Dust collector
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Dust collectors are classified into two types based on whether liquid is involved in the dust removal process: dry dust collectors that do not use liquid, and wet dust collectors that use liquid. (II) The main components of the dust collector: the inlet through which the dust-containing gas enters the dust collector ； The dust removal space used for solid-gas separation, or dust removal chamber ； Exclude the dust discharge port for the dust separated and the outlet for the relatively clean gas after dust removal. (III) Dust removal process 1. Capture and separation process ① Capture and displacement stage. Essentially, it is the stage of dust concentration. Dust that is uniformly mixed or suspended in a carrier medium enters the dust removal chamber of the dust collector. Due to the action of external forces, the dust is pushed toward the separation interface; as the dust moves toward this interface, its concentration increases, thereby preparing further for the solid-gas separation. ②Separation phase. When a high-concentration dust flow moves toward the separation interface, two mechanisms come into play: first, the capacity of the carrier medium to transport dust gradually reaches its limit; in terms of the behavior of the dust – whether it remains suspended or settles – settling becomes the dominant trend, and through this settlement the dust is separated from the carrier medium ； Secondly, in high-concentration dust flows, the tendency of dust particles to diffuse or aggregate is toward aggregation; the particles can aggregate with each other as well as on solid interfaces. 2. The dust removal process refers to the process in which the separated dust is discharged through the dust outlet after passing through the separation interface. 3. The process by which the relatively purified airflow, after dust removal during the exhaust process, is discharged from the exhaust outlet. Single-cylinder cyclone dust collector: 1. The different inertial centrifugal forces generated by the airflow in the single-cylinder cyclone on dust particles and air enable the separation of these particles from the airflow. The structure is shown in Figure 5-4. 2. Working principle: The dust-laden airflow enters the cylindrical chamber through the inlet pipe at a speed of 12–25 m/s in a tangential direction, and then moves downward in a spiral pattern between the outer cylinder and the central exhaust pipe. Incentrifugal force is generated during the rotation. The dust particles are influenced by the airflow and rotate downward within it ； In the other direction, it is affected by centrifugal force and gradually spreads outward toward the cylinder wall. Ultimately, it collides with the inner wall of the outer cylinder, slides down along this wall, is collected at the ash discharge port at the middle bottom, and is discharged from there. Due to their low mass, gases are hardly affected by centrifugal force; they move toward the center of the dust collector as the cone shape narrows, and under the influence of the resistance at the bottom, they rise upward, forming an upward swirling current that is discharged from the upper end of the exhaust pipe, thereby achieving dust removal. 3. Features: Simple structure and easy maintenance. III) The water film dust collector uses spraying or other methods to create a thin layer of water film on the inner wall of the collector, effectively preventing secondary dust generation caused by the rebounding and erosion of dust against the wall. (1) Vertical swirl water film dust collector: The air intake for a vertical swirl water wax dust collector can be from the tangent direction or from the center, and rotational motion is achieved through guide vanes. Water spraying can be carried out in ways such as surrounding spraying, central spraying, and upper-peripheral spraying. Figures 5-6. The dust-laden gas enters the dust collector tangentially from the lower part of the cylinder and rises in a rotational motion. The dust separated by centrifugal force is flung against the wall of the device, absorbed by the water film layer, and carried away with the contaminated water; the purified gas is discharged from the upper part. (2) The horizontal rotary water film dust collector is also known as a water drum dust collector or a rotary cylinder water film dust collector; its structure is shown in Figure 5-7. It consists of components such as a horizontally placed obovate shell, an inner cylinder, spiral guide vanes, and a water tank. The spiral blades are wound around the inner tube and fixed to the inner wall of the casing, thereby dividing the gap between the casing and the inner tube into a spiral-shaped channel that connects the air inlet and the air outlet. Dust removal process: The dust-laden airflow is introduced tangentially at one end of the dust collector; after hitting the water surface, it flows within the spiral channel. The airflow impacts the water at the bottom of the housing, creating splashes and mist that move along with the airflow, thereby forming a water film on the