Some relevant knowledge about circulating fluidized bed boilers

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http://bbs.hcbbs.com/archiver/tid-54020.html There are some items here; I will add more as follows: Circulating fluidized bed boiler technology is an efficient, low-pollution clean coal combustion technology that has developed rapidly over the past few decades. Internationally, this technology has been widely commercially applied in areas such as power station boilers, industrial boilers, and waste treatment, and it is being developed for large-scale circulating fluidized bed boilers with capacities of hundreds of thousands of kilowatts. Research, development, and application in this field in China are also in the early stages; hundreds of circulating fluidized bed boilers have already been put into operation or are under construction. It is foreseeable that the coming years will be a crucial period for the rapid development of circulating fluidized bed technology. Based on the publications issued in our country over the past few years regarding the theoretical design and operation of circulating fluidized bed boilers, the following information is provided on the principles, characteristics, startup, and operation of such boilers: I. Working principle of circulating fluidized bed boilers: (1) Fluidization process: When a fluid flows upward through a particle bed, its state of flow changes. At low flow rates, the particles remain stationary, and the fluid flows only through the gaps between them. When the flow velocity increases to a certain level, the particles are no longer supported by the distribution plate, but are instead held up entirely by the frictional force of the fluid. At this point, for an individual particle, it no longer relies on its contact surface with other neighboring particles to maintain its spatial position. On the contrary, after losing its previous mechanical support, each particle can move freely within the bed ； As a whole, the bed layer exhibits many properties similar to those of a fluid. This state is known as fluidization. The minimum velocity at which a granular bed transitions from a static state to a fluidized state is called the critical fluidization velocity. The properties of a fluid-like fluidized bed are mainly as follows: (1) The static value at any height is approximately equal to the weight of the solid particles per unit bed cross-section above that height. (2) No matter how the bed is inclined, the surface of the bed remains horizontal, and the shape of the bed retains the shape of the container ； (3) The solid particles in the bed can be discharged like a fluid through holes at the bottom or sides ； (4) Objects with a higher density than that of the bed layer will sink inside the packed bed, while those with a lower density will remain on the surface of the bed ； (5) The particles within the bed are well mixed, with the particles evenly dispersed throughout the bed layer; this is referred to as \"dispersed\" fluidization. Therefore, when the bed is heated, the temperature throughout the entire bed is essentially uniform. In normal gas and solid phases, the gas does not flow evenly through the particle bed. A portion of the gas forms bubbles that escape via short circuits through the bed, while the particles move in clusters in a turbulent manner. The porosity within the bed varies depending on location and time; therefore, this type of fluidization is referred to as \"agglomerative\" fluidization. The combustion process of coal is a gas-solid fluidization process. II. Principles and characteristics of circulating fluidized beds: The flow behavior of the solid particle bed in a circulating fluidized bed varies depending on the airflow velocity. As the gas flow velocity increases, the solid particles exhibit solid bed, bubbling fluidized bed, turbulent fluidized bed, and pneumatic conveying states, respectively. The rising phase of a circulating fluidized bed typically operates in a rapidly fluidized state, and the development of the hydrodynamic characteristics of this rapidly fluidized state is crucial for the circulating fluidized bed. At this stage, the solid fuel is fluidized by air currents with speeds greater than the terminal velocity of individual fuel particles, causing them to move up and down in the form of particle clusters, which results in high levels of mixing. The particle clusters move in all directions, constantly forming and breaking apart; in this fluid state, the airflow can also carry a certain amount of large particles, even though their terminal velocity is much greater than the mean aerodynamic velocity. In this gas-solid flow mode, there is a significant difference in the velocities of the gas and solid phases, that is, a relative velocity. The circulating fluidized bed consists of a fast-fluidized-bed (rising section), a gas-solid fuel separation device, and a solid fuel return device. The characteristics of a circulating fluidized bed can be summarized as follows: (1) There is no interface like that in a bubbling fluidized bed; solid particles fill the entire upward flow region. (2) There is strong fuel backmixing; particle clusters are continuously formed and broken apart, moving in all directions. (3) The relative velocity between the particles and the gas is high, and it is related to the bed porosity and the particle circulation rate. (4) The operating fluidization velocity is 2-3 times that of a bubbling fluidized bed. (5) The bed pressure drop varies with the fluidization velocity and the mass flow rate of the particles. (6) Good lateral mixing of particles. (7) Intense particle backmixing, external circulation of particles, and good lateral mixing ensure a uniform temperature distribution throughout the entire rising section. (8) By changing the amount of material stored in the rising section, the residence time of the fuel in the bed can be adjusted within the range of a few minutes to several hours. (9) The overall flow pattern of the fluidized gas is plug flow. (10) The fluidizing gas can be introduced at different heights within the reactor as needed. III. Main types of fluidized bed fueling equipment: Fluidized bed operation was initially used primarily in the chemical industry. Starting from the 1960s, fluidized beds were applied to coal as a fuel source, and they quickly became one of the three main fueling methods – along with fixed-bed fueling (laminar combustion) and suspended combustion (coal powder combustion). The theory and practice of fluidized bed combustion have also **contributed to the development of fluidization science. Today, fluidized bed combustion is one of the main application areas of fluidization, and it is receiving