Analysis and Application of Deaerator Pathway for Industrial Boiler Feed Water

Oxygen removal is a very critical part of the boiler feedwater treatment process. Oxygen is the main corrosive substance in the water supply system and boiler. The oxygen in the feed water should be quickly removed. Otherwise, it will corrode the boiler feed water system and components. The corrosion product iron oxide will enter the pot, deposit or adhere to the boiler tube wall and On the heated surface, iron scale which is difficult to transfer and poor in heat transfer is formed, and corrosion causes pits on the inner wall of the pipe, and the drag coefficient increases. When the pipeline is seriously corroded, even a pipeline explosion may occur. 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 are required to remove oxygen. Many boiler feedwater treatment workers have been searching for efficient and economical oxygen removal methods for many years. This paper introduces the main methods of main deaeration of boiler feed water, and combines the adjustments and improvements made on the basis of these methods in recent years. These methods are analyzed and summarized for reference by boiler feedwater treatment workers.
1 Analysis of the oxygen removal pathway 1.1 Physical methods According to Henry's law, any gas is present on the water surface at the same time, the solubility of the gas is proportional to its own partial pressure, and the solubility of the gas is only related to its own partial pressure. Under a certain pressure, as the water temperature increases, the partial pressure of water vapor increases, and the partial pressure of air and oxygen becomes smaller and smaller. At 100 ° C, the partial pressure of oxygen is reduced to zero and the dissolved oxygen in the water is also reduced to zero. When the pressure on the water surface is less than atmospheric pressure, the solubility of oxygen can reach zero at lower water temperatures. Thus, as the water temperature increases, the solubility of oxygen therein is reduced, and oxygen in the water is allowed to escape. In addition, oxygen molecules in the surface of the water are discharged or converted into other gases, so that the partial pressure of oxygen is zero, and oxygen in the water continuously escapes. The physical method of removing oxygen is to use physical methods to precipitate oxygen in water. Commonly used are thermal oxygen removal method, vacuum oxygen removal method and analytical oxygen removal method.
1.2 Chemical methods The chemical method is used to remove oxygen. The chemical reaction is mainly used to remove the oxygen contained in the water, so that the dissolved oxygen in the water is converted into a stable metal or other chemical compound before entering the boiler, thereby eliminating it. Chemical deoxidation method and steel dust removal method.
1.3 Electrochemical method Boiler feed water deoxidation, in addition to chemical methods and physical methods, electrochemical methods can also be used. Electrochemical deoxidation is the principle of applying electrochemical protection to electrochemically etch an easily oxidizable metal, allowing oxygen in the water to be consumed and removed. Compared with the above oxygen removal method, the method has the advantages of simple equipment, convenient operation and low operation cost, and can be widely applied to water supply and deoxidation of low-pressure boilers and hot water boilers. However, although the electrochemical oxygen removal method has no mature experience at present, according to the situation of trial production, its economic applicability is obvious.
2 Comparison and analysis of oxygen removal methods 2.1 Thermal deaeration Thermal deaeration generally has atmospheric thermal deaeration and jet thermal deaeration. The principle is to heat the boiler feed water to the boiling point, so that the solubility of oxygen is reduced, the oxygen in the water continuously escapes, and the oxygen generated on the water surface is removed together with the water vapor, and various gases in the water (including free CO2, N2) can be removed. ), such as water treated with ammonium sodium ion exchange, can be removed after heating 3. The water after deoxidation does not increase the salt content, does not increase the amount of other gases dissolved, and the operation control is relatively easy, and the operation is stable and reliable. It is the most widely used oxygen removal method at present. In order to ensure the reliable effect of the thermal deaerator, the following conditions should be met in the design and operation: a. Increase the contact area between water and steam, and distribute the water flow evenly. b. Ensure that there is a pressure difference between the dissolved pressure of oxygen in water and its partial pressure on the water surface. c. Ensure that the water is heated to the boiling temperature of the deaerator operating pressure, typically 104 ° C. Thermal deaeration technology is a commonly used mature technology, but there are still some problems in practical application: First, the soft water temperature after desulfurization by heat is high, and it is easy to reach the vaporization temperature of the boiler feed water pump, so that the feed water is in the process of transportation. It is easy to be vaporized; and when the heat load changes frequently, the management can't keep up. When the oxygen removal water temperature is <104 °C, the deoxidation effect is not good. Secondly, this method of deaeration requires high equipment layout, increases capital investment, and is inconvenient in design, installation and operation. In order to achieve the purpose of softening water vaporization in the feed water pump, the oxygen removal method generally requires a high-position configuration of the deaerator, which causes great noise and vibration during use, which causes inconvenience. Thirdly, the self-consumption steam volume of the boiler room is increased, and the effective external steam supply is reduced. Fourth, for small quick-load boilers and low-temperature oxygen removal, thermal deaeration has certain limitations, and it cannot be used for pure hot water boiler rooms. For a boiler that adopts thermal deaeration, when a new boiler is installed, the atmospheric thermal deaerator is installed on the ground, and the dehydrated high-temperature softened water delivery pipe passes through the soft water tank to exchange heat with the water in the soft water tank. Then, it flows to the boiler feed water pump and enters the boiler through the economizer. This improvement can first reduce the vibration and noise of the boiler room, improve the working environment of the boiler room, and reduce the engineering cost of the boiler room. Secondly, through the heat exchange in the soft water tank, the water temperature in the soft water tank is increased, the heat is not wasted, and the temperature of the deaerator inlet water is also reduced, and the time during which the deaerator heats the influent water to the saturation temperature is also shortened. Conducive to achieving the desired oxygen scavenging effect.
