Deoxygenation is a crucial step in boiler feedwater treatment. Oxygen is the primary corrosive substance in feedwater systems and boilers, and it must be rapidly removed from the feedwater. Otherwise, it will corrode the boiler's feedwater system and components. The corrosion product, iron oxide, will enter the boiler and deposit or adhere to the boiler tube walls and heating surfaces, forming insoluble and poorly heat-transferring iron scale. Furthermore, corrosion can cause pitting on the inner walls of pipes, increasing the resistance coefficient. Severe pipe corrosion can even lead to pipe explosions. National regulations stipulate that steam boilers with an evaporation capacity of 2 tons per hour or more and hot water boilers with a water temperature of 95°C or higher must undergo deoxygenation. For many years, boiler feedwater treatment professionals have been exploring efficient and economical deoxygenation methods. This article introduces several major deoxygenation methods for boiler feedwater and analyzes and summarizes these methods based on adjustments and improvements made in recent years, providing a reference for boiler feedwater treatment professionals. [b]1 Analysis of Deoxygenation Pathways[/b] 1.1 Physical Methods According to Henry's Law, the solubility of any gas present on the water surface is directly proportional to its partial pressure, and the solubility is only related to its own partial pressure. Under a certain pressure, as the water temperature increases, the partial pressure of water vapor increases, while the partial pressures of air and oxygen decrease. At 100℃, the partial pressure of oxygen drops to zero, and the dissolved oxygen in the water also drops to zero. When the pressure on the water surface is less than atmospheric pressure, the solubility of oxygen can also reach zero at lower water temperatures. Thus, as the water temperature increases, the solubility of oxygen decreases, allowing oxygen in the water to escape. In addition, oxygen molecules in the space above the water surface are expelled or transformed into other gases, resulting in zero partial pressure of oxygen, and oxygen in the water continuously escapes. Physical deoxygenation methods utilize physical methods to release oxygen from the water. Commonly used methods include thermal deoxygenation, vacuum deoxygenation, and analytical deoxygenation. 1.2 Chemical Methods Chemical deoxygenation primarily utilizes chemical reactions to remove oxygen from water. Dissolved oxygen in the water is converted into stable metal or other chemical compounds before entering the boiler, thus eliminating it. Common methods include chemical deoxygenation and steel scrap deoxygenation. 1.3 Electrochemical Methods In addition to chemical and physical methods, electrochemical methods can also be used for boiler feedwater deoxygenation. Electrochemical deoxygenation applies the principle of electrochemical protection, causing electrochemical corrosion of an easily oxidized metal, consuming and removing oxygen from the water. Compared to the above deoxygenation methods, this method has simpler equipment, is easier to operate, and has lower operating costs, making it widely applicable to feedwater deoxygenation in low-pressure and hot water boilers. Although there is currently no mature experience with electrochemical deoxygenation, its economic practicality is evident based on trial use. [b]2 Comparison and Analysis of Deoxygenation Methods[/b] 2.1 Thermal Deoxygenation Thermal deoxygenation generally includes atmospheric thermal deoxygenation and jet thermal deoxygenation. The principle is to heat boiler feedwater to its boiling point, reducing the solubility of oxygen and causing it to continuously escape from the water. The oxygen produced on the water surface, along with water vapor, is then removed. This process also removes various gases from the water (including free CO2 and N2), such as water treated with sodium ammonium ion exchange, which can be removed by heating. Deoxygenated water does not increase salinity or the amount of other dissolved gases. Operation and control are relatively easy, and the operation is stable and reliable, making it the most widely used deoxygenation method. To ensure the reliable effect of the thermal deaerator, the following conditions should be met in its design and operation: a. Increase the contact area between water and steam, and ensure uniform water flow distribution. b. Ensure a pressure difference between the dissolved oxygen pressure in the water and its partial pressure on the water surface. c. Ensure that the water is heated to the boiling point at the deaerator's operating pressure, typically 104℃. Thermal deoxygenation is a widely adopted and mature technology, but it still has some problems in practical applications: First, the temperature of the soft water after thermal deoxygenation is relatively high, easily reaching the vaporization temperature of the boiler feedwater pump, causing the feedwater to be easily vaporized during transportation; moreover, when the heat load fluctuates frequently and management cannot keep up, the deoxygenation effect is poor when the deoxygenated water temperature is below 104℃. Second, this deoxygenation method requires high-level equipment placement, increasing infrastructure investment and making design, installation, and operation inconvenient. To achieve the purpose of vaporizing the softened water in the feedwater pump, this deoxygenation method generally requires the deaerator to be configured high, which generates a lot of noise and vibration