Introduction to the technology of Harbin Boiler circulating fluidized bed boilers

Thread Content

Introduction to the technology of Harbin Boiler Group’s circulating fluidized bed boilers: The technology behind these boilers stems primarily from technical cooperation with foreign companies, the introduction of foreign technologies, and collaboration with domestic research institutions. Taking into account the domestic market conditions as well as the specific requirements of users, Harbin Boiler Group integrates the technologies obtained through cooperation and introduction in a cohesive manner, and carries out comprehensive optimization designs to develop circulating fluidized bed boiler technologies that feature Harbin Boiler Group’s strengths and suit China’s national conditions. This approach provides a technical advantage that helps Harbin Boiler Group enter and dominate the domestic circulating fluidized bed boiler market. Over the years, Harbin Boiler Group has built on its existing foundations, drawn on the operational experience gained from multiple boilers in use, and continued to innovate through reform. It has introduced new technologies and products, thereby enriching its design concepts and solutions and meeting the various needs of different users. To date, the fuels designed by Harbin Boiler include bituminous coal, sub-bituminous coal, lignite, anthracite, coal gangue, coal slime, as well as a combination of coal and gas for combustion, covering a wide range of fuel types ； The return valves used include single-path return valves and double-path return valves ； The air caps used include large-diameter bell-type air caps and pigtail tube-type air caps ； The slag coolers used include air-water combined slag coolers, drum slag coolers, and spiral slag coolers ； The ignition methods employed include bed-top ignition, bed-bottom ignition, and combined bed-top+bed-bottom ignition ； Coal feeding methods include front-wall coal feeding, rear-wall coal feeding, and combined front-wall + rear-wall coal feeding. The following provides a detailed overview of the technical improvements made to the Harbin Boiler Works’ circulating fluidized bed boiler technology: 1. Separator – Harbin Boiler Works has optimized the design of the separator by utilizing imported technologies in order to improve its separation efficiency. The main optimization measures include: a) Tilting the flue at the inlet of the separator downward, so that the flue gas entering the separator moves at an angled downward direction; this gives the solid particles in the gas downward momentum, which facilitates gas-solid separation. b. Biasing the separator’s central cylinder can reduce wear on it and improve the flow field around it, thereby enhancing separation efficiency. c. A unique vortifier (central cylinder) design effectively controls the flow velocity of the upward airflow, reduces the entrainment of particles by turbulent airflow, and improves separation efficiency. d. The flue at the separator inlet is designed as an acceleration section to increase the flue gas velocity at the separator inlet, which facilitates gas-solid separation. After optimization, the separation efficiency of the separator can reach over 99.5%, with a particle size distribution of d50=10–30μm and d99=70–80μm. An efficient separator is an effective measure to reduce the combustible content in fly ash, and it is also a crucial guarantee for achieving a high cycle rate. Based on the existing high-temperature adiabatic separators, Harbin Boiler Group introduced a steam-cooled separator design and applied it to the design of multiple boilers. The outer wall of the air-cooled separator is made of tubes and flat steel, with insulation material and T-shaped protective plates on the outside ； The inner wall is lined with only a thin layer of refractory material. The main advantages of the steam-cooled separator are: 1) It has a very thin lining of wear-resistant refractory material (60 mm); compared to high-temperature separators, this reduces the amount of wear-resistant refractory material needed, thereby lowering the initial investment. 2) Due to the thin lining of wear-resistant and fire-resistant material in the steam-cooled separator, along with its certain heat transfer capacity, it allows for faster changes in flue gas temperature; this facilitates quicker startup of the boiler and saves a significant amount of oil used for starting. 3) The air-cooled separator features a membrane wall structure, which not only increases the heating surface area but also reduces heat loss, thereby improving the efficiency of the boiler. 4) The outer wall of the air-cooled separator uses the same conventional insulation material as that of the furnace water wall, which is economical and reliable. 2. Return valve: Haga designs single-channel and dual-channel return valves for boilers of different capacities. The so-called dual-path return valve is one that has only one feed port connected to the separator, along with two return pipes connected to the furnace, thereby ensuring a uniform distribution of the circulating material within the furnace. The dual-return material valve facilitates the overall layout of large-capacity units, while ensuring that the boiler has a sufficient number of feeding interfaces, thus resulting in a simple and rational overall structure for the boiler. 