Classification of circulating fluidized bed boilers

1. CFB boilers can be classified into medium-temperature separation and high-temperature separation types based on the temperature of the flue gas at the separator； 2. Classified by the fluidization state inside the furnace, circulating fluidized bed boilers can be divided into bubble beds, turbulent flow beds, and rapid flow beds ； 3. Classified by the high, medium, and low levels of material circulation ratio, they can be divided into: circulating fluidized bed boilers with a low circulation ratio (K<15), those with a medium circulation ratio (K=15∽40), and those with a high circulation ratio (K≥40) ； 4. Classified by the amount of material carried per unit of flue gas at the furnace outlet: low-carrying-rate circulating fluidized bed boilers, medium-carrying-rate circulating fluidized bed boilers, and high-carrying-rate circulating fluidized bed boilers.

Over the past 20-plus years, to develop and improve circulating fluidized bed combustion technology, industrial countries around the world have made substantial investments in terms of technology, human resources, and financial resources; yet it is still the more developed industrial countries that lead the way in this field. In the early days, the main foreign research and development organizations as well as manufacturers included Germany’s Lurgi company, Finland’s Ahlstrom company, the United States’ Foster Wheeler company, Germany’s Babcock company, the United States’ Battelle Research Center, and Sweden’s Studsvik company. Through continuous development, mergers, and reorganizations, Alstom in France and Foster Wheeler in the United States have now become the two companies with the strongest production capacity for circulating fluidized bed boilers in the world. The main domestic manufacturers include Dongfang Boiler (Group) Co., Ltd., Harbin Boiler Factory Co., Ltd., Shanghai Boiler Factory Co., Ltd., Jinan Boiler Group Co., Ltd., and Wuxi Huaguang Boiler Co., Ltd. Some of them collaborate with research institutions, drawing on advanced foreign experiences to develop circulating fluidized bed boilers with unique characteristics for China, while others work together with large foreign manufacturers of such boilers to produce larger-scale circulating fluidized bed boilers. Although it is under development. There are many companies and manufacturers that develop and produce circulating fluidized bed boilers, but in terms of the design characteristics of these boilers, there are mainly the following types. 1. The Pyroflow type circulating fluidized bed boiler developed by Aoson Company. Since the early 1970s, Aoson Company has invested significant human and financial resources in establishing large-scale testing facilities dedicated to developing circulating fluidized bed technologies suitable for various types of fuels. The largest circulating fluidized bed boiler of 420 t/h, produced in the late 1980s, was won by this company for the Neijiang project and Maoming project in China. The typical structure of the Bailu Bao circulating fluidized bed boiler is shown in Figure 4-2, and its main technical features are as follows: Figure 4-1 Pyroflow-type circulating fluidized bed boiler. (1) There is no external heat exchanger; the circulating fluidized bed boiler mainly consists of a combustion chamber. It consists of a high-temperature cyclone separator, a return conveyor, convective flues, etc. Some are also equipped with a cold slagger, using a straight high-temperature cyclone separator; the return of solid particles is achieved through an annular seal. (2) The combustion chamber is divided into upper and lower sections. The lower section consists of an extension of the water wall, a steel plate enclosure, and a refractory brick lining. Around the upper part of the furnace, there is a membrane-type water wall. The first and second superheaters are located at the top of the furnace or above the rear flue; the second superheater is made of steel pipes and is placed in the middle of the furnace. This is a design feature unique to the Bailebo type circulating fluidized bed boilers. (3) A high-temperature cyclone separation method is employed, with the maximum inlet flue gas temperature reaching 950°C. This separator can be installed in front of the furnace, on either side of the furnace, or between the furnace chamber and the tail flue, offering flexible installation options. A high cycling rate is often used, achieving a separation efficiency of 99%. (4) Primary air is supplied at the furnace bottom, while secondary air is supplied above the dense phase zone. The primary air ratio is 40% to 70%, with about 50% being the typical value. Bed temperature control and rough adjustment of superheated steam temperature are achieved by adjusting the ratio