during operation, causing inconvenience. Third, it increases the boiler room's self-consumption of steam, reducing the effective external steam supply. Fourth, thermal deoxygenation has certain limitations for small, quick-installation boilers and applications requiring low-temperature deoxygenation, and it cannot be used in pure hot water boiler rooms. For boilers employing thermal deaeration, when installing a new boiler, the atmospheric thermal deaerator is installed on the ground. The deaerated, high-temperature softened water is then transported through a soft water tank, where it exchanges heat with the water in the tank before flowing to the boiler feed pump and then through the economizer into the boiler. This improvement firstly reduces boiler room vibration and noise, improving the working environment and lowering the boiler room's construction cost. Secondly, the heat exchange in the soft water tank increases the water temperature, preventing heat waste and effectively reaching the deaerator's inlet water temperature. This shortens the time it takes for the deaerator to heat the inlet water to saturation temperature, thus improving the desired deaeration effect. 2.2 Vacuum Deaeration: This is a medium-temperature deaeration technology, generally performed at temperatures between 30℃ and 60℃. It can achieve deaeration at low water surface temperatures (60℃ or room temperature). For thermal boilers and steam boilers with large load fluctuations where thermal deaeration is ineffective, vacuum deaeration can achieve satisfactory deaeration results. Compared with thermal deoxygenation technology, its heating conditions are improved and the self-consumption of steam in the boiler room is reduced. However, most of the disadvantages of thermal deoxygenation still exist. Moreover, the high-level arrangement of vacuum deoxygenation has higher requirements for the operation and management of key equipment such as jet pumps and booster pumps than thermal deoxygenation. The low-level arrangement also requires a certain height difference and has high requirements for the operation and management of key equipment such as jet pumps and booster pumps. In addition, heat exchange equipment and circulating water tanks are added. Vacuum deoxygenation can utilize low-grade waste heat and can use jet heaters to heat softened water; it can also be installed in stages and at low levels, with reliable deoxygenation, stable operation, simple operation, and wide applicability. Since my country’s energy conservation work has been vigorously carried out, the use of this method for deoxygenation in industrial boiler rooms has been increasing. 2.3 Chemical deoxygenation (1) Steel scrap deoxygenation: Water passes through a steel scrap filter, the steel scrap is oxidized, and the dissolved oxygen in the water is removed. There are two types: independent and attached. This method requires a water temperature greater than 70%, with the best effect at 80-90℃. The deoxygenation effect is worst at 20-30℃. The use of steel scraps requires compaction, the tighter the better. The higher the oxygen content in the water, the lower the flow rate is required. Since the application of steel scrap deoxygenation, there has been little improvement or enhancement, and the deoxygenation effect is not very reliable. It is generally used in small boiler rooms with low requirements for feedwater quality, or as makeup water for thermal networks, and as supplementary deoxygenation after thermal deoxygenation of high-pressure boilers. It is generally only used as an auxiliary measure. (2) Sodium sulfite deoxygenation, which is an in-furnace chemical deoxygenation method. Because oxygen is the main corrosive substance of boilers in the feedwater system, it is required to remove oxygen from the feedwater quickly. Sodium sulfite is generally used as a deoxygenating agent. 2Na2SO3+O2→2Na2SO4. Usually, the dosage is required to be larger than the theoretical value. The higher the temperature and the shorter the reaction time, the better the deoxygenation effect. The effect is best when the boiler water pH=6. If the pH increases, the deoxygenation effect will decrease. Adding copper, cobalt, manganese, tin and other catalysts can improve the deoxygenation effect. This method has low investment, is safe and relatively simple to operate because sodium sulfite is inexpensive. However, the dosage of this method is difficult to control, the deoxygenation effect is unreliable, and it cannot guarantee that the standard will be met. In addition, it will increase the salt content of the boiler water, resulting in increased sewage discharge and heat waste, which is uneconomical. Therefore, this method is generally used in small boiler rooms and some thermal systems with high water quality requirements as an auxiliary deoxygenation method. (3) Hydrazine (hydrazine) deoxygenation. At present, this method is mostly used as an auxiliary measure after thermal deoxygenation to completely remove residual oxygen in the water without increasing the salt content of the boiler water. When the pressure is greater than 6.3 MPa, sodium sulfite mainly decomposes into highly corrosive sulfur dioxide and hydrogen sulfide. Therefore, hydrazine is often used for high-pressure boilers. Hydrazine reacts with oxygen to generate nitrogen and water, which is conducive to hindering the further development of corrosion. Because hydrazine is toxic and easily volatilized, it cannot be used for deoxygenation of drinking water boilers and domestic water boilers. Many boiler plants are restricting or no