer reaches 15% at 90 °C for 2 hours), the dispersion function is reduced, and it may accelerate the corrosion of the substrate by hydrochloric acid, which is not conducive to subsequent use and increases ash adhesion.

[0037] Preferably, the temperature of the first acid treatment is 40-60°C and the time is 30-180 min. These conditions are beneficial for accelerating the reaction of organophosphonic acid with Ca. 2+ Fe 3+ The complexation reaction rate is reduced, the ash removal time is shortened, and the stability of polycarboxylic acid is still guaranteed (degradation rate <5%), which can meet the treatment requirements of different ash thicknesses.

[0038] Step (S3) Second acid treatment In this invention, step (S3) inherits the deep cleaning effect of the first acid treatment in step (S3), that is, after the ash is loosened by ultrasonic pretreatment and the ash is thoroughly removed by the first acid treatment, the surface of the substrate is clean and a slight passivation film is formed. The core purpose of the second acid treatment is surface modification, to build an anti-adhesion protective film on the clean substrate surface and extend the subsequent ash accumulation cycle.

[0039] In some embodiments, the second acid solution is selected from at least one of organophosphonic acid and polycarboxylic acid. The phosphonic acid group (-PO3H2) in the organophosphonic acid forms a coordination bond with the hydroxyl group (-OH) on the substrate surface, and / or the carboxyl group (-COOH) in the polycarboxylic acid forms a hydrogen bond with the amino group (-NH2) of the organophosphonic acid, thereby forming a dense protective film of 5-10 nm, which significantly reduces the adhesion of dust to the substrate surface.

[0040] In some embodiments, the total concentration of organic acids (i.e., at least one of polycarboxylic acid and / or organophosphonic acid) in the second acid solution is 2-10 g / L, preferably 3-7 g / L, and more preferably lower than the total concentration of organic acids in the first acid solution. If the concentration of the second acid solution is too low, it is not conducive to the formation of a continuous and complete protective film, which may reduce the anti-adhesion effect; if the concentration of the second acid solution is too high, it is easy to cause the protective film layer to be too thick, which may increase the thermal resistance and affect the thermal conductivity of the heat storage element.

[0041] In some embodiments, the second acid treatment also employs an immersion method. The amount of the second acid solution is sufficient to submerge the heat storage element. For example, the ratio of the volume of the second acid solution to the mass of the first treatment element (or simply the mass of the clean heat storage element) can be (5~20) L: 1 kg. This amount is directly related to the target of the second acid treatment (the clean heat storage element), and the amount must ensure that the inner wall of the element's pores is completely submerged in the acid solution to form a uniform protective film.

[0042] It is worth noting that the second acid solution does not include hydrochloric acid. This design is closely related to the characteristics of the preceding process and the protective film formation mechanism: First, after the first acid treatment, a trace amount of passivation film (formed by the reaction of organophosphonic acid with the substrate) may remain on the substrate surface. The acidity of hydrochloric acid will corrode this passivation film, destroying the cleanliness of the substrate surface, which conflicts with the goal of protecting the substrate after the preceding cleaning process; Second, the H+ released from hydrochloric acid... + It will compete with the phosphonic acid group of organophosphonic acid and the carboxyl group of polycarboxylic acid for adsorption of the substrate hydroxyl group, and at the same time destroy the electrostatic interaction of hydrogen bonds, resulting in the inability of coordination bonds and hydrogen bonds to form stably, and ultimately failing to build a continuous and dense protective film; thirdly, the residual salts from the previous ultrasonic pretreatment and the alkaline impurities from the first acid treatment have been completely removed, and hydrochloric acid is not needed for dissolution. In fact, adding hydrochloric acid will have a modifying effect.

[0043] In some embodiments, the mass ratio of polycarboxylic acid to organophosphonic acid in the second acid solution is 1:(1~3). This ratio is logically consistent with the component ratio of the first acid solution, ensuring that the organophosphonic acid provides sufficient coordination bond binding sites, while the long-chain structure of polycarboxylic acid optimizes the density of the protective film, making the porosity of the protective film <1%. This effectively prevents dust particles such as SiO2 in the flue gas from adhering to the pores of the element, avoiding pore blockage and affecting heat exchange efficiency, which corresponds to the goal of restoring heat exchange performance after the previous cleaning.

