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1 Introduction Boilers are heat exchange devices used for generating steam or hot water. With the development of production and the improvement of people’s living standards, they have been increasingly widely used in modern industry, hotels, and daily life, becoming one of the important thermal equipment for developing the national economy and improving working and living conditions. Industrial boilers generally refer to boilers used in industrial production for heating, boiling, drying, as well as for heating buildings and providing hot water for daily use. In Hainan, sugar factories are the sectors where industrial boilers are most commonly used, followed by hotels and resorts. Sugar factories account for the largest total capacity as well as the highest capacity per boiler; the steam generation capacity of each boiler is typically 20 t/h or 25 t/h, with larger ones generating over 40 t/h. Boilers in other industries have smaller capacities, and those used in hotels are usually fuel-fired boilers with a steam generation capacity of 1 t/h or 2 t/h. During operation, boilers develop scale due to various factors; this scale includes types such as sedimentary scale and rust scale. Scale has very poor thermal conductivity; its presence leads to worse heat transfer on the heated surfaces, an increase in exhaust temperature, and higher fuel consumption ； At the same time, it causes overheating of the metal in the heated surfaces; the high temperature reduces the strength of the metal. Under the pressure of steam, the overheated areas deform and bulge, which can even lead to explosion accidents. When there is a lot of scale, it can also block the pipes and interfere with the normal circulation of the boiler water. Local corrosion such as that under deposits also occurs beneath the scale layer. To eliminate the hazards of scale, once scale forms in a boiler, it needs to be cleaned and removed. We have been engaged in research and implementation of chemical cleaning for equipment such as boilers for many years, and have encountered various types of industrial boilers and scale. Through research and extensive practice, we have accumulated some experience in analyzing, identifying, and removing different types of scale. This article aims to share some of this information for mutual discussion. 2 Mechanism of scale formation: During the operation of industrial boilers, scale forms in areas such as the boiler drum, tube walls, and steam drum due to factors such as substandard feedwater quality and poor operational management. The mechanism behind scale formation is quite complex. 2.1 Water quality for feedwater Industrial boilers almost always use raw water or softened water as feedwater, and there are various reasons why feedwater can cause scale formation in the boilers. The formation of scale is a process of deposition of insoluble salts; when the temperature of the boiler water rises, the salts in the water become concentrated, and when their concentration exceeds the solubility limit at that temperature, deposition occurs ； Some salts, such as calcium sulfate, magnesium sulfate, and calcium phosphate, experience a decrease in solubility and precipitation as the temperature rises ； In the furnace water, precipitation also occurs when the silica concentration is high relative to the alkalinity ； Soluble bicarbonates, such as calcium bicarbonate and magnesium bicarbonate, decompose when heated, producing insoluble salts that also lead to deposition. For example, the severity of scale formation is closely related to the quality of the water supplied to the boiler; the boiler water can be either raw water or softened water. Raw water: Also known as unprocessed water, it is natural water that has not undergone any treatment (such as river water, lake water, groundwater, etc.). It is generally obtained from local water sources (surface water or groundwater) or the urban water supply network. The quality of this water varies greatly; tap water sourced from cities or their suburbs, after being filtered, has a more stable quality, while groundwater directly used has a higher hardness. Some facilities draw water from nearby untreated river waters, whose quality is unstable; such water contains suspended particles, colloidal substances, and various soluble impurities. Especially during the rainy season, the water is mixed with sediment, making it yellow and turbid. We have encountered a factory that used this water as feedwater in rainy weather; boilers using such water are highly prone to the accumulation of sedimentary scale, or a mixed scale formed by a combination of sediment and scale. Softened water: Sodium-ion exchange water or water treated in the furnace is