3. Large-diameter bell-type air caps: Drawing on experience from operating units and incorporating imported technologies, a design for large-diameter bell-type air caps has been developed. The advantages of these air caps are as follows: 1) The use of a combination structure of an inner tube and an outer cover results in reasonable resistance design, which facilitates low-load operation of the units. 2) The air vents are located on the side, so they won’t get blocked. 3) The inner tube and outer cover are connected by a threaded structure, facilitating replacement. 4) The outer cover is made of cast steel structure, featuring high hardness and wear resistance. 4. Wind-water combined slag cooler: By summarizing the experience of operating units and combining it with imported technologies, the wind-water combined slag cooler was developed. It is divided into three chambers in total. The first chamber uses pneumatic selective cooling; during the pneumatic cooling of the ash, the finer bottom slag (containing unburned particles, unreacted limestone particles, etc.) can be sent back to the combustion chamber ； In the second and third chambers, the pipes installed for heat exchange with the ash allow the ash to be cooled to below 150°C, after which it can be discharged to the ash removal system. Each chamber is equipped with an independent air distribution plate and bellows; the air distribution plate is of steel plate construction, on which large-diameter bell-shaped air nozzles are mounted. At the same time, a 200-mm-thick wear-resistant and fire-resistant material is laid on the air distribution plate, and its slight inclination facilitates the directional flow of slag. Each chamber is equipped with a bottom slag discharge pipe, while the third chamber also has an overflow ash pipe. The air distribution for the three chambers comes from the cold slag fluidization fans connected in series with the main fan. The working fluid inside the heated surfaces of the cold slagger tubes is demineralized water, which comes from the heat recovery system; after heat exchange, it is sent back to that system. Depending on the amount of slag discharged by the boiler and its cooling conditions, the amount of cooling water supplied to the slag cooler can be adjusted accordingly. Due to the very low water temperature (around 30°C), a large heat transfer temperature difference can be achieved, resulting in good ash cooling efficiency. All three chambers of the cold slagger are in a bubbling bed state, with a very low fluidization velocity (≤1 m/s). In addition, anti-wear fins are welded to the tube bundle, which prevents the tubes from wearing out and thus ensures the safety of operation of the slag removal system. The main advantages of the fengshui combined cold slagger are: 1) strong ash and slag cooling capacity, and high compatibility with various coal types. 2) Simple structure, easy to operate and maintain. 3) Compact structure with small floor area ； The main drawback of the wind-water combined slag cooler is its sensitivity to the particle size of the bottom slag; when there are many large particles, slag clogging is likely to occur. 5. Ignition method: Depending on the type of coal used in its design, the Haga boiler employs bed-top ignition, bed-bottom ignition, or a combined bed-top+bed-bottom ignition method for starting up. The advantage of igniting on the bed is that the flame from the oil gun directly heats the bed material, resulting in a high heat load and simple and convenient operation and maintenance ； The advantage of ignition under the bed is that the hot flue gas is introduced through the air distribution plate and flows through the bed material entirely, resulting in high heating efficiency and fast heating speed ； Combined ignition above and below the bed is suitable for fuels with low volatility; this combined ignition method allows for faster heating of the bed material, reduces startup time, and saves fuel used for starting. In summary, through nearly a decade of effort, HAGO has accumulated considerable experience in the design and manufacture of circulating fluidized bed boilers, and has mastered sophisticated technologies for their design and production. The Hagao circulating fluidized bed boilers, which are manufactured using imported technology or through collaborative design, will meet the growing market demand in China, allowing domestic users to obtain boilers of international advanced standards without having to make substantial investments. III. Measures to Ensure Proper Boiler Combustion During the design of the boiler, in order to reduce the amount of combustible material in the bottom ash and fly ash and to ensure efficient combustion, Harbin Boiler Group adopted the following technical measures: 1. Sufficient furnace height To allow fine particles with a size smaller than the d50 value determined by the separator to have sufficient time for complete combustion within the furnace, designing a sufficient furnace height and volume helps to improve the combustion efficiency and reduce the carbon content in the fly ash. The actual furnace height is Hf = 37m–39m (from the air distribution plate to the top of the furnace). This height is slightly higher than that of boilers using pulverized coal (CPC) for combustion, as such coal is generally more difficult to burn completely. 