[0044] Preferably, the mass ratio of polycarboxylic acid to organophosphonic acid is 1:(1.2~2.2) (different from the preferred ratio of 1:(1.5~2.5) for the first acid solution, to meet the requirements for protective film formation). At this ratio, the surface energy of the protective film is reduced to 30~40 mN / m (approximately 70~80 mN / m for untreated substrates), the contact angle between dust (mainly SiO2) and the protective film is >90°, the adhesion is significantly reduced, and the cleaning cycle can be significantly extended.

[0045] In some embodiments, in step (S3), the temperature of the second acid treatment is 20~80 °C and the time is 10~360 min. This temperature range is beneficial to the activity of organophosphonic acid and polycarboxylic acid, avoids molecular chain breakage caused by high temperature, ensures stable binding of coordination bonds and hydrogen bonds, and allows the inner wall of the pores to be fully covered with a protective film.

[0046] Preferably, the second acid treatment temperature is 25~45 °C (more preferably 5~15 °C lower than the first acid treatment temperature) and the time is 30~120 min. At this lower temperature, the coordination bonds between organophosphonic acid and the hydroxyl groups of the substrate, and the hydrogen bonds between polycarboxylic acid and organophosphonic acid, form more slowly and uniformly, which can construct a protective film with better density and stronger adhesion, and lower thickness uniformity error, avoiding accelerated dust accumulation due to insufficient local film thickness; at the same time, this temperature range does not require additional adjustment of heating equipment parameters, and can be achieved by fine-tuning only.

[0047] Drying treatment before and after step (S3) (preferred step) In some preferred embodiments, step (S3) may first dry the first processing element obtained in step (S2), then perform a second acid treatment, and finally perform a second drying. These two drying steps are optional and are only used to further improve the dust removal effect and the stability of the protective film.

[0048] (1) First drying (before the second acid treatment) In this invention, the purpose of the first drying step is to remove residual acid and moisture from the surface and gaps (or channels) of the first processing element. After the first acid treatment (e.g., immersion), regardless of the structure of the heat storage element, trace amounts of acid and moisture may adhere to its surface. If it directly enters the second acid treatment, the residual acid may compete with the second acid for adsorption on the substrate, or cause acid superposition. This may increase the risk of corrosion, especially for sensitive substrates such as low-alloy heat-resistant steel, and may also interfere with the formation of coordination bonds and hydrogen bonds in the protective film.

[0049] In some embodiments, the temperature of the first drying step is 60~150 ℃, and the time is 30~360 min. Too low a temperature will reduce drying efficiency and prolong the production cycle; too high a temperature may cause some substrates (such as galvanized steel) to oxidize and discolor, affecting thermal conductivity; too short a time will result in incomplete drying, and residual moisture will still interfere with subsequent processing; too long a time will result in energy waste.

[0050] Preferably, the temperature of the first drying step is 80~120 ℃, and the time is 60~180 min. Under these conditions, the moisture content in the first processing element can be reduced to ≤0.5%, there is no liquid residue on the surface and in the pores, and the substrate does not oxidize or discolor (e.g., stainless steel substrates do not show oxide film thickening at 120 ℃). It is also compatible with ordinary hot air ovens, matching the cycle time of the preceding first acid treatment (30~180 min), and does not affect continuous industrial production.

[0051] Second drying (after the second acid treatment) In this invention, the purpose of the second drying is to fix the protective film formed by the second acid treatment and enhance the adhesion between the film and the substrate. Regardless of the structure of the heat storage element, the adhesion between the film and the substrate after the second acid treatment still needs to be stable and cured to prevent the film from falling off during subsequent assembly, transportation, or flue gas erosion, thereby ensuring the long-term effect of "dust removal and dust prevention".

[0052] In some embodiments, the second drying is performed at a temperature of 60-150 °C for a time of 30-360 min.

[0053] Preferably, the second drying temperature is 70~110 ℃, and the time is 90~210 min. This condition, through "medium-temperature slow drying", allows the protective film to bond more tightly with the substrate, which will not cause the film layer to pyrolyze, and can ensure that the film layer is stably fixed on substrates of different thicknesses and structures, without the need for additional high-temperature equipment.

[0054] In some implementations, the heat storage components, after being dried and optimized twice, were tested for two weeks in a simulated flue gas device (flue gas flow rate 10L / min, dust content 50 ppm). The ash content of a single component was ≤14 mg, and there was no obvious ash accumulation or blockage in key parts such as corrugated gaps and honeycomb channels. The heat exchange efficiency was restored to more than 95% of that of a new component. Even without drying treatment, the basic dust removal effect can still be achieved, with only a slight decrease in the stability of the protective film and its long-term resistance to ash accumulation.