commonly used, with the former being the most widely applied. Water treated with sodium ion exchange resin generally has a hardness level that meets the requirements of industrial boilers; as long as the boiler is drained regularly, scale does not tend to accumulate. However, in some units, due to the small capacity of the water treatment equipment, the amount of water that can be treated is insufficient; as a result, some raw water is added to the furnace, which accelerates the deposition of scale. Water treated by chemical dosing in the boiler often has difficulty in terms of controlling its hardness and alkalinity, due to reasons such as insufficient dosage, untimely addition of chemicals, or inadequate waste discharge. Boilers using such water are more prone to scale formation compared to those using ion-exchanged water. When phosphates are used as water treatment agents, hard, adhering brown iron phosphate scale may also form. 2.2 Corrosion and corrosion products During operation, boilers are subject to various forms of corrosion due to factors such as dissolved oxygen in the boiler water, scale and sludge, acids, alkalis, salts, and temperature. The boilers in sugar mills have their own inherent characteristics: they are in use for a short period each year (4–5 months), and are shut down for a long time (7–8 months). During these shutdown periods, no protection is provided, which accelerates their corrosion. The basic form of corrosion is an electrochemical process, in which iron loses electrons to form ions; the type of corrosion products depends on the factors causing corrosion. Dissolved oxygen can cause cathodic depolarization in both neutral and alkaline as well as acidic media, leading to the corrosion of steel. This results in the formation of relatively loose hydroxide or iron oxide III corrosion products; for example: (in neutral and alkaline media) (in acidic media). Corrosion beneath scale layers is very likely to occur, and this type of corrosion can take the form of alkaline corrosion or acidic corrosion. The main components of the corrosion products in alkaline corrosion are also iron oxides, but severe alkaline corrosion can cause caustic embrittlement of the metal. Acid corrosion is the depolarization of hydrogen (2H++2e→H2), and the corrosion products are mainly soluble salts. High-temperature corrosion occurs in boiler metals due to direct oxidation at high temperatures or interaction with oxygen-containing substances, as well as as a result of temperature differences in hot water boilers; the corrosion products in all these cases are iron oxides. The colors of iron oxides range from black to yellow-brown to brick red, which is why the inner surfaces of some boiler chambers and drum sections appear red-brown in color. 2.3 Management Aspects Poor operation and management of boilers can also lead to or accelerate the formation of scale. Operation and management involve various aspects; among these, water treatment and boiler operation are the most important. If water treatment is not properly managed and the water quality fails to meet the required standards, either regularly or occasionally, this will accelerate the formation of scale. Failing to drain the water at regular intervals while operating the boiler will also accelerate the formation of scale. 3 Types of Scale and Cleaning Strategies The types of scale (including water sludge, mud, corrosion products, etc.) and the chemical cleaning strategies for them are summarized as follows: 3.1 Carbonate scale Its main components are carbonates of calcium and magnesium, with calcium carbonate being the predominant component; its mass fraction is often above 50%. This type of scale is white, relatively loose, and soluble in acids, especially hydrochloric acid. For example, the scale in the boiler of a sugar factory is in the form of loose white flakes that crumble under light pressure; when placed in dilute hydrochloric acid, it produces numerous bubbles. The soluble components account for 70.5% (by mass), while the insoluble components remain in powder form within the acid and settle at the bottom of the container. This is a typical case of carbonate scale. Numerous tests and production practices have shown that this type of scale is easy to remove. Any of the following methods can be used for cleaning. 