Flue gas desulfurization

It is recommended to read \"Flue Gas Desulfurization and Denitration Technologies and Engineering Examples\" written by Zhong Qin.

It is now relatively mature, with lime or ammonia methods being the main ones used

Many environmental protection projects are now using the ammonia-based desulfurization method.

Environmental regulations favor ammonia-based desulfurization; pneumatic lime emulsion desulfurization is also widely used, but it increases dust levels, which is not approved by environmental agencies

This post was last edited by 181128425 on 2009-7-30 21:53. Wet calcium-based desulfurization is used in large power plants; this technology has disadvantages such as a complex system, high requirements for operation and control, high investment costs, large land usage, difficulties in utilizing by-products, high operating expenses, and the risk of secondary pollution. Small power plants used to employ limestone injection inside the furnace for desulfurization, with mediocre results. Now, environmental regulations are stricter; not only are efficient desulfurization devices required, but online monitoring systems connected to the environmental protection agency must also be installed. Our facility uses a relatively advanced ammonia-based desulfurization process. The principle behind this method is to use ammonia (ammonium hydroxide or liquid ammonia) to remove sulfur dioxide from exhaust gases, resulting in the intermediate product sulfurous acid. This unstable sulfurous acid is then oxidized into the stable substance sulfuric acid, which is subsequently crystallized to produce ammonium sulfate, an agricultural fertilizer. The desulfurization efficiency of this method averages over 95%, which is **below** the sulfur dioxide emission standards set by environmental authorities; thus, it truly achieves the goal of turning waste into a valuable resource.

Limestone-gypsum remains the most widely used method. The ammonia method is limited by the source of ammonia and is suitable for power plants located near chemical industry clusters. There are also many magnesium topical products available abroad! Dry-process activation of coke was experimented with in China during the 1950s and 1960s, but it was still foreign countries that first achieved successful industrialization! The dry method using activated coke can not only remove sulfur and sulfur oxides but also eliminate dioxins, and it is possible to recover sulfuric acid as well – it’s quite good. But the costs are high, control is difficult, and the risks are also great. The magnesium method is similar to the lime blast furnace method in that it can recover sulfuric acid, but the cost is very high. The ammonia method is equally difficult to control. All three of the above save water! The limestone-gypsum process requires a large amount of water; if the water balance is not properly controlled, the problem becomes even worse! Fortunately, it has a low cost and is simple to operate, which is why it’s the most widely used!

Given the current situation, and considering the economic crisis, many companies that produce ammonia have reduced their production or even stopped operating. The limitations of this approach become more apparent; the lime-based method for desulfurization is not very effective, and it also causes significant pollution. The dry method using activated carbon represents a promising direction for the future. If it is widely used in China, it will have a minimal impact on the environment, and its economic benefits are also considerable.

The last edit to this post was made by 181128425 on 2009-7-30 at 21:53. In domestic power plants, the limestone-gypsum wet flue gas desulfurization process has been the dominant method since the 1970s. There are many methods such as the ammonium method, the magnesium method, and the sodium method. In my opinion, various methods should be determined based on the specifics of each project, and it’s not possible to generalize regarding their advantages and disadvantages. For example, in the auxiliary power plants of some coking plants, it is entirely possible to make use of coking wastewater by employing the ammonia method ； If the power plant is a coastal power plant, the seawater method can be considered ； If built in the city center, to avoid the impact of wet flue gas rainfall on surrounding residents, semi-dry methods such as circulating fluidized bed should be employed.