[0055] Example 1 (IE1) Step (S0) Prepare the heat storage element to be cleaned. ① Take a Q235 carbon steel test piece with dimensions of 100mm×100mm×5mm, grind and dry it to constant weight, and the net weight of the substrate is about 393g; ②Preparation of simulated ash: Calcium sulfate accounts for 13.67% of the total ash mass, and the remaining components are mixed in the following ratio: silicon dioxide: aluminum oxide: iron oxide: alkaline impurities = 5:3:1.5:0.5. After grinding evenly, it is prepared as a 10wt% anhydrous ethanol suspension. ③ After immersing the test piece in the suspension, pull it out at a uniform speed, dry it at 105℃, and repeat the operation until the total ash mass of the test piece is stable at 24g (the ash content accounts for 5.76wt% of the total mass of the heat storage element to be cleaned); the resulting ash is a dense, adhesive hard shell that cannot be removed by conventional compressed air purging.

[0056] Step (S1) Ultrasonic pretreatment Prepare a 0.5 mol / L sodium chloride solution, immerse the heat storage element to be cleaned in it, set the ultrasonic power to 200W and the treatment time to 30 min, and remove it after treatment to obtain the pretreated element.

[0057] Step (S2) First acid treatment The pretreatment element was immersed in a first acid solution at a temperature of 50°C for 120 minutes. After treatment, it was removed to obtain the first treated element. The first acid solution is composed of acrylic acid-maleic anhydride copolymer (MA / AA), hydroxyethylidene diphosphonic acid (HEDP) and hydrochloric acid, with the mass ratio of MA / AA to HEDP being 1:2; The total concentration of organic acids (i.e., MA / AA and HEDP) was 10 g / L, and the concentration of hydrochloric acid was 0.03 mol / L. The volume ratio of the first acid solution to the ash mass in step (S1) is 167 mL: 1 g.

[0058] Step (S3) Second acid treatment The first processing element was dried in a 100°C oven to remove surface moisture; the dried element was then immersed in a second acid solution at a temperature of 40°C for 90 minutes. After processing, the element is removed and dried again in a 100℃ oven to obtain a cleaned heat storage element; among which, The second acid solution is composed of MA / AA and HEDP in a mass ratio of 1:2, with a total concentration of 5 g / L; The ratio of the volume of the second acid solution to the mass of the first processing element (which is substantially equal to the mass of the substrate) is approximately 10 L: 1 kg.

[0059] Example 2 (IE2) The procedure is the same as in Example 1, except that: In step (S2), the organic acid used in the first acid solution is only acrylic acid-maleic anhydride copolymer, and hydroxyethylidene diphosphonic acid is not used.

[0060] Example 3 (IE3) The procedure is the same as in Example 1, except that: In step (S2), the organic acid used in the first acid solution is only hydroxyethylidene diphosphonic acid, and acrylic acid-maleic anhydride copolymer is not used.

[0061] Example 4 (IE4) The procedure is the same as in Example 1, except that: In step (S2), the mass ratio of acrylic acid-maleic anhydride copolymer and hydroxyethylidene diphosphonic acid in the first acid solution is 1:1.

[0062] Example 5 (IE5) The procedure is the same as in Example 1, except that: In step (S2), the mass ratio of acrylic acid-maleic anhydride copolymer and hydroxyethylidene diphosphonic acid in the first acid solution is 1:3.

[0063] Example 6 (IE6) The procedure is the same as in Example 1, except that: In step (S2), the concentration of hydrochloric acid in the first acid solution is 0.1 mol / L.

[0064] Example 7 (IE7) The procedure is the same as in Example 1, except that: In step (S2), the concentration of hydrochloric acid in the first acid solution is 4 mol / L.

[0065] Example 8 (IE8) The procedure is the same as in Example 1, except that: In step (S2), the temperature of the first acid treatment is 90°C and the time is 2 hours.

[0066] Example 9 (IE9) The procedure is the same as in Example 1, except that: In step (S3), the temperature of the second acid treatment is 120°C and the time is 2 hours.

[0067] Example 10 (IE10) The procedure is the same as in Example 1, except that: In step (S0), calcium sulfate is further added to the ash raw material used for ash collection in Example 1 to adjust the calcium sulfate content in the ash after ash collection, so that the mass ratio of each component in the non-calcium sulfate component in the ash after ash collection is basically the same, and the total mass of ash in the sample is approximately the same as that in Example 1 (within ±2g); at this time, calcium sulfate accounts for 33.48% of the total mass of ash in the ash of the element to be cleaned.