increasing attention. Based on their hydrodynamic characteristics, fluidized bed combustion equipment can be divided into bubbling fluidized bed boilers and circulating fluidized bed boilers. Depending on their operating conditions, they can also be classified as atmospheric-pressure or pressurized fluidized bed boilers. Thus, fluidized bed combustion boilers can be categorized as atmospheric-pressure bubbling fluidized bed boilers, atmospheric-pressure circulating fluidized bed boilers, pressurized bubbling fluidized bed boilers, and pressurized circulating fluidized bed boilers; these types are currently in the stage of industrial demonstration. (IV) Characteristics of circulating fluidized bed boilers: (1) Operating conditions of circulating fluidized bed boilers: Parameter Value Temperature (°C): 850–950 Bed pressure drop (KPa): 11–12 Fluidization velocity (m/s): 4–6 Particle concentration in the furnace (kg/m3): 150–600 Particle size at the bottom of the furnace (μm): 100–700; 10–40 Particle density in the upper part of the furnace (kg/m3): 1800–2600 Molar ratio of Ca/S: 1.5–4 Fuel particle size (mm): <12 Wall heat transfer coefficient: 210–250 Particle size of desulfurizing agent: around 1 mm (2) Characteristics of circulating fluidized bed boilers: A circulating fluidized bed boiler can be divided into two sections. The first section consists of the furnace (fluidized bed), equipment for separating gas and solid materials, equipment for recycling solid materials (such as cyclones), etc. These components together form a circulation loop for solid materials. The second section is the convective flue, which is equipped with superheaters, reheaters, economizers, and air preheaters. In a typical circulating fluidized bed boiler combustion system, the primary and secondary air required for combustion are supplied from the bottom and side walls of the furnace respectively. The combustion of fuel takes place mainly within the furnace, with water-cooled walls arranged around it to absorb part of the heat generated by the fuel. The solid materials carried out of the furnace by the airflow are collected in a gas-solid separation device and then returned to the furnace via a return mechanism for further combustion. The basic characteristics of circulating fluidized bed combustion boilers can be summarized as follows: 1. Low-temperature, controlled combustion: Circulating fluidized bed combustion is a type of fluidized combustion process in which high-speed flue gas comes into close contact with solid particles carrying strong turbulent disturbances inside the furnace, resulting in extensive particle recirculation. At the same time, the vast majority of the high-temperature solid particles are captured outside the furnace and sent back into it to participate in the combustion process again, thus enabling repeated cycles of combustion. Obviously, the time during which the fuel burns in the furnace is extended. Under this combustion mode, the temperature level inside the furnace is limited by the optimal temperature for desulfurization, typically around 850°C. This temperature is much lower than that in conventional coal-fired boilers (usually 1300–1400°C), as well as lower than the ash fusion point of ordinary coal (1200–1400°C), which eliminates the various problems associated with ash melting. This low-temperature combustion method has many advantages: slag formation and the precipitation of alkali metals inside the furnace are significantly reduced compared to coal-fired boilers, the sensitivity to ash properties is decreased, there is no need for a large space to cool the hot ash, and the generation of nitrogen oxides is low. It can also be combined with an inexpensive and efficient desulfurization process for the furnace interior structure. From the perspective of combustion reaction kinetics, the combustion reaction in a circulating fluidized bed boiler is controlled within the power combustion zone (or transition zone). Since the combustion temperature in a circulating fluidized bed boiler is relatively low, and there is intense mixing of a large number of solid particles, the combustion rate under such conditions depends mainly on the rate of chemical reactions, which in turn is determined by the combustion temperature level. Physical factors related to combustion are no longer the dominant factors controlling the combustion rate. The fuel burnout rate in circulating fluidized bed boilers is very high; typically, well-performing boilers can achieve a combustion rate of 98-99% or more. 2. High-speed, high-concentration, high-throughput fluidization cycle process for solid materials: The solid materials in a circulating fluidized bed boiler (including fuel char, desulfurizing agents, and inert bed material) undergo an external circulation that consists of the furnace, separator, and return device. At the same time, the materials in the circulating fluidized bed boiler participate in both internal and external circulation within the furnace. The entire combustion process as well as the desulfurization process are gradually completed within the dynamic cycle of these two forms. 3. Intense transfer of heat, mass, and momentum: In circulating fluidized bed boilers, large amounts of solid material pass through the furnace under intense turbulence. By manipulating operational parameters, it is possible to adjust the amount of material in circulation as well as the distribution of material within the furnace, thereby adapting it to different combustion conditions. Under this arrangement, the transfer of heat, mass, and momentum within the furnace is very intense, which results in a uniform temperature distribution throughout the furnace height; practice has indeed confirmed this. 4. Comparison of circulating fluidized bed boilers with other boiler types: The combustion of solid fuels can generally be divided into layer combustion, fluidized bed combustion, and suspended combustion; fluidized bed combustion can further be divided into bubbling fluidized bed combustion and circulating fluidized bed combustion. To understand the advantages of circulating fluidized bed boilers as well as the issues that require further research, it is necessary to compare them with other types of boilers. (1) Comparison of combustion processes: Characteristic values for layer-fired boilers, circulating fluidized bed boilers, and suspended solids combustion boilers – Average diameter of fuel particles (mm): <3000, 0.05–0.10, 0.02–0.08. Wind speed in the fuel chamber area (m/s): 1–3, 3–12, 15–30. Solid particle movement: Stationary; most particles move upward, some downward. Heat transfer coefficient from the solid layer to the heating surfaces: 50–150, 100–250, 50–100. Degree of wear: Low, medium, low.

