Circulating fluidized bed (CFB) boilers, with their unique advantages, are a coal-fired technology that is currently being developed competitively both domestically and internationally. However, it is well known that CFB boilers generally face three major challenges during startup and operation: difficulty in ignition, susceptibility to coking, and severe wear – the so-called "three hurdles." Overcoming these three hurdles has become one of the main issues in the widespread adoption of CFB boilers. [b]I. Ignition Hurdle[/b] Ignition and startup methods vary depending on the type of coal and boiler structure, but the commonalities remain. Domestic 35-75t/h CFB boilers generally use light diesel oil for ignition, with two methods: bed ignition and under-bed ignition. Under-bed ignition is more widely used due to its advantages of fast ignition, labor-saving, and fuel-efficient operation. In actual operation, some users, due to insufficient preparation or lack of operational experience, often experience furnace flameout or coking accidents during ignition. From ignition and trial operation to steam connection, it often takes ten, twenty, or even more attempts, affecting the overall project progress and wasting significant manpower and resources. How can I successfully pass the ignition stage? 1. After the boiler installation is completed and passes acceptance, a cold-state test should be conducted first. The purpose is to check the fluidization status of the furnace, understand the resistance characteristics of the air distribution device, identify problems in the boiler's design and installation, and propose solutions. The cold-state test mainly includes: ignition oil gun atomization test, air distribution uniformity test, air distribution plate resistance characteristic test, and material bed resistance test. 2. After the drying and boiling furnace is completed, the ignition scheme is determined based on the cold-state test parameters. Before ignition, a layer of ignition bed material is laid on the furnace bed, generally with a thickness of about 350-800mm. If it is too thick, although the initial ignition is relatively stable, the required fluidizing air volume for ignition is large, the heating time is long, and it is also prone to uneven heating. If the bed material is too thin, although the ignition time is short and oil is saved, the air distribution is uneven, and localized blow-through of the bed material may cause coking. Furthermore, the bed temperature is unstable in the initial ignition stage and is easily affected by coal shortages or ash blockages, leading to flameout or coking accidents. The particle size of the substrate is generally between 0 and 13 mm. If it is too fine, a large number of fine particles will be carried away by the fluidizing air, making the substrate layer thinner; if the particles are too coarse, a larger air volume is required to fluidize the substrate during startup, making ignition and temperature rise difficult. Generally speaking, the fine particles in the substrate are in the upper layer during fluidization, serving as the ignition source during the ignition period, while the larger particles play a role in absorbing fuel heat during deflagration and storing heat to maintain the bed temperature after their own combustion. The calorific value of the substrate should generally be controlled within the range of 2093-4186 KJ/Kg (500-1000 Kcal/Kg). If the calorific value is too high, the temperature rise rate during ignition is too fast, making ignition difficult to control and easily causing overheating and coking; if the calorific value is too low, it is difficult to raise the bed temperature, and volatile matter may precipitate and burn out, but the bed temperature still cannot reach the ignition temperature. 3. The ignition process is divided into three stages: substrate preheating, ignition, and transition. First, start the induced draft fan and primary air fan, opening all dampers to the normal fluidization position determined by the cold test, maintaining a certain negative pressure in the furnace, and then add oil guns. Carefully observe the flue gas temperature at the generator outlet (≤950-1000℃); otherwise, open the cold air damper to lower the temperature. The preheating process of the bottom material should be slow, controlling the bed temperature using oil and air volume. When the bed temperature reaches 400-450℃, small amounts of coal can be added intermittently, closely monitoring the bed temperature changes. When the bed temperature rises above 700℃, if the coal feeding is normal and combustion is stable, the oil guns can be disconnected. Generally, when the bed temperature is below 300℃, the temperature rise is rapid due to the high heat absorption of the material. The temperature rise slows down at 300-450℃, and above 450℃, the temperature rise accelerates again after a period of coal feeding, indicating that the added coal has begun to ignite. When the bed temperature approaches 600℃, a large amount of coal added to the furnace begins to ignite. At this time, the fluidizing air volume should be increased to control the temperature rise rate and prevent coking. Secondary air can be used for combustion assistance when the boiler load reaches 30%-40% or higher. It is important to note that high-calorific-value bituminous coal should be used as the ignition fuel, and coal should not be mixed with coal gangue, gasifier slag, limestone, or other difficult-to-burn fuels or raw materials. For a successful ignition process, attention should be paid to the bed thickness, bed material screening characteristics, and bed material properties and proportions. Strict control of the ignition air volume is crucial during operation. Practice has shown that the ignition characteristics of each type of circulating fluidized bed boiler differ, requiring operators to continuously explore and summarize in actual operation to find the optimal ignition and heating scheme to ensure successful ignition on the first attempt. II. Coking Issues During normal operation, the furnace temperature of a circulating fluidized bed boiler is generally controlled at around 850-950℃. In actual