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In recent years, with the rapid economic development, a large number of boilers that use raw coal as fuel have been installed. The air pollutants emitted by these coal-fired boilers cause significant harm to the surrounding environment. However, the main way to reduce or minimize the pollutants released by such boilers is through various smoke removal devices attached to 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. As of August 1, 1992, **the maximum allowable emission concentrations for boilers were increased from 200 mg/N.m3, 400 mg/N.m3, and 600 mg/N.m3 in Zone 1, Zone 2, and Zone 3 respectively, to 200 mg/N.m3, 300 mg/N.m3, and 400 mg/N.m3. For newly installed or replaced boilers: the maximum allowable dust emission concentrations in Zone 1, Zone 2, and Zone 3 are 100 mg/N.m3, 250 mg/N.m3, and 350 mg/N.m3 respectively. As a result, the standards for control become higher, which necessarily leads to the development of a range of dust collectors that meet current environmental protection standards in order to achieve smoke and dust removal. The dust collectors installed in boilers can be divided into two main categories: ① Dry dust collectors: including gravity sedimentation chambers, inertial dust collectors, electrostatic dust collectors, bag dust collectors, and cyclone dust collectors. ②Wet dust collectors: include spray towers, impact dust collectors, Venturi washers, 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 from the outlet on the other side. (As shown in the figure below) 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 motion to separate the dust from the gas. Generally, some kind 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 a sudden change in direction. 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 as the first stage in multi-stage purification processes, in concentration equipment used for purification, or in combination with other purification devices. Louver-type inertial dust collectors are the most commonly used. (As shown in the figure below), 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 complete system. 3. Cyclone Separator – Working principle: The working principle of a cyclone dust collector is as shown in the figure below; dusty gas is introduced from the inlet between the shell of the dust collector and the exhaust pipe, resulting in a rotating downward outward flow. Dust suspended in the outward rotating current is moved toward the wall of the device under the effect of centrifugal force, and then moves to the lower part of the dust collector along with the outward rotating current, where it is discharged through the dust outlet. 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 large and heavy settle due to gravity, which 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 pronounced 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; however, 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 Effect of thermal motion – Light and small dust particles (under 1 micron), carried by air currents, move very closely along the flow lines of those currents and are able to pass around 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; they can capture even fine dust at a rate of over 99%, and this efficiency remains relatively stable. A schematic diagram of a bag filter is shown below: 5. Electrostatic dust removal. Working principle of an electrostatic dust collector: When gas containing dust particles passes through the high-voltage electric field formed between the cathode wire (also known as the corona electrode), which is connected to a high-voltage direct current power supply, and the 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 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. The schematic is as follows: 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-Advantages of the electrostatic precipitator as a dust removal device: (1) 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 required purification efficiency. ⑵ The resistance loss is low, generally below 20 millimeters of water column. Compared with cyclone dust collectors, even when taking into account the power consumption of the power supply units and shaking mechanisms, the total power consumption remains relatively low. ⑶ They can operate at high temperatures; for example, SHWB-type circuit dust collectors are ideally suitable for operation at 250°C, while other types can handle temperatures of 350–400°C or even higher. ⑷ The range of processing gases is wide. ⑸ Fully automatic operation control can be achieved. Disadvantages of electrostatic precipitators: (1) The equipment is relatively complex, requiring a high level of skill for 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 influenced by operating conditions such as gas temperature and humidity. For the same type of dust, different results are obtained when it is used under varying temperatures and humidities. Some dusts perform well at certain temperatures and humidities, but at other temperatures and humidities, changes in the dust’s electrical resistance make it almost impossible to use an electrostatic precipitator. ⑷ The initial investment is high, and horizontal electrostatic precipitators require a large amount 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 filter dust from high-temperature gases, and gravel layers (particle layer dust collectors) or fiber layers (bag dust collectors), which are used to purify gases, are based on the same filtration theory. The filter elements of ceramic filters currently use high-density materials, and the ceramic filter elements produced are mainly of three types: rod type, tubular 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 reaches the end of the longer channel where the clean gas is present; one end of the clean gas channel is sealed so that the clean gas can flow into the clean gas collection box. The dust particles captured in the short channels are removed regularly 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, inert (louver) water film, and centrifugal water film dust collectors. Below is a schematic diagram of a CLS-type water film dust remover: The dust-containing gas is introduced tangentially from the bottom of the cylinder, rising while rotating; the dust particles are separated by centrifugal force and thrown against the inner wall of the cylinder, where they are captured by the water film flowing on that inner 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 introduces 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. (end)