longer using it. 2.4 Deoxygenation by Desorption Deoxygenation is a relatively advanced technology that has emerged in recent years. Its working principle involves intensely mixing and contacting oxygen-free gas with the feedwater to be deoxygenated, causing dissolved oxygen in the water to be released into the gas. This cycle is repeated to achieve deoxygenation of the feedwater. Deoxygenation by desorption has the following characteristics: 1. The water to be deoxygenated does not require preheating, thus not increasing the boiler room's self-consumption of steam; 2. Deoxygenation equipment occupies less space and consumes less metal, thereby reducing infrastructure investment; 3. The deoxygenation effect is good. Under normal conditions, the residual oxygen content after deoxygenation can be reduced to 0.05 mg/L; 4. The disadvantage of deoxygenation by desorption is the complexity of equipment adjustment, and the need for a sealed piping system and deoxygenated water tank. Current deoxygenation methods generally use new deoxygenation deaerators, replacing the original boiler flue gas heating with a heater, and using activated carbon with a catalyst as a reducing agent, thus greatly reducing the equipment's footprint. Inside the deoxygenation unit, baffles are added to control water flow, and small holes and perforated pipes are added to allow the oxygen-containing gas in the water to escape fully, achieving a very good deoxygenation effect. Desorption deoxygenation equipment is small, easy to manufacture, consumes little steel, has low investment, is convenient to operate, and runs reliably. It does not require chemicals, reducing environmental pollution, and can deoxygenate at low temperatures with good deoxygenation effect. Currently, it is widely used in hot water boilers and single-layer industrial boilers in China. Its disadvantages are that it can only remove oxygen from water and cannot remove other non-condensable gases, resulting in an increase in carbon dioxide content; the water tank surface cannot be sealed, sometimes allowing the deoxygenated water to come into contact with air, thus affecting the deoxygenation effect. As early as the 1960s, many boiler rooms at home and abroad widely adopted this technology, but because the reactors at that time were located in the flue, they could not adapt to changes in heat load. Therefore, the use of this technology was once limited. In the 1990s, a second-generation desorption deoxygenator with a centrally located electrically heated reactor was developed, leading to significant progress in this technology. In particular, the new desorption deaerator developed by Tsinghua University and the Design Institute of the Ministry of Machinery and Electronics Industry has overcome the shortcomings and deficiencies of the original system. It separates the heating furnace from the reactor; the heating furnace heats the gas exiting the desorption deaerator, and the heated gas is deoxygenated as it passes through the reactor, ensuring that the oxygen-containing gas in the water to be deoxygenated is fully desorbed, guaranteeing operational reliability and deoxygenation effect. Furthermore, its size and power consumption are smaller than the original equipment. The new desorption system eliminates the need for a deoxygenation water tank, solving the sealing problem of the original tank. Operation in multiple boiler rooms has proven that the desorption deaerator is simple to operate, has low investment, reliable operation, and good performance. However, it also has the problem that many factors affect deoxygenation and it can only remove oxygen, not other gases. 2.5 Resin Deoxygenation: When water passes through the resin layer, the dissolved oxygen in the water is reduced from zero oxidation state to negative two oxidation state, forming oxides (copper oxide). After the resin becomes ineffective, it can be reduced with hydrazine, and Cu2+ is absorbed by the exchange groups on the resin. During use, it should be noted that the effluent contains trace amounts of hydrazine and cannot be used as drinking water. The deoxygenated water tank should be isolated from air, and two deoxygenated tanks are required to ensure a continuous supply of deoxygenated water. The Y-12 series of redox resin deoxygenators, patented products developed by the Twelfth Research Institute of the Ministry of Electronics Industry, has been applied in hot water boilers at Tsinghua University and Beijing Third Machine Tool Plant, achieving excellent deoxygenation results with residual oxygen in feedwater of 0.06–0.02 mg/L. It is currently being promoted for use in small hot water boilers. The steam and hot water produced by this deoxygenation method must not come into contact with drinking water or food, and the investment and land occupation are relatively large, generally making it unsuitable for widespread application in industrial boilers. 3. Conclusion There are various methods for boiler feedwater deoxygenation. To achieve efficient, economical, stable, and safe operation, it is necessary to consider the boiler type and actual conditions, taking into account the boiler's thermal parameters, water quality, tonnage, load changes, economic conditions, etc., and selecting the appropriate method based on local conditions. For feedwater deoxygenation technology, we must constantly pay attention to new technologies, new materials, and new achievements, be bold in exploration and innovation, and seek methods with good deoxygenation effect, reliable operation, simple management, and low investment requirements.