As an important energy production equipment in industrial production, boilers are widely used in thermal power generation, petrochemical industry, ships, industrial and mining production, and domestic heating and other fields. At the end of 2013, the General Administration of Quality Supervision, Inspection and Quarantine of China provided data showing that there were approximately 2 million boilers in use in China, including approximately 64 120000 units, with an annual growth rate of about 15%. When the pressure inside the boiler exceeds the strength of the boiler's allowable materials, the boiler materials rupture, and the steam containing enormous energy is suddenly and instantly released, directly causing an explosion accident. Therefore, protecting the strength of boiler materials from multiple aspects such as design, manufacturing, use, and maintenance is the fundamental starting point to prevent boiler explosion accidents. Over the past 10 years, about 50% of total accidents were caused by scaling and corrosion caused by water quality, which reduced material strength and led to boiler explosions. From this, it can be seen that ensuring the safe operation of boilers, especially medium and high pressure boilers, is of great significance to the development of the national economy and the safety of people's lives and property.

1. Analysis of boiler scaling and corrosion

The boiler feedwater system often contains cations such as calcium magnesium ions and divalent trivalent iron ions, as well as anionic groups such as hydroxide, carbonate, and sulfate ions. In medium and high pressure boiler systems, after high-temperature and high-pressure evaporation and continuous concentration, the concentration of certain impurities in the boiler water can reach 50-300 times that of the feedwater. If left untreated, it will inevitably cause corrosion, scaling, and salt accumulation in the boiler materials, which can directly affect the heat transfer of the furnace tubes, increase energy consumption, block the drainage and sewage system, and increase the cost of pollution control. Severe scaling and corrosion can reduce the thermal conductivity of boiler metal, and uneven temperature can cause local deformation, bubbling, serious damage to boiler equipment, and even pipe explosion accidents.

At present, adding chemical agents is a common way to solve the scaling of water quality, corrosion of metal materials, and growth of microorganisms in boilers. Usually, boiler water changes the physicochemical properties of scale substances by adding a quantitative chemical scale inhibitor, and is discharged from the boiler through boiler discharge, thereby preventing or reducing scaling inside the boiler. Based on this situation, it is not difficult to see that the development process of boiler water treatment technology is the process of developing high-performance boiler water treatment agents.

2. Development history of boiler water treatment agents

The use of ammonia and hydrazine, non-toxic new deoxidizers, thermal deoxygenation, and oxidative treatment are mature and widely used deoxygenation technologies in boiler systems. The difficulty of boiler water treatment technology lies in the research and development of descaling and corrosion inhibitors, especially in medium and high pressure boilers. The research methods for corrosion and scale inhibition technologies are becoming increasingly diverse, and research results are gradually increasing. This includes inorganic salt corrosion inhibitors, corrosion inhibitors, green corrosion inhibitors, and environmentally friendly corrosion inhibitors, which have been successfully applied in different corrosion media. With the development trend of environmental protection and green industry, scale inhibitors have gradually evolved from phosphorus and nitrogen containing systems to safe, non-toxic, non bioaccumulative, and biodegradable green scale inhibitors.

2.1 Conventional treatment agent for boiler water

The conventional treatment agent phosphate is actually a single functional scale and corrosion inhibitor. In order to maintain the content of trisodium phosphate in the boiler water below the critical value when phosphate is hidden, and at the same time, it is required that the sodium hydroxide content in the boiler water be less than 1 mg/L to prevent corrosion and calcium scale caused by the hidden acidic phosphate in the boiler. With the increasingly strict environmental requirements of the country for phosphorus emissions, researchers have developed low phosphate salt water treatment agents to further reduce the acidic corrosion caused by hidden phosphate. The probability of acidic phosphate corrosion in this technology is very low, but it has high requirements for the water quality of the feedwater.

Throughout the decades of development in boiler water treatment technology, phosphate treatment technology is a widely used and mature boiler water treatment process in high-pressure steam drum boilers. However, the drawbacks of phosphate treatment technology cannot be avoided, that is, the hidden phenomenon of phosphate in boiler water, which leads to acid phosphate corrosion in the boiler, making it difficult to avoid. It is easy to cause salt accumulation in the superheater and turbine of medium and high pressure boilers, and the rich oxygen pollution of phosphate in water.

2.2 Multifunctional corrosion and scale inhibitors

A multifunctional boiler water treatment agent, which is a type of agent that can achieve various effects such as corrosion inhibition, scale inhibition, and coagulation of suspended solids. In terms of scale inhibitors, Professor Tan Wei's research group at Tianjin University conducted experiments comparing the scale inhibition rates of inorganic scale inhibitors sodium carbonate and scale inhibitors. The experimental results showed that the scale inhibitor effect was much better than that of inorganic scale inhibitors. Based on this, a scale inhibition composite formula was studied, and the scale inhibition effect of the phosphine/sodium carbonate composite formula was better for water with a total hardness of 7 mmol/L. In order to improve the corrosion inhibition performance of the formula, studies have shown that adding zinc salts or polymer dispersants in combination can enhance the synergistic corrosion inhibition effect. Water treatment agents are suitable for pH>8 A water system of 5, which is basically consistent with the required acidity and alkalinity of boiler water quality, is suitable for the requirements of boiler water treatment.