of primary to secondary air in the furnace. The fuel burns not only at the bottom of the furnace but also rises with the airflow and burns throughout the entire furnace, resulting in a relatively uniform flue gas temperature along the height of the water-cooled wall. At low load, the material circulation rate decreases, combustion concentrates in the lower part of the furnace, and a gradual transition to bubbling bed operation mode takes place. It can operate at high cycle rates using a variety of fuels, ranging from petroleum coke with virtually zero volatiles to oil shale, anthracite, wastewater wood, peat, lignite, and coalite with ash contents exceeding 65%. Bituminous coal, high-sulfur coal, industrial waste, etc. (6) The load regulation ratio is 3:1 or 4:1. The load change rate is 7%/min when loading, and 10%/min when unloading. (7) The boiler’s layout is compact, resulting in much lower steel consumption and plant electricity usage compared to Lurgi’s circulating fluidized bed boilers. Based on statistics from the operational furnaces, the boiler’s output, parameters, thermal efficiency, fuel adaptability, operation control, and emission levels are all at satisfactory levels, with an availability rate as high as 98%. Therefore, AOSUN’s BAIHUBAO type circulating fluidized bed boilers enjoy a high reputation worldwide and hold a large market share. The drawback of this type of boiler is that the manufacturing process for the secondary superheater is extremely complex; any minor defect can lead to rapid wear and tear. To overcome the high manufacturing costs of air-cooled cyclones, the Finnish company Ahlstrom proposed the Pyroflow Compact design concept. Figure 4-2 shows the structure of the Pyroflow Compact circulating fluidized bed boiler. The Pyroflow Compact circulating fluidized bed boiler is unique in that it uses a square separator. The separation mechanism of the separator is essentially no different from that of a circular cyclone; the shell still uses the FW-type water (steam) cooled tubular wall design. However, it is distinct due to its planar structure. The wall of the separator forms part of the furnace wall water circulation system, which means that there is no need for thermal expansion joints between it and the furnace. At the same time, the square separator can be arranged closely against the furnace chamber, thereby significantly reducing the volume of the entire circulating fluidized bed boiler and resulting in a very compact layout. Furthermore, to prevent wear, a thin layer of fire-resistant material is applied to the water-cooled surface of the square separator; the separator also serves as a heat transfer surface, thereby accelerating the startup and cooling rates of the boiler. A cooled separator is used, allowing the temperature inside the separator to be effectively controlled, thereby eliminating the risk of coking in the separator and the material legs. The manufacturing cost of water-cooled or steam-cooled square cyclones is roughly equivalent to that of adiabatic cyclones; however, considering the significant savings in insulation and refractory materials for the former, the actual cost ends up being lower. Furthermore, it reduces heat loss and improves boiler efficiency. Furthermore, due to the reduced insulation thickness, the start-up and shutdown speeds can be increased. During these processes, the rate of temperature rise of the bed material no longer depends on the refractory materials, but rather on the safety of the water circulation, thereby **reducing** the start-up and shutdown times. Figure 4-2 shows the structure of the Pyroflow Compact circulating fluidized bed boiler. 2. Jinan Boiler Group Co., Ltd. is one of the earliest manufacturers in China to develop CFB technology. Since 1986, when it collaborated with the Institute of Engineering Thermophysics of the Chinese Academy of Sciences to develop and produce China’s first 35 t/h circulating fluidized bed boiler, the company has subsequently developed boilers with capacities of 75, 130, 220, 240, and 450 t/h. To date, Jinan Boiler Group Co., Ltd. has produced over 700 circulating fluidized bed boilers with various parameters. Among them, the 75t/h circulating fluidized bed boilers have been awarded the title of “**Key New Products”; more than 400 of these boilers have been sold. There are 30 units of the 130t/h type and 32 units of the 240t/h type, accounting for over 50% of the total number of small and medium-sized circulating fluidized bed boilers in the country. The company is currently moving forward in the development of larger circulating fluidized bed boilers. The circulating fluidized bed boilers produced by Jinan Boiler Group Co., Ltd. have the following features: (1) The furnace water wall, fluidized bed, air distribution plate, and air