inner wall of the housing at a speed of 3–5 m/s. When water droplets, water films, and dust come into contact, the dust particles are captured. Under the effect of centrifugal force, both dust particles and water mist are flung onto the water film on the inner wall of the casing, and then flow into the lower water tank. (IV) Bag filters: Bag filters are filtration-based dust removal devices that use organic or inorganic fiber filters to remove dust from gases. The structure of the pulse jet cleaning bag dust collector is shown in Figure 5-8. It consists of an upper box, a middle box, a lower box, and control valves. Dust removal process: The dust-containing gas enters the middle chamber through the inlet, passes from outside the filter bags into their interior; the dust is trapped on the surface outside the filter bags, while the purified air enters the bags. From there, it moves upward to the upper chamber and is finally discharged through the exhaust pipe. Advantages of the electrostatic dust collector: ① High separation efficiency, enabling effective removal of fine particles ； ②Large gas handling volume with low resistance ； ③Suitable for high temperatures and corrosive gases ； ④Low operating costs. 2. Disadvantages: ① High investment costs, large-sized equipment, and large floor space required ； ②High requirements for equipment manufacturing, installation, and maintenance. ③ It is relatively sensitive to the properties of dust. (1) Separation mechanism of electrostatic precipitators 1. Gas ionization process: If high-voltage direct current is applied between two specific electrodes, an electric field is created; when the voltage across this electrode system exceeds the critical value, the gas is ionized, resulting in corona discharge. The electric field strength is very high within a very small area around the cathode, sufficient to ionize the gas. After corona appears, two regions are formed within the electric field. The first is the corona region near the electrode, which lies within a range of 2–3 mm from the surface of the electrode. In this region, the sufficiently high electric field strength at the surface of the cathode causes the gas to ionize, generating a large number of positive ions and electrons. If a negative voltage is applied to the electrode at this time, negative corona discharge occurs; electrons move toward the positive electrode while positive ions move toward the electrode itself. The other region is the corona outer zone, which occupies most of the space between the electrodes; no gas ionization occurs in this zone. 2. The charging process of dust particles occurs when ions or electrons generated in the corona region enter the area outside the corona and collide with neutral molecules, thereby charging the dust particles. The magnitude of the charge is related to factors such as particle size, electric field strength, thermal energy of the ions, and residence time. It is generally believed that there are two mechanisms for dust particle charging: electric field charging and diffusion charging. The former is due to ions colliding with dust particles under the influence of an electric field, thereby adhering to the dust particles and acquiring a charge. The latter is due to the irregular thermal motion of ions, gas diffusion, and collisions and adhesion with dust particles, which result in the charging of the dust particles. 3. During the dust collection process, once the dust particles are charged, they move under the influence of an electric field in the direction dictated by the polarity of their charge, toward the electrode with the opposite polarity, where they then deposit on the surface of that electrode. The velocity of dust particles moving perpendicular to the electrodes is usually referred to as the drive velocity. The magnitude of the driving velocity is related to factors such as the charge on the dust particles, the electric field velocity, and the properties of the gas. When negatively charged dust particles come into contact with the collection electrode, they immediately lose their charge and become neutral particles that adhere to the surface of the electrode. Then, the vibration device causes the electrodes to vibrate, causing dust particles to fall off the electrode surface and into the ash collection hopper at the bottom of the electrostatic precipitator, where they are continuously removed.