2. A uniquely designed high-efficiency separator improves the separation efficiency of the cyclone separator, allowing more fine particles to be separated and used in cyclic combustion, thereby effectively reducing the combustible content in fly ash. Practical operation has shown that the following design features have a significant effect on improving the separation efficiency of cyclone separators: a) The flue at the inlet of the separator is inclined downward, causing the flue gas entering the separator to move at an angle downward; this imparts downward kinetic energy to the solid particles in the gas, thereby aiding in the separation of gas and solids. b. By offsetting the center cylinder of the separator so that the center of the flue gas flow within the separator aligns with the center cylinder, wear on the center cylinder can be reduced. Additionally, the flow field around the center cylinder is improved, gas flow fluctuations are diminished, and the separation efficiency is enhanced. c. A unique guide vane (central cylinder) design effectively controls the flow velocity of the rising vapor stream, reduces the entrainment of particles by the turbulent vapor flow, and improves separation efficiency. d. The flue at the separator inlet is designed as an acceleration section to increase the flue gas velocity at the separator inlet, which facilitates gas-solid separation. 3. Reasonably design the air distribution and fluidization speed to enhance the penetration and entrainment effect of the secondary air, so that the air in the dense phase zone mixes evenly with the fuel and limestone, thereby improving combustion efficiency. 4. Rational design of the dense-phase zone structure: Based on the characteristics of the coal, the following key dimensions were designed: 1) A combustion chamber with a rectangular cross-section, along with air distribution plates featuring a high width-to-depth ratio, which enable proper distribution of the secondary air and provide sufficient penetration capability to enhance mixing within the furnace and facilitate complete combustion of the fuel. 2) Reasonably design the height of the conical section in the dense-phase zone to increase the height of the operating bed layer, which facilitates the complete combustion of the fuel ； 3) The dense-phase zone of the furnace features a conical cross-section design, which directs the material flowing back along the wall toward the center of combustion. Combined with the inclined jets generated by the secondary air flow, this promotes lateral mixing of the circulating material, resulting in more uniform mixing of the gas-solid two-phase flow and thereby improving combustion efficiency. 5. Select a higher operating bed temperature to enhance the combustion of carbon particles. 6. Reasonably design the structure of the furnace top: By taking into account the end-effect at the furnace top, the concentration distribution of materials inside the furnace can be improved, which facilitates the complete combustion of carbon particles. 7. The coal feed port should be located far away from the slag discharge port. The coal feed port is located on the front wall, while the slag discharge port is located on the side wall ； Alternatively, the coal feed port can be located on the rear wall, while the slag discharge port is placed on the front wall; this prevents coal from being discharged directly, increases the residence time of carbon particles in the furnace, and reduces the carbon content in the slag. IV. Anti-wear technology for non-metallic wear-resistant and fire-resistant materials. The anti-wear issue in circulating fluidized bed boilers is a key factor that hinders the development of this boiler technology. The effectiveness with which wear problems are addressed directly affects the success of CFB boiler design and, in turn, impacts the availability of CFB boiler units. Wear in circulating fluidized bed boilers mainly occurs in the combustion chamber and the material circulation circuit of the separator; in addition, the convection ducts at the rear of the boiler suffer from the same type of wear as in coal powder boilers. Based on the characteristics of CFB boilers and their wear patterns, we employ non-metallic wear-resistant and fire-resistant lining technologies in areas prone to wear such as the combustion chamber and separator to prevent wear. The wear rate is a function of the solid concentration, velocity, particle properties, and flow channel geometry; therefore, wear in CFB boilers occurs in areas related to these factors, such as the lower part of the combustion chamber, around the return device and the combustion chamber outlet, at the inlet of the separator, on the cylindrical surface facing the separator inlet, and around various openings in the combustion chamber. Typically, in CFB boilers, non-metallic wear-resistant and fire-resistant materials are used to create wear-resistant linings in the following areas. 