chamber all adopt a membrane-type water wall structure, forming a single unit as a result of which the furnace has excellent airtightness. (2) A high-temperature cyclone separator is used as the separation device, which offers high separation efficiency and results in low carbon content in the fly ash. A water-cooling jacket is installed at the lower part of the separator to control the temperature of the material leg and the recycler, ensuring the stability and reliability of the circulation system. (3) By reasonably using non-metallic expansion joints and stainless steel bellows expansion joints, the boiler achieves proper overall expansion and good sealing performance. By using either under-bed air duct ignition or a combined under-bed and above-bed ignition method, the ignition success rate is achieved at 100%, thereby saving fuel used for ignition. (4) By investigating a large number of CFB units in operation, the patterns of wear can be identified, allowing for proactive preventive measures to be taken: reducing the flow velocity of the flue gas in the design, installing anti-wear covers and flow-blocking baffles, and applying anti-wear and refractory materials or performing metal thermal spraying on areas prone to wear. This helps to prevent or reduce wear, thereby extending the continuous operation period and the lifespan of the boiler. (5) The Distributed Control System (DCS) enables the automatic control of the circulating fluidized bed boiler. 3. Main technical specifications of domestic small and medium-sized circulating fluidized bed boilers. The 75t/h circulating fluidized bed boiler is the most representative model among domestic circulating fluidized bed boilers; almost all domestic manufacturers of A and B class boilers, as well as some manufacturers of C class boilers, are actively developing their own versions of such boilers. More than 600 units of 75t/h circulating fluidized bed boilers are in use across the country. In commercial operation, due to various factors, the performance levels vary greatly, with significant differences observed in boiler startup, boiler operation, auxiliary equipment operation, as well as related technical aspects. The circulating fluidized bed boilers put into operation in the early stages had the following problems: Regarding the boiler structure itself: ① The water wall and furnace walls were severely worn, resulting in a very short maximum continuous operating time for the boiler ； ② Coking occurs inside the boiler, with varying degrees of coking taking place both during startup and while the boiler is in operation ； ③ Boiler efficiency is generally low; aside from a few boilers whose thermal efficiency reaches 90%, most boilers have low efficiency and are not competitive with coal-fired boilers ； ④ Some boilers experience blockages in their cold slag pipes ； ⑤ Wear of economizers is a fairly common issue ； ⑥ The overall sealing performance of the boiler is poor, especially at the junction between the dense-phase zone and the dilute-phase zone. The burners of boilers put into operation after the late 1990s have relatively good performance, whereas those of boilers commissioned in the mid-1990s have slightly poorer performance. Regarding the circulation loop: ① The shedding of the lining inside the cyclone separator is a fairly common issue, and the conflict between resistance and efficiency is quite prominent ； ② Inertial separators have many problems, such as burning out and falling off. Wear and other issues are severe; performance needs improvement, and the material still requires further study ； ③ There is ash buildup in the material legs ； ④ Re-ignition occurs in the cyclone separator, leg, and return system. Regarding auxiliary equipment: ① For most boilers, spiral coal feeders are used for feeding coal; the initial diameter chosen was too small, so it had to be increased. This situation has also occurred with boilers of other capacities. ② The output capacity of the crushing system is insufficient, and the ring hammers in the crusher wear out severely, requiring replacement every year. ③ Most users use fan model No. 16, but the power of the motors used with them varies greatly, ranging from 250 kW for the smaller ones to 500 kW for the larger ones. I. The total air volume for primary and secondary air is 0.11 mm3/h, distributed in a 50:50 ratio; the actual output of the primary air fan varies significantly, with values ranging from 70% at the higher end to 35% at the lower end ； The actual output of the secondary fan is 30% to 65%; although it is sufficient to meet the air requirements for combustion, it has a significant impact on circulation, even leading to operation characteristics similar to those of a bubbling bed. Based on the operation of circulating fluidized bed boilers in our