In recent years, with the rapid economic development, the number of boilers that use raw coal as fuel has increased significantly. The air pollutants emitted by these coal-fired boilers cause serious harm to the surrounding environment. However, the main way to reduce or minimize the pollutants emitted by such boilers is through various smoke removal devices installed alongside them. The performance and efficiency of these dust collectors are key factors determining the extent of damage that a boiler can cause to the surrounding environment. Dust collectors can be divided into two main categories: ① Dry dust collectors: including gravity settling chambers, inertial dust collectors, electrostatic dust collectors, bag filters, and cyclone dust collectors. ②Wet dust collectors: include spray towers, impact dust collectors, Venturi scrubbers, foam dust collectors, and water film dust collectors, among others. Currently, the most commonly used ones are cyclone separators, electrostatic precipitators, and bag filters. The following is a brief introduction to various dust collectors: 1. Gravity dust removal – A purification device that utilizes the difference in specific gravity between dust and gas, allowing the dust to settle naturally from the gas due to its own gravity; it is commonly referred to as a sedimentation chamber or settling chamber. It is a relatively primitive purification device with a simple structure, large size, low resistance, easy maintenance, and low efficiency; it can only be used for rough purification. The working principle of the gravity dust settling chamber is as shown in the figure below: Dust-laden gas enters the settling chamber from one side at a uniform horizontal velocity V, while the dust particles settle downward at a settling velocity V. After operating for time t, the dust particles settle to the bottom of the chamber. The purified gas is discharged through an outlet on the other side. 2. Inertial dust removal — An inertial dust collector is also known as an inert dust remover. Its principle is to use the difference in inertial forces between dust and gas during movement to separate the dust from the gas. Generally, some form of obstacle is placed in front of the dusty airflow to cause a sharp change in the direction of the airflow. At this point, due to the greater inertial force acting on the dust particles compared to that on the gas, these particles separate from the airflow and are removed, while the purified gas is discharged after changing direction rapidly. The following figures show several common types of bag filters. This type of dust collector has a simple structure and low resistance (10–80 millimeters of water column); its purification efficiency is also low (40–80%). It is commonly used in the first stage of multi-stage purification processes, as well as in concentration equipment during purification or in combination with other purification devices. Louver-type inertial dust collectors are the most commonly used. (It is suitable for purifying air containing non-sticky, non-fibrous dusts, and is usually used in combination with other types of dust collectors to form a system. 3. Cyclone Separator – Working principle: The working principle of a cyclone dust collector is as shown in the figure below; the dust-laden gas is introduced between the shell of the dust collector and the exhaust pipe, resulting in a rotating downward outward flow.) Dust suspended in the outward swirling flow is moved toward the wall of the device under the effect of centrifugal force, and then carried by the outward swirling flow to the lower part of the dust collector, where it is discharged through the dust outlet holes. The purified gas forms an upward swirling flow and is discharged through the exhaust pipe. Application scope and features: Cyclone dust collectors are suitable for purifying dry dust that is non-sticky and non-fibrous, with particle sizes greater than 5~10 microns. It is a purification device with a simple structure, easy operation, high temperature resistance, low equipment costs, and low resistance (80–160 millimeters of water column). Cyclone dust collectors are the most widely used type of purification device. 4. Bag dust removal technology – Working principle: (1) Gravity sedimentation – When dust-laden gas enters the bag dust collector, dust particles that are larger in size and have a higher specific gravity settle downward due to gravity; this is exactly the same mechanism as that in a sedimentation chamber. ⑵ Screening effect – When the particle diameter of the dust is larger than the gaps between the fibers of the filter media or the gaps between dust particles on the filter media, the dust is retained as the air flow passes through; this is known as the screening effect. This effect becomes more significant as more dust accumulates on the filter media. ⑶ Effect of inertial force – As the airflow passes through the filter media, it can flow around the fibers, while larger dust particles, due to the effect of inertial force, continue to move in their original direction and thus collide with the filter media and get trapped. ⑷ Thermal motion effect – Light and small dust particles (under 1 micron), carried by air currents, move very closely along the flow lines of the air and are able to pass around the fibers. However, after being struck by gas molecules in thermal motion (i.e., Brownian motion), they change their original direction of movement, which increases the chances of contact between the dust and fibers, allowing the dust to be captured. The finer the diameter of the filter media fibers, the smaller the porosity, and thus the higher the capture efficiency, which is more conducive to dust removal. Bag filters have been widely used in various industrial sectors for a long time to capture non-adhesive, non-fibrous industrial