1. Overview: The limestone-gypsum flue gas desulfurization technology has been developed over several decades; it is a mature and reliable technique with a wide range of applications. According to available data, this process accounts for over 85% of the market share in this field. Given that the reaction principles are largely similar, this training material summarizes some general rules and design guidelines that are applicable to various limestone-gypsum flue gas desulfurization technologies currently in use in the market, including spray towers, bubble towers, and liquid column towers. 2. Typical system configuration The typical limestone/lime-gypsum wet flue gas desulfurization process is shown in Figure 2-1; the scope of the desulfurization units actually used varies depending on the specific conditions of the project. 3 Reaction Principle 3.1 Absorption Principle The absorption liquid is atomized through nozzles and sprayed into the absorption tower, where it disperses into fine droplets that cover the entire cross-section of the tower. These droplets come into countercurrent contact with the flue gas inside the tower, where mass transfer and absorption reactions occur, and SO2, SO3, HCl, and HF in the flue gas are absorbed. The oxidation and neutralization reactions of the SO2 absorption products take place in the oxidation zone at the bottom of the absorption tower, ultimately resulting in gypsum formation. To maintain a constant pH value of the absorption solution and reduce limestone consumption, limestone is continuously added to the absorption tower. At the same time, the absorbent slurry inside the tower is constantly stirred by mixers, oxidizing air, and the tower’s circulation pump, in order to accelerate the distribution and dissolution of the limestone within the slurry. 3.2 Chemical Processes The chemical processes in the forced oxidation system are described as follows: (1) Absorption reaction The flue gas comes into effective contact with the circulating slurry sprayed from nozzles inside the absorption tower; the circulating slurry absorbs most of the SO2. The reactions are as follows: SO2 + H2O → H2SO3 (dissolution); H2SO3 ⇋ H+ + HSO3- (ionization). Mechanism of the absorption reaction: The absorption reaction is a process of mass transfer and absorption. The absorption of SO2 by water involves the absorption of a gas component with moderate solubility. According to the double-membrane theory, the mass transfer rate is controlled by both the gas-phase mass transfer resistance and the liquid-phase mass transfer resistance. The absorption rate = absorption driving force/absorption coefficient (where the mass transfer resistance is the reciprocal of the absorption coefficient). Measures to enhance the absorption reaction: a) Increase the partial pressure (concentration) of SO2 in the gas phase, thereby increasing the driving force for gas-phase mass transfer. b) Adopt counter-current mass transfer to increase the average mass transfer driving force in the absorption zone. c) Increasing the flow rates of the gas and liquid phases; high Re numbers alter the interface between the gas film and the liquid film, thereby facilitating intense mass transfer. d) Enhanced oxidation accelerates the ionization and oxidation of the dissolved SO2; once sulfurous acid is oxidized, its concentration decreases, which in turn promotes the absorption of SO2. e) Increase the pH value to reduce the reverse ionization process and enhance the driving force for liquid-phase absorption. f) With a constant total absorption coefficient, increasing the gas-liquid contact area and prolonging the contact time, such as by increasing the liquid-to-gas ratio, reducing the droplet size, and adjusting the spacing between the spray layers. g) Maintain a uniform flow field distribution and spray density to improve the efficiency of gas-liquid contact. (2) Oxidation reaction: HSO3‑ + 1/2O2 → HSO4‑. Some of the HSO3‑ is oxidized by the oxygen present in the flue gas in the spray area of the absorption tower, while the remaining HSO3‑ is completely oxidized by the oxidizing air in the reaction tank. The reaction is as follows: HSO4‑ ⇋ H+ + SO42‑. Mechanism of the oxidation reaction: The mechanism of the oxidation reaction is essentially the same as that of the absorption reaction; the difference is that in the oxidation reaction it occurs continuously in the liquid phase, whereas in the absorption reaction it occurs discontinuously in the gas phase. The absorption of O2 by water is the absorption of gas components with low solubility; according to the double-membrane theory, the mass transfer rate is controlled by the mass transfer resistance of the liquid membrane. Measures to enhance the oxidation reaction: a) Lowering the pH value to increase the solubility of oxygen; b) Increasing the excess coefficient of the oxidizing air to raise the oxygen concentration; c) Improving the uniformity of oxygen distribution, reducing the average bubble size, and increasing the gas-liquid contact area. (3) Neutralization reaction: The absorbent slurry is introduced into the absorption tower to neutralize hydrogen