(2) Comparison of desulfurization processes: In coal-fired boilers, calcium injection desulfurization involves directly injecting calcium-based desulfurizers (such as limestone, dolomite, or slaked lime) into the furnace. At high temperatures, these desulfurizers undergo reactions as follows: At 500°C–900°C, CaCO3 transforms into CaO(S) + CO2(g); at the same temperature range, MgCO3·(OH)2 converts into CaO(S) + MgO(S) + 2 CO2(g); and Ca(OH)2 turns into CaO(S) + H2O(g). At typical combustion temperatures, the combustion process is essentially complete within less than 200 milliseconds (with desulfurizer particle sizes around 10 μm). After combustion, porous calcium oxide particles are formed. Once CaCO3 is generated as a result of combustion, it reacts with sulfur dioxide to form calcium sulfate: CaO(S) + SO2(g) + O2(g) → CaSO4(S). According to experiments conducted on coal-fired boilers using calcium injection, the optimal injection temperature is around 1100°C. A particle size of limestone between 8–10 μm yields better desulfurization results. The utilization rate of the desulfurizer is generally around 20%, with a desulfurization efficiency of 50%. In the combustion desulfurization process of circulating fluidized bed boilers, a desulfurizing agent (lime or calcine) is fed into the furnace, where it reacts with the sulfur dioxide gas generated during combustion to achieve desulfurization. Similar to coal-fired boilers, once the desulfurization agent enters a circulating fluidized bed boiler, it burns to form calcium oxide, which then reacts with sulfur dioxide gas. In circulating fluidized bed boilers, due to their unique design and operating conditions, the entire main circulation loop of such boilers operates within the optimal temperature range for desulfurization (850–900°C). At the same time, due to the internal and external circulation of solid materials within the furnace (through separation and return devices), the residence time of the desulfurizer in the furnace is **extended**, with the average residence time typically reaching several dozen minutes. Furthermore, the intense turbulent mixing within the furnace is also highly beneficial for the combustion desulfurization process in circulating fluidized bed boilers. When the Ca/S ratio is between 1.5 and 2.5, the desulfurization efficiency can typically reach 90%, while the utilization rate of the desulfurizing agent can reach 50%, which doubles the desulfurization effect compared to that achieved using coal dust. (3) Comparison of the main technical and economic indicators for various types of boilers: Boiler model, Main technical and economic indicators – YG-35/39-M3: Circulating fluidized bed boiler; BG-35/39-M: Pulverized coal boiler; L-35/39-W/I: Chain grate boiler. Actual thermal efficiency (%): 87.8, 87.9, 650. Types of fuel used: Low-calorific-value coal, low-calorific-value coal, low-calorific-value coal. Lower calorific value (KJ/kg): 2173, 62200, 321736. Coal consumption per hour (kg): 495, 948, 838, 707. Standard coal consumption per hour (kg): 368, 436, 776, 468. Total power consumption of auxiliary equipment (KW): 470, 587.1, 362.3. Total standard coal equivalent of auxiliary equipment consumption (kg): 1002, 351, 45. Total standard coal consumption per hour (kg): 3872, 4218, 6613. Standard coal consumption per ton of steam produced (kg): 110.69, 109.25, 188.94. Combustion efficiency (%): 98–99, 98–99, 88.1. Load regulation range: Large, Large. Adaptability to changes in coal type: Adaptable to a variety of coal types, Suitable only for a single coal type. Operation and maintenance complexity: Moderate, High. Cost of boiler equipment (in ten thousand yuan): 82.68, 978, 65.9. System investment cost (in ten thousand yuan): 245, 400, 200.7. Steel consumption for the boiler (tons): 157, 165, 186. Sulfur dioxide emissions: Can be reduced by adding limestone; All emissions occur. Nitrogen dioxide emissions: Less generated, More generated, Relatively more generated. Fly ash emissions: Relatively low. Note: The investment cost for boilers is estimated based on values from the early 1990s. Comparison of circulating fluidized bed boilers with other types of boilers: Boiler characteristics – Chain grate boiler, Pulverized coal boiler, Circulating fluidized bed boiler. Height of the furnace bed or fuel combustion zone (m): 0.215–40, 27–45. Cross-sectional wind speed (m/s): 1.24–8, 4–6. Excess air coefficient: 1.2–1.3, 1.2–1.25, 1.15–1.3. Cross-sectional heat load (MW/m²): 0.5–1.5, 3–5, 4–6. Coal particle size (mm): 6–32, Below 6, Below 0.1. Load regulation ratio: 4.1, 3:4.1. Combustion efficiency (%): 85–90, 95–99, 99. NO2 emissions (PPM): 400–600, 50–200, 400–600. Desulfurization efficiency inside the furnace: Low, 80–90. As can be seen from the table above, circulating fluidized bed boilers are clearly superior to other types of boilers. V. Advantages of circulating fluidized bed boilers: Due to their unique hydrodynamic properties and structure, circulating fluidized bed boilers possess many advantages, which are briefly described below. 1. Fuel adaptability: This is one of the main characteristic advantages of circulating fluidized bed boilers. In a circulating fluidized bed boiler, fuel accounts for only 1%-3% by weight of the bed material; the rest are non-combustible solid particles such as desulfurizers, ash, or sand. The special hydrodynamic properties of circulating fluidized bed boilers enable excellent mixing of gas, solids, and solid fuels. As a result, once the fuel enters the furnace, it quickly mixes with a large amount of bed material; this causes the fuel to be heated to a temperature higher than the ignition temperature, without a significant drop in the bed temperature. As long as the calorific value of the fuel is sufficient to provide the heat needed to heat both the fuel itself and the air required for ignition, circulating fluidized bed boilers do not require any supplementary fuel or special materials. Circulating fluidized bed boilers can use not only high-quality coal but also various low-quality coals, such as coal with high ash content, coal with high sulfur content, coal with both high ash and high sulfur content, coal gangue, peat, as well as oil shale, petroleum coke, slag, bark, waste wood, garbage, and so on. 2. High combustion efficiency: The combustion efficiency of circulating fluidized bed boilers is 97.5–99.5% higher than that of chain grate boilers, and it can rival that of coal powder boilers. The high combustion efficiency of circulating fluidized bed boilers is attributed to the following characteristics: good mixing of gas and solids, a high combustion rate; especially for coarse powder fuels, the majority of the unburned fuel is recycled back to the furnace for further combustion. Meanwhile, circulating fluidized bed boilers can maintain a high combustion efficiency over a wide range of operating conditions. This is even the case when burning fuels with a high content of fine powder. 