1) Water wall air distribution plate. 2) Surface of the water-cooled walls surrounding the lower part of the combustion chamber. 3) The water-cooled screens arranged in the combustion chamber, the lower surface of the superheater screens, and the surfaces of the water-cooled walls surrounding them through the wall. 4) The area around the smoke outlet of the combustion chamber and the inner surface of the flow channel at the smoke outlet. 5) The entire inner surface of the separator. 6) Inner surfaces of the material legs and material return devices. 7) Inner surface of the flue gas duct at the separator outlet. 8) Inner surface of the inlet to the tail convective flue. 1. Design of the anti-wear structure for the combustion chamber 1) Since the material concentration in the dense-phase region at the bottom of the combustion chamber is very high, mixing and turbulent flow are intense, making this area highly susceptible to wear. Therefore, a wear-resistant and fire-resistant cast material of a certain thickness is used as lining in the lower dense-phase zone; these wear-resistant and fire-resistant materials are fixed by metal pins welded to the surface of the tubes. 2) The combustion chamber is equipped with water-cooled screens and secondary superheater screens, both of which are located in a gas-solid two-phase flow field and are prone to wear. Especially at the wall-penetrating areas, wear is more severe due to changes in the flow field. Therefore, wear-resistant castables need to be applied in these areas. 2. Anti-wear design of the material return device: A structure that combines wear-resistant materials with insulation materials is adopted, and the main forms are as follows: 1) Wear-resistant brick lining + insulation bricks; the gap between the wear-resistant bricks is 2 mm, and expansion joints are provided at regular intervals to accommodate thermal expansion. At an appropriate height, high-temperature heat-resistant steel brackets are installed to transfer the weight of the wear-resistant bricks to the steel shell in layers. 2) Wear-resistant brick lining + insulating castable: suitable for areas where the steel shell has a complex shape or for other parts that are not suitable for use with insulating bricks. The gap between the wear-resistant bricks is 2 mm, and expansion joints are provided at appropriate intervals; brick supports are installed at regular intervals to allow for load distribution. 3) Wear-resistant castable + insulating castable: suitable for areas on the surface of the wear-resistant lining where the geometry is complex, as well as for the top surface of the equipment. The most common design for this structure involves using “Y”-shaped anchors arranged in a regular pattern to secure the wear-resistant lining. An asphalt layer 1 mm thick is applied to these anchors to address the thermal expansion differences between the metal anchors and the wear-resistant castable. Stainless steel fibers are added to the wear-resistant castable in a proportion of 2%, and expansion joints must be provided in the wear-resistant lining. 3. Anti-wear design of the separator: The separator is a key component in circulating fluidized bed boilers, and its separation efficiency plays a crucial role in both the particle size distribution and the amount of material in the circulating fluidized state. If the wear-resistant and fire-resistant materials inside the separator fall off or form coking, it will directly affect the separation efficiency, disrupt the normal balance of the circulating materials, and impact performance parameters such as the boiler load ； The broken pieces of wear-resistant and refractory materials that fall off end up in the return material system, disrupting the fluidization state of that system until normal material return is no longer possible, which leads to the forced shutdown of the furnace. Therefore, it is very important to prevent such wear-resistant and refractory materials from falling off. For air-cooled separators, a structure with a thin layer of wear-resistant cast material as the lining is primarily used, which is secured with stainless steel studs. Through years of technical research and practical operation experience, Harbin Boiler Group has accumulated extensive expertise in the design of wear-resistant and fire-resistant materials for CFB boilers. In multiple circulating fluidized bed boilers with capacities of 220t/h to 480t/h, designed and manufactured by Harbin Boiler Group, strict requirements were applied throughout the process, from the selection of wear-resistant and fire-resistant materials to the structural design. As a result, excellent performance was achieved after the boilers were put into operation, with no accidents occurring that could affect their normal