ions, thereby maintaining a certain pH value in the absorbent solution. The neutralized slurry is recycled in the absorption tower. The neutralization reactions are as follows: Ca2+ + CO32- + 2H+ + SO42- + H2O → CaSO4·2H2O + CO2↑; 2H+ + CO32- → H2O + CO2↑. The mechanism of neutralization reactions involves the dissolution of limestone, as well as neutralization and crystallization processes. Since limestone is relatively insoluble, the key in this process is to increase its solubility and to ensure that the gypsum produced by the reaction crystallizes as quickly as possible, thereby reducing the supersaturation of gypsum. The neutralization reaction itself is not difficult. Measures to enhance the neutralization reaction: a) Increase the activity of limestone by using limestone of high purity and reducing impurities. b) Refine the limestone particle size to increase the dissolution rate. c) Lower the pH value, increase the solubility of limestone, and improve the utilization rate of limestone. d) Increase the residence time of limestone in the slurry tank. e) Increase the solid concentration of the gypsum slurry to increase the crystallization attachment surface and control the relative saturation of gypsum. f) Increase the solubility of oxygen in the slurry, displace CO2 dissolved in the liquid phase, and enhance the neutralization reaction. (4) Other side effects: Other pollutants in the flue gas, such as SO3, Cl, F, and dust, are absorbed and captured by the circulating slurry. SO3, HCl, and HF react with the limestone in the suspension according to the following reactions: SO3 + H2O → 2H+ + SO42–; CaCO3 + 2HCl → CaCl2 + CO2 + H2O; CaCO3 + 2HF → CaF2 + CO2 + H2O. The impact of side reactions on the desulfurization process and relevant considerations: Desulfurization is a relatively complex reaction process, and some of the side reactions either facilitate the progress of the reaction or hinder it. The following reactions should be taken into account during the design phase: a) Reaction of Mg. The Mg element present in the slurry comes mainly from impurities in the limestone. When the soluble Mg content in the limestone is high (in the form of MgCO3), MgCO3, being more reactive than CaCO3, will participate in the reaction first, which is beneficial for the process. However, an excessive amount of MgCO3 can lead to the formation of large quantities of soluble MgSO3 in the slurry. An elevated concentration of SO42– in the solution results in a reduced driving force for the chemical reaction involved in SO2 absorption, thereby worsening the efficiency of SO2 absorption. On the other hand, an increase in the Mg+ concentration in the absorbent tower slurry leads to an increase in the amount of MgSO4(L) present in the slurry. An increase in SO42- levels in the slurry makes it more difficult to oxidize the suspension within the absorbent tower, which in turn requires a significant increase in the amount of oxidation air used. The principles behind the oxidation reactions are as follows: HSO3‑ + 1/2O2 → HSO4‑ (1) HSO4‑ ⇋ H+ + SO42‑ (2) Since reaction (2) is reversible, from the perspective of chemical kinetics, a high concentration of SO42- hinders the progress of the reaction toward the right. Therefore, spray towers generally control the concentration of Mg+ ions; when it exceeds 5000 ppm, it is necessary to discharge more wastewater. At this point, the control criterion is no longer that CL- should be less than 20,000 ppm. b) The reaction involving AL: AL mainly comes from the fly ash in flue gas. When the concentration of F ions reaches a certain level, soluble AL forms aluminum fluoride complexes (colloidal flocs) which coat the surface of limestone particles, leading to obstruction in the dissolution of limestone. In severe cases, this can result in serious accidents due to a significant deterioration of the reaction process. c) Reaction of Cl: In a closed or nearly closed system, the operation of the FGD process leads to very high concentrations of dissolved chlorides in the absorbent solution, as these are absorbed from the flue gas. These dissolved chlorides result in high concentrations of dissolved calcium, primarily calcium chloride. If such high concentrations of dissolved calcium ions are present in the FGD system, it leads to a reduction in the amount of limestone that can dissolve, and this is due to the \"common ion effect.\" Under this effect, the dissolved calcium from calcium chloride hinders the dissolution of calcium carbonate in the limestone. Maintaining the CL ion concentration between 12,000 and 20,000 ppm is an important factor to ensure the proper progress of the reaction. 4 System Description 4.1 Composition of the FGD System The flue gas desulfurization (FGD) unit employs an efficient limestone/gypsum wet process, and the entire system consists of the following subsystems: (1) SO2 absorption system, (2) flue gas system, (3) limestone slurry preparation system, (4) gypsum dewatering system, (5) water supply and discharge system, (6) wastewater treatment system, (7) compressed air system. 