country since the late 1990s, these boilers are able to reach their rated output and can handle a load of up to 110%; their efficiency can reach 90%. When the furnace temperature is properly controlled and an appropriate limestone desulfurization agent is used, with a calcium-sulfur ratio of 1.5 to 2, the desulfurization efficiency can reach 80%. For example, the YG75/5.29 type circulating fluidized bed boilers produced by Jinan Boiler Group Co., Ltd. are designed for coal particles with a size range of 0–13 mm; the height of the dense phase bed is approximately 4 m, and the circulation rate is 20–25. Using the under-bed ignition technology, the cold start time is 4–6 hours, the hot start time is 1–1.5 hours, and the flame-out time is 8 hours. The test results show that the oxygen content in the flue gas at the furnace outlet is 4%–5%, the fly ash fraction is 40%–60%, the carbon content in the fly ash is 3%–5%, the carbon content in the slag is less than 2%, and the plant electricity consumption is 15%. Maximum continuous operating time is 4000 hours, with a maintenance cycle of 4 months.

Based on the flow pattern in the fluidized bed and the type of bed material, fluidized bed boilers can be divided into three main categories: bubbling bed boilers, circulating bed boilers, and pressurized fluidized bed boilers. The circulating bed boilers include the following types: 1. Multi-particle fluidized bed boiler, 2. Pyroflow circulating fluidized bed boiler, 3. Circofluid circulating fluidized bed boiler

In recent years, several types of fluidized bed boilers have been introduced in our country. In terms of the arrangement of the heating surfaces, there are those with tubes embedded in the dense-phase bed, as well as those without such tubes； The fluidization velocity can range from as low as 3–4 meters per second to as high as 5–6 meters per second ； There are more types of separators, such as high-temperature cyclone separators ； Types such as medium-temperature cyclone separators, horizontal cyclone separators, planar flow louver systems, and trough-shaped steel separators are all referred to as circulating fluidized bed boilers. However, from a mechanistic perspective, it remains to be determined whether it belongs to CFBB. As is well known, fluidized bed boilers are divided into two main categories: bubbling fluidized bed boilers (BFBB) and circulating fluidized bed boilers (CF-BB). To date, there is no clear and authoritative classification system between the two. Some suggest classifying them based on the fluidization velocity, but from the perspective of gas-solid two-phase dynamics, wind speed only becomes meaningful in relation to particle size and density. Others propose distinguishing between them based on whether the dense phase region is a bubbling bed, a turbulent bed, or a rapid flow bed; however, boilers use fuels with high screening forces, and those using coal ash as the bed material have a bubbling bed in their dense phase region, so this classification method is still incomplete. Some people also use whether there is a cycle of gray as a criterion, and so on; in all these cases, attention is divided among different aspects. In the author’s view, we might as well classify them based on the mechanism of combustion. In bubbling bed boilers, combustion primarily takes place in the dense-phase region at the lower part of the furnace. As recommended in China’s \"Technical Manual for Industrial Boilers (Volume 2)\", for ordinary gangue bituminous coal, lean coal, and anthracite, the proportion of combustion that occurs in the dense-phase region ranges from 75% to 95%; the air required for combustion is also supplied to the bed mainly through primary air. In circulating fluidized bed boilers, the proportion of primary air is generally between 50% and 60%. The combustion fraction in a dense-bed reactor fluctuates around the values mentioned above, influenced by factors such as fluidization velocity, fuel particle size and properties, bed height, and bed temperature. The remaining fuel burns in suspension in the dilute-phase region at the upper part of the furnace; therefore, in terms of the combustion mechanism, the BFBB is similar to a stratified combustion furnace, while the CFBB is more similar to a chamber combustion furnace. There is a significant difference between the two in this regard, so it seems more reasonable to classify them based on this criterion. Boiler systems with a bubbling fluidized bed have a high combustion share in their dense bed phase, necessitating the use of embedded heat-exchanging surfaces to absorb the heat released from combustion. The heat transfer coefficient of buried tubes is as high as 220–270 KW/MC, which is far higher than the 100–500 KW/m2°C value for the heating surfaces in CFBB furnaces. Although the