As a new technology, polymer water treatment agents optimize the scale and corrosion inhibition effects of boiler water by adjusting the types of polymer monomers, polymer functional groups, and molecular weight, further promoting the technological development of boiler water treatment. Most polymers can prevent the polymer and calcium ions from forming gel due to the hydrophilic group in their structure, which makes the polymer have better scale inhibition ability. Research has found that the principles of polymer scale inhibition and corrosion inhibition lie in solubilization, electrostatic repulsion, and crystal distortion.

Polymer compounds of ether based polysaccharides have also been applied in water treatment research. This type of chitosan adsorbs on the active sites of calcium carbonate crystals, forming inclusions in the middle of inorganic salt crystals, causing lattice distortion that slows down the growth rate of calcium carbonate crystals. However, the unstable properties, easy decomposition, and large dosage of drugs of polymers limit their range of use. Although studies have shown that chemical modification can be carried out through the hydroxyl and amino groups distributed on the chitosan macromolecular chain to prepare chitosan such as hydroxypropyl chitosan, maleic anhydride chitosan, and acrylic chitosan to increase the water solubility of chitosan and improve its scale inhibition performance, the limitations of modified polymer water treatment agents have prevented their widespread application in medium and high pressure boiler systems.

The boiler water treatment of carboxylic acid copolymers can form chelates or complexes with metal ions in the boiler water through carboxylate ions, which hinder the precipitation of scale through electrostatic repulsion. Domestic institutions such as East China University of Science and Technology have also begun to study the performance of carboxylic acid polymers in water treatment, and have preliminarily explored the impact of molecular structures of phosphonic acid polymers and carboxylic acid polymers on water treatment performance. At the same time, many studies have reported the formation of scale substances such as calcium phosphate, calcium sulfate, calcium carbonate, and even magnesium hydroxide by improving the functional groups of carboxylic acid polymers. The practical exploration of adding acrylic acid to polymers indicates the technological progress and good social and environmental benefits of polymer water treatment agents.

Polymers containing sulfonic acid functional groups are another common polymer boiler water treatment agent. Betz Lab and Calgo in the United States have continuously disclosed various multifunctional carboxylic acid polymer scale inhibitors and dispersants containing sulfonic acid salts. The molecules copolymerize sulfonic acid monomers with acrylic acid, and then use them in combination with enaminomethylene phosphate to improve the effect of inhibiting calcium scale and stabilize the state of phosphorus in water, ensuring the corrosion and scale inhibition performance of polymer treatment agents in multiple aspects. The development of sulfonic acid polymer boiler water treatment in China started relatively late. Yi Longxin and others have expanded their research on the scale and corrosion inhibition performance of sulfonic acid polymer boiler water, with rapid development momentum, good results, and a wide range of applications. Up to now, they are in a national position in industrial water treatment. The current outstanding achievement is that the addition of AMPS monomer, 2-acrylamide-2-methylpropylsulfonic acid, to copolymers has a good effect, pioneering the research on scale inhibition and dispersion of water-soluble polymers containing sulfonate functional groups. By mixing strong and weak acids, precipitates can be dispersed, metal ions can be stabilized, and calcium and iron can be hindered from scaling. In recent years, new series of polymer scale inhibitors and dispersants containing sulfonates have been continuously developed in China. Common monomers such as acrylic acid, maleic anhydride, tartaric acid, and itaconic acid are used to copolymerize with sulfonates, mostly binary or ternary copolymers. Calcium phosphate and calcium carbonate scale agents have shown good effects and have been tested and applied in medium and high pressure boilers.

At present, Western Europe and Japan have begun to restrict the use of phosphorus based water treatment agents, causing a wave of development and research in environmentally friendly water treatment in various countries. Dow Chemical reported in a patent published in 2015 on the application of hydrophobic modified epoxy chloropropane polyurethane copolymer scale inhibitor. The industry university research cooperation group of Gyeonggi University in Japan reported β ——  A mixed treatment agent for manganese dioxide nanoparticles and carbon nanotubes and its preparation method. China has also launched green and environmentally friendly scale inhibitors, with research groups represented by East China University of Science and Technology and Wuhan University of Engineering developing polyaspartic acid, polyepoxysuccinic acid, and γ ——  Polyglutamic acid, one of the three major types of environmentally friendly water treatment scale inhibitors, has a scale inhibition rate of up to 99% for calcium carbonate 7%, but they all use industrial circulating water systems, and have not yet been involved in medium and high pressure boiler water systems.

3 Conclusion

The progress and rapid development of contemporary technology have promoted people's awareness of water conservation and environmental protection. With the increasing emphasis on environmental protection and stricter technical requirements from countries around the world, regulations and standards for low phosphorus, limited phosphorus, and banned phosphorus have been introduced both domestically and internationally. This means that water treatment technologies that have both corrosion and scale inhibition properties can no longer keep up with the pace of green technology. People must not only pursue technological performance, but also pay attention to the development of environmentally friendly and non-toxic water treatment technologies. Medium and high pressure blast furnaces play a crucial role in modern industries such as petrochemicals, aerospace, and thermal power generation. They also require higher technical, safety, and environmental requirements for the treatment of boiler water chemicals and process methods. Although significant achievements have been made in the development of industrial cooling water, circulating water, and low-pressure boiler water treatment agents domestically, the development of medium and high-pressure boiler water treatment agents is still in the laboratory stage. Further research on environmentally friendly pressurized boiler water treatment agents should be accelerated to improve product quality, reduce production costs, expand application scope, and ensure the safe and stable operation of medium and high-pressure boilers in industrial production through scientific and technological means.