4.2 SO2 Absorption System The flue gas enters the absorption zone of the absorber through the inlet; as it rises, it comes into countercurrent contact with the limestone slurry. As a result, most of the pollutants contained in the flue gas are removed by reacting with the suspended limestone particles in the slurry. The treated, clean flue gas then passes through a demister to remove water droplets before entering the flue. The material of the absorption tower body is carbon steel lined with glass flake. The flue gas inlet section of the absorption tower is made of corrosion-resistant and high-temperature resistant alloy. The upward flow velocity of the flue gas in the absorption tower is 3.2–4 m/s. The tower is equipped with spray layers; each set of spray layers consists of a main slurry distribution pipe with connecting branch pipes and nozzles. The layout of the spray components and nozzles is designed to evenly cover the cross-section of the upper region of the absorption tower. The spray system is designed on a unit basis, with each spray layer equipped with an absorption tower slurry circulation pump connected to it. Each absorption tower is equipped with multiple slurry circulation pumps. The number of slurry circulation pumps in operation is determined based on changes in the boiler load and the requirements for the flow rate of the absorption slurry. While ensuring the desired absorption efficiency, the most energy-efficient pump operation mode can be selected to save energy consumption. The recycled slurry that has absorbed SO2 falls into the reaction tank of the absorption tower. The reaction tank of the absorption tower is equipped with multiple mixers. The oxidation blower forces oxidizing air into the reaction tank. The oxidizing air distribution system uses a nozzle type; the oxidizing air is injected through distribution pipes onto the pressure side of the mixer blades, where it is dispersed into fine bubbles by the pressure and shear forces generated by the mixer, thereby being evenly distributed throughout the slurry. A portion of the HSO3‑ is oxidized by the oxygen in the flue gas in the spray area of the absorption tower, while the remaining HSO3‑ is completely oxidized by the oxidizing air in the reaction tank. The absorbent (limestone) slurry is introduced into the absorption tower to neutralize hydrogen ions, thereby maintaining a certain pH level in the absorbent solution. The neutralized slurry circulates in the absorption tower. The absorption tower discharge pump continuously transfers the absorption slurry from the absorption tower to the gypsum dewatering system. The flow rate of the discharged slurry is controlled via a slurry discharge control valve to maintain the concentration of the circulating slurry at approximately 8–25 wt%. The flue gas after desulfurization passes through a demister to reduce the amount of water droplets it contains; the amount of water droplets at the outlet of the demister is not more than 75 mg/Nm3. The two-stage demisters are arranged in a traditional top-mounted configuration at the top of the absorption tower or outside the tower; they are made of polypropylene and have a Z-shaped design. Both stages of demisters are washed with process water. The rinsing process is automatically completed through program control. The flue side plates and bottom plate at the inlet of the absorption tower are equipped with a process water flushing system, and the flushing is carried out automatically on a periodic basis. The purpose of flushing is to prevent the gypsum slurry ejected from the nozzle from drying and sticking inside the inlet flue. An emergency cooling system is installed in the flue at the inlet of the absorption tower, with the emergency cooling water supplied by the process pump. This system is activated when the flue gas at the inlet of the absorption tower becomes excessively hot due to an accident involving the equipment upstream of the absorption tower, and the bypass dampers fail to open in time, or when all of the circulation pumps of the absorption tower are shut down. 4.3 Flue Gas System: The hot flue gas coming from the boiler is pressurized by a booster fan and then enters the cooling side of the flue gas heat exchanger (GGH). After being cooled by the GGH, the flue gas enters the absorption tower, flowing upward through the spray layer where it is cooled to saturation temperature; meanwhile, the SO2 in the flue gas is absorbed by the limestone slurry. The flue gas free of SOX and other pollutants is heated to over 80°C by a GGH and then discharged through the chimney. GGH uses the heat carried by the hot flue gas to heat the cold, cleaned flue gas coming out of the absorption tower. Under design conditions and without an additional heat source, the GGH can raise the temperature of the exhaust gas to over 80°C. The pressure loss of flue gas passing through the GGH is maintained by an online cleaning system. During normal operation, the cleaning system requires steam soot blowing 3 times a day. In addition, the system is equipped with an online high-pressure water cleaning device (used once approximately per month). A telescopic cleaning device is installed at the flue outlet where the hot