3.1.1 Acid cleaning: Carbonates decompose when exposed to acids, producing soluble salts and carbon dioxide; for example, when hydrochloric acid is used, the reaction equation is as follows. Detergents can be either pure hydrochloric acid, sulfamic acid, or a mixture of hydrochloric acid with organic acids such as citric acid, with an appropriate amount of corrosion inhibitor added. The relationship between acid concentration and scale thickness is shown in Table 1. Table 1 Relationship between acid concentration and scale thickness. Scale thickness /mm, Hydrochloric acid (or sulfamic acid) concentration /g·L-1, Mass fraction of corrosion inhibitor in the cleaning solution (%): 10, 120, 0.8. The cleaning temperature can be room temperature or elevated to 50–60°C; the cleaning method can involve static immersion, pumped circulation, or a combination of immersion followed by circulation. During circulation, the flow rate of the acid should not be too high (0.2–1 m/s) to prevent accelerated metal corrosion. The cleaning time depends on the boiler’s capacity, the cleaning temperature, and the thickness of the scale. The general principle is to use a large capacity, low temperature, and a long pickling time; the amount of acid to be added and the end point of cleaning are determined by monitoring the changes in the acid concentration in the cleaning solution. For small boilers with a evaporation rate of 0.5–2 t/h, it is usually possible to observe the scale removal through the manhole. Under normal circumstances, as long as the construction is carried out strictly in accordance with the specifications, satisfactory results can generally be achieved. For the boiler of a sugar factory mentioned above (with an evaporation capacity of 40 t/h), we used hydrochloric acid for cleaning: it was first soaked at room temperature for a period of time, after which each drain pipe was circulated using a pump. After acid cleaning, rinsing, washing, and passivation were carried out, and practice has shown that this method yields good results in removing scale. 3.1.2 Quercitric acid method: The main component of quercitric acid is tannic acid, which possesses astringent and penetrating properties. It can penetrate into the interior of scale, loosening it; it can also reach between the scale and the metal, where it forms a protective layer of tannic acid on the metal surface. This disrupts the connection between the scale and the metal, allowing the scale to be removed. Additionally, quercitric acid can react with carbonate scale to form tannates, thereby altering the crystal structure of the scale. Its descaling process is as follows: the amount of tannin used is determined based on the volume of water in the boiler, with 5–10 kg added per ton of water; a base (sodium carbonate, caustic soda, or trisodium phosphate) is then used to raise the pH value above 7.0. The amount of base used is generally 1/2 to 1/3 of the mass of tannin. The boiler operates under pressure (0.49–0.78 MPa) for 72–170 hours. The descaling effect of this method is inferior to that of hydrochloric acid and aminosulfonic acid, but tannin does not corrode the boiler, and no passivation treatment is required after cleaning. 3.1.3 Alkaline cooking: Pure carbonate scale can be subjected to pressure cooking using 10–20 g/kg of trisodium phosphate or a mixture of this substance with sodium hydroxide; the specific procedures can be found in the alkaline cooking process described later. Alkaline boiling provides poor cleaning efficiency, but it does not cause corrosion to the equipment; moreover, the boiler does not require passivation treatment after alkaline boiling. 3.2 Sulfate scale: Its main component is calcium sulfate, which often accounts for more than 50% by mass. This type of scale is hard and dense, yellowish-white in color; it is insoluble in organic acids but can dissolve slowly in hydrochloric acid. This type of scale is best cleaned using hydrochloric acid; the required acid concentration is higher than that for scale formed from carbonate solutions of the same thickness. Heating to 50–60°C and employing a cyclic cleaning method yields the best results. The cleaning time is generally longer than that for carbonate scale, and the cleaning efficiency is lower compared to that in the case of carbonate scale. Next, the alkali washing method is used; sodium hydroxide and trisodium phosphate are commonly employed for this purpose. Trisodium phosphate reacts with scale as follows: the calcium phosphate that is formed is also insoluble in water, and it cannot be allowed to accumulate, so it must be rinsed away promptly. Alkaline descaling can be divided into alkaline washing and alkaline boiling; the formula for the alkaline solution is the same, namely: NaOH: 3–5 g/kg and Na3PO4: 5–10 g/kg. The operation process is as follows. 3.2.1 Alkali washing: Temperature of 90–95°C, cycle duration of 8–24 hours; the alkali concentration is measured once per hour to determine whether the amount of chemical used is sufficient and whether the alkali washing process is complete. After alkali washing, drain the alkali solution and rinse with water until the pH of the water at the outlet is