heat transfer coefficient in the dilute phase region of BFBB is lower, the proportion of heat absorbed in this phase is relatively small. Overall, for boilers with smaller capacity, the BFBB design results in less steel being required for the heating surfaces. In BFBB, combustion takes place mainly in a bed of coal particles with a relatively large average size; the coal crushing equipment is simpler, electricity consumption is low, the fluidization velocity is low, fine coal particles remain suspended for longer periods of time, and the furnace height is also reduced. Although the buried pipes experience wear, with proper measures to prevent wear and tear, horizontally buried pipes can generally last for five years, while vertically buried pipes can last... By using fly ash recycling at the tail end, the combustion efficiency of BFBB can reach 97%; if a separator is installed at the furnace outlet to enable hot fly ash recycling, this efficiency can go as high as 98–99%. However, in this case, the purpose of installing the separator is mainly to improve combustion efficiency, rather than, as in the case of CFBB, to change the combustion and heat transfer mechanisms within the furnace. The sectional heat load of CFBB is 2–3 times that of BFBB (the total heat load from top to bottom, rather than per layer), which facilitates larger scale operations. It ensures uniform temperature within the furnace, results in low emissions of air pollutants, and achieves high combustion efficiency (over 99%), representing an improvement over BFBB technology with superior performance. However, since the separator cannot capture fine coal particles, a larger furnace is required, and higher standards are needed regarding the particle size of the coal as well as operational control. This leads to high investment costs and complex technology; therefore, CFBB does not offer significant advantages for boilers of medium and small capacity. As a result, some foreign researchers believe that BFBB is suitable for capacities below 50 t/h, while CFBB is appropriate for capacities above 220 t/h, with both types coexisting in the 50–220 t/h range. Over the past many years, our country has built nearly 3,000 boiling furnaces (i.e., BFBBs). Although they have played a significant role in burning low-quality coal, they have been operating at a low level, producing large amounts of fly ash and high carbon content, resulting in low boiler efficiency. Coupled with insufficient investment in dust removal, the issue of smoke and dust control has not been properly addressed, which has led to a somewhat poor reputation for these boiling furnaces. After the emergence of CFBB, many manufacturers began to market circulating fluidized bed boilers, introducing various types of such boilers. For example, Tsinghua University developed a circulating bed boiler with a low carry-over rate, while Harbin Institute of Technology and Beiguo Corporation created circulating bed boilers equipped with buried tubes and trough-type separators; in fact, all of these are variations of BFBB. But they are improved boiling boilers that raise boiling boiler technology to a higher level; these types of boilers have great viability in the field of industrial boilers and cogeneration boilers. Therefore, we should celebrate the new achievements of BFBB, restore its rightful place and reputation, and develop this type of BFBB within certain boiler capacity ranges. Our country has the largest number of BFBBs in the world, and thanks to its long-term operational experience, the improved BFBB technology is quite mature. The CFBB technology still needs to be improved and refined. When choosing among various types of furnaces, it is first necessary to determine whether it belongs to the BFBB category or the CFBB category; only after that should other technical parameters and reliability factors be considered. The sections that follow in this article are focused on CFBB, while those techniques that are common to both categories apply as well. Fluidization velocity: The most direct and significant impact of fluidization velocity on CFBB is its effect on the lifting, bending, and entrainment of the circulating material. As V increases, the amount of entrainment rises at a rapid pace. In early foreign CFBB technologies such as Lurgi technology, V was as high as 8–12 M/S; however, issues such as wear and increased energy consumption resulting from these high flow rates led to a reduction in V to around 6 M/S at present. CFBB technology development in China started relatively late, and initially, due to concerns over the aforementioned problems, some furnaces were designed to operate at lower V values (4–5 M/S). It was found that insufficient circulation of material occurred under these conditions, and improving the wind speed greatly