flue gas enters the GGH, for routine soot blowing and online water flushing. Cleaning devices are equipped with separate, telescopic spear-shaped tubes and driving mechanisms with separate auxiliary steam and water nozzles. The GGH is equipped with an online flushing water pump, which provides high-pressure flushing water for online cleaning. The automatic soot blowing system ensures that the heating surfaces of the GGH are not clogged, maintaining the promised net flue gas exit temperature. Automatic soot blower control. After the GGH is shut down, the heat exchange elements can be cleaned using a low-pressure water cleaning device. This low-pressure water cleaning device is used twice a year. The two fixed water flushing devices on each GGH are used for offline flushing. Each fixed water cleaning device is equipped with a straight tube fitted with nozzles, from which flushing water is evenly sprayed onto the heat exchange surface through the nozzles spaced at regular intervals. A sealing system is installed to ensure that the air leakage rate of the GGH is less than 1. The flue is equipped with a baffle system to facilitate normal operation of the FGD system as well as bypass operation in case of an accident. The baffle system for each FGD unit includes one baffle for the raw flue gas at the FGD inlet, one baffle for the clean flue gas at the FGD outlet, and one baffle for the bypass flue gas; these baffles are of the double louver type. During normal operation, the FGD inlet and outlet dampers are open, while the bypass dampers are closed. In the event of a fault, open the flue gas bypass damper doors and close the FGD inlet and outlet dampers; the flue gas then passes through the bypass duct to bypass the FGD system and is discharged directly into the chimney. All baffles are equipped with a sealing system to ensure zero leakage. For the sealed air system, there are two sealed air fans with a capacity of 100 each (one as a spare) along with a secondary electric heater, ensuring that the heating temperature remains at least 70°C. The flue includes the necessary flue gas passages, flushing and discharge funnels, expansion joints, flanges, guide plates, gasket/bolt materials, and accessories. Under the BMCR operating condition, the gas flow velocity at any position in the flue does not exceed 15 m/s. The flue is equipped with appropriate sampling interfaces, testing interfaces, and manholes. For the FGD system of each boiler, a booster fan (BUF) with 100% BMCR flue gas flow rate is installed in the dry flue gas section upstream of the absorption tower. The booster fan is an axial flow fan with adjustable moving blades. Including electric motors, sealed air systems, etc. 4.4 Limestone Slurry Preparation and Supply System: The limestone delivered by truck is unloaded into the hoppers in the limestone slurry preparation area, and then conveyed to the limestone storage tank using a bucket elevator (the capacity of this tank is designed based on the required amount of limestone). From the outlet of the limestone storage tank, the material is fed into the wet limestone grinder via a belt weigh feeder. The ground limestone enters the grinder slurry circulation tank, from where it is pumped to the limestone cyclone by a slurry circulation pump. The qualified limestone slurry flows out of the cyclone’s overflow port and into the limestone slurry tank, while the unqualified slurry returns to the grinder’s inlet for further grinding. The system is equipped with a limestone slurry tank, and each tower has 2 limestone slurry supply pumps. The absorption tower is equipped with a limestone slurry delivery pipe, through which the limestone slurry is conveyed to the absorption tower. From each delivery pipe, a recirculation pipe branches off and returns to the limestone slurry tank to prevent the slurry from settling within the pipes. The amount of limestone slurry required for desulfurization is controlled jointly by the boiler load, the SO2 concentration in the flue gas, and the Ca/S ratio, while the amount of limestone slurry that needs to be prepared is controlled by the liquid level in the limestone slurry tank. The concentration of the slurry is measured using a densitometer, and this measurement is used as feedback to determine the number of cyclones to be used. 4.5 Gypsum Dewatering System: The 25 wt% concentrated gypsum slurry generated by the unit’s FGD is pumped from the bottom of the absorption tower, using gypsum slurry discharge pumps installed there (two such pumps per tower, one in operation and one as a backup), to the gypsum slurry cyclone. The system is equipped with 2 sets of gypsum cyclone stations; the underflow from these 2 sets of cyclone stations flows automatically into 2 vacuum belt dryers. The designed filtration capacity of each vacuum belt dehydrator is 75% of the total amount of gypsum from the desulfurization systems of 2 units. The gypsum dewatering system includes the following equipment: — Gypsum cyclone station — Vacuum belt filter — Filter cloth washing tank — Filter cloth washing water pump — Filtrate tank and agitator — Filtrate water pump — Gypsum cake washing water