3. Efficient desulfurization: Circulating fluidized bed boilers are more effective at desulfurization compared to other types of boilers; typical desulfurization efficiency in such boilers can reach 90%. Unlike the combustion process, the devolatilization reaction proceeds more slowly; in order to fully convert calcium oxide (obtained from burning limestone) into calcium sulfate, the sulfur dioxide gas in the flue gases must have sufficient contact time with the desulfurization agent as well as a maximum possible reaction surface area. Of course, the interior of the desulfurization agent particles cannot be completely divided; the average residence time of gases in the combustion zone is 3–4 seconds. In circulating fluidized bed boilers, the particle size of limestone is usually 0.1–0.3 mm. In terms of both the efficiency of the desulfurization agent and the rate at which sulfur dioxide is removed, circulating fluidized bed boilers are superior to other types of boilers. 4. Low nitrogen oxide (NO2) emissions: Low nitrogen oxide emissions are a very attractive feature of circulating fluidized bed boilers. Operating experience shows that the nitrogen dioxide emissions from circulating fluidized bed boilers range from 50–150 PPM or 40–120 mg/mJ. Reasons for low NO2 emissions: First, combustion at low temperatures, during which nitrogen in the air generally does not produce NO2; second, staged combustion, which prevents the nitrogen in the fuel from converting into NO2 and allows some of the NO2 that has been formed to be reduced. 5. Low emissions of other pollutants: The emissions of other pollutants from circulating fluidized bed boilers, such as CO, HCl, HF, etc., are also very low. 6. High combustion intensity and small furnace cross-sectional area: A high heat load per unit area of the furnace is one of the main advantages of circulating fluidized bed boilers. The sectional heat load of a circulating fluidized bed boiler is approximately 3.5–4.5 MW/m2, which is close to or higher than that of coal powder boilers. 7. Fewer coal feeding points: Due to the larger furnace cross-sectional area, as well as the excellent mixing and expanded combustion zone, the number of coal feeding points required is reduced; only one coal feeding point is needed, which also simplifies the coal feeding system. 8. Simple fuel preprocessing system: The coal particle size in circulating fluidized bed boilers is generally less than 12 mm; therefore, compared to coal-fired pulverized coal boilers, the system for preparing coal powder is significantly simplified. Furthermore, circulating fluidized bed boilers can burn coal with high moisture content directly (the moisture content can exceed 30%). When burning coal with high moisture content, no special treatment system is required. 9. Easy comprehensive utilization of ash and slag: Due to the low-temperature combustion process in circulating fluidized bed boilers, along with favorable combustion conditions within the furnace, the carbon content in the boiler ash and slag is low, making it easy to achieve their comprehensive utilization. For example, ash can be used as a cement admixture or as a building material; moreover, thorough thermal treatment also facilitates the extraction of rare metals. 10. Large load regulation range and fast load adjustment: When the load changes, it is necessary to adjust the coal feed rate, air volume, and material circulation rate; the load adjustment ratio can reach (3-4):1. Additionally, due to the high wind speed across the cross-section, high heat absorption, and ease of heat absorption control, the load adjustment rate of circulating fluidized bed boilers is also very fast, generally reaching 4% per minute. 11. No buried heat-exchange tubes in the circulating bed: Circulating fluidized bed boilers do not have buried tube heat exchangers in their beds, so there is no issue of wear. Additionally, they can be started immediately during startup, shutdown, or coking treatment, as well as after a long period of idling. 12. Moderate investment and operating costs: The investment and operating costs of circulating fluidized bed boilers are slightly higher than those of conventional coal-fired boilers, but 15-20% lower than those of coal-fired boilers equipped with desulfurization systems. VI. Issues requiring further research in circulating fluidized bed boilers: To optimize the design and operation of circulating fluidized bed boilers and fully utilize their advantages, in-depth research is still needed in the following areas. 1. Separation of circulating materials: The separation devices in circulating fluidized bed boilers can be classified into high-temperature, medium-temperature, and low-temperature separators based on the operating temperature, and they can also be divided into types such as cyclone separation and inertial separation depending on the mechanism of separation. Based on the current operation of circulating fluidized beds, high-temperature cyclone separators are relatively mature. However, the wear problem when using high-ash fuels has not yet been resolved. Moreover, the volume of the separated material is also extremely large, roughly on par with the diameter of the furnace chamber. Due to the limitations imposed by the maximum size of cyclone separators, large-capacity circulating fluidized bed boilers must be equipped with multiple separators. Due to the thick anti-wear refractory material lining the cyclone separator, which results in high thermal inertia, the startup time of the boiler is prolonged. The dynamic characteristics of load variation deteriorate; therefore, the use of inertial separators is worth considering, as such devices are relatively simple, compact, and easy to arrange in terms of structure. The flow resistance is also relatively low. Furthermore, it should not be used in low-temperature separators. In line with the development requirements of circulating fluidized bed boilers, a material separation device that features efficient design, small size, low resistance, minimal wear, and ease of manufacturing and operation is required. 