2.2 Vacuum deoxidation This is a medium temperature deaeration technology, generally carried out at a temperature of 30 ° C ~ 60 ° C. It can realize the deaeration of water at low temperature (at 60 °C or normal temperature). For steam boilers with large thermal boiler and load fluctuation and poor thermal deaeration, vacuum deaeration can be used to obtain satisfactory oxygen removal effect. Compared with the thermal deaeration technology, its heating conditions have been improved, and the steam consumption of the boiler house has decreased. However, most of the disadvantages of thermal deoxidation still exist, and the high-level arrangement of vacuum deaeration, the operation management jet pump, The requirements for critical equipment such as pressurized pumps are higher than thermal deaeration. The low position also requires a certain height difference, and the operation management requirements for key equipment such as jet pumps and pressure pumps are also high. In addition, heat exchange equipment and circulating water tanks have been added. Vacuum deaeration can utilize low-grade residual heat, can be heated to soften water by jet heater; it can be installed in graded and low position, reliable in deaeration, stable in operation, simple in operation and wide in application range. Since the vigorous development of energy conservation work in China, industrial boilers have been increasing their oxygen consumption by this method.
2.3 Chemical deoxidation (1) Steel scraps remove oxygen, water passes through the steel scrap filter, steel scraps are oxidized, and dissolved oxygen in the water is removed. There are two types of freestanding and attached. The water temperature requirement of this method is greater than 70%, and the temperature is best at 80 to 90 °C. The temperature of 20 ~ 30 °C is the worst oxygen removal effect. The use of steel scraps requires compaction, the tighter the better, the greater the oxygen content in the water, the lower the required flow rate, because the steel dust removal and improvement has not been improved since the application, the oxygen removal effect is not reliable, generally used in the feed water Small boiler rooms with low quality requirements, or as replenishment water for thermal networks, and supplementary deaeration after high-pressure boiler deoxidation, generally only as an auxiliary measure. (2) Sodium sulfite deoxidation, which is an in-furnace dosing method. Because oxygen makes the main corrosive substance of the boiler in the water supply system, it is required to quickly remove oxygen from the feed water. Generally, sodium sulfite is used as an oxygen scavenger, and 2Na2SO3+O2→2Na2SO4 usually requires a larger dosage than the theoretical value. The higher the temperature, the shorter the reaction time and the better the oxygen removal effect. When the pH of the furnace water is 6, the effect is best, and if the pH is increased, the oxygen removal effect is lowered. Adding copper, cobalt, manganese, tin, etc. as a catalyst can improve the oxygen removal effect. The method has low investment, safety and simple operation because of the low cost of sodium sulfite. However, the dosage of this method is not easy to control, and the oxygen removal effect is not reliable and cannot be guaranteed. In addition, it will increase the salt content of the boiler water, resulting in an increase in the amount of sewage and waste of heat, which is uneconomical. Therefore, the method is generally used in a small boiler room and some thermal systems with high water quality requirements as an auxiliary oxygen removal mode. (3) Hydrazine dehydrogenation, this method is currently used as an auxiliary measure after thermal deaeration to achieve complete removal of residual oxygen in the water without increasing the salt content of the furnace water. When the pressure is greater than 6.3Mpa, sodium sulfite is mainly decomposed into highly corrosive sulfur dioxide and hydrogen sulfide. Therefore, for high-pressure boilers, hydrazine is used, and hydrazine reacts with oxygen to form nitrogen and water, which is conducive to hinder the further development of corrosion. Because hydrazine is toxic, it is easy to volatilize and cannot be used for deaeration of drinking water boilers and domestic water boilers. Many boiler plants are being restricted or no longer in use.