operation, coking accidents can occur both during the ignition and heating phase and during normal operation. Once coking occurs, it will seriously affect the safe and economical operation of the boiler equipment, and the removal of coke can easily damage components such as the air distribution plate, air cap, furnace walls, and water-cooled wall tubes. Coking is mainly classified into two types: high-temperature coking and low-temperature coking. High-temperature coking is a common accident during the ignition and heating stage. During heating, the coal undergoes deflagration, causing the bed temperature to rise rapidly. When the temperature reaches above the ash melting point, a single coke lump forms on the surface of the furnace. During normal operation, if the bed thickness is not properly controlled, the feeder and blower do not automatically adjust properly, or the air distribution valve opens too wide or too abruptly, a large amount of high-temperature ash separated by the separator enters the furnace, causing overheating and coking. Low-temperature coking generally occurs during the ignition and heating stage. If the bottom material is too thin and uneven, or if the bituminous coal is improperly spread, high temperatures can easily form in localized areas. At this time, the fluidizing air volume is low, and heat transfer is not timely, leading to the formation of coke lumps in certain areas. [b]Practice shows that the main factors affecting coking in circulating fluidized bed boilers are as follows:[/b] 1. Excessively high furnace temperature, exceeding the ash melting point of the fuel coal; 2. Too thick or uneven material layer, resulting in excessive or insufficient fluidizing air volume; 3. Thickness and calorific value of the ignition bed material, particle size of the coal fed into the furnace, ash melting point, etc.; 4. Worker skill level, degree of factory automation, and accuracy of instrument readings. During the ignition and heating stage, the combustible material needs to ignite and burn within a very short time, which can easily cause the bed temperature to rise rapidly and enter the deflagration stage. During this stage, the heat absorbed by the bed material itself is much less than the heat released. If the excess heat is not carried away by the air in time, it will inevitably cause coking in the bed. Therefore, controlling deflagration is an essential and important means during ignition and heating. If the calorific value of the ignition bed material is too high, the temperature rise during deflagration will accelerate, and the deflagration time will be prolonged. Therefore, once a rapid temperature rise during deflagration is detected, the oil gun should be stopped as soon as possible. In addition, adjusting the air volume early according to the initial temperature rise trend during deflagration is also important for controlling deflagration. After successful ignition, the separation device is put into operation. Under load, the amount of circulating ash in the return feed pipe and the bed temperature changes should be constantly monitored. Based on operational experience, the bed thickness should be strictly controlled, and the appropriate ash discharge time should be determined. Ash discharge should be controlled according to fuel properties, load, and bed temperature fluctuations to prevent excessive ash from entering the furnace, which could lead to uncontrolled bed temperature and coking. During normal furnace pressure control, cold air should be strictly avoided from entering the furnace, as this may cause unburned combustibles to burn, resulting in localized overheating and coking. In short, controlling a stable bed temperature is key to preventing coking in the furnace. Factors affecting furnace temperature mainly include fuel calorific value, air volume, and return feed volume. In actual operation, fuel quality often changes, even with a constant coal feed rate, which can cause changes in bed temperature. Furthermore, changes in the particle size of the coal entering the furnace will cause changes in the return feed volume. Under constant load, an increase in air volume will also change the bed temperature (the bed temperature decreases under constant bed pressure). To ensure a stable bed temperature between 900℃±50℃ during operation, it is generally not necessary to control the temperature by changing the circulation rate, but rather by controlling the air volume and coal volume. During stable load operation, the bed temperature can be adjusted by slightly changing the air volume and coal volume, or by changing both simultaneously. When the bed temperature is high, reduce coal or increase air volume; when the bed temperature is low, reduce air volume or increase coal volume. When the boiler is running at full load, the air volume can generally remain constant. When the bed temperature fluctuates, it can usually be stabilized by changing the coal feed rate. [b]III. Wear and Tear[/b] Domestic circulating fluidized bed boilers typically use a relatively low circulation ratio, with a flue gas velocity in the furnace of approximately 4.5-5 m/s, resulting in relatively minor wear. However, in localized areas and at locations with reduced cross-sections, the degree of wear can be tens or even hundreds of times greater than normal. Common areas with severe wear include: embedded tubes, furnace walls, water-cooled wall tube systems, separators, superheaters, economizers, and air preheaters. (I) Wear of Embedded Tubes Embedded tubes are directly arranged in the boiling zone above the furnace air distribution plate. The degree of wear is considerable, as can be seen from the following functional relationship: Where: E—wear amount ωT—fluid flow velocity in the boiling bed DP—average particle size Vf—particle concentration in the boiling bed The greater the airflow velocity, particle diameter, and ash particle concentration, the greater the wear amount. Flue flow velocity and particle diameter have the greatest impact on wear amount, followed by ash particle concentration, with the least impact. Therefore, for circulating fluidized bed boilers designed with embedded tubes, the following measures should be mainly taken: 1. Reduce the particle size of the coal fed into the furnace. Although the wear amount has a square relationship with particle diameter and a cube relationship with airflow velocity, the smaller the particle size, the lower the required airflow velocity. Therefore, reducing the particle diameter not only reduces wear on the heating surface itself, but also reduces wear on the heating surface of the embedded tubes. 