2. Selection of the solid particle concentration in a circulating fluidized bed: The solid particle concentration in a circulating fluidized bed has a significant impact on the combustion process, desulfurization process, and heat transfer process. However, it is very difficult to determine the appropriate solid particle concentration in a circulating fluidized bed. At present, an important parameter for the particle concentration inside the furnace used by various manufacturers of circulating fluidized beds is the circulation ratio. The circulation ratio of some domestic circulating fluidized bed boilers is usually below 10, whereas that of foreign boilers often reaches 50 or even higher. When analyzing the operating process of a circulating fluidized bed boiler, it is necessary to consider not only the internal circulation of the material but also the external circulation; the internal circulation of the material becomes more prominent at high wind speeds. Therefore, the proper selection of the solid particle concentration in a circulating bed has an impact on a range of factors such as combustion desulfurization, heat transfer, wear, and energy consumption. 3. Arrangement of the heating surfaces inside the furnace and temperature control: To maintain the temperature inside a circulating fluidized bed boiler within a certain range, it is necessary to absorb some heat from the solid particle circulation loop. Currently, there are mainly two methods for heat absorption within the furnace: one is to install water-cooled walls or partition walls inside the furnace chamber ； Another approach is to arrange some heating surfaces inside the furnace (such as superheaters), and then place a fluidized bed heat exchanger in the solid material circulation circuit. Both forms are feasible. However, these two methods employ different approaches to bed temperature control; the former relies primarily on adjusting the amount of return material in order to modify the concentration of solid particles in the bed, thereby changing the heat transfer coefficient of the water wall. Thus, the heat absorption in the furnace is altered to control the bed temperature; otherwise, the bed temperature can be controlled simply by adjusting the ratio of the amount of solid material entering the fluidized bed heat exchanger and that returning to the furnace, which provides relative flexibility and is particularly suitable for large-capacity circulating fluidized bed boilers. 4. Determination of operating wind speed (or cross-sectional heat load): The operating wind speed in a circulating fluidized bed boiler is an important parameter. The normal operating wind speed is 4-10 m/s. An increase in operating wind speed makes the furnace more compact. The sectional heat load increases accordingly; at this point, to ensure that the fuel and limestone particles have sufficient residence time and to provide enough heating surfaces, it is necessary to increase the height of the furnace. This not only increases wear but also raises the cost of the boiler. The fan power will increase, and the plant electricity consumption will rise accordingly. However, if the wind speed is too low, the advantages of the circulating fluidized bed cannot be utilized; therefore, there should be an optimal operating wind speed for all types of fuels. 5. Return material mechanism: In a circulating fluidized bed, the solid materials that have been separated must be sent back into the furnace through a return material mechanism. The material return mechanism should also be able to adjust the amount of material returned flexibly. However, due to the high temperatures and significant wear in such mechanisms, the use of conventional mechanical valves for adjustment can easily lead to problems such as sticking and poor operation. Currently, non-mechanical valves are commonly used in circulating fluidized beds. (The L valve) and the fluidized bed return mechanism serve to regulate the material flow rate, while also preventing the fuel from flowing back into the separator in the combustion chamber, thereby avoiding short circuits. Currently, many manufacturers keep the return mechanism a secret. 6. Wear of components in circulating fluidized bed boilers: Due to the high particle concentration and high operating wind speed in circulating fluidized bed boilers, wear on these boiler components is relatively severe. Wear is mainly related to wind speed, particle size, and the unevenness of the flow field; wear is proportional to wind speed and concentration. During design, sudden expansions and contractions in the flue gas corridor should generally be avoided. Current research is relatively weak. 7. Low-pollution fuels: Circulating fluidized bed boilers have seen rapid development. An important reason is the low-pollution fuel properties of circulating fluidized beds. There is currently consensus regarding desulfurization research, but much still needs to be studied in terms of the optimal temperature for desulfurization and the efficient use of desulfurizing agents. Further research is needed on aspects such as reducing NO2, bed temperature, flue gas recirculation, ammonia injection, and the impact of desulfurization agents on NO2. 8. Design of the heating surfaces at the tail section: In current circulating fluidized bed boilers, the design of the heating surfaces in the tail flue is generally given insufficient attention; further research is needed to determine how to arrange these heating surfaces more reasonably. 9. Dust removal: Electric dust removal is now widely used in domestic tail flues. VII. Development of circulating fluidized bed boilers: Abroad, research began in the 1960s; it was Oy Ronka in Finland that developed the first commercial circulating fluidized bed boiler, which was modified from a fuel-fired boiler with a thermal power output of 15 MW. Subsequently, various types of circulating fluidized bed boilers were manufactured in the United States by Battelle, as well as in Germany, Sweden, Canada, Italy, and other countries. The largest of these was a circulating fluidized bed boiler designed for a power generation capacity of 165 MW, manufactured in Canada in 1993. Domestically: (1) The main institution is the Northeast Electric Power Institute, which, in July 1986, developed a bubbling fluidized bed boiler of 10 t/h in collaboration with Jilin Boiler Factory. Since then, series of boilers with capacities of 4, 6, 10, 15, and 20 t/h have been produced; these boilers operate at a steam pressure of 1.27–2.45 Mpa and a temperature of 194–350°C. Meanwhile, the Department of Thermal Engineering at Tsinghua University has also developed fluidized bed boilers of a similar design. (2) In 1984, the Institute of Engineering Thermophysics of the Chinese Academy of Sciences undertook the project \"Research on Combustion Technology for Fluidized Bed Boilers Using Coal\" assigned by the Science and Technology Commission; in 1985, it developed a 10t/h circulating fluidized bed boiler in collaboration with Kaifeng Boiler Factory. A **patent was obtained