2.4 Analyze a relatively advanced technology that has emerged in recent years in deoxygenation and deoxidation. The working principle is understood to be that the oxygen-free gas is strongly mixed with the feed water to be deaerated, and the oxygen dissolved in the water is resolved into the gas. Go, so cycle to make the water supply to deoxidize. Desorption and deoxidation have the following characteristics: 1. The oxygen-removing water does not need pre-heat treatment, so it does not increase the self-consumption of steam in the boiler room; 2. The desorption and deoxidation equipment occupies less land and the metal consumption is small, thus reducing the capital investment; The oxygen removal effect is good. Under normal circumstances, the residual oxygen content after deoxidation can be reduced to 0.05mg / L; 4. The disadvantage of desorption and deoxidation is that the device adjustment is complicated, the pipeline system and the deaerator tank should be sealed. The current analytical deaeration method generally adopts a new analytical deaerator, replaces the original boiler flue gas heating with a heater, and uses activated carbon plus a catalyst as a reducing agent, thereby greatly reducing the equipment footprint, and increasing the partition control within the analysis. The water flows, and the small holes and the holes are added to make the oxygen-containing gas in the water fully escape, achieving a good oxygen removal effect. The desorption and deoxidation equipment is small, easy to manufacture, consumes steel, has low investment, convenient operation, reliable operation, no chemicals, reduces environmental pollution, can remove oxygen at low temperature, and has good oxygen removal effect. At present, it has been widely used in domestic hot water boilers and single-layer industrial boilers. The disadvantage is that only oxygen can be removed from the water and other non-condensable gases can not be removed, and the carbon dioxide content in the water is increased; the water surface of the water tank cannot be sealed, and sometimes the water after deoxidation is in contact with the air to affect the oxygen removal effect. As early as the 1960s, many boilers at home and abroad used this technology extensively, but since the reactor was installed in the flue, it could not adapt to the change of heat load. Therefore, the use of this technology was once limited. By the 1990s, a second generation desorbent deaerator with a centrally located electric heating reactor was developed, which made this technology a great development. In particular, the new desorption deaerator developed by Tsinghua University and the Design and Research Institute of the Ministry of Electrical and Mechanical Engineering has overcome the original deficiencies and shortcomings. The heating furnace is separated from the reactor, and the heating furnace heats the gas from the deaerator and desuperheater. The gas is deoxidized through the reactor, so that the oxygen-containing gas in the deoxygenated water can be fully desorbed, thereby ensuring the reliability of operation and the effect of removing oxygen. And the size and power consumption are smaller than the original equipment. The new desorption system eliminates the need for an oxygen-removing water tank and solves the problem of sealing the original water tank. The operation of several boiler houses proves that the desorption and deaerator is simple in operation, low in investment, reliable in operation and good in effect. However, at the same time, there are many problems that affect the deoxidation factor, only the removal of oxygen, and the inability to remove other gases.
2.5 Resin deoxidation When water passes through the resin layer, the dissolved oxygen of water is reduced from zero valence to negative divalent to form an oxide (copper oxide). After the resin is deactivated, it can be reduced by hydrazine, and Cu 2+ is absorbed by the exchange gene on the resin. In use, it should be noted that the effluent contains trace amounts of strontium and cannot be used as drinking water. The deaerator tank should be insulated from the air, and two deaerator tanks should be provided to ensure continuous supply of deoxidized water. The Y-12 type redox resin deaerator series patented product developed by the 12th Research Institute of the Ministry of Electronics Industry was applied in hot water boilers of Tsinghua University and Beijing No. 3 Machine Tool Plant to obtain residual oxygen of 0.06-0.02 mg/L. Excellent oxygen scavenging effect. It has been promoted and used in small hot water boilers. The steam and hot water produced by the method of deaeration are not allowed to be in contact with drinking water and food, and the investment and land occupation are large, and it is generally not suitable for promotion and application on industrial boilers.
3 Houyu boiler water supply deoxidization methods are various, in order to be efficient, economical, stable and safe operation, it must be combined with the boiler type and actual conditions, according to the boiler's thermal parameters, water quality, tonnage, load changes, economic conditions, etc. Use. For the water supply and deoxidation technology, we must always pay attention to new technologies, new materials, new achievements, dare to explore and improve innovation, and seek ways to improve oxygen removal, reliable operation, simple management, and low investment.

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