Boiler deaeration technology

You’re absolutely right; I completely agree

Well, during the study*, the induction was good…

Thermal deoxidation is the mainstream method, chemical deoxidation serves as a supplement, while vacuum deoxidation is a newer approach: lol

Thermal deoxidation is indeed the mainstream method, and it provides good deoxidation results.

Agreed; the thermal deaeration system is relatively simple and has low costs.

Chemical deoxygenation is also a good option, and its cost is not high either

Generally, a combination of thermal deoxidation and chemical deoxidation can be considered; thermal deoxidation allows for the reuse of low-pressure steam from systems such as boiler blowdown systems, while chemical deoxidation can make use of hydrazine.

Just to add to the excitement, let’s go all out~ The application of boiler feedwater deoxidation technologies: This article analyzes several commonly used methods for deoxidizing boiler feedwater, such as physical deoxidation, chemical deoxidation, and electrochemical deoxidation, and outlines the appropriate applications for each of these methods. On this basis, their advantages and disadvantages were identified, and suggestive recommendations were provided for the selection of boiler feedwater deoxidation methods. In the boiler feedwater treatment process, deoxygenation is a very critical step. Oxygen is the main corrosive agent in the feedwater system and boilers; it must be removed from the feedwater as quickly as possible. Otherwise, it will corrode the components of the boiler’s feedwater system. The corrosion products, namely iron oxide, will enter the boiler and deposit or adhere to the boiler tube walls and heating surfaces, forming iron scale that hinders heat transfer. Moreover, corrosion can cause pits to form on the inner walls of the pipes, increasing the resistance coefficient. When pipeline corrosion is severe, pipeline explosions can even occur. **It is stipulated that steam boilers with an evaporation rate of 2 tons per hour or more, as well as hot water boilers with a water temperature of 95°C or more, must be deoxygenated. For many years, numerous workers in boiler water treatment have been seeking efficient and economical deoxygenation methods. This article introduces several major methods for deoxidizing boiler feedwater, and analyzes and summarizes these methods in light of the adjustments and improvements made over recent years based on them, to provide reference for those working in boiler feedwater treatment. 1 Analysis of deoxygenation methods 1.1 Physical methods According to Henry’s law, when any gas is present above the water surface, its solubility is proportional to its partial pressure; moreover, the solubility of a gas depends only on its own partial pressure. At a certain pressure, as the water temperature rises, the partial pressure of water vapor increases, while the partial pressures of air and oxygen decrease. At 100°C, the partial pressure of oxygen drops to zero, and its solubility in water can also reach zero at lower temperatures. In this way, as the water temperature rises and the solubility of oxygen in it decreases, the oxygen in the water can escape. Additionally, the oxygen molecules in the air on the water surface are removed or converted into other gases; physical deoxygenation methods involve using physical techniques to remove oxygen from water. Common methods include thermal deoxygenation, vacuum deoxygenation, and stripping deoxygenation. 1.2 Chemical deoxygenation: Chemical methods are used for deoxygenation, primarily by employing chemical reactions to remove the oxygen present in water. This allows the dissolved oxygen in the water to be converted into stable compounds of metals or other chemicals before it enters the boiler, thereby eliminating it. Common methods include chemical dosing for deoxygenation and the use of steel scrap for deoxygenation. 1.3 Electrochemical deoxygenation method: For deoxygenating boiler feedwater, in addition to chemical and physical methods, electrochemical methods can also be used. Electrochemical deoxygenation utilizes the principle of electrochemical protection to cause electrochemical corrosion in a metal that is prone to oxidation, thereby consuming the oxygen in water and removing it. Compared with the aforementioned deoxygenation methods, this approach features simpler equipment, easier operation, lower operating costs, and can be widely applied to deoxygenating the feedwater in low-pressure boilers. However, although there is currently no mature experience with electrochemical deoxygenation, based on trials and tests, its economic viability is quite evident. 2 Comparison and Analysis of Deoxygenation Methods 2.1 Thermal Deoxygenation Thermal deoxygenation generally includes atmospheric thermal deoxygenation and jet thermal deoxygenation. The principle involves heating the boiler feed water to its boiling point, which reduces the solubility of oxygen; as a result, oxygen continuously escapes from the water. The oxygen that forms on the surface of the water is then removed along with the steam. This method also eliminates various gases present in the water, including free CO2 and N2. Gases such as these can also be removed from water that has been treated using the ammonium-sodium ion exchange method, even after heating. Deoxygenated water does not increase the salt content nor the concentration of other dissolved gases. Its operation is relatively easy, and it provides stable and reliable performance; it is therefore the most widely used deoxygenation method at present. To