improved the situation; nowadays, V has also been increased to 5.5–6 M/S, bringing it closer to the levels seen in foreign furnaces. Coal particle size and coal quality analysis: The fluidization speed of CFBB is very high; even larger bed material particle sizes can be fluidized. As shown in the literature, the particle size range of coal fed into the furnace can be 0–12, 0–20, 0–25 MM, etc., with the allowable ranges varying depending on the manufacturer and type of coal. This allows for a wider range of fuel particle sizes compared to BFBB, and the maximum allowable particle size is also larger. However, according to our research and some foreign literature reports, the average particle size of the fuel used in CFBB is actually much smaller than that in BFBB. The average fuel particle size of BFBB is 1–2 MM, while that of CFBB is only 300–400 UM. Strictly speaking, CFBB requires a large proportion of fine particles in the fuel whose terminal velocity is less than the fluidization velocity; this allows such fine coal particles to be carried into the suspension zone for combustion once they enter the furnace, and it also helps to increase the amount of material in circulation. The effect of fuel particle size is mainly reflected in its impact on the fraction of combustion in the dense-bed phase and on the material balance; with finer fuel particles, the fraction of combustion in the dense-bed phase is smaller, and the amount of material in circulation is greater. The determination and selection of the particle size distribution of the fuel fed into the CFBB are related to the choice of fluidization velocity; it is evident that particle size has a significant impact on both. The selected particle size distribution should ensure that, under the determined fluidization velocity, sufficient finely divided coal particles are blown into the suspension zone, so as to maintain an adequate combustion rate in the upper part and to form enough bed material to preserve material balance. Other main factors affecting the particle size of the fuel fed into the furnace are the pyroexplosive properties of the coal and its volatile content; for coals with strong pyroexplosive properties, a larger particle size can be chosen. Once these larger coal particles enter the furnace, they undergo pyroexplosion, which increases the amount of material produced, allowing for a looser particle size distribution in the coal fed into the furnace. I. Secondary air ratio: The air required for combustion is divided into primary and secondary air, which are supplied to the fluidized bed combustion chamber from different locations. This creates a reducing atmosphere within the dense phase bed, enabling staged combustion. This approach helps to reduce the formation of thermally induced NOX, which is one of the main advantages of CFBB. However, the purpose of dividing the air into primary and secondary air goes beyond this. The primary air ratio (the proportion of primary air in the total air volume) directly determines the extent of combustion in the dense phase bed. Under the same conditions, a higher primary air ratio results in a greater degree of combustion in the dense phase bed; in such cases, more cool circulating material is needed to return to the dense phase bed in order to absorb the heat generated by combustion and maintain its temperature. If there isn’t enough circulating material, the temperature of the fluidized bed will rise too high, making it impossible to add more coal and thus limiting the load capacity. The material used for cooling the bed can come from the cooled circulating ash collected by the separator, or from the circulating ash that falls along the membrane walls surrounding the furnace – this ash is cooled as it comes into contact with the membrane walls during its descent. From the perspective of combustion and thermal balance in a dense-bed reactor, the lower the primary air ratio, the fewer requirements there are regarding the material balance of the circulating ash. However, the selection of the primary air ratio is also influenced by factors such as the particle size and properties of the fuel. A low primary air ratio means that a smaller proportion of large particles in the fuel should not be carried up into the suspension zone for combustion; otherwise, these large particles will not burn completely due to a lack of sufficient oxygen, resulting in very high carbon content in the discharged bed ash. The primary air ratio is generally set at around 50%, while for anthracite it can be above 60%. Secondary air is generally injected into the furnace above the dense-phase bed; this serves to supply the air required for combustion and also helps to disturb the flow, thereby enhancing the mixing between the gas and solid phases. The lower part of the