for this technology; subsequently, in collaboration with Jinan Boiler Factory, series of circulating fluidized bed boilers with capacities of 35 t/h, 50, 65, 75, and 130 t/h were developed. These boilers feature a multi-stage separation system comprising a first-stage vaned diffuser and a second-stage cyclone separator, and they are in use at the Sannō Kaku Thermal Power Plant. (3) Although Tsinghua University started working on circulating fluidized bed technology later than the Chinese Academy of Sciences, it has developed rapidly. The basic concept is to employ two-stage separation: a column plate inertial separator combined with an S-shaped separator, which offers the advantage of lower resistance; patents have been filed in China, Japan, and the United States respectively. In 1989, in collaboration with Foster Wheeler and Japanese company Ishikawajima-Harima Heavy Industries, 20 t/h boilers were manufactured by Jiangxi Boiler Factory, while four 35 t/h boilers were produced by Sichuan Boiler Factory. The pilot fluidized bed boiler and the 75 t/h circulating fluidized bed boiler have since been put into operation. (4) Since the 1980s, Zhejiang University has been conducting research on circulating fluidized bed combustion technology, covering aspects such as the characteristics of circulating fluidized beds, heat transfer properties, desulfurization and denitration capabilities, wear characteristics, separators, return material systems, ash cooling devices, as well as steam and power co-generation technologies related to circulating fluidized beds. The university has obtained a patent for a \"high-temperature material cooling device.\" Currently, it is working together with Hangzhou Boiler Factory sowie relevant universities and companies in Canada on the joint development of 220 t/h circulating fluidized bed boilers designed for burning high-sulfur coal. In addition, Harbin Institute of Technology, the Thermal Engineering Research Institute of the Ministry of Electric Power, Taiguo, Cheguo, and certain other organizations are all researching, developing, and manufacturing 35, 65, 75, and 220 t/h circulating fluidized bed boilers. In general, circulating fluidized bed boilers in the 1990s should meet the following technical standards: (1) combustion efficiency of 100% ； (2) Plant efficiency greater than 40% ； (3) SO2 emissions less than 10PPM ； (4) NO2 emissions are less than 30PPM. The main features of the high-efficiency and low-pollution fluidized bed combustion technology for coal-water mixtures developed by Zhejiang University are as follows: First, the agglomeration phenomenon during the combustion of coal-water mixtures is a very important phenomenon in this combustion process. Experiments show that for a considerable portion of coals, when coal-water mixtures composed of fine particles are fed into a high-temperature fluidized bed in a large-volume aggregated state, they tend not to be reduced back to fine particles after drying, but rather rapidly form larger agglomerates with certain strength and wear resistance. Furthermore, the coal-water mixture can also form larger lumps by sticking to other particles within the bed, either repeatedly or through adhesion. We call this phenomenon coagulation clumping. Clusters formed by agglomeration are called agglomerates. The presence of coal-water mixture agglomerates has a significant impact on ensuring the stable operation of the fluidized bed. Agglomeration tends to cause an increasing trend in the particle size of the fluidized bed material; large agglomerates can easily deposit within the fluidized bed, gradually compromising its quality and making stable operation of the fluidized bed combustion difficult. However, the presence of this agglomeration effect ensures that the fuel will not be suspended even at higher operating wind speeds. Therefore, the strong agglomeration phenomenon has a very important positive impact on the fluidized bed combustion of coal-water mixtures. It not only provides favorable conditions for organizing normal combustion within the fluidized bed, but also creates favorable conditions for reducing the loss of fuel due to entrainment during fluidized bed combustion at higher cross-sectional heat loads. This is because the loss due to the volatilization of combustibles usually accounts for the vast majority of the losses in the combustion efficiency of fluidized bed boilers. This loss is notable. II. In order to mitigate the impact of large agglomerates on stable combustion in weight-driven fluidized beds, compared with conventional fluidized bed combustion, a certain degree of segregation tends to occur within the fluidized bed due to the wide particle size distribution of the fuel used. That is, those with a larger particle size are more concentrated at the bottom of the furnace. In the bed, agglomerate formations can occur, which are sometimes more than 10 times larger in size than the particles of the coal-water mixture. Such large agglomerates, even before they have dried out after formation, quickly settle to the distribution plate area at the bottom of the fluidized bed. The low temperature in this area results in a very slow rate of combustion reactions; even the evaporation of water involved in combustion is slow. This leads to a low fluidization wind speed and poor fluidization quality, thereby reducing heat transfer in this area. Heat transfer processes, particle movement, and their interactions. This prevents the large agglomerates deposited near the air distribution plate from having either the chance to burn out or from being broken apart and worn down. As it keeps accumulating, it continuously deteriorates the fluidization quality at the bottom of the fluidized bed, and this issue gradually spreads from the bottom upward throughout the entire bed layer, resulting in the inability of the entire fluidized bed to operate stably and continuously. So, how can we prevent large agglomerates from sinking to the bottom of the fluidized bed and allowing them to circulate properly within the bed? The differential weight fluidized bed technology can be employed, which refers to a fluidized bed system composed of different particles with significant differences in density. Particles with high density tend to distribute in the lower part of the bed, while particles with low density tend to distribute in the upper part. In practice, fuels that are dense, have good wear resistance, and are inexpensive are chosen as the basic bed material for fluidized beds; fluidized beds made from such materials have a higher apparent density. The agglomeration of the coal-water mixture creates a \"buoyancy effect\" that prevents the agglomerates formed during operation from