[0068] Example 11 (IE11) The procedure is the same as in Example 1, except that: In step (S0), calcium sulfate is further added to the ash raw material used for ash collection in Example 1 to adjust the calcium sulfate content in the ash after ash collection, so that the mass ratio of each component in the non-calcium sulfate component in the ash after ash collection is basically the same, and the total mass of ash in the sample is approximately the same as that in Example 1 (within ±2g); at this time, calcium sulfate accounts for 29.82% of the total mass of ash in the ash of the element to be cleaned.

[0069] Example 12 (IE12) The procedure is the same as in Example 1, except that: In step (S0), calcium sulfate is further added to the ash raw material used for ash collection in Example 1 to adjust the calcium sulfate content in the ash after ash collection, so that the mass ratio of each component in the non-calcium sulfate component in the ash after ash collection is basically the same, and the total mass of ash in the sample is approximately the same as that in Example 1 (within ±2g); at this time, calcium sulfate accounts for 7.88% of the total mass of ash in the ash of the element to be cleaned.

[0070] Example 13 (IE13) The procedure is the same as in Example 1, except that: In step (S1), the sodium chloride solution is replaced with a potassium chloride solution with a concentration of 0.8 mol / L.

[0071] In step (S2), the concentration of hydrochloric acid in the first acid solution is 0.035 mol / L.

[0072] Example 14 (IE14) The procedure is the same as in Example 1, except that: In step (S1), the concentration of the sodium chloride solution is 5 mol / L.

[0073] Example 15 (IE15) The procedure is the same as in Example 1, except that: In step (S2), the total concentration of MA / AA and HEDP in the first acid solution is 0.10 g / L.

[0074] Example 16 (IE16) The procedure is the same as in Example 1, except that: In step (S3), the total concentration of MA / AA and HEDP in the second acid solution is 0.10 g / L.

[0075] Comparative Example 1 (CE1) The procedure is the same as in Example 1, except that: In step (S1), a sodium chloride solution is used to replace a calcium chloride solution of the same concentration.

[0076] Comparative Example 2 (CE2) The procedure is the same as in Example 1, except that: In step (S1), the sodium chloride solution is replaced with an equal amount of water.

[0077] Comparative Example 3 (CE3) The procedure is the same as in Example 1, except that: In step (S3), the second acid solution also includes hydrochloric acid with a concentration of 0.03 mol / L.

[0078] Test case Using the heat storage elements after cleaning treatment in the examples and comparative examples as test objects, their cleaning effect, substrate protection and resistance to secondary dust accumulation were tested. The results are shown in Table 1 below.

[0079] I. Dust Removal Effect Test Weigh and record the total mass of the heat storage element before cleaning (referred to as m1), the total mass of the heat storage element after cleaning (referred to as m2), and the net mass of the substrate (referred to as m0). Calculate the ash removal rate using the following formula: Ash removal rate = (m1-m2) / (m1-m0)×100%.

[0080] II. Substrate Corrosion Testing A metallographic microscope with a magnification of ≥200x was used to conduct a comprehensive observation of the substrate surface of the heat storage element after cleaning, focusing on checking for damage such as corrosion spots, scratches, and oxide layer breakage, and recording the surface condition of the substrate.

[0081] III. Resistance to Secondary Ash Accumulation Test under Simulated Flue Gas Environment ① After cleaning, fix the heat storage element in the test chamber of the simulated flue gas device, ensuring that the element is in the core area of ​​flue gas flow and is firmly fixed without shaking; ② Introduce simulated flue gas with a flow rate of 10 L / min and a dust content of 50 ppm. The dust composition is consistent with the original ash content in the heat storage element to be cleaned in step (S1). Introduce the flue gas continuously for 2 weeks. ③ After the test, remove the heat storage element, dry it in a 100℃ oven for 120 min, cool it down and weigh it. The mass increment is the mass of the adhering ash within two weeks (i.e., the amount of adhering ash in Table 1 below).