dusts and vapors, with the ability to capture dust particles as small as 0.1 microns. However, when using it to treat gases containing water vapor, dew formation should be avoided. Bag filters have a very high purification efficiency; even their ability to capture fine dust particles can reach over 99%, and their efficiency is quite high. 5. Electrostatic dust removal: The working principle of an electrostatic dust collector is as follows – When gas containing dust particles passes through the high-voltage electric field formed between a cathode wire (also known as a corona electrode) connected to a high-voltage direct current power supply and a grounded anode plate, corona discharge occurs at the cathode, causing the gas to be ionized. The negatively charged gas ions then move toward the anode plate under the influence of the electric field force; during this movement, they collide with the dust particles, charging those particles with negative electricity. The now-charged dust particles also move toward the anode under the influence of the electric field force. Upon reaching the anode, they release the electrons they are carrying, and the dust particles settle on the anode plate, while the purified gas is discharged from the dust collector. Based on the common types of electrostatic precipitators used in China, they can be roughly classified into the following categories: according to the direction of airflow, they are divided into vertical and horizontal types; according to the type of collection electrode, they are divided into plate-type and tube-type; and according to the method used for removing dust from the collection electrode plates, they are classified as dry or wet types. 1-Anode ； 2-Cathode ； 3-Cathode support 4-Anode upper support ； 5-Insulated support ； 6-Quartz insulating tube ； 7-Cathode suspension tube ； 8-Cathode support frame ； 9-Top Plate ； 10-Cathode Rapping Device ； 11-Anode Rapping Device ； 12-Remove cathode from stand ； 13-Anode suspension weight ； 14-Shell 15-First distribution plate at the inlet ； 16-Second inlet distribution plate 17-Outlet distribution plate ； 18- Ash removal device: Advantages of electrostatic precipitators ⑴ High purification efficiency, capable of collecting fine dust particles larger than 0.01 microns. In the design, different operating parameters can be used to achieve the desired purification efficiency. ⑵ The resistance loss is low, generally below 20 millimeters of water column. Compared to cyclone dust collectors, even when taking into account the power consumption of the power supply units and the shaking mechanisms, the total power consumption remains relatively low. ⑶ They allow for high operating temperatures; for example, SHWB-type circuit dust collectors are ideally suitable for operating at 250°C, while other types can handle temperatures of 350–400°C or even higher. ⑷ The range of process gases is large. ⑸ Fully automatic operation control can be achieved. Disadvantages of electrostatic precipitators: (1) The equipment is relatively complex, requiring a high level of skill in its transportation, installation, and maintenance. ⑵ There are certain requirements regarding the specific resistance of dust, so there is a certain degree of selectivity with regard to dust; it is not possible to achieve a high level of purification efficiency for all types of dust. ⑶ It is greatly affected by operating conditions such as gas temperature. For the same type of dust, different results are obtained when it is used under varying temperatures and humidity levels. Some dusts perform well at certain temperatures and humidity levels, but at other temperatures and humidity levels, changes in the dust’s electrical resistance make it almost impossible to use electrostatic precipitators with them. ⑷ It requires a large initial investment, and horizontal electrostatic precipitators take up a lot of space. ⑸ At present, in some enterprises, the practical performance does not meet the design requirements. 6. High-temperature ceramic dust collectors: For coal-fired combined cycle power generation systems (IGCC), it is very important to develop high-temperature gas purification systems that can meet both the requirements of gas turbines and environmental protection standards; this is one of the key technologies for the true commercialization of coal-fired combined cycle power generation technology. High-temperature ceramic filters are currently widely regarded as the most promising high-temperature dust removal devices. Both ceramic filters, which are used to filter dust from high-temperature gases, and gravel layers (granular filter) or fiber layers (bag filter), which are used to purify gases, are based on the same filtration principle. The filter elements of ceramic filters currently use high-density materials, and the ceramic filter elements produced are mainly of three types: rod type, tube type, and cross-flow type. The figure below shows a cross-flow ceramic filter element, which is composed of thin, porous ceramic plates that, through sintering, form a ribbed structure with channels. The dust-containing gas enters the filter from the end of the short channel, and after being filtered in each channel, it moves to the end of the longer channel where the clean gas is present. One end of the clean gas channel is sealed, allowing the clean gas to flow into the clean gas collection box. The dust particles captured in the short channels are removed periodically through reverse pulse air currents. 