settling at the bottom of the bed. This gives the agglomerates the opportunity to gradually disappear through processes such as burnout, wear, and fragmentation, thereby not posing a threat to the stable combustion in the fluidized bed. III. Coarse-grained feeding: A characteristic of the heterogeneous fluidized bed is its ability to ensure that materials of various particle sizes and agglomerates can circulate properly within the bed without settling. This means that there is little need to worry about the impact of the feed on the stable operation of the fluidized bed; instead, the feed issue can be handled in as simple a manner as possible, depending on the requirements regarding combustion efficiency. To make full use of the agglomeration properties of the coal-water mixture, larger-sized agglomerates of fuel can be formed within the fluidized bed, thereby reducing the loss of combustible material due to suspension in the gas flow and improving combustion efficiency. A larger feed particle size can be employed so that, during combustion, aside from a very small amount of fine-particle fuel being carried out of the fluidized bed by the gas flow, most of the fuel burns gradually within the bed in the form of agglomerates, thus ensuring high combustion efficiency. IV. Operation with discontinuous slag discharge. In conventional fluidized bed combustion, apart from a portion of the burned ash being carried away by the gas flow, a considerable amount continues to accumulate within the bed. To prevent the fluidized bed layer from rising indefinitely, slag is typically removed continuously or periodically. For the heterogenous fluidized bed approach, there are certain disadvantages to using continuous slag discharge. One of these disadvantages is that the high-density bed material used will be discharged from the fluidized bed along with the ash and slag; if it cannot be recovered, this will result in high consumption and elevated operating costs. If recovery is attempted, it will make the process equipment overly complex. Another drawback of slag discharge is that it can lead to significant losses of combustible materials, as the fuel stays in the fluidized bed for a relatively short time. On the other hand, fuels with high ash content take longer to burn; large agglomerates can take dozens of minutes, or even over an hour, to burn out. As a result, it is easy for these agglomerates to be discharged from the bed before they have had time to burn completely. Therefore, operating without slag discharge not only prevents the loss of bed material and ensures stability in the properties of the bed layer, but it also has another significant advantage: it avoids fuel loss. It is possible to use larger-grained feed materials, which reduces the amount of fuel that is suspended in the air; this allows the fuel particles to remain in the furnace for a sufficient length of time to burn gradually, thereby improving combustion efficiency. V. Gradual air distribution to reduce NO2 emissions: The fluidized-bed combustion method employs low-temperature combustion, with a combustion temperature of around 900°C. This creates favorable conditions for nitrogen oxide reduction during combustion; as a result, the nitrogen oxides generated by combustion mainly come from the nitrogen in the fuel – fuel NOx. Meanwhile, the nitrogen oxides formed from nitrogen in the air at high temperatures are usually less than 5-10%. To further reduce NOx emissions, the use of staged air distribution allows the excess air coefficient in the fluidized bed layer to be kept below 1, thereby creating a reduction zone. The additional air required for complete combustion of the fuel is supplied to the furnace as secondary air at the upper part of the bed layer. As a result, the presence of this reduction zone within the fluidized bed layer helps to suppress the formation of NOx, and it also promotes the reduction reactions of NOx with substances such as coke, H2CO, and CHNH3, further reducing NOx emissions. VI. Efficient desulfurization: For a considerable portion of coal-water mixtures, the sulfur content is quite high, and at the same time these mixtures have a low calorific value. Therefore, efficient desulfurization techniques suitable for fluidized bed combustion of such coal-water mixtures are also very important. Traditional fluidized bed combustion desulfurization typically involves feeding a desulfurizing agent (limestone) directly into the furnace, where it decomposes into calcium oxide at high temperatures. Calcium oxide then reacts with sulfur dioxide generated during combustion to form calcium sulfate, which is discharged from the furnace as solid ash. Since the desulfurization reaction causes an increase in the volume of solids, if the limestone particles are large, the reaction proceeds slowly and the desired desulfurization effect is not achieved; on the other hand, if the limestone particles are too fine, it increases the suspension resistance. To resolve this contradiction, the coagulation and agglomeration of the coal-water mixture can be utilized, so that the crushed desulfurizing agent is evenly mixed into the coal-water mixture before being fed into the furnace. In this way, after the fuel enters the furnace, it forms uniform coal-water mixture agglomerates containing desulfurization agents, which can remain in the furnace for a sufficient amount of time. By using this method, it is possible to use desulfurization agents with a smaller particle size, thereby improving the efficiency of desulfurization while ensuring thorough reaction and thus achieving efficient desulfurization. In summary, the coal-water mixture, along with the high-efficiency and low-pollution sulfurized bed combustion technology, can utilize large-grain, high-level feeding as well as high-temperature agglomeration properties to reduce entrainment and improve combustion efficiency; it is thus a promising and practical approach worth promoting. Various factors affecting desulfurization efficiency: 1. Influence of sulfur content: Sulfur dioxide emissions are proportional to the sulfur content in coal. During combustion, about 28.5% of the sulfur in the fuel remains in the ash, while 71.5% is emitted into the flue gases as a gas. The SO2 emission concentration at this time depends largely on the degree of precipitation of inorganic sulfur and the desulfurization capacity of the fuel itself. II. Influence of bed temperature: The impact of bed temperature lies mainly in altering the reaction rate of the desulfurization agent, the distribution of solid products, and the characteristics of pore clogging, thereby affecting the desulfurization efficiency and the utilization rate of the desulfurization agent. It