[0082] Table 1:

Claims

1. A method for removing ash from heat storage elements inside an air preheater, characterized in that, The method includes the following steps: (S1) The heat storage element to be cleaned is subjected to ultrasonic pretreatment in a non-calcium chloride salt solution to obtain a pretreated element. (S2) The pretreatment element is subjected to a first acid treatment using a first acid solution to obtain a first treatment element; (S3) The first processing element is treated with a second acid solution to obtain a heat storage element after cleaning. The first acid solution is selected from at least one of polycarboxylic acid, organophosphonic acid and hydrochloric acid; preferably, it contains organophosphonic acid, polycarboxylic acid and hydrochloric acid simultaneously. The second acid solution is selected from at least one of polycarboxylic acid and organophosphonic acid, and does not contain hydrochloric acid; preferably, it contains both organophosphonic acid and polycarboxylic acid, and does not contain hydrochloric acid.

2. The method according to claim 1, characterized in that, In step (S1), the chloride salt is selected from at least one of sodium chloride, potassium chloride, aluminum chloride and zinc chloride, preferably at least one of sodium chloride, potassium chloride and aluminum chloride; Preferably, the concentration of the chloride salt solution is 0.05~2 mol / L, more preferably 0.2~2 mol / L.

3. The method according to claim 1 or 2, characterized in that, In the first acid solution in step (S2), The organophosphonic acid is selected from at least one of aminotrimethylphosphonic acid, ethylenediaminetetramethylphosphonic acid, hydroxyethylidene diphosphonic acid, diethylenetriaminepentamethylidene phosphonic acid, and potassium salt of hexamethylenediaminetetramethylidene phosphonate, preferably selected from at least one of hydroxyethylidene diphosphonic acid and aminotrimethylphosphonic acid; and / or The polycarboxylic acid is selected from at least one of polyacrylic acid, polymethacrylic acid, acrylic acid-maleic anhydride copolymer, maleic anhydride-vinyl acetate copolymer, and polyaspartic acid, preferably selected from at least one of acrylic acid-maleic anhydride copolymer and polyaspartic acid.

4. The method according to any one of claims 1 to 3, characterized in that, In the first acid solution in step (S2), The concentration of at least one of the polycarboxylic acid and / or organophosphonic acid is 5-20 g / L, preferably 8-15 g / L; and / or The concentration of the hydrochloric acid is ≤0.1 mol / L, more preferably 0.005~0.08 mol / L, and even more preferably 0.01~0.05 mol / L; Preferably, the mass ratio of the polycarboxylic acid to the organophosphonic acid is 1:(1~3), more preferably 1:(1.5~2.5).

5. The method according to any one of claims 1 to 4, characterized in that, In the first acid treatment in step (S2), The temperature of the first acid treatment is 20~80℃, preferably 40~60℃; and / or The duration of the first acid treatment is 10-360 min, preferably 30-180 min; and / or The ratio of the volume of the first acid solution to the mass of ash in the heat storage element to be cleaned is (100~300) mL:1g, preferably (150~200) mL:1g.

6. The method according to any one of claims 1 to 5, characterized in that, In the second acid solution in step (S3), The concentration of at least one of the polycarboxylic acid and / or organophosphonic acid is 2~10 g / L, preferably 3~7 g / L, and more preferably lower than the concentration of polycarboxylic acid and / or organophosphonic acid in the first acid solution; Preferably, the mass ratio of the polycarboxylic acid to the organophosphonic acid is 1:(1~3), more preferably 1:(1.2~2.2).

7. The method according to any one of claims 1 to 6, characterized in that, In the second acid treatment in step (S3), The temperature of the second acid treatment is 20~80℃, preferably 25~45℃, more preferably 5~15℃ lower than the temperature of the first acid treatment; and / or The second acid treatment time is 10~360 min, preferably 30~120 min; and / or The ratio of the volume of the second acid solution to the mass of the first processing element is (5~20) L: 1 kg.

8. The method according to any one of claims 1 to 7, characterized in that, In the heat storage element to be cleaned in step (S1), The ash content in the heat storage element is 1~30 wt%, preferably 2~15 wt%; and / or The calcium sulfate content in the ash is ≥10 wt%, preferably 10~90 wt%, and more preferably 10~50 wt%.

9. The method according to any one of claims 1 to 8, characterized in that, In the ultrasonic pretreatment in step (S1), The ultrasound power is 20~500 W, and the duration is 5~120 min.

10. The method according to any one of claims 1 to 9, characterized in that, The first processing element is dried before the second acid treatment; and / or, the heat storage element after ash removal is dried. Preferably, the drying temperature is 60~150 ℃ and the time is 30~360 min.

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