functioning. V. Performance requirements for wear-resistant and fire-resistant materials in CFB boilers In CFB boilers, due to the presence of large amounts of material, severe wear problems occur in areas such as the dense-phase zone at the bottom of the furnace, the flue gas separator at the furnace outlet, the return valve, and the slag cooler. The wear is particularly severe in the dense-phase zone where the material concentration is high, in the separator area, and on the ceiling of the furnace. Therefore, in addition to further addressing issues such as the design of wear-resistant structures, the properties of wear-resistant materials are also extremely important; their various performance parameters directly affect the service life of these materials. Drawing on the operational experience of multiple boilers and by adopting ALSTOM’s CFB boiler technology, Haga has set strict requirements for the performance parameters of wear-resistant materials. In addition, to reduce heat loss from the furnace wall, Harbin Boiler uses a thicker insulation layer for it: in the dense-phase region at the lower part of the furnace chamber, the insulation thickness is 300 mm ； The insulation thickness in the remaining parts of the furnace chamber is 200 mm, and it is 200 mm in the separator as well as in the inlet and outlet flue ducts, to ensure that the temperature on the outer surface of the insulation layer does not exceed 45°C. VI. Sealing design of boilers A good boiler seal is a prerequisite for civilized production in power plants as well as for the safe operation of staff. Due to the large amount of material present in circulating fluidized bed boilers, especially in the combustion chamber, separator, and return valve, which are part of the high-temperature material circulation circuit, poor sealing can lead to significant leakage of flue gas and fine particles, thereby having a negative impact on the operating environment. Therefore, it is essential for designers to address the sealing issues in circulating fluidized bed boilers. Based on cooperation with foreign boiler manufacturers and the adoption of advanced technologies, as well as its own years of practical experience, Haga has developed a reliable design method for sealing circular fluidized bed boilers: 1. Sealing of the combustion chamber. During the operation of a circular fluidized bed boiler, the combustion chamber operates under positive pressure, with a large number of solid particles in motion. Additionally, the combustion chamber is equipped with water-cooled screens or double-sided water-cooled walls, as well as secondary superheater screens; in boilers equipped with a reheat system, high-temperature reheater screens are also present. All these elements pose significant challenges to the sealing of the combustion chamber. The Harbin Boiler uses advanced sealing technology to effectively address the sealing issues in the combustion chamber. The main sealing design methods include: 1) The water wall adopts a full-mode wall structure with hermetic full welding, and experience shows that this is an effective sealing structure. 2) A reliable wall box structure along with expansion joint design is used at the points where the screen-type heating surfaces pass through the walls, in order to address the expansion differences between the superheater screens and the water wall. Fill the sealed box with fire-resistant insulation material to improve the working conditions of the metal in the box, prevent it from oxidizing easily, and extend its service life ； On the secondary superheater panels and at the areas where the high-temperature reheater panels pass through the roof, metal expansion joints made of stainless steel are used to prevent the sealing boxes from being damaged. Additionally, fire-resistant insulation material is filled inside the entire sealing box to reduce the temperature in that area and thereby extend the service life of the expansion joints. Sealing between the bottom water-cooled air chamber and the water-cooled wall. Since the water-cooled air chamber is welded together with the water-cooled wall, its upper surface serves as the air distribution plate for the combustion chamber and is entirely covered with wear-resistant and fire-resistant materials, resulting in excellent sealing at that location. 2. Use of non-metallic expansion joints: Since the boiler is equipped with expansion centers – specifically, one at the center line of the combustion chamber, one at the center line of the separator, and one at the center line of the tail flue – three-dimensional thermal expansion differences arise at the interfaces between the combustion chamber and the separator, between the separator and the tail flue, and between the separator and the return valve. Such three-dimensional thermal expansion differences cannot be addressed using conventional metallic expansion joints; instead, non-metallic expansion joints must be used. Through its collaboration with PYROPOWER, Harbin Boiler Group acquired the design technology for sealing structures using non-metallic expansion joints, and applied this technology to the CFB boilers developed independently by Harbin Boiler Group, achieving excellent operational results. Currently, in China, in-depth research has also been conducted on the structural properties of this type of non-metallic expansion joint, and it has been put into production and use. In the 220 t/h CFB boilers used at the Liaohe Oil Field, domestic non-metallic expansion joints have replaced the imported counterparts. It can be said that the performance of these domestic expansion joints is now on par with that of imported products, while their price is much lower. 