CFBB furnace is often designed to be tapered, and the secondary air can be divided into several streams that are introduced from different heights in order to maintain a relatively uniform flow velocity of the smoke gases within the furnace. The location of the secondary air inlets also has a significant impact; if they are placed in areas with high ash concentration in the transition zone above the dense-phase bed, more carbon particles and material can be blown into that space, thereby increasing the fuel fraction and material concentration in the upper part. The importance of the separator for CFBB is beyond doubt; without a separator, there would be no CFBB. For this reason, considerable attention has been devoted to the research and development of separators both domestically and internationally. The type and structure of the separator constitute one of the distinguishing features among the CFBB variants. The main performance indicator of the CFBB separator remains the separation efficiency; it must have a high enough efficiency to, first, provide sufficient recycled material, and second, collect fine carbon particles to send them back to the furnace for re-combustion, thereby improving combustion efficiency. The main material in the CFBB cycle is particles with a size of 200–300 WM. The designed separator not only achieves an extremely high separation efficiency for this particle size (>99%), but the d50 value should also be as low as possible in order to increase the burn rate of carbon. Analysis of the carbon content in CFBB fly ash shows that it reaches a peak at a certain particle size, after which it decreases. The particle size at which this peak occurs is closely related to the efficiency of the separator. The separators currently used in CFBB can be mainly divided into two categories: cyclone separators and inertial separators. Generally, cyclone separators have higher efficiency but larger size, while inertial separators have slightly lower efficiency yet smaller dimensions, which makes the boiler structure more compact. Based on the operating conditions, separators can be divided into two main categories: high-temperature separation and medium-temperature separation. In terms of their impact on boiler performance, high-temperature separation is superior; this is because the solid material concentration in the CFBB furnace is high, resulting in poor mixing within the furnace and higher CO concentrations. The secondary combustion in a high-temperature separator helps to reduce CO concentrations, and the resulting temperature increase facilitates the reduction of N2O, thereby lowering its emission levels. When selecting a separator, the capacity range of the boiler must also be taken into account, along with a technical and economic comparison. For example, in the case of small industrial furnaces, cyclone separators are used; considering that both the cyclone chamber and the material legs need to have a certain height, the furnace chamber must also be sufficiently tall. Otherwise, reducing the height of the cyclone chamber and material legs will inevitably affect their performance. At this point, a comprehensive technical-economic analysis should be conducted. The ash return device: In the ash circulation system of CFBB boilers, aside from a few mechanical valves (such as Luirgl’s conical valves), mechanical exhaust valves are generally used, such as J-type valves, L-type valves, V-type valves, etc. Non-mechanical valves have no moving parts, and their opening and closing are controlled by the air supply; their advantages are self-evident. Non-mechanical valves are divided into two main categories: self-balancing and adjustable. Valves such as J-valves, V-valves, and LOOP seal seal ports belong to the self-balancing type, meaning that the flow rate is automatically adjusted based on the incoming flow rate; these valves have limited capability for regulating the flow rate themselves. The L-valve, on the other hand, is of the adjustable type, allowing the flow rate to be adjusted as needed. From his own experience, the biggest issue with L-valves during operation is the measurement of the material level in the vertical section of the valve. If the material level in this vertical section is too low, the purge air may not carry the ash out through the horizontal section, but instead blow upward from the vertical section. This not only fails to ensure proper sealing of the valve but can also lead to coking, so this issue needs to be given attention. In the design of non-mechanical valves, it is important first to select an appropriate ash flow cross-section. Second, if the returning ash is at high temperature, it is also necessary to calculate the thermal balance within the valve: oxygen in the loosening air comes into contact with carbon in