2. Wear-resistant fins are installed on severely worn sections of the buried tube heating surface to protect the tube surface from wear. A particle film composed of particles is also formed between the fins, buffering the wear of the fins. 3. The buried tube material can be an alloy material with high temperature, high hardness, and good oxidation resistance. To reduce costs, at least alloy steel should be used for the fin material. Thickening the tube wall and applying a high-temperature wear-resistant coating to the tube wall surface are also possible measures. 4. During actual operation, the bed pressure in the wind chamber should be kept below the specified value. If it does, slag must be discharged to maintain the material layer thickness and reduce wear on the buried tube. 5. It is hoped that relevant equipment manufacturers will avoid designing buried tube heating surfaces in boiler design. (II) Furnace Wall Wear Currently, different boiler manufacturers have different methods in furnace wall design and the selection of wear-resistant materials, but these methods also have some shortcomings. The selection range of refractory materials should be relatively large, but in actual operation, various furnace wall wear phenomena always occur, even leading to collapse accidents. Relevant data indicates that for high-temperature and easily eroded areas of the furnace, different refractory and wear-resistant materials should be used according to their respective wear characteristics. Materials such as widely used silicon carbide bricks, corundum bricks, and high-alumina bricks (Al2O3≥65%) can be used locally depending on the location of the furnace wall. HF-135 high-temperature strength castable and SiC castable using phosphoric acid solution as a binder are ideal choices, generally having a lifespan 2-3 times longer than phosphate concrete. Simultaneously, the quality of furnace construction should be particularly emphasized during boiler equipment installation. (III) Wear of Water-Cooled Wall Tube Systems: Whether it is a membrane water-cooled wall or a bare tube water-cooled wall, wear occurs to varying degrees during actual operation. Since the four corners of the furnace have the highest chance of forming eddies, the most severe wear often occurs in these areas. Some operating plants apply high-temperature refractory and wear-resistant coatings to the heated surfaces at the four corners of the main bed after adding grinding strips. This measure can also be taken for some areas locally subjected to flue gas erosion. In addition, it is recommended that the four corners of the membrane water-cooled wall (including the tail flue, etc.) be made into arcs with a certain radius during boiler design to minimize flue gas disturbance. (IV) Separator Wear Regardless of the technology of Tsinghua University, the Chinese Academy of Sciences, or other scientific research and design units, the flue gas separators vary in type and arrangement, but their basic principle is the same. To achieve the purpose of separating flue gas and ash particles, the flue gas must form a certain vortex. At this time, the flue gas velocity increases, which strongly scours the inner wall of the separator. Over time, this will inevitably lead to wear inside the separator. In order to reduce the high-temperature wear of the separator, the main separation device, such as cyclone separator and planar flow separator, is arranged in the medium or low temperature zone as much as possible during boiler design. However, this arrangement greatly increases the wear of the flue gas on the first-stage superheater. Therefore, a louvered or labyrinth-type first-stage separation device with relatively low resistance is generally set at the furnace outlet and the inlet of the first-stage superheater. Regardless of the type of separator, its inner wall is lined with high-temperature and wear-resistant materials. According to information, materials such as SiC tiles and high-alumina tiles are widely used. With the continuous research and development of high-temperature and wear-resistant materials in my country, it is believed that the wear problem of circulating fluidized bed boiler separators will be completely solved in the near future. (V) Wear of superheater, economizer, and air preheater Wear of the three major components at the tail of the boiler (superheater, economizer, and air preheater) causing tube rupture is a major headache for users. For a long time, some boiler manufacturers and users have taken measures such as thinning tube bundles, adding protective plates at bends, and adding stainless steel anti-wear plates at the windward side, and have also installed anti-wear plates locally and carried out abrasion spraying treatment, but the fundamental problem has not been solved. In particular, the wear is more serious when the coal fed into the furnace is mixed with coal gangue or gasification slag. It is recommended that users take measures as early as possible when ordering boiler products, negotiate with boiler manufacturers on the characteristics of fuel, and take effective anti-wear measures to extend the service life of the boiler as much as possible. IV. Conclusion 1. Familiarity with the basic principles and operating points of circulating fluidized bed boilers is the foundation for successful ignition and startup; ensuring qualified coal particle size is a condition for normal fluidized combustion. 2. Controlling a stable bed temperature is crucial to preventing coking inside the furnace. 3. Reducing the circulating fluidization velocity is a major factor in minimizing wear on heating surfaces and furnace walls; therefore, low-ratio circulating fluidized bed boilers are generally preferable when selecting a product.