3. Sealing design of the tail flue: The arrangement of the heated surfaces in the tail flue is similar to that in conventional coal-fired boilers, and Harbin Boiler Group has more experience in the sealing design of tail flues. 1) The tail flue enclosure wall employs both a patterned wall and shielding plates as structural elements; both are airtight welded structures that ensure the sealing of the enclosure wall. 2) For the sealing of the wall-penetrating structure of the ceiling-mounted pipe in the tail flue, a traditional segmented seal box design is employed. Fire-resistant insulation material is also installed inside the sealed box. 3) For the sealing of the wall-penetrating structure of the superheater located on the rear wall of the tail flue, foreign secondary sealing technology was adopted; that is, in addition to the traditional segmented sealing box structure, a large cover box was added to enclose both the superheater header and the segmented sealing box. By adopting the aforementioned sealing structure design, the manufacturing quality of the boiler has been improved, with particular emphasis placed on controlling the quality of installation of the sealing structure on-site; this ensures good sealing performance of the boiler and creates a clean working environment for the operators. VII. Explosion-proof and wear-resistant measures for the furnace water wall, tube banks, and superheater: The unique combustion method of circulating fluidized bed boilers results in a large amount of material present in their combustion chambers; in particular, the lower dense-phase region during combustion is an area characterized by intense turbulence, mixing, and entrainment of high-concentration materials ； At the same time, research results and operational experience show that severe wear tends to occur in areas where the gas-solid two-phase flow field changes, such as at the bends of the water wall (access holes, thermal measurement ports, etc.), at the points where tube banks pass through walls, and at the boundary between the dense phase and lean phase zones in the lower part of the furnace. Therefore, strict anti-wear measures must be taken in these areas to ensure the proper operation of the unit. Hagao has extensive experience in non-metallic anti-wear design, and it has improved upon this by actively absorbing and applying the anti-wear design techniques of foreign companies. To date, no severe wear has been observed in the CFB boilers designed and manufactured by Harbin Boiler Group. 1. Anti-wear design of the superheater screen 1) The anti-wear mechanism of the superheater screen should be designed appropriately, as it is located in a flow field of gas-solid two-phase flow moving upward; in particular, its lower end is directly exposed to the erosion caused by this gas-solid two-phase flow, posing a serious risk of wear ； Furthermore, at the wall-penetrating section, due to the change in the flow pattern of the gas-solid two-phase flow, phenomena such as bypass flow, backflow, and pulsation occur, making wear more likely. Therefore, reasonable anti-wear measures must be taken. A certain thickness of wear-resistant and fire-resistant material is laid at the lower part of the superheater screen and at the locations where the water wall passes through the walls, which effectively addresses the issue of wear. 2) Reasonably design the inlet and outlet header configurations for the superheater panels in order to reduce hydraulic imbalances, avoid the formation of offset tubes, and prevent overheating of the tubes in the panel-type superheater ； At the same time, because the temperature distribution in the combustion chamber of a circulating fluidized bed boiler is very uniform and the heat load is low, it is also possible to reduce the likelihood of tube failure in the screen superheater. 2. Anti-wear design for the openings in the dense-phase zone at the lower part of the furnace: There are numerous opening structures in the dense-phase zone at the lower part of the furnace, including manholes, thermal measurement ports, and secondary air inlets. To prevent wear in these areas, a proven anti-wear structural design from abroad is employed; the areas surrounding these openings are covered with wear-resistant and fire-resistant materials. Asphalt is applied or ceramic fiber paper is used to cover the secondary air ducts and thermal measurement sleeves, in order to address the expansion issues between the metal sleeves and the wear-resistant and fire-resistant materials. With this design, wear has never occurred in these areas of the CFB boilers manufactured by Harbin Boiler. 