is generally recognized that the optimal desulfurization temperature for the desulfurization bed is 890°C. III. Impact of particle size: A smaller particle size of the desulfurization agent results in better desulfurization performance in a circulating fluidized bed, as the separation and return systems ensure the circulation of fine particles; therefore, a limestone particle size of 0–2 mm, with an average of 1–1.5 mm, is considered ideal. IV. Influence of oxygen concentration: The oxygen concentration level in the furnace is related to the excess air coefficient, whether segmented combustion is used, the feeding method, the furnace wind pressure, and the distribution of feeding points. A circulating fluidized bed is generally in an oxidizing atmosphere, and appropriately increasing the excess air coefficient is beneficial for desulfurization efficiency. For example, when the excess air coefficient increases from 1.0 to 2.0, the equivalent SO2 emission concentration drops from 345 PPM to 315 PPM. V. Impact of segmented combustion: Segmented combustion causes significant changes in the oxygen concentration distribution within the furnace, which may have a negative effect on desulfurization in circulating fluidized beds; moreover, the desulfurization efficiency also depends on the location where the secondary air is introduced. VI. Impact of wind speed inside the bed: For circulating fluidized bed boilers, from an operational perspective, changing the wind speed means changing the load. An increase in the primary air volume or the ratio between primary and secondary air, as well as an increase in wind speed, leads to an increase in the circulation rate and a longer residence time for the desulfurization agent. This increases the concentration of the desulfurization agent in the suspension space, without having a negative impact on the desulfurization efficiency. VII. Influence of the circulation ratio: As the circulation ratio increases, the amount of limestone required to achieve an desulfurization efficiency of 80% decreases. In other words, the higher the circulation ratio, the longer the residence time of limestone in the bed due to the recirculation of fly ash, which improves the utilization rate of the desulfurizing agent. This is especially true for smaller particles; since the sulfation reaction occurs at a relatively slow pace, after 30 seconds, the utilization rate of limestone, without taking wear into account, is only 0.2–0.4. By extending this time to one hour, the utilization rate can be increased. Therefore, raising the circulation ratio also increases the particle concentration in the suspension, thereby enhancing the desulfurization efficiency. VIII. Influence of sulfur dioxide residence time in the furnace. In circulating fluidized bed boilers, the particle concentration in the suspension zone is high; moreover, the utilization of the suspension section increases the reaction time for sulfur dioxide. In circulating fluidized bed boilers, the residence time of SO2 is determined by the ratio of the furnace height to the gas velocity. The furnace height must be such that the SO2 residence time is at least 3–4 seconds, but it should not be too long either. IX. Influence of feeding methods Feeding involves two aspects: coal and limestone feeding. This feeding can be carried out at the same location or at different locations, and it can also be done above or below the bed. In terms of the feeding location and mechanism, there are methods such as feeding from the front wall, from both the front and rear walls, as well as feeding through a circuit sealer. Additionally, the number of coal feeding points varies. The feeding method has a significant impact on combustion and gas emissions. Experience gained from the operation of circulating fluidized bed boilers in U.S. power plants shows that a 1:1 distribution of coal feed between the front and rear walls is optimal, while a 2:1 distribution with coal feed to the front wall and the circuit sealer is insufficient. When coal is fed using only a loop sealer, SO2 emissions are high. When all the input comes from the front wall, both NOx and SO2 are at their highest levels. However, the CO level is the lowest, indicating that limestone must be fed simultaneously with coal to achieve satisfactory desulfurization results. Operational experience has shown that when coal is fed in a balanced manner to the front and rear walls, the utilization rate of the desulfurization agent is highest, resulting in moderate SOx emissions. But CO emissions are high. X. Impact of load variations: It is generally believed that when the load of a circulating fluidized bed boiler varies over a fairly wide range, the desulfurization efficiency remains relatively constant. However, in more extreme situations, such as a sudden drop in load, significant changes in bed temperature, gas velocity, hydrodynamic factors, and the concentration of SO2 in the flue gas within the phase-out zone can occur, leading to a noticeable decline in desulfurization efficiency. Nitrogen oxide emissions in circulating fluidized bed boilers and their impacts. In circulating fluidized bed boilers that are in operation, the NOx and N2O emission levels are 50–150 PPM and 25–100 PPM respectively. The most significant characteristic of nitrogen oxide emissions is their strong sensitivity to combustion conditions, bed temperature, and air volume. (1) Effect of temperature: There is consensus regarding the impact of temperature on nitrogen oxides: as the operating bed temperature increases, NOx emissions rise while N2O emissions decrease. Attempting to control NOx by lowering the bed temperature results in an increase in N2O emissions. On the other hand, the control of the bed temperature during operation is also influenced by the load and combustion efficiency; a low bed temperature leads to an increase in CO concentration. Although this facilitates the reduction of NOx, it results in incomplete combustion. Therefore, the optimal bed temperature is around 850°C, at which the conversion rate of fuel nitrogen into N2O is highest, with N2O levels reaching 200–250 PPM. (II) The effect of the excess air coefficient: If staged combustion is not employed, the overall excess air coefficient has a similar effect on both NOx and N2O. As the excess air coefficient decreases, both NOx and N2O emissions decline; when the excess air coefficient increases, the effect on NOx and N2O emissions weakens. When the excess air coefficient is too low or too high, the CO concentration increases, which facilitates the reduction and decomposition of NOx and N2O. In regions where O2 is less than 1.5% or CO is around 1%, low-oxygen combustion (O2 at the furnace outlet)