7. As an example of a wet dust collector, a water film dust collector will be introduced below: It utilizes dust-laden gas to impact a water film created on the inner wall of the dust collector or on other special components by certain methods; the dust is captured by this water film, thereby purifying the gas. Such purification equipment is called a water film dust collector. These include various types such as impact water film dust collectors, inert (louver) water film dust collectors, and centrifugal water film dust collectors. The dust-containing gas is introduced tangentially from the lower part of the cylinder, rising in a rotational motion; the dust particles are separated due to centrifugal force and thrown against the inner wall of the cylinder, where they are captured by the water film flowing along that wall. They then move with the water flow to the bottom cone and are discharged through the dust outlet. The formation of the water film layer is achieved by several nozzles located at the upper part of the cylinder, which spray water tangentially against the wall of the vessel. In this way, a thin layer of water flowing downward in a rotational motion always covers the inner wall of the cylinder, thereby improving the dust removal efficiency. This type of wet dust collector has a simple structure, requires less metal, and consumes less water. Its disadvantages are its large height, difficulty in installation, and water leakage observed during actual operation. The above describes several common dust removal devices used in engineering. In practice, the choice of which device to use should be determined based on their respective advantages and disadvantages as well as the actual circumstances. Currently, most large power plants use electrostatic precipitators or bag filters. 1. Dust removal efficiency: Dust removal efficiency refers to the ratio of the amount of dust captured by the dust collector to the amount of dust that enters the dust collector. Based on the overall dust removal efficiency, dust collectors can be classified into: low-efficiency dust collectors (50–80%), medium-efficiency dust collectors (80–95%), and high-efficiency dust collectors (over 95%). 2. Dust removal resistance: Resistance refers to the pressure loss when air flows through the dust collector. Based on the level of resistance, dust collectors can be classified as follows: low-resistance dust collectors (ΔP); these can achieve approximately 80% desulfurization and up to 95% dust removal efficiency. The price of WMC marble water film desulfurization dust collectors: such collectors are generally used for treating flue gas from industrial boilers, and their price depends on the volume of flue gas and the specifications required by the user. It usually ranges from tens of thousands to several million yuan; for example, a 2-ton boiler requires a cost of around 50,000 yuan, while a 4-ton boiler costs around 70,000 yuan. This dust collector belongs to the category of wet dust collectors and is often used in situations where high dust removal and desulfurization requirements do not exist. Its actual dust removal efficiency is between 80–90%, while the desulfurization efficiency is around 80%. In addition, supporting facilities such as sedimentation tanks and alkali solution tanks are required. Due to the limitations of the device itself, it is not recommended for use in applications requiring online monitoring or in equipment such as manual-fired furnaces with high flue gas volumes. This device is an obsolete product, but due to its low cost, it still has a certain market share at present. There is also a semi-dry dust removal and desulfurization system that combines atomized desulfurization with bag filter dust removal; it can achieve a dust removal efficiency of over 99% and a desulfurization efficiency of around 90%. Moreover, no sedimentation tanks are required, which reduces water consumption. It is considered one of the more advanced dust removal and desulfurization systems available today. The cyclone dust collector features a simple structure, small size, no need for special auxiliary equipment, low cost, moderate resistance, no moving parts inside, and easy operation and maintenance. Cyclone dust collectors are generally used to capture particles larger than 5–15 microns. Their dust removal efficiency can reach over 80%; improved versions of these cyclone dust collectors available in recent years can achieve an efficiency of over 85%. The disadvantage of cyclone dust collectors is their low efficiency in capturing particles smaller than 5 microns. Article source: China EIA Network. This post was last edited by Welcome on 2009-3-30 20:00.]
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