3. Wear protection design at the boundary between the dense-phase zone and the dilute-phase zone in the lower part of the furnace. The boundary between these two zones in the lower part of the furnace (i.e., the end point of the wear-resistant and fire-resistant materials in that area) is a zone prone to wear. The material flowing downward along the water wall causes particle rebound at the junction point, and irregular movement of the particles occurs as well; therefore, wear is likely to occur in this area. To address the wear issue in this area, many boiler manufacturers at home and abroad have conducted extensive research and applied various solutions, and this problem has now been largely resolved. The Haga boiler adopts a mature design structure from foreign companies, and no wear issues have been observed in the CFB boilers that are already in use in this region. VIII. Measures to prevent ash blockage in the fluidized-bed type air-water combined slag cooler 1) The slag cooler contains a large amount of low-temperature bottom slag during operation, which allows the high-temperature ash and slag discharged into it to cool down more easily compared to this large amount of low-temperature bottom slag. By the time the high-temperature ash and slag reach the air distribution plate, their temperature has dropped below the ignition temperature of the carbon particles; as a result, they do not burn or clog, and no ash blockage occurs. 2) Water-cooled heating surfaces are arranged in the fluidized bed slag cooler, which provides strong cooling capacity; at the same time, the fluidization speed is low and less air is required for fluidization, thus preventing combustion or coking. 3) The air distribution plates in the cold slag holder are equipped with large-diameter bell-type air nozzles, which ensure even air distribution and sufficient fluidization, thereby preventing the accumulation of bottom slag on the air distribution plates and avoiding coking. 4) The air distribution plate of the cold slag holder is slightly inclined toward the slag discharge port, which facilitates the flow of larger slag particles toward this port for timely removal, thereby preventing coking caused by bed material accumulation. 5) The slag discharge port of the cold slag holder is equipped with a slag poking hole, which allows for the timely removal of ash buildup at that port, ensuring the proper operation of the cold slag holder. 6) The particle size of the coal fed into the furnace should be less than 7 mm; this ensures proper fluidization in the dense phase zone of the furnace, preventing the formation of slag composed of large particles. This is essential for ensuring that the slag cooler functions properly without getting clogged. 7) The head and flow rate of the fluidization fan in the slag cooler are designed with a large margin, providing ample room for adjustment in case abnormal operating conditions occur in the slag cooler. IX. Measures to prevent excessive bed temperature and coking 1. When designing the furnace, appropriate heat transfer coefficients should be selected by fully considering factors such as changes in the type of coal used and variations in boiler load, to ensure that there are sufficient heating surfaces inside the furnace. 2. The method of adjusting the ratio of primary and secondary air is effectively utilized to keep the bed temperature within the designed range. To this end, air nozzles with anti-clogging structures are employed in the design, and sufficient pressure head is provided for the secondary air, thereby ensuring that both the volume of primary and secondary air can be adjusted. 3. During design, sufficient margin is provided for the output of the cold slagger; therefore, the slag discharge rate can be increased or decreased to adjust the bed pressure, which in turn changes the amount of ash remaining in the furnace and the composition of the bed material, thus affecting the heat material circulation rate and enabling the adjustment of the bed temperature ； 4. When designing the air distribution plate and bellows, ensure a uniform wind speed so that the bed material does not accumulate in certain areas of the air distribution plate. 5. The interlock protection system ensures that during boiler startup and operation at low load, the amount of primary air entering the air distribution plate must be greater than the minimum specified value, in order to maintain an adequate fluidization wind speed inside the furnace. 6. Take the following measures to prevent a reducing atmosphere from forming inside the furnace, as such an atmosphere easily leads to coking on the furnace bed: 1) A cross-limiting function has been added to the control systems for air volume and fuel supply, meaning that when the boiler’s load is increased, air is supplied first and then coal ； To reduce the load, coal consumption is reduced first, followed by wind volume, in order to avoid a situation of excess fuel when the boiler load changes. 2) Monitor the air-coal ratio during the stable operation of the boiler using an oxygen level transmitter, and utilize the oxygen correction function in the air supply system to ensure sufficient excess air in the furnace. 3) Add a signal representing the air-coal ratio to the interlock protection system; an alarm is triggered when this value falls below a certain level, and the main fuel supply is shut down when it drops to another specified level. In summary, on the one hand, reliable measures must be taken in the boiler’s structural design and control system design to prevent overheating and coking, and on the other hand, efforts should be made to avoid deviations from the designed operating conditions during boiler operation. Once the bed temperature becomes too high or coking occurs, the most effective measure is to quickly reduce the coal feed rate, or increase the air supply rate – in particular, the amount of primary air flowing through the air distribution plate should be increased, while the amount of secondary air should be reduced accordingly. X. Control methods for boiler operation: control methods under conditions such as cold start, load adjustment, and shutdown. Compared with conventional coal-fired boilers, the biggest difference in terms of control methods between circulating fluidized bed boilers and the latter lies in the monitoring, regulation, and control of the circulating material, which is a feature specific to circulating fluidized bed boilers. Specifically, CFB boilers focus on the monitoring, adjustment, and control of bed temperature, separator inlet temperature, air-coal ratio, and bed pressure ； Attention is paid to monitoring the air flow rates that affect material fluidization, circulation, and combustion, in order to ensure a stable and sufficient circulation of hot materials, thereby facilitating the fuel combustion and heat transfer processes on the combustion side of the boiler. The adjustment and control characteristics of CFB boilers (which differ from those of conventional PC boilers) are summarized as follows: 1. The bed temperature is maintained by adjusting the ratio of the primary air flow passing through the air distribution plate to the secondary air flow that enters the furnace directly, keeping it within the temperature range most favorable for limestone desulfurization inside the furnace. 2. Maintain a constant bed pressure in the furnace by controlling the selective wind-water combined cold slag system, thereby ensuring ash balance and the composition of the bed material inside the furnace. 3. Control SO2 emissions by adjusting the amount of limestone fed into the furnace. 4. The signal of excessively high flue gas temperature at the inlet of the cyclone separator, or the signal of low primary air flow rate through the air distribution plate, is used to trigger the main fuel trip (MFT) mechanism, rather than the signal of loss of furnace flame; this serves as one of the important means for furnace safety protection. 5. The output regulation range of the oil burner is large, allowing for precise control of the combustion rate in the furnace. This ensures that the temperature of the wear-resistant and fire-resistant materials inside the furnace changes in a controlled manner, thereby preventing damage. 6. Both the bed temperature and bed pressure measuring elements should be equipped with heat-resistant, wear-resistant, and clog-resistant features to ensure that the measured data are accurate, reliable, and representative. Apart from these main differences, the DAS, MCS, and BMS systems of CFB boilers are not fundamentally different from those of conventional PC boilers, so they will not be listed one by one here. During the cold-start-up and shutdown processes of a circulating fluidized bed boiler, two points need to be considered in terms of control: 1) The fans should be started in a certain order (and stopped in the reverse order), which can be achieved through the fan interlocks in the BMS. 2) The bed material is heated by the fuel-based start-up burner (SUB), and then coal is added to increase the load; the timing of adding and removing coal is determined based on the bed temperature, which is also achieved through the interlock conditions in the BMS. When the bellows pressure under the air distribution plate is above the minimum value and there is no MFT, by activating SUB to raise the bed temperature to 450°C–650°C (the specific value depending on the coal quality), coal feeding can be permitted. By burning oil and coal together to raise the bed temperature to 760°C, all SUBs can be removed, and the normal bed temperature can be maintained through coal combustion. For certain reasons (including normal shutdowns), oil should be supplied to ensure proper combustion of the coal before the bed temperature drops to 650°C ； If no SUB is supplied and the bed temperature is below 650°C, an MFT must be initiated via the BMS to cut off the coal feed. Note: The temperature setpoints mentioned above are merely examples; the actual setpoints depend on the fuel properties and should be determined through experiments during the unit’s commissioning.