ensure reliable performance of thermal deaeration equipment, the following conditions must be met in its design and operation: a. Increase the contact area between water and steam, and ensure even distribution of the water flow. b. Ensure a pressure difference between the dissolved oxygen pressure in water and its partial pressure at the water surface. c. Ensure that the water is heated to the boiling temperature at the operating pressure of the deaerator, which is generally 104°C. Human-powered deoxygenation is a widely used and mature technology, but there are some issues in its practical application: firstly, the temperature of the soft water after thermal deoxygenation is relatively high, and it can easily reach the vaporization temperature of the boiler feedwater pump, resulting in the feedwater being prone to vaporization during transportation ； Moreover, when the heat load changes frequently and management fails to keep up, the deaeration effect is poor when the deaerator water temperature is as low as 104°C. Secondly, this deoxygenation method requires the equipment to be installed at a high position, which increases capital costs; it is also inconvenient for design, installation, and operation. To achieve the vaporization of softened water in the feed water pump, this deoxygenation method generally demands that the deoxygenator be placed at a high altitude, and it generates significant noise and vibration during use, causing inconvenience. Third, it increases the steam consumption for internal use in the boiler room, reducing the amount of steam available for external supply. Fourth, for small, quickly installable boilers and applications that require low-temperature deoxidation, thermal deoxidation has certain limitations, and it cannot be used in pure hot water boiler rooms either. For boilers that use thermal deaeration, when a new boiler is installed, an atmospheric thermal degasser is placed on the ground, while the pipeline carrying the deaerated, high-temperature softened water passes through a soft water tank to exchange heat with the water in that tank, before flowing to the boiler feed pump and entering the boiler via the economizer. Such improvements can first reduce vibration and noise in the boiler room, thereby improving the working conditions there, and they also lower the construction costs of the boiler room. Secondly, through heat exchange in the soft water tank, the water temperature in this tank increases; no heat is wasted. This temperature also corresponds to the inlet water temperature of the deaerator. By heating the incoming water to its saturation temperature, the deaerator reduces the time required for this process, which helps to achieve the desired deoxygenation effect. 2.2 Vacuum deoxidation: This is a medium-temperature deoxidation technique that is generally carried out at temperatures between 30°C and 60°C. It enables deoxidation at low temperatures near the water surface (at 60°C or room temperature). Vacuum deoxidation can achieve satisfactory results in both thermal boilers and steam boilers where deoxidation is difficult due to large fluctuations. Compared to thermal deaeration technology, its heating conditions have been improved and the amount of steam consumed internally by the blast furnace has decreased; however, most of the drawbacks of thermal deaeration still remain. Moreover, due to the elevated installation location of vacuum deaeration, higher requirements are placed on key equipment such as jet pumps and pressure pumps in terms of operation and management compared to thermal deaeration. A low-mounted layout also requires a certain height difference, and it imposes high demands on the operation and maintenance of key equipment such as jet pumps and pressure pumps. In addition, heat exchange equipment and a circulating water tank have been added. Vacuum deoxidation can make use of low-grade waste heat, and jet heaters can be used to heat softened water ； It can be installed at different levels and in low positions; it provides reliable deoxygenation, stable operation, simple operation, and a wide range of applications. Since the vigorous promotion of energy conservation efforts in our country, the use of this method for deoxidization in industrial boiler rooms has been increasing. 2.3 Chemical deoxygenation (1) Deoxygenation using steel scrap: Water passes through a steel scrap filter, where the steel scrap is oxidized, thereby removing the dissolved oxygen from the water. There are two types: standalone and attached. For this method, the water temperature must be above 70%, with temperatures of 80°C to 90°C yielding the best results. The deoxygenation effect is worst at temperatures of 20°C to 30°C. The steel scrap used must be compacted as tightly as possible. The higher the oxygen content in the water, the lower the flow rate required. This is because, since its introduction, deoxygenation technology has seen little improvement, and its effectiveness is not very reliable. It is generally used in small boiler rooms where high standards for water quality are not required, or as supplementary deoxygenation for water supplied to heating networks, as well as as an additional deoxygenation step after thermal deoxygenation in high-pressure boilers; it functions mainly as an auxiliary system. (2) Sodium sulfite deoxidation, which is an in-furnace chemical dosing deoxidation method. Since oxygen is the main corrosive agent in water supply systems for boilers, it is necessary to remove oxygen from the water promptly. Sodium sulfite is generally used as a deoxygenizing agent: 2Na2SO3 + O2 → 2Na2SO4. Usually, the amount of this chemical added is higher than the theoretical value. The higher the temperature, the shorter the reaction time and the better the deoxygenation effect. The best effect is achieved when the pH of the boiler water is 6; as the pH increases, the deoxygenation efficiency decreases. Adding catalysts such as copper, cobalt, manganese, and tin can improve the deoxygenation effect. This method has low investment costs due to the low price of sodium sulfite, is safe, and its operation is relatively simple. However, the amount of chemical added in this method is difficult to control, the deoxygenation effect is unreliable, and it is not possible to ensure compliance with standards. It also increases the salt content in the boiler water, leading to greater waste of water and heat, which is uneconomical. Therefore, this method is generally used in small boiler rooms and some thermal systems with high requirements for water quality as an auxiliary deoxidation method. (3) Hydrazine deoxygenation: Currently, this method is primarily used as an auxiliary measure after thermal deoxygenation, in order to completely remove residual oxygen from the water without increasing the salt content in the boiler water. When the pressure exceeds 6.3 Mpa, sodium sulfite mainly decomposes into highly corrosive sulfur dioxide and hydrogen sulfide; therefore, for high-pressure boilers, hydrazine is often used, as it reacts with oxygen to produce nitrogen and water, which helps to prevent further corrosion. Due to its toxicity and volatility, hydrazine cannot be used for deoxygenation in drinking water boilers or domestic water boilers. Many boiler plants are restricting or discontinuing their use. 2.4 Analytical deoxygenation: Analytical deoxygenation is a relatively advanced technique that has emerged in recent years. Its working principle involves strongly mixing oxygen-free gas with the feed water that needs to be deoxygenated, thereby causing the oxygen dissolved in the water to be released into the gas phase. This process is repeated to achieve deoxygenation of the feed water. The advantages of analytical deaeration are as follows: 1. The water to be deoxygenated does not require preheating, thus it does not increase the steam consumption in the boiler room ； 2. The deaeration equipment requires less space and uses less metal, thereby reducing capital investment: 3. It provides excellent deaeration results. Under normal conditions, the residual oxygen content after deoxygenation can be reduced to 0.05 mg/L; 4. A disadvantage of deoxygenation is that the equipment requires complex adjustment, and the piping system as well as the deoxygenation tank must be sealed. The current methods for analytical deoxygenation typically use modern analytical deoxidizers, with heaters replacing the traditional boiler flue gas heating. Activated carbon combined with catalysts is used as a reducing agent, which helps to **reduce the space required by the equipment. Dividers are installed inside the deoxidizer to control water flow, and small holes and perforated tubes are used to ensure that the oxygen-containing gases in the water can escape fully, thereby achieving an excellent deoxygenation effect. Analysis shows that deoxygenation equipment is small in size, easy to manufacture, requires less steel, has low investment costs, is easy to operate, reliable in performance, does not require the use of chemical substances, reduces environmental pollution, and can remove oxygen at low temperatures with good deoxygenation effects. It is currently widely used in domestic hot water boilers and industrially boilers with a single-layer arrangement. Its drawback is that it can only remove oxygen from water, not other non-condensable gases, resulting in an increase in the carbon dioxide content in the water ； The water level in the tank cannot be sealed, which sometimes allows the deoxygenated water to come into contact with air, thereby affecting the deoxygenation effect. 2.5 Resin deoxygenation: When water passes through the resin layer, the dissolved oxygen in the water is reduced from zero valence to negative two valence, forming oxides (copper oxide). Once the resin becomes ineffective, it can be restored using hydrazine, and Cu2+ is absorbed by the exchange sites on the resin. Be careful when in use: the output water contains trace amounts of hydrazine, and a continuous supply of deoxygenated water cannot be guaranteed. The Y-12 redox resin deaerator series, a patented product developed by the 12th Research Institute of the Electronics Industry Department, has been used in Tsinghua University and Beijing’s third machine tool hot water boilers, achieving an excellent deaeration effect with residual feedwater oxygen levels of 0.06–0.02 mg/L. It has currently been adopted on small hot water boilers. 3. Conclusion: There are various methods for deoxidizing boiler feed water. To ensure efficient, economical, stable, and safe operation, it is necessary to take into account the type of boiler as well as actual conditions, and to select the appropriate method by considering factors such as the boiler’s thermal parameters, water level, capacity, load variations, and economic considerations, adapting the choice to the specific situation. Regarding water deoxygenation technologies, it is necessary to stay attentive to new technologies, materials, and advancements, and to be bold in exploring, improving, and innovating in order to find methods that offer good deoxygenation results, reliable operation, simple management, and low investment requirements.