the ash and burns, with the heat released being partially converted into the enthalpy of hot flue gas, while the remaining heat heats the circulating ash, turning it into its sensible heat. The temperature rise of the ash should be controlled to prevent coking due to excessively high ash temperatures; this is also one of the reasons why water-cooled footings have been developed abroad in recent years. Heating surface wear: The BFBB dense-phase bed is equipped with embedded tube heating surfaces, which are constantly subjected to a certain degree of wear due to the scouring by the fluidized bed material. The wear of BFBB mainly occurs in the areas where tubes are embedded; since there are no heat-sensitive surfaces with embedded tubes in the dense-phase bed of CFBB, the wear problem is not resolved as a result. Due to some shortcomings in the design, severe wear can occur in any part of the furnace and ash handling system. Mechanistically, metal wear can be divided into two categories: one is the gradual loss of mass of metal components due to friction as the metal surface is abraded by solid particles; the other category involves the formation of an oxide layer on the metal surface. This layer is very hard but brittle, and under the action of particle abrasion, it peels off in small amounts and quickly. A new oxide layer then forms on the exposed metal surface, and wear proceeds through this process. The table below shows a comparison of the hardness of the oxide layer with that of some other materials (3): Table 1: Material Hardness Table (at 20°C) Material: Limestone silicate, Steel, Coating, Oxide film; Hardness (HV): 140–160, 800, 130–250, 500–1800, 600–1800. It can be seen that the hardness of the oxide film is extremely high; therefore, if an oxide film can be formed on the surface of pipes, it is highly beneficial for reducing wear. The formation rate of the oxygen film is very important; if it is lower than the wear rate, no oxide film can form on the metal surface. Experiments have shown that an oxide film tends to form more easily when the wall temperature is above 300 degrees Celsius. The dense-bed reactors of CFBB are generally in a reducing atmosphere, which is unfavorable for the formation of an oxide film on the metal surface; wear-resistant materials can be used to coat the tubes in order to prevent severe wear. At the boundary between the reducing and oxidizing atmospheres, wear increases due to the fluctuations at this interface, and it should be treated in the same way as the reducing zone. Areas such as the junction between the vertical section and the converging section of the lower furnace wall, the furnace top, and the furnace outlet are prone to severe wear; therefore, structural considerations or anti-wear measures should be taken during design. The wear of the convective heating surfaces at the tail section is also an issue that must be taken seriously; several CFBB units put into operation in China earlier have already shown signs of wear. Some people believe that since CFBB is equipped with a separator, the ash concentration in the exhaust flue is lower than that in BFBB; however, this view is incomplete. The presence of a separator that collects the ash and returns it to the furnace leads to an increase in the ash concentration inside the furnace. Separators are designed to deal with this high ash concentration, and to maintain the ash circulation required for normal operation, the separation efficiency needs to be over 99%. Despite such high efficiency, the absolute amount of ash that fails to be collected by the separator due to the high concentration in the furnace can still be quite large. The ash concentration in the exhaust flue remains high as well, owing to the high levels of ash inside the furnace. In the tail flue, the smoke flows downward; the particles move along with the smoke while being affected by gravity. The absolute velocity of these particles is equal to the velocity of the smoke plus the size of the particles, which leads to severe wear on the heating surfaces at the rear of the equipment, such as the economizer. If there is a large gap between the elbows and the wall surface of the tube bundles in the tail heating surfaces such as the economizer, a flue gas corridor is formed, which accelerates wear. The wear rate of the metal wall is proportional to the velocity to the power of 3–3.5, and is proportional to the square of the diameter of the ash particles. When designing the tail flue, the above factors should be fully considered, an appropriate wind speed should be selected, and a reasonable structure should be designed to avoid severe wear of the heated surfaces.

Based on the movement characteristics of the sulfurization bed, it can be divided into bubbling beds, turbulent beds, and rapid beds.