pump — Waste water cyclone station feed tank — Waste water cyclone station feed pump — Waste water cyclone station — Gypsum conveyor — Gypsum storage area (1) Gypsum cyclone station and waste water cyclone station: The slurry at the bottom of the cyclone station, which has been concentrated to a concentration of about 55%, flows naturally to the vacuum belt filter; the overflow from the cyclone station flows naturally to the waste water cyclone station feed tank. Part of this overflow is pumped to the waste water cyclone station using the feed pump, while the remaining part overflows into the filtrate tank. The overflow from the wastewater cyclone station flows into the wastewater tank, and is pumped to the wastewater treatment system via a wastewater pump, while the underflow goes into the filtrate tank. (2) Vacuum belt dehydrators: 2 sets of dehydration systems are installed, with a capacity sufficient to handle 75% of the total gypsum output from the 2 desulfurization units. The vacuum belt dryer and vacuum system are designed for this capacity. The underflow slurry from the gypsum cyclone station is dewatered by a vacuum belt dryer to a concentration of 90% solids and 10% moisture; the dewatered gypsum is then washed to reduce the Cl‑ concentration in it. The filtrate enters the filtrate water recovery tank. The dehydrated gypsum is conveyed to the gypsum storage area via a gypsum conveyor belt for stacking. Through optimized design, the gypsum warehouse facilitates loading of gypsum transport vehicles without causing pollution to the factory environment. Industrial water is supplied as sealing water to vacuum pumps, and then collected in the filter cloth washing tank for washing the filter cloths. The water used for washing the filter cloths is collected in the filter cake washing tank for washing the gypsum filter cakes. The filtrate, wash water, etc., collected in the filtrate tank are pumped by a filtrate pump to the limestone slurry preparation system and the absorption tower. 4.6 Water Supply and Discharge Systems 4.6.1 Water Supply System The water source drawn from the power plant’s water supply system is supplied to the desulfurization island to meet its needs for industrial and process water. The main users of industrial water are: demister flushing water and vacuum pump sealing water. The cooling water from the cooling equipment is routed to the absorption tower’s drainage pit for reuse. The main users of process water include (but are not limited to): · Water for limestone slurry preparation ； ·Flue gas heat exchanger flushing water ； ·Washing water for all slurry transfer equipment, transfer pipelines, and storage tanks. Process water/industrial water enters the process water/industrial water tanks on the island, and is then sent to each water usage point in the FGD area via the process water/industrial water pumps and the demister flushing pumps. The associated piping within the system, along with its measuring and control instruments. 4.6.2 Emission System: A common accident slurry tank is provided within the FGD island. The capacity of this accident slurry tank should be sufficient to meet the requirements for draining the slurry from a single absorption tower during maintenance, as well as for other slurry drainage tasks; it also serves as a source of gypsum seeds when the absorption tower is restarted. When the slurry tank of the absorption tower needs to be emptied for maintenance, the gypsum slurry from the absorption tower is transferred to the emergency slurry tank, where it can be used as seeds for the next FGD startup. The accident slurry tank is equipped with 1 slurry return pump (to send the slurry back to the absorption tower). During shutdown, the slurry pipes and slurry pumps of the FGD unit need to be flushed; the flushing water is collected in sump tanks located in various areas, and then pumped to the accident slurry tank or the absorber tank. 4.7 Compressed Air System: The gas required for the instruments on the desulfurization island, as well as for miscellaneous purposes, is supplied by a compressed air system installed on the island, with a pressure of around 0.85 Mpa. Gas storage tanks with sufficient capacity should be installed as required; the pressure stabilization tanks for instruments and the storage tanks for miscellaneous uses should be set up separately. The air supply capacity of the air storage tank shall be sufficient to ensure that, in the event that all air compressors stop operating, the stored air in the tank can meet the air consumption required to keep all desulfurization control equipment functioning for at least 15 minutes. Special pressure-stabilizing air storage tanks should be installed for pneumatic protection equipment and air consumption points located away from the air compressor room. The operating pressure of the gas storage tank is taken as 0.8 MPa, with the minimum pressure not falling below 0.6 MPa.

Our company has also recently installed a catalytic flue gas desulfurization unit that uses